Spoolin wrote:are u @!#$ kidding me there is psi = hp correlation its called 7-10hp per psi. Why don't u learn a little about boost before you open your mouth. Go on any forum that knows cars www.dsmtuners.com and say that same @!#$ and you will be band. Idiot

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Hi guys, at Doug's invitation I thought I would drop in and contribute if I may. I wrote the article on the turbo Buick site, and I'm glad I did since I've "met" (electronically anyway) lots of interesting people from all over the world who share our interests.

Just to let you know where I'm coming from, I'm not a automotive turbo expert. I am a chemical engineer that does design work in the refining and petrochemical industry. As such I have to deal with things like centrifugal compressors and turbines (among many other things). Biggest one I've done so far is a 4 stage compressor boosting from 5 psi to 600 psi that took a 30,000 hp turbine to drive it! I'm telling you this so you can judge whether or not to believe anything I say. Too many internet experts out there that might know what they are talking about, and then again they might not. I think I know these things, but my knowledge isn't specifically automotive in nature. The same principles apply though.

Anyway, on to work. DCJ98GST, I don't think I like the conclusion you arrived at with your example. I hope you don't mind me poking at it for a second. You came to the conclusion that a bigger turbo might flow 15% more at the same boost pressure. I think you made a bad assumption that led you to that answer, and that was the charge air temperature.

If you run through the math, you find that while an increase in compression efficiency does lower the compressor outlet temperature, its not that big of a gain.

The outlet temperature is only dependant on two things: the efficiency and the pressure ratio (or the outlet pressure/inlet pressure). And if we keep the boost level constant, we only have the efficiency to deal with.

At 15 psi boost the pressure ratio will be about 2.25. With a 60% compression efficiency and a 80F inlet temperature, the outlet temperature should be about 315F. Suppose you put some new turbo on and it has a compression efficiency of 70%. The new outlet temperature is about 280F. 35F drop is pretty good, but is it really worth? Turns out the colder outlet temp increases the air density by 4.5% If you had a nonintercooled car, that should mean (all else being equal) a 4.5% increase in mass air flow, and a 4.5% increase in power. Think about that, 10% better efficiency leads to a 35F drop in temperature, but that by itself is only worth 4.5% more air flow on a nonIC car.

But toss the IC into the equation. An air/air IC can at best get the air down to ambient temperature. If you study heat exchanger theory, you find that dropping the inlet temperature by 35F doesn't mean the outlet temperature also drops by 35F. The IC kind of dampens things, on the one hand it doesn't have as much heat to transfer with the colder inlet, but on the other hand the closer you get to ambient air temperature the harder it is to get there. Net effect, if you drop the inlet temperature by 35F then the outlet may only drop by 20F. And what is that worth? A 20F drop in the IC outlet only increases the charge air density (and therefore mass air flow and therefore power) by a little over 3%.

To sum all that up, on an intercooled car, an increase in compressor wheel efficiency from 60% to 70% (a pretty big jump) will only net a 3-4% gain in air flow *from temperature effects alone*.

When you changed the charge air temperature from 165 F to 120 F you cheated. That is why you got the big air flow increase. In reality it will be a lot less.

But still, people put on bigger turbos and do see more power at the same boost level. If its not air temperature, then what is it? I think it's two things (and this is my opinion, remember my qualifications, so take it for what its worth), and they are both on the exhaust side.

As you so correctly mentioned, when you swap turbos you swap turbine sides as well as compressor sides. A better flowing turbine side will reduce exhaust backpressure. Less backpressure means less exhaust left in the cylinder after the exhaust valve closes, which means more room for fresh air on the intake stroke. This means increased volumetric efficiency, or more air flow at the same charge air temperature and boost level. It's like adding more cubic inches! I believe that if you kept the compressor side exactly the same and changed the turbine side to something more freely flowing, you will see a power increase. And you do. Bigger turbine housings give a power increase, even though you haven't changed the compressor side at all. Might add some lag and such, but ultimately power is improved. It's similar in concept to getting rid of a cat converter or a restrictive muffler. Lower backpressure = improved VE = more air flow = more power. So thats one thing.

The other thing I think does come from improved compressor efficiency, but it isn't a temperature thing. It takes power to turn that compressor wheel, hp that is extracted from the exhaust by the turbine. If you improve the compression efficiency from 60% to 70% you need something like 20% less hp to drive the compressor wheel. Power to drive that wheel comes from exhaust temperature, exhaust flow, and exhaust pressure. If you reduce the power required by the compressor, then the exhaust flow that the turbine wheel needs goes down (ie more exhaust through the wastegate) and the exhaust pressure required goes down. And again, the lower backpressure means improved VE, more air flow, and more power.

Sorry this has been such a long book, but to sum up: IMO, a bigger turbo at the same boost level makes more power, but not due to temperature effects, but mostly due to improved VE. If you change compressor sides and don't change the turbine side, I don't think you'll see nearly the gains that someone who gets a whole new unit sees.


I could on, but I think I've done enough damage here for today

John