Thread: How about this cam in a 283?

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The motor is indeed just a pump. Consider that your lungs are one cylinder. You open your mouth and breathe in a lungful of air, then you close your mouth. You have trapped the maximum amount of air. If this air were an explosive mixture, you would get the max amount of push on the piston. Now, let's say that as you were taking a breath, you didn't close your mouth and started to breathe out. You would lose a lot of the air that you just breathed in and once you did close your mouth, you would only have a fraction of the combustible mixture that you might have had if only you had closed your mouth at the right time.

So it is with a cam. You have to know when to close the intake so as not to push the mixture that you just drew into the cylinder back up the intake tract, thus losing some of the bang and also disrupting the vacuum signal to the carburetor.

To answer your question about a blower, I think you'll find that in talking to the blower guys, they'll tell you that you really don't need a hell of a lot of cam to make a blower motor work. Just a little over stock will work real well.

Back to being just an air pump..... the more cfm of air you move through a motor, the more horsepower you can make. In order for a 283 to make as much power as a 350, you have to spin it faster. A 350 turning 6,000 rpm's will take in 607 cfm. To take in the same amount of air and theoretically be capable of making the same power as the 350 at 6,000, the 283 would have to turn 7,400 rpm's. Now, here's where the rub come in.....you can't use a regular street grind cam in the 283 and spin it 7,400. A 9.0:1 283 with the proper cam will make power to about 5,000 rpm's, so you are out of cam and you need to spin it another 2,400 rpm's to make the power that you could with a low-rpm 350. So, you say to yourself, well, I'll just cam the hell out of it and it will rev to 7,400 and make power. BZZZZZT. Wrong answer. The intake closing point is so late on this l-o-o-o-o-ong cam, that you are not capturing enough compression to make a decent bang when the plug fires. In order to make this cam work and make power to 7,400 rpm's, you would have to increase the static compressio ratio to something on the order of 11.5 or better. It's late and I'm not figuring all this stuff out to the letter right now, but I hope you get the idea of what I'm trying to say.

If I have not made this clear, please say so. I have a lot of patience and will keep giving examples over and over until you guys understand this cam thing.

Shawnlee gets it, you other guys can too.......

Thanks Tech,,,I'm teachable also. The ICP point you made,,,I just need to know,that is based on providing torque which is 'all important' for a street vehicle? So,won't more gear and torque converter make up for this or would that be 'putting a bandade' on it and not using all parts that are compatible? The reason I ask (though I agree with you),is that I have read of engine builds that have an ICP that lowers their compression below 8:1 yet the horsepower increases??? Evidently that horsepower is only good at high RPM and reduced at the best???

Yes, I can see how you could delay the ICP and produce more power at the top end, because the more agressive grind would allow the motor to rev higher. The higher it revs, the more power it makes. Like you said, it is just an air pump. If it were my motor, I would want to make more power throughout the entire rpm band where I could use it, not just at the top end to make a big splash and sell magazines.

I have gone back and added more info on the last post.

Well at high RPM instead of spitting it back out the motor is spinning so fast that the late closing point helps fill the cylinder because the time available to fill it is shortened. But you have brought up such a valuable point that I wonder why the cam mfr's. don't cover this subject more? Many of them only list lift and duration figures and I bet that there are many guys running cams that have mega lift but low operating compression due to ICP. Richard I'd like to see you due an article on this or at least a 'sticky' on here.

"I'd like to see you due an article on this or at least a 'sticky' on here."

I'll try to put an article together in the next few weeks and archive it on another forum where I put all the tech stuff I write....

I think maybe the cam companies already have enough to deal with, there are so many aspects to grinding a cam. The other thing is......who's gonna care? All the customer has to do is pick up the phone and call his favorite grinder for a recommendation. There are very few individuals who want to know this stuff and even fewer who are equipped with enough reasoning power to figure it all out and take advantage of the information. There are how many members on this board....17,000?? I can count on one hand the number of individuals who have shown enough interest to ask questions to the point of getting me excited about teaching this dung. By the way, thanks to those of you who have expressed a genuine interest.