GE-60 or S-cams on a 3.2?

[Roamer]

Can GE-60 or S-cams be used on a 3.2 liter Motronic (stock pistons), assuming custom engine management?

[KobaltBlau]

Originally posted by Roamer
Can GE-60 or S-cams be used on a 3.2 liter Motronic (stock pistons), assuming custom engine management?

Don't think so. I've heard web-cam 20/21 are the biggest you can clear. there was a thread with this info very recently.

[Carrera3.5L]

Even if you get rid of the stock Motronic (which won't work with those cams) and get some sort of stand alone system or carbs, you still have the problem of the piston shape that the CIS/Motronic cars have. It is a high dome on one side that channels the fuel to one side of the combustion chamber where the single plug is. This high dome will interfere with the valves on high lift cams. You would need to use the hemi-style piston with the deep valve pockets to use these cams.

This is why I am using the carb/MFI piston with the stock Motronic for my 3.5L. If I ever want to get rid of the Motronic, I would just need to change the cams and not the pistons as well...

Ralph

[camgrinder]

The problem with both the GE-60 and the old S grind is the lobe centers.
Motronic cars have 113 lobe center cams which cut the lift at overlap down. The ge-60 has a 102 L/C and the S cam has a 97 L/C. Both cams have a lot more lift at overlap which decreases the valve to piston clearance.
The 964 grind or the 20/21 grind are the most popular for use with stock pistons.

[Wayne 962]

You might be able to get away with machining the tops of your Motronic pistons, but I'm not sure. You have to leave at least 5mm of material between the top and bottom of the piston...

-Wayne

[Roamer]

In light of these well educated responses, it might make sense to look into new pistons with greater relief and higher static compression, say a 9.8:1 (to stay with a single plug) then combine that with a higher lift cam like the S or GE-60 (or some variant on either).

Ralph, how do the carb/MFI pistons you mentioned differ from a hemi style (which I believe may interefere with the flame propogation)?

Thanks guys!

[Carrera3.5L]

Originally posted by Roamer
In light of these well educated responses, it might make sense to look into new pistons with greater relief and higher static compression, say a 9.8:1 (to stay with a single plug) then combine that with a higher lift cam like the S or GE-60 (or some variant on either).

Ralph, how do the carb/MFI pistons you mentioned differ from a hemi style (which I believe may interefere with the flame propogation)?

Thanks guys!

The Carb/MFI piston is what is referred to as the hemi-style. This piston shouldn't be used with the stock Motronic system and single plug for some of the reasons you mention. It should only be considered as an alternative when changing to twin-plug ignition (like I am). In my opinion (and that of some other engine builders), the carb spec piston will work better with Motronic and twin-plugs than the stock shaped piston does. My buddy who was the head engine builder at Andial/Porsche Motorsport NA for over 15 years confirms this is the way to go, as the 962's all used a version of Motronic (albeit more advanced) and used this hemi-style piston shape with twin-ignition. Only the very early 962 engines with single ignition used a wedged dome piston similar in shape to the stock Motronic. I don't have a picture to reference that shows the contrast right now, but I think that Wayne's excellent book shows what I am referring to.

BTW, my buddie's 3.4L twin-plug with Motronic uses the hemi-style piston and with 964 cams, headers by George and Steve Wong's magic the car runs awesome.

Ralph

[Roamer]

Thanks for the clarification Ralf. I believe that the "Max Moritz" piston design that is oft referenced uses a Motronic dome with greater valave releif and higher compression. This may be the key to using more aggressive cams.

Does anyone have any suggestions on whether I should go beyond a GE-60 or S cam on a street driven 3.2? I plan on raising the rev limiter (and replacing the key valvetrain components and rod bolts to acccomodate the increased revs) to 7-7500 rpm to take advantage of the new cams, should I get more agressive in my cam selection to generate more hp? Or do I risk a loss of too much low-end torque?

[Roamer]

John, IYO, how would an Elgin Super C-2 cam function on the engine I am descirbing?

[Henry Schmidt]

Valve pockets in your stock pistons will allow for greater lift and duration cams and still provide adequate compression with today's fuel. Remember that with the Motronic injection wide lobe center is crucial. Low rpm intake pulses will make the engine impossible to enjoy.
If you choose to run new pistons the key is a piston design that has a bump as flat as possible and still creates the pop you want. I have even run stock 3.6 pistons and an otherwise stock Carrera and the results were amazing

[KobaltBlau]

Henry, did you use standard 3.6 cylinders and heads as well? head gasket? single or dual ignition?

TIA!

[Henry Schmidt]

We used modified 100 mm 3.6 Carrera II Ps & Cs on an otherwise stock 3.2 Carrera. We even built one with 20/ 21 Web cams and SSI Exhaust. Great fun!!

[camgrinder]

The super C2 cams work great with 9.8-1 compression if you retain the injection and get a chip.
If you change over to some PMO carbs you would need another cam. I have done some wide lobe center (108) Mod-S cams that work
great with the PMO's and 10.5-1 CR.

[KobaltBlau]

Originally posted by Henry Schmidt
We used modified 100 mm 3.6 Carrera II Ps & Cs on an otherwise stock 3.2 Carrera. We even built one with 20/ 21 Web cams and SSI Exhaust. Great fun!!

Nice. How did the one with 20/21 and SSI run? namely what rpm did it run out of steam?

Have you ever put 3.6 cyls on a 3.0? custom pistons would be needed, I'd guess....

TIA, and thanks for everything so far!

[Henry Schmidt]

6400 RPM and it was done.
3.0 rods have 22 mm wrist pins. 3.6 pistons have 23 mm wrist pins. You will need to change the pin fit in the 3.0 rods. Because of the extra rod length you will also find the engine starts to get very tall.

[Roamer]

Henry, the motor sounds interesting, but is it nothing more than a very high comp 3.5 (100mm P&C's on 3.2 crank and rods is usually a 3.5)?

John, thanks for the info, it would retain the stock injection and some form of modified engine management would be in use. As a follow-up, how much valve relief should I be seeking in the piston crown?

[Henry Schmidt]

Originally posted by Roamer
Henry, the motor sounds interesting, but is it nothing more than a very high comp 3.5 (100mm P&C's on 3.2 crank and rods is usually a 3.5)?

In fact the engine is not high compression, that was the point. The piston dome on a C2 is completely different from most anything you will find in any other engine [see illustration]. As you see the dome is concave and this recess is where the fuel will collect making a very nice single plug combustion chamber. Because the wrist pin location is higher in the piston ( to compensate for longer stroke/ same rod scenario) and the fact that there is less stroke in the 3.2 the end result is a fairly large deck height and great valve to piston clearance. Mearured compression ratio was 9.6.
Just a high compression 3.5 ? maybe not.

[KobaltBlau]

Originally posted by Henry Schmidt
6400 RPM and it was done.
3.0 rods have 22 mm wrist pins. 3.6 pistons have 23 mm wrist pins. You will need to change the pin fit in the 3.0 rods. Because of the extra rod length you will also find the engine starts to get very tall.

Thanks again, Henry. I see why the 3.6 pistons are attractive (rather than custom) based on your later response. Do you put in ARP rod bolts on the stock carrera 3.2 rods?

Maybe I'll stop asking you questons on this thread now

[Roamer]

Henry, Intereseting . . .

How involved is the "modification" process?

[KobaltBlau]

Henry has been very nice, but, Roamer, I bet we are getting close to trade secrets...