[hgerhardt]

My 428 with Ross flat tops and zero-decked block and 72cc E-heads is 11.4:1 and is happy with 91 octane. Cam is 248/254 @.050 and 114 LSA.

So much of this is dependent on cam timing. It's the dynamic CR that's important, not static. Dynamic CR is determined from when the intake closes on the compression stroke. The later the intake closes, the less dyn CR you'll have.

An example: this engine with the same cam originally had 87cc E-heads (9.7:1-ish) and I accidentally figured out that it ran better on 87 octane than 91. With the 72cc heads, throttle response is sharper, and I picked up around 1/2 sec in 1/4 mile. Much happier motor!

[torqjunki]

Originally Posted by hgerhardt

My 428 with Ross flat tops and zero-decked block and 72cc E-heads is 11.4:1 and is happy with 91 octane. Cam is 248/254 @.050 and 114 LSA.

So much of this is dependent on cam timing. It's the dynamic CR that's important, not static. Dynamic CR is determined from when the intake closes on the compression stroke. The later the intake closes, the less dyn CR you'll have.

An example: this engine with the same cam originally had 87cc E-heads (9.7:1-ish) and I accidentally figured out that it ran better on 87 octane than 91. With the 72cc heads, throttle response is sharper, and I picked up around 1/2 sec in 1/4 mile. Much happier motor!

Can you post up details on the 248/254 cam your running please sir? Working on similar combo.

[hgerhardt]

Originally Posted by torqjunki

Can you post up details on the 248/254 cam your running please sir? Working on similar combo.

Here you go. As you can see, it's ground 5 deg advanced, so I installed it straight up.

Also, all of my experience with this engine is with the older E-heads. Both my 87cc and 72cc heads are 27+ years old. In fact, other than replacing the valve springs a few years ago, It hasn't been apart in 25 years.

[torqjunki]

Thanks!! I like it. I don’t see LSA

[hgerhardt]

Originally Posted by torqjunki

Thanks!! I like it. I don’t see LSA

It's there on the cam card, you just have to do some math to see it. (109+119)/2 = 114.

What I like about this cam is how well the engine likes to rev. 4" stroke (434 cu in.). Enough torque to get wheelspin (with 255-section tires) by hitting the gas at 55 mph in 2nd gear (which is pretty ridiculous on the street!). So it makes good torque plus has that nice, high-RPM lunge (shift at 6500).

And even though these 72cc heads have the old combustion chamber, max power happens at 31 deg total timing. With the old 87cc heads, I needed 37 deg. So good quench combined with high compression seems to work here. (11.4:1 on the 72cc and 9.7:1 on the 87cc)
Note that I did not just swap heads when going from 87 to 72. I also decked the block to make it parallel to the crank but also got the pistons slightly above the deck (forgot exactly how much). So I increased compression plus quench at the same time.