I am putting a Comp Cam 274, 230/236 and Edelbrock RPM heads, 70/170 on my 355 SBC. Question is which roller rockers will work best for me, 1.5 or 1.6. Please advise and thanks lots. Need power coming strong between 2500 and 5000.
The best way to determine your net valve lift is to look at the flow numbers of the heads. If the heads peak their flow at .400" valve lift, then choose the rocker ratio that puts you just above .400" lift. Edelbrock heads peak their intake flow at around .500" lift and exhaust at around .600". Its perfectly fine (if you need the extra exhaust flow) to use different ratios on the intake valves and exhaust valves if you care to fine tune the combo, but if your engine only needs the flow that is supported by .450" lift, adding more might not help power.
It also depends a little bit on the cam's duration. Higher lift rockers simulate a tiny bit more duration since they get off the seat faster. If your engine is suited for more cam, the higher lift will show more of a power increase than if it doesn't need more cam... but that's getting pretty picky since its a subtle difference.
I forget the optimum intake/exhaust ratio but its something like 75 or 80%, meaning you want your exhaust flow to be 75-80% of your intake flow. Many heads don't offer that which is why many cams have more duration in the exhaust side to help that flow.
Hi Curtis, gross cam valve lift is .487, heads flow 237 in and 173 exau. @.500lift(best flow numbers) per Chevyhighperformance mag. Will the 1.5 or 1.6 rockers work best, or should I ask, what differance in performance will there be between 1.5 and 1.6 rockers? These Edelbrock heads have a 73% dif between intake and exaust flow, not too bad! Thanks for the above answer by the way! Bob
Opening the valves higher than the maximum flow point of the head port is very beneficial. Remember the intake valve opens it maximum lift at about 105* ATDC and the piston is already slowing down after its maximum velocity point of 90* ATDC. By opening the valve farther, that means it hits the heads max flow point sooner and maintains that flow longer = more power.
That also explains why advancing the cam makes more power. The valve is opening closer to the piston max velocity point.
example= for the same duration cam, if you open the valve to max flow at say .500, it is only open for one degree at that flow level. If you open the valve farther, say .550, in the same duration, the valve might be flowing its max for about 12 degrees.... that means more power. :thumbup:
So .487" lift as measured with 1.5s would be .519" lift with 1.6. Provided your heads can take the .519" lift I'd say go for the 1.6s. The E/I flow ratio is about 73%, so you have a good cam choice with more exhaust duration. That should take care of helping the exhaust. Since they both peak flow aroung .500", taking it to .519" would go just over the top of the curve and maximize time at peak flow lifts. EDIT: Xntrik has a good explaination of why above ^^^
Since you are giving the engine more air, it will act like adding more flow which means a tiny shift up in RPM of the peaks (but very little... like 50 rpms) and in general a broadening of the curves leading up to and above the peaks. The peak hp (I'm guessing) should increase 5-10 hp with similar gains in torque. Low end torque will probably not be affected much at all; just at the peak and above.