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**JJ77** · 04-18-2005, 10:29 AM

I believe there is .012 clearance between the valves and pistons

**LQ1** · 04-18-2005, 05:07 PM

I ran a couple calculations, and going off the specs on this site that the DOHC motor is at 9.5 CR, bore 3.622 and stroke 3.307. If you took off .012 on the heads that would bump the ratio to 9.8. If you took off 5 thousandths that would bump it to 9.6. 8 thounsanths off, which my machinist wanted to do on my heads to get them back to even would make about a 9.7 CR.

**BlueShift** · 04-18-2005, 05:12 PM

Ah, good stuff, very useful - I actually have the auto spec motor at 9.25:1. Can you point me towards where I can learn to do that nifty CR calculation too? I'm pretty sure the bore and stroke on here is correct, but the rest of the stuff is for a later generation motor? Dunno - heard the torque specs and all that don't match up to the 91-93 motor.

**LQ1** · 04-18-2005, 05:28 PM

Ok, re-ran the numbers, and going .012 will get you up to 9.6, .005 to 9.4, .008 to 9.45, .010 will bring you up to 9.5.

**BlueShift** · 04-18-2005, 09:53 PM

Great! The only thing to contemplate now is if I should mill .010" or .012".

Is .002" of valve clearance even worth it? You'd figure rod stretch would still result in messed up valves in the event of a belt break.

One lingering question - anyone know where you gotta start going to better gas? The Manual spec motors, were they 87 octane? How about the later year motors (9.7:1, right?)?

[edit]: Can we confirm that there's a .012" valve to piston clearance on the 91-93TDC? Thanks again!

**DrRodey** · 04-19-2005, 12:51 AM

LQ1...... What formula are you using to calculate compression ratio? Teach us o' Master of the numbers.

**Fierobsessed** · 04-19-2005, 01:30 AM

Im cross posting what I wrote in the Fiero forum, should give you some Ideas.
there are three ways I know of to raise compression, decking the head is one, Chaniging the piston is another, and increasing displacment is the third. You have to keep in mind that when you deck the 3.4 DOHC's heads, that the heads are moving closer to the block and tension will be decreased on the timing belt. and all head decking movements are X4 as great to the belt.(because there are 2 heads, and the pulley factor). The stock displacment per cylender is 558.4 CC's. and if you have 9.25:1, you subtract one from the compression ratio and devide the cylender displacment by that value to find the total volume of the combustion chamber + head gasket + piston at TDC. (9.25 - 1 = 8.25, 558.4 / 8.25 = 67.7 CC's) So now if you deck the head, you are cutting back at this number, .010" deck on the head takes 1.7 out of the 67.7 CC's, so now with 66 CC's (558.4 / 66) + 1 = 9.46 is your new compression ratio. Boring changes only the cylender displacment, but that still plays a part in the little formula .030 bore will add about 6.7 CC's to the cylender displacment, so now your lookling at (565.1 / 67.7) + 1 = 9.35 as your new compression ratio. Both of these mods combined will get you 9.56:1 so you see how effective some small changes are on comopression. If you want more info on how to calc these values just ask I'll be happy to help. I wouldn't worry so much about the interference thing. The engine will never contact valves to pistons if the belt doesn't break. So just be cautious and maintain the belt at regular intervals and you will be ok.

**LQ1** · 04-19-2005, 10:34 AM

I did basically what was done above, only I used cubic inches, volume of 1 cylinder(V1), volume of combustion chamber(V2).

((1/2*Bore)^2)*(3.14)*(Stroke)=V1

V1/CR=V2

V2=((1/2*Bore^2)*(3.14)*(Height of combustion chamber)

Solved for V1=34
Took the stock CR, 9.25 divided into V1 and got V2 which i believe is 3.68.
Worked through the third equation backwards to get the height of the combustion chamber .357. Then, with that number subtract from it how much material milled off say .357-.010=.347. Take that new height, and put it into the third equation to get the new V2. Work the second equation backwards to solve for CR=V1/V2. Which comes out to about 9.51:1 CR.

EDIT: I went through the equation again increasing the bore by .030, and the CR came out to 9.54:1.

**SappySE107** · 04-19-2005, 12:29 PM

Wouldn't you have to actually cc the head and piston/bore to know your final compression. The machine shop I use told me that the .012 clearance is without the headgasket, but when you factor in rod stretch at high RPM, you are going to risk hitting the valves once you deck them anything at all.

Im a bit lost on how decking the heads is going to lower tension on the belt. We have a tensioner, and it does compensate for a bit of movement (why the stock belt stretches). All you are really doing is limiting how long the belt is good for due to stretch, not wear. Thats how I see it anyway.

I have my heads decked .011 and everything is working fine so far. I hope to keep my belt changed every 25k though, not 60k. After checking the timing on the cutlass im tuning now, its obvious that even when the belt stretches, the cam banks don't get even timing. 8 retarded on the rear bank, 4 retarded on front intake and 2 retarded on the front exhaust. I think thats how it was, its 88 42 or 88 24. Either way, thats a stetched belt:P

**LQ1** · 04-19-2005, 11:52 PM

So how thick is the headgasket? I got some new ones laying around but dont have a mic.

As for the rod stretch, let me see if what I learned in physics can show something here(no sarcasm intended this would be my first time doing this calculation towards somethign usefull):

A force in torque, converted to Newtons, and lengths converted to centimeters, with a cross-sectional area of 2"(this is a random guess) and the Young's Modulus for cast iron, I figure at 4500RPMs at peak torque the rod would stretch about 2.85*10^-8 inches. Or, .0000000285 inches.

Errors that can arrise in my equations used to figrue this out:
Rod cross-sectional area
Ignition timing on this engine @ 4500rpms

**BlueShift** · 04-20-2005, 04:12 AM

My stocker headgasket I just pulled out measures juuust over .060" on my digital caliper. Probably 060" compressed if I have to guess. So does that mean we have an extra .062" between actual valves and pistons with it in place with a .010" deck? Are we doing the calcs with the gasket figured in?

**SappySE107** · 04-20-2005, 03:03 PM

Im pretty sure its at 7000 when the rod is stretching more. I only know that the general rule is .030 quench area for lower RPM motors,and .040 for higher RPM. God knows I don't remember where I read this or if its true but yeah, thats what im going on. Besides, its the only way I can figure for people bending valves at high RPM when the belt breaks, yet most everyone else has no problems.

Im sure there are other factors as well.

Head gasket I believe is .040. .052 is what the machine shop told me was the clearance I think. Never have doublechecked this though since I dont have a motor here to test with.

**dohcfiend** · 04-20-2005, 04:54 PM

don't mean to hijack but it is related to the thread somewhat. i have my heads at the machine shop getting the checked out and the guy said if they are warped he can take 5 thousandths off of it and make it right again if need be. i told him they are interference heads(96-97) he said it is a gimick by gm to get people to buy neww heads and they should be fine with 5 thousandths removed if need be.

what do u guys think?

**SappySE107** · 04-20-2005, 07:25 PM

Its possible. I never understood why 96-97 was interference since the valve reliefs are still there. Again, we need to really check the clearances to know for sure.

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3.4L DOHC Head Decking? Compression Ratio?

3.4L DOHC Head Decking? Compression Ratio?

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