[DaveN]

So,here I am again rebuilding my engine after I blew a head gasket. I had a Wide fire in there but now I am not sure what to choose. Should I go stock, Wide fire again or Cometic composite? My mechanic recall a real pain of a boosted BMW and it would never retain a gasket until they went to a composite.

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[flash]

wide fire - no question. the metal is the key. more metal means more heat dissipation.



the key is to maintain compression ratio. if your head is shaved, then no wide fire for you.



also, the block surface has to be perfect for forced induction. if there is any pitting or uneveness, then any gasket will fail. this is a problem that some will face when they remove the head to freshen it up, thinking that they are "fixing" a problem, when in reality they are causing one. a new gasket will not seat as well against a pitted head as an old gasket that seated up as those pits developed.



you have to do both sides to really get it right, unless you are lucky enough to have a perfectly smooth block

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[tamathumper]

I've heard of a couple of WideFire head gaskets failing. Do people perhaps think by going to a WideFire they can get away without resurfacing, or based on your comment flash, does resurfacing negate WideFire as an option?

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[rl968]

I have a wide fire and the head was machined. Only been a year, so no long term data from me.

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[banditsc]

I have a cometic on mine, it's presently doing it's job.

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[flash]

rl - we are talking about boosted cars. a shaved head will raise the compression. this is NOT a good idea for a boosted car, unless you are prepared to take the necessary precautions.

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[Lear35A]

Sure a shaved head or block will raise the compression. But what compression is ok? Everything Porsche publishes says our cars when stock are 11:1. But apparently that isn't true. Something in the low to mid10s is more accurate.



So what does this mean for head gaskets? Can we use the stock head gasket thickness (1.1mm) for shaved heads and blocks until we get up to a true 11:1? If not, why not? Is a true 11:1 ok for boosted engines?



Does anyone other than a long retired Swabian engineer really know?

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[flash]

our engines are 11:1 at minimum spec. that's how they got to "lie" about it.



some engines can run boosted at 11:1. some can't. too many other factors to have a one size fits all number.



for our engines, it's all about final compression. you have to factor in the boost amount, then add the raise in compression from the shaving (which can be a half point), and do the math from there, and see if your rods can handle it and such.

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[Lear35A]

You don't have to factor anything. You cc the cylinder head after any shaving; you cc the piston tops; you measure where the piston top is in relation to the top of the block. Go on line to Diamond Pistons and plug in this data along with bore, stroke and head gasket thickness you will use. Out pops the CR. Whether or not an engine is boosted does not change the CR.





If it's 11 are we good to go? Or should our target be lower?

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[flash]

yes, boost factors in. your final cylinder pressures and dynamic compression ratio are the key. boost is a huge factor there.



static compression ratio is essentially irrelevant.



there is no magic number. you have to factor in timing, cams, overlap, materials of the pistons, block, rods, etc, and a number of other factors, to determine what maximum static compression ratio any engine can handle. it varies from engine to engine, based on those things.

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[DaveN]

Ok, so it seems that I can either Widefire or Cometic the engine, probably back to the Widefire. Head is not going out for any dressing. What about the use of the copper spray some have eluded to in previous discussions. Seems like it's a good thing?

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[flash]

you escaped pitting on the head? quick - run out and buy a lottery ticket

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[Lear35A]

[quote name='flash' timestamp='1358793335' post='137738']

yes, boost factors in. your final cylinder pressures and dynamic compression ratio are the key. boost is a huge factor there.



static compression ratio is essentially irrelevant.



there is no magic number. you have to factor in timing, cams, overlap, materials of the pistons, block, rods, etc, and a number of other factors, to determine what maximum static compression ratio any engine can handle. it varies from engine to engine, based on those things.

[/quote]

I'm not sure I agree that what you're calling static compression is irrelevant. However, I'm still trying to get more of a Yes/No opinion here, so let me word this a little differently.

With a fresh stock engine, stock cam timing, stock pistons, no special coatings, etc, etc; do you think the stock rods (or even the R2 rods) are ok with the stage 2 D1R SC installation at a true 11:1 "static" CR? Please assume that the stock head was milled to achieve the combustion chamber volume necessary to get the exact static CR of 11:1. The only variable is the static CR.

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[flash]

not likely. we already know that it was a problem with stage 1. stage 2 adds some fuel, but not enough to handle more compression. stage 3 would be a bit safer, but possibly still "on the edge" if you were driving hot. you could try higher octane fuel, but it might not work.



the bottom line is that in order to bump the compression, and run the D1R kit, some additional timing has to come out to prevent detonation

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[Lear35A]

But that's my point. You wouldn't be bumping the compression. Everything Porsche put out on the car says 11:1. And I meant Stage 3. Sorry. Can't keep up with all the revisions.

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[flash]

sigh - one more time



the 11:1 figure represents the engine at MINIMUM deck and block height. that means AFTER you have shaved it as far as you can before throwing it away. that figure does NOT represent how the engines were built from the factory. this is not a common practice, but i have seen it before. back in the days of muscle cars, manufacturers were known to "fiddle" with the specs that were published. this allowed them to meet certain regulations, but still be able to provide the performance numbers.



i was very clear from day 1 that shaving the head is NOT recommended, as it would raise the static compression. this is also why i recommended a run-rite treatment to remove the carbon from the pistons, which can also raise the static compression ratio. the D1R kit is designed to run on the compression ratio of a "normal" engine with full new head and block specs, regardless of what that compression number is in reality.

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[KRAG50]

My thoughts, I blew my head gasket tracking the car, I have the D1R SC unit, before the install I had my head dressed and installed a Wide fire gasket. Still blew at <acronym title='wide open throttle'>WOT</acronym> due to detonation and the lean condition as described by Flash, also note the because the compression was higher due to .005 to .006 remove from the head. This time around I needed to dress again, about .002 or .003 I bought a Cometic which is thicker than the 1.1mm stock, I believed it worked out to 1.43mm thick, which should put me back to just a touch below stock static compression.

The Cometic piece is a nice bit, the MLC is the type, I think, all metal and really designed for this type of application.

The engine is almost back together so no real hard data.

FYI I've also gone the stage 3 route.

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[Cloud9...68]

I'd never heard the term Dynamic Compression Ratio before, so I googled it, and found this concise summary:



Dynamic Compression Ratio (DCR) is an important concept in high performance engines. Determining what the compression ratio is after the intake valve closes provides valuable information about how the engine will perform with a particular cam and octane.

Definition: The Compression Ratio (CR) of an engine is the ratio of the cylinder volume compared to the combustion chamber volume. A cylinder with 10 units of volume (called the sweep volume) and a chamber with a volume of 1 has a 10:1 compression ratio. Static Compression Ratio (SCR) is the ratio most commonly referred to. It is derived from the sweep volume of the cylinder using the full crank stroke (BDC to <acronym title='top dead center'>TDC</acronym>). Dynamic Compression Ratio, on the other hand, uses the position of the piston at intake valve closing rather than BDC of the crank stroke to determine the sweep volume of the cylinder.

The difference between the two can be substantial. For example, with a cam that closes the intake valve at 70º ABDC, the piston has risen 0.9053" from BDC in a stock rod 350 at the intake closing point. This decreases the sweep volume of the cylinder considerably, reducing the stroke length by almost an inch. Thereby reducing the compression ratio. This is the only difference between calculating the SCR and the DCR. All other values used in calculating the CR are the same. Note that the DCR is always lower than the SCR.



That last sentence is the key - people who have calculated their engines' compression ratios by measuring the volumes of their combustion chambers and piston tops are consistently getting numbers that are lower than what Porsche has published. So, it doesn't seem that the difference between static and dynamic CR is behind the discrepancy between Porsche's numbers and what people are measuring.



But regarding this discussion, the last sentence of Flash's post seems to sum it up: If you do something to raise the CR of this engine (e.g. mill the head), and add a D1R super charger, you're risking putting excessive strain on certain parts of the engine.

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[Lear35A]

It's nice that Porsche has the thicker 1.4mm gasket. That additional thickness effectively lowers the STATIC CR by 0.35. I intend to use one.

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[Eric_Oz_S2]

I am running the stock gasket on my rotrex supercharged s2 without problem and it is heavily tracked in a hot climate with 7psi boost. It seems the problems reported here are not related to the gasket type, but the conditions causing the gasket to blow. Shaving the head by .003 inch would raise the cr by .07. I really don't think this is the issue. Why are the head gaskets blowing? Detonation of course. Fix the fuelling, intake temp or ignition timing problem - don't look for a magic head gasket solution.



After a year of heavy track use i inspected my cylinders and they were perfect and unmarked. Rod bearings are still near the minimum end of the specified gap tolerance (no evidence of wear). I would be confident then that the rods are still true as well.



For the record, the calculated change in cr between the stock 1.1mm and 1.4mm gasket is 0.40.



Lear - you will loose power by dropping the cr unless you adjut the boost level and ignition timing.

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