How much power is 1lb of boost worth?

[mark '87 930]

I recently made some changes to my setup and now it seems as though i am boosting at slightly lower levels (9 instead of 11 psi).

Does anyone know what the relationship is between 1psi and horsepower?

[sammyg2]

Too many variables, but you can make an estimate with enough information like displacement, intercooler efficiency at both pressures, turbo efficiency at both pressures, back pressure, etc.

My guess (not knowing your set up) would be about 15 hp per lb.

[mark '87 930]

It's a stock car with ghl headers and exhaust... nothing else is changed.

Thanks for the estimate though.. gives me a general idea.

[Emission]

Interesting.

So, stock 930 boost is about .8 bar (11.76 psi at sea level).

My aftermarket spring brought it up to 1.0 bar (14.7 psi at sea level).

That means it was good for about 45 horsepower over stock (under optimal conditions)?

Too bad I chose longevity and put my stock spring back in...

[Hugh R]

1 Bar=14.7 psi. So 1 lb is 1/14.7 or about 7% increase max.

[Chuck Moreland]

Actually I think it is 1 / (14.7 * 2) ~ 3.5% max.

Remember the 1 bar boost augments 1 bar atmospheric.

[RarlyL8]

Nice catch Chuck. Is that curve/line actually linear given the design restrictions?

[sammyg2]

That is assuming that an engine running at atmospheric pressure (normally aspirated) is 100% volumetrically efficient.
I've turbocharged 3 different engines that started out as normally aspirated and every time I found that the best part of adding positive intake manifold pressure is the engine does not run out of breath at 5000 rpm, it continues to breath very well all the way to redline. I had a stock 2 liter type 4 in a 914 that wouldn't pull much past 5200 rpm. When I added a turbo, it pulled until the valves floated (around 6500) without a hint of falling off.
My stock SC engine pulls stronger and stronger to 7000. That's when I chicken out and shift before i break something. Before the turbo it fell on it's face at 5600.

In fact, I would venture to say that the first pound of pressure makes the most difference and it tapers off gradually from there.

[mppickett]

There's an old saying in the forced induction world: "It's flow, not boost." If you've done things like add efficient headers, port/polish the intake or increased the size of the throttle body you may see boost levels drop and power go up. Essentially boost levels are back pressure. If you open up the system to flow more air, you'll see the boost level go down but the power rise

[kuehl]

mark '87 930

just for fun and giggles, try this calculator:

http://www.csgnetwork.com/blowerhpcalc.html

[wswilburn]

If you were not flow limited, the increase in power would be linear, because the power would be proportional to the amount of air you can fit into a cylinder, which is proportional to the pressure. At sealevel, the ratio to normally aspirated would be (14.7+1)/14.7=1.07, 7% increase, as pointed out by Hugh.

An interesting number to remember is that the energy content of a stochiometric gas/air mixture is about 20 hp/1000rpm/liter for atnospheric pressure. This doesn't take into account the Carnot (thermal) efficiency of the engine, which is approximately 1-R^-0.4, where R is the compression ratio. This is 0.6 for a compression ratio of 10:1. So ideally, you can make about 12 hp/1000rpm/liter for a normally aspirated engine. Of course, flow limitations keep the power from increaing linearly with engine speed. To the engine, it looks like atmospheric pressure drops as engine speed increases at high RPMs due to flow impedance in the intake and exhaust.

Turbos help in two ways. First, they effectively increase the atmospheric pressure, and the power is proportional to this. Second, as pointed out by Mike, and maybe more important, they help you with flow limitations.

Turbos usually reduce the CR to reduce stress on the engine (and maybe to prevent detonation, anyone know?), but since the you are only losing as R^-0.4, instead of linear, it is a net gain.

[slow&rusty]

Based on several dyno trips and well tuned car with well set-up breathing (exhaust flow is a big factor).

1psi = approx. 8-10hp

Yasin

[Hugh R]

Originally posted by Chuck Moreland
Actually I think it is 1 / (14.7 * 2) ~ 3.5% max.

Remember the 1 bar boost augments 1 bar atmospheric.

Respectfully disagree. We're talking increase, and that would be theoretically the maximum. Inertia, drag, and other losses not withstanding.

[sammyg2]

Here's a few turbo calculators I have played with in the past. They are good but they require accurate information input to provide accurate output.
http://www.turbofast.com.au/javacalc.html

[Porsche_monkey]

Originally posted by mppickett
There's an old saying in the forced induction world: "It's flow, not boost." If you've done things like add efficient headers, port/polish the intake or increased the size of the throttle body you may see boost levels drop and power go up. Essentially boost levels are back pressure. If you open up the system to flow more air, you'll see the boost level go down but the power rise

What he said. Less restrictions, less boost. Not necessarily less HP.

[sammyg2]

Based on some assumptions like 80% VE, 40% intercooler efficiency, 55% turbo efficiency, I get a 17hp drop going from 11 to 9 psi boost.
Of course those numbers i put in are guesses and are very conservative. With a more effcient intercooler, a more efficient turbo, and a better VE the hp per lb will go up a great deal.
As an interesting side note, gonig from -1 psi (NA) to +1 psi gives an increase of 21 hp.

[Craig 930 RS]

On a car with moderate modifications, .1 bar usually equals 20 hp.
All else being equal, etc blah blah