3.4L DOHC Head Decking? Compression Ratio?

[Fierobsessed]

I was stating that decking lowers belt tension, it may not be a big factor. The tensioner according to factory specifications, is to be cranked with a wrench so that it bottoms out the snubber, the square key thing on the tensioner should be facing 3 O'clock. So decking .010 will take .040" of tension out of the belt, probably insignificant, but still something to take into account. I firmly believe that all the interference talk is about 75% BS. Sure the valves can hit pistons, but there is more clearance then people think. I took my 96 heads and pistons. Set the valves at .370" off the seat (witch if the lifter was solid, would still just come shy of that number) Then took a piston and placed it up against the head, To my suprise there was still alittle clearance from the piston to the valve, not much, but any is plenty. This doesn't take into account the head gasket or if the piston is under deck. Now, If lets just say at 7000 RPMs the valves were overshooting the .370, and the valve got stuck or the belt let go at just the right time, there is a slight chance that valves could contact pistons. Quad 4's have a much more aggressive valve angle and protrude into the combustion chamber a good amount, the pistons only have 2 valve reliefs on the intakes. They are only there to take up any possible valve float. If the chain were to break, the motor would be toast. Believe it or not, you could get away with using flat tops or non valve relieved pistons in the 3.4 DOHC, and you will never have a problem with it, but if the belt goes, there is probably a 75% chance you'll hit valves, an there is a slight risk if the valves float of hitting the pistons.

Oh yea almost forgot, as far as I can tell, all sources indicate that the head gasket is .060" I measured .070" on a used one, so it sounds reasonable.

Im feeling generous, here are some specifications that I have for the 3.4 DOHC, they should help you find the info you are looking for. Anything that is inaccurate, I appologize ahead of time. The compression height I measured does NOT match any catalogs, or specifications I have ever read. So, don't know what to believe, but the accuracy of my micrometer.

Compression height 1.476" (37.5mm)(as I measured it)

Bore 3.622 (92mm)

Stroke 3.310 (84mm)

Block height 8.818" (224mm)

compression ratio 9.7:1

Head gasket .060" (1.524mm) = 10.1 CC's (assuming its 3.622" 92mm diameter)

Combustion chamber 51.5 CC's (calculated, not measured)

Displacement per cylender 558.4 CC's

Total chamber volume at TDC 64.2 CC's (calculated, not measured)

Pin Diameter .9055" (23mm)

Rod length 5.700" (144.78mm)

Crank end Journal Diameter 2.000" (50.8mm)

Ring pack 2- 1.5MM 1- 4.0mm

piston out of deck by .013" (was .013" = 2.2 CC) (calc'd by deck, stroke, rod, comp ht)

Piston CC's 2.7 cc's (between valve reliefs, and dome top, total added) (measured)

.???" big end Rod width.

6647.61 mm˛ = area of 92mm cyl.

TDC Volume make up:

Compression ratio 9.7:1
Piston CC's 2.7 CC's (marginally offset by piston over deck + piston dome)
Head gasket 10 CC's
Head Chamber 51.5 CC's
_________
Total 64.2 CC's

[LQ1]

I have read that a normal quench area is about .060". Mr Lingenfelter and Yunick both said that .040" and possibly .037" would provide very beneifical for an engine, but you have to know what you are doing. Shrinking the quench area will add a slight increase in compression, but the power gained is from the quench providing more turbulence in the air/fuel mixture. The turbulence causes the air/fuel to mix better and provide for a more efficient explosion. I still do not think the rod will stretch much at all based on the forces applied to it. You would need some where around 2,000,000,000 ftlbs of tq to stretch a rod .040". While this is more of a stationary number, I still would find it hard to believe to stretch a rod moving at 7000rpms.

Increasing the quench is as easy as getting a thinner gasket or decking the heads. If the gasket is .060" plus a valve to piston clearance of .012" then there is some room to play with.

[Kohburn]

since we are discussion compression ratios.. anyone running a thicker copper headgasket for boost?

[SappySE107]

http://www.hotrod.com/techarticles/82378/

Just saying they they do strech and compress under loads. I never said how much.

[LQ1]

I believe they do stretch and compress, I was just attempting to show that it wouldnt be enough to cause the pistons to hit the valves.

I was considering doubling up on the headgaskets, but with a .060" thickness that would based on the calcs used in this thread drop the compression to almost 8 even.

[BlueShift]

I dropped by the machine shop today, and talked to one of the machinists there. I told him that though I still have to check some stuff (have them CC the chambers, learn the calculations and run the numbers myself, amonst other things), that two different people arrived at .010" deck bumping me to around 9.5:1 CR from 9.25:1. He says "That's bullshit - on those motors over there (V8's I think), I machine like 20 or 30 thousandths off the block/heads to get a quarter point of compression, there's no way".

Does the TDC have a small quench area/combustion chamber or something? I know that trying to compare how much head decking will raise CR on a V8 is like comparing apples to oranges, but .010" is just a little more than a cleanup cut, isn't it? I can see why he'd be hesitant to believe it, but we just proved otherwise mathematically, did we not?

[Fierobsessed]

There are two differences, .010" off the head of a 3.4L is a bigger change to a small motor, then .010" on a huge 5.7L If you had a engine with 4' stroke and a 4' bore, .010" decking won't make a squat of difference. Even more accuratly, you get less then half the compression change by decking a 425 CI engine an equal amount as a 207 CI engine.
Also, The higher the compression ratio, the less change it takes to effect the compression ratio significantly, but that is the lesser of the two factors.
lemme quantify what I am stating by applying my little formula to a SBC.

4" bore 3.48" stroke 8.0:1
4 / 2 = 2 * 2= 4 (redundant figure eh?)* 3.14 * 3.48 = 43.731
43.731 CI per cylender displacment.
8.0 - 1 = 7.0
43.731 / 7.0 = 6.247 CI Top Dead Center volume
Decking .010" = .1256 CI
6.247 - .1256 = 6.1214
43.731 / 6.1214 = 7.144
7.144 + 1 = 8.144 Compression ratio.
So you can see that .010" mill on the 350's head is good for 0.15 CR increase, while the same mill on a 3.4 is good for a 0.21 CR increase. Ok so its not such a huge difference, but you can see it though. You gained almost a 1/4 point, while the SBC gained alittle more then half of a quarter point. You can see that if he did deck the SBC .020" that he would have indeed increased compresson .29:1

You cannot fool math. it is always accurate.

[BlueShift]

There are two differences, .010" off the head of a 3.4L is a bigger change to a small motor, then .010" on a huge 5.7L If you had a engine with 4' stroke and a 4' bore, .010" decking won't make a squat of difference. Even more accuratly, you get less then half the compression change by decking a 425 CI engine an equal amount as a 207 CI engine.
Also, The higher the compression ratio, the less change it takes to effect the compression ratio significantly, but that is the lesser of the two factors.
lemme quantify what I am stating by applying my little formula to a SBC.

4" bore 3.48" stroke 8.0:1
4 / 2 = 2 * 2= 4 (redundant figure eh?)* 3.14 * 3.48 = 43.731
43.731 CI per cylender displacment.
8.0 - 1 = 7.0
43.731 / 7.0 = 6.247 CI Top Dead Center volume
Decking .010" = .1256 CI
6.247 - .1256 = 6.1214
43.731 / 6.1214 = 7.144
7.144 + 1 = 8.144 Compression ratio.
So you can see that .010" mill on the 350's head is good for 0.15 CR increase, while the same mill on a 3.4 is good for a 0.21 CR increase. Ok so its not such a huge difference, but you can see it though. You gained almost a 1/4 point, while the SBC gained alittle more then half of a quarter point. You can see that if he did deck the SBC .020" that he would have indeed increased compresson .29:1

You cannot fool math. it is always accurate.

That's what I was thinking. On a bigger displacement motor, .010 isn't much. He talked about how the chamber is a little tight on these motors anyways, which is also probably is why so little deck does so much.

BTW, I'm hoping to get some CC numbers in in not too long... then I'll run those through these calculations for a little more accuracy and all that.

On a side note: can anyone confirm what fuel grade the manual spec motor uses? 87? I'm going to make a big assumption (and therefore possibly make an ASS out of U and ME) that my motor will run on the same, despite using a different approach to get that compression. I'm not trying to be really cheap, it's just that CA gas prices are unreal right now at about 2.60/Gal for 87!

[SappySE107]

You can run regular with decking no problem. I prefer midgrade as its noticable, but premium is a waste.

[93GTP]

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.

Yes, you would have to cc the head to be completely accurate. The calculations that have been used here are treating the combustion chamber as being cylindrical, when it is actually hemispherical. So as you mill more and more off the head the compression ratio starts to raise faster (the volume of the chambers is decreasing at a faster rate). But for how little is being milled off (0.012" max) I would say that it is still fairly accurate.

I ran the numbers for 0.012" of decking as well and they matched lq1's for the 9.5:1 engine, going from 9.5 to 9.8:1. But with the 9.25:1 engines I calculated a 9.5:1 not 9.6 as he got. I am thinking of getting the heads decked too on the '92 5-speed Z motor I'm going to rebuild.

Ben, you say you have yours decked 0.011" with no problems? This was on a 5-speed engine correct?