Head decking & how it effects compression ratio, & timing
#1
Elite Member
Thread Starter
iTrader: (15)
Join Date: Sep 2007
Location: Incline Village, NV
Posts: 2,034
Total Cats: 5
Head decking & how it effects compression ratio, & timing
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?
Solomiata : Engine : Miata MX5 camshaft specifications
Shaving the head .010" will increase the compression ratio about a 1/4 point.
A .020" shave will net you about a degrees of timing retard in addition to raising the compression ratio
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?
#2
Senior Member
iTrader: (1)
Join Date: Jun 2006
Location: Outside of the Loop-ATL
Posts: 761
Total Cats: 1
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?
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?
#3
Moderator
iTrader: (12)
Join Date: Nov 2008
Location: Tampa, Florida
Posts: 20,650
Total Cats: 3,010
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.
#4
Elite Member
iTrader: (1)
Join Date: Feb 2008
Location: Birmingham Alabama
Posts: 7,930
Total Cats: 45
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.
#6
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.
#8
Elite Member
Thread Starter
iTrader: (15)
Join Date: Sep 2007
Location: Incline Village, NV
Posts: 2,034
Total Cats: 5
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.
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?
#10
2 Props,3 Dildos,& 1 Cat
iTrader: (8)
Join Date: Jun 2005
Location: Fake Virginia
Posts: 19,338
Total Cats: 573
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.
#12
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.
#16
Elite Member
Thread Starter
iTrader: (15)
Join Date: Sep 2007
Location: Incline Village, NV
Posts: 2,034
Total Cats: 5
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?
Any other ideas?
Thread
Thread Starter
Forum
Replies
Last Post