Compression ratio

[Slice]

Almost finished with 500 stroker 6 pack build.  I'm trying to decide what compression ratio to go with.  The heads are eddy rpm's, the cam duration is .231/.239.  From what I understand felpro gaskets can put me at 10.4:1 or 10.1:1. 

Can someone tell me how much difference this will make and how risky it is running 10.4:1 with my setup???  It is mostly a street car and though I want power, I don't want to worry about overheating or detonation.

[FLG]

10.4:1 on aluminum heads should be just fine, most ive seen run 10.5:1 or could even squeeze out 10.7:1 with the aluminum heads.

[BSB67]

Almost finished with 500 stroker 6 pack build.  I'm trying to decide what compression ratio to go with.  The heads are eddy rpm's, the cam duration is .231/.239.  From what I understand felpro gaskets can put me at 10.4:1 or 10.1:1. 

Can someone tell me how much difference this will make and how risky it is running 10.4:1 with my setup???  It is mostly a street car and though I want power, I don't want to worry about overheating or detonation.

Need more info on your cam and its installation.

[MSRacing89]

We run 11:1 with aluminum heads on pump gas with a just slighty bigger cam (.240ish).  It has .140 quench dome pistons on stock stroke motor.

Your stroke may build more cylinder pressure but you should be ok at 10.5:1 +.  Aluminum heads eat about a full point of compression.

http://www.projectpontiac.com/ppsite15/compression-ratio-calculator  Try this link....input as much info as possible.  Dynamic Compression is the real number that determines your limitations with pump gas.  Max DC of 8.5:1 - 9:1.

[frederick]

How far down the hole are the pistons and how thick are the two gaskets?

[gtx6970]

493 stroker ,11.5 compression , Indy aluminum heads and I think it's in the 700-730 lift range roller cam

[Slice]

Thanks for the responses.   I'm not sure how far in the hole the pistons are, but I'm having the short block assembled for me, and my builder told me I had a choise between 2 felpro gaskets.  One would yield 10.4:1 and the other 10.1:1.  Based on Eddie Perf RPM 84cc heads.

The cam I'm using is a Lunati hyd roller #60312. 

Lobe Sep................110  106 Center line
Ex closes................5.5 ATDC
Ex opens.................53.5 BBDC
In opens..................9.5 BTDC
In closes.................41.5 ABDC

Lift   .535/.550

Dur @ 50  231/239

[BSB67]

The cam is pretty small, but the hyd roller has more seat timing than an flat tappet for a given duration @ 0.050".  The actual seat timing for the intake closing is 67 ABDC.  I think 10.4 will be close.  If it pings, move the ICL back to 110.

[MSRacing89]

I would say if you are not going to take the time to figure out if your DCR is in the correct range then just go with 10.1:1 and call it good.  You seem pretty concerned about it so unless you take the time yourself to calculate it you just better go the safe route.  That combo will have plenty of torque one way or the other.

[Slice]

Thank you all for the replies.  I'm gonna go 10:1 just because the car will see amlost all street use and cruising and I dont want any headaches.  I'd rather lose the occasional race than be stuck overheating somewhere.

[morepower]

I built mine for 10.5 to 1.  zero deck with a -11 cc dish. edelbrock perform rpm's heads.  I have not ran the motor yet but im not worried about it being too much

[Chryco Psycho]

you are more likely to get detonation with a 10:1 engine & poor quench than a 11:1 build with tight quench
using alum heaads I would highly recommend the Cometic gasket to allow for the different expansion rates of alum & iron .
overheating has little to do with higher compression , more to do with water flow & radiator efficiency

[femtnmax]

Several excellent comments just posted.
Your engine builder should be able to tell you the exact compression ratio and quench.  Proper quench is important, but takes time to set up properly.  As Chryco said you can run higher compression with no detonation if quench is designed into the engine build.
Using the KB-silvolite calculator:  
http://kb-silvolite.com/calc.php?action=comp

Feeding in your numbers, then morepower's numbers:  Morepower has the better combination because his has quench built in, while your combo has no quench benefit.
Your combo for 500 CID with 4.35 bore and 4.15 stroke:
cyl head vol= 84
piston head vol=4  flat top piston
head gasket thickness=.04
head gasket bore=4.45 (best guess), like I say a good builder will have all the exact data
cyl bore=4.35
deck clearance=.018 piston down in the bore, info from 440 source web page
stroke=4.15
rod length=6.76
intake closing=67 abdc

RESULTS:  static compression = 10.86, and dynamic = 8.5
But quench clearance is 0.018+.04 = .058 which is way to loose, no quench benefit at all.  Can you say possible engine knock even with premium gas.

Morepower's build: same 500 CID with 4.35 bore and 4.15 stroke:
cyl head vol= 84
piston head vol=11  dished piston
head gasket thickness=.04
head gasket bore=4.45
cyl bore=4.35
deck clearance=.0.00 piston top flush with block deck
stroke=4.15
rod length=6.76
intake closing=67 abdc

RESULTS:  static compression = 10.6, and dynamic = 8.3
Quench clearance is 0.00+.04 = .04   this is perfect clearance for better combustion, less chance of detonation.

Your engine builder should take the time to do this right.
Changing head gaskets to drop the compression will only make the quench clearance worse.  And consider that 10:1 static means actually 9:1 when you account for heat loss of the aluminum heads.  Heck, old work trucks had more than 9:1 compression.  Your engine would sound tough, but not have much for real performance for the $$$ you have spent.