cc'ed my motor check my math....

[Z3_GoCar]

Since my motor needed to have some material taken off the deck. So, I figured I needed to check the static compression ratio. It was originally built by Alan Jones for TC Kline's World Challenge team in January 2001, so it was a bit of an unknown, other than the 84mm piston overbore. So here's what I measured:

Piston dome and a compressed 0.070" head gasket measured 7.2cc
The head measured 36.8cc
So the total top end measured 44cc

The bore has been enlarged 0.003" so it's now 3.310" or 84.074mm and the stoke is 84mm, so the swept volume is 466.329cc. So the effective compression ratio is now 11.6:1

Now come the reason I'm asking for confirmation....

To get the Compression ratio back to an IT legal 10.7:1 I determined I'd need the head gasket to be about 0.029" thicker, maybe a little less because it's not perfectly circular, but for the moment let's just assume it is the same diameter as the bore. The equation I solved is:

10.7 = (36.8 + X ) + 466.329 / ( 36.8 + X) => X= 11.275cc

So, I need to increase the head gasket thickness to gain ~4.1cc
so assuming the head gasket is cirular and the same diameter as the piston I get an increase of 0.07385cm or 0.029" Does this sound reasonable?

One final item to discuss is, anyone run a multi-layer steel head gasket? I was orginally planning on running an oe gasket, but the only source of a thicker oe gasket is Victor-Rienz, which are known to not last under race conditions.

[JeffYoung]

Technically, what you are doing is not legal. You can only "adjust" compression ratio by shaving the head. You can't save it down too much, or use higher compression pistons, and then build it back up via a thick head gasket.

That said, there are places you can special order head gaskets of varying thicknesses, although different materials are required at higher levels of thickness and they can get pricey (and no, I've not done this to the TR!).

Good luck with it.

Jeff

[Z3_GoCar]

I discussed getting a second motor, but it was a non-starter with my better half. Part of the problem is that the aluminum block that's in my car is a bit of a rarity only being used in Z3's so spares come at a premium ~$2k for a untouched salvage yard motor. So, I'm making this as legal as I can. As I mentioned, there's an oe gasket solution, because the head and deck have been shaved for flatness not neccessarily for compression. It's just that I don't trust the gasket manufacture.

[JeffYoung]

I hear you and I wouldn't protest.

Do some google searching. There are some custom gasket manufacturers out there that will make pretty much anything. Not cheap, but cheaper than what you describe below.

I discussed getting a second motor, but it was a non-starter with my better half. Part of the problem is that the aluminum block that's in my car is a bit of a rarity only being used in Z3's so spares come at a premium ~$2k for a untouched salvage yard motor. So, I'm making this as legal as I can. As I mentioned, there's an oe gasket solution, because the head and deck have been shaved for flatness not neccessarily for compression. It's just that I don't trust the gasket manufacture.

[Z3_GoCar]

I hear you and I wouldn't protest.

Do some google searching. There are some custom gasket manufacturers out there that will make pretty much anything. Not cheap, but cheaper than what you describe below.

I was considering Cometic:

http://www.cometic.com/sportcompact.aspx

[xr4racer]

I highly recommend Cometic, call them and ask for Ashley, she will tell you if the thickness you need is available. If they can do it you will have it in a week or so and not terribly expensive either, just had Viper head gaskets made for a customer car at $130/ each

matt

[Greg Amy]

I hear you and I wouldn't protest.

In reality, it would be pointless anyway. CR would either be checked with a Whistler or by puling the engine apart and manually cc'ing the engine, including measuring the actual thickness of the head gasket (which will even expand slightly after removal. In either case, you'd be legal.

Technically, [putting in a thicker head gasket] is not legal.

Let me intorturate that. Gaskets are free, yes? Ergo a thicker head gasket is free. The part you're thinking of is where it states:

"Where a factory specification for original cylinder head thickness can be proven, a tolerance of .025 inch less than the service limit will be permitted."

Given gaskets are free, and given no further head gasket requirements, there's no limit to a thickness MORE than the service limit, thus thicker head gaskets are allowed... "If it says you can..." Or am I missing another limiting rule somewhere...?

That limitation on the thinner head gasket should be deleted; it's pointless. If the CR is legal, it's legal. I can't even remember why it was put in there, but I suspect it had to do with the allowance for overbore. Read the rules for what they "say" and not what they "mean" and it's apparent that there was never an express allowance to increase the compression ratio half a point (e.g., "engine's CR can be raised half a point over stock"); the CR allowance is there to accommodate other allowed mods, such as overbore which tends to increase compression ratio ("in no case shall the CR be raised more than half a point")... - GA

GA

[chuck baader]

For the M20 motors, BMW makes a second, thicker, head gasket. Check. Also, VAC motorsports would be a good choice for a MLS gasket and I think they do custom.

For the CR calk, http://www.rbracing-rsr.com/compstaticcalc.html

CB

[JeffYoung]

I agree with you that, other than modifying pistons, we should be able to get to the correct CR anyway we want.

That said, I do think it is illegal. The GCR only allows you to take up to .025 off the head to get the added .5 compression bump. If you take more off and then try to correct with a thick gasket, I don't think that is technically legal although there is no reason in my view why it shouldn't be.

In reality, it would be pointless anyway. CR would either be checked with a Whistler or by puling the engine apart and manually cc'ing the engine, including measuring the actual thickness of the head gasket (which will even expand slightly after removal. In either case, you'd be legal.



Let me intorturate that. Gaskets are free, yes? Ergo a thicker head gasket is free. The part you're thinking of is where it states:

"Where a factory specification for original cylinder head thickness can be proven, a tolerance of .025 inch less than the service limit will be permitted."

Given gaskets are free, and given no further head gasket requirements, there's no limit to a thickness MORE than the service limit, thus thicker head gaskets are allowed... "If it says you can..." Or am I missing another limiting rule somewhere...?

That limitation on the thinner head gasket should be deleted; it's pointless. If the CR is legal, it's legal. I can't even remember why it was put in there, but I suspect it had to do with the allowance for overbore. Read the rules for what they "say" and not what they "mean" and it's apparent that there was never an express allowance to increase the compression ratio half a point (e.g., "engine's CR can be raised half a point over stock"); the CR allowance is there to accommodate other allowed mods, such as overbore which tends to increase compression ratio ("in no case shall the CR be raised more than half a point")... - GA

GA

[GKR_17]

Given gaskets are free, and given no further head gasket requirements, there's no limit to a thickness MORE than the service limit, thus thicker head gaskets are allowed... "If it says you can..." Or am I missing another limiting rule somewhere...?

9.1.3.D.1.o
"Cylinder head gasket(s) may be replaced with any gasket(s) having the same compressed thickness as stock."

[Ron Earp]

On my first 260Z motor I used these guys:

http://www.headgasket.com/whoweare.html

They did a fantastic job. Verified how thick it needed to be and built the gasket from copper. Fit perfectly.

[tom91ita]

i think you might be missing the volume between the piston and the cylinder wall down to the top of the top piston ring. this should be part of the denominator in the calc.

this is part of the calculation as well. might give you a very slightly thinner gasket.

there was a spreadsheet somewhere at scca.com at one time.

not excel but try this http://www.scca.com/documents/Club%2...FCompRatio.pdf