Volvo Project - Part 2 [ February 7th, 2011 ] By: Mark Ozimek Posted in » Ramblings

I mentioned in part 1 that I have a hard time making up my mind. This is the story of how I came to decide what turbo should get bolted up to the engine to get me to where I want to go. Be forewarned: A lot of text lies ahead. I’ll do my best to be interesting as I tell the (not so) enthralling tale.

As a point of reference in all this, for those who are unfamiliar with the Volvo powertrain, the stock shortblock seems to be good for around 600hp without sleeving, assuming the engine tune is good and heat is managed properly. Beyond that, the cylinder liners have a tendency to crack where they touch the next cylinder. The 5 speed transmission, M56H, is reliable for around that much as well, and can handle more, although gear and bearing life is rapidly declining at that point.

Originally, I was aiming for around 350whp, maybe a bit more, with a 56 trim Garrett GT2871R tucked away behind the engine. Let’s take a look at how the engine matches up with the compressor map. I made some very basic and incorrect assumptions that will get me into a ballpark estimation, such as the pressure ratio across the turbine being equal to the pressure ratio across the compressor. That will give a rough feel for where the boost threshold lies.

This is at 21psi, with a 7000rpm rev limit. Because I am looking to make this last a reasonably long time, I am choosing to keep the shaft speed around 90% of the maximum listed on the compressor chart. For the GT2871R, this is a whopping 120,000rpm! This allows for some special circumstances, like driving up mountains, to avoid overspeeding the turbo to hit the higher PR needed to get target boost in thinner air.

Anyway, onto the actual graph. As you can see, this turbo looks pretty well matched to the engine I want to build, although it is just a bit on the small side for peak power. The spool-up is based on the 0.64 A/R turbine housing flow curve that Garrett provides. Volvo uses a T3 flanged manifold, so I would get this turbo with the T3 based 0.63 A/R turbine housing, but that shouldn’t noticeably change spool.

That is just about enough airflow for about 400bhp without pushing the turbo too hard, or around 340whp. Being a FWD car, that seemed pretty reasonable figure. More would only really be usable at very illegal speeds, or on a pretty high speed track. The real nice thing about the GT2871R was that it should be making as much boost as I wanted by around 3000rpm, which is perfect for the highway, where the engine sits at 3000rpm as the car cruises at 75mph in 5th gear. Stepping up to a GT3071R or GT3076R will bring the boost up to 3500-3750rpm, which may be a bit too late for my tastes, despite the possibility of a bit more power and a cooler running engine from less exhaust restriction on a small turbine wheel.

I thought I had my turbo picked out, and had everything picked out to support it; ATP ultimate internal wastegate, the actuator, an adapter flange, the hose kit needed to get all the fluids to and away from it, the whole nine yards.

Fast forward a few months, and Garrett announces the GTX3582R, 3076R and 3071R. With a redesigned compressor wheel, they give about a 20% boost in max airflow from each turbo over the GT turbos they replace. Curiously enough, they switched from 12 split blades to 11 equal height. That will certainly affect how the compressor wheel performs. Plus they added “extended tips”, which basically just makes the compressor wheel bigger than its advertised exducer size.

Older “GT” compressor wheels look like this:

Newer GTX:

The basic sizes of the wheels remained about the same, and overall efficiency didn’t change noticeably. The general operating window got pushed to higher PR and more flow, including shifting the surge line up. By by pushing the compressor map to the right with the same turbine wheel, the compressor will be operating in a slightly less efficient spot during spool-up. I suspect this will push the boost threshold up in the RPM range a bit, as there will be more energy required from the turbine to compress the same amount of air to the same PR.

Despite previously ruling it out because of the spool time, the GTX3071R seemed like more viable alternative. It suddenly offered a much higher power potential without a significant impact on spool from before. Despite being “slow” compared to the 2871R, I reasoned that having boost by 3500-3750rpm could be doable for a DD. That still left me with about half of my total RPM range in boost, which is far from being a spiky peak hp dyno monster.

Not long after that, I found out about BorgWarner’s EFR line. There were a couple things that I really liked about what BW did with them. First, they made a really light turbine wheel, and kept the size up. This improves the turbine efficiency, and increases the amount of torque the exhaust gas should be exerting on the turbo shaft. This, along with the reduced rotating mass compared to the typical Inconel turbine wheel, should greatly improve transient response, and reduce backpressure a lot while keeping a configuration that still allows a respectable boost threshold.

In playing around with Matchbot, it seems that the EFR7064 will spool around 2750-3000rpm, and the 7670 will spool around 3250-3500rpm. As far as turbo performance goes, the 7064 stacks up pretty well against the GT2871R; similar boost threshold, potentially faster transient response, and can supply a few extra lb/min of airflow at the top end. The Garrett is better than the BW at lower pressure ratios. The most pressure I want to run on the GT2871R is about 21psi, from what we saw on the chart before. The improved performance of the 7064 at higher PR and higher flow means that I could run about 25psi and get a reasonable improvement in power without compromising the spool.

In the end though, I ended up settling on the EFR 7670. Here are the operating points found through the matchbot program, targeting a peak boost of 30psi, the points are at 2750, 3000, 3250, 3500, 6000 and 8000rpm. As you can see if you can squint hard (or right click and open the image to see the original size), it can make 30psi by 3500rpm and hold it to 8000rpm without overspinning the turbo:

I decided that having full boost by 3500rpm, going through the peak efficiency islands of the compressor wheel, and a potential for 500+whp was a good compromise, despite being more power than I should really be trying to push out of the block, and even more than I should be trying to put down to the front wheels of a street car. Logic be damned, I’m gonna do what I want! Plus, the EFR series has the distinct advantage of having a built-in recirculating BOV, and a high-flow IWG with an actuator that comes with the turbo. Those two things save enough money to make the higher cost of the EFR worthwhile.

So, one step of the project out of the way! I know what turbo I’m going with now. It’s time to make the rest of the engine support my goals. I’ll save that for part 3, since this is already a tl;dr post.

3 Comments

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!

July 3rd, 2008 | 3 Comments

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