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This is just some of my current observations and if you have different experience or information feel free to jump in.

Ever since I got my new Comp cam I have been wondering about the specs on it. Its a symetrical grind with [email protected] and .560 lift. Most of the small block Ford cams I have seen are split duration cams with a little more duration and lift on the exhaust side. The general explanation for this has been that the heads don't flow as well on the exhaust as they do on the intake so the split duration cam helps make up the difference.

My earlier post regarding head flow and the optimum 80% ratio seems to bear this out when you are dealing with the stock heads and even most of the aftermarket heads. Of all the heads I listed, only the Windsor Jr's and Performer RPM heads hit the magical 80% flow ratio out of the box. Most of the other heads could be brought up to that ratio(or close to it) with some extra exhaust porting work.

I went back and did some research on stock engines and saw every combination of cams including intake biased grinds, symetricals and exhaust biased grinds on different engines. I found out that the A and C code 289's both had cams that favored the intake side. The 271 horse 289 however had a symetrical cam grind. An extensive tuning project for the K code in my engine book recommended going .002 tighter on the intake lash which would give it a little bit more intake lift. The big block motors(428, 429) tended to have asymetrical cams favoring the exhaust side. The 390 had a symetrical grind. This didn't seem right with what I thought I understood so I had to be missing something there.

My Performer RPM cam was a split duration cam with more on the exhaust side and I was running ported stock 302 heads. However, the new Comp solid cam that was recommended for my current setup is a symetrical grind that has the same duration and lift on both sides. I happen to be running Windsor Jr heads which flow out of the box right at 80% but something was still missing and it finally hit me. Flow is not just in the heads, DUH! Flow involves the whole system including the intake, carb, heads and exhaust. Consequently, you can't just pick your cam based on the head flow numbers. You have to look at the rest of the package that you intend to run.

And in conclusion ladies and gentlemen(LOL), to know what sort of cam you need you have to have a really good idea of how the whole system will flow. If you run a dual plane and a small venturi carb you might actually need an intake biased cam like the A and C code 289's used. If you have a lot of intake and good flowing exhaust but you are running stock heads or aftermarkets that are smallish on the exhaust side then you probably will need an exhaust biased cam. If you have ported heads or aftermarkets that flow at the magical 80% optimum and your intake and exhaust are well matched then you probably need a symetrical cam. If you are running a boosted application(blower, turbo or NOS) you will most definitely want an asymetrical cam with more duration and lift on the exhaust side.

Going back and applying this to my set up, on my intake side I run a Victor Jr single plane and a 650 double pumper Holley. This part of my setup is good for 7000 rpms so plenty of flow there. My Windsor Jr's flow right at the optimum 80% factor out of the box. I run open headers so I don't think I have to worry about any exhaust bottlenecks, LOL. Based on all this the symetrical grind Comp cam is perfect for my setup. I guess somebody was watching out for me on this one, LOL.





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As you you, Chevies had better exhaust ports in the past. A lot of cam companies simple used lobes designed for them on Fords because they allready had them and didn't want to spend the r&r for the Ford profiles.

I'm suprised that you're using the Jr, heads. World product usually recommend the Jr's for mild 289's & 302's, and recommend the Sr's for most applications.

Now, if you REALLY want to read something that'll change your way of thinking about cylinder heads and flow, and make you wonder about everything you've read in the past, go to www.theoldone.com. Very interesting reading!

Tom
You can do anything you want to......ONCE!
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The degrees of lobe seperation have a lot to do with how an engine performs also.... A camshaft with a 104/106 degree lobe seperation will be "doggy" at a low RPM, but will come on strong at upper RPM's.

Most street cams have a 112/110 degree lobe seperation.

Camshafts with a lobe seperation of 110/112/116 will have a broader power band and have a smoother transition from low-mid-high RPM's, but lack the punch at the high RPM's

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Good points all. Sometimes I have seen posts here where people want a cam reccomendation for their basically stock engine...with the plan that they will change the cam now, and replace the carb, intake, and heads latter. Seems to me this is an impossible task.

BTW Johnny. Your torque convertor seems to work fine based on 30-feet of test driving I did last night backing out of the barn and then pulling back into the barn. Thanks again for helping me out with that!

Phil

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