Hello everyone!

I was at the machinist's shop dropping off a set of big block heads for a valve job, and he told me he had a deal I may be interested in. He had a rebuilt and upgraded 351W that someone dropped off to build, didnt come to pay for, and finally abandoned it. It's been in the corner for a year and a half, so i got it for the cost of parts. it is an F4TE roller block.

It has a set of forged pistons, it's zero decked, the pistons look to be flattops with small reliefs cut in them to account for valves, and I was told that with a set of 64cc heads, the compression was 10.5 to 1.

I'm planning on this being a street driven engine, carburated, and I have been looking at aluminum heads for it. I'm quite pleased with the AFR heads, but the two options are a 60cc head, or a 72 cc head. From what i understand, the 60 cc head will bump my compression ratio higher, and the 70cc heads will lower it. But i guess my question is, how much higher or lower will it go? Will the 72 cc heads get my compression ratio to around 10 to 1 or 9.5 to 1, or will it dip lower? will the 60 cc heads push me above 11 to 1? My build that was in my head was hoping to run on pump premium gas (91 octane or 93 octane) which seemed like 9.5 to 1 ratio would have been ideal.

If anyone would have more info on what these heads would do for my compression ratio, I'd love to hear it, I'm a bit over my head, i think.

 

8 cc's in a 351 will probably change it by a half point at best. I did the math and the 72's will net you about a 9.5 to 1 ratio.. Put in a wide LSA EFI cam, high rise dual plane intake topped with a vacuum sec 650 carb and you'll have a torque monster. If you go higher on the ratio, you'll need a narrower LSA cam to bleed off some pressure and less timing to run it. Otherwise with the EFI cams you'll need to run pump premium gas in it.

 

What vehicle will it go into? Manual or automatic? 2.73:1 or 4.11:1 gears?

60cc = 11:1. You can generally run higher compression with aluminum heads but 11:1 would be pushing it on pump gas. 10:1 is probably okay, 9.5 should be no worries.

Check the dynamic ratio too with some different cam specs. 8.0:1 or below dynamic is the rule of thumb (7.0:1 in a heavy hauler) with iron heads.

https://www.uempistons.com/index.php...tors&type=comp

 

going in a 3500 lb truck, with a manual overdrive trans, and probably a 3.73 gear.

its going to be street driven, but on the edge of "probably shouldnt be street driven"

 

You can't run any more compression with aluminum over iron heads. Once the engine's hot, it's hot. I'm running 10.4 to 1 in my 331 with aluminum heads and it requires no less than 92 octane. Feed it 87 and it'll self destruct. Did that once when I first got it together 13 years ago (that's what was in the tank) and it sounded like it was crushing gravel. Ain't fed it nothing but 91-93 mixed since. The cam's a Z303 with a 112* LSA if I recall, I've run the same ratio in a 390 with narrower LSA cams and got by feeding them both (two different motors) 87-89. BBoth had the stock iron heads.

 

yeah, i've heard the argument about that you can run more compression on lower octane gas, even if it is true, im not going to take the chance. I want this thing to get a steady diet of 91-93 octane fuel.

 

Well, with aluminum heads and dynamic CR of 8.0:1 or lower you should be able to run 87 all day long.

 

I guess you lost me on the term "dynamic compression ratio". care to enlighten me?

 

If both valves are closed when the piston starts moving up from BDC, the compression ratio is the static calculation (total volume:combustion chamber volume). But since valves don't close instantly due to lobe profile and timing, the achieved compression ratio is always less due to escaping gasses. That's the dynamic CR.

 

The dynamic is more affected by the valve overlap than when the piston starts moving up the cylinder. By that time the intake valve has closed. The time when the intake and exhaust valves are open at the same time (exhaust closing, intake opening) is the overlap and is affected by the LSA width.

 

Dynamic CR is a function of static CR plus rod length and when the intake valve closes, it calculates where the piston is in the cylinder when the intake fully closes and how much it's actually compressing the a/f.

Intake centerline and duration determine when the intake closes.

 

All that is only a part of the equation. You also need to calculate (if that's even possible) the airflow moving through the intake and carb, which constantly changes with rpm and how that airflow affects the amount of air actually getting into the cylinder. Even after all that, the ignition timing also affects what octane fuel is required to prevent detonation or preignition.

 

Whatever, with 72cc aluminum heads and 280°+ cam (i.e. street driven, but on the edge of "probably shouldnt be street driven") in a 3,500lb vehicle that engine will run on 87 octane no worries.