Thread: COPO heads? Rod Length on 496?

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Hey Guys! It is good to be up and running again. I have two questions for everyone today.
1- I tore down my 396 to find it had closed chamber, rec. port heads, with a casting of 3919840 - When I looked up what these were, in the comments section of the website I used, it said these were COPO heads. I do not want to run rec. port heads on my 496 that I am building, so I was going to trade these for some 781 or 049 casting oval port heads. My question is this, if these are COPO heads, does that mean they are worth more than any old oval port casting head? I really dont need them, but I dont want to get screwed into giving something of value for a dime-a-dozen set of heads.
2- My second question is what is the rod length supposed to be for a 496 stroker? I have seen some advertised with standard length, and other's with a 6.385 length, which is right?

Thanks alot guys, I look forward to your responses!

It depends on the compression height of the piston. You start with a 9.8 deck height, then subtract half of your stroke, and then your piston's compression height, and then .025" for the deck clearance. That will give you an approximate rod length.

"I do not want to run rec. port heads on my 496 that I am building"

Why not?

Hey Tech:
To be honest, the only reason for that would be what I had a local hot-rodder advise me. He said that for a streetcar, with somewhere between 9.5:1 and 10.5:1 C.R., that rec port heads are to much, causing a loss of torque, is this inaccurate?

No, that's not necessarily inaccurate. It's true that too much port volume for a given displacement will slow the speed of the mixture going past the valve and result in inadequate cylinder filling at lower rpm's.

But here's my take on your situation. These heads were designed to feed a 396, probably at an elevated rpm level in a hi-po factory piece. But you don't have a 396, you have a 496. That's a 25% increase in displacement, so the heads will seem smaller to the motor than they did on the 396 and build max torque at a lower rpm level than they did in the 396.

I guess it all depends on what you want to do with the motor, but I don't think I'd go to the time, effort and expense of building a 496 stroker and then trade off some nice heads for oval port units unless I wanted a low rpm stump-pullin' motor. Now, if a stump puller is what you're after, then by all means use the oval port heads.

If it WAS a little slack on the bottom with the rectangular heads, which I don't think it will be, I'd adjust the combination with a looser converter.

I WOULD use a dual plane, 180 degree intake manifold such as this from Chevrolet....
3933163 High Rise Intake Manifold, Rectangular Port
Your big-block Chevy will breathe easy with this high-rise aluminum intake manifold. This manifold's dual-plane design produces impressive low-speed torque and plenty of high-rpm horsepower. It can be used with high-performance cast iron and aluminum cylinder heads with rectangular intake ports. The carburetor mounting pad is machined for a standard flange Holley four-barrel.

and a Barry Grant 850 annular discharge carb. I'd build it 9.5:1 with a 0.040"- 0.050" squish, zero deck, headers and a hydraulic roller cam something like this...
http://www.cranecams.com/?show=brows...tType=camshaft

2,500 converter and a 3.70 gear should do the trick.

run the rect ports if they are in good shape but big O ports are very good 781 the gm stock head had some very good numers on them and can make big power .all the engine i have built used both O and rect . i am building a big block rigth now and it will have O ports on it this a 513 and have it is going in a heavy car and for rod length the depends what you want to spend i have built them with stock 6.135 and 250 long6.385 in short deck and in the tall deck 6.635 to 6.800 if you do not have alot of money then you can get in the 496 with out alot of money a good set of stock rods with good rod bolts in them. but the scat I beams do look good for the money$275 so at this piont go 6.385 scat rods pistons hyper pistons $285 or $550 for srps it has a lot to do with how much out have to spend but 496 can be built for not a lot of money and the o ports can and will support a 496 with some big 2.190 intake and 1.880 ex and a bowl job

I don't know about those being COPO heads but they are for the higher hp bbc engines from 67 to 69. They have 107cc chambers and are closed with rect ports as you said. They also made some aluminum heads like those although I don't think the 840s had the same porting as the 842s.

You have almost 500 CI so you will have gobs of torque whether you run oval or rect port heads. You will move the curve to the right with those heads and left with oval ports. You will make more HP at higher RPMs depending on your cam, duration, tranny, car wt, rear end yada.

If those heads are nice use them since you already have them. You just need to adjust your cam lift, duration, ramp, roller, hyd, mech, other valve train parts depending on how much you want to spin it. For anything over 6200 then I say go with the rect ones. If less then you'll enjoy the streetability more with the ovals. But if you're using mild cams, CR etc then you won't notice a lot of difference in the seat of your pants with either. JMO.