I completed an engine rebuild last year on my '68 Chevelle; originally with a 396/325. This is what I did; Bored 0.0030 over, crank 0.0010 under; flat-top standard pistons; Carter AFB carburetor 750CFM part #9755. Worked on carb and installed Metering Rods Cruise 0.068-Secondary 0.046; Metering Jets Primary 0.107-Secondary 0.104. Needle & Seat 0.111. Edelbrok Intake Performer RPM Air Gap #7562. Ignition Accel Street & Strip. Genuine GM Aluminum heads, rectangular port, intake @ 2.19, exhaust @ 1.88, Open chamber @ 116cc size. Camshaft & Lifter kit is Edelbrock Part #2162. Stock exhaust manifolds. Here's my problem. Ever since the rebuild, I have very poor throttle response and often backfires throught the carb. I must wait until engine is up to operation temp before I have any performance at all. I've explained my problem to others who have suggested that my heads are a mismatch. They came from a 454 blown engine and domed pistons. I'm told that these are primarily my problem because they can't develop the necessary high comp. ratio needed for these heads. I'd like to get other opinions before I make any major alterations. Can anyone lend some experience/assistance/advise. It is much appreciated.

too little compression and too large of runners on heads to develope much tourqe. i would try a small chamber oval port head. those heads are just too big for your little 396

A flat-top piston and a 116cc chamber you have about 7.6:1 compression, and coupled with a 218°/228° cam it's not going to run very well.

Originally posted by Terry Snelgrove:
I completed an engine rebuild last year on my '68 Chevelle; originally with a 396/325. This is what I did; Bored 0.0030 over, crank 0.0010 under; flat-top standard pistons; Carter AFB carburetor 750CFM part #9755. Worked on carb and installed Metering Rods Cruise 0.068-Secondary 0.046; Metering Jets Primary 0.107-Secondary 0.104. Needle & Seat 0.111. Edelbrok Intake Performer RPM Air Gap #7562. Ignition Accel Street & Strip. Genuine GM Aluminum heads, rectangular port, intake @ 2.19, exhaust @ 1.88, Open chamber @ 116cc size. Camshaft & Lifter kit is Edelbrock Part #2162. Stock exhaust manifolds. Here's my problem. Ever since the rebuild, I have very poor throttle response and often backfires throught the carb. I must wait until engine is up to operation temp before I have any performance at all. I've explained my problem to others who have suggested that my heads are a mismatch. They came from a 454 blown engine and domed pistons. I'm told that these are primarily my problem because they can't develop the necessary high comp. ratio needed for these heads. I'd like to get other opinions before I make any major alterations. Can anyone lend some experience/assistance/advise. It is much appreciated. I would like to use my rectangular port Edelbrock intake. I'd also like to use aluminum heads to match. Can you suggest a suitable Edelbrock dressed head? Also, do I need open or closed chambers?

Thanks,
Terry

with flat top pistons you need closed chamber heads 110cc or smaller.

Put the correct high dome pistons in it and get the compression up close to 10.0/1. The stock exhaust manifolds are going to choke it down though. The rec ports work on a 396 above 6500 RPM. 6500 RPM will need headers to work right also.

from what i understand you are running alum. heads and they are rectangle port and you are running a rectangle port intake.

this can be made to work on the street just fine but you need the compression up. the low compression is really the killer here. running a 7 to 1 something compression ratio will make that thing run like a dog. as you allready know.

being the heads are alum. and running a 218/228 @ .050 cam. you really need to change those pistons out. you can buy the factory replacement(375hp) 11.0 to 1 trw slugs for your motor and end up with about 10.5 to 1 compression with those heads.

this will really boost your torque and fix the ill effects of backfires. it will run on 91 octane fuel. even with the smaller cam. you will have to rework the dist. some to get the timming curve right.

this would not be the best set up but it will work fine.

the other option would be to go with some closed chambered heads that are in the 90 some cc range. and changed intake to a dual plane oval port design. this would provide more streetable torque. and you can keep your current pistons you have now and run a steel shimmed head gasket to get your compression in the higher 8 to 1 range. close to 9 to 1 and of course this would allow for 87 octane fuel. timming would not have to be so much dialed in as with the rectangle set up i described above.and you would be fine with the stock exhaust manifolds.

it just depends on what set up you can live with. if i was to go the rectangle port design i would change out the cam to be more matched to the rpm range those heads can run.and put some headers on there.

either way the compression is to low. and theres just to many mismatched parts in your combo right now. just have to decide which way you want to go. higher rpm or lower rpm street torque.