Stock 2g compression vs 1g compression.

[dharris89]

I'm about to put a stock 6 bolt motor in my car. I'll be using my 2g head and I'm running a 16g and I'm getting ready to invest into a bastard 20g. I don't know everything, but will there be a difference in power? I know the 6 bolts are 7.8 and 2g are 8.5. Just curious.

 

[BogusSVO]

First remember to ream out the head bolt holes in your 2g head to .500
http://www.dsmtuners.com/forums/cylinder-head-short-block/341885-oil-port-mod-2-4g63t.html

The 6 bolt rating of 7.8 and the 8.5 of the 2g are just factory target numbers. Your real world numbers will be different, could be higher or lower depending on different variables.

Some will say but the "Butt" dyno a higher comp has the better off boost drivablity.

Lower compression can be simpler to tune out knock.

Until you measure the volumes and run the math, you have just a guess at what your CR is.

Funny you post this today, I just did a write up on CR.
http://www.dsmtuners.com/forums/articles-engine-fuel/468895-how-figure-compression-ratio.html

 

[gsexxx]

You can run 2g pistons on 1g rods. To do this you need to bore out the wrist pin hole in the rod for the larger pin size of the 2g piston. It seems to be the best/most durable stock piston/rod combo and has proven to hold 500hp+.

However, with a 16g a stock 6 bolt block will be fine.

 

[MachineGunSquad]

I have a 2G head on my 6 bolt, didn't have any issues with the head studs fitting?

 

[BogusSVO]

I have a 2G head on my 6 bolt, didn't have any issues with the head studs fitting?

It is rare that there is not some sort of binding issue. The 6 bolt has a 12mm stud and the head bolt holes are 13mm

The 7 bolts have a 11mm stud and the head bolt holes are 12mm.

 

[MachineGunSquad]

Yeah, thought the same. but just would make sure before reaming them out.

 

[Azumaninjay]

Whats the standard compression for a stock 2g. Vs stock 1g? My 91 GSX is reading 90psi in all 4 cylinders after rebuild. (After about 200 miles) It seems low to me, but I'm not sure what the norm is.

 

[taz4432]

The service limit is 121psi on a stock 1G block (7.8 CR)...i.e. anything below 121psi is supposed to be rebuilt. A 'perfect' engine will read 164psi.

For the 8.5CR engines, the 'perfect' value is 178psi and the service limit is 133psi.

 

[Azumaninjay]

I see... well I guess 90 will have to do for now. Thanks for the clarification.

 

[BogusSVO]

I see... well I guess 90 will have to do for now. Thanks for the clarification.

The engine would have to spin a bit for it to even start at 90 psi

Either the gauge is off, or the test was done incorrectly.

 

[Cdc1221]

So from your calculations, my "8.3:1" advertised CR is most likely closer to 8:1 since my head has been machined once, and my block has never been resurfaced. Very cool writeup, as usual dale

 

[Azumaninjay]

The engine would have to spin a bit for it to even start at 90 psi

Either the gauge is off, or the test was done incorrectly.

The test is super easy, I will try a new gauge. The car runs strong with no real issues to speak of, So I imagine its fine.