Anybody who's built a 3.4L develop one or have one handy? Thought I'd ask before making yet another measurement.

Check over at Pelican in the engine rebuild forum. Lots of 3.4s over there.

Run the numbers you have in the formula:

Bore (mm) X Bore (mm) X Deck Height (INCHES) X .01996 + Deck volume in CCs

So, 98 X 98 X (.XXX") X .01996 = your deck vol.

That formula only works for a 4 cyl. So are we talking about a 4 cyl?

A /6 is harder to measure because the pistons are domed. Really any engine I wouldn't rely on someone else's measurements, there's a lot of variables that can change things.

These pistons have a dome that's not a standard spherical section. I can come pretty close with and estimate but if someone already knows this I'd rather have the exact data for these formerly popular pistons. I know all of the other numbers to generate a CR for the cam design including the dome volume. A pic might help explain my question.

Click to view attachment

I'm building one of these now. The advertised CR is 9.8, so that was good enough for me. I measured the piston/cyl height difference using a feeler gauge. Then measured the head/piston clearance with putty. Even if I did go to the effort of measuring CR I'd still verify old school as above. I found that the limiting factor in selecting cyl base shims to be in clearing the head versus piston wedge near the valve pocket. That part of the piston got closer to the head than at the circumference.

I know these are the 9.8:1 spec pistons but still need to measure absolutely everything fro myself. Putty is my friend and is definitely gonna happen with the buildup, but that's after the cams and springs are specified and installed. The motor came to me with 3L heads... not the early ones with big ports. I'm using 3.2L heads now and might, although these pistons aren't supposed to need it, relieve the heads a bit around the inside circumference of the dome. The 3L heads were. Both 3 and 3.2 heads are 90cc give or take what one does with the valve seating and piston relief.

The cam and proper springs are the outcome of this and I'm with you, gotta measure it all to get the base shims just right. It's funny how very little change in deck height will move the cr up and down.

Thanks for the info gents, including the bump JT!

I did a search over at Pelican for threads posted by one of the members who I know that built a 3.4 stroker.

He says his deck height was 0.99mm with a 1mm shim in the thread. There is another thread somewhere on Pelican where he mentions the factory spec as 1mm for the 3.4 with the Max Moritz pistons. He used the 9.8:1 Max Moritz pistons, but his measured CR wound up being over 10.6:1, so something went goofy somewhere.

http://forums.pelicanparts.com/porsche-911...-4-numbers.html

This build was pretty well documented on Pelican if you want to search his other posts. The torque numbers on his dyno were impressive when it was done.

Not sure if that helps.

Thanks. I'll be wandering over there to check it out. From the reading I've done so far the nearly ideal deck height is in that ballpark at ~.045in, or 1.14mm which should provide the proper valve clearance as well. I'll want to see what stroke his crank has as well.

It was a 74mm crank w/Pauter rods. Base motor was '78 3.0 with the big port heads. The huge compression bump was never really explained by any of the engine gurus there. At ~10.7:1 you obviously need twin plugs & race fuel. It's just something to be aware of if you are expecting ~9.8:1

Funny you're getting that high of CR as the dome on those pistons don't look very large. Must be the pic.
The best way to do this is to dry fit the pistons and heads with a bit of grease and CC the heads with a Burette. This has to be done at an angle on a stand but you can do it without the crank and rods in .005 increments, Then you would know the exact CR for certain deck heights to aim for.