Edelbrock RPM Head CC Selection on Pump Gas

[LR1970]

Hi everyone,

I am trying to figure out which Edelbrock head to choose. My car is a 72 Roadrunner with 440, 727 and 3.9 rear gears. I am using the revised purple shaft 509 cam, Edelbrock RPM inlet, Holley performance 750 vac carb and with headers and 3in exhaust. I know there are better cams out there but this is the one I have to use at the moment. I will be using pump fuel with 98 octane here is Australia.

I have calculated my compression as follows.

88cc = 9.84
84cc = 10.19
84cc = 10.39
75cc = 11.11

I'm thinking the 84cc will probably be the way to go. I'm wanting to bolt them straight on without any machining to the heads and inlet manifold.

Thanks in adavance

Leigh

 

[mentor70]

if your engine is working now, i would see what your cranking cylinder pressure is now on the current setup. my suspicion is that the cranking compression is around 145-155, if so you could prolly get away with the 75cc heads. the original .509 was ment to run with an 11.1 compression.
the alloy heads will pull alot of heat out of the engine, and if i had your fuel quality here, i would deff. run my cylinder pressure close to 200 psi.

not knowing which pistons you have makes me question just how much compression you actually have, so i would make the call on cylinder pressure VS calculated compression ratio.

 

[MOPOWER871]

you guys have 98 octane out of the pump , nice ! whats the price of gas down under ?

 

[Garys1969RR]

I'm running 13 to 1 C/Ratio on my 451 with a 509 cam. That cam likes compression, tall gears and Hi Stall T/Converters. I mix fuel to get 98 octane, and have no pinging issues at 5600 feet. I also run .037" of squish which helps with all around performance, and allows for higher cyl pressures with out getting any pinging. I would go with the 75cc heads, and use zero deck clearance with a .040" head gasket. That will get you good squish/quench. Gives the engine very good throttle response, as the mixture burns better on every firing stroke. I get really good off idle throttle response from 1200 RPM and up. Run at least 20 degrees initial timing, 36 to 38 total.

 

[moper]

The 98 octane down under is the equal to the domestic 91-92 octane. It's just normal pump hig htest, but the octane is measured in a different way.
ersonally, I would not just bolt on any aftermarket head assembly without having a quality shop check them out. But in terms of choice for this application I'd use the 84cc.

 

[Kern Dog]

you guys have 98 octane out of the pump , nice ! whats the price of gas down under ?

The octane numbers used outside of the USA are calculated differently. Chances are that the OP is referring to a MON (Motor octane number) rather that a RON (Research octane number) In the USA, pump gas octane ratings are calculated by adding the MON and the RON then dividing them by 2 for an "average". The MON is often a higher number. It is not a hard and fast rule, but the MON is often as much as 8 points higher, meaning the 98 octane might be equal to 90-91 American premium.

The revised 509 cam his a wider LSA, allowing better idle quality and a slightly improved and broadened power curve. I agree that to avoid detonation, try to keep the cranking compression under 175. If the engine has flat top pistons at zero deck, the Edelbrocks with 84 cc chambers and a .039 Fel Pro gasket will be higher than either of the numbers that you listed. I had 11.22 CR when I was at .012 below deck using Edelbrock heads!

Some will say that the 509 needs more compression. Well, it does run stronger with more CR but they will still make power with less CR and NOT knock or ping as much. Jim LaRoy did a build with a 7.5 CR 440 with a 509 and made almost 400 HP !
The 88 cc heads are open chamber. I'd avoid them because they do not have "quench pads". These do increase compression but also provide swirl/quench which is great for power. No matter what, heed the warning:
Do NOT build the the engine to run right up to the edge of detonation.
It makes far more sense to be at least 3/4 of a point below that point. The trouble is, it is difficult to know exactly where that might be. I knocked with 11.2 with aluminum heads. I tried backing off the ignition timing, changing to a bigger cam, carburetor tuning...It still knocked. I switched to thicker head gaskets, lowering the CR to 10.07. This allowed me to optimize the spark timing. It now makes more power than before even with less compression.

 

[Garys1969RR]

Hi LR, hey do you know how far below deck your pistons are at now? My 77-440 had pistons that were .120" or more below deck, plus they were dished. What is the deck clearance for a 72-440? This will play a big role in final head choice, as well as thickness of the head gasket.

 

[Garys1969RR]

Using the United Engine (KB Pistons) calculator, with 84 cc heads. .039 head gasket, zero deck, and 4 cc for valve reliefs, I get 10.22 to 1 C/Ratio. 75 cc heads get you to 11.1. So your figures appear pretty close to me if your pistons are indeed at zero deck clearance. I use a gasket diameter of 4.4.

 

[IQ52]

The octane numbers used outside of the USA are calculated differently. Chances are that the OP is referring to a MON (Motor octane number) rather that a RON (Research octane number) In the USA, pump gas octane ratings are calculated by adding the MON and the RON then dividing them by 2 for an "average". The MON is often a higher number. It is not a hard and fast rule, but the MON is often as much as 8 points higher, meaning the 98 octane might be equal to 90-91 American premium.

The revised 509 cam his a wider LSA, allowing better idle quality and a slightly improved and broadened power curve. I agree that to avoid detonation, try to keep the cranking compression under 175. If the engine has flat top pistons at zero deck, the Edelbrocks with 84 cc chambers and a .039 Fel Pro gasket will be higher than either of the numbers that you listed. I had 11.22 CR when I was at .012 below deck using Edelbrock heads!

Some will say that the 509 needs more compression. Well, it does run stronger with more CR but they will still make power with less CR and NOT knock or ping as much. Jim LaRoy did a build with a 7.5 CR 440 with a 509 and made almost 400 HP !
The 88 cc heads are open chamber. I'd avoid them because they do not have "quench pads". These do increase compression but also provide swirl/quench which is great for power. No matter what, heed the warning:
Do NOT build the the engine to run right up to the edge of detonation.
It makes far more sense to be at least 3/4 of a point below that point. The trouble is, it is difficult to know exactly where that might be. I knocked with 11.2 with aluminum heads. I tried backing off the ignition timing, changing to a bigger cam, carburetor tuning...It still knocked. I switched to thicker head gaskets, lowering the CR to 10.07. This allowed me to optimize the spark timing. It now makes more power than before even with less compression.

Almost nothing! At 7.8:1 compression that 440 made 449HP!

Sincerely,
J. La Roy

 

[Kern Dog]

Thanks, Jim. I knew you surprised many with that setup !

 

[LR1970]

Thanks for everyone's replies, so 98 octane is not really 98! So I wonder how low our 91 ethanol blend really is?

I'm using ICON pistons, IC9953+30, the valve reliefs are +5.6 and I did my calculations at zero deck height. Gasket of .38. I have also been reading that aluminium heads absorb 1 point of compression, so 11 to 1 becomes 10 to 1, any opinions about that?

Thanks agin

 

[IQ52]

Old wive's, er, mechanic's tale.

http://www.hotrod.com/how-to/engine/ccrp-0602-iron-versus-aluminum-cylinder-heads-test/

http://www.superchevy.com/how-to/83858-iron-vs-alloy-engine-heads/

These tests were more about whether or not iron made more power than aluminum with the same airflows and compression ratio because of the aluminum's heat transfer rate. As a side, one of the tests tried to ascertain the detonation resistance and mentions their test observations......no detectable difference at their compression ratios.

More importantly I watched Kern Dog's struggle with compression ratios and his aluminum heads and I completely agree with him, don't take it to the edge. I've built a lot of pump gas 440s and 500s in the 10:1 range that make 590-650hp.

We just finished a 10:1 compression 383 that ran aluminum heads and a custom solid street roller and made 483 lb-ft and 531 hp........ah, er, well, it was a small block Chevy.....but you get the point, don't go for high compression unless you are class racing.

 

[mentor70]

im running that same pistons in my latest 440, i had my Stealth heads CNC ported by Hughes engines. i had the chamber CCs set and 78CCs

thats what my local machine shop and i thought would be the limit on 91 Cali piss fuel.

 

[IQ52]

im running that same pistons in my latest 440, i had my Stealth heads CNC ported by Hughes engines. i had the chamber CCs set and 78CCs

thats what my local machine shop and i thought would be the limit on 91 Cali piss fuel.

I've got you at 10.7:1 compression if you are at zero deck. Is that true? If so, what cam are you running? Because, that is pretty edgy without some cam duration.

 

[mentor70]

As i recall im .005 below deck with a 243 @ .50 Lunati voodoo roller.

theres also a little method to the madness, if i end up moving back to my home state of Utah, the 4200 feet of elevation will nullify the added squish.

 

[Kern Dog]

I still say play it safe. What are the gains of running such a high CR? Are they worth the risk of detonation and piston damage?

 

[mentor70]

big torque and a crisp snappy throttle response if the tune is right!
My dynamic compression is around 9.2 to 1 with the cam im running.