Actual compression with 3.4 max moritz?
 

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!

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

Those pistons were works of art.

BTW: What induction will the 3.4 use?

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.

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

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

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

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

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

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):


http://forums.pelicanparts.com/uploads/dings2.jpg

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



http://forums.pelicanparts.com/uploa...1069134036.jpg

Tom

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.

Which?

sorry.. the flat headed one

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

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

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?

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.

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

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

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.

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 ?

I'd twin plug it and enjoy the high compression. :)

I had to run 1.5mm of shim to maintain 9.5:1. I found that shims are available in 1mm and 0.5mm and can be stacked. The subject was faily well covered here:

http://forums.pelicanparts.com/showthread.php?s=&threadid=124956&perpage=20&highl ight=shim&pagenumber=2

That seems high for a piston with a fairly low dome. What were your dome and head CC measurements?

What is your deck height?

Tom

The following method measures the compression volume, exactly, without any math or measurements that are hard to make.

The engine must be out of the car and on an engine stand. The engine must be in good condition, ie good rings and valves. Remove the spark plug from the cylinder to be measured (its a good idea to measure all of them, as it will vary). Turn the engine so that piston is on TDC and both valves closed. Rotate the engine on the stand until the open spark plug hole is facing up and the spark plug mating surface is horizontal.

Now for the measurement. Obtain one of the following: A syringe, calibrated in CC, preferably a large one, ie 50 to 100cc. OR a beaker that is calibrated in cc, OR one of those long tubes that are calibrated in CC with a petcock on one end.
Next fill the cylinder (I will use the syringe for an example) up with one of the following: water with a small ammount of antifreeze mixed in OR water with a small ammount of alcohol mixed in, OR very light weight oil, like diesel fuel (if you are worried about rust or leakage into the crank case).

Now start filling the cylinder with fluid, until it is full, ie fluit level with the lower edge of the spark plug hole. How much fluid did it take? Thats the EXACT ammount of cc in the compressed volume. All varitaitons in pistons, valves whatever are taken into account. Very simple and Very accurate.

Next rotate the engine on the stand untill the fluid drains out, completely. If you used water, follow up with some compressed air to help dry the cylinder out and a follow with a couple of squirts of oil and rotations of the engine to spread it around.

Finally you can make the compression calculations (sorry you do have to use math at this point). You know the bore, the stroke, and the exact compressed volume, thats all you need.

If your a real purist, and anal about it, you could actually measure each bore to the nearest 10 thousands of an inch and the stroke of each throw, to the nearest thousandth, but knowing compression ratios to 5 decimal places is of questionable value.

TRE any follow up on the engine you were building? Dyno numbers and specifics would be great.

Jack,
Doesn't the spark plug take up some volume that wouldn't be measured using the fluid method?
-Chris