kerplunk318

iTrader: (3)

I've been driving myself crazy looking over this quench and compression dilemma.

I want to get some individual opinions...

Bore: 4.155
Stroke: 3.75
Rod: 5.7
Piston: forged SRP 5cc dish
Heads: 64cc aluminum chamber smoothed
Gasket: .041 x 4.2
Cam: howards roller 234/242 @ .050

Car has a 4.10 and a 6 speed, probably weighs around 3200.

I will run 93 on the street, and either e85 or 100 octane at the track.

I can deck the block .020 to get the quench right, but the compression will be about 11.5 what do you all think?

old z mzn

iTrader: (1)

are you including the head gasket

kerplunk318

iTrader: (3)

yes: .025 in the hole plus .041 gasket, or .005 in the hole plus .041 gasket.

It's a 406 if there's any confusion.

TTOP350

iTrader: (8)

I'd go 6" rod and 11.5-1, if the combustion chamber is a new design.

Orr89RocZ

iTrader: (20)

tight quench, 11.49 to 1 compression option. 93 on street should be fine with conservative timing, slightly richer tune perhaps, and step or 2 colder plugs.

What size heads on the car? I'd run more cam than that too or at the very least, go 108-110 lsa. If planning on E85, run more compression if possible.

93 should run it tho as it sits.

kerplunk318

iTrader: (3)

I wish this was a new build but unfortunately I already have all these parts on hand.

Heads are performer RPMs with the ports cleaned up a little. Planning on colder plugs. What will the tighter LSA do for me?

Orr89RocZ

iTrader: (20)

tighter lsa will lower the dynamic compression ratio, which will help alittle on pump gas driveability. Also give you some more torque in the midrange.

1989GTATransAm

iTrader: (3)

I agree with Orr. The 93 octane will help. Also look into a cold air intake system if you do not already have one.

kerplunk318

iTrader: (3)

http://www.compcams.com/Technical/FAQ/LSAproperties.asp

Isn't dynamic and effective compression the same idea?

Will definitely look into some cold air ducting.

vetteoz

iTrader: (13)

No
CR is what you have if the piston moves full stroke with the valves closed.
However, the longer the cam duration, the longer the valves are held open as the piston moves up so compression only starts when the valves are actually closed.
Hence DCR; the actual "true" compression.
That is why engines with big cams need more static compression to get back what is lost by the valves closing later ;
conversely a longer duration cam will help you out if you have too much compression

Read for a better understanding
http://members.uia.net/pkelley2/DynamicCR.html

There is a nice calculator at bottom of page you can download and install.
It allows you to input different cam specs to see what the resulting DCR change is


http://racingarticles.com/article_racing-10.html
http://www.camaros.net/forums/showthread.php?t=124935
https://www.thirdgen.org/forums/powe...ce-vs-dcr.html

86LG4Bird

iTrader: (1)

Why would you use an .041 head gasket?
If you have guaranteed flat deck and head surfaces, use an .015 gasket. If not, an .026 gasket at the most. Shoot for a quench of .035

If you can't do either for some reason, then the answer to your post question is.... the .046 quench with 11.49 CR.
.066 quench is simply a bad plan.

kerplunk318

iTrader: (3)

I can't seem to find a .015 gasket with any bigger than a 4.1 bore

86LG4Bird

iTrader: (1)

sorry about that.....I missed your bore size.
Call Cometic then; they can get you just about any gasket.