I'm pretty sure that's EXACTLY what he did.

Why don't you read up a little...lots of info on this forum, but if you come into class thinking you're the professor, you won't learn shit.

Either your reading comprehension skills or your memory are failing you desperately. Smurf asked for several bits of info between his first two posts. Are you sure he didn't say he wanted to go 10 to 1?
And we were pointing out that doing something potentially counterproductive was a fool's venture. Further, it should not be undertaken by anyone needing a reliable street car that is already REQUIRING A LOAN to reassemble.
And I know a couple of Puerto Rican guys that build 9 and 10 second forced induction drag cars that don't have a clue why most of the things they do even work. They just copy the successes of others. Building a powerful engine isn't difficult at all. Explaining the intricacies of the relationships between the components and the variables involved can be somewhat more consuming.

True; so maybe I introduced my question all wrong. I probably should have introduced myself and my situation first instead of posing it like a performance mod-type question.

I was loosing coolant, about .5 gallons every month for like 3 months w/ no external leaks. I was letting my fiance' drive the car while I'm away at school and I didn’t have time to look at it. Compression test: the middle two cylinders had low compression, about half as #1 and 4; probably a head gasket ---> took off head and just planned on replacing gaskets. Looked at the pistons...

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Doesn’t look the greatest, and I didn’t know if there was ring involvement, and I seriously started to question what was done to the car before I got it. Take the motor the rest of the way down, and the first compression rings were carbon-caked onto the piston seemingly not holding compression, w/ blow-by to ring #2. Have a friend who knows my situation, said he’d do my head-work for free, including a port job (who would turn down a free port job), and take a couple thousands off to the head to make sure the surface is true, especially since it over-headed. What is the stock compression? If it’s 9.5:1; take off the most minimal amount possible. If it’s 9:1, there is some wiggle room, up to 9.5 because I’m not going to run high boost. I know everyone here would want me to run higher boost or help me make more power, but this will be my daily driver for the next 2 years and I’ll only have all of a parking lot and a socket set to fix the car in 2 months b/c I’m moving.

I was hoping someone who has machined enough miata heads would just say: 8.8:1 and don’t take it past 5 thousandths b/c you run into clearance problems or push the compression too high to reasonably run boost.

Let me know what you think.

Not sure where you got that number. What Gompers tossed your way started at the '94 (1.8) model year. Not sure about yours, but my '90 1.6 came with 9.4:1 pistons.
Reading comprehension is your friend on this site, Smurf'n. Your initial guess of 9-ish:1 was pretty good, though. You might want to check the Power Mods section at miata.net for more answers on head milling as related to compression ratio.

Leave the compression also, mill head only if nessessary (if the surface is warped). Spend your time balanceing the stock rods and piston. Clean up the head (ports, bowls for casting inperfections. Then wait till you get the funds to up-grade. cause there will be no turning back once you take off .030-.050 from the deck :(, but replacing the head.

get a real job too, so you don't have to get a ghetto loan.

you dumb shit. have you ever heard of a thing called dynamic compression? whether you raise the boost level or the compression ratio you are effectively doing the same thing to cylinder pressure. soooooo either way; fucktard, you have to do something in the way of more fuel / less timing. so if you don't have the means to run more boost, you don't have the means to run more ccompression.

hustlers advice is sound lower compression means larger volume. so in turn for a given dynamic compression ratio, a lower static compression ratio will allways yeild more power because there is more room to cram in air and fuel. think about it like this; if somebody were to kick fuck your anus with a pound of C4 on their boot and light it off there would be a big explosion. now if that same guy were to put two pounds of the stuff on his boot and proceed to stomp a mudhole in your rectum there would be a bigger bang. he ain't gunna to put it up there with any more pressure than before because his leg is only so strong. there will just be more of it under the same pressure. the only reason one should mill the head is to make it have a flat surface again.

So this thread is not completely worthless...

If anyone actually wants to know how much decking their head will effect their compression ratio, I just did the research myself, broke out a spreadsheet and answered my own question.


Stock compression on a 1.6 is 9.5:1
Stock gasket thickness is 1mm (or .040 in)
Bore/stroke 78mm x 83.6mm

Calculate the stroke volume and combustion chamber volume. (and i'm going to assume that decking the head would give comparable combustion chamber volume changes as changing the head gasket thickness).

I used HKS's info on their head gaskets to make sure the compression ratio and thickness of the head gasket change would be a linear relationship to make the math easier... didn't see any reason for it not to, but its good to have at least 3 points on a curve.

their gaskets: 1.2 mm = 9.37:1 CR
2.0 mm = 8.72:1 CR

With the spreadsheet. I made CR a function of head gasket thickness (and/or change in combustion chamber volume) using the stock parameters. I plugged HKS's info in and got:
1.2 mm = 9.33:1
2.0 mm = 8.73:1
So a tad off for the 1.2mm measurement, but good enough for government work.

Now, all I had to do was plug in various height changes that could be made after a head is machined.

So from my original question before going crazy-off topic...

How far to machine to 9.5:1?
Retarded question knowing now that that is the stock compression (which is why it was my first question). I think, then, it would be really, really good to know how far you can deck the heads before you hit 10:1.

According to my calculations; I figured it to be around 0.55mm or (0.0217 in).

Now you now you loose/gain about 0.5:1 compression ratio per .55mm or .0217 inches assuming the stroke volume isn't changed from stock 1.6. That may help if you have your heads decked and add a different size head gasket and want to know where your compression ratio will land after both are done.

The heads were decked today, and it took 0.012in to achieve a true surface. All things being equal, that puts my compression ratio to about 9.77:1.

If you want to discuss boost vs. compression physics, you already have a thread that's great. Use the search option and find "Compression Ratios and Forced Induction."

If anything, someone should at least recognize I've gained about a quarter point in compression; according to the rule of 4%, that's at least a 1% net gain (rough estimate I know) and the heads will flow better being ported, both increasing my engine efficiency. No one else cares but me, but if the option is to put the engine back together stock, or get free horsepower (although small) I'll take the free horsepower.

No measureable gain, just a bigger war against Pump gas vs. more boost = Detonation. It's your car so do what you like, although .012 is more then the machine shop usually takes off to true the head. Some times you have to do what you have to do to get it right. Well good luck with that rebuild, and hurry up summer is almost here.

Thanks LowBoostn. I was hoping someone would recognize that 12 thousands is a bit much to take off, or caution me about valve clearance and let me know how much clearance is in the 1.6. The engine did overhead the first time the coolant was low and after pulling the head off, the head gasket didn't look that bad, provoking me to look further for the significant compression loss. I thought I may have to get close to the tolerances if the head was warped or misshaped and wanted to know how far the shop could go (original intent of the thread) and if that would significantly affect compression.

It should be together next week and we'll see how it looks after the break-in and tune.