3900 V6 9.8 compression. Is it static or dynamic?

[Joseph Upson]

Got the crank back today. 3.555" stroke. I also got a chance to check deck clearance again and this time I measured both sides of the piston inboard and out while pressing down on one side. I measured .031 and .049 to come up with .040 deck clearance so with what has been previously been measured we can be sure the quench area stock is at least .100 unless Cometic confirms that the stock gasket thickness is .051 which is what they list the 3900 gasket the offer as. From what I've read on MLS gaskets to date they do compress a little which is what accounts for some of their superior sealing characteristics.

[SappySE107]

Where does Cometic list stock gasket measurements for the 3900?

[Joseph Upson]

Where does Cometic list stock gasket measurements for the 3900?

I said if they confirm that the stock gasket thickness is .051. That's what they have listed for the gasket they sell on their site, it doesn't state whether it is stock or not. If I call them and they say it's the stock thickness then it's confirmed. They list the same thickness for the 3400 gasket.

http://www.cometic.com/domesticauto.aspx

[SappySE107]

Yeah and we all know the stock 3400 is .051" thick too.

[Joseph Upson]

Yeah and we all know the stock 3400 is .051" thick too.

I don't know that. Looks like you've got a fat gasket. Seriously it appears you are measuring over the embossed sealing material which will easily compress when the heads are torqued down. The literature I've read on multi-layer steel headgaskets indicate that more than one type of metal is used in the composition of the gasket and that one has a spring like composition to allow for some compression when the heads are torqued down on it. I measured an area that is not coated and although I could be wrong I'd expect the number I measured to decrease some under head bolt torque.

Here's a couple of examples of the LS1 MLS gaskets with .051 or less compressed height. The 3900 heads were designed after the LS1 so maybe they didn't stop there.

http://www.summitracing.com/parts/NAL-12589227/ This one references GM's part # and is a GM gasket.

http://www.trickflow.com/egnsearch.asp?N=400098+4294925237+115+328295&autoview=sku

[Driver_10]

Here is a good Hot Rod article on compression dated 2009. It also gives a lot of insight on why the 3900 quench area is relatively high compared to what we should expect.

Excerpt pg 3:

" If quench is so good at suppressing detonation and allowing the use of higher CRs for more power and better mileage, why doesn't the factory make it tight to start with? In a nutshell the answer is emissions. "...Tight quench over too large an area (such as seen in a typical small-block Chevy or Ford of the pre-1997 era) causes unburned hydrocarbon emissions to go up..." However, quench is a key element toward fast burn and this in itself can lead to the successful use of a higher CR just as we see with the LS1/6 family of engines. For modern engines, the trend has been to use a more open chamber with less quench area, but to make the quench action more active by tightening it up as necessary. Although high compression benefits fuel mileage, it can bring about a dramatic increase in oxides of nitrogen, which is the primary cause of smog. Offsetting this is the fact that because a fast-burn chamber requires less ignition advance, the amount of cylinder pressure and temperature generated to develop a certain amount of output is less, so in that respect oxides of nitrogen are lowered."

http://www.popularhotrodding.com/eng...tio/index.html

Ok... this statement.

Im not quite sure how this would cause an increase in HC's. I mean, the end gasses are cooled at the quench area to prevent unwanted premature ignition at the outer edges of the chamber. The ignition event (unless the chamber design is ubber crappy) should promote flame propegation throughout most of the AF charge within the chamber. (possibly a slight increase in CO, but thats why we have cat converters)

I understand the principle of "improved chamber design" needing less "help" to accomplish high cylinder pressures during combustion through faster flame-front travel (AKA fast-burn), but I cant quite picture that big a difference in the production of HC's if they were using a tight quench + modern "fast-burn" chamber design instead.

If anything, with a tighter quench I think that there would be an increase in efficiency just due to the fact that you could get away with leaner AF ratios while at cruise and part throttle. Come to think of it... the 3500, with its "loose" quench, runs fairly rich, no?

[Joseph Upson]

I think we better focus on the gasket compressed thickness issue to get accurate compression ratio calculations.

[CNCguy]

A used gasket from an '06 3900 measures .055" with micrometers in the uncoated areas. The middle and outer layers measure .039" and .008" respectively. Coated areas around the water passages and head bolts mic .060"~.061". A new gasket would probably measure thicker in the coated areas. Installed thickness is probably in the .057~.060 range with .055" being the minimum since there is no way the head bolts and aluminum heads could generate enough force to displace the metal gasket. I will see if I can measure the installed thickness on my engine the next time I drive the MG to work (when it quits raining).

[Joseph Upson]

A used gasket from an '06 3900 measures .055" with micrometers in the uncoated areas. The middle and outer layers measure .039" and .008" respectively. Coated areas around the water passages and head bolts mic .060"~.061". A new gasket would probably measure thicker in the coated areas. Installed thickness is probably in the .057~.060 range with .055" being the minimum since there is no way the head bolts and aluminum heads could generate enough force to displace the metal gasket. I will see if I can measure the installed thickness on my engine the next time I drive the MG to work (when it quits raining).

Thanks CNC, I just got off the phone with Cometic and the rep stated the advertised 3.9 gasket is based off O.E. specs as close as they can get .051-.052 and that the new uncompressed gasket would be thicker.

[dskopek]

A used gasket from an '06 3900 measures .055" with micrometers in the uncoated areas. The middle and outer layers measure .039" and .008" respectively. Coated areas around the water passages and head bolts mic .060"~.061". A new gasket would probably measure thicker in the coated areas. Installed thickness is probably in the .057~.060 range with .055" being the minimum since there is no way the head bolts and aluminum heads could generate enough force to displace the metal gasket. I will see if I can measure the installed thickness on my engine the next time I drive the MG to work (when it quits raining).

I bought a set of Cometic head gaskets for a 3.4 from Ben in .030 thickness. Before installing they measured about .036 to .038 depending on where you check. After install I show .031 to .032 on a few places that I can get to the block and head. Seems to be really close to what they say it should be

[SappySE107]

Makes sense that it compresses but I wasn't expecting .020". Stock 3400 is .060". They sell .060" thick gaskets. Thats a recipe for piston to head contact at higher RPM at their advertised .051" thickness.

I measured .065" inside the fire ring as well on my new 3900 gasket.

[gectek]

FYI, we have measured the LS (ALL) GM gaskets at .053 on a daily basis by us here at SD. They say .051. If anyone measures less than that they are mistaken.

[Joseph Upson]

I figured it out, the static compression in a VVT motor might be relatively high, but the dynamic compression in a VVT motor is variable. As the cam is retarded, the dynamic compression drops, so the high static compression may only be at work for a short while, probably at startup when the cam is fully advanced.

As soon as the cam starts to retard the dynamic compression drops and so does the risk of detonation. This can serve as an alternative method to pulling timing to stop detonation as well as be the reason behind some high compression VVT motors like the 3.6L at 10.2:1 being able to run 87 octane fuel at coolant temps well in excess of 200 deg.