PT

Just wonder what is the consensus on highest safe compression ratio on our car, converted to EFI with twin plug AND it will be driven hard on track?

If we limit boost to 1 bar or below, would 8.5:1 be max or closer to 9:1 is still safe?

thanks...

Sameer

I'd stick with 8:1

pete77930

awww, why would you want to increase compression??? turbo lag induced power slides are soo much fun.

Sameer

Pete,
nice avatar

pete77930

Thanks. that was taken in the barn a few months ago when the R1 was getting fitted with a micron full exhaust. the bike wheelies in 3rd gear so much easier now. I need to get it rejetted here soon for smoother throttle response though, its a little twitchy coming out of turns.

edpurplett

.. imho.. you can run to 8.8-8.9 ...although I havent talked to anyone who has run that CR on a 930.... BUT you dont dare run over .8-.9 bar regardless,,... it's what I have on my current rig.. having said that I had 7.5-1's on my 930 TT twinplug.... but ran 1.25+ bar....

125shifter

Here's a boost vs CR vs intercooler efficiency chart from A. Graham Bell's book "Forced Induction Performance Tuning"

DonE

Good graph, but its generally accepted for a water-cooled engine. Good book too.

125shifter

I figured it wouldn't be too far off for an air cooled Porsche. I wouldn't think an air cooled engine could run more boost than water cooled and I believe it's accepted that we can run 7.7:1 at 1 bar with a 70% efficent intercooler. This scenario matches the graph and is about what I will have.

I like all of Bell's books: 2-stroke, 4-stroke, and force inductions although I don't have his modern tuning book.

nathanUK '81 930 G50

David, is that 98RON UK grade 98RON or your USA 98RON ?

How efficient is a Kokeln IC ?

TIA

125shifter

That's UK, I think it's close to 93 in the US.

I don't know about the Koklen, you could guess 60-65% to be safe.

Sameer

Yes 98 UK would be somewhere btwn 93-94 US grade

Geoffrey

From the testing I've done, I believe that the cylinder head temps are one of the key measurements to watch. I like 8.0:1 engines and they run fin at .8bar within a range of cylinder head temps that are considered acceptable. Above about .8 bar, the cylinder head temps are too high (in my opinion). I'd take an 8:1 @ .8bar engine over a 7.5:1 @ 1bar engine any day since you'll get more area under the curve. I believe you can run 8.5:1, however, you'd want to run even less boost and you'll get a diminishing return on HP with compression compared to boost.

PT

Geoffrey, thanks for the response. Kind of what I was thinking but want to ping the group to see if anyone had success with running higher than 8:1 CR.

thanks...

DonE

Agreed. Geoffrey is spot on regarding heat - its all about heat management, not pressure (even though they are directly related). In a water cooled engine we can evacuate the heat better, and therefore run higher boost and/or CR.