View Single Post
turboteener turboteener is offline
Registered User
turboteener's Avatar
Join Date: Jan 2002
Posts: 301
While I find it an interesting chart, it is a little to theoretical. (sorry I got long winded)

One thing the chart does not account for dynamic compression, which will radically alter the results. Now I did a few numbers and here are how some things compare.

Now you can see the differences in compression ratio, piston speed, and intake valve dynamics. All pistons speeds were calculated at 7000rpm except for the Baby 935 which was at 8000.

Now the 3.0 street engine will have a much higher dynamic compression ratio because of the camshaft timing than say the 935, but the 3.6 Turbo will probably have about the same dynamic compression with higher boost levels.

Now comparing boost between street motors and race motors is pretty much useless. Even comparing it between different generation street motors, say 3.0-3.6 is relatively useless. Because changes in camshaft timing, piston speed, compression ratio, intercooler design, intake design, exhaust manifolding turbo design, etc will all alter the measured restriction in the intake path (boost level). Pressure ratios would actually be a better way of comparing street engines, but even that is kinda irrelevant, except to show how improvements were made as Porsche learned more about turbo charging.

I suppose on a theoretical level you could perhaps calculate effective pressure, but it will be different in reality, because, for example the K27 will flow more airflow at the same boost levels than say a K26. So in the case of VoitureLTD he will have a different mean and maximum cylinder pressure than that of a stock 930. His turbo will pump more airflow into the cylinder than a stock turbo.

Also timing curve, piston speed, intake valve opening and closing points,cylinder head and piston design, rpm, etc will alter detonation resistance and cylinder pressure levels. This will also alter the burn speed (although somewhat controlled by fuel used) and combustion efficiency.

Maybe I wandered off topic. If so nevermind, I didn't mean to sidetrack the discussion.

Last edited by turboteener; 03-08-2008 at 09:34 PM..
Old 03-08-2008, 09:09 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #13 (permalink)