JE Pistons vs Deckheight Mod for Compression

AFM744 · 02:23 PM

I just spent an hour on the phone with Ted at German Precision, a very knowledgible, helpful and generous guy. I'm glad to be taking him some business with my head rebuilds.
I want to increase my compression, and we had a lengthy discussion about custom JE 8:1 pistons vs milling into the heads, offsetting wristpin bushings, and clearancing the pistons to achieve same. I know opinions on JE are very polarized on this forum, though they'd achieve a quick and easy compression bump. I also know my Mahle pistons are tip-top, broken in and mated to these cylinders.
Long-term build plan is EFI, port&polish, 964 cams, GT35R.
Anybody with input? Thanks!

AFM744 · 10-03-2008, 01:04 PM

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C'mon guys, you gonna make me post this on the Engine Building forum? I'll get scolded again for using The Wrong Studs, or chastised for not sending my heads to [insert shop here].

GJF · 10-03-2008, 01:28 PM

Well.......... I have JE's and have used them for years in many different turbo cars. I would change pistons rather than have all that machine work done. Plus once you cut the heads then you are done with them there is no going back. With the cost of a set of JE's you are not out that much , especially compared to a set of Mahle P&C's with the desired compression. I take it you are going to do a rebuid or just the top end? You should definetly do ARP or Raceware rod bolts and head studs.

AFM744 · 01:58 PM

Your build is one of the examples I'm hoping to emulate. Yes I'm doing a total rebuild, though the bottom end is totally copacetic and will only be reassembled with ARP rod bolts and studs.
Ted assures me he can take .5mm off the heads and .5mm offset of the wristpin bushing (and clearance the piston edge) and I'll have the same ~1-point bump in compression. I spent the morning talking with him at his shop. He's starting with the requisite valve guides and rocker refurbs.
I also recognise that you and several other active contributors on this forum have had excellent luck with your JEs. Thank you for sharing!

GJF · 10-03-2008, 02:07 PM

OK, Here is something I always do on a turbo motor. Especially 911/930 Porsches. When I have my pistons made I lower the rings as low as they can be on the piston so there is more meat at the crown. Typically when the valve reliefs are cut in IMHO there is too much material taken away, and IF you deck the piston higher and need to clearence the valve pockets further it will make the crown at the vale relief's even thinner. If the engine might lean out, the valve relief is the first place to melt. By lowering the ring lands, it gives the piston more crown and gives alittle insurance to the pistons. I also am a fan of the anti-detonation grooves at the crown. Just a suggestion.

AFM744 · 10-03-2008, 02:13 PM

Excellent points on the pistons. Mind sharing who had your pistons made, or did you source directly from JE?
In tangent, I cc'd my heads last night. 80ccs is what I got. The stock piston domes displace 3ccs, the stock headgasket (965) is .45mm. I math'd this all out and I got between 7.1 and 7.4:1 depending how I interpret the deck height. So factory static spec is 7.0 and people here saying c/r is actually in the high 6s? The math is the math, right?

GJF · 10-03-2008, 02:30 PM

Typically the the values of all piston manufactures are ever so slightly lower than advertised. The math is the math. I have bought JE's through many different vendors. The last pistons I had done I had Mike Bruns at JB Racing order them for me. It was easy for me to have him order them as he did have to set the clearence on the JB cylinders to them anyways. I will say this......I will be a JB Racing cylinder customer forever.

lucittm · lucittm's Garage

You mentioned rocker refurb. What is Ted doing for your rockers? I didn't think there was much to do on those.

Thanks,
Mark

AFM744 · 10-03-2008, 03:00 PM

My rockers are fine, except that I'm going from 930 cams to 964 cams. Consensus is, regardless of follower condition they should be reground when going onto a different cam so they can mate to the new cam.

GJF · 10-03-2008, 03:09 PM

Quote:

Originally Posted by AFM744

Excellent points on the pistons. Mind sharing who had your pistons made, or did you source directly from JE?
In tangent, I cc'd my heads last night. 80ccs is what I got. The stock piston domes displace 3ccs, the stock headgasket (965) is .45mm. I math'd this all out and I got between 7.1 and 7.4:1 depending how I interpret the deck height. So factory static spec is 7.0 and people here saying c/r is actually in the high 6s? The math is the math, right?

You also need to add the deck volume. Porsche sets factory pistons .040.

AFM744 · 10-03-2008, 03:12 PM

I did. My deck height seems a little tighter than that (0.25mm), but I factored that in there. Thanks again!

David · David's Garage

I have a nice set of 98mm Mahles on the shelf. I went with JE's so I wouldn't have to mill the heads for compression.

DonE · 10-03-2008, 04:34 PM

My heads were milled by the previous owner. This shortened the distance between the cams and the crank thereby putting additional slack into the cam chain. I recently had to install larger idle arm pulleys to take the slack out. Put the work into the pistons and I would make sure JE knows you want factory deck height with your OEM heads. Squish is critical to these motors on boost.

AFM744 · 09:10 AM

DonE, yours is another build I've followed. I suppose my machinist would argue the relevance of .030" on chain length, but I also recognise that when we change deck height and cam height we have to think about a LOT of other little details. Yours is a compelling point, thank you. Bringing the valves and pistons a total of .060" closer will certainly require machining valve pockets on pistons that were not designed for such.