Piston dome volume of 3.2SS?

ChrisBennet · ChrisBennet's Garage

Hi guys,

I really need you help on this.

Question: Does anyone know the dome volume for a JE 9.5:1 or 10.5:1 piston as used in a 3.0 to 3.2 conversion?

Background:
I'm building up a 3.2 short stroke motor for a friend and I've hit a snag. The pistons are supposed to be 9.5:1 but I'm getting a compression ratio of 10.5:1 which seems like too much of a coincidence.
It's possible that we got the wrong pistons but my first instinct is that I'm having another um "learning experience" - I learn from my mistakes.

The vendor is going to check with JE for me but the sooner I can get to the bottom of this the better.

Here are my figures if anyone would be so kind as to check them.

Stroke: 70.4cc
Head vol: 87.8cc
Deck: 1.0mm
Dome vol: 39.5cc
Bore: 98mm

Hopefully I've pumped enough good karma into the system that some will come back to me on this.

thanks in advance,
Chris

kenikh · 03-25-2005, 10:56 AM

Maybe this is karma's way of telling your buddy he needs twin plugs?

1meansc · 03-25-2005, 10:56 AM

Hi Chris
I have just done a similar conversion.
I would just like to know which cylinder heads you are using. The normal volume of a 911 SC head is approx 90 cc.
So, there is one of two possible errors here. Either you have made an error with your cylinder head volume, or you are using the JE pistons suited to the SC, but are using the wrong heads.
Markus

ChrisBennet · ChrisBennet's Garage

Quote:

Originally posted by 1meansc
Hi Chris
I have just done a similar conversion.
I would just like to know which cylinder heads you are using. The normal volume of a 911 SC head is approx 90 cc.
So, there is one of two possible errors here. Either you have made an error with your cylinder head volume, or you are using the JE pistons suited to the SC, but are using the wrong heads.
Markus

Hi Markus,
These '82SC heads have been chamfer to remove the overhang you get when you put a 95mm diameter head on a 98mm cylinder so they should CC bigger than 90cc, not smaller. The sealing surfaces would have been "cleaned up" which would reduce the volume some, but I wouldn't think it would be that much.

Did you CC your's with a plastic plate on top to reduce the miniscus (sp?) effect?

-Chris

1meansc · 03-25-2005, 08:01 PM

Hi Chris,
I used plastic plate. I have been involved in a friends rebuild as well. He did the twinspark mod and had the minimum amount of material removed around the second spark plug, to keep the head volmume as low as possible, and he measured around 93cc.
My heads were converted to twin plug, and had an additional 0.5 mm machined off them to increase compression, and they only measured 89.5cc.
The SC head volume is recorded in the book "How to rebuild & modify 911 Porsche engines" as approximately 90cc.
I hope someone else could assist with info as well.
Markus

ChrisBennet · ChrisBennet's Garage

Thanks Markus.
When I got back tonight there was an email waiting from the vendor - they checked with JE and yes they sent the wrong pistons.
Glad I caught it.
-Chris

Jeff Alton · 05-03-2006, 08:48 PM

Chris, I hate to bring back an old thread and test your memory skills.....

But, did these infact turn out to be 10.5 pistons?? How did you measure the dome volume? I am interested as I just cc'd some heads for fun (practice) and used your dome volume (I have 98mm 10.5 JE) and put it through the calculator and it looks like I need to get to around 91cc in head volume, which should not be a problem for the 3.2 heads with twin plug....

Cheers

ChrisBennet · ChrisBennet's Garage

Hi Jeff,
I'll check my notes tomorrow - unless I forget to. ;-)
-Chris

Jeff Alton · 05-03-2006, 09:07 PM

You are up late!! I just found another thread with some pics you posted of measuring the volume around the dome. You gave a pretty good description there and it coincides with what I was going to do!!

You are one of the guys I seem to count on to answer my stupid questions!!

Cheers

ChrisBennet · ChrisBennet's Garage

What is sad is the reason I know the answers to the stupid questions is because I made the stupid mistakes.

I'm trying to our little company off the ground so I have to put in some long hours. On the upside, no one is shooting at me and I have a great commute - 12 steps from here and I'm in bed!
nite!
-Chris

ChrisBennet · ChrisBennet's Garage

Jeff,
Yes, both sets of "9.5:1" JE's I got for this 3.0->3.2 conversion measured 10.5:1.
The piston dome volume was ~39.5cc
The head volume was ~87.9cc
Actual deck height was .97m left bank, .92mm right bank. (The 1.0mm in my original post was a guess.)

I ended up reducing the piston dome volume to 32.7cc to get a compression ratio of ~9.5:1. The motor runs great.
-Chris

Vin-barrett · Vin-barrett's Garage

Chris

I contacted JE a couple days ago about them shipping the wrong pistons. Looks like they are doing something about it.

Here's what I got back:

Hello Vincent,

We now laser etch job numbers on all of our pistons. If you are concerned, give me a call with the job number and I will confirm.

Thank you,

Sean Crawford
Sales
JE Pistons
714-898-9763 x273
714-891-3233 fax

Henry Schmidt · Henry Schmidt's Garage

JJust a note: most JE pistons designed for increased bore size take chamber diameter into account. On the 98 they usually have a low area around the piston top before the dome starts. This eliminates the need for chamfering the heads.
It has been my experience that 911 SC heads CC between 89-91 cc even when twin plugged.

Jeff Alton · 05-04-2006, 07:07 AM

The stock set of 3.2 heads that I CC'ed last night were 89cc. At 89 it looks like I have about 10.8 so hopefully when I get me twin plugged ones back next week the have a little more volume...

Cheers

ChrisBennet · ChrisBennet's Garage

Quote:

Originally posted by catca
The stock set of 3.2 heads that I CC'ed last night were 89cc. At 89 it looks like I have about 10.8 so hopefully when I get me twin plugged ones back next week the have a little more volume...

Cheers

Just so there is no confusion you asked about the 3.2 conversion so the numbers I gave you previously were SC heads.

The head volumes for the 3.2 heads in my 3.4 twin plug conversion were: 91.2, 91.3, 89.6, 92.2, 91,2, 89.8 after twin plugging. I didn't CC these heads myself, my head guy did. I had to replace a couple of wrecked heads so he CC'd the to get them in the ballpark with the other 4 heads.

These heads are not chamfered. I think I will be getting them chamfered though after seeing how nicely any foreign bits get trapped between the piston and the edge of the head.

-Chris

Jeff Alton · 05-04-2006, 09:16 AM

Super, I just measured the dome volume and got 40cc, so as long as I keep 1mm deck height AND I can get 91-91.5cc in the heads once I have them back, that will put me right at 10.5:1 which was the target. If the heads end up a little bigger in volume by .2 or .3 that should be fine also......

Cheers