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DW SD DW SD is offline
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Join Date: May 2004
Location: Encinitas (San Diego CA)
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Originally Posted by hobieboy View Post
I thought I knew but now I can't explain it...

2 turbos, same boost. One big, one smaller. Why would a bigger turbo make more hp if both can hold same boost and same RPM?

If you look at the ideal gas law this might explain the issue. I think it is about heat added through the turbocharger.

Please excuse me, as I haven't studied this stuff in a long time. I finished engineering school in 1994 and don't practice.

However, from the idea gas law.... pV = nRT .

T = absolute temperature
n = relates to number of molecules of air.
V = we can consider the engine a fixed volume at a snapshot in time and rpm (even though this changes relative to volumetric efficiency, cam timing, lift, etc),
p= pressure
R = ideal bas constant.

We would agree that the more air / gas mixture at the appropriate ratio which you can burn, the more HorsePower (HP).
n = pV/RT. If R and V are constants and then R & V drop out leaving a constant ratio between n and p / T
n ~ p / T
since n has direct relationship to HP (more air burned correlates to more HP)
then we reduce this to HP ~ P/T.

conclusion: if absolute temperature were higher then HP would drop. Conversely if pressure increases, temp is fixed, HP goes up.

A drop in temperature of 20 F degrees to 150 F from 170 F is about 11 degrees on the absolute scale. 338.5 K from 349.6 K, which equates to 3.0% more molecules of air to burn.
1971 RSR - interpretation

Last edited by DW SD; 07-15-2008 at 05:18 PM..
Old 07-15-2008, 05:12 PM
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