Compression and Cylinder Base Gaskets

[Superman]

Perhaps I should post on the Engine Rebuild Forum, but I'm guessing I can get my answer here.

When I rebuilt my engine a few years ago, the heads were flycut and the cylinder tops were made square. The result was that I needed to use a cylinder base gasket that was twice as thick as usual. I actually used two normal ones. Anyway, that is the thickness of the cut that was made on the heads. My engine's stock compression ratio is 9.3:1. If I had used just one cylinder base gasket, my compression ratio would have been increased. And there is a chance that the valve-to-piston clearance might not have been enough. And perhaps my chain wheel carriers might have ended up at a different angle. I am installed 20/21 cams at the time of rebuild.

Questions:

How much would my compression ratio have increased?
Would my chain wheel carrier angles have been poor?
I wonder if I would have had enough valve-to-piston clearance.

[911pcars]

Originally posted by Superman
Questions:
How much would my compression ratio have increased?

You need the formula for compression ratio (includes bore, stroke, combustion chamber volume and head gasket thickness).

If you know for sure the current CR is 9.3:1, you can extrapolate backwards to determine combustion chamber volume. Subtract the base gasket volume from the calculated cylinder volume to arrive at the slightly lower compression ratio.

"Would my chain wheel carrier angles have been poor?"
Depends on how much closer the cam is to the crankcase center line. You can observe the distance between the idler arm sprocket and the cam box wall as well as observe how far the tensioner piston extends from the tensioner housing. You must also adjust the cam box so the cam remains centered in its opening.

"I wonder if I would have had enough valve-to-piston clearance."
Easier to measure if you replace the valve springs with lightweight test springs. Then you can manually push the valve open at different piston positions while measuring travel with a dial indicator. Cam and crank shouldn't be connected during this test. There are other methods to accomplish this as well.

Sherwood

[Superman]

So, "it depends....."

BTW the distance that the cam has moved closer to the crank, for purposes of giving me input about the chain wheel carrier.....would be the thickness of a cylinder base gasket. There is enough information in my post to get an answer beyond "it depends....."

I wonder, what is the RANGE of changes in compression ratio when the cylinder head is moved one base gasket-length closer to the piston. My guess is that I'd still be under 10:1. And I suspect that the chain wheel carrier angle would not change drastically. My sense is that I could have gotten away with this, and that the only real issue would be the valve-to-piston clearance thing. And I suspect that few people are going to be able to answer that one. But I think I'll get an answer, or at least an approximation, on the other two. Perhaps I'll just call John.

[The Turk]

Check this site out, it’s a C/R calculator.

http://www.csgnetwork.com/compcalc.html

I’m currently building a 3.5 turbo motor, when I plug my info in and compared it to the same info except adding .25mm for the additional base gasket, my compression ratio goes from 7.45:1 to 7.32:1, using this info (guestimate) you probably would be at 9.43:1, maybe a little less because you’re a 3.0.

The deck height on a 911 motor can vary .5mm without any adverse effect on the chain wheel angle or the big o-ring seal going to the chain housing, so I doubt .25mm would have been a problem.

The valve clearance should always be checked if you change cams or adjust deck height, I just puttied the 1 & 4 piston tops and did a mock build timing the cams and spinning the motor over carefully.
My clearances came in at the bare minimum so I’m dropping my pistons off at the machine shop today to have some small “eyebrows” cut, a little miss-shift insurance.

[Scott Clarke]

Deck height must also be considered. Excessive deck height can actually increase the risk of detonation while lowering CR. This is a lesson I recently learned in this thread: 9.8 or 10.1?

-Scott

[Superman]

Thanks, Gary. That answers my questions.

Now.....what the heck is "deck height?" I mean, the crank and rods and piston configuration is such that the piston's lowest point is X and the highest point is Y, and what we're talking about here is the position of the cylinder "dome" vis-a-vis the top of the piston. What other variable is this "deck height" and what effect is it thought to have?

[Superman]

I know I'm anticipating here, but if somebody's going to try to assert that cutting cylinder tops and head mating surface material causes its own effect, I'm gonna have a hard time buying it without a really interesting explanation. If my cylinder mating surfaces were cut (which they were) and if the cylinder tops were also cut (which they may have been, I don't recall), that's all immaterial if enough base gasket thickness is used so that the top of the cylinder head combustion chamber surface is placed back into the same location vis-a-vis the piston top as before.

Talk to me.

[Scott Clarke]

Super-
You are correct that the base gaskets can compensate for the machining of the spigot faces and cylinder tops. Thew relationship of the outer top edge of the piston to the upper edge of the cylinder is deck height. Have a look at Grady's post in the above referenced thread for an explanation of the geometric issues occurring at the edges of the combustion chamber. He really went to town!
-Scott

[The Turk]

Base gaskets are offered in various thickness, .25mm (standard) .5mm and 1mm to adjust the deck height after machining of case spigots or cylinders, there are different opinions on deck height but I believe most experts agree that 1mm is good, you can go .25mm on either side of that.

In my case it was recommended that I run a .5mm base gasket due to the limited surface area after the spigots were opened up to accept the 3.5 cylinders, so I had to do mine backward.

I measured the deck height using the .5mm base gasket. then I sent the cylinders out to the machine shop to have material removed from the bottom to give me a deck height of .95mm.

Here’s a picture so you get the idea, the correct method is to measure on the side of the piston not the top as shown in the picture, flattop pistons are very easy, it gets more complicated with dome pistons.