[BruceWilkie]

I found the following old post on YB very interesting. Especially the last paragraph which I've highlighted. Thought I would share and see where the discussion leads.

I prefer to think of turbos in terms of airflow because that is what produces the power in an engine. The math involved in determining how much power a given turbo will make on an engine can be confusing because there are alot of variables involved.

Compressor airflow is a function of 4 things:
Intake air density, Volumetric Efficiency, Engine Displacement, Engine RPM

Intake air density is a function of 2 things:
Intake pressure, intake manifold temperature

Intake temperature is a function of 4 things:
ambient temp, turbo pressure ratio, intercooler efficiency, compressor efficiency

Engine horsepower is a function of 3 things:
Compressor airflow, air-fuel ratio, brake specific fuel consumption

BSFC is a dependent on the pressure delta in the engine (intake manifold pressure - exhaust backpressure). As your backpressure gets significantly higher than intake manifold pressure the pumping losses in the engine go up and the bsfc increases. When the BSFC increases the horsepower for a given compressor flow decreases. The standard of 10hp for every 1 lb/min of compressor flow assumes a bsfc = 0.5, when the backpressure becomes very high the bsfc can drop to 0.6 so 1 lb/min of airflow now makes 8.33hp.

[Travis Q]

I'm not going to say much on this, seems like every time I get into a technical discussion I get my head bit off, but I will give you this much:

I have never seen any gasoline powered turbocharged engine on an engine dyno have a brake specific of 0.6 (when tuned to an appropriate AFR). I have probably only seen a handful over 0.52, and those were intentionally safe on the fuel tune. 0.6 is what I see with a well tuned ProCharger engine with a good head and cam combo.

I have seen gasoline engines with backpressure ratios over 2:1 make over 11 hp
per lb/min of air. Of course, the air consumption part is calculated and some assumptions must be made in the calculation, but still.

I agree, pumping losses go up as backpressure increases. But I see that two tenths of an AFR in the wrong direction makes a LOT bigger difference than backpressure does. Now, if your camshaft is out in left field...........

TQ

[tininjun67]

Travis your accomplishments speak for themselves. if anyone has an argument for you, they need to get their head checked. if someone is running faster than you are, they can talk. I'm pretty sure it's a short list.

[Travis Q]

Thanks for that.

I want to clarify that I wasnt talking about anyone here on this board. More on other boards like YB and a few others.

I try my best not to talk out of my rear end (Rodney may disagree!! LOL). If I post something on a message board, I try really hard to be sure that it is factual information that I (and others) have seen with my own eyes, and information that can be backed up by data 100%. The reason I make those statements is not to show how "big and bad" I am, or "how much I know" (or think I may know), its usually to help clear up misinformation. And there's a LOT of that out there, particularly as it relates to turbocharged engines. People spend a lot of money and waste a lot of time because of misinformation. And I don't know anyone that has an excess of either one. And Bruce - Im in no way referring to this post, or any of your past posts. Hope you don't take it that way. I know what you're fishing for, and I admire it.

I cant give answers to very many questions, but I can sure share with you what I have seen!

[Travis Q]

Another little bit.....

Backpressure in and of itself doesn't kill horsepower. However, it dictates what your camshaft has to look like.

And that is what matters on the track!

To say it another way: You can still make a HUGE power NUMBER with excessive backpressure. But it may not be fast on the track because of what the cam has to be like.

I'm shutting up now!

[BruceWilkie]

Travis, thanks for sharing. Real world experience beats theory, cant argue with that.

I have to wonder how bad the differential pressure would have to be to affect bsfc as much as his example. All he stated was "very high backpressure". I would think it would have to be more than just a too small turbine.

Agree camshaft timings can change things alot. On the phone with a friend testing with DOHC 4cylinder turbo last fall. He was swapping exhaust cam to intake position and put intake on exhaust side then spare exhaust cam to exhaust side(same durations vs split) trying various timings. One of the combos with one of the cams moved near 15 degrees from "normal"(full sprocket tooth) put the AFR way rich. (I think it was the exhaust side advanced and maybe intake retarded.) He readjusted the fuel tables and the whole powerband shifted over 1500 rpm higher with a bunch more top end power. It was still climbing at his 8000 rpm redline. Seem to recall intake manifold pressure decreasing slightly from other pulls too. He likely has notes(I should have, I was figuring his LSA's and installed centerlines for him)He was on the dyno all day. Sure like what you can do with twin cams.

I can call and find out the details if you want.

Last we talked about it he was going to have different cams ground. He did find near 40 hp over stock cam configuration but it wasnt a powerband that suited his car's use or its gearing. Its likely on his back burner while he works on his turbo'd 455.