Piston Styles

jjrowe · 09-18-2006, 09:45 AM

I have a question on pistons that I'm hoping someone may have also had. I just purchased a new piston set for my 1976 2.7 CIS 911s. The part # on the box is 911.103.949.01, this is the correct part. My issue is that the pistons are of the older style with a uniform dome at the top with notches for the valves (right piston in photo). This is the early style according to waynes rebuild book. The ones that I pulled off had a steep concave surface without notches for the valves which I understand is the CIS style (middle piston in photo). This is also the style that Wayne shows in the rebuild book for my engine year. I wonder if porsche superceded the CIS style pistons with the notched ones.

Anyone know for sure before I put them on?

Thanks,
Jesse

kenikh · 09-18-2006, 10:38 AM

That pic is just too small. Can you post something larger?

jjrowe · 09-18-2006, 11:13 AM

I am looking for something bigger. This pic is in Wayne's rebuild book so you can see it there if you have it.

Jesse

jjrowe · 09-18-2006, 03:45 PM

Here are pics

My new ones are this style:

My old ones are this style:

kenikh · 09-18-2006, 03:58 PM

Your news ones aren't CIS, then, as far as I know.

76911 · 09-18-2006, 04:21 PM

The pistons with the valve relief in them are for carbs or mfi.They still
are 8.5:1 cr

CliffBrown · 09-18-2006, 04:34 PM

Jesse

You definitely have the wrong pistons for use with a CIS system, would probably work just not as efficient and I'd suggest a check for any interference with valve to piston.

From the photos it looks like your cylinders are Alusil. Can you confirm? My question concerns the type of piston (material not design) used with Alusil cylinders.

You might do a search there is plenty of info here on the forum regarding the use of alusil and nikasil cylinders

jjrowe · 09-18-2006, 04:45 PM

Cliff,

These pictures are not actually of my pistons, I found them on this site for reference. My new ones are mahle nikasil and are the correct size for my 2.7. I wonder if the factory just quit making the special ones for CIS. I don't think there were any 2.7s that were not CIS (other than the RSR) so I can't imagine they mixed them up with some other ones. I will definitely check the clearance once I get the timing set. I am thinking of going with EFI in the not so distant future so these may be OK for now.

Jesse

EPorsche · 09-18-2006, 08:14 PM

Jesse,
I have the same pistions in my 2.7 with 143 and 144 cams (I think CIS) w/TBitz EFI. The car runs fine and pulls strong but the idle is just a little rough but stable. I'm also in the middle of tuning. If you use the same cams then you will have problems with valve clearance. I had my pockets cut about 1 to 2mm deeper to pass the valve clearnace check. The pictures are before I had the pockets cut deeper.

911pcars · 09-19-2006, 12:23 PM

These are new? Instead of returning them, I'd seriously consider using them. If the cylinders are Nikasil, and the pistons are forged, you have RS pistons/cylinders. Get some Webers/PMOs and some MFI/carb cams and you will have an engine more responsive than the CIS-emission version.

One drawback would be CA smog inspection.

Sherwood

Henry Schmidt · Henry Schmidt's Garage

The flat 90s with valve pockets are Euro 90s also know as RS spec. The same pistons that were used in all street 2.7 MFI engines.
These engine were build from 72-77.
They (the pistons) will work just fine in a CIS engine. You would be hard pressed to see a difference even on a dyno.
As for valve to piston clearance, these pistons offer more clearance than the CIS piston. You can run up to a 911"S" grind with no clearance issues. As you probably know, the 2.7 CIS cam is one of the lamest cams as for as lift and duration.

If they are new and paid for use them. This will allow you to upgrade your cams without clearance issues. If you stick to CIS try 911SC grind.

jjrowe · 09-19-2006, 04:10 PM

Thanks Henry, that is the best news I've heard all day. I have almost assembled the motor with these pistons. I was somewhat worried about having a clearance issue, but from the look of them I figured I'd be OK so I went ahead.

Jesse

jjrowe · 09-24-2006, 08:09 AM

Henry,

As far as upgrading to SC cams, how much difference would this make from my stock cams (1976 911s CIS). Can I have mine reground or try to purchase some used?

Henry Schmidt · Henry Schmidt's Garage

Stock SC cams will not fit in your cam towers.

Call John @ http://forums.pelicanparts.com/drcamshafts.com for options on cams. He is as good as anyone I have seen when it comes to cam selection.

camgrinder · 09-25-2006, 10:06 AM

The stock 911105143 and 911105144 cams can be ground to SC specs.

And once again, Thanks Henry!