the motor I have now has a compression ratio of 9.5/1 wich I hear is decent for an engine. I'd like my new engine to be 350cu and have a favorable compression ratio.I don't want to be timid with the heads/cam,I want this thing to crank! i already have the shorty headers and I'm definitly using an aftermarket intake. how high could I reasonably go? I don't plan on using nitrous or any power adders,just carburation I still don't have a clue what heads I'm using,but I will be porting and polishing whatever I use,all I know about this is that the piston hieght and the cc's of the chamber affect the compression and higher compression can improve economy and increase performance... I'm assuming that stonger pistons/rods become neccesary at some point.

how can I know what pistons/heads will get the compression ratio I want?

 

You won't know your final compression ratio until you know the combustion chamber size. Piston manufactures will usually list compression ratios for their pistons depending on different combustion chambers sizes. The most common size of SBC heads are 58, 64, 68 and 76cc. Since there are too many variables from engine to engine, they will give a compression ratio for each of these combustion sizes. How much the block has been decked, head gasket thickness etc will vary the compression ratio.

Higher compression builds torque however higher compression also increases the chances of preignition (spark knock) from lower octane fuel. With the best pump gas available at around 91-92 octane, you're limited to 9-10:1 compression ratio.

 

Not true, you can run a higher compression ratio with alot of duration that helps to bleed off clyinder pressure.

 

thanks guys!! I'm glad LottaBalls brought up the cam duration,I know theres a lot involed in calculating compression and the cam will definitly play a huge role in how it deals with higher compression. it's hard for me to believe that 10:1 is the highest,because my 305 H.O. has the 9.5/1 pistons in it and runs.....ahem....ran fine and 87 was the factory recomended fuel. it really did run it's best with just 87 gas. well,this isn't going to be easy, I know. I've decided on a 350,for bang for the buck factor,it'll be easy to get 350 block. the idea here is to build something thats made from the a common casting and stock & aftermarket parts that's not the typical everyday 350. if I can run that compression up, I will. If I can use the lumpiest cam I can find and still keep an idle (I don't think the 84 computer will know the difference) I will. the whole reason that i really want to keep this a N/A engine is because I'm sort of new to building my own engine and I want real power out of it,not just go by what some crate motor says on the box. I shouldn't have to do to many external mods to a good motor. I'll put my same old carb (re-built) and V-belts and air cleaner and my headers which won't be craving attention coated with flat black paint. stock plug's n wires and all. a real engine shouln't mind the OE stuff. if I have it in before summer's over,you'll definitly see quite a few import wanna-be's down at the beach selling roses to pay for gas because I took all their money .......ah ha ha I'm eivil and I don't care!!

 

Aluminum heads allow for higher compression ratios without knock as well, since they don't retain as much heat as an iron head. I've seen some tricks people have done to raise the compression they can safely run. Ceramic coated pistons, removing sharp edges on pistons and in combustion chambers, and the like. Do a search, I'll bet you can find some of 'em.

 

thanks Mark, sounds like the extra $$ for Al would be a good investment in my case. wouldn't my Hedmen shorty 1 5/8" headers help to lower combustion temps as well? I used to have "power secrets" by smokey yunic it had some decent info on building the SBC up from scratch,that was in my car when it was stolen.....just like everything else. good thing my soul wasn't in the car,they might have pawned that too! believe me,I'm searching,you all have been a great help. I hope that other people here have found some of this info useful as well.

what kinds of compression ratios is everyone here running?

 

Sure, build a motor with exessive compression, then overcam
the hell out of it as a bandaid solution to your bad engineering.
You'll end up with a half sorta fast car that eats gas like its
going out of style.
Cam choice should be based on induction/ exhaust flow
and tuniing, as a complete system. With close consideration to the usable rpm range and intended use.
A good general guideline for compression ratio for a street
conventional smallblock is 9.5/10.0 :1 for cast iron heads
and about .5 cr more for aluminum heads. This assumes you can/will run 92+ octane gas.
Put your money into a set of the best cylinder heads that you can afford, and build from there. Cam choice should come up
further down your list as you design your motor.

 

In the lastest hotrod, there is a nova that runs 9NA with 11 to 1 ratio, pump gas , alot of duration and did the entire power tour. I believe it was a smallblock but I'm not sure.

 

Higher altitude can get away with more compression also. My engine is 10.3:1 and I only run 92 oxygenated pump gas with no knocking. As mentioned in my sig, Calgary is 3500 feet elevation and on race days we're usually over 5000 feet density altitude. I could probably bump up the compression to 11:1 and still be able to use pump gas here.

 

I'd go along with 10.30:1 cr at high altitude
but ya won't get much farther than that without compromizing the tune. (overrich, retarded timing etc) There will be a definate
limit on the practical compression you can run on a certain
octane gas. (without power compomize)
If you were at sea level, on a good day
(density altitude) with low humidity you'd need 100+ octane gas and would go noticably faster with optimized jetting and timing.
You could build a radical motor wirth 13.0:1 cr and putt putt
around town at part throttle with very conservative timing,
on 92 octane gas. But you'd never be able to run that same motor
at full throttle with optimum ignition timing and jetting for best power without knock.