Two Guys Rally

Rear Wheel Steering: Why Not? [ July 21st, 2008 ] By:Mark Ozimek

I was pushing a cart around at work today at a rather rapid rate, and no, I was not racing a co-worker, I swear! It was one of those carts that had fixed front wheels and rear wheels that were free to pivot. Being the forever analytical engineer I am, I noticed how much the cart liked to turn, and how much the front wheels slid. I didn’t give it much thought until I hopped into my car to go home. Then: “Why don’t we see many, if any cars with rear wheel steering?”.

Obviously, it must have some sort of major handing characteristic that is undesirable, or we would have seen it in racing a long time ago. It is kind of hard to predict what such a car would handle like without testing it out. I would say drive a normal car in reverse, but that’s a little different because the suspension is designed in such a way to make the wheels center when going forward. In reverse, the car wants to turn more into the direction it’s turning.

From an overall physics standpoint, it’s rather curious to think about. Normal cars turn by pulling the front end in towards the center of the turn, the rear wheels just follow along. With a car that has rear wheel steering, the back end is let loose, and follows the front tires that stay on the same line. This should sound vaguely familiar, as that is what happens when the car oversteers. To be precise, it’s very very similar to what is happening when the car oversteers just enough that the turn can be held by keeping the front wheel’s axis in line with the center of the turn.

Knowing that, it’s pretty easy to understand why we don’t see it in cars; The setup is highly unstable. However, this could be a very good thing for certain types of racing, if the driver is up to dealing with the demands. Rally is one where it’s benefits could be seen greatly. Going around hairpin turns would be a breeze, just keep the front wheels on the line you want to take, and pivot the rear out, just like what happens through careful use of throttle, steering and handbrake use on normal cars. If the front tires start sliding, simply turn a little harder to pivot the car in some more. If the back slides out, turn less. Very intuitive, as opposed to countersteering and managing throttle input to keep the back from spinning around, or juggling weight transfer through braking to manage understeer.

Someday I’ll have to try racing a rear wheel steering car to confirm my suspicions. Until then, just a thought to keep in the back of your mind, instead of taking for granted that cars should always use the front wheels for steering. Also, think about how easy parallel parking would be!

The Volvo Chronicles: Worn Suspension Parts [ July 14th, 2008 ] By:Mark Ozimek

Since there are more things going on with my S70 T5 than just some missing power, I decided it would be wise to just label everything that relates to my car the same way. After all, it’s a ten year old car with 150,000 miles on it. Things are gonna break, and I’m gonna have to replace stuff and write about it.

So what happened this time? Well, I had to get my car inspected, in order to remain road-legal. So I drop my car off at a local mechanic in the morning. After a few hours, I get a call from the mechanic, informing me that my inner tie-rods had too much play in them, and he couldn’t pass the car as it was. In case you are unsure, the tie-rods are the beams that connect the steering rack to the wheels so that you can turn the car. They have to allow movement in a few axes to account for turning and suspension travel. The parts the rub together are usually the parts that break after a while. If you break the rod itself, I’ll be very impressed.

Suspicious, because I never noticed excessive slop in the front end, I went down to check it out, and sure enough, there was a lot of play. I don’t know exactly how much is passable, but the wheels were pivoting about the vertical axis enough to move the front and rear parts of the tire tread around 3/8″ to 1/2″ on the driver’s side, a little less on passenger.

I give him the go ahead to replace the inner tie rods, and asked him to change out the outer tie-rods too, since it’s only 5 minutes of extra work once you’re in there. If the inners are worn out, the outers are likely to be pretty bad too. Unfortunately, since the car was already in the shop, and I was on a tight time schedule, I didn’t do the work myself. It’s not a very hard job, just somewhat time consuming, and you should get an alignment afterwards.

Curious to see how much of an effect the worn parts had, I took the car out for a spirited drive afterwards. The difference was almost surprising, especially over rough surfaces. I had never thought about it before, but when turning on a road that has bumps, or is generally rough, my S70 made some clunking noises, and tended to skip sideways every now and then.

Now with the new tie-rods, the car was much more settled and predictable, and a few of the clunking noises I had grown accustomed to and thought nothing of had disappeared. Of course, it wasn’t all better, since all the other bushings and ball joints are probably pretty old too. Someday I’ll have to take the time to rip apart my suspension and replace all the other worn parts.

Moral of the story? Check the amount of play in the suspension part joints, even something that was relatively unnoticeable could actually be a pretty serious issue!

Performance Loss Hunt: Part 3 [ July 8th, 2008 ] By:Mark Ozimek

A while back, I made two posts about my car, and how there is a lack of power in the top end compared to what it used to feel like. I verified that the exhaust isn’t causing significant restriction and that the turbo is making about as much boost as it should be.

Since this is turning into a guess and check thing, mostly because my car is 10 years old and has almost 150,000 miles on it, and I don’t know how it was treated for the first 130,000 miles of its life, I said “To hell with it”, and ordered two things that I highly suspect to be contributing to the problem: Vacuum hoses and a CBV diaphragm.

Unfortunately, the vendor I’m getting the CBV diaphragm from does not have any in stock, nor does anyone else that I could find, replacing that part will sit on the back burner for now.

The silicon vacuum hoses from StylinMotors came in the other day, and sat in a corner of my apartment until I had the time to start ripping junk out of my engine compartment to get access to some of the hoses. Thanks to Independence Day being on Friday, I got a three day weekend to have fun. First order of business was figuring out what each hose does, and where it needs to attach to. Ideally, I would be able to just take one hose out, cut a new one to match and install. Knowing what everything does is something important to me, so I couldn’t make it that easy for myself.

After a couple minutes of fun wrestling with worm gear clamps and torx screws, the engine compartment of my S70 looked like this:

(click for larger image)

Although it looks like a disaster, all I really did there was take out the intake filter box and two intercooler pipes that were in the way of some hoses I needed to get to. Judging from the hose clamp style, the hoses are the original parts that were on the car when it rolled off the factory floor.

After prying the clamp off, I found something quite comical and frustrating at the same time. The hoses had rotted into place! I had to cut off every single hose I changed, since they would not come off any other way. Unfortunately, this meant that some of the hoses that are in tight spots did not get changed, since I couldn’t fit my knife into the area. I still plan on changing them though, I just need to remove more parts that get in the way.

Afterwards, the hoses in that picture had been replaced with silicone parts:

While changing out hoses, I found something very interesting. In the first engine picture, there is a small white thing on the very left edge in the center of the picture. This is a check valve that only allows air to flow in one direction. That hose comes from the intake manifold and leads downward to a T junction. The hose going to the right has another check valve, and connects to the intake hose just before the compressor inlet. The other hose goes to a solenoid that is part of the onboard fuel vapor recovery system.

Since the check valves are aligned in such a way to only allow air to be pulled out of the solenoid that is attached to a carbon filter, a broken valve from the intake manifold means that boost pressure can leak out of the intake manifold to before the compressor inlet or into the carbon filter. Both of these are things that should be avoided due to loss of efficiency and contamination of the fuel vapor recovery system.

Either way, I replaced the hoses I had relatively easy access to. Some will require the removal of the intake manifold, another is attached to the compressor housing, which the bottom part of the intake hose blocks, there is even a hose that runs over the top and back down to the back of the engine to the fuel pressure regulator. I’ll try to address the rest of these when the CBV diaphragm comes in.

So with all that said and done, did it fix the problem I’ve been seeing? Well, no. It actually did some things I didn’t really expect. Acceleration from a stop is now much smoother and more consistent as the engine speed increases. Fuel economy on the highway seems to have gone up by one or two MPG, but it is still too early to tell for sure. The most interesting is that the brakes feel much more responsive now. My suspicion is that there was a/some vacuum leak(s) that allowed air into the system causing minor problems, but not enough to make the ECU freak out. Knowing that is more motivation to go back and replace the rest of the hoses, since they surely have leaks too.

However, the top end power is still lacking, so the hunt to restore my engine to normal continues! I was joking with Charles earlier that I’m probably gonna replace everything under the hood short of the engine itself before I fix the problem.. I suppose time will tell. Until then, remember that preventative maintenance is the best thing to do to keep your car performing as it should.

Turbochargers! Part 4 [ July 3rd, 2008 ] By:Mark Ozimek

Last time, I left off talking about how intake and exhaust restrictions should be minimized with a turbocharged setup to increase efficiency and overall power at the same boost level. I meant to cover how the engine must be changed to accommodate a turbocharger, but discussing efficiency ended up to be more involved than I thought it would be.

At any rate, there are quite a few things that must be considered, mostly to prevent the engine from going KABOOM! The things I will cover this time around are:

1. Ignition timing
2. Air/Fuel Ratio
3. Compression ratio
4. Boost control

Now, when I said KABOOM, I meant it quite literally. The primary concern with increasing the boost is knocking or detonation, which is when the air/fuel mixture explodes, instead of burning outward from the spark plug like the engine is designed for. The explosion flame front travels a lot faster than combustion, over 300 m/s compared to around 30 m/s for combustion. This causes a rapid spike in pressure before top dead center (TDC), which can be pretty damaging to the engine. Here is a handy diagram from Volvo that shows what happens when knocking occurs:

As you can see, the expanding circles represent areas that are burning. When knocking occurs, there is detonation instead of, or in addition to the normal combustion.

First, we must understand why knocking occurs. It really boils down to one thing: Excessive temperature. This high temperature can be caused by a few things, like intake temperature being too high, compression ratio too high, excessively hot cylinders and pistons. Something as unavoidable as the increase in temperature and pressure from the normal combustion can cause detonation in another part of the cylinder. Lower octane fuel also burns more readily, contributing to knocking. As quick side note, this is why most turbocharged cars recommend premium fuel.

Increasing the boost pressure increases the final pressure and temperature within the cylinder significantly, which as we now know, greatly increases the chances of knocking. In order to increase the boost a lot to make more power, we must try to prevent knock.

The first way to prevent knocking is to retard the ignition timing. The ECU usually does this on the fly based on signals from the knock sensors mounted to the cylinder block. Retarding the timing may seem counterintuitive, since waiting longer to ignite the mixture means the temperature and pressure is higher, because the mixture is still being compressed. Ignition is almost always before TDC, and ignition timing is measured in degrees before TDC, a negative value indicating that the timing has been retarded to after TDC. Once again, a nice little diagram from Volvo for visual reference:

The curve indicates pressure within the cylinder, a spike occuring after ignition, and a sudden drop-off when the exhaust valve opens at the very end of the cycle.

However, later ignition means that the hot gas from combustion stays in the cylinder for less time, reducing temperature, preventing knock. It also means that if knocking persists, the pressure build-up occurs later, while the piston is traveling downward, decreasing the intensity of the pressure spike. However, the negative side to retarding timing is reduced power output, so we want to run it as close to the optimal timing for the RPM as possible without causing knock. This will typically range from 40º to 30º before TDC, depending on the geometry of the cylinder head and piston.

Another easy way to prevent knocking is to richen the Air/Fuel Ratio (AFR). When there is more fuel present, the final exhaust temperature is ultimately higher, but the extra fuel acts as a thermal damper of sorts, since it takes more energy to heat up more fuel during compression. The temperature during compression is what is ultimately what determines if there will be knock. Making the AFR too rich will also reduce power, and cause the engine to consume a lot more fuel, a doubly bad thing to do. Despite this, many ‘high performance’ tunes will richen the mixture significantly to allow much higher boost levels to be run without knocking.

Getting into the engine itself, there is a very straightforward way to allow for a lot more boost without causing knock. Just reduce the compression ratio, it reduces the final pressure within the cylinder, preventing knock. This is why most turbocharged engines have pretty low compression ratios compared to their normally aspirated (NA) counterparts. For example, my S70 has a compression ratio of 8.5:1, while the NA version of my engine has a compression ratio of 10.3:1. Charles’ WRX has a compression ratio similar to mine, at 8.2:1, and the NA Impreza of the same vintage has a compression ratio of 10:1.

The reduced compression ratio compensates for how there is almost twice as much air in the cylinder as there would be at WOT without the turbocharger. The nice thing is that the final pressure within the cylinder is higher with a turbocharger than is possible with an NA engine, because of how the air is cooled down in the intercooler between the two compression stages. Without getting into the specifics of the math, a higher pressure usually yields a higher efficiency, meaning the engine extracts more power out of a certain amount of fuel. In theory, a turbocharger can be used to increase the fuel economy when trying to reach a specific horsepower target. In reality, turbocharged cars often get worse gas mileage due to the lower compression ratio, and tuning of the ECU for extra power over efficiency.

The other important factor that has not been discussed in detail yet is controlling the amount of pressure that the compressor makes. As I explained in part 1, the compressor and turbine wheel are attached by a shaft. To control the amount of boost the compressor makes, the speed at which it is spinning must be controlled somehow. This is usually done by letting exhaust around the turbine wheel, through something called the wastegate, instead of forcing it through the wheel. The external view of a wastegate that is integrated into the turbine housing looks like this:

(click for larger picture)

The wastegate itself is a vent hole right before the turbine wheel that allows exhaust flow into the exhaust pipe with a valve that is pulled closed by an actuator. In the above picture, you can see a rod come out of the right side of the picture, and end at a small arm. That rod and arm are connected to the wastegate and the actuator. The actuator is vacuum driven in this case, boost pressure is supplied to a solenoid that is controlled by the ECU, with two output ports, one to the unpressurized portion of the intake, and the other to the actuator. The solenoid bleeds off pressure as needed so that the actuator can be controlled by the ECU as the boost level changes.

How the wastegate is controlled changes some important factors, such as how quickly the boost pressure ramps up, if there is “overshoot”, where the turbo temporarily exceeds the target boost level, and so forth. In most stock turbo setups, the wastegate starts opening at a pressure significantly below the target boost level, causing a slower increase up to the maximum boost. This gives the least amount of overshoot, which is good for safety reasons, but bad for performance. One option is to increase the pressure required to start opening the wastegate, which will decrease spool time, but potentially creating overboost situations that may damage the engine if you’re running close to the limit of it’s capabilities.

Another choice that must be made is the wastegate type. There are some turbos that do not have an internal wastegate, and require an external one, either from something like the 5 bolt Garrett flange that has a port for an external wastegate, or by using an exhaust manifold with a tube coming off to go to a wastegate. Typically, the more air the wastegate can flow, the better control over boost pressure there is, to a point. Once there is too much flow, it is hard to have fine control over the boost levels. If there is not enough flow, boost will creep above the target level, which is not good.

So not much math this time around, although if anyone wants me to, I’d be happy to review my thermodynamics notes and explain the Otto cycle and why higher combustion temperature and pressure is better. There is always more stuff to cover on turbochargers, so stay tuned for part 5, coming soon!

Aerodynamics: An Introductory Rant [ June 24th, 2008 ] By:Mark Ozimek

I’m going to take a brief break from the turbocharger! series to make a little segue into what I hope to by my next topic that I will take a close technical look at: Aerodynamics.

Really, we only care about two things here:

1. Downforce/Lift
2. Drag

Hopefully I’ll explain how these two things come about in a pretty simple fashion that explains it a little bit better than “Oh, the body pushes air out of the way.” However, before that, I have noticed some very disturbing trends among the modding community.

The most incomprehensible to me is the addition of only a rear wing to a front wheel drive car. This is really a double negative. The wing creates downforce in the rear of the car, behind the rear wheels. The wheels act as a fulcrum, and this downforce that is generated in the rear actually lifts the front of the car up at speed. Totally counter-productive, since it will decrease traction on the drive wheels, and increase the amount of understeer. Keep in mind that front wheel drive cars already tend to understeer a lot, a wing in the back will just make it worse.

The other negative of the rear wing is one you will have to deal with almost every time you try to generate downforce by pushing air up: Lots of drag. The engine has to put out a bit more power to overcome the extra drag, which increases with the square of velocity. So now we have a situation where when the wing is putting down the most downforce is when the front tires are applying the most force to the ground to propel the car forward through the air. If this downforce was applied up front, or if it was a rear wheel drive car, this would be a great thing. Since it’s not, the drag slows the car down, and increases understeer through turns even more!

The other are body kits that add ‘features’ to the body that don’t actually do anything. Some things are useful, like a front bumper that allows less air under the body of the car should reduce drag, however, many are just as counter-productive as a rear wing on a FWD car. A classic example is the intake vent on the hood and before the rear wheels on the some of the more recent generations of Mustangs. The protrusions do exactly that, protrude into the air stream. This adds extra turbulence to the airflow over the body, which is almost always a bad thing since turbulence usually increases the amount of drag on the car. It’s possible to use turbulence to your advantage, but that’s a complex topic to cover, and requires some pretty precise placement of fins to make the air go where you want it to, think F1 for an example here.

One thing that Charles said to me while we were discussing this article is that we have a pretty utilitarian view on what looks good on a car. If it improves performance somehow, we’re almost always for it. If it does nothing, is counter-productive, or just adds weight, we typically hate it and immediately reject the idea. The things that make the car ‘go fast’ also look/sound good to most people, since we associate that with performance race cars. Maybe what we should do when we get our car to modify into a rally car is mask the things that make it look fast. Sleeper race cars, ready GO!

Turbochargers! - Part 3 [ June 20th, 2008 ] By:Mark Ozimek

Welcome back to the Turbochargers! series, where I get to have fun rambling on about one of the most effective ways to create a lot more power from an internal combustion engine. If you haven’t done so already, I recommend reading part one and part two and grabbing a snack before continuing on with this one.

There are several things I left open ended in part two that I would like to cover this time around. First is compressor efficiency, and other things, like how having a turbocharger affects the engine itself, will follow.

Simply put, the efficiency of a turbocharger is how much work is put into compressing the air compared to how much work would have been done in an ideal world. What is different about the ideal world? Well, things like turbulence, heat transfer between the blades and the air, the effects of sound, air’s high and low pressure points within the compressor wheel, and so forth. Nothing can ever be 100% efficient, so we just try to get as close as possible. Newer turbos are generally more efficient than older ones, thanks to improvements in modeling technology, more experience in design, improved bearing tech, and stronger materials to name a few.

The efficiency of the turbo really affects two things:

1. Exhaust pressure right before the turbine inlet
2. Intake air temperature after the compressor wheel

Both of these are very important things to keep as low as possible. I’ll touch on exhaust pressure (commonly referred to as back pressure) more later, since it ties in with a few other important things regarding turbo selection and engine design. The intake air temperature is pretty obvious, the lower the temperature at a given pressure, the more dense the air is, which means more air can get into the cylinders per stroke, mass-wise. This ultimately means more power, if it is not immediately obvious why, I have gone over the effects of temperature on engine performance in more detail before.

I won’t get into detail on the theory behind the calculations involved with efficiency and intake air temperature, but if you really need to know this stuff for some bizarre reason, go do some research on adiabatic compression. For the calculation, you need to know 4 different things to find the compressor outlet temperature, which I will designate as To for temperature outlet:

1. Ti: Compressor inlet absolute temperature (ie: Kelvin or Renkin, add 293.15 or 457.69 to Celcius and Farenheit, respectively)
2. Pi: Compressor inlet absolute pressure (ie: psia)
3. Po: Compressor outlet gauge pressure (absolute works too, but you will have to modify the equation)
4. n: Compressor efficiency (ranges from 1 to 0, typically around 0.7 to 0.6)

This may seem a little messy, but it is straightforward. Plug the values into the upcoming equation and you have the outlet temperature.

We can find the pressure ratio to help us simplify the final equation, and help us relate to the compressor maps, since they are given in terms of airflow and pressure ratio between the inlet and outlet pressures:

Pressure ratio (Pr) = (Po + Pi) / Pi

The equation used for finding the compressor outlet temperature:

To = ((Ti*Pr)^0.283)-Ti)/n+Ti

So if we have a car that is running with an 80ºF inlet temperature, 14.2 psia inlet pressure, 10.0 psig outlet pressure and 70% efficiency…

Pr = (10 + 14.2) / 14.2 = 1.704

To = (((80ºF + 459.67) * 1.704^0.283) - (80ºF + 459.67)) / 0.7 + (80ºF + 459.67)

Do the math and you get To to be 665.2ºR. The units are significant here, since we did all the temperatures in absolute value due to the ratios involved, the result is an absolute value. To convert, just simply subtract the number needed to convert it back to relative, 459.67 for imperial units (Fahrenheit and Rankine) and 273.15 for metric (Celsius and Kelvin)

So the outlet temperature is 206ºF, I usually round to the nearest integer, since these calculations are hardly accurate due to the complexities involved. Either way, that is pretty warm, eh? It gets much hotter with more boost and less efficient compressors. This is what we use intercoolers for.

Many intercoolers are rated up to a certain horsepower, but I find this a rather silly notion. The calculations involved with the temperature drop across the intercooler are quite complex due to the nature of the geometry of the intercooler, and I will omit them simply because we usually don’t know things like the fin height, depth, thermal resistance between the plate and fin, and so forth. It is possible to calculate the outlet temperature based on an airflow speed through the intercooler, speed of the intercooler through the air, and a lot of geometry, but it’s still an estimation at best.

So when picking an intercooler, my advice is to use as big of an intercooler as will fit in the area you’re working with, since bigger intercoolers can remove more heat and usually have a smaller pressure drop across them, which means your turbo can do less work to get the same pressure at the intake manifold. Just remember that the more volume it has, the more air must be put into it when the boost pressure increases (read: throttle response time increases)

In a similar vein, be careful of how much tubing is used to install the intercooler. The bigger the diameter, the less restriction, which is always good, but there is more volume. To avoid excess restriction, try to use as few bends in the intake path as possible, and when you need them, use a bend with as large of a radius as will fit, since that will give the least restriction to the airflow. The whole idea with the intake is to allow it to flow as freely as possible without increasing the volume, thus lag, too much. This is something that you will have to figure out on your own, or talk to other people who have done similar modifications on the same car as yours to find their opinion on how to set things up.

The same thing applies to the exhaust side of the engine too. There are two evils with exhaust restrictions, reduced power and increased exhaust gas temperatures.

I see people say things like “This engine needs a little bit of back pressure to perform properly”, and then I end up laughing a lot. The camshaft profile was designed to create optimal torque with some specified amount of back pressure. Reducing the pressure may reduce torque, but only because that is how the cam profile is set up. Change the profile some and you will end up with more power with less pressure. I’m not going to get into cam profiles yet, since it is an area that is beyond my understanding for now. With turbocharged engines, this is not a concern at all, since the turbine creates an enormous amount of back pressure.

This pressure is created by the work needed to spin the compressor wheel, and the geometry of the turbine wheel and housing. The smaller the overall turbine assembly is, the more pressure it generates at a given airflow. This is why larger turbos tend to generate more power at the same boost level as a smaller turbo. However, as we went over in part two, a larger turbo almost always spools later in the RPM band. This means that while the peak power will be higher, the total amount of energy the engine is capable of putting down to the road is lower.

Getting back to what I was talking about before with back pressure, with a turbocharged car, it is best to keep the back pressure as low as possible, since the turbine generates a substantial amount of pressure for the engine to deal with. This pressure is not constant either. Increasing the boost increases the back pressure even more, since neither the turbine wheel or the compressor wheel are 100% efficient.

In addition to this, the turbine creates energy through the difference in pressure between the inlet and outlet of the turbine wheel. Once again, due to the nature and inefficiencies of the turbine, every small increase in pressure after the turbine wheel creates a larger increase in pressure before the turbine wheel.

Why is this so bad? Well, as I pointed out before, you can make more power with less backpressure. You may have to modify the cam profile to make full use of it, but the net result is more power, which is our goal. The other is equally important. Higher exhaust pressures increase the exhaust gas temperature (EGT) with everything else being held constant. When pushing an engine close to it’s limit, a close eye needs to be kepts on the EGTs to make sure that ridiculous things like melting a piston or warping the manifold don’t happen. Plus, lower EGT’s typically mean a longer engine lifespan, since there is less thermal stress on the parts.

So how to reduce exhaust pressure? Quite simple really, use the biggest diameter exhaust pipe you can fit into the car, straight-through mufflers are a huge plus, use as few bends as possible, and possibly most importantly, the part known as the downpipe must be capable of supporting the airflow.

The downpipe is often the most restrictive part in the exhaust after the turbo (known as the turbo-back, all the parts after the turbine housing) aside from the mufflers, because the exhaust is the hottest in that part. Hot air means low density, which means a high volume for the same mass. This low density creates a high airflow velocity, and drag increases exponentially with velocity. Just like the rest of the air stream, try to ensure that the downpipe has a large diameter, smooth bends, a smooth interior surface (roughness causes more turbulence, which almost always increases the resistance to flow), and the turbo will thank you.

Well, I think that’s enough for this time around. I didn’t cover quite as much as I wanted, but the topics I did cover were gone into a lot of detail, which is good. On the plus side, I already have a few ideas for part four. I always welcome comments, questions or suggestions, so feel free to ask and I’ll do my best to help you out.

So what is everyone’s personal motto for the next month? Less restriction is better!

WRC Italy [ June 16th, 2008 ] By:Mark Ozimek

I finally got the time to watch EuroSport’s coverage of the WRC Italy race, well the review of it at least. As Charles pointed out the other day, Rally isn’t the most spectator friendly sport, since the race occurs over so much distance, and takes up a lot of time due to everyone racing against the clock instead of next to each other like most other motor sports. As a result, most people only have the time to watch the highlights of every race. Now I need to watch the races that were in Greece and Turkey.

A funny thing about rally races is how much effort it takes to stay in the lead. After the first day, the two Subaru cars were doing quite well, but by the end of the second day, Solberg had some unknown issues, like he often does, and Atkinson just wasn’t keeping up with the pace, and just like that, Subaru is basically out of the running. Atkinson ended up in 6th overall.

Loeb managed to win the event, as he has been doing quite often lately. The combination of his driving skills, the partnership between him and his co-driver, and the reliability of the Citroën car seems to work out to get him on the podium pretty consistently.

Also putting in a solid effort are Hirvonen and Latava. Ford has been dominating the top 10 in the recent WRC events with sheer numbers, getting four of the top 10 places. It is quite amazing how many Ford teams are competing this year. I’m not going to complain though, since this much involvement from an American car company can only be a good thing for bringing the sport here to the USA.

So if you haven’t watched the Italian WRC yet, I recommend doing so, it was a pretty exciting race in a fairly tough course. A few large jumps and rather bumpy terrain puts a lot of strain on the suspension, and there are the typical crashes and component failure that goes with the car abuse that is known as rally racing. Definitely good times to be had by all!

Tires: Often Overlooked [ June 13th, 2008 ] By:Mark Ozimek

It seems to me that when people want to upgrade their car to give better performance, they focus a lot on creating more power. Sometimes there will be brake upgrade or suspension modifications, but rarely do I hear or read people talk about the best performing tires for their car, and I really don’t know why.

When it comes to making a car move somewhere, accelerate, decelerate, turn, slide, whatever it is you make your car do, the tires are what provide the traction to do so. Perhaps one problem is that there are performance tires for almost every application. High performance dry tires, mud tires, snow tires, rain tires, hell, even ice tires. There are so many choices to choose from, It makes me wonder if many just pick out any old all season tire for the ease of use and practicality. Even then, within a segment, tires are all different. Some are softer, providing more grip and faster wear, the tread design is different, the sidewall thickness is different, even the contact patch width varies between tires of the same ‘width’.

But do not let this faze you. Tires are, in my opinion, the most important piece of equipment on the car to understand. Now I’m not asking that everyone knows which compound and tread combination will give the best traction on a dry surface at a 10º slip angle, but please, just use common sense and get a nice set of tires.

Why do I bring this up? Well, I finally got the opportunity to take my winter tires off my car the other weekend and put the all seasons that were on the car when I bought it back on. The difference in handling, and general amount of traction available is incredible. The snow tires I had on were Dunlop SP Winter Sport 3D’s, and they should really be called all season tires with a tread that makes it good in snow. In comparison, the actual all seasons have a much less aggressive tread. As a result, the predictability in hard cornering is much better, and all around traction when the roads are dry is far superior to the snow tires. But then when the roads get wet, the snow tires deal with the water much better, no doubt thanks to the deeper tread.

So yeah, next time you start budgeting out parts for your car, be sure to include a nice set of tires that work well in your application. For Charles and I, we’ll probably aim for a set of all seasons with good mud and dirt capabilities for the rally car.

Update on Performance Loss Hunt [ June 3rd, 2008 ] By:Mark Ozimek

I have been troubleshooting my car for the last week or so, as I had talked about previously. I had some time recently to take a closer look at the engine, thanks to school being done for the quarter and getting a week break before I started work.

I started off by taking a closer look at the list of things I had suspected to be the problem:

1. A hole or tear in an air hose.
2. Compressor bypass valve (CBV) failure.
3. Turbo control valve (TCV), AKA boost control solenoid (BCS) failure.
4. Significant restriction in the exhaust system.

In the previous post, I ruled out number four by detaching the exhaust after the turbine outlet temporarily.

Since then, I have checked for vacuum leaks and made sure the TCV functions properly by checking manifold pressure throughout the RPM range. The pressure gauge indicated good idle vacuum at around 19 mmHg, which is approximately where it should be. Though this basically rules out vacuum leaks, a cursory glance at the vacuum lines showed that if any are leaking, the cracks are small enough to not be seen at idle.

The turbo was generating around 10 psig of boost at wide open throttle (WOT) by 2300 rpm, and held up until around 5500rpm, when I shifted into third gear. Because of this behavior, I believe that the TCV is operating properly in that the wastegate is held to a position that will allow just enough air around the turbine wheel to generate the 10 psig of pressure the ECU calls for.

This leaves the CBV or something else that I didn’t immediately think of. Without explaining my suspicions to him, I took my dad out for a drive to see if he heard anything strange, as he has better hearing than I do. After after accelerating hard in second gear from around 20 to 45mph, he pointed out a wooshing noise that started soon after I got on the gas. I listened more carefully for the noise he was describing, and it sounded exactly like the CBV on Charles’ WRX after we removed the Snorkus.

So from this, I am certain the CBV diaphragm is leaking a lot, which means that a lot of compressed air is going back to the compressor inlet. The compressor must then spin faster to make the same amount of boost at a higher mass flow rate. This means two things, both pretty bad. First is an obvious one; The faster the turbine/compressor wheels spin, the more wear there is on the turbo overall. The second is not quite as obvious. Since the turbo is now doing more work to move that extra air through the CBV, the wastegate is not open quite as far as before, which results in more backpressure on the engine. This reduces power and increases exhaust gas temperatures a bunch, especially in the top end when the turbo has to do the most work.

What’s next? I need to get ahold of a CBV diaphragm for a Mitsubishi TD04HL-16T turbo to replace the current one, and see what happens from there. Just for the sake of being thorough, I should replace the vacuum lines too. Age and heat does wonders for rubber, and I’m sure that even if the idle vacuum is good, there are tiny leaks here and there, which bothers me to think about. As with all other works in progress, I’ll update you guys on anything that happens.

Hunting for Performance Loss Causes [ May 20th, 2008 ] By:Mark Ozimek

Recently I have noticed a lack of power in the upper RPM range of my S70. Considering that it is now 10 years old and has just over 146k miles, I’m sure there are a whole bunch of things that are causing problems in one way or another, from dirty fuel injectors to leaks in the air hoses. Part of being a good driver or co-driver on a rally team is being able to quickly and accurately diagnose faults with your car that impact the performance. Also, being the engineering oriented car enthusiast that I am, I like knowing what is going on under the hood of my car.

So immediately, there is a list of probable causes to this performance reduction.

1. A hole or tear in an air hose.
2. Compressor bypass valve (CBV) failure.
3. Turbo control valve (TCV), AKA boost control solenoid (BCS) failure.
4. Significant restriction in the exhaust system.

A hole or tear in an air hose can cause a vacuum or boost leak, depending on where it is. This will cause the reading from the MAF sensor to be incorrect, and will throw off the base pressure upon which some of the turbocharger’s boost control devices operate on. A significant leak will cause the ECU to throw an error and turn on the Check Engine light, as well as significantly decrease fuel economy, neither of which have happened, so it’s actually rather likely that this isn’t the cause.

The compressor bypass valve (CBV) is a known weak spot, being integrated into the compressor housing, it has to endure a lot of heat and vibration. Tears in the valve diaphragm develop over time and allow air to circulate back to the compressor inlet when the engine is under load, which we definitely do not want.

Another part that is known to fail on this car is the turbo control valve (TCV), or boost control solenoid (BCS). Either name works, they’re the same thing. It operates based on a duty cycle from the ECU and the pressure difference before and after the compressor, and sends the ‘resultant’ pressure to the wastegate actuator to control the boost pressure. If this solenoid is stuck open, the wastegate will open before it should, and the turbo will never develop significant amounts of boost.

A restriction behind the turbocharger can also cause a significant performance decrease, for example, the catalytic converter failing and clogging up. Any backpressure on the turbine wheel increases how hard the turbine must work to generate a certain boost level by a lot, so this is also a sensitive spot. It is also the easiest to check for, and is what I am doing today.

As you can see, it is most likely related to the turbocharger. Unfortunately Volvo did not see the need to include a factory boost gauge in the instrumentation cluster, nor do I have a gauge on hand to test to make sure the pressure is at 10 psig like it should be.

So, with limited diagnostic resources at hand, there are really two choices I have; Check for leaks, or see if there is a restriction in the exhaust. I decided to check for the latter, since checking for vacuum leaks would entail replacing hoses to see if anything changes, and I do not have appropriate hose with me.

Starting off, there is a nice heat shield around the exhaust manifold and turbocharger to keep the engine bay temps down a bit, so I’ll remove that to get to the downpipe which I will unbolt to allow for gas to flow directly out of the turbine housing, bypassing any restriction. Be sure to do this work when the car is cold, as the exhaust system can get VERY hot.

Now with the heat shield out of the way, I have access to the bolts holding the downpipe on. A 13mm socket fit perfectly. After loosening the bolts, I pulled the downpipe off the turbo outlet flange:

After pulling it back enough to create a significant gap, I started the car, and went for a short test drive. I could tell that a significant amount of air was bypassing the exhaust system through that gap from the sound of the air blowing through, and the exhaust smell in the cabin. I ended up opening the windows to get some fresh air, since carbon monoxide poisoning sucks.

After doing some hard acceleration after the car had warmed up, I found that there was very little difference in the performance of the car. It had become significantly louder though, quite similar to the sound of a diesel truck, and surely in violation of noise ordinances. Fortunately, that rules out restriction in the exhaust causing the performance issues, since it would have been an expensive fix, working against the money Charles and I are trying to save up for the rally car. So I put everything back together. Always make sure to use proper torque for tightening things up in the exhaust to prevent leaks or cracking. Volvo specifies 30 Nm of torque for the downpipe bolts.

Before putting the heat shield back on, I decided to investigate two of my other leads: the CBV and the TCV. The CBV is totally caked in dirt and oil, which makes me highly suspicious of a leak there. The only way to find out if it has failed would be to replace it, which I plan on doing since it is an inexpensive part.

The TCV was harder to look at, since it was hiding underneath the air intake hose. It did appear normal aside from the electrical tape, but looks can be deceiving, so this is next in line if replacing the CBV diaphragm doesn’t do anything.

It looks like I need to brush up on my diagnostic skills a bit, since I was not able to find the fault in an afternoon’s work. When I find out what is wrong, I’ll post a guide on what happened and how to fix it, since boost related performance issues appear all the time on turbocharged cars as they age.