is my compression too high?? non z-car engine

[mobythevan]

I just re-installed a rebuilt engine into my truck and had a couple questions. First of all the engine was built about 5 years ago so I have forgot some things about it, but here is what I know.

 

355 small block chevy

iron heads (I think 67cc)

268 comp cam

Flat top pistons with four valve reliefs

600cfm holley vac secondaries

stock iron manifold

 

The engine seems to be running good now that I have put it back in service. I measured cranking pressure with the egine warmed up and it showed 150psi. I am going to measure cold cranking pressure tonight, but is this going to be too much pressure? I am still a little soft on compression, pressure, etc. I am at 6000' altitude.

 

If this pressure is too high can I go up a size or two with the camshaft to get by for now. I am not sure what to run on timing either or what octane of gas to run. I was going to start experimenting. I was hoping the high altitude would help me run a little higher compression without detonation.

 

Open to any suggestions, but keep in mind this is a budget truck and I am trying to keeep from changing out heads and big dollar items. I was hoping camshaft at worse, or comprimise and not run as much timing, etc.

[rustorbust]

I have a 400 small block with 64cc heads and flat tops and i can run on 91 without pinging so i think you shouldnt have no prob.

 

James B.

[Pyro]

150 isn't that much. You should be able to run the cheap gas at that pressure with a little timing retard.

 

200 psi is about the limit for 92 octane. I have read that 175 is about perfect for a street engine.

 

If you are using the old style cast iron heads then you should have 64 cc or 76 cc combustion chambers. What are the casting numbers on the heads?

 

Set the ignition timing at 40 "total" degrees and 4500 rpm and let her rip. If the engine pings under a load then drop a few degrees.

 

A set of headers will help make some more power.

 

Make sure the distributor vacuum advance is connected to the correct carb port. The correct carb port will NOT have any vacuum at idle.

 

Holleys normally come jetted rich. Read a plug and make adjustments. I bet you will need a 68 or 69 jet in the primaries. Power and mileage go up if the mixture is right.

[mobythevan]

The iron heads I have came from some company that rebuilds heads and these have been milled to 67cc I believe.

 

BTW, on the vacuum source from the carb, I am running a Holley 4160 carb with it hooked to the port on the metering block, should I use a different port. I'll check it out tonight.

 

Thanks

[Pyro]

That is right. The port on the metering plate which is right above the idle mixture adjustment screw. It is a common mistake on chevys to place the vacuum advance on the wrong port. If the wrong port is used, the timing will retard instead of advance when the throttle is openned. Not good for performance!

 

FYI, Fords work opposite of chevys, that is, fords use the vacuum port that has vacuum at idle.

[Kevin Shasteen]

.., 355 small block chevy-268 comp cam-stock iron manifold w/iron cylinder heads.., it showed 150psi.., is this going to be too much pressure? I am at 6000' altitude...

 

I would say it all depends on your staying in the 6000 ft altitude. Without knowing your actual compression ratios anything is just a guess.

 

A 268 Adv. Duration cam is not that radical; yes it is radical compared to a smog motor's factory cam but on its own it is not that radical.

 

The only real concern is if you ever decided to come back down to earth & slum w/the rest of us common folk at sea level: then your engine will see increased cylinder pressures.

 

The reason for this is that for every 328 ft. of change in elevation your engine will see a .01 change in pressure. So if your elevation decreases 328 ft. your cyl. pressure will increase by .01 and if your elevation increases by 328 ft. your cyl. pressure will decrease by .01 percent.

 

6000 ft to sea level will gain you 18 sets of 328 ft segments. If your 355 is a mere mortal of a 300hp engine then, by coming down to seal level, your engine will have gained 50hp.

 

If your engine is a 400hp rip-snort'n go getter then at sea level you will see an increase to 469hp...possible detonation if your compression ratio's arent matched appropriately.

 

Still, without knowing your other engine parameters all I can say is that 150psi is not that big of a deal when combined w/a 268 cam and a Total Chamber Volume somewhere in the 67-76cc range.

 

I would be concerned if you were in the 185-200psi range-but with 150psi you should be okay.

 

Kevin,

(Yea,Still an Inliner)

[Pyro]

I think your 355 is running about 10.0:1 cr.

 

Here are my calulations:

 

cylinder volume = 727cc

head gasket volume = 8cc

piston down in the hole 0.020"= 4cc

piston reliefs volume= 2cc

combustion chamber volume = 67cc

 

(727+8+4+2+67)/(8+4+2+67)=808/81=9.97

[mobythevan]

Thanks for the input guys, that was my feeling but I wanted a little comforting. 10:1 has to be real close because I remember crunching numbers and showing 10.6:1 not including the head gasket and valve reliefs. They would lower the ratio a little so that should be real close. The truck will hit the road this weekend and I start experimenting with timing and octane.

 

thanks