|
|
|
|
| Author |
|
|
Registered
|
Need Head info differenecs between a 3.0 and a 3.2?
I am still in the process of getting my 3.4 up and running. The sticking point is the 98 mm Mahle P&Cs came out with a 10.67:1 compression ratio on my engine on a dry run. They are suppose to be 9.8:1 on a 3.2 Carrera. I had planned the entire car and spec'ed the engine around 9.8:1 compression using a 3.2 crank and rods and a 1mm or less shim.
Is there a difference in head volume (smaller volume in the 3.0 heads in comparison to the 3.2s) that could account for the gain in compression? At the moment I have around $7K in parts (plus labor) that don't add up to a usable engine. Lots of "don't know" coming from the wrench and parts supplier. Any one know the answer or know who to ask? |
||
03-31-2004, 11:03 AM
|
|
|
Irrationally exuberant
|
I forget what the head volume of a stock 3.2 is. My twin plug 3.2 heads were around 90cc's.
How did you arrive at your compression number? -Chris
__________________
'80 911 Nogaro blue Phoenix! '07 BMW 328i 245K miles! http://members.rennlist.org/messinwith911s/ |
||
|
03-31-2004, 11:22 AM
|
|
|
Registered
|
My numbers, also on twin plugged heads are: 58.0cc cylinder head volume at TDC.
The compression was the actual measured numbers done by my shop...twice. |
||
|
03-31-2004, 11:58 AM
|
|
|
Irrationally exuberant
|
rdane,
I looked in my notes. My 3.4 has 98mm bore, 74.4 stroke, 1mm deck height, 39.7cc piston dome and "10.5:1" JE's. I get ~10.7:1 CR and my head volume at TDC is a 58cc also. Coincidence? Hmmm, I wonder if you really got 9.8:1 pistons. -Chris
__________________
'80 911 Nogaro blue Phoenix! '07 BMW 328i 245K miles! http://members.rennlist.org/messinwith911s/ |
||
|
03-31-2004, 01:01 PM
|
|
|
Licensed User
Join Date: Feb 2003
Location: ....down Highway 61
Posts: 6,506
|
Quote:
Last edited by Shuie; 03-31-2004 at 01:18 PM.. |
||
|
03-31-2004, 01:13 PM
|
|
|
Registered
|
Chris, the conclusion my wrench and I both came to two weeks ago was the pistons were wrong. They went back to the supplier with a cylinder, rod and head to have them check the compression ratio.
I know a little more than I did earlier. The pistons have now been double checked (more like quadruple) by a dry run and a comparison with a new set of Mahle 9.8:1 and are the stated Mahle 9.8:1. My heads are now 86cc instead of the original 90cc. The loss is due to resurfacing. If the deck and cyclinders were resurfaced also I'll have another bump in compression from the machine work tolerence stack. My case was unmolested and unopened when we started the process. In a mock up on a standard case, my heads came out with 10.27:1 compression. On my actual case from my car they measured 10.67:1. Nothing really settled today. But I believe the pistons are correct. Thanks for the insights guys. Last edited by rdane; 03-31-2004 at 03:09 PM.. |
||
|
03-31-2004, 02:45 PM
|
|
|
Irrationally exuberant
|
It sounds very frustrating. I hope my silly questions haven't made it worse. In case it will help, here are the measurements from my 3.4 twin plug for comparison:
Head volume: ~90cc Deck volume: 9.27cc (@ 1mm deck height) Displacement: 560.96cc Piston dome volume: 39.7cc Cylinder volume @ TDC: 57.8cc Cylinder volume @ BDC: 618.8cc CR: 10.7 : 1 My actual deck height is 1.23mm for a CR of 10.4 : 1. I came to the conclusion years ago that when it comes to motors "Trust No One, Measure Everything Yourself."
__________________
'80 911 Nogaro blue Phoenix! '07 BMW 328i 245K miles! http://members.rennlist.org/messinwith911s/ |
||
|
03-31-2004, 03:28 PM
|
|
The Blue Pheonix
