If you were building a motor (289, 302) for street use would you shoot for 10:1 or 9:1 Here is what I have sofar: NOS 5.0 roller block Std. bore 68 C8OE 302 4V 54cc chamber 3angle valve job new stock size valves & new springs. 289 crank ground 10/10 New rods 5.315 in length Ford size crank end and .927 pin end Complete roller cam setup from a 1986 5.0 Reman 302 Duraspark distributor with steel gear Stock factory alum.4 barrel off a 1984 mustang Edelbrock 600 cfm carb with a 1" four hole spacer 4 speed toploader 8" rear with ls 3.40:1 gears So the motor will be a long rod 289
YES,,,, oh wrong poll,,,real vague here,,What are you doing, how do you want it to run, Street use mileage expected? oh yea the pocketbook thing. You get the right spec's, an 10:1 compression will run pump gas easy enough. Plan your attack carefully and make it all work together!
I say 9:1, cause you don't need as much cam to get the most of it, and you did say street use, but it helps to have more specifics, like what kind of street use are you talking about? Do you have to worry about emmisions?
i would go for 9.75 :1 just to be difficult seriously... i have 9.5:1 flat top pistons.. i dont know what my final compression ratio is.. i wish i knew, and i wish i could find out but i have heard.. the more compression you have, the more power... and if you arent running nitrous.. go high.. if you want to run nitrous.. go low
Have you ran a compression test? Cyl pressure is very important, too high cyl pressure, you will need a big overlap cam to drp the cyl pressure down to street levels. Read somewhere,, thats how they get 12.5 to 1 engines to run on 87 octane and run like a scalded dog. If I had to guess, your engine is in the 195 psi range????
Why would you want 10:1 compression in anything other than a race motor? Unless you are running aluminum heads, which are more forgiving with high compression ratios, 10:1 is going to require premium fuel all the time, which still may not be enough to prevent pre-ignition (knocking/pinging) without drastically backing off the initial timing. I am using '66 cast iron heads, and I built the motor with 9.5:1, which is the max I would want with those heads...
10:1 will be fine with iron heads. What pistons do you plan on using? You're going to need cu$tom pistons most likely to use that rod length. Why not pick up a 3.400" stroke crank for under $200, use those rods and make a 347? Back to your question...whatever you choose, definetly go with around 10:1 compression. Zero deck the block as well and use a .039" compressed thickness head gasket. Clean up ALL sharp edges in the chambers and the piston tops, then you might have to run a little colder spark plug and pull a little bit of timing out. A camshaft will make it or break it...call a cam company and get their opinions because no matter how much we think we know about cams, someone always knows more. Just picking an off-the-shelf cam is usually a waste of time, IMO. Seriously though, I would look hard at putting a 3.400 stroke crank in there instead of the weeny 289 crank. You'll like the 100 lb-ft increase in torque on the low end....as well as a VERY fast revving smallblock. 347's are very quick revving engines!
More compression=More power, but on todays fuel and for long term street reliability 9 to 1 and let the engine live.
Check out the Calculator on the KB Piston Web Site http://kb-silvolite.com . With "out of the box" Flat Top Pistons (6.5 cc piston Volume), 0 Deck, .039 Gasket, 54cc Heads, 4.060 Head Gasket Bore Opening, 4.030 Bore the Compression Ratio is 9.727. Increase the head gasket thickness to .050 (?stock) and the Ratio is 9.441 I like IT but I don't know what your cam specs are and that would be the deciding factor. The Calc. also has a Dynamic Effective Compression Ratio using Rod Length and Cam Timing.........Good Stuff
You didn't specify what 86 roller cam you're using, but I wouldn't go over 9 to 1 with either the base or GT/HO roller. I've got a std 30 over 5.0 in my 89 Ranger with an F4TE roller (this is the one used in all the 94-up pickup/vanm 5.0's and the Explorer/Mountaineers) with 1.7 rockers, ported E7 heads and it's got an affinity for 89 octane fuel with a 9 to 1 comp ratio.
