What did Chrysler do different in the 383's in 1968 to get 9.2:1 comp ratio in the 2bbl engine and 10.0:1 in the 4bbl? In my understanding they all have the 906 casting heads. Is the difference piston height, head gasket or some other factor I didn't cover?

I just picked up a 383 and I have never played with one in my adult life. I used to take them apart for my dad when I was very young but I dont remember the specifics of the engine.

I am also curious if a set of closed chambered heads will bolt on to a stock bottom end 383 without piston to valve clearance issues. Simple English: If I bolt on a set of 915's will the valves hit the pistons?

 

i took the heads off my 68 newport 383 2 bbl and it had dished pistons. but i dont know what the cr was.
if you dish the pistions u lower the cr.

 

ok, i havent picked the engine up yet... or taken it apart.

 

915s r just a closed chamber head no threat of the valves hitting

 

The difference is in the pistons, and not the heads. You should have no trouble mounting the closed chamber heads on your 383 engine if the pistons are stock. If you are still unsure, once you have the old heads removed, put some modeling clay on the top of a piston, and then mount the new head on (with a gasket), and torque it down lightly. Turn the engine by hand at least 3 full turns. If the engine turns a certain distance but won't turn any further, then you have an interference problem (unlikely). After turning the engine, remove the head, and you'll see marks left on the clay from the valves. This will give you an idea of how much clearance you have.

You need mount only 1 head, and the engine will turn easily by hand if the spark plugs are removed.

 

yep it's the compression height of the pistons that makes the difference . std pistons sit about .080" down the bore so you'll have heaps of clearance . if you can afford new pistons get some zero deck height ones .

my deck has had .003" taken off just to square the decks & clean up the new sleeves . & with 74.6 cc closed chamber Indy ez heads + .040 fel-pro head gaskets & 12cc dome top pistons with 6cc valve reliefs in the pistons , & the valve reliefs i ground into the tops of the bores i ended up with 10.7:1 compression .
i rebuilt & extensively ported a set of 906 heads that ended up with 86 cc chambers . in the end i couldn't use them because i wouldn't of had enough compression to make the 557 mopar cam work properly .

 

Ok, I was hoping i would be able to put my closed chambered heads on this 383 i found. I haven't picked it up yet so i dont know what has been done to it internally. I found it on craigslist and the ad said it was rebuilt 10 years ago and never installed or something like that. I just want to make sure that i have enough compression to run my .510 lift cam.

 

is that a hyd cam ???? they usually have a lot of duration those cams ??
you could also use a .020" head gasket . or use kb hyperutectic pistons . you gota do the math really to know or you & we are just guessing . it's very important to match the cam & compression .

 

Yeah, i need to get the engine and crack it open before i decide what i will do with it. The cam is a hydraulic with a decent amount of duration.