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Old 09-02-2009, 02:16 AM   #1
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Default Head decking & how it effects compression ratio, & timing

Taken from Solomiata cited here:
Solomiata : Engine : Miata MX5 camshaft specifications

Quote:
Shaving the head .010" will increase the compression ratio about a 1/4 point.
Quote:
A .020" shave will net you about a degrees of timing retard in addition to raising the compression ratio
The head i am having built had to get decked .020, which means ill be left with a 9.2:1 motor rather than a 8.7:1. My timing wont be spot on, which i guess could be cured with a set of adjustable gears. But my main concern is how safe a lets call it 9.5:1 motor would be with a 2860 absurd flow strapped to it.

I don't like the idea of a thicker headgasket, and i guess the other option would be getting another 99 head and throwing all the supertech hardware into that.

Thoughts guys?
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Old 09-02-2009, 10:01 AM   #2
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Quote:
Originally Posted by thesnowboarder View Post
Taken from Solomiata cited here:
Solomiata : Engine : Miata MX5 camshaft specifications





The head i am having built had to get decked .020, which means ill be left with a 9.2:1 motor rather than a 8.7:1. My timing wont be spot on, which i guess could be cured with a set of adjustable gears. But my main concern is how safe a lets call it 9.5:1 motor would be with a 2860 absurd flow strapped to it.

I don't like the idea of a thicker headgasket, and i guess the other option would be getting another 99 head and throwing all the supertech hardware into that.

Thoughts guys?
As for the whole 9.5:1 I wouldn't worry about it. Think about all the people that are running the 99+ block are running 9.5 or more so you be fine.
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Old 09-02-2009, 02:11 PM   #3
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I'd personally try to find somebody who was going to remain naturally aspirated and see if they would swap with you. That decked head would be ideal for them. I've had roughly half a point make the difference between running 87 octane and 92 octane on a 5.0 Ford I built for my brother many years ago.

You will be able to run a little less boost and a little less timing and make a few less horses with the decked one. That is your trade off. Or you might make the same power with the same tune but have less margin for error.

Life is a series of trade-offs.
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Old 09-02-2009, 02:18 PM   #4
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I suck, so my head was machined about .035-.040 and the block deck machined another .005 or so. At 15* timing, I sense a little pinging in really hot weather on regular gas. Premium helps a bit. On a turbo, I wouldn't really want any higher compression. My vote is new head.
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Old 09-02-2009, 03:08 PM   #5
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i say you are fine
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Old 09-03-2009, 01:52 AM   #6
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If you can get a .020ish thicker head gasket I'd do it... That minimal amount of difference in HG thickness won't make any difference in its sealing ability but could make a fair amount of difference in your power output and what gas you'll have to run. We juggle HG thicknesses all the time at work to get CR's and/or piston to valve clearances where we want.
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Old 09-03-2009, 01:55 AM   #7
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Bill, I've seen several folks say that the reduction in squish zone causes a lot of detonation when you deck our heads and then use a thicker head gasket.

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i say you are fine
I say you're an idiot.
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Old 09-03-2009, 03:10 AM   #8
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Originally Posted by Rennkafer View Post
If you can get a .020ish thicker head gasket I'd do it... That minimal amount of difference in HG thickness won't make any difference in its sealing ability but could make a fair amount of difference in your power output and what gas you'll have to run. We juggle HG thicknesses all the time at work to get CR's and/or piston to valve clearances where we want.
A fair amount of difference? This part worries me slightly, i would like to be able to run pump 91 pretty much all the time. Safely. Will a thicker HG mean ill have to fill the car with something higher?

My builder mentioned something about shims, i kind of ignored that thought but does anyone know anything about this and how safe/unsafe it would be?
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Old 09-03-2009, 08:36 AM   #9
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Thicker gasket means lower compression ratio, not higher, so that might be a solution to the problem.
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Old 09-03-2009, 10:49 AM   #10
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Quote:
Originally Posted by Savington View Post
Bill, I've seen several folks say that the reduction in squish zone causes a lot of detonation when you deck our heads and then use a thicker head gasket.
Quote:
Originally Posted by sixshooter View Post
Thicker gasket means lower compression ratio, not higher, so that might be a solution to the problem.
thicker head gasket reduces compression
thicker head gasket reduces squish and quench area effectiveness

the latter tends to have a greater negative effect is what Sav is suggesting.


I vote for hogging out the combustion chamber.
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Old 09-03-2009, 11:20 AM   #11
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I vote for 11:1 pistons.
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Old 09-03-2009, 12:26 PM   #12
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I feel the only potential problem would be if you were to also install high lift turbo cams. High lift, long duration with almost no overlap. i.e. piston to valve contact. Otherwise You can work around all the other stuff so any manifold and turbo combo is not going to change a thing and will be safe. Unless you are asking for the impossiable then nothing is safe.
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Old 09-03-2009, 01:02 PM   #13
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Dude, just buy shorter rods, duh.
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Old 09-03-2009, 06:24 PM   #14
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It CCed to 8.5:1 so i have nothing to worry about. Thanks for the replies guys
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Old 09-03-2009, 08:03 PM   #15
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Quote:
Originally Posted by thesnowboarder View Post
It CCed to 8.5:1 so i have nothing to worry about. Thanks for the replies guys
How'd it do that?
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Old 09-03-2009, 11:21 PM   #16
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The pistons were ordered for a 94-95 motor, apparently the 99+ heads have slightly larger combustion chambers? At least that is what my builder is assuming, since it was decked and no porting involved tha'ts the only logical thing we can both think of.

Any other ideas?
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