What is the maximum static compression ratio with aluminum heads?
 

I realize there are many different variables going into an engine build. Equally there are many variables involved in determining a good compression ratio. I'm starting to piece together a 462 build for my '79 pro-touring Trans Am. I'm certainly going to use a FAST EZ EFI system in conjunction with an Edelbrock Torker II intake manifold. I'm eyeing a completely forged bottom end from Kaufman. My question is what is the recommended static (and dynamic) compression ratio to shoot for using aluminum heads. I've read plenty regarding the heat sink properties of aluminum and the diminished performance it causes over iron. Consequently, additional compression is warranted to match performance over a similar flowing iron head. I'm wondering if I should aim for the 72-74 Kaufman D-port / Edelbrock or the lower compression 85-87 of the same units. I'm also considering running a supercharger in the future which may factor into the decision. Any insight or suggestions? I also have access to a set of ported 6x-8s and untouched 4xs.
Thanks in advance, and sorry for being long-winded.

Sounds familiar! And yes, too many variables to state any 'magic' number, every build is different. The basic rule of thumb does apply though, add a point for aluminum heads. Many these days are shooting for roughly 10.2 - 10.7 on a performance street build, though it requires careful planning and execusion when you lean towards the bigger numbers. For the most part though, SCR is a small portion of the formula.

Adding forced induction changes a lot of things, and it's best to plan for it AND execute it at the same time.

My suggestion is to either follow a proven build or contact a professional builder for advice. You don't want to rely on hear-say (or 'here-say') from internet chatter, especially when your dumping that kind of coin into a build.

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I ran 11.1:1 on 89 octane and 38 degrees of timing with a 350 SBC in a Nova. The trick was that I had a great cooling system, Brodix RR200's with CNC profiled chambers, excellent ignition and a pretty stout cam. 298/302, 256/[email protected] .562/.562 lift on a 108 L/S.

I tried to get it to detonate and I couldn't. I once put it in 5th gear, dropped to around 1500 RPM and floored it up a big hill in the middle of summer boooooooooooooooggggggggggggg. I figured if that didn't make it detonate, nothing would. It never did. Ive had the heads off of it and the pistons looked fine. No signs at all of anything bad. I often wondered if I could have gotten away with another full point of compression. I still have the engine so maybe someday ill find out.

the one lingering problem I had was that it would occasionally after a long drive like to run on for just a second. I always shut it off with the clutch though. I never did figure that out.....

Safely. 10.50-1
You can run more depending on the can profile.

I run 10.75-1 with a 110 LSA for ET's. and no issues.

Have seen upwards of 12-1.

Coating piston tops allows more compression but can sacrifice heat in the chamber which makes power.

Lot of thought goes into a combo for max kill.

The super charger "trumps" the entire deal. You will have to take out some compression to effectively manage any boost, how much depends on the final boost the engine will see at peak load?

You can make PLENTY of power on low octane fuel without one, and still have a engine that is relatively "mild" at a glance, and conservative drivetrain parts.

This is one of the best attributes of running a Pontiac engine right to start with.

You can get-er done with less of everything, and still have a dead solid reliable set up that will provide many years of enjoyment without a lot of wrench turning on it.

My engine uses a 76cc version of the KRE heads, right at 11 to 1 compression, and easily manages 87-89 octane fuel with zero issues.

Since many folks think that I operate "outside the box", these engines run fine down around 10 to 10.5 to 1 compression, just giving up a slight amount of power and rougher idle quality.

In any case, what you want to do is easy, choose the cam wisely, and keep the quench distance tight, zero decked block, etc......Cliff

@ PDude: how does coating the top of the piston take heat out of the chamber? I thought it kept more IN the chamber (in addition to some incremental protection).

Not fighting, just asking...

Prevents from heat-soaking the crown of the piston, and pushes it out the exhaust...

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I appreciate the feedback and I realize the question is slightly loaded. I will most likely be utilizing the Kaufman's experience to build this engine. I figure I can bleed off excessive compression with an agressive cam profile and still make decent power on pump gas with higher compression. I'll probably go with the KRE or Edelbrock 85-87cc heads to allow extra room if and when I add boost. My rough math put compression around 10.3 with the 85cc KRE D-ports.

Thanks again and any additional info is always appreciated.

With boost, you want lower SCRs, which allows targeting a specific DCR. No way can you apply any boost on pump gas to a 10.3 SCR engine. I think like 9:1 is roughly max target SCR for boosted engines, and lower to allow for more boost.

'Bleeding' off compression with a cam is a farce for the most part too, fyi...

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Agreed.

We are NOT trying to use a bigger cam to "bleed off compression", at least just so we can manage the larger cam. The purpose, or goal, is to choose the cam so that the engine makes about the same dynamic compression thru peak torque, as a lower CR engine with a smaller cam in it. We also use a wide LSA cam, to spread out that compression (also flattens out the torque curve at the same time) to produce more average power, and help with idle quality and top end charge.

The end result is that the higher compression ratio engine with larger cam that idles about the same, similiar off idle power, etc. The larger cam makes for more upper rpm power, and more average power, all else being equal.

The contraversy, and debates rage on about the ideal CR for pump gas with these engines, and there are several schools of thought on the subject.

All I know for sure is what we do here, and we're not done testing in that area.

My next engine will step things up another notch. Haven't ironed out all the specs as of yet, but I'm looking at 11.5 compression with about 10 degrees more cam than I'm running now, on a 114 LSA....Cliff

being this is in the street section,a lot depends on the quality of gas you can buy from any pump in your area.I know our calif 91 is WAY diff from the 93 that can be bought in other states.Also the air temps and the type of driving like stop and go traffic in 100 degree heat.Just too many things to take into account.Tom

I know with my engine combination it has ran on 87 octane with no problems in any temperture.85cc KRE heads on a 462 with "Old Faithful" cam.Usually I buy 91 octane at the pump for piece of mind.

What's your SCR?

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My motor manages 10.7:1 on 91 and 93 just fine. Haven't pushed it to far with octave limits yet. Makes. A butt load of power everywhere. Pretty happy on how it turned out.

Hey cliff, who is doing the machine work on your next rotating assembly?

HWYSTR455, would you happen to have one of those color boost charts for iron heads?

I pulled that chart right from the Procharger website, and honestly didn't notice it said for aluminum heads until you mentioned it! But in general, subtract a point for iron heads, would give you a rough idea. Maybe in other areas on the Procharger site you can find guidelines regarding iron heads and SCRs, I haven't looked in a while.

There are places on the 'net that give general guidelines for calculating DCRs, and supercharging engine theory as to what DCRs are acceptable when designing boosted engines, you could search it. I believe the general rule of thumb is 16-18:1 DCR is max on pump gas. The buick guys seem to have broken that rule and gotten away with it though, but it's a ragged edge.

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"Hey cliff, who is doing the machine work on your next rotating assembly?"

Not sure, it's still at least a year off before I start on it?

Since I've completely exhausted all the Pontiac cranks around here, I'll buy the entire spinning assembly balanced and ready to go. It will have a 4.21" stroke forged crank, stock length Pontiac rods, probably forged "I" beams, and a forged piston with two valve reliefs.....Cliff

Probably a good idea. You never know what you are getting sometimes. Might be someone's headaches.

Agree'd. I'm done with anything used from anyone at this point, and rest assured, I'll be going over the new stuff with a microscope!.....Cliff

11.25 to 1 with KRE D-ports. 30 degrees timing. flat tappet 296/300@adv and 264/[email protected]" on a 108 (installed at 108).

I do feel that I'm right on the edge. I am thinking about getting the 74cc's opened to 80cc.