piston choices 80SC

[trond]

To get a bit more bulge in the midrange I am changing to higher lift cams (DC13s). I already have the EFI, SSIs and EDIS, and the next step appears to be pistons and cylinders. I have the small port heads and narrow 2R runners.

However my own experience with just upping capacity without also improving the breathing has not been too good on the two other engines I have done. Those engines were a Kawasaki (749cc-810 cc) and a Ducati (649-749 cc) and both of them turned into tractors, with need of head work (after which they went fine). I am not racing and I am already struggling to stay below our local go-straight-to-jail speeds. For those reasons I want to keep the RPM range of the engine as it is, and don’t want to move the useful power band in any direction. It would be more fun with a bit more cam but there are no tracks within 2 days travel so it will remain a back road second vehicle and grunt and useful range is more important.

I have no reason to believe my existing cylinders are anything but good and it appears to be a shame to replace them. New 95 mm pistons might or might not be justified to get a better shaped combustion chamber and increase compression. As I already have the 9.3:1 CIS pistons, moving to the easily available JE 9.5:1 carb/MFI pistons seems to be a very small upgrade and maybe not justified.

So I am seeking advice and opinions. Is there a real benefit with using a non-CIS piston dome? What other piston choices are there and how much CR makes sense with 91/92 octane (PON) and slow cams ? The EDIS makes retarding ign to avoid detonation at max load easy.

Regards

[brads911sc]

With your induction you have lots of choice. I went with bored / plated 3.2 Mahle cylinders and 9.5:1 JE's... 98mm. Was 1/2 the price of Nickies/JE's, or MM Mahle. May want to talk to Henry at Supertec.. He has done this many times.

[trond]

Hmm. I'll look into that. I would have preferred cast 9.8CR pistons. The Euro SCs had 9.8 CR cast Mahles but CIS shape.. Don't know if thats significant or not
As it stands its either 9.5 CR forged/98 mm replated, or 95 mm Euro pistons in existing cylinders

Forged pistons typically need more clearance because the higher expansion coeficient with the high Silicone contents. Is this a real issue or not ? Is your engine noisy when cold ?

I am still concerned my narrow heads and runners might limit breathing with the slightly larger capacity

Trond

[Flieger]

The JE alloy is different (lower Silicon) than the MAHLE so JE pistons need larger clearance in MAHLE cylinders when cold because the higher Silicon alloy expands less with increasing temperature.

JE pistons and LN Engineering's "Nickies" cylinders are matched alloys so there are no piston slap issues while cold.

[brads911sc]

Using Mahle with unmatched alloy isnt what i said.

I said have the Mahle's bored and replated and then run JE's. For what its worth... the company that plates the Nickies also did my replating of the Mahle (call Don at EBS).. So no issues. Same plating. Same lifetime warranty. no piston slap.

Originally Posted by Flieger

The JE alloy is different (lower Silicon) than the MAHLE so JE pistons need larger clearance in MAHLE cylinders when cold because the higher Silicon alloy expands less with increasing temperature.

JE pistons and LN Engineering's "Nickies" cylinders are matched alloys so there are no piston slap issues while cold.

[E Green]

Rothsport has custom made CP European 9.8 to 1 forged pistons made with the CIS dome. We have these for the 3.0L and the 3.2L engines. We require you send your cylinders to us for honing as our pistons are made slightly larger to work well in worn/honed cylinders. We can also CNC port your small port heads to the 78/79/EURO large port. This will net 5 to 8 HP over the small port. We have been using this combination for several years on our 3.0L and 3.2L 911 spec race engines as well as numerous street engines both on pump 91/92 octane. BTW the 3.0L Mahle euro pistons are very weak as they are cast and not forged.

Thanks... Earl
Rothsport Racing

[E Green]

[E Green]

[trond]

I don't have CIS, but EFI.

It looks like it is either JEs or 98 mm conversion.

Given my cams and small port heads higher CR 95 mm pistons appears to be the way to go.

[trond]

I was reading in Waynes Engine rebuild book last night and noticed the shape of the 3.2 Motronic pistons are shaped similar to the CIS pistons and it says in the text this is because of gas pooling and to assist with mixing in the combustion chamber with the batch fire injection.

My Megasquirt EFI is batch fire too, similar to Motronic, so a CIS/Motronic shape might be good ? Any viewpoints on this ?

[Flieger]

Sounds like sound engineering logic to me.

[tom1394racing]

Just remember that the 3.0 CIS/3.2 DME dome shape will limit the amount of valve lift you can use without getting piston to valve interference.

[brads911sc]

Yes. When I called camgrinder and told him where I wanted my torque, he recommended a cam, what compression ratio he liked for that cam, etc. The cam left me one piston choice. JE 9.5:1. None of the stock pistons 3.0 or 3.2 would tolerate the lift... So do this in a big picture way not peice by peice kind of way.

Originally Posted by tom1394racing

Just remember that the 3.0 CIS/3.2 DME dome shape will limit the amount of valve lift you can use without getting piston to valve interference.

[trond]

Same as with me, I too was recommended a specific cam to go with my target rpm range and that’s what I ordered. I am sticking with the plan and a moderate CR increase is part of the plan. I considered for a short while changing plan and go with 3.2SS/different cam/porting/induction but decided against it. Making an engine run at its optimum is more important to me than max power. For the roads I would be as fast with a 2 liter so no need to break the bank.

The last days I have been reading more.

Some say that the CIS/Motronic pistons were shaped that way was to squish mixture towards the single, eccentric plug.
Others believe it was to enhance mixing/swirling with CIS and/or batch fire Motronic.

If the correct answer is achieve squish towards one single plug, then cars with CIS shaped pistons should stay with single plug. A change to different pistons (like 10.5 CR JEs) and dual plugs would be great.

If the answer is CIS shaped pistons work better with batch fire injection then it would be better to stay with CIS shaped pistons (and single plug)

[brads911sc]

I doubt you will be able to tell the difference. With a recurved Dizzy, hotter cam, higher compression... the shape of the dome is really a marginal impact at best. There may be an optimal way to do this, but doubt youll notice so I wouldnt spend any extra money. The beauty with EFI is that you can program it to smooth out any rich / lean areas created by the piston shape...

[tom1394racing]

Another option to consider with the stock piston is to machine a valve pocket on the high end of the dome to increase piston to valve clearance.