Choosing the right compression
 

I know, I know - lots of info on this subject. And I've read a lot too.

But following this thread

http://forums.pelicanparts.com/911-engine-rebuilding-forum/955955-budget-engine-upgrade-what-would-you-do.html

I've decided to do the right thing for my winther project: Choose the right parts for the right engine. So, if that means a top end rebuild - so be it.

I'm aiming (hoping) for 280HP (would love to have 300...) in a 2000lb hot rod:

http://forums.pelicanparts.com/uploa...1501096122.jpg

Now, I already have EFI (Tbitz conversion) and fully programmable CDI with "dual spark" (not dual plug) from classic retrofit , 42mm headers and Dansk 2 in two out. Engine is right now 3.2SS with 98mm bore - unknown compression and piston design, but I suspect it's wedge designed max moritz. I would like to run ITB (quite possibly Al Kosmals solution) and I need to decide on new pistons and cams. I have to set the limit somewhere, so there will be no twin plugging. I'm fundamentally against the 3.6 swap (thats cheating), also I like the hot rod look - and I keep the car as matching nr's.

So finally the question: With AFR on 12.5 (isch) at WOT, EFI with ITB on a 98mm bore 3.2SS with suitable ignition advance for 98RON octane and probably 993SS cams - what compression should I aim for???

I was thinking 9.8:1 but a friend of mine is running 10.1:1 on his 2.7 engine using modified JE pistons. I also know a guy running 10.5:1 (or so he says) on single plug?! Is that possible with the above configuration? Do I gain anything at all by raising comp 0.3?

98 RON octane? Where are you located?

Northern Europe. And actually, It's 99 RON

The stock 3.2 euro piston available from our host is 10.3:1
Piston / Cylinder Set 990174915 - Mahle - 99-0174-915 | Pelican Parts

I know of at least one person running these on a single plug 3.2 in the us where the best fuel available is 93 R+M/2 (I think this is approx RON 98) ... Most people say that's a no no. I guess it depends on your risk aversion. These things have gotten so expensive, that making one go boom isnt a small expense.

Remembering that the more overlap you have on the cam, the lower the dynamic compression actually is.... but the increased VE at high RPM makes a more dense high RPM charge. Without study, calculation, or simulation, I don't think you will get a definite answer.

I was going to try to make a pump gas tune for my 12.5:1 twin plug, but I ended up selling it before I put any effort into it.

Consider adding a knock controller/detector.

I have for my race engine I originally bought a vampire which I believe you could use, but I needed an intercept version

My vampire is forsale

http://forums.pelicanparts.com/porsche-911-used-parts-sale-wanted/957941-j-s-safeguard-vampire-forsale.html

Mike