Whats a safe CR for pump 89 or 91 octane gas with Iron or Aluminum heads? Car is street driven and will never see a track since there is nothing close by. Thanks

Depends a lot on your cam. Specs?

A wimpy cam with mild compression can put out a higher cranking compression than a big cam with high compression.

Im rebuilding the engine and havnt picked out a cam yet. I know im going with a hyd roller, not sure on specs yet. Im trying to figure out if I should go with aluminum or iron heads. The car will only be a street driving/cruising car so the cam will not be radical. I want a nice idle reliable engine and make decent vacuum at idle. It will also have EFI

Generally, aluminum heads will allow you to run a bit more compression than iron heads would for the grade of gas you want to use. Higher compression will also allow a more radical cam but you'll get a rough idle and low vacuum.

If you just want to cruise around as you said without a radical cam, then aim for a compression in the 9s - adequate for many nice street cams and you won't have to worry about detonation. Perfect with EFI too.

First decide how many cubic inches you want to run. Then decide how much cam you want. Then pick between iron or aluminum heads. You can plan compression around all of those factors.

Its bored .030 over stock stroke. Any suggestions on a hyd roller cam? For compression i should aim for around 9 with iron heads and around 10 with aluminum?

I still don't know what kind of engine you're building.

What engine?
I prefer to use closed chamber heads and flat top pistons setup with about 0.040" quench.
For the street, with premium pump gas, I would try to get about 9.0:1 for mild/economy/RV type engines using a small cam around 205-210 duration @ 0.050", 9.5:1 for mild street performance using a cam around 215-225 @ 0.050" duration, 10:1 compression if using a hot street cam in the 230-245 @ 0.050" duration, 11:1 for a street/strip or mild bracket engine with a 250-260 @ 0.050" duration cam.

Depending on the engine and heads, you may need dished or "reverse dome" quench pistons (when using open chamber heads) to get the desired compression ratio and good quench. The heads can also be milled to reduce chamber volume or small amounts of material can be removed from the chamber to slightly increase the chamber volume.

The main thing to avoid are pistons that sit too far down the cylinder bore.

You may want to focus on choosing your camshaft first so you can use the specs and calculate your actual compression numbers.

What options are you considering that will effect your compression ratio? I'm assuming pistons and cylinder head choice.

There are two basic types of compression:

1) Static Compression (this is the number that most use when BS-ing about their engine) Static compression numbers do not consider valve opening and closing events. Compression with valves shut all the time.

2) Dynamic Effective Compression does take into consideration the opening and closing of valves (controlled by the camshaft). It is important to note that DE Compression numbers are on a different scale that Static numbers. For example you can only run about 8.3 to 8.4:1 D.E Compression on pump gas before running into problems.

I built a pretty hot 347ci small block Ford a few years back. With aluminum heads I was running about 8.4:1 D.E (or a tad less) which resulted in about 210-212psi in each cylinder. I ran 91 octane California gas and it didn't ping, but I new was pushing it.

If you're talking static numbers anything in the 9s will almost certainly be fine on pump gas regardless of your cam specs. As you get into the mid to high 10s (static compression) calculating D.E compression is a good idea. Depending on your stroke, cam specs, pistons, combustion chambers, head material, and gasket thickness you might find that you have less actual compression that you thought.

I once calculated the D.E compression on my last 440 combo and I was surprised that with the cam I was using I had far less actual compression that I thought. Even though I had a Static Ratio of 10.9:1 (flat tops with milled heads) my D.E ratio was in the mid 7s if I remember correctly. I had been running 91 octane gas in that 440 and probably could have easily ran 87 or 89 without any issues.

There are D.E compression calculators available for free online. You'll need several specs including bore & stroke, head gasket thickness as well as actual head gasket bore diameter. Full cam specs, valve relief specs (or other piston head info) as well as how far your piston is in the hole at TDC are also needed to calculate true compression.

Good luck.
-Dan

Ah sorry the motor is a 340, .030 over, stock stroke crank, the piston comes out of the bore .025 at tdc.

Perfect, I'd run the open chamber Edlebrock aluminum heads. I run them and my pistons which are KB243 stick out .029 out of the bore and my car runs fine on 91 octane fuel. I figured it to be near 10.5-1 or there abouts.

I've driven my car all over the country in extreme conditions, it's been tested with many different fuels from all over the country. So I'd say I have tested that combo more than most here. it's a good one,imo.

What cam are you using? With what i found with one rod lookingcrank bent, crank journals pretty worn and needing new pistons anyways im think im going to just buy a stroker rotating assembly withe the forged crank and pistons. Questions is should i go with flat top pistons or play it safe and go with dished. Still need to figure out on a roller cam to determine dynamic compression but i have a couple in mind by lunati and hughes. And i will be going with eddy aluminum heads.

Anymore input on flat top or dished pistons?

I prefer tiny chambers with a dished piston, imo promotes better, less turbulant flame travel. I've built smallblocks with dome/big chamber, flat/medium chamber/ & dish/small chamber, I always get away with more compression on less octane with dish/small chamber. Cam overlap is also crutial!

thanks. Okay so the plan is to go with a 416 stroker w/ forged dished pistons, Eddy aluminum heads that have 63 cc combustion chambers. For cams there are a couple I was looking at the huges http://www.hughesengines.com/Index/produ...mp;partid=26506
or Lunati http://www.lunatipower.com/Product.aspx?id=1549&gid=289

Any opinions on those two cam choices?

What is the vehicle, application, gearing and stall.
for a 412" those cams are pretty mild, but should produce a really good torque curve for a daily driver, or truck.

Its a 72 challenger, not daily driver but only street driven, stall will be replaced so im ooen on the gearing will be 3.55:1

???

Both of those cams are pretty mild. To me those two cam choices look like a waste of money to buy in a roller, when a hydraulic flat tappet of comparable spec is so close in lift. For a 10:1 aluminum headed 416 with a 3.55 gear and a half decent stall I would be looking at something more like lunati voodoo 60403.

http://www.lunatipower.com/Product.aspx?id=1545&gid=287