How can I drop my CR?
 

I am running 11-1 CR on Alum Performer RPMs. I am considering either stroking my engine to 393 using a Scat crank (this allows me to keep the 5.956 inch forged Eagle rods) or adding the new Novi 1200 for the 351W. Either way, my current CR is too high for either of these applications. I know I can get new cyl. head gaskets from Cometic that have a compressed gasket thickness of .080 and that this can drop my CR. What other options are there if I need to drop a full 2 points in the event I go with the compresser?

Just a quick note of caution. I bought a stroker kit from Speedway which has the scat crank, h-beam rods and cast flat top pistons. This was advertised as a 10:1 compression piston and may very well have been in a 351 engine, but on the 393 it worked out to 11.9:1. Massage the numbers before you buy your pistons to be sure you get the compression you want. The more you stroke it the higher the compression for the same piston. Otherwise no issues with the scat crank or other components.
h dog

Mach 1,

I am running 11:1 compression on my stroker that is stroked out to 416 and have been since 1996. I have never had a problem of any kind. Just the usual crappy California gas and I need to use racing gas if I plan to run it hard.

Ron

here is the problem
 

I am already at 11:1 and if I up the displacement to 393, I simultaneously increase the CR by about 2.5 points. You and I are running 11:1 just fine right now. But 13.5:1 is just asking for trouble.

I have "pop up" pistons and really small, 63 cc, combustion chambers on mine. I to am looking to drop the C.R. from 12.5 to 10 or so.

I'm going with a thick head gasket and "milling" my pistons to a flat top. I think that will work. However, I seriously considered buying NEW dished pistons if thats what it took to get it down.

I am sure I read somewhere about the importance of 'squish'. Basically you want as much of the mixture as possible squashed into the chamber as possible and not sitting around in the remaining disc shaped area above the piston where it does not burn very well. When you fit dished pistons, or thick gaskets or skim material off flat-top pistons you are decreasing (or increasing) the squish area and the resulting engine may not run so well. That being so, the best method of lowering the CR must be to change or modify the heads which may be easier said than done. I'm sure someone can fill us in about this issue... I may be talking complete bollocks!

you got me stumped
 

I know that you can mill cylinder heads to reduce the chamber volume giving you an increase in compression but I don't know what you would do in reverse. I would hate to have to dump my pistons. they are brand new and cost $500 just in parts. I am probably being too optimistic to think I can achieve a cheap fix that will allow me to bolt on a supercharger without my engine going ka-BOOM!

There is something to be said for the "squish area". I wonder what kind of "stock" pistons were used originally? When the C.R. WAS reasonable!

Better to have a dished piston and lower compression than a 12.5 to 1 C.R. that blows head gaskets if not pistons!!! In that respect, the hell with "squish". :D

Mach 1,

I mis-read your post. Don't know where my head was but it sure wasn't in my answer. You are right about me being wrong. Ernie has some good advise about lowering the compression.

Ron :CRY:

There is always talk about the benefits of 'unshrouding the valves' by which is meant milling the chamber to make more space between the chamber wall and the open valve. This operation, if possible, would lower the CR without effecting squish AND you might realise some flow benefit.

If you go with the stroker kit, you'll have to buy new pistons anyway. Get pistons with the CR dialed in as the beginning.

Jack21 has the first right answer of the bunch. You cannot change to a stroker crank without getting new pistons. Think about it, the pin has to move closer to the top of the piston or the piston goes above the deck at TDC...

When you get the crank, just order the pistons you want. Make sure you carefully calculate the CR based on piston dish volume, deck height, gasket thickness, actual bore diameter, actual stroke, and head chamber volume to get it right.

Hound dog above has sound advice not to just go with what is being advertised. This is especially important in a blown application.

Good luck.

Here is the thing
 

The Paxton Novi 1200 installation instructions say that you have to use stock or lower than stock compression for carburated SBFs. My 69 Mach I was an M code 351W, which means it was originally running 10.7:1 CR on iron heads. I swapped these out for alum heads. Now the rule of thumb is that you can run 1 point higher CR on alum heads than normal because the heat dissipates so fast. This means I comfortably could have upped to 11.7:1 but all I did was up my CR by .3. And is there any real difference between 10.7 and 11? Theoretically, the alum heads should allow me to bolt on that Paxton and run 5 lbs of boost without courting disaster. I keep tryign to call Paxton but can never get through. They say they will call you back but I am 9 hours ahead of them and I don't want them to call at 2 in the morning for me when it is 5 pm for them.

The final word
 

I finally got through to a human at Paxton who told me that 9.5:1 - 10:1 is the absolute max, with 9.5:1 being the conservative estimate for the max static CR to run their blower at 6 lbs boost. The tech guy told me to run 9:1 max. Even if I put on the Cometic gaskets, the lowest I can really get to is 9.9:1, which, he said, is pushing the envelop. I guess if I am going to spend the 3 large on a compressor and installation, I might as well swap out the pistons while I am at it. Assuming, of course, I end up doing it. After all, I already am at 438 hp at the crank and 351 at the wheels. Maybe I will just have to learn to live with what I got.

:CRY:

You have three options as I see it.

1: Built the stroker 393W, max CR 10:1, and leave it alone. No hair drier.

2: Add hair drier, max CR 8.5:1. Have piston tops and combustion chambers ceramic coated for added insurance.

3: The 429CJ was an option on '69 Mustangs. Todays 460 is a stroker 429. Get a 460, modify it to your liking, and bolt it in. This could be your most cost effective route. Plugs and oil for maintenance, and better resale value.

Jack:

I used to work in DC and lived in Lorton (not in the prison, though). Where abouts in NOVA are you?

I am thinking that I should just make do with what I have for the foreseeable future. The engine is fresh, with less than 50 miles on it. It dynoed at 351rwhp, which, accounting for 20% drive train loss from my C-6, means 438 at the fly. The reality check is that I have already spent $35 large on this car in the last year and this work done here would be more than 2x what it would cost there. I still have more flywheel hp than the new vette and more than the original viper. You see, I have been bitten by the bug and I am a true believer in the old man's sage observation about horsepower that if "some is good, more is better, and too much is just enough." I've done a lot of cipherin and figgerin and the stroker seems to be the most cost effective way to go. I will probably go that route down the line.

Perhaps you can buy off the shelf pistons with the C.R. I can't, I got to have customs made. Chris at Shelby American will be making mine to my specs.

Someone brought up squish
 

Someone brought up squish earlier, so I thought I would clarify a bit. The man you're probably speaking about is Larry Widmer of NHRA Boss fame.

He was and is against even measuring the static compression ratio, all he cares about is the cylinder pressure (dynamic compression). The squish, refers to quench, and his goal is always to design the combustion chamber (the piston is the floor of the chamber) to deliver the best mixture possible, to the smallest area possible infront of the exhaust valve(s). If you can make that mixture burn quickly, and keep extra pockets of mixture from forming, there is no chance at detonation. Because of this, he ran 25:1 compression ratios with no problem, on pump gas. They called the squished mixture, "instant octane." There is a lot more to it, but his "soft head" technology has trickled into many things over the last 30 years, and I don't know of anyone else that has caused as many rules to be changed in NASCAR; for instance, you can only run a 12:1 CR ratio now.

If anyone has questions about it, I can try to answer them, I really do enjoy the subject...

ImportGuy
Quote " He was and is against even measuring the static compression ratio" This is absurd. If your building a street engine you have to define a livable compression ratio of, for example, 10 to 1. Same for a race engine. If you want 14 to 1, this must be determined before hand. Cam selection is critical as the intake and exhaust opening and closing events before and after top dead center determine cylinder pressure. I remember reading an article, maybe 20 years ago, on the soft head. My thoughts about it then was nonsense, nothing has changed. He was describing a cylinder head with an intake valve perpendicular to the piston and a very small exhaust valve in the middle of a trough opposite the intake valve. Now, there were your spark plugs, one at either end of the trough. I can't imagine two flame fronts propagated at either end of the trough meeting in the center. Can you? He was also talking about incredible cam dynamics and valve motion requiring multi lobes on the intake side for different velocities concerning the opening and closing events of the intake valve. He was also claiming air fuel ratios of 18 to 1 and very little ignition advance! " He ran 25 to 1 compression ratios with no problems on pump gas" I thought he was against measuring static compression? That compression ratio along with the cam with little or no overlap, as he prescribed, would create cylinder pressures in excess of 300 PSI. Does anyone produce this head? I would like to look at it, touch it. "I don't know of anyone else that has caused as many rules to be changed in NASCAR" He didn't have a thing to do with that rule change. At the time they were running 18 to 1 compression ratio and more. Engines were blowing, leaving oil and debris on the track. To dangerous. The ruls board noted this and made the 12 to 1 change. If you could let me know what the technology was that trickled in from the design of his soft head and any other rules he had changed in NASCAR I would be delighted to know. Also, any independent tests run by SAE (Society of Automotive Engineers) or any other group that you know of, I would enjoy reading their findings.A continued conversation would be enjoyable.

Thank you;
Lyman