Reducing compression ratio - for forced induction on 930.20 motor
 

Hello gents,

I am in the process of rebuilding my motor and looking to supercharge it initially and turbocharge it later (or Turbocharge it from the beginning).

My cylinders are in mint condition and my pistons are in very very good condition too.


Since I have a 930.20 motor the compression ratio is 10.3/1, which needs to be decreased especially since the climate in Greece can get really hot.


You have any suggestions for dropping the compression ratio using my pistons?
Maybe thicker gaskets or that doesn't work on the aircooled motors?
My mechanic told me that at the base of the cylinder there are copper rings that can be found in 0.25 or 0.50? Fitting 0.50 could lower the compression ratio? Any idea? If so by how much? Maybe machine the pistons would be a solution or this is not a good idea as we will weaken the pistons? Anyone done it? Any company that does it?


If the option above (maintaining my pistons) is not possible? What about buying a new set of pistons (in which case I will go twin-plug) with lower compression ratio whilst maintaining my cylinders? What pistons would you suggest?

Lastly would it be a solution to buy 3.0 Turbo pistons since they have the same bore ie 95mm? What other parts would need to be bought ?

Most importantly what would you guys do to have a cost effective but correct engine build to accommodate forced induction, trying to at least keep the cylinders that I have (replacements are not cheap!! and I find it a pity to change them since mine were found in top condition).


All the best,
Alexis

hi
 

Hello
Are you sure that you have compression ratio is 10.3/1 ?
is that EURO engine ? something does not add up.
there are old engine the fuel wasn't that good - most of them are 8.5 : 1
check again

3.3 turbo pistons and cylinders are bolt on.

Yes it is a 3.2 Carrera N/A Euro Spec !!
Hence the compression ratio.

I am sure it is 10.3/1 whilst American cars are 9.5/1

The idea is to try and keep my cylinders.

I have some 930 pistons and cylinders I just posted for sale.

https://forums.pelicanparts.com/porsche-911-used-parts-sale-wanted/1058127-3-3-pistons-cylinders.html

You could go with some custom pistons.

Thank you for your reply.
I will check with my mechanic and I will advise you if they could work for us.
I think I want to go with new pistons to be honest for piece of mind.

What brand would you recommend for custom pistons?

JE Pistons makes flat top pistons in a range of compression ratios for turbo applications. They still have valve reliefs in case you are running a high lift cam. The website isn't quite as friendly as it used to be, but depending on the bore size you want, they have 7.0:1 all the way to 11:1 in half point increments. And they can always make a custom piston as well. CP Carrillo and Mahle are two other manufacturers that make an excellent product.

It's hard to get much lower on compression ratio by adding the cylinder base gaskets, since you want to keep the deck height in the .040-.060" range. Replacing pistons is a better option.

930.20 is a ROW (Rest of World) 3.2L. It does have a 10.3 compression ratio. For the 95mm bore there are these options:

Porsche
USA 3.2L Carrera pistons - 9.5 compression ratio
USA 3.0L Turbo pistons - 930/52 engine, 6.5 compression ratio

JE Pistons
353243S - Can be machined to 8.0 Compression ratio

you might consider building a 3.4 and doing other engine mods. it will be a much more fun car to drive.
if its not EFI you might consider converting

Thank you for your input.

Checking with JE and Mahle.

Will definitely replace pistons ideally twin plug.
I hope I will hold myself and not change the cylinders - if I do it will be 3.4...

I shall keep you posted gents.

Stay safe.

I believe that the pistons from Mahle all used the same forging die for 95mm pistons. Consequently, if you look at the underside of the pistons, they should look identical. Therefore the differences between compression ratios is simply the shape of the dome. For a turbo/supercharged engine, I would want the compression ratio to be around 8.0-1 for high boost and maybe around 8.5-1 or a little higher for lower boost engines. The excess material can be machined off the domes, as long as the minimum safe thickness of the piston dome is not compromised.
I would start by figuring out what camshaft I would want to run and have the piston valve reliefs fly-cut to accommodate the camshaft, if necessary. Then just mill the piston top off until you reach the desired compression ratio. Easy to do by any qualified machinist with a piston vise and if you have good pistons, why not use them?

Another point on going to 98mm. In my personal opinion, since you are going straight to a boosted application, I would save your money and retain your current cylinders and only work on the pistons. The boost will be plenty compared to the small bump in displacement. I only went with 98mm since I already had fresh 98mm cylinders from a different project I was contemplating.

Also, since you have a stock setup, you should be able to get values for your cylinder head volume, deck height, and piston dome volume. With that, you can calculate what change in piston dome volume you need to achieve your desired compression ratio. That way you can see how feasible it would be to machine your current pistons to suit.

Or, if you buy new pistons, know which generation you want. The previous gen JE pistons were still pretty good, but last I checked were like $500-600 cheaper than their current line.

I'm in the same situation as you, I have a 930/20 engine and planning on turbocharging it, I started by getting a very clean set of US specs 9.5 P/C set and will use .5 mm base gasket to get the ratio down to 9:1. With the current EFI technology, you can get away with relatively high boost levels with such ratio maybe upward of 10 psi or so.

Hello Kamaro,

I have received a downpayment for my car.
Hence my supercharger kit is now up for sale:

https://www.ebay.co.uk/itm/202983390978


Think its a good - trouble free upgrade for our cars.
Mind you I will go 930 / 964 Turbo next.

All the best,
Alexis