hey guys - i have a '72 SS with a 468 in it, and it was recently rebuilt at a reputable shop here in town. the first time it was built, the final compression ratio came out to be 10.1:1 thanks to a set of 049 GM heads and SRP pistons.

the cam was a COMP hydraulic roller, with 244/244 duration and .612/.612 lift.

shortly after being built, the engine grenaded. turns out the dynamic compression was too high to support pump gas. it did run good for a while, and felt real strong, despite the detonation going on.

now it has been rebuilt with a new set of open chamber GM heads (oval ports - i don't know the cast #), same cam, same pistons. now the compression ratio is calculated to be 9.1-9.3:1.

i've had the car for about week, and it feels a slight bit slower than before. i guess that is expected, but i thought since we were going with a set of heads that would let the flame burn more efficiently and the way it should, i wouldn't see a performance decrease.

it's not really an option to go back and have it built again, because the engine is built and finished and we don't have the cash to throw around. i will get the casting #'s off the heads so i could give you guys all the complete specs.

Well, its the dang oval ports! Kidding.

There is no way a 10.1 motor should have grenanded with such a big cam. Wierd. Nevertheless, Your problem is now too little cylinder pressure. A 244@.050 hyd cam is going to be a turd in a low compression motor.

Are you CERTAIN the motor grenaded from detonation, or was that what the machinist said? Unless you threw a bunch of timing at it, and crap fuel, I can't see it. I sure as heck would doubt that as a cause without independent proof/analysis, especially if that conclusion came from the same folks that built it. Frankly 10:1 seems spot on right for such a big cam.

Since you have a new 9:1 motor, I;d put a suitable cam, along the lines of the Lunati A7, in there and get some torque back. That cam only sports 223@.050 and will make the car faster.

well, the machine shop said the compression was making it detonate. i don't see how this was happening either. truth be told, the engine broke SIX times. he made it right every time, but i'm thinking he just bumped the compression down and put something out he knew wouldn't break.

he didn't run any wild timing and we ran supreme gas from exxon in it, so i dunno.

would the car be faster back w/ 10:1 or with 9:1 and a new cam?

Doesn't make sense to me either, but what you experienced may have occured from lack of quench. It is possible that the motor had a large piston dome but wound up down in the block about .020 or more. Add in a .041 or thicker head gasket to reduce compression, run it too lean and without enough cooling system and you will promote detonation. So I guess anything is possible.

Detonation would have to be fairly sever though. You didn't say this, but were you running Nitrous?

Originally posted by four o two:
well, the machine shop said the compression was making it detonate. i don't see how this was happening either. truth be told, the engine broke SIX times. he made it right every time, but i'm thinking he just bumped the compression down and put something out he knew wouldn't break.

he didn't run any wild timing and we ran supreme gas from exxon in it, so i dunno.

would the car be faster back w/ 10:1 or with 9:1 and a new cam? first, it sounds like you are going to the wrong machine shop.
I suspect that the tune is WAYYYY off for a 10:1 iron headed motor to grenade from detonation.

There are quite a few variables in detonation. Spark plug heat range, engine temperature, fuel and the ACTUAL cylinder pressure under the particular operation conditions, as well as how much load is on the motor.

Maybe your tune is too lean, how hot does this motor run?

Infact what are all the specs on the tune.

guys - thanks for the input. what sucks is i'm out of cash, and the engine has been rebuilt six times - we finally realized the block was causing the problems, so build #5 was a 10:1 setup, which mysteriously destroyed itself. after that, for rebuild #6, he bumped the compression down to around 9:1 by putting on a set of open chamber heads.

i think it would really put him in a spot to go back and say i'm not happy with what i've got - but then again, maybe i should.

as far as the 'tune' goes, he swore it was spot on. the carb is a street demon 850, and i don't think it was running lean. temps always stayed 180-190 degrees.

i'm just a little perturbed that i've been through all this crap with the rebuild and it hasn't turned out to what i was expecting... i'm a college student finishing up my CS degree, i go to a 4 year school and have to devote all my time to studies, so i don't have a job or extra money to have this thing remedied or take it to another shop. :mad:

Chris,

Something doesn't add up. The cam you are referring to has a seat duration of 302 - 304 degrees? I can't see how the motor was detonating with 10/1 cr and that cam. Are you sure you weren't revving it too high and using a cast piston? I have seen cast pistons break at 6000 RPM.

How did he lower the compression ratio if the pistons are the same? 049 heads ARE open chamber heads, which heads did he put on the last time? Sounds like he is trying to cover his butt.
Ron

chris,
did you take the bowls off an verify the jetting actually is the factory baseline.
Have you played with the jetting.

where is the jetting now?

ToocoolZ28 , sorry - the heads were not open chamber. i could've sworn the stock 402 heads off of the SS were 049's. guess not. all i know is they were closed chamber, and now the ones on it now as open.

also guys, i can't tell you where the tuning is right now - i'd have to consult with the shop since he jetted it and stuff. (i'm not too keen on all that stuff)

my father and i built the first 402, so i know quite a bit about engines, but not all the extensive info like how the carb is setup and stuff. i will try and find out for your guys...

gosh, i sound ignorant. graemlins/clonk.gif

Sorry that you didn't try a different Mechanic after the second go-a-round..

pdq67

here's some more info guys:

i'll go ahead and give the entire engine specs

454 bored 60 over
Demon 850 - the front jets are Holley #88 with a #37 squirter (i'll get rear jet sizes tomorrow)
Weiand Stealth intake
cam is hydraulic roller w/ .616/.616 lift and 244/252 duration
heads are GM open chambered heads w/ a 119cc chamber, 2.19/1.88 valves (i'll get cast # tomorrow)
pistons are SRP domed. they create 10:1 with a 109 cc head and 9.1:1 with a 119 cc head.

also, consider this: the cranking pressure of the motor with 10:1 compression was 210 psi. this is why we think the motor was detonating and thus shelling the bearings. the old heads were angle milled (this was done when they were on the 402 motor) and had only a 109 cc chamber (closed). the motor now generated about 150 psi cranking pressure.

as far as running lean, the plugs have been inspected and they show a good a/f mixture.

any ideas? what are my options? i CAN NOT afford a rebuild. i just don't understand why this engine wouldn't handle 10:1? (cylinder pressure)

#88 front jets, any reason you went up 3 jets from the out of the box settings. Plugs reading lean?
WHat plugs, heat range, brand and style.

does it ping only when up to operating temperature? or all the time?

Is it using Oil? How much? every race motor uses a little oil, but alot and you will dilute the mixture.

jetted up to get rid of hesitation. the cam, according to the mechanic needed more fuel...

plugs are AC Delco, R45T

210 psi is alot for pump gas. Either the cam was out of phase, or your static calculations were off.

tell me about it - i just can't figure out why it was pulling that much pressure when the CR was "supposed" to be 10:1. i didn't do the calcs, the mechanic did.

i really hate to be stuck with this thing. i am going to gather all the specs and give a guy up in Durant a call sometime this week.

Six times is ridiculous! I'd say if the builder did the tune he should correct the problem. 210 psi is problems for pump gas, I bet it was well above 10-1 static. Does it rev OK? make good power at higher rpms?

revs fine and makes good power up top - it's just that it seems slow than it was when it was making 210 cyl. pressue on pump gas. right now the cylinders all give about 150 psi. is there a way to calc. CR from that?

Just read your posting of the orginal engine specs. You had a lot more than 10/1 to start with. Depending on the closed chamber head (oval port I assume?) the heads were anywhere from 98CC to 101CC (my 290's are actually CC'd at 104 after polishing the chamber and opening the combustion chamber around the valves and no more than .008 cut on the face). Even the closed chamber rec ports only started at 106CC. When you angle mill, you take a lot of volume out of the chamber - around 8-10CC. It is possible that your heads were down around 92 - 94 CC pushing you CR near 11/1 maybe higher depending on things like piston/deck ht. and whatever the actual piston you have.

For the low compression motor, you too much cam.