3900 V6 9.8 compression. Is it static or dynamic?

[Guest]

The mistake with my crankshaft regrind caused me to do some thinking. I'm not exactly sure but I recall the 3900 combustion chamber volume being about 53cc, if that's exact, or close then my engine in stock form couldn't have had 9.8:1 static compression. I measured .064 gasket thickness and estimated from the crankshaft regrind that the piston sat about .031 below the deck surface for a near .100 quench area height. That coupled with a 53 cc coumbustion chamber and 18 cc piston dish should produce compression in the 8ish range.

Since camshaft grind affects compression perhaps the full advanced cam position equates to a 9.8:1 static compression levels which probably only exists at idle and just off idle then changes as the camshaft is varied giving the impression that the engine is running that kind of impressive combination in the same area that a fixed cam would, or is lagged behind rpm to limit efficiency equivalent to that ratio in areas where detonation is likely to occur with low grade fuel.

That's probably how the 3.6 DOHC is able to run 10.2:1 with 87 octane by varying cam position to reduce dynamic compression under load on low grade fuel to prevent detonation.

This is important because if in fact the static compression was arrived at creatively by GM, just using the piston volume to estimate your compression ratio in a build up could get you in trouble, especially if you increase dish volume to lower compression ratio from what you think is 9.8:1 instead of possibly a real 8.8:1. GM advertises the connecting rods as 5.9" but there are two confirmed measurements of 5.827".

[SappySE107]

You can run over 11:1 on a 3100 with 87 octane and a good sized cam for that compression. I don't think a 3.6 DOHC has to do anything special to run 10.2:1 on it. I am sure they programmed the cam timing for each application but I have never seen a company list a compression ratio as anything but static.

[Superdave]

I've never seen a manufacturer rate something in dynamic, it's always static. Maybe you are onto something though, the 4.2 in my trailblazer is rated at like 10.3:1 but has a variable exhaust cam.. It runs fine on 87.

On another note, i wish i had known you lived in Tampa, i was there last week.. i could have bought you a beer or 6.. haha

[Joseph Upson]

I've never seen a manufacturer rate something in dynamic, it's always static. Maybe you are onto something though, the 4.2 in my trailblazer is rated at like 10.3:1 but has a variable exhaust cam.. It runs fine on 87.

On another note, i wish i had known you lived in Tampa, i was there last week.. i could have bought you a beer or 6.. haha

That's the home base but I'm in Georgia right now and don't need anymore good habits, like beer drinking. Given the typical compression rating being static, you still don't get 9.8:1 compression with the numbers I listed. Maybe the engines with true 5.9" connecting rods have 9.8:1 compression and the others with 5.827" rods like mine less. The connecting rod to spec mismatch alone is good cause to question the actual compression ratio.

[SappySE107]

a 3.4 DOHC has a 54.5cc chamber. The 96 3.4 DOHC is 51.5 (cloverleaf shape) You think the 3900 is that size?

[Joseph Upson]

a 3.4 DOHC has a 54.5cc chamber. The 96 3.4 DOHC is 51.5 (cloverleaf shape) You think the 3900 is that size?

Just going by what I seem to recall for the combustion chamber, I don't know it's 53 for a fact, but that's the number I recall seeing posted.

[Superdave]

It doesn't look that big.. 35cc maybe?

[ForcedFirebird]

I don't see the 3900 heads having 53cc chambers, that's larger than the iron head chambers, isn't it?

[Joseph Upson]

I don't see the 3900 heads having 53cc chambers, that's larger than the iron head chambers, isn't it?

Yes, thanks for reminding me of those relics, it's more realistic now. The quench area is still nearly .100 and that's a lot. Hope I can get ahold of another crank without having to disassemble and rob my perfectly good 3500 for another shot at the offset grind I was trying to achieve. Maybe I'll be able to find a rod combo to allow use of the 3.48" stroke crank in the 3500 in exchange.

What's compression height on the 3500 piston Superdave?

[Superdave]

I'll check tonight.. i know i posted it somewhere but i don't remember where.

[SappySE107]

36.2cc chamber for the 3900 heads.

[Joseph Upson]

36.2cc chamber for the 3900 heads.

Thanks

[ForcedFirebird]

36.2cc chamber for the 3900 heads.

Beat me to it, lol. I was going to try to find time today to measure...

[bszopi]

If we can get all the measurements for the 3900 in this thread, I will add the values to the CR Calc database for future use.

[Joseph Upson]

My second crankshaft is at the machine shop for a second attempt at getting the stroke I requested so that I can build the high compression engine I want for boost. I just received a call from the shop stating they can't stroke the crank to the theoretical 3.56" I wanted (minus .00X" clean up) because the crank stroke is not 3.31" stock as advertised. It's 3.290". This leads one to question wether the static compression ratio really is 9.8:1. The connecting rods are advertised at 5.9" but have been measured at 5.827".

I'll have to break one of the assemblies down from the recent engine I purchased to take another measurement since someone else stated their engine actually has 5.9" connecting rods.

There is also the possibility and likely hood that the 9.8:1 compression is dependent upon camshaft position since the engine has a variable camshaft.