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New guy here. Great board with a tremendous amount of experience with my favorite subject. I'm hoping to benefit from a little of that experience. I'm about to choose pistons for my 289 buildup, and want to run as much compression as I can.

So, here's everything that I can think of that will affect my choice. How high would you go?

'68 289, .030 over, will probably go with KB hyper pistons unless another dish/dome works better
'69 Windsor heads, pro ported w/ 1.90/1.60 Ferrea valves. Milled to around 52-53 cc (accurate measurement coming soon).
.040 quench
C3AED forged rods, polished and shotpeened, ARP bolts
Cast 1M crank shotpeened
Balanced to 1/2 gram
Comp Cams XE274 hyd flat tappet, anti-pumpups, roller rockers
Edlebrock F4B and Holley 650 DP
1 5/8 long tubes to a short, straight 2.5 inch exhaust w/ straight thru mufflers
Motor is going into a 2400 lb car w/ wide ratio Toploader and 2.88 rear gears (road racer).

I'm running at sea level, can get 94 octane gas within 3 miles and 100 octane within 6. (I'd rather stick with the 94, in case I get too far from the 100 station).
No A/C, so I don't see too many 90 degree weather cruises in my future. Car is strictly a weekend cruiser whose biggest exertion will be 15 to 20 minute exhibition sessions at open track day maybe twice a year.

Thanks in advance!
 

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Id stick to 10:1. probably coul go higher with the 94...But Why push it to the limits if your going to get predetonation? If your going to always run the 100 you can probably get away with alot higher without a problem. Stick to the 10:1 and youll be happy. Good Luck
 

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You can reasonably expect to get no more than 3-5% torque/horsepower increase per full point increase in compression. I would go with no more than 10 to 1 on an iron head motor that is street driven to any extent, with the combo you quoted. Personally, for street duty, I'd shoot for more like 9.5 to 9.75 and bump the ignition timing up until I got maximum output. Racing is a different game and everybody runs closer to the edge.

With everything tuned perfectly, and with consistently high quality 94 octane pump gas, you MIGHT get away with 10.5 in that light car with low axle gearing and conservative ignition timing. However, with even a little preignition occuring, you'll lose far more than the 2% or so you gained with the 1/2 point increase. If you get into detonation, you'll make very little power and it's only a matter of time before you break something.

The potential to MAYBE gain 2% (6 horsepower on a 300 horse motor) with the extra 1/2 - 3/4 point compression is not worth the downside risk of losing even more horsepower than that, or damaging the engine.

With your final combo assembled, run a static compression test (spinning the engine with the starter, all spark plugs removed). If you have no more than 160 lbs. pressure, you'll likely be okay. If you're pushing 170 lbs or more with those iron heads, you're looking for trouble. 100 octane territory. If you're just a bit high, sometimes you can find a thicker head gasket that will add enough volume to solve the problem.

Steve Amos
 
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