JE pistons with CIS?

914efi · 09-11-2006, 01:17 PM

Is it unwise to use JE pistons with CIS? There has been plenty written about the piston shape being optimized for CIS, but the JEs would not have this shape. My goal would be to do a c/r increase along with my rebuild. I plan on stock cams.

cnavarro · 09-11-2006, 01:21 PM

JE can do a CIS profiled dome which is different from the more common hemi dome prevalent on most JEs. I would stick to around 9.5:1 if keeping it single plug. I have purchased them from EBS Racing in the past; I'm not sure if Pelican has it or not.

2.7RACER · 09-11-2006, 01:31 PM

Art,
I put in JE 9.5:1 pistons in my 2.7L with CIS three years ago.
Trying to gain back what I lost from being over 5,000 feet.
I didn't dyno it but I don't think I gained much.
The next year I put in a solex cam from John Dougherty and webers.
The engine came alive.
Can't pass smog though. Doesn't matter for me since I live in a county in New Mexico without smog testing.
Primarily a race car today. 95% track. No heat, no air.
At 5,000 feet I should have gone to a higher comp.
Listen to Charles Navarro, He knows.

safe · safe's Garage

Doug, so the CIS will still work with the non cis dome JEs?

cnavarro · 09-12-2006, 11:04 AM

It will, but the CIS specific design doesn't shroud the valves as much as the normal hemi piston, so the CIS piston has a better flame front propagation, but again, I have had customers use the normal piston with no problem.

2.7RACER · 09-12-2006, 01:04 PM

Here is a pic of the 9.5 JE's.

Compared with the stock 8.5 Mahles.

safe · safe's Garage

I was thinking it would be possible to use JEs or 2.7RS pistons with a hotter cam and still retain the CIS with the addition of Tony's EFI kit.
A slightly raised idle would be ok.

The reason would be to make the engine look stock for the visual inspection. The only other demand it would had to meet is < 3% Carbon-monoxide.

matty74 · matty74's Garage

There is an article on pelican re best CIS rebuild using JE pistons and 964 cams shafts if you want HP and economy it is the way to go. search the articles archive it will point you in the right direction

914efi · 09-14-2006, 04:57 AM

Thanks all.

The more I think about this, the more I think the piston shape was probably designed more for emissions, economy or fuel grade than any particular CIS issue. Fuel and air in are the same no matter what injection system is used. They might have been trying to run very lean for economy and the shape helped with pre-ignition or similar issues.

safe · safe's Garage

The design of the CIS dome are to help with the atomization of the fuel since the CIS fires fuel into the intake continuously. You probably get a cleaner burn with the CIS dome.

914efi · 09-14-2006, 06:42 AM

The dome design can't affect atomization.

I would expect CIS atomization to be better than modern EFI anyway because the pressure is higher.

Henry Schmidt · Henry Schmidt's Garage

The CIS dome has more to do with detonation than anything else.
CIS engines run lean by design and this lean running condition preffers a squish chamber dome.

If you keep the mixture on the rich side any dome will work.

If you adjust the mixture for smog requirements be careful with the timing so as not to detonate the motor especialy if you raise the compression.