Turbo Boost and FE

[GasSavers_Gollum]

Originally Posted by Hal9000

My apologies if I sound terse here, I'm at work and have to be quick.

Gollum, I think that you'll find that an apples to apples comparison between a turbo specific exhaust system and an NA specific exhaust system will surprise you in terms of backpressure. Yes turbos add some backpressure, but so do normal mufflers. Turbo exhausts are designed to be as low restriction as possible after the turbo since that's necessary for the unit to function properly. In a good turbo exhaust, the turbo itself is the ONLY major restrictions.

Regarding catalytic converters since it came up. Newer generation catalytic converters are much less restrictive than older ones used to be. I read one article that said the increase in backpressure through a good cat is equivalent to less than a 1/4" restriction in the exhaust system.

dkjones, sure turbo engines can still run manifold vacuum. The question is when does the vacuum disappear and either become neutral or switch to a positive pressure situation? Ideally, you want that to happen at the same approximate throttle position you'd find yourself at during cruise. That's the whole point.

In terms of lag... 95% of the DIY and aftermarket turbo systems I've ever seen have had oversize turbochargers, intercoolers, intake piping, or all three. Of course they'll have lag. I'm not going to concern myself with problems associated with design flaws that wouldn't be incorporated into a system that I'd build for myself though. A properly sized turbo system should have little or no turbo lag under any circumstances. Doing it right involves quite a bit of planning and mathematics, but it's not difficult as much as it is labor intensive.

That was a very good post, and I can agree with all of it. Though here's a question I'll pose -

If a turbo is going to be right at the brink of negative to positive pressure, then will it run out of steam in the higher RPM? The turbo planned for a FE build up would have to have a VERY wide map to work without a significantly large intercooler. Most of the map calculations I'm comming up with to create pressure at cruise RPMS are dangerously close to the surge limit on the turbo. I'd really like to see what you'd recommend for a given setup.

Oh, and mufflers don't create much of any backpressure when they're straight through There's some pretty decent straight through mufflers now days that work well enough for street vehicles not to be too loud and buzzy.

I completely agree about most turbo setups out there. A T3 on a honda is just way too big for most street cars. My 2.8 liter inline 6 does just fine with the stock T3 up to around 270hp, then a T3/T4 50 trim will go from 250 to 400hp easily.

But I have to be going around a 100mph cruise to get my turbo to positive pressure. I know with a modern ball bearing turbo that wouldn't be the case, but how much different would it really be?