Max compression ratio for regular gas?

There's lots of good information here.

Generally speaking, a mild cam iron head SBC is covered for all fuel formulation variances at 9.0:1. Keep in mind that fuel formulations vary by region - and by season. Some areas will run 10% ethanol, others run MBTE; these do affect preignition and HP output.

Also remember that the DCR goes up over time as the combustion chamber and piston top collect carbon deposits. An engine on the ragged edge of preiginiton when first built will be pingin' at 25K miles.

finally - don't trust yer year as a knock sensor. A single small hot spot will cook a piston in a few thousand miles - I kept a reminder of that on my desk for years to illustrate the need to manage comp ratios on pump fuel.

 

Brett,
As per your PM, our Speed Pro 283 CID piston has -2.5cc per the valve reliefs.

Your compression ratio will as follows depending your cylinder head chamber CC, factory 9.025 deck height and our FelPro composite head gaskets which are .039 compressed:

58CC head - 8.88-1
60.5 CC head - 8.68-1
64CC head - 8.36-1
68CC head- 8.02-1
72CC head- 7.70-1
76CC head - 7.42-1

Hope that helps!
Glenn

 

FWIW, a hemi (hemispheric combustion chamber) will tolerate more compression than a wedge or bathtub chamber.

One more reason to spring for FIREPOWER!

 

What is the seat to seat duration of your cam? That has a lot more to do with dynamic compression that the @50 duration.

Did you a a cc for the space above the rings, and figure out the gasket cc's? A standard felpro chevy replacement gasket is 4.185" bore diameter and about 0.038" thick.

Also, how far is the piston down in hole? Most replacement pistons are about 0.025" down in the hole. What is the CH of the pistons you are using? If you are running this far down in the hole with a standard replacement head gasket, replace it with a thin steel shim gasket. You will have a quench distance much much better and be less likely to detonate.

Did you CC the heads? The only heads rated at 53cc's are 601 castings and they are actually about 55cc's from the factory. If you do have 601 castings, they have a good chamber and are less detonation prone than other heads.

I did some calculating using the specs you gave and the assumptions above. You should be around 9.45:1 true compression ratio. With a 270 seat duration on the intake, you will have a DCR of around 7.65:1 and be fine on pump gas.

 

The quench area of small block cylinder heads handles compression buch better than most. I made a mistake of putting 305 heads on a 350 with flat tops. probably came out with at least 10:1. Didnt have any problem with detonation whatsoever. The only indication that the CR was so high was that I had a very bad hot start problem. Had to put two batterys on the truck like a diesel so it would start when it was hot. Put 350 heads back on it and got rid of that problem.

 

Rick: Unfortunately I did'nt completely blueprint this motor. My machinist assembled it, and from his specs on gasket,deck height, etc. I have come up with 9.33:1 and 8.22 dynamic.
Its a mild RV cam I've had sitting around forever. I do have "601" heads and did'nt realize they were so hi-po 'till it was all together,
thats what prompted the "worry" .
I'm ok now we're gonna slap it in and run it

 

lets get that bitch together and smoke some big tires...

 

When gas is $3.50 a gal., which is what I just paid, what's another $.20 for super?

 

You build a nice motor and worry about the price of gas. I think that cheap peotection for the enging, plugs, etc. With most states demand 10% Ehtanal[sp]in you gas I run the better gas Just for that reason.

acechev55

 

The word is frugal . Or cheap ass if we know each other .
I am building it to be an economical long distance cruiser, having to use premium would defeat the purpose and was not in the orriginal plan. If I was building it for maximum horsepower & torque I would not have built a 283.

 

More variables than just compression ratio. Vehicle weight, gear ratio, barometric pressure humidity, operateing temperature. I could go on and on.

I run 11.5:1 with aluminum heads on premuim, but I can run on mid grade when its cold out. With the same engine in a lighter vehicle and a deeper gear I ran on mid grade year around. Just for an example of what I'm saying.

That said you should be OK with your setup.

 

compression don't start till the intake valve closes. If you think you are that close, install a cam with 6 more degree's of intake duration. Cast iron heads don't like crank pressures above 160 psi with crappy gas. Putting on a thicker head gasket can drop the compression, but could induce detonation. What's the quench?

 

Duh, wear'd ya go 2 skool bud? A 9 to 1 kumpreshun rashio wud meen 90 ahktain, wooden tit?

 

I've got a 12:1 Lincoln 430 that I haven't bothered to get running for this very reason. I figured it wasn't really drivable other than a blast down the 1320