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Resident Ford Nut
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Discussion Starter · #1 ·
I've never thought about it; what determines the opening and closing events of the valves on the cam card? IE set intake opening at 28* as opposed to 34*. I'm assuming it has the do with the Crank/piston position to achieve max cylinder pressure or some other thing, but what determines it ? :)bulb

You can retard or advance the cam to move the power around but for the most part doesn't the power stay about the same you just move the rpm. At some point though I guess the Cylinder pressure will drop off and you'll lose power.

I figure someone here would know, even if Straub doesn't see this.

Sleeper CP :D
 

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Resident Ford Nut
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Discussion Starter · #3 ·
Not lobe sep. but a cam card may read : intake opens at 27* BTDC Closes 70* ABDC Exhaust opens 77 BBDC Closes 26* ATDC. That's part of lobe sep.

The question is, what parameters are used by the cam grinder to determine the desired opening point as far a degreing in a cam. You may need to advance or retard your cam a few degree's to match the cam card. Why is it desired to open the intake at 27* as opposed to 24* or 32* BTDC ?

I hope that makes sense :)bulb

S CP :D
 

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I believe he is refering to the .050 opening. If the cam is symetrical, as the duration gets longer, the valve has to open sooner, I.E. 34* as opposed to a shorter cam that would open at 28*. The intake opening is a product of both the duration, and the LSA. Two other wise identical cams having differant LSAs will have differant opening points. A .050 opnening at 34* with a 108* cam, will be at 28* on a 114* So the question becomes, does the cam open at differant points because it is shorter, or because it has a differant LSA



 

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Premium Member
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Not lobe sep. but a cam card may read : intake opens at 27* BTDC Closes 70* ABDC Exhaust opens 77 BBDC Closes 26* ATDC. That's part of lobe sep.

The question is, what parameters are used by the cam grinder to determine the desired opening point as far a degreing in a cam. You may need to advance or retard your cam a few degree's to match the cam card. Why is it desired to open the intake at 27* as opposed to 24* or 32* BTDC ?

I hope that makes sense :)bulb

S CP :D
The point the cam grinder gives may be just a be the split overlap (straight up) location. Isky, to my knowledge does not grind cams with an advance ground in unless it is specified on a custom. If the grinder feels the cam will perform better advanced, they may grind it that way. Where the point actually belongs is a matter of everything from engine CID, the entire induction, compression, rod ratio, trans type, car/boat, weight, use, gearing, etc,etc,etc. And this is where a guy like Cstraub earns his $$$$



 

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Resident Ford Nut
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Discussion Starter · #6 · (Edited)
Thanks GN-7,

You are correct it's just a function of math. I was just asking for personal reference.

A cam with an intake duration of 280* at .050 and a lobe sep of 114* would have an intake opening point of 26* if ran straight-up.

280/2 -114* lobe sep= 26*

I just wanted to know how that number was derived, now I know. And then there's the deal with the cam having max lift at peak piston velocity,roughly 70-75* crank rotation or when the big end of the rod is at 90* to the crank. Blah, blah, blah.......

S CP :D
 

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Here's one event to consider:

IVC can be hugely important. Think about which way the piston is moving. Wait a little too long and you lose air/fuel (compression too) you had in the combustion chamber. Don't wait long enough and you miss out on some air/fuel you could have used to your advantage.

I say you set IVC where you want it to be vs "it just ends up where it is because of LSA or etc."

Remember, one thing effects the other....but IVC is a big one. I'm not speaking of .050" here either.
 

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Sit N' Spin
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Speaking of "advance being ground in"...

What is the real reason behind cam manufacturers make some cams that have advance "ground in"? I have the Comp Cams XM278H cam and that one has 2* ground into it (112 LSA, 110 ICL, 278 int/292 exh @006", 234 int/244 exh @ 0.050"; 564 int/568 exh lift).
 

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Speaking of "advance being ground in"...

What is the real reason behind cam manufacturers make some cams that have advance "ground in"? I have the Comp Cams XM278H cam and that one has 2* ground into it (112 LSA, 110 ICL, 278 int/292 exh @006", 234 int/244 exh @ 0.050"; 564 int/568 exh lift).
Back in the 60s, cams weren't ground with "advanced". And the vast majority of the cam purchasers just grapped them and slammed them in. Then along came Comp Cams, with there "Energizer" series of cams, and sold the living hell out of them. What made them different? Well they may have had a little quicker ramp, but more than any thing, they were ground 4* or more advanced in most cases. Why? Because they knew that a huge number of buyers were buying cams too big for their motors, and knew that if they advanced them, they would run better, mainly on the street. Many of them had wider LSA also. And they made a huge name for themselves. No cam needs to be "ground" advanced. It is all in where you install it. The cam manuf. may know a certain profile works better advanced in most cases and grind it that way. Companies like Isky don't play that game except on cams for a certain application, like upgrade cams for 5.0 fords and LSs with minor mods and EFI. IMO true racing cams should be ground straight up and let the engine builder and cam grinder decide how it should be installed.



 
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