3900 V6 9.8 compression. Is it static or dynamic?

[Driver_10]

Not at all, approximately .090 quench area is stock for the 3900 and from my experience it is very resistant to detonation. You may not recall but I ran 7 psi on the stock motor before intercooling with 212 degree inlet temps datalogged and mid grade fuel with poor tune to boot. It's terrible by early iron head stats but GM has done their homework with the head design as it doesn't seem to be a problem at all. My concern was with making it even greater by using the first offset ground crank which would have taken it to ~120.

Wow..No shit, huh


If you got away with 7lbs, "non-intercooled" on mid-grade and a .090 quench, then I should have no issue tuning on mid-grade with 14lbs and a .050 quench. Hell, maybe even regular unleaded and meth would be good.

Had me worried for a minute. I was shooting for a .050-.045 quench based off of an advertised stroke and .070 head-gasket. (+/- .005 to account for any rod-stretch and expansion at high rpm)

.090 quench?, That would have been "knock city" under heavy boost on a hot day (I wont be running any less that 14.6 PSI to prevent compressor surge)

Glad you had that verified.

Not at all, approximately .090 quench area is stock for the 3900 and from my experience it is very resistant to detonation. You may not recall but I ran 7 psi on the stock motor before intercooling with 212 degree inlet temps datalogged and mid grade fuel with poor tune to boot. It's terrible by early iron head stats but GM has done their homework with the head design as it doesn't seem to be a problem at all. My concern was with making it even greater by using the first offset ground crank which would have taken it to ~120.

[ForcedFirebird]

Man, we just rebuilt a 4.6 that got water in the cyl and bent 2 rods. Had I remembered this thread, I would have measure the deck height, but I will have to say it didn't appear to be out of the norm. I would like to further investigate this large quench you talk about.

[Driver_10]

The used gasket I just measured is about .065" and that's measuring over the embossed sealing material that I know would be compressed more than what my caliper is doing if the heads were torqued down. .070 would mean the quench area is even worse than thought, eventhough GM refers to the cylinder heads now as "mist quench".

The machinist appears to have been mistaken, I received another call today stating that the stroke is going to be pretty much what I wanted. You don't suddenly recover .020" stroke out of thin air. What's nice about it is that they're actually talking to me about what's going on, something the last guy didn't do.

Had me worried for a minute. I was shooting for a .050-.045 quench based off of an advertised stroke and .070 head-gasket. (+/- .005 to account for any rod-stretch and expansion at high rpm)

.090 quench?, That would have been "knock city" under heavy boost on a hot day (I wont be running any less that 14.6 PSI to prevent compressor surge)

Glad you had that verified.

Having the piston nearly contact the head at peak power revolutions is ideal, period, GM would not have done otherwise since it has been prooven time and time again.

You would think, but in addition to the head gasket thickness the piston sits about .030 in the hole so quench is definately greater than .070. Despite that fact combustion chamber design comes into play as well so the extra quench area is probably negated. The Ford 4.6L runs a quench area greater than .100. smaller diameter cylinder bores also add a positive contribution to the quench effect I have read, so that may allow greater quench without loss of effect when compared to an engine with the same quench but larger bore.

[robertisaar]

mmmm...... direct injection.....

[ForcedFirebird]

Not the first time GM lied. Look at the 3.4/3400, it's advertised as a 207, and is a 204 (much closer to 3.3l than 3.4).

To measure compressed thickness, you have to insert a feeler guage and/or shims between the head and block after clamping.

Having the piston nearly contact the head at peak power revolutions is ideal, period, GM would not have done otherwise since it has been prooven time and time again. I can't wait to see the LLT in a wrecker exchange and get to dissect it.

stock gasket is .070" thick.

The used gasket I just measured is about .065" and that's measuring over the embossed sealing material that I know would be compressed more than what my caliper is doing if the heads were torqued down. .070 would mean the quench area is even worse than thought, eventhough GM refers to the cylinder heads now as "mist quench".

The machinist appears to have been mistaken, I received another call today stating that the stroke is going to be pretty much what I wanted. You don't suddenly recover .020" stroke out of thin air. What's nice about it is that they're actually talking to me about what's going on, something the last guy didn't do.

[SappySE107]

stock gasket is .070" thick.

They also said the 3.6 had forged pistons. Instead of telling us possibilities and theories, why don't you measure everything and give us a break down to the real compression. No one (repeat, no company) posts compression ratios as dynamic. No one.

I believe you, I'm just having a problem with believing GM. I'm questioning more the possiblity the static compression may not be what they say it is.
So far with my measurements along with those the machine shop have come up with plus what you guys have provided:

I found a better compression calc than the one I had been using and arrived at the following
using:
cylinder bore: 3.899 I measured
gasket bore: 3.974 I measured
gasket thickness: .051 WOT and Cometic
rods: 5.827 I measured plus someone elses confirmation
piston to deck: .030 I measured +/- a small variation ~.002
stroke: 3.290 machinist
piston cc: 18 forum members
head cc: 36.2 form members
Intake valve close: 30 deg atdc forum members

http://kb-silvolite.com/calc.php?action=comp

Static = 10.116: 1
Dynamic = 9.105:1 here the calc states to add 15 to int close angle. With my cam fully retarded it read 7.694:1 with an value of ~70 used, 42.3 + 15 + 12.7

Fully advanced: 37.3 value for intake, (with adjusted specs for 9.80:1 static), 8.939:1 dynamic. Fuel economy was excellent with the cam advanced.

The power loss when comparing the fully retarded cam run against the fully advanced run using the GTech accelerometer supports the trend as the more the cam was retarded the more the compression droped.

My setup should turn out to be: static 12.275:1 and dynamic 10.272:1
Based on an exact 9.8:1 compression ratio my mods will yield: 11.781:1 static and 9.905:1 dynamic
My pistons will be notched bringing the 9 cc volume up to at least 11 dropping my static compression down to at least 11.5:1 where I want it.

[SappySE107]

They also said the 3.6 had forged pistons. Instead of telling us possibilities and theories, why don't you measure everything and give us a break down to the real compression. No one (repeat, no company) posts compression ratios as dynamic. No one.

Changing the ICL of a cam can effect dynamic CR, but not static. GM has never released an engine and publicized dynamic CR for it. The piston sticks .020 (at least) in the hole. I have measured that myself.

We have not proven it and there is no rule that states GM has to state static compression ratio as opposed to at a specific VVT cam location/dynamic.

Things that support the possibility the advertised compression could be inaccurate or deceptive on purpose are: Giving the impression that only the 3900 in an SUV came with a steel crank in literature a few years back, advertised 5.9" connecting rods and 3.31" stroke both of which have been inaccurate on my engine having been measured at 5.827" and 3.290" respectively. The last two are crucial inaccuracies because if you buy replacement parts based on GM's documented specs you would have serious problems come time to assemble everything as those numbers are not even close to advertised.

[gectek]

Changing the ICL of a cam can effect dynamic CR, but not static. GM has never released an engine and publicized dynamic CR for it. The piston sticks .020 (at least) in the hole. I have measured that myself.

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.

[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.