JE pistons - Can I machine them to increase compression?
 

Hi folks. I have a dedicated AX car. Naturally aspired 2.7 with JE 10.5/1 pistons in a standard 90mm bore cylinder. I'm pulling it apart right now to machine for twin plug and "while I'm in there" decided to increase compression. Is there a known technique for increasing compression by machining the pistons and lowering the deck height? Or, does JE make a piston set at 12.5/1 compression for just such an upgrade.

Any tips and trix would be appreciated. Just looking for an increase in performance.

Thanks,

Mike

Machining the pistons can only remove material; doing so on the dome can only increase chamber volume which -- all else (bore & stroke) remaining constant -- will lower CR, not raise it.

IMHO if you want to 'raise compression' on your existing motor you'd be better off considering:

1) New custom JE pistons or
2) Raising compression pressure rather than ratio via custom cam grind/profile/timing

Good luck

Well you can lower the pin height in the piston......... But that would introduce clearance problems in an otherwise stock head/combustion chamber.

Agreed the common way to increase static compression is to fatten the piston or fill up the combustion chamber. That's why high compression pistons have the large valve reliefs. The area around the piston where the valves extend toward the piston need room to clear around the fattened piston. I've seen kidney shaped combustion chambers that were created by welding them up to increase compression.

Also keep in mind that increasing the compression by a large amount also greatly increases your octane requirement.

Additionally, you can shave the heads to reduce combustion chamber volume. This does reduce piston/head clearance and can create some cam chain tension issues. Check with Henry Schmidt at SuperTec to ask about the shim spacers that move the cam towers back out to the correct location to offset the shaved heads.

IMHO it will be very difficult to keep the existing parts and gain 2 points of compression. Only custom pistons will get you there.

head work...shave the deck to decrease volume at TDC.

I'd order new JE pistons with the correct specs you're looking for. Bet you can sell your current JE's for a pretty good price and recoup some of the cost of the new pistons. And I suspect that net-net you don't spend much more than if you set out to do all the head and barrel machining. Plus you'll be doing it the right way.

But as Kevin said, 12.5:1 is a lot of compression, gonna need to haul some good 110 octane to the autocrosses with you.

Thanks, folks. Looks like new pistons will be the answer. I wonder if JE has a profile to modify these pistons to get what I am looking for. I am aware of the octane and clearance issues as well as the cam chain problems one can run into. I was hoping someone would have a "Oh, that's easy. We do that by..." but it looks as if calling on JE will be the least tricky solution. Or, I stick with what I have and lose 200 pounds.

Maybe posting a WTB in the classifieds will yield something too. I'll try that as well.

If anyone still has ideas please keep them coming.

Thanks,

Mike

Whether you shave the deck of the heads...or remove some material from the base of the cylinders (maybe thinner shims) you will run into a problem with the cam chains eventually.
If you move the heads inwards...the cam chain tensioners will need to move further to keep the chains tight.
There is only a small amount you can get out of this arrangement.
New high compression pistons would be the way to go...but even then...you are limited by the head design (dome) and the valve angles.
Newer style heads would have a better valve angle and probably better flow if ported correctly.
All things to think about.
Bob

Another way of dealing with the cam chain length is a larger idler gear. I believe German Precision makes them.

Are you doing your own engine work?

I can think of two expert commercial contacts that have a lot of experience with JE's and Porsches. I would start with them.

One is Engine Builder's Supply. They are friends. And are expert and supportive.

The other is LN Engineering.

As for machining changes, Ted Robinson at German Precision and Mike Stimson at Pacific Motorsports are excellent resources as you put your plans together.

Note that quite a few of us in the SF Bay Area with performance Porsche engines have gradually moved away from JE products and toward custom Mahle products. Certainly in the larger sizes. We have seen a relatively recent series of failures in wrist pin boss areas and domes. In my case, several of my large JE"s failed in the dome area at about 30 hours (at RR IV last year at Laguna). Most use was under 7000 rpm. This was in a 3.8 liter application, but we have seen failures other sizes as well.

Of course if you are going to have an engine built, you cannot go wrong with Jerry Woods in Campbell, or Mat Lowrance at Reno Rensport. Both have extensive experience and an impressive array of customers spanning street cars, valuable historics, DE cars and racers (including but not limited to the front part of the grid at PRC races).