Brian M.

I'm currently undergoing a top end rebuild on my 1987 930 and the mechanic said if I wanted to raise the compression to 7.5/1, now is the time to do it while the heads are still at the machine shop. I know the best way to raise compression is to get new pistons and cylinders, but I don't want to spend the extra $3000-$4000 right now. What do you guys think about shaving the heads? Should I shave them or leave them alone? The other mods I have are K27-7200, Kokeln intercooler, GHL headers/muffler, SC cams. Thanks for any advice.

Brian M.

PorschePhD

It has been done for years this way. While I perfer the the piston change for added CR if you are tight on money do the heads. Just make sure that you send the chain housings in along with the heads to have them shaved as well. You don't want to get stuck with that mess. Just make sure everything will match and you will be set.

Luke

why would you you want to increase your CR? You have the displacment to make fat tq down lower, you'll only shorten your peak hp with a higher CR.........

Sameer

You increase compression for more torque, there is quite a noticable difference off the line from a traffic light. The bigger displacement is more for horsepower. Go for it. Many of us take this approach.

Geoffrey

Luke,

Actually a turbo car is pretty weak down low due to the extra weight the car has and the low compression. It is not uncommon for a 3.2 Carrera to be able to out accelerate a turbo car off of a corner until the car comes on boost. This is one of the reasons proper gearing is so important.

The displacement increase from 3.3 to 3.4l really is rather minimal, only about 68ccs. The advantage of the 3.4l is 1) in the better cooling of the cylinders due to the 360degree cooling fins. 3.3l have no cooling fins on the top and tend to become oval over time. 2) The 3.4l has higher compression. I measured the stock setup to be 6.5:1 compression (factory listed at 7.0:1) and the 3.4l to be 7.2:1 (listed as 7.5:1) with a stock 90cc head and .038" deck height. You do really notice the extra compression, but it still does not run from a 3.2 Carrera "off boost".

What do you mean by "shorten your peak HP with a higher compression ratio"? Since HP is a mathmatical calcualtion based on torque, and RPM, and higher compression does in general make more torque, you will get both more torque and more hp all else being equal. Would you explain?

Brent 930

Geoffrey,

Mahle now has 3.3L fully finned cylinders as a direct replacement for the stock setup. It's the same package for the 91-92 C2T which I put on mine since I didn't go with 3.4L but wanted to upgrade. Still don't think going higher CR and having to twin plug is worth it when I can run 1bar otherwise. Sure I lose throttle response but save thousands!

Geoffrey

Cool.

The 3.3l pistons and the 3.4l pistons have virtually no dome, so twin plug benefits both about the same. Everything is a trade off.

Luke

how much of a CR bump are you talking about? .5:1 can't be more than 4 or 5 hp off boost, and .5:1 will severally limit the amount of boost you can run, resulting in a lower peak hp.

Geoffrey

How severely limitied with the boost be?

PorschePhD

Not very.....I assure you that.

Luke

yeah, i guess 6.5-7:1 is a really low starting point, but it just seems the CR/boost trade offs are more ideal for trubocharged economy daily drivers or rally cars w/ 34 restrictors

I couldn't imagine anybody wanting to go any higher than 7-7.5:1

still on pump gas? How much of a bump? Do you have any charts or numbers for us to compare?

PorschePhD

I have plenty of charts and numbers. What do you want?? There are also several people on this board with before and after dynos of motors we have built with a bump in compression. You ask why would anyone want to go higher than 7:5:1??? Well the new TT runs 9:4:1 and we still carry 1.2 bar with an AFR of 12.2-12.5 and a EGT of 1340ish and near 700TQ. It is done on a daily basis. All this on pump gas...All of my motors are built on pump..

Brent 930

Stephen,

Isn't there a big difference to what you can do between running CIS or EFI? I guess the way I looked at it is you get 10 more hp and better throttle response with the bump in compression but I get 20-30 more hp running 1bar vs. .8bar on pump gas with the stock CR. I realize you still can hit the limits of CIS with running .8bar but it will be more expensive. I also have stock intake and exhaust ports and you know how much power I dynoed at with running 1bar. Then again it just depends on what you want.

Geoffrey

Luke,

Compression ratio isn't the limiting factor in a 911 turbo engine. The fact that it is air cooled and the head to cylinder sealing surface is. You can run 1.0 - 1.2 bar max unless you plan on having a 30 hour engine. Everything needs to be taken into consideration, timing, AFR, compression and octane all play to the engine characteristics. You can safely run 8.0:1 compression with the right timing, right cams, right boost pressure and have a very, very nice engine that makes a lot of power and is reliable.

Don't draw any conclusions based on experiences with 951s, they are completely different.

EFI lets you control the engine fuel and ignition more exactly so you can better approach the edge of the engine.

A930Rocket

I'm not ready to upgrade until I have to do so, but what does the 3.3L fully finned cylinders (with pistons incl?) cost for future reference?

Jim