Head compatability with Mahle 2.8L cylinders

[911SCfanatic]

What heads are compatable with new Mahle 2.8L (10.3:1) cylinders? The cylinders are machined flat with no groove. The hole spacing appears to align with the stud holes on the case. (I was going to use 2.7L heads and have them machined for my MFI injectors).

[andrew15]

I'm pretty sure that the 10.3:1 Mahle 92mm piston you have were made to have 10.3:1 with the larger RSR heads. I don't believe that Mahle supplied different pistons for the early CE-ring and later No-CE-ring cylinders.

Lucky for you I just used a set of these in my 2.6 rebuild and have some compression measurements handy. The main one is piston dome volume - I have 29.01cc for that. Using a 2.7 head volume of 68cc, and a stroke of 70.4mm, I get a 11.25 compression w/ 1mm deck height.

Regards,
Andrew M

[Scott Clarke]

Fanatic-
The outside diameter should be the same for a 92mm cylinder as for a 90mm. So, you just need to be concerned about establishing the right piston dome volume if you have 2.7 heads. You don't mention what pistons you intend to use. Andrew has told you about Mahle pistons, but JE is also a possibility. JE can make a piston of any dome volume you like, so you can start with head volume and ideal deck height (most say 1mm) and establish the dome volume that will deliver the compression ratio you seek. It sounds like, if you have Mahle, you will have too much compression, at least for single plug.

2.2 or 2.4 heads should also work, and have the same size valves and combustion chamber, provided you have them machined for the larger cylinders. I did this for my 2.5. The head volume was 66.8 cc. This is a little different from Andrew's spec, which is to be expected as the surface where the cylinders rest on the heads had to be machined as part of the operation to make the cylinders fit. I would be sure to measure the volume of whichever head you use as a first step. Also measure the piston dome volume of your pistons, regardless of source. My pistons are being significantly modified as we speak due to an error.
-Scott

[Steve@Rennsport]

Originally posted by 911SCfanatic
What heads are compatable with new Mahle 2.8L (10.3:1) cylinders? The cylinders are machined flat with no groove. The hole spacing appears to align with the stud holes on the case. (I was going to use 2.7L heads and have them machined for my MFI injectors).

Bill:

As others have mentioned, any 2.2-2.4-2.7 head can be made to work with the Mahle 92's with some machine work to the squish bands.

The reason that Mahle deleted the CE grooves was to prevent the all-too-frequent cracking and breaking off of the cylinder liner between the CE ring groove and the bore, from detonation.

I hope you are going to run twin-ignition and modify your heads as needed to get around 10.25:1 for pump gas. We also machine the piston domes to reduce compression in these small chambered 2.8's for street use and if properly done, really helps cylinder burn patterns.

[bertj]

Originally posted by Steve@Rennsport
Bill:

As others have mentioned, any 2.2-2.4-2.7 head can be made to work with the Mahle 92's with some machine work to the squish bands.

I hope you are going to run twin-ignition and modify your heads as needed to get around 10.25:1 for pump gas. We also machine the piston domes to reduce compression in these small chambered 2.8's for street use and if properly done, really helps cylinder burn patterns.

Hi Steve,

What is a squish band?

Is it possible to modify ONLY the combustion chamber area in the heads to reduce the compression to 10.25:1? Or do you have to cut the piston dome also?

What modifications would be involved to the heads?

Is there enough "meat" in the piston dome to cut it down without hurting the integrity of the piston dome?

I have a Mahle 2.8 RSR piston/cylinder set with 10.3:1 C.R. (for the original RSR heads). I plan to use a twin plug distributor, but with 2.7 heads and I want to run street gas if I can reduce the C.R. to acceptable levels.

Thanks for your advice.

[Steve@Rennsport]

Bert:

The squish band is the area of the head near the outer edges that is shaped to force the fuel/air mixture toward the spark plug(s) as the piston approaches TDC. This reduces (not eliminates) pockets of air/fuel left at the outer edges that can initiate detonation. Careful control of deck heights and matching the profile of the squish bands helps reduce these engine's propensity to detonate with higher compression ratios.

To reduce the nominal 11.2:1 CR that these 2.8's have with OEM heads, one must modify the heads, spark plug reliefs, and pistons to get a reasonable compression ratio suitable for pump gas, even with twin-ignition. You cannot do it by machining the heads, alone.

You need to have around 10.25:1 to run pump gas in hot weather,....

[bertj]

Now I understand. I guess there will be more machine work than I anticipated. But these projects always cost more time and money than planned.

Thanks for your explanation.