Max compression ratio for regular gas?

I just calculated the compression ratio for my fresh 283 and came up with 9.33:1 Is this too high to run regular gas?
Its a mild build, .040 over 4 valve relief flat tops, mild RV cam, 53cc 1.84/1.5 iron 305 heads, old Weiand dual plane, 500 cfm Edelbrock, rams horn manifolds. Its going in my '63 C10. It's not meant to be a screamer, just a good running street truck. With the skyrocketing price of gas I definately do not want to have to run premium.
Anyone with firsthand experience? Thanks.

 

You can run 87 octane IF your cam grind allows you to bleed off some cylinder pressure, OR if you have a perfect tune, good ignition and strong pistons. Check out the story about how cylinder pressure can be tuned

 

I've always figured, multiply your compression ratio by 10, and that's the octane you should run. Not scientific, but a good rule of thumb.

 

I can't Imagine a problem with that...most street engines for the last couple decades have kinda had 9:1 for a benchmark. yer right there...

 

The cam specs are .429" lift and 214 deg duration @50 on a 111 deg lobe center.

 

Not bleeding off too much cylinder pressure, there...but, you should be fine, 9.3:1 is not very high...good tune, not too much advance, good ignition...your good, IMO.

 

I've been debating on weather I need to change anything before I put it in (its all assembled) your putting my mind at ease.

 

Much has to do with what it is installed in. My Old Roadster had a '57 PowerPack 283 punched to 292 with a solid lifter cam, 10.5:1 forged pistons, a single 4bbl, and never saw any gas more expensive than regular unleaded. Was very fast and had no problems.

 

I am at just about 9.5:1 on my daily 302, very very mild cam. It sits like 12-14 degrees initial advance and only makes noise when I get a bad batch of 87. At the track though I'm always on 93+

 

I had a '67 Chevy with a 283 and a '65 pickup with a 283 and both of them had 9.25:1 compression. I always ran on the cheap stuff, but the cheap stuff used to be more like 88 or 89 octane. If I was going to do a lot of mountain driving or hauling a heavy load, I might mix in some premium gas to keep it from pinging when pulling a heavy load up steep hills. Usually it only pings when you "lug" the engine, like going too slowly up a steep hill in high gear on a hot day. Otherwise you probably won't have any problems with the cheap gas.

Did you put in some hardened valve seats? Those older heads don't last forever on unleaded gas, because they don't have the induction hardened valve seats that came out around 1970 or 1972 or so.

 

yeah I have heard the same thing. 10 octane per 1.00 of compression ratio is good to go by. example 9:1 = 91 octane

 

That rule is fairly accurate for wedge-type chambers & the avg. bore diameter around 4"- 4.060" or less....not so much on other chamber designs, or fairly big bores. For what you are using, & doing, you'll be fine....and...

so long as you are willing to experiment at the beginning with your advance curve & initial timing, you won't have too much of a problem with any gas, even swill.

Hardened valve seats are always a good idea if you are doing some extensive head work anyway...I will say, though, that with all the cylinder heads I've run across, only three had erosion problems caused by unleaded....in those cases they were marine engines, or used for extensive towing...in other words, high EGT/high load/lower rpm settings for prolonged periods of time. I have never seen ordinary street/passenger car seats have a problem...even with pre '70s heads used with bad gas.

 

Your fine. Conservative cam. Compresion is right on for a hot rod. Like all cars, do a good tune up and keep it up. IF and I say IF you hear ANY pinging at all, try stepping up the octane and adjust your timing accordingly.

Bottom Line. Your good to go! Have fun!

 

I ran a 351w with 11.0 to 1 and a 3/4 cam whatever the hell that is and ran 93 octane with no detonation, and I ran the shit out of it. So you should be fine. My daily is a 50 chevy truck with a mild 350 9.5 to 1 and no problem there.

 

The weight of the car, gearing and ability to load the engine play a role too.

A heavy car that really puts a load on the engine will not tolerate low quality gas like a light weight car will
.

 

Got a compression tester? If cranking pressure is below 200psi with throttle blades open, you're OK. If not, retard the cam timing to delay yht closing of the intake valve. That will do the trick.

 

I'm in the same boat with my 235 - about 9.3 or 9.4 compression and I hope like hell I can run regular. Should be OK, right? any helpful comments on this compression with this engine?

 

Thanks for the responses guys. The heads are late model 80-86 305 heads(thanks Tudor) so the hardened seats are not an issue. The truck currently has 4.10 gears so lugging should not be a problem.
The "worrying" started after I picked up the motor Tuesday, I had not figured the comp ratio yet (dumb ass) and asked my machinist, he did a quicky on the calculator and came up with 9.5:1 .
I was hoping for between 8.5-9 but had only told him what it was going in and what we were doing with it(dumb-ass). Looks like we should be ok.

 

what you want is a dynamic compression ratio of 8.0:1 to run 87-octane pump gas. there are numerous dcr calculators on the web (kb pistons for example) that will allow you to figure out your dcr given your static cr and cam events.


hope this helps,
ed

 

Hot Rod Magazine fooled around with extremely long rods in an sbc about 10 years ago and achieved over 400 hp with Vortec 305 truck heads, mild street cam and 11 to 1 CR on 87 octane with no preignition. The article set off a controversy but you couldn't argue with the numbers. The premise was that with a high enough rod/stroke ratio (1.8 - 2.0) very long rods dramatically increase piston dwell time at TDC.

 

aluminum heads = you can run higher compression too...right?

 

Put it more that with aluminum heads you have to (or should) run more compression, to compensate for the greater heat rejection of the heads.

 

I think it will be right on the edge of detonation on 87 octane....as mentioned be prepared to play with the timing or add some premium if needed, especially when hauling. The rearend gear ratio will help.

Or move to the mountains, I've had a few that didn't ping here at 4500 ft elevation, but did down in Phoenix at 1200

 

I ran the #s thru the KB calculator and came up with a dynamic cr of 8.22, as you said squirrel "right on the edge" . I'm going to go back and tripple-check all the #s and then make a decision. Now would be the time to change something (thicker head gasket?) if I'm going to

 

or you could put a bigass cam in it....to match the rearend gears....

 

If you use a thicker head gasket, you are in effect moving the quench surface of the head farther away...which may aggravate the tendency to detonate. If you still need to deck the block, you can play with that, but if the machine work is done...and you don't feel comfortable about the dynamic CR being where it is....

...then I'd step up a notch on the cam....either add some overlap, or mess around with a slightly different intake valve closing. Basically, bleeding off a little cylinder pressure. Try this in the calculator with your figures: http://store.summitracing.com/partdetail.asp?part=CCA%2DCL12-235-2&autoview=sku & check Comp's site to get the valve timing.

My personal opinion is that you'll be OK as is, but if you are worried, now's the time to make changes.

 

going a little bigger on the cam will bleed off some compression

 

Don't step on the gas real hard. Use higher octane fuel when feeling racey.

 

Aluminum heads help for disspersment of Heat.
The days of an 11:1 Duntov Carb engine are gone thanks to our fuel. ( For some of those who have a radio, turn it off. Put the hammer down going up a hill. That sound of BB's in a coffee can aint good! )
9.5 to one is a good common everyday fuel that can run on 87 all day long. you havent ventured far from stock.

And most of the time, magazine build ups aren't realistic. The real key to not having a problem with iron stuff is QUENCH. Zero deck clearance will ward off those bad little gremlins that detonate inside.

Mike

 

well, My 10.5 to 1 289 in my comet ran on pump..and so does the 14 to 1 version in it now..haha...I didn't say it runs the greatest..but judging from the posi shot in front of a friends house tonite..I'd say it pulls pretty well to 7500 hahhaa..and there are guys to back me up haha....