702 heads on a low RPM 496?
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702 heads on a low RPM 496?
I have a 1966 396 with 702 heads in a 1972 K10 longbed that I drive for fun and plow my driveway with. Looking at sensible options for giving it a little more power.
I’ve read that the closed chamber 396 heads can do ok on a 454. I ran the numbers on a compression calculator and it seems like you could even use them on a 496, but does anyone actually have any experience with this? Sometimes things like piston and combustion chamber shapes can make what seems reasonable on a calculator turn out to be a bad idea in practice.
Just toying around here. Thinking of a baby flat tappet hydraulic or maybe hyd roller, dual plane intake with functional heat riser, exhaust manifolds with straight 2 inch duals, maybe even modifying a thermac air cleaner to fit my 3310 holley.
Sort of like if GM had built a stock 496 for these trucks. Shift at 5000 rpm. Hyper or 4032 pistons, cheaper I beams, cast crank, nothing crazy.
I’ve read that the closed chamber 396 heads can do ok on a 454. I ran the numbers on a compression calculator and it seems like you could even use them on a 496, but does anyone actually have any experience with this? Sometimes things like piston and combustion chamber shapes can make what seems reasonable on a calculator turn out to be a bad idea in practice.
Just toying around here. Thinking of a baby flat tappet hydraulic or maybe hyd roller, dual plane intake with functional heat riser, exhaust manifolds with straight 2 inch duals, maybe even modifying a thermac air cleaner to fit my 3310 holley.
Sort of like if GM had built a stock 496 for these trucks. Shift at 5000 rpm. Hyper or 4032 pistons, cheaper I beams, cast crank, nothing crazy.
Re: 702 heads on a low RPM 496?
i was actually looking at rotating assemblies for this and the flat tops seem to come in at around 10:1. not sure if that is a great idea or not. i ran a 10:1 iron head 350 for years on unleaded with cast pistons driving like an idiot and never managed to blow it up. maybe if i err on the side of running a bit rich and keep it at 34 degrees total it will be ok. we are 2000 feet above sea level and i run 91 octane ethanol free gas in the 396 now.
come to think of it i don’t even know what the CR is on the 396, i bought the truck with the engine in it.
does a 496 wear out quicker than a 454 due to side loading issues, or does running a little longer rod more or less solve that issue? this engine will see some miles.
come to think of it i don’t even know what the CR is on the 396, i bought the truck with the engine in it.
does a 496 wear out quicker than a 454 due to side loading issues, or does running a little longer rod more or less solve that issue? this engine will see some miles.
Re: 702 heads on a low RPM 496?
Do some chamber work to lower the comp down to 9.6:1. We just did a 496 with semi closed chambers, flat tops...9.6:1, no port work, hyd. Isky cam and rpm intake...made 472HP and 548TQ. Cam is small 221@ .050.
Re: 702 heads on a low RPM 496?
that sounds like a very sweet engine. the cam is probably one step up from what i’d want to run, can’t see any reason to run a big cam with exhaust manifolds. i need to do some research on manifolds as well, not sure if there are any better than the ones i already have.
been around pontiacs a long time and small blocks even longer, but this is my first big block chevy. i really like it, pretty sweet for what it is. but i know there’s a lot more to be had from a modern big chevy than what i’m getting from this old 396!
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Re: 702 heads on a low RPM 496?
Going 100 bigger will fry the tires.i82much wrote: ↑Sat Dec 19, 2020 4:26 pmthat sounds like a very sweet engine. the cam is probably one step up from what i’d want to run, can’t see any reason to run a big cam with exhaust manifolds. i need to do some research on manifolds as well, not sure if there are any better than the ones i already have.
been around pontiacs a long time and small blocks even longer, but this is my first big block chevy. i really like it, pretty sweet for what it is. but i know there’s a lot more to be had from a modern big chevy than what i’m getting from this old 396!
The last one I did at 489 ran 10.14 at 3300 lbs with driver and those dreaded heads...And a .600 lift and 260@ .050 cam
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Re: 702 heads on a low RPM 496?
i’m thinking i could plow snow pretty fast with a 489 like that.af2 wrote: ↑Sat Dec 19, 2020 8:15 pmGoing 100 bigger will fry the tires.i82much wrote: ↑Sat Dec 19, 2020 4:26 pmthat sounds like a very sweet engine. the cam is probably one step up from what i’d want to run, can’t see any reason to run a big cam with exhaust manifolds. i need to do some research on manifolds as well, not sure if there are any better than the ones i already have.
been around pontiacs a long time and small blocks even longer, but this is my first big block chevy. i really like it, pretty sweet for what it is. but i know there’s a lot more to be had from a modern big chevy than what i’m getting from this old 396!
The last one I did at 489 ran 10.14 at 3300 lbs with driver and those dreaded heads...And a .600 lift and 260@ .050 cam
by “those dreaded heads” did you mean closed chamber ovals?
Re: 702 heads on a low RPM 496?
flat top pistons-massage a couple of CC's, bore notches-massage a few more, cylinder head-check on dish face valves, the actual chamber volume is probably higher than the listed specs, probably hours of hand work, I would stay under 10:1 even at your altitude
Re: 702 heads on a low RPM 496?
sounds like a lot of work and i'm blessed with two young children, which means i'm not blessed with much free time
i wonder if i can order a custom 4032 piston for a reasonable price? i don't need the world's greatest piston for this application. if i could find a custom piston that would let me run closed chamber heads and .060 over pistons (4.155), i could just keep the 396 block and run a 4.00 crank. or maybe the 396 block can be clearanced for a 4.25 crank, i dunno.
almost seems like it makes more sense to sell the running 396 out of the truck complete if i want to do this, find a 454 block and some open chamber oval ports and do it that way.
i wonder if i can order a custom 4032 piston for a reasonable price? i don't need the world's greatest piston for this application. if i could find a custom piston that would let me run closed chamber heads and .060 over pistons (4.155), i could just keep the 396 block and run a 4.00 crank. or maybe the 396 block can be clearanced for a 4.25 crank, i dunno.
almost seems like it makes more sense to sell the running 396 out of the truck complete if i want to do this, find a 454 block and some open chamber oval ports and do it that way.
Re: 702 heads on a low RPM 496?
Why not use the parts you have just pick up some later model oval ports. Stroke it and use a set of off the shelf pistons and the large open chamber heads that are 118cc or 124cc to yield a mid to high 9 compression ratio.
Re: 702 heads on a low RPM 496?
that's a good idea. i was just looking and it appears KB makes a hyper piston for just this purpose, would work with the open chamber heads but not my 702's.
i need to check the numbers on the block, i assume the block and heads go together but i don't think i ever looked at the block casting number. i'd feel a little bad about separating the block and heads like that, although i guess i could always just hang onto the heads so i wouldn't feel guilty.
maybe get the block sonic checked and see if it is one of those thick wall jobs, the heads are 1966 so if the block goes with it there's a chance.
Re: 702 heads on a low RPM 496?
Going to 4.25" is a lot for a 396 block period. It has been done successfully but I would have it sonic checked.
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Re: 702 heads on a low RPM 496?
I would like to hear more about the combo. What was the head dreaded casting number and what kind of work was done to the heads?af2 wrote: ↑Sat Dec 19, 2020 8:15 pmGoing 100 bigger will fry the tires.i82much wrote: ↑Sat Dec 19, 2020 4:26 pmthat sounds like a very sweet engine. the cam is probably one step up from what i’d want to run, can’t see any reason to run a big cam with exhaust manifolds. i need to do some research on manifolds as well, not sure if there are any better than the ones i already have.
been around pontiacs a long time and small blocks even longer, but this is my first big block chevy. i really like it, pretty sweet for what it is. but i know there’s a lot more to be had from a modern big chevy than what i’m getting from this old 396!
The last one I did at 489 ran 10.14 at 3300 lbs with driver and those dreaded heads...And a .600 lift and 260@ .050 cam
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Re: 702 heads on a low RPM 496?
Keep in mind most performance piston compression listings are zero deck and a .040” head gasket, so you throw.020” deck and the compression falls pretty quick. Deck according to your compression target.
Re: 702 heads on a low RPM 496?
I built a truck 496 earlier this year with a 221 @ 050 Howard’s HFT cam for a 79 1 ton truck. Idle and low rpm manners are like a stocker. I don’t think there’s any benefit to a smaller cam in a 496.i82much wrote: ↑Sat Dec 19, 2020 4:26 pmthat sounds like a very sweet engine. the cam is probably one step up from what i’d want to run, can’t see any reason to run a big cam with exhaust manifolds. i need to do some research on manifolds as well, not sure if there are any better than the ones i already have.
been around pontiacs a long time and small blocks even longer, but this is my first big block chevy. i really like it, pretty sweet for what it is. but i know there’s a lot more to be had from a modern big chevy than what i’m getting from this old 396!
Re: 702 heads on a low RPM 496?
i’ve been using .039 for the gasket and .01 for the deck clearance when doing compression calculations. i have done a lot of reading on the subject of quench distance on Pontiac engines but I don’t know how much that information translates to a big chevy. The conventional wisdom on the Pontiac side seems to be that using a thicker gasket or having pistons down in the hole are generally not good ways to reduce compression. Is that true on the big chevy as well?rustbucket79 wrote: ↑Sun Dec 20, 2020 11:31 pm Keep in mind most performance piston compression listings are zero deck and a .040” head gasket, so you throw.020” deck and the compression falls pretty quick. Deck according to your compression target.
seems like i’ve read about guys for some applications having the pistons fly cut to reduce compression, but not sure if that applies here. i guess they just make the valve reliefs deeper? or maybe machine off part of the dome?