Actual compression with 3.4 max moritz?

[TRE Cup]

Does anyone know what the actual measured compression ratio is on a 3.2 crank/ 98 mm max moritz motor is? Not what it says on the box, but actually measured it with stock 3.2 heads
Thanks!

[dd74]

If I give the right answer, do I win the engine?

Those pistons were works of art.

BTW: What induction will the 3.4 use?

[rdane]

I will know shortly what it is on '79 SC heads. Suspect that isn't much help. But I have heard as low as 9.0/9.3:1 from the 9.8:1 set. I'll share as soon as I know. Please do the same.

I was told by two reputable sources (Andial and EBS) that the compression was low enough that the Porsche "special works" shop never bothered to twin plug them as a factory upgrade.

[TRE Cup]

We are using the stock 3.2 injection with extrudehoned manifolds, big bore throttle body, cams, headers, and steve wong chip. If we can get the compression up close to 9.5-9.7, we expect about 250-260 hp
we'll do the measurements and share

[emcon5]

For what it's worth, the 98mm Maz Moritz set I had the dome volume measured 28CC, but the valve pockets had ben recessed slightly for hotter cams. With a 90CC 911SC head they came out to ~8.6:1 with a 40 thou deck. I believe they were supposed to be 9.3:1 when new.

Punching the longer swept area for a 3.4 into my CR spreadsheet, it comes out to 8.9:1 with the dome measuremnt I made. Should be slightly higher without the deeper valve pockets.

The Mahle 98mm RSR has a dome of 37.3CC, which on my engine came out to 9.8:1

Tom

[Wayne 962]

There is much confusion on this topic. While the "Max Moritz" pistons and cylinders were available many years ago, it's not a wise idea to label all 98mm big bore kits as "Max Moritz" sets. In reality, Mahle makes several different big bore kits in the 98mm size. I have one, for instance, that is 10.5:1, and I had a higher compression one and a lower compression one to choose from when I bought those from Andial.

-Wayne

[TRE Cup]

Good point , Wayne- I'll get the mahle numbers off the pistons to see exactly what they are

[Wayne 962]

Let me know if you find a cross-reference on these - I have not been able to, and even the guys at Andial confused me when I called them last year. I even asked the guys at SEMA this year, and they gave me that blank stare back...

-Wayne

[emcon5]

The Max Moritz pistons I am refering to had a wedge shaped dome, exactly like the picture in BA's book.

This is a 98MM Max Moritz piston (with deeper valve pockets, and some damage from chewing up something metal):




The 98MM RSR pistons have a more traditional dome, with deep valve pockets. Here is an RSR piston:





Tom

[TRE Cup]

Tom
That is exactly what we have with a 22 mm pin. One thought was to rebush the 3.2 rod (23 mm pin) and offset bore it to get another 1 mm height into the chamber. We'll have to see where it measures out.

[emcon5]

Originally posted by TRE Cup
Tom
That is exactly what we have with a 22 mm pin.

Which?

[TRE Cup]

sorry.. the flat headed one

[rdane]

I am also using the flat head 98 Max Moritz CIS/Motronic pistons. Suppose to be 9.8:1. We'll see. I am also using a 3.2 crank and 3.2 Pauter billet rods on my original '79 3.0 case and heads with CIS. Doing some flow work, head work, h/performace grind on the valves, bored throttle body, boat tailing the case, half mooning the Pistons, ceramic and DFL coatings on the pistons, 20/21 cams, ARP bolts and SSIs.

Waiting to get a measure off a set P&C to decide on twin plug. 9.5 and above it'll be twin plugged. 9.4 and down no. Please feel free to comment

[TRE Cup]

rdane
are you talking about cutting the skirts on the pistons? or the cylinders?

[rdane]

Cutting the cylinders.....it was a brain lapse. Piston's coating is done and they are uncut

By the look of the MM piston's profile I would suspect a better combustion chamer and a better flame front over CIS and Motronic pistons. Your take gentlemen?

[TRE Cup]

i agree that the flame front would be better than stock or RSR. Most likely the result of a single plug intent of the design.

[Wayne 962]

My set looks like the 2nd pictures. They were available from Andial in 11.8 (I think), 10.5 (what I have), and under 10...

-Wayne

[emcon5]

Originally posted by Wayne at Pelican Parts
My set looks like the 2nd pictures. They were available from Andial in 11.8 (I think), 10.5 (what I have), and under 10...

Those are the 10.5 ones. Unless things have changed, the 10.5 is with smaller volume RSR heads.

On my engine, they came out to 9.815:1

Tom

[rdane]

Mine came out 10.67:1 on a '79 with a 1mm shim.

Tolerance stack on the rebuild, new cylinders, machining the case and heads ran it from the stated 9.8:1 to the fuigure I posted.

Eliminated the single plug idea.

[rdane]

After seeing the dry assembly and the 10.67:1 compressioin ratio I am rethinking how to finish the engine. Any one know how much shim I can easily add to lower the compression back to 9.5/9.8?

Or should I just follow through now and add the twin plugs since the mill work is already done and leave it at 10.6:1 ?