JoeMag

Going to be doing an engine rebuild over this winter and thinking about shaving the heads to raise the compression a little. Read an old post from Stephen that indicated raising CR 0.5 points is not an issue, and to make sure chain housings were sent out with them for shaving also.

My car has 964 cams in it. Is there going to be any piston/valve clearance issues by going up 0.5 points of compression?

As far as piston's go, which I really don't plan on getting new ones of, but does anyone make something that's lower than CR 8:1 other than stock? ...my car is still CIS.

Sameer

Many of us run 7.5 comression. Got mine from Mahle and I'm sure JE does a pretty good job on it too.

Geoffrey

Joe,

According to Porsche, you can make ONE .25mm or about .009" cut on the head. The later 3.6 N/A engines you can take TWO cuts. On the later heads, I have found that if you take .75mm or about .030" off the heads, it raises the compression about .6:1. However, I don't recommend shaving the heads at all with the exception of cleaning up the surface to ensure they are straight and flat. The reason being that it weakens the sealing surface since it places the surface very close to the first cooling fin which becomes very thin.

If you want to raise your compression with pistons, Mahle makes a 98mm piston and cylinder set in 7.5:1 compression which is really more like 7.2:1 when measured. Additionally, a stock setup, while advertised at 7.0:1 is more like 6.6:1 as measured. Your other alternative is JE pistons which are available in many different compression ratios. You should not have any piston to valve clearance issues with the 964 camshaft and any of the combinations mentioned.

PorschePhD

If you would rather do the cheaper way for servicing down the road do the cylinders. These are really worthless on the 3.3. Finned one side, etc. So if you ever choose to do something different it is easy to replace those. I do recommend of course a new set of pistons with the calculated CR already setup. However the prior will work as well.

Geoffrey

Shaving the cylinders will give you minimal return in that you still need to maintain the proper deck clearance which is 1mm or .039". If you run less than that with stock rods, you may end up with pistons hitting the cylinders. With a good steel rod (like Carrillo or Arrow) and max RPM under 7200, you can get away with a .032-.035 deck clearance since the rods don't stretch as much much as stock rods. If you do shave the cylinders, you may need to shave the pistons as well to maintain proper deck clearance which is effectively increasing the net dome volume of the piston. The concern here is to not negatively affect the squish area of the piston. If however, you are above the recommended deck clearance and are not able to get close with the various base gaskets available, then as Stephen mentions, simply shaving the cylinders to get to the minimum deck clearance will increase compression. However, returns are minimal, in the .2:1 range unless the pistons are cut as well.

JoeMag

Thanks folks. ...if I do the 7.5:1 piston's, will I have any issues with CIS with enrichment? I've read through other posts that 8:1 is pushing it with CIS.

Sameer

With CIS I'd stick with 7.5 unless your going twinplug and keeping it below or at 1 bar.

PorschePhD

You will need to send the fuel head out to be modified. You should also twin plug the system.

edpurplett

.. Geoffrey/Stephen...

Wouldn't .5mm base shims solve the deck clearance issue and also tend to prevent the .25mm squish issues which I think we've all seen? Though I totally agree with don't shave heads unless you have to... particularily on 500+ HP engines.. let alone all the aircooled 3.6 heads .. with all their issues to begin with..

e

Ed

Geoffrey

Deck clearance is deck clearance and can be obtained in a variety of ways. Porsche provides three sizes of base gaskets (.25, .5, 1.0mm)to be able to arrive at or close to the minimum deck clearance. If you are at 1mm clearance and you cut the cylinders to raise the compression, then you drop below the minimum deck clearance and you will need to add a thicker base gasket. No gain. If you cut the cylinders AND cut the pistons to regain the deck clearance rather than add a base gasket, then you effectively increase the net dome volume of the piston and therefore increase the compression.

m42racer

As stated, there are many ways to obtain what you want. Its always a compromise. Raising the CR does not always gain anything either. Run as high as you can with ALL factors considered. But raising it just to raise it may not gain what you are after. There are diminishing returns on these Chambers when raising the CR. It has been well proven that these engines respond better to more air, than squeezing the air you already have. Less heat, less dependacy on Fuel and the ability to run more Ignition lead, and the Cam choice opens up too.

The problem you will always be faced with is the Piston To Valve Clearance, and the Piston to Head clearance. As soon as you lower the clearance, you also decrease the ability to time the Cams to where you may require the power band to be. Included in this you decrease the ability to have certain Cam centerlines, etc.

The only way to get what you may really want, without making too many compromises is to have Pistons made to suit. For example, a 98.00mm bore you want to run more Dome Volume, run a bigger Cam with more lift etc, and soon you find that the limit is around 12.5:1.0 as the Valve Pockets lower the Dome Volume as you add Cam. Change the timing and gain back volume with less pocket, and you lose performance. Fill the chamber with Piston, up goes the CR but you have to consider the Spark plug location etc. This is where the dome configuration and shape comes into play too. The chase continues.

Another way to add CR is to increase the Bore or stroke. Less piston is required to obtain the same CR ratio, but more $ is required.

JoeMag

Regarding the 7.5:1 piston's, could I safely run those in stock 930 cylinders?

SoCal Mike

The 7.5:1 are for use with 98mm cylinders that take you to 3.4l. You might be able to get some custom JE 7.5's for your stock cylinders, but as Stephen has mentioned, the stock "half-finned" cylinders leave something to be desired, and more compression would certainly be going the wrong direction with them.

special tool

These are all excellent solutions for a 930 owner who like to wax nostalgic. Porsche stopped using these solutions with the introduction of digital management.
You can get a fully programmable Motronic engine management package for your car that will cost 1/4 the cost of compression and twin-plug mods and offer 10 times the driveabilty.
But if you are really into nostalgia, use the Porsche mods from 1975 and you will get your severly compromised, dangerous driveabilty and get to spend MUCH more money for your troubles.

Also - keep in mind that an engine that benefits from the more exact control of modern managment last much longer than engines with antique fuel systems that run 17:1 afr at some loads and 10:1 (or worse) at others........