Octane and compression?


Well-known member
If I build my 65 200 engine with a CR of about 9.5:1 will I be able to run anything lower than 93 octane? So since gas prices are so high what would happen if I ran like 87 octane?
8) the detonation resistance of an engine depends on many things, compression being only one of them. cam timing, ignition timing, fuel quality, engine load, atmospheric, etc. all come into play. if you select a cam that has more valve overlap, you can run higher compression and a lower octane fuel as the cylinder pressure is lower even though static compression is higher. larger rod length/stroke ratio also allows you to use lower octane and higher compression. with 9.5 compression you can run 87 octane, just reduce ignition lead a bit.
compression, cam timing, ignition timing, fuel quality, engine load, atmospheric conditons...you named 'em! Here are a few more aside:-

About 25 years ago engine builders started to find out ported and polished heads on stock cammed engines were more prone to detonation than stock headed, stock cammed engines. They also found the flame front travel was critical in getting the engine to handle poorer grade gas that was only available, not the 120 Sunnoco or 105 octane that you could by in the 60's.

The detonation resistance of an engine is related primarily to three things:-

1. Charge temperature. Old MGB's with exhast heated intake manifolds could detonate very easily on even 89 octane because the heating could loose 6 points of detonation resistance. That 89 octane was like an 83 octane because of the heating factor that made the fuel closer to its point of burning.

2. Mixture motion in the combustion chamber. Today, auto makers stuck with non-pentroof chamber 2-valve heads use mixture motion to elevate the compression ratio and thus maximum torque and power. The slightly rough surfaces and pockets around the valves are there for low end torque and swirl. Smooth them out, and you'll loose power and be more prone to detonation.

3. Plug position, Some engines, like Pontiac 350/400/455's would produce more power with a lower compression ratio because the flame front was going from the exhast to the intake, not from the intake to the exhast. When extra area was presented to the flame by deapening the piston or chamber, the previously screwed up flame front could then work in the right direction. This is why pistons in NASCAR SVO block 351 V8's are relief cut to allow for flame travel. A 1970 TransAm Boss 302 and a 2003 Aussie AVSECO racer run the same combustion chamber and valve angles and valve sizes, but the pistons and chamber volumes are all altered to optimise flame front propagation.