I am building a 383 and I am curious to what compression ratio I can get away with on pump gas. Most of the stations around me are 91 octane but I can get 93 right next to my house. The combination that I am going to be running is as follows.

224/236 @ 50 .49x/.49x lift flat tappet
H859CP30 ST Speed Pro Pistons (Compression Distance 1.425/Dome volume 12.5cc)
Scat 9000 3.75 Crank
Ported 083 heads (58cc)
89 Truck block all cleaned up
5.7 powder metal rods

I have been doing some reading on quench but I am not 100% understanding it. I am not sure if the pistons are going to be sitting up over the block or not so I do not know what gasket I am going to be running. I am thinking that it will be right around 10.25:1 comperssion. My biggest concern is with the iron heads and is it going to be ok to run with pump gas. Any help at all would be greatly appreciated. Thanks in advance.

PS Yes the 083 heads for now until I can save some money up for Brodix.

 

There's a very useful excel based spreadsheet out there called engincalculator.xls, I don't have the link but if you do a search on "compression ratio" it should come up.

With a zero decked 383 using 12.5cc dome pistons and 58cc heads, compression will be over 15:1.

I don't know what you can get away with on 91/93 but I hear to shoot for between 9.5-10:1 for an iron head SBC on pump gas. My 383 should (haven't cc'ed the heads yet) be right around 9.7-9.8:1 (if I EVER finish it dang it)

 

10:3:1 Here on 87!!! Havent had any knocks yet, but I only have about 2 hours on the new motor lol. I'm just using up last years full tank of gas.

 

They arent actually domed they are recessed.



This is what they look like. I have asked around and it should be right at 10.24:1 but I am not sure if the motor will hold up to that compression with iron heads and 91 octane.

 

Those look like mine.

 

The %&^$^#ing computer lost my entire response and it was long, so I'll make the re-type shorter.

http://homestead.juno.com/jlstrey/fi...calculator.xls

There is the calculator.

10.25:1 works out with those pistons assuming the nomimal theoretical piston depth of 0.025" in a 383. Because this value can vary, along with actual measured chamber cc and gasket volume and can affect actual compression ratio you need to measure everything to verify what you really have. It is not uncommon for heads that are to have 58cc chambers to actually measure between 55-60cc, your deck height may (and probably is) different.

Bottom line--don't count on compression being exactly 10.25:1, measure everything and verify it since you may get an answer that 10.25:1 is fine but when you start looking at it you're really closer to 11:1 with your actual parts.

From what I've read here there are guys with iron headed pump gas motors over 10:1 but not too far over and the higher you go the more critical the details like proper quench height, combustion chamber surface finish, cam specs, etc become. I hear to shoot for 9.5-10:1 for iron headed street motors; I've also heard that it is better to achieve proper compression ratio with a smaller chamber+larger dish, so maybe you could work the numbers with an 18cc dish if you can eventually get the aftermarket heads you want with a small enough chamber to work with the 18cc dish later on.

I've been looking into this too as I was worried about detonation with iron heads as well. All this info is from what I've read so take it for what its worth and good luck.

 

Thanks for the honesty. I have been reading a little bit and I am very leary about what people type. There are a lot of internet racers that know "everything" and then you either try it or go to work something out and then find out the hard way. The next step in my process is getting the head gaskets, I think, and then I will check out what what compression is going to be. The one thing I just thought of though is figuring out quench. Wouldn't I need to assemble the motor to find out how far the pistons are going to over/under shoot the deck height and then buy a gasket from there? Also proper quench should be between .012 to .060 right? If that is right I would like to shoot for .040.

 

No .012" quinch and your piston will hit your head! minimum quench is .035" maximum should be .050".