Matching 3.0 Euro pistons to 3.2 cyls?
 

Does anyone see any problem with matching 95mm 3.0 Euro pistons (9.8:1) to a set of 95mm Mahle 3.2 cylinders?

I am considering using this P&C set up on a 79 Euro SC engine...

Further, I am also going with 40 mm Webers, between a 20/21 and Early S grind cam, single plug, MSD ignition, steel head studs, and Raceware rod bolts..

Thoughts?

As long as the piston to cylinder clearance is correct you should be fine.

The 40mm Webers are a little small for the setup you are proposing. 46mm Webers would be a better choice.
I'd also double check with Web Cam about piston to valve clearance with the Euro 9.8:1 pistons.
Should be a fun motor.

If you're going to 40mm webers, this should really be a street-only engine.. above 6k rpm it will be falling off in power. I would run the Mod-911S cams, but Mod-S cams won't work with those CIS pistons as-is. The most you can run, pretty much, is the 20/21 cam (unless you cut valve pockets in the pistons, which is definitely possible). The heads you have are great for a hot 3.0...they really like the webers/hot cam/pistons combo.

If you end up with a 20/21 cam, you'd probably be better served with EFI or CIS than Webers.....

Raceware or ARP rod bolts are great but really are not necessary for this type of engine...7k + rpm regularly, then you need them (or if you are planning to miss a shift). They are reusable so they pay for themselves in two uses (compared to factory bolts) but it will be a long while (hopefully) before you need to do rod bolts over again! :)

Steel head studs are fine for this..MSD works well, set the limiter!

Thanks for responding guys!

So then by using these pistons I can expect a drop in power after 6k? How much of a benefit does cutting valve pockets on the pistons give me power-wise (assuming I keep all of the other components discussed above including Mod-S cams)?? Do you know of a source cutting valve pockets? Estimated cost?

Thanks again.

The pistons will not cause a drop in power above 6,000.
The 40mm Webers will cause a drop in power after 6,000, just like Scott said.
Cutting valve pockets is done to prevent the pistons from hitting an open valve.
It will actually lower the compression slightly.

Right. This is the trade off for running a bit more cam. But, Euro pistons should still leave you in the 9.3 range, maybe a little more.

Hmmm, well then I'm probably better served going with a bigger bore, 98 mm p&c setup, 3.2 SS route vs. what in my mind would have been something similar to Wayne's High Performance 3.0??

Or is my problem really just sticking with 40mm vs. using 46mm webers or ITBs of the same size with EFI?? But then does that also mean twin plugging the heads plus dizzy or Crankfire ignition?

Is this what you all mean when you talk about the rebuild "slippery slope?"

A 3.2 SS will be more expensive; more expensive P/C set, and you have a bit of head chamfering to do.

Twin plugging is recommended for a motor with 98mm+ bore. Having a very good single plug system, however, may allow you to stay with single plugs (MSD, Iridium plugs, Magnecor wires, etc.).

You have to outline what you want. CIS and EFI on a CIS or 3.2 manifold will give you a very reliable, nice engine, price is pretty reasonable. ITB or carburetors and a hot cam will get you more "rawness," power and sound. Personally, I love an engine of the latter disposition, and find them worth the extra $.

There is no strict formula. If you want to rev it to 7k and have it making power there, you need a good cam like a Mod-S or something even hotter. The problem with this is that a motor above 2.5L or so will start to run out of breath at that RPM, because of the carbs. You have to consider the fact that a 3.2 SS will run out of breath even sooner using 40mm webers. Solution? The easiest one in my eyes is boring the carbs to 46mm. Or, you can simply order the ITBs in a larger size for your engine...shouldn't cost more for a different size.

I have a client who came in with a stock '79 (big port) 3.0 CIS engine, using 40mm Webers with no other mods. The car is still fairly quick because of the torque, but it just does not have the power at the top-end. He says he short-shifts it at 5500 because it stops pulling, and I don't doubt it. I am going to rebuild this engine to be a single-plug 3.0L, with 9.8:1 JE pistons, hot cams (DC62?) and 46mm Webers. It's not in his budget to make it a twin-plug, non-expense-spared engine but when it's done, it should keep pulling to 7400 or so, and have that early 911S "cammy" feel that it currently lacks. Boring the carbs is not a cheap proposition, but it is cheaper than buying used Weber 46 carbs and rebuilding them.

Welcome to the slippery slope! :)