Turbo Boost and FE

[GasSavers_Hal9000]

I haven't gone as far as selecting a turbo yet since I'm still on the fence about actually turbocharging my mpg car, but it's an '88 crx with a D15. I'll probably end up with a very small turbocharger. But one of my other cars is a cosworth/mercedes that I'm building for bonneville. It's got a 2.3L 16V engine that I'm pairing up with a hybrid Holset HX 35/40 turbo. This setup should yield roughly 400-450 whp, although with some lag.

I agree with you about the stright through mufflers, most of the newer ones are pretty good. But NA exhausts and even some factory turbo exhausts are usually longer and have more bends than what I would build for a turbocharged engine. In general terms though, even the restriction of exhaust gasses through a turbocharger can be a good thing. A big problem that many garage tuners create for themselves by adding high flow exhausts to NA cars is overscavenging the head. That'll kill your FE and low end torque. Most street engines work best with some moderate level of backpressure unless you're only worried about top speed and high rpms. Zero backpressure is typically a bad thing except for a very few special conditions.

Many older turbos might run out of gas at high rpm, but I'd say that careful selection of even an older turbo could alleviate the problem. The first time I heard about this phenomenon was close to 20 years ago, and turbo design has changed quite a bit in the intervening years. Also, keep in mind, that the boost produced isn't usually linear. It really ramps up steeply in the upper flow ranges in most cases. What I'm talking about is not necessarily having the turbo beginning to spool up and produce real boost at low throttle settings. It's just kind of spinning lazily along and just barely producing pressure.

For turbos with good wide ranges, I'd personally start by looking at the Holset HX series units that have a "power ring" or "map enhancement ring". Basically it's a slit in the intake housing that allows air to be pulled in from the side partway down the inducer. This isn't state of the art technology, but it's new enough that you won't find it on the old t3's and t4's. I can't explain how it works, but it widens the useful gasflow range that the turbo works at substantially. This is basically why the honda guys have been able to get away with running HX 35's off dodge cummins trucks with any level of success. Schwitzer and I think the newer GT series Garrets also carry the same technology. What it will allow you to do is get away with a larger turbo that won't surge at high rpm, but will still spool at a relatively low rpm. Of course, header design, wastegate placement (be sure to go w/ external wastegates since the internal ones usually affect turbo efficiency and increase the rpm that the turbo begins to spool up at by a few hundred rpm) and many other factors can really affect things. I can't think of a way that IC size would affect things except to change the volume of the intake (more volume takes longer to pressurize => lag) and temperature. But those are more performance that FE issues in my opinion.