Head gasket thickness

decoupe

decoupe

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My m90 block is still in the car with the b35 head being reconditioned, the cam getting a regrind and it was suggested to use a thinner MLS head gasket to increase the compression ratio. Going from the 1.7mm standard to 0.8mm (thinnest they can make) should get my CR from 9.0 to 9.5:1 and cost about $250. This has to be confirmed if possible by testing the valve clearance with a lump of clay on intake and exhaust. They also said this might change the cam timing which I also assume a 282 regrind will also change (yes - no?).

So some questions.

Is an increase of a half point (5% or so) worth the effort - about 10-15rwhp if it is proportionate to hp produced on this setup.

What is cam timing? I assume it is the relative position of the valves when fully open and the pistons at TDC and making sure they don't meet suddenly. The stock method of setting the cam timing is the dowel of the camshaft sprocket at 7 o'clock and the four bolts at 90 and 180 relative to the cylinder head deck. Correct?

What do I need to accommodate the changes in valve timing from the regrind (if this is the case) and from changes to the CR from a skim job on the head and the thinner head gasket.

I have all winter to get this sorted which shouldn't be a problem if I stop taking things apart.

Thanks
 
That is cheap HP! However, it comes at the price of robustness. A .031" gasket can be unforgiving in a road car. You will want to make sure that the deck is flat. This assumes that the head will be dead flat after the rebuild.

Don't worry about valve collisions unless you're way (multiple) teeth off. You want to get the cam timing right for proper dynamic compression ratio. I dialed my cam, using the VAC cam sprocket, because I decked the block and shaved the head. Lobe center for the M30 is 110, running the Schrick symmetric. FWIW, I advanced it by 2-degrees to favor bottom end punch.
 
MLS head gasket and Cam Timing

Vraned,

Thanks for the comments.

The head will be flat but as I'm leaving the block for now I'll have to check it in place with a mechanics flat edge and hope for the best. Is the problem with very thin gaskets that the two surfaces have to be perfect and parallel or is there some inherent problem with failures or longevity?

Static versus dynamic CR? I'll try some searches on this.

Is there some method of calculating the advance you used of 2 degrees on the cam or is it arrived at through trial and error (pardon the expression)? I'm slowly chewing my way through the 120 page Camshaft and Valve Timing document by "Racer Brown" that was linked to on an earlier post here.

Thanks again,

Doug
 
The calculations I have seen indicate an increase of 2% in hp for an increase of 1/2 of a point in compression?
Keep in mind that the m90 engine was 9.3 : 1 compression...
 
I don't think half-point increase in compression is gonna result in 10-15 hp at the rear wheels based strictly on the compression. It's not a linear relationship. This subject is debated ad nauseum on performance and racing websites, and it's been tested since the dawn of IC engines. Bowling and Wallace have calculators to show some estimates which fall far short of your expectations.

What it will do is bring the dynamic compression back up to near where it was when you were closing the intake earlier with the smaller cam.
 
Further to Mikes comments, if you end up with an old engine and old engine management and almost 10:1 compression- it becomes more problem than benefit for 4 hp.
Can you still get 93 octane?
Do you have adjustable timing?
 
Is there some method of calculating the advance you used of 2 degrees on the cam or is it arrived at through trial and error (pardon the expression)?
Click to expand...

Doug, yes, there is a method. If my understanding is correct, I believe it goes something like this. Essentially you use a dial caliper set on top of the valve springs to find the halfway point between when the inlet valve starts to open and the exhaust valve closes at the TDC of the exhaust stroke. Under normal settings the crank timing TDC mark on the dampener should align with the mark on the timing cover when the valves are centered. So to advance the cam 2° you need to put a mark on the dampener 4° to the right of the TDC mark on the dampener and then ensure the cam is at the middle point between I valve opening and E valve closing when the 4° mark aligns with the timing cover TDC mark.


Another easier way, as I have been instructed by Paul Burke, is to set the intake valve lash at 0, then use the caliper to measure .050 of intake valve lift just after it starts opening. At this point the dampener TDC mark should be 10-11° before the timing cover TDC mark. This is as noted for his N21 cam. I am not sure how this applies to other cams.

BTW, you will need the adjustable cam sprocket.
 
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