cam question

[bill jones]

-an example is an old Crane R-268-408-8.
-268 degrees at .050" on the intake and the exhaust.
-.408" lobe lift intake and exhaust.
-a 108 lobe spread.
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-with stud mounted rockers and 1.5 rocker ratio intake and exhaust, with the pushrod length what I consider acceptable:

-242int & 241ex degrees duration at .100" lift (measured at the valve raceready).
- 93int & 84exh degrees duration at .500" lift, .598"int & .577"exhaust is max valve lift.
-the max VALVE lift centerlines measured at .050" down from max lift are 105int & 112exh.
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-same cam with 1.65 rockers and again the pushrods are what i considered acceptable:

-247int & 246exh at .100" valve lift.
-110int & 109exh at .500" valve lift.
-intake centerline was 104 & exhaust was 112.
-max lift at the valve was .645"int & .640"exhaust.
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-if you were to calculate the .408" times 1.5 and subtract .024"int and .026"exh lash you should have .588"int & .586" exhaust but I actually had .598"int & .577" exhaust.
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-then with the 1.65 rockers I should have had .649"int & .647" exhaust.
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-The small increase at the .100" lift point is about 5 degrees, and half of that is happens during the overlap at near TDC.
-The VALVE is moving at something like .008" per degree so we have reduced the valve to piston clearance by about .020" just with the rocker change.
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-This same cam when checked at the lobes showed:
-at .050" lift had 267.5 degrees for the intake and 270 for the exh where it was supposed to have 268.
-at .350" lobe lift the duration was 88degrees int and 91 exh.
-the max lobe lifts were pretty close at .409" for the int and .408" for the exh.
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-I have seen issues with special order cams like what 70MC has mentioned, about the dowel or keyway not where we may like it to have been placed.
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-I once ordered a custom cam for a BBC where they forgot to to change the cam grinder from doing a SBC and I ended up with the lobe spread ground at something like 101 rather than 110.
-Now that was disaster waiting to happen had I not checked the lobe centerlines of BOTH intake and exhaust and then when you get to the checking the other bank there are surprises galore.
-Same deal with the oldsmobiles having 39 and 45 degrees lifter banks and I think the older hemi's have lifter angle differences also, and getting the cams mixed up.
-Then the BBF 428/460 and the 360/390 Fords have the retarded crank sprockets right from the factory that can create problems.
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-All of this sort of stuff is caught by measuring at the lobes but if we get serious enough to check the cam timing.
-A lot of engines have no issues with valve to piston clearance or cam lobe to rods but then again there are a lot that do so we need to check it.
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-I have a LT1 project in the shop right now where the guy has changed the cam, it runs fine, but he installed it dot to dot has no idea how it is phased but it must be fairly close to right.
-He wants to install another set of heads that have larger diameter valves and the Ferrera valves margin thicknesses are about double the thickness of stock valves.
-So when we check the fit of the valves into a stock 4 eyebrow piston with the head sitting on bench we have absolutely zero clearance on the intake even with a .038" gasket.
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-The problem is:
-A-the valves are too thick.
-B-the valve seats weren't sunk deep enough.
-C-the valve notches don't fit the intake valve at all as far as diameter and concentricity is concerned.
-These particular problems have nothing to do with the camshaft phasing but if I hadn't KNOWN about the problem and just happened to catch it the guy would have ended up with a perfectly junked engine.
-We can totally fix the problem by cutting the circumferences of the valve notches of the pistons while they are still in the block, and not have to do anything to the valves or seats, or we can sink the seats .100" and machine the valve margins.
-So what do you do? Just forget the new heads and live with what you have, or do something?
-I vote to do something.

[idunno]

When you order a custom grind from any cam co. you need to specify advance or retard in the relationship to the pin indexing.A consciencous cam grinder may ignore the wishes of the customer of not wanting to move the cam and grind the lobe in position that best matches the core its going on to. In other words if you have a core that is supplied to the cam co. that has .400 lobe lift int and ex. and a 108 lobe sep and the customer wants a 106 installed on a 100 c/l the lobe will be ground off set and making the heat treat thinner on one side of the cam than the other. This happens on all cams by the way.If it gets way off from the core specs the base cir diameter will be smaller and the heat treat depth will be shallow. The cam will last longer if not moved so much just to save a little effort .The cam rotates 360 degress and can always be moved to put it in on what ever centerline you want.I would rather move mine with a bushing,lower crank gear or both if I have to to get a better heat treat on my cams.Keep in mind the core heat treat depth on a un ground core is only around .060 .So if you increase lobe lift by .025 and move lobe sep 2 degrees you have already removed alot of material from the core before advancing the lobe more on the pin location.

[OldSStroker]

When you order a custom grind from any cam co. you need to specify advance or retard in the relationship to the pin indexing.A consciencous cam grinder may ignore the wishes of the customer of not wanting to move the cam and grind the lobe in position that best matches the core its going on to. In other words if you have a core that is supplied to the cam co. that has .400 lobe lift int and ex. and a 108 lobe sep and the customer wants a 106 installed on a 100 c/l the lobe will be ground off set and making the heat treat thinner on one side of the cam than the other. This happens on all cams by the way.If it gets way off from the core specs the base cir diameter will be smaller and the heat treat depth will be shallow. The cam will last longer if not moved so much just to save a little effort .The cam rotates 360 degress and can always be moved to put it in on what ever centerline you want.I would rather move mine with a bushing,lower crank gear or both if I have to to get a better heat treat on my cams.Keep in mind the core heat treat depth on a un ground core is only around .060 .So if you increase lobe lift by .025 and move lobe sep 2 degrees you have already removed alot of material from the core before advancing the lobe more on the pin location.

You make some good points.

.060 case depth prior to grind isn't necessarily industry wide. I assume you mean effective case (Rc50) and not total. 8620 billet steel cams may have more case depth than that. I've seen cast cores that had mid 50s hardness over .100 deep on the finished lobes.

Many cam companies have a selection of cores with various lobe heights and lobe separations so that they can grind a large selection of lobe profiles and separations on them. Every once in a while you might order a combination that only works on a core they are out of. It's Murphy's Law of cam building. Usually if they need to move the advance number to fit it on a core, they will discuss this with the engine builder ordering the cam before they grind it.

Of course, your results may vary, depending on the cam company.