Deck Height vs Squish Area
 

I am assembling my 87 930 engine using a set of 98 mm RUF pistons and cylinders (CR = 7.5 to 1). I am also using head gaskets for the C2 turbo (.51mm thick) to help prevent any cylinder head leaks.

Measuring multiple times I get:
Deck height = .61 mm (measuring with caliper from top of piston to top of cylinder with piston @ TDC)
Squish area = 1.37 mm (measuring with the solder method)

As per Wayne's book the "deck height" should be between 1.25-1.50 mm. Which measurement do I use?

A second question is should I preassemble to measure the piston to valve clearance. With stock CIS cams and flat top pistons what are the chances that I would not have adequate V to P clearance.

If you're moving your heads out by a half a milimeter, then your compression ration and deck height will change. In this case, I would look at stretching the rod to make up for the head gasket.

Where did you get the info to add this gasket to the top?

On your second question, CIS profiles are very mild, and typically don't even come close to the pistons, so you're probably okay. Especially if you are adding that gasket in there without doing anything else, then you are giving yourself a lot more clearance.

-Wayne

Sorry, I left out the fact that the heads were cut .5 mm to compensate for the head gaskets (.5mm thick).
Randy Greff (Greff Motors, New Orleans) uses this technique on his 911 race motor which runs close to 13 to 1 compression. The motor had head leaks before the gaskets were installed.

Re: Deck Height vs Squish Area
 

Don't understand your question?

-Wayne

Do I use
1) the measurement from the top of the piston to the top of the cylinder with piston @ TDC (.61mm)
2) the measurement of the thinnest point of the solder placed on top of the piston and smashed by the head (1.37mm)

Vic

Victor,
If I understand the concept correctly - the key concern is piston to valve clearance and less so with the actual deck height.

You do want to know deck height, dome volume, etc to accurately calculate the compression ratio. Other than that, if you are sure that there's no P-to-V clearance issue, I don't think the actual deck height is THAT critical.

No, the issue of deck height is important, because you don't want the top of your piston hitting your cylinder head! (or even getting too close). Plus, as was mentioned, it does affect your compression ratio, and all of the other calculations as well.

Deck height is defined as the height from the top of the piston to the cylinder head. If you're using flat-top pistons, it's real easy to measure. I have a picture in the book on this...

-Wayne

Therefore the solder method that Competition Engineering uses and is discribed in Wayne's book would be the most accurate measurement for Deck height. It takes into account all different shapes of cylinder heads and pistons, and also measures any cylinder head gaskets that may be used.

Although the height from the top of my piston to the top of the cylinder is only .61 mm, once the head gasket and head is installed the "height from the top of the piston to the cylinder head" is 1.37 mm which is fine.

Vic

Note: this measurement is not an exact science either...

-Wayne