Keep or change cam - Pontiac GTO Forum
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post #1 of 4 (permalink) Old 12-07-2012, 08:03 PM Thread Starter
 
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Keep or change cam

Would like to get some advice from you motor guys about my cam. I didn't know which cam to choose for my motor because I'm not that knowledgable about cam specs so I left it up to my builder to choose. I told him to pick a cam that would match up good to my heads and intake but nothing to drastic cause its basically going to be cruiser. The motor is built, broken in and sounds really good but now I have people telling me that he should of put a bigger cam in. Wht do you guys think. Will it be worth buying and installing a bigger cam or will the gains not be enough for the money and labor. I woul like to have a happy medium between low end torque and getting the most out of the engine. Here are my engine specs. Any advice would be appreciated thanks guys. 67 400 block .30 over, stock crank cut20/20, stock rods, forged speed pro pistons, Molly rings, comp cams extreme energy 268h-10, comp mag pushrods, comp lifters, edelbrock 72cc heads gasket match (no porting I believe), heartland sharp rockers(believe 1.65), edelbrock rpm intake gasket match, Holley 650 double pumper, melting oil pump and miladon pan, msd dist, 6al box and wires, ngk plugs Doug's 1-7/8 headers. If anyone cam give me torque/horsepower estimate to that would be great.
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post #2 of 4 (permalink) Old 12-07-2012, 10:03 PM
 
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Sounds like a great combo, except the melting oil pump, lol..
Drive the car and find out if you want a bigger cam. Big cams compromise driveability, and won't make them a pleasure to drive. A guy told me he had a CC 268 and it sounded pretty healthy, he may have been lying.

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post #3 of 4 (permalink) Old 12-08-2012, 08:45 AM
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The biggest problem I see is the stock rods. Because of that I'd be very cautious about revving the engine past 5000 rpm, so that means you don't want to go crazy with a fat cam that shifts the torque curve towards higher rpm. Normally I don't like Holleys much but in this case, having that small one on there is going to tend to restrict air flow at RPM and that's going to help protect the motor. Do you happen to know whether or not your block has been zero-decked? That'll make a difference on compression ratio. If it hasn't, your compression is pretty much in the sweet spot for aluminum heads and pump gas. If it has, it's a bit on the high side but still manageable with the cam you have.

I ran your specs through Engine Analyzer, and I've got good news and bad news. The good news is that it predicted 444 HP, 449 lb. ft. of torque. The bad news is that 444 HP didn't happen until 5800 RPM, and there's no way I'd risk spinning those stock rods that high, ifn' it were me. That's even with that little Holley choking the motor from 4600 rpm on. An 800 cfm QJet would free up at least another 10 HP, but it would come in right where you don't want it - 5800 rpm and above.

Other considerations. The cam you have is going to make just under 15" of manifold vacuum at idle, so if you've got power brakes I'd be very hesitant to go with any "more" cam than that because you're already borderline on being able to operate vacuum assisted power brakes.

Your friends who've been recommending more cam don't know what they're talking about. They're probably chevy guys. What you've got is already "too much" for how the rest of your engine is built, in my opinion. You'll be able to "use" all the torque (that 449 lb. ft. comes in at around 3700 rpm) but you won't be able to "use" the power, at least not safely.
Now, swap out those factory rods for a set of good forged rods and that changes everything. You'd be able to safely wind that motor to whatever limits would be imposed by the rest of the valve train and the induction system. Idle vacuum would still be a concern for power brakes, and also you'd have to start thinking about rear gear and torque converter stall (if it's an automatic), but at least you could quit worrying about "abnormally disassembling" the engine.

If you're going to keep the rods you have, I'd consider "dropping back" to something like a Comp Cams XE250 grind. That would put the power peak right where you could use it all. 415 HP right at 5000 rpm, or just a hair over. Torque would be 446 at 3400. It would probably have a dead smooth idle and would make over 20" idle vacuum - more than enough for brakes. You'd have yourself a sleeper -- it would run right with, or maybe even quicker, than the cam you have now at the same rpms. No, it wouldn't "sound as good" as what you have now at idle, but on the track it would run every bit as quick. On the street, stoplight to stoplight, the XE250 would put a pretty good hurtin' on what you have now because it makes more torque earlier in the rpm range. By the time the XE268 starts making good torque, it's going to be looking at the XE250's tail lights. Take your pick

Bear

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post #4 of 4 (permalink) Old 12-08-2012, 06:06 PM Thread Starter
 
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Bear thanks for the advice and the time to respond. I'll prob stick with the cam now and see how it feels with the car. I'm never going to race the car. It will never see over 5000 rpm. The only racing it will see is when me and my buddy's screw around with each other on the way to shows. I do have power brakes so I wouldn't want to go bigger cause I don't want to put an assist in the car. After a while if I get the itch for more power ill get a striker kit with forged components and dished pistons for the 72 heads, but that won't be for a while. Thanks
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