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

Z3_GoCar

Z3_GoCar

New member
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.
 
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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
 
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.
 
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.

Z3_GoCar said:
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.
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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
 
JeffYoung said:
I hear you and I wouldn't protest.
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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.

JeffYoung said:
Technically, [putting in a thicker head gasket] is not legal.
Click to expand...

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
 
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.



Greg Amy said:
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
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I think its .025 beyond the discard limit... An old vw 8v head that equals .054 total removed from a flat, never machined head...there is a min thickness of a head, say a head minimum is 5.200 ...so it can be machined to 5.175...

A possibly stupid idea popped in to my head, can you open up the cyll above the top ring with a ridge reamer to ad volume?
 
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Greg Amy said:
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...?
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9.1.3.D.1.o
"Cylinder head gasket(s) may be replaced with any gasket(s) having the same compressed thickness as stock."
 
GKR_17 said:
9.1.3.D.1.o
"Cylinder head gasket(s) may be replaced with any gasket(s) having the same compressed thickness as stock."
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Ah.

Yep, I think those rules are dinosaurs from "the old days" when IT cars were remnants of Showroom Stock, and they wanted to allow servicing of spent engines ("back then" you could not blueprint or rebuild a SS engine.) Inferences of that include the restrictions on thickness of the gasket as well as "in no case" on the compression ratio.

But you work within the rules ya got...! Were I king, I'd kill the rules that control that kinda stuff. If the CR's legal, it's legal, I don't care how you get there.

GA
 
I think the current piston rule is fine. We had some debate a few years ago about how to handle 1mm pistons where there's no "factory equivalent", but I recall that got resolved to most everyone's satisfaction...? The big thing with pistons is weight; unless you're ready to increase the performance envelope the "factory equivalent" thing works pretty good...

It still amazes me that all these years later we still get fresh perspectives on the rules that makes us go "huh..." And we still find remnants of dinosaurs that because 'forest for the trees' we never "saw" before ...

GA
 
... rules need to be clear and up to date. I don't encourage us to keep the "old days" rules on the books and then not follow them. It makes us look hypocritical.
 
While I agree, the 'all emissions equipment' and 'gaskets are free' thought processes are simply born from 'what we think we remember'.

People should read and follow the rules. Easy really.
 
I've been meaning to write in on the head gasket thing for a long time. stupid rule. when I can get a thinner gasket to achieve all or part of my 0.5CR bump and not have to remove material from my head, I just made my motr last longer (more trips to the shop). decking of the block might also need to be looked at but I think this is less critical, because my block is iron.
 
Chip42 said:
I've been meaning to write in on the head gasket thing for a long time. stupid rule. when I can get a thinner gasket to achieve all or part of my 0.5CR bump and not have to remove material from my head, I just made my motr last longer (more trips to the shop). decking of the block might also need to be looked at but I think this is less critical, because my block is iron.
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I agree, and this goes back to my 'creep' thread. I am not in favor of changing a rule because 'people are doing it anyway' but because it makes sense now that 'technology' has advanced enough to make something easier and cheaper.
 
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