Timing question concerning Edelbrock RPM FE heads

[Joe-JDC]

I spent the morning at a dyno session on a 390 FE with my ported Edelbrock RPM heads, my ported Offenhauser Port O Sonic intake manifold, and a relatively low compression build.  Parts were from 1986, (Forged Seal Power pistons, 5/16" rings, etc.) and solid flat tappet camshaft for street duty.  Compression worked out to 9.8:1, and dynamic was 8.05.  Carb was a 750, but still had 2.1" vacuum at top of pull.  Tried timing down where most aluminum heads like it~36*, made more power at 38*, increased to 40*, and it made more power still.  Dropped timing back, and lost power, so went back to 40* and it repeated highest power pulls.  First for me.  Anyone else had a similar timing requirement with your RPM heads?  Camshaft was degreed, advanced, etc., damper correctly marked.  Joe-JDC 

[blykins]

Makes sense with the lower compression.

[plovett]

Yes.  About 20 years ago I had a 428 dynoed, with Edelbrock heads, and it liked a lot of timing.  I hesitate to answer because it sounds dubious for sure and I don't want to argue what happened.  But I wanted to share my experience.   The key points in my mind were that it was low compression and dyno-ed on race gas.   The compression ratio was about 9.2 or 9.3 to 1.    I don't remember the exact octane of the race gas.  110?  I don't know much about race gas, but that was what they had at the dyno and we used it to "be safe".   Cam was solid flat with 276/284, 242/246.   Anyway, the power kept going up as we increased timing, all the way to 45 degrees total.  I got kind of freaked out at that point and stopped.  As I remember I backed it off to around 40 degrees driving on the street with pump gas.  It seemed to work fine.  For sure we should've run it on the dyno with pump gas, too.  I don't remember the quench distance on that combo.  It could have been fairly large?

I know the first thing that will be mentioned is, was the balancer reading right?   I think it was.  I had lots of very knowlegeable engine guys around helping and we checked those things.   I used the same heads, similar but different cam, same balancer, same pointer, same distributor, etc. on a later rebuild with a true 10.5:1 compression and dynoed with 91 octane pump gas, it made best power at 36-37 degrees. 

I think the low compression and high octane of the original combination as dynoed wanted a lot timing.  Also possibly a suboptimal quench distance?  I don't know for sure about that.  I know people argue about the effects of octane, burn rate, and such.......

pl

[SMA390]

Just curious ...what kind of HP/TQ numbers are you seeing with those heads and intake

[Barry_R]

Timing is combination dependent.  All the guidance, computer models and directional trends in the world go away once the engine is running. 
At that point you just "feed the dog what it wants to eat".

Back when they allowed us to run EFI and engine management at EMC I used to develop by making a bunch of dyno pulls at one degree increments and then stich together a timing curve that blended the "best" at each RPM point.  Ended up with "S" shaped timing curves that looked pretty weird sometimes - pulling timing back at torque peak and then adding it back in at higher RPM.

[Joe-JDC]

Just curious ...what kind of HP/TQ numbers are you seeing with those heads and intake

 ~430tq/460hp as installed in car with no spacer, 750vs Summit carb, stock crankshaft, stock rods with new bolts, 0035 cylinder wall clearance, .015" in hole, running water pump,   nothing fancy, just plain old school 390 shortblock with new heads and P-O-S intake port work.  Joe-JDC

[Stangman]

very nice Joe

[turbohunter]

and P-O-S intake port work.   Joe-JDC

LOL

[427John]

Since fast burn chambers require less timing to make peak power and more conventional "slow burn" chambers require more,does it make sense that for a given CR something else that would stabilize and possibly slow the burn rate such as high octane race fuel would require or at least tolerate more timing?

[winr1]

 ~430tq/460hp  ... nice ..

If ya had a .041 head gasket and piston .015 in hole would that account some for 40 total timing ??

Or same piston, 0 deck, same combustion chamber with camshaft  that had compression worked at 9.8:1, and dynamic was 8.05 ??

Or same piston, 0 deck with larger combustion chamber that had compression worked at 9.8:1, and dynamic was 8.05 ??


Or a dish piston that was 0 deck and still had compression worked at 9.8:1, and dynamic was 8.05 ??

What octane fuel ??



Ricky.

[plovett]

I also imagine there is a lot more variation in the formulation of race gas than there is in pump gas.   That is, variation of factors other than octane, that affect the burn rate. 

paul