Originally Posted by HotRod.com

A: For the reasons above, I didn’t want two totally divorced turbo systems. Besides, when I started this project four years ago, the aftermarket was clearly more oriented to the single TB.

Q: As we photographed the car, you mentioned choosing (among other things) gear ratio, intercooler size, and the backpressure-to-boost-ratio work to select turbine nozzles. How did you decide on these? Are any of them slightly overkill with some extra built in, or are they just right for the job and will need to be changed if something else changes?

A: Intercooler size is somewhat like fuel-line size or air conditioning condenser size—you almost can’t make them too big. The entire goal for an intercooler is maximum cooling and minimum pressure drop. As you make them bigger the cooling always gets better, but just making an air-to-air intercooler taller eventually starts increasing pressure drop, which of course is bad. There’s some juggling there where it actually gets worse. The water intercooler I selected is the biggest one I could squeeze under the fender without mods, and the beauty is that it’s exactly twice the size of many 5.0L and 5.7 Chevy aftermarket water-air intercoolers, but they are just dying for more available room. At twice that size, it calculated out pretty well. Once you decide to intercool, it’s a bad place to cut corners to try and save a buck.

Gear ratio is, on the other hand, just like you said; you size it for the job and it might need to be changed if something else changes. I make plots of this all the time. If you are a drag racer the trap speed is very much tied to the power production in the last quarter of the track. If you’ve just shifted into the next gear at the 900 foot point and are groaning the car at 4000 rpm (not peak power in this example!!) your trap speed will clearly suffer. Optimum gear for this car today is 3.60 if drag racing were my only criteria. As soon as I add 50 horsepower, this figure drops (like maybe to 3.40) because you still want to cross the traps at the power peak, and obviously the increase in power will manifest itself as more trap speed—hence the gear-change requirement.

Also, in a turbo car, boost vs. rpm is a transient event, so with a 5-speed, you really have five different torque curves to design for. By gearing the car steeper (4.11, 4.56, etc), engine rpm grows faster than boost can keep up if you will, so in a rpm-sense, this is slowing down boost response. Add on top of this the fact that the effective mass of the rotating group of the engine goes up with gear ratio squared. So there are two downsides to more gear, while the obvious up-side is the increase in torque multiplication (which is why people do it in the first place). This is definitely a juggling act, and turbo cars for this reason tend to do better than expected with slightly taller gears because they have two downsides fighting the up-side, not just one vs. one when you’re normally aspirated.


Turbine nozzles are also a compromise. Much like selecting a camshaft, you’re essentially trading top-end for bottom-end power. This one is very hard to do analytically before you build your turbo system, so you shoot from the hip as best you can, then test the system once it’s together. I use the back-pressure/boost ratio as one of the evaluation tools.

Here’s a little background to appreciate the exchange of bottom end for top end: If you run little nozzles the boost response is early and you’ll make more power at 2,000 rpm than you would have with a bigger nozzle because of the increased boost. However, the downside is that your back pressure will be higher with the small nozzles on the top end because the wastegates are open and less and less of your exhaust has to do "all the compressor work." What do I mean? If memory serves me correctly, it takes around 60 to 70 compressor horsepower to supply my engine with 11 psi compressed air at 6,000 rpm from the turbo’s compressors. Where does this energy come from? Exhaust heat (which is "free") and exhaust back pressure (not free). With really big turbine nozzles, (no wastegates), 100 percent of my exhaust generates this work and back pressures the motor minimally, less than 11 psi, in fact. With really small turbine nozzles, the majority of exhaust is going through the wastegate (not through the turbine) to keep the compressor from overspeeding. Thus, if you will, "40 percent" of my exhaust has to do all the compressor work. If there’s less flow through the turbine but you need the same power, it must be accompanied by a higher pressure—you guessed it—back pressure.

Anyway, the point is that I do measure the back pressure and compare it to boost because turbo cars are notorious for having exhaust go up the intake during overlap. Lots of cars have back pressure that is two, even three times boost pressure. This is why turbo cars have the reputation of loving cams with tons of lobe separation, which minimizes the time when both valves are open and this problem occurs. If you’re Joe Bonneville, you’ll run huge nozzles, have low back pressure to boost, and traditionally run closer lobe centers just like anyone in the normally aspirated racing world, since your back pressure to boost ratio is much like theirs. Back pressure testing is important on n/a cars, but even more important on turbo cars as it helps you size the turbine nozzles.