Two Guys Rally

Loeb Wins Again! [ August 18th, 2008 ] By:Mark Ozimek

The WRC was tearing it up through the beautiful German countryside near Trier over this past weekend. The course has an interesting variety of surfaces; back-country roads that are covered in leaves and dirt on Friday, concrete roads on Saturday, and asphalt on Sunday. Through all this, the terrain varies from being in the woods to out in large fields to being in an old city. If you haven’t gotten a chance to watch the race, I highly recommend finding a way to watch it as soon as you can.

So what happened? Well, Loeb managed to snag 1st place in the first 13 consecutive stages! By the end of stage 13, he had a 43 second lead over Sordo in second place, who managed to stay in second place overall throughout the race, aside from temporarily losing it to Hirvonen in stages 5 and 6.

By the end of the rally, the podium was taken by Loeb in 1st, Sordo in 2nd and Duval in 3rd. Unfortunately WRC hasn’t made their official highlights video, but this one is pretty cool, and shows the German scenery well.

Suspension Setup Basics [ August 15th, 2008 ] By:Mark Ozimek

I’ve heard that a few of our readers would like to know a little more about things like camber and toe, and the effects the basic suspension settings have on vehicle stability and control. Before reading this, keep in mind that the optimal setup for any combination of car and road can vary a lot. This is just a guide to help understand what three settings do:

1. Camber
2. Toe
3. Caster

There are many more variables in the suspension setup, but these three seem to be the most easily changed, and have the largest effect when tuning the car.

Camber is the angle of the wheels from vertical when viewed from the front. Negative camber means the top of the wheels is closer to the center of the car than the bottom. Positive is the opposite, with the top of the wheel further away than the bottom. The measurement is degrees off from vertical.

Usually the suspension in a car is designed to decrease camber as the suspension compresses. This way, when the body rolls as it goes through a hard corner, the outside suspension compresses and pulls the top of the wheel in, the inside decompresses and pushes the top of the wheel out, counteracting the roll from the body, keeping the tire closer to perpendicular with the road.

The main idea behind changing the camber angle is to maximize the tire’s contact patch for when you need it most. Typically it is set slightly negative to maximize traction during hard cornering. The downside is less traction when traveling in a straight line.

Positive camber causes more wear on the outside edge of the tire, while negative camber causes more wear on the inside edge of the tire.

Toe is the angle between the wheels and the car’s centerline when viewed from above or below. Toe-in means the tires point inwards, ie front of the tires are closer to the car’s centerline than the rear of the tires. Toe-out is opposite, with the front of the tires out and the rear in. The measurement is degrees off from parallel with the car’s centerline.

Toe mostly affects straight line stability and turn-in response. Toe-in improves straight line stability, negating the effects of things like surface irregularity, bumps, crosswind, and generally makes the car want to travel in a straight line.

The downside of this is that the turn-in response is reduced. Consider that the inside tires must travel through a smaller radius when turning than the outer tires. When turning with toe-in, the inside front tire will have a smaller angle of turn than the outside tire, meaning that it wants to go through a larger radius, and is fighting against the outside tire during a turn. As the weight is transferred to the outside tire, the effects of the inside is reduced.

Conversely, with toe-out, the car will be unstable at high speeds, anything that transfers weight to one side of the car will make the car want to turn in that direction because the tire is pointed outward. Keeping this in mind, it seems a contradiction that toe-out improves steering response. Remember what I mentioned before about the inner and outer tire’s turning radii. With toe-out, the inside tire tries to turn a tighter turn than the outside tire, which is exactly what we want. This way, the tires aren’t fighting against each other until the weight transfers to one side.

However, just like camber, any toe away from 0º increases wear on the tires; Toe-in causes more wear on the outside edge of the tire and toe-out causes more wear on the inside edge of the tire.

Caster is slightly more difficult conceptually, and it only applies to the steering wheels. The angle between the axis upon which the wheel turns and vertical is caster. The best example I can think of is a bicycle. The front wheel rotates about an axis that is not vertical, but is angled so that the axis of rotation is in front of the contact patch. When viewed from the side, positive caster means this axis of rotation is tilted backwards, the top is towards the rear of the car and the bottom is forward. Negative camber is when this axis is tilted forward.

What does this do? Well, when the contact patch is behind the steering axis (Positive caster), the wheels want to travel in a straight line, and will have a tendency to center when turning. As you would expect, the opposite is true with a negative caster, the wheels want to turn away from going straight and more in the direction that they are currently turning.

Negative caster was used a lot back in the 70’s and earlier to make the feel of the steering lighter, since less force is needed to turn if the wheels want to go in that direction. The problem there is that negative caster gives some instability when going in a straight line.

Almost all modern cars have positive caster to improve stability and ease of driving at speed. Although the steering wheel will be more difficult to turn, power steering helps that.

Aerodynamics: Drag [ August 10th, 2008 ] By:Mark Ozimek

Aerodynamics is quite an interesting subject, and also one of the more complex. Seeing as I’m still learning this stuff myself, this series will be an introductory lesson on aerodynamics, I’ll just cover the basic concepts that are a good framework to understanding a lot of other important things.

An important thing to keep in mind is that aerodynamics is more or less the study of how fluids move (aka: fluid mechanics), with the fluid in this case being air, and a car’s body pushing the air out of the way. As long as you think of it as air being pushed around, the rest of the concepts are pretty straight forward.

First up is drag. Drag is a force opposing motion. In the case of aerodynamic drag, it’s the force applied against the car as it moves through the air. There are a few variables that affect the aero drag. The faster you go (velocity, or V), the more drag there will be. Also, more total surface area and frontal area increases drag. The frontal area (Af) can be thought of how much area the car takes up when viewed from the front. Or if the car is moving sideways, the side area would be used, or some combination thereof. There is a coefficient of drag (referred to as Cd) that is a function of the body shape. The final important factor is fluid density. The density of air varies with altitude, temperature and humidity, as I have pointed out in the past.

There is an equation that puts all of these things together to find the aerodynamic drag that will be seen:

Drag = (Density / 2) * Cd * Af * V²

Just make sure the measurement system is the same for all and the answer will be a force. As an example, let’s look at how much drag there is on a VW Golf GTI from the late 80’s going 80mph. I have a book here, Theory of Ground Vehicles by J. Y. Wong that has a list of different cars and their Cd and Af. The GTI has a Cd of 0.35-0.36, and Af of 1.91 m². 80mph is 35.76 m/s. I’ll assume standard temperature and pressure, so the density of air is 1.292 kg/m³.

Drag = (1.292 kg/m³ / 2) * 0.35 * 1.91 m² * (35.76 m/s)² = 552.2 kg-m/s² = 552.2 Newtons

Now for some fun with math to see what this means. Let’s convert the force and speed into power.

552.2 Newtons is the same as 124.1 pounds of force. 80mph is 117.3 ft/s, multiply the two together to get 14,556.9 lb-ft/s. There are 550 lb-ft/s in a horsepower, so this hypothetical GTI needs 26.5 hp to overcome aerodynamic drag at 80 mph. If we increased the speed to 100 mph, that number changes to 51.7 hp! Note that this is power at the wheels, and is neglecting any incline, rolling resistance, drive train resistance, and so forth that increase the power requirements at the crank.

I’m going to cover lift and downforce in a later article, but while it may seem obvious, one major cause of drag is fins and spoilers that create downforce while the car is moving. The extra turbulence and changes in airflow usually turn up as an increase in the Cd. Why is this? Well, the fins are designed to push air up as their way to get downforce. When a car is going down the road pushing lots of air upwards, there will be similar amount of drag. Let’s look at F1 cars, since they make good examples. The car is basically covered in wings that make enough downforce to allow the car to drive upside down at speeds over something like 100mph. They have to make a tradeoff when setting up for every race to balance between downforce and drag, which effectively means they have to choose whether the car can corner faster, or have a higher top speed. Rally faces a similar dilemma, although in rally, there is a lot less space for wings, and there are many slow hairpin turns where wings don’t do a lot, so the emphasis on wing setup is diminished.

So hopefully now it is obvious why when driving down the highway, you hit a speed where the gas mileage suddenly drops off really fast: The power required to overcome aero drag increases with the cube of velocity! Stay tuned for the next part of aerodynamics, and feel free to suggest topics that you would like to hear from me on!

The Volvo Chronicles: CBV Diaphragm [ August 2nd, 2008 ] By:Mark Ozimek

Last time I posted about my S70’s engine, I had just replaced some vacuum lines. Although it helped, there’s still something strange going on. I had ordered a new Compressor Bypass Valve (CBV) diaphragm to replace the worn out one that is allowing air to escape from the compressor outlet back to the inlet. Unfortunately it was out of stock everywhere I looked, so I had placed an order and waited for the part to arrive.

Now, a few weeks later, I get home from work to find a small package at the door. “What could it be!?”, I wonder. Opening up the bag reveals the impossibly rare diaphragm:

(Click for larger view)

What this diaphragm does is open and close to allow air out of the compressor outlet back into the inlet to prevent compressor surge when there is a sudden decrease in airflow through the engine at higher loads. The actuation method is pretty simple. There is a spring that pushes the valve assembly closed under most conditions. The pressure from the compressor pushes against this spring. There is a air hose that runs to the intake manifold that allows for a change in pressure behind the diaphragm. When near or at WOT, the pressure in the intake manifold is close to that of the compressor outlet, meaning the spring holds the valve shut. When the throttle is suddenly closed so that there is a lot more pressure in the compressor outlet than in the intake manifold, the spring can no longer hold the seal closed, allowing air to flow past it.

Over time, the heat, oil, cyclic action, vibration, dirt, etc., break down the diaphragm, it stiffens up, develops holes and tears, which generally decreases its performance. When there are holes in it, air can always escape through the CBV, significantly decreasing turbo performance and efficiency.

So I plan to change the diaphragm in my car soon. It isn’t a terribly complex job, there is just very little room to work, as it is wedged between the turbo housing and engine block, with a bunch of stuff in the way from both the top and bottom.

It’ll be a fun project to work on, and I’ll be sure to make a post when the old one gets replaced!

Rally in X-Games! [ July 28th, 2008 ] By:Mark Ozimek

As pointed out here in passing before, we’re excited for the 2008 X-Games and the rally event that it is going to have. Just a reminder to everyone, the X-games is coming up very soon, starting on July 31st, this Thursday, running to Sunday, August 3rd.

A large part of the philosophy behind TwoGuysRally is to attempt to create a larger Rally awareness and fanbase in the USA. Media coverage is sorely lacking, and the general popularity of the sport is just pathetic compared to many other countries in the world, especially when you look at European countries. How can we fix this? Well, now that the opportunity is presented to us, go watch the rally coverage along with all of your friends who have even the slightest interest in motorsport! The more people you recruit into the crazy sport where we toss cars through trees at high velocity in the snow, the better! I mean seriously, who doesn’t enjoy the insanity of the co-driver’s pace notes, and his reaction to unforeseen events, forbid they actually happen.

Unfortunately, I’m having difficulty finding exactly when the Rally is going to be shown. Anything that covers X-Games should show it, or some of it. I know that it will be airing on ABC on Sunday sometime, in the middle of Skateboarding. So make plans to sit near a TV this Sunday to watch it, I know Charles and I will be!

Calm The Eff Down: An Addition [ July 26th, 2008 ] By:Mark Ozimek

Charles made a mention about keeping calm to improve your driving a few days back. There are two things that brought this article to my attention recently.

First would be the F1 race in Germany last weekend. For those who follow the F1 races, notice how much Filipe Massa’s driving improved when he stopped pushing the car too hard. It is possible, and perhaps too easy to create strange and unsettling handling issues that unnerve you when pushing too hard. This will just make you nervous and lose confidence in the car. Stay calm, don’t let the pressure affect your driving negatively. I realize this is easier said than done, and requires some practice, which brings me to my second point.

The other relates to driving on normal roads with everyone else. A major problem with cars is that we treat them as our own personal space, when we have to share that space with others on a public road. Invariably, we get frustrated and annoyed with other drivers who don’t do what we want them to. I saw a few good examples of this on the drive home from work today. There was a large backup caused by an accident, two lanes closed on a three lane highway. When it came time to merge to the one lane, everyone went from just driving normally to honking, swearing, cutting each other off, and generally carrying on. Just calm down, use a blinker, and try to set a good example for other drivers on how to behave. This prevents a lot of stress, as well as potential accidents. Both are good things to avoid! By mastering your patience with the insanity of the general population, you will take a large step forward to mastering your calmness on the race course.

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!