Cam information Duration and Advance & Retard

[Dragon]

From Crane Cams:

What is Camshaft Lobe Separation and how does it affect the engine?

Lobe separation is the distance (in camshaft degrees) that the intake and exhaust lobe centerlines (for a given cylinder) are spread apart. Lobe separation is a physical characteristic of the camshaft and cannot be changed without regrinding the lobes.

This separation determines where peak torque will occur within the engine's power range. Tight lobe separations (such as 106?) cause the peak torque to build early in basic RPM range of the cam. The torque will be concentrated, build quickly and peak out. Broader lobe separations (such as 112?) allow the torque to be spread over a broader portion of the basic RPM range and shows better power through the upper RPM.

How does Advancing or Retarding the camshaft's position in the engine affect performance?

Advancing the cam will shift the basic RPM range downward. Four degrees of advance (from the original position) will cause the power range to start approximately 200 RPM sooner. Retarding it this same amount will move the power upward approximately 200 RPM. This can be helpful for tuning the power range to match your situation. If the correct cam has been selected for a particular application, installing it in the normal "straight up" position (per the opening and closing events at .050" lifter rise on the spec card) is the best starting point.

Enjoy

[FORD428CJ]

You didnt talk about Duration. That is just as important as the rest of the info given~

DURATION: Duration is how long the cam holds the valves open. It's expressed in degrees of crankshaft rotation (remember, the cam rotates at half the speed of the crank). A 280-degree-duration cam holds the valves open longer than a 260-degree-duration cam. Holding the valves open longer allows more air and fuel into the engine and also allows more to get out through the exhaust. Longer duration (higher number) improves top-end power but almost always sacrifices low-end torque. Lower duration improves low-end torque and makes the car idle better, but it limits top-end power, and you can get only so much valve lift with a short duration cam due to the rate-of-lift limitations of the lifter. Roller cams, which we'll discuss below, have the advantage of allowing high rates of lift with relatively short duration.

The confusing thing about duration is the difference between "advertised" and "at .050-lift" duration. At .050-lift duration is measured from the point where the cam moves the lifter up .050 inch until .050 inch before the lifter is all the way back down. Most cam manufacturers differ in where they start and finish measuring for advertised duration. Some start at .004-inch lift, some at .008-inch and some measure it somewhere in between. That's why the .050-lift numbers are the best to go by. A 280 cam (advertised duration) from one manufacturer could actually have less at-.050 duration than a 278 cam from another, due to the different points at which the companies measure advertised duration


http://www.hotrod.com/techarticles/113_ ... ology.html

[FORD428CJ]

Also Duration @ .050 tells ya how fast the valve opens and closes!

[My427stang]

Both of you guys posted good stuff and needless to say, are educating the young guys.

However, I have to bring up a huge pet peeve that I have.

If the correct cam has been selected for a particular application, installing it in the normal "straight up" position (per the opening and closing events at .050" lifter rise on the spec card) is the best starting point.


What you are trying to say there is true first of all. However, the term "straight up" as almost always misused nowadays.

Straight up means that a cam with a given LSA, is installed on a centerline that equals its LSA. In other words, a cam with an LSA of 110, would be installed at an intake centerline of 110 degrees and an exhaust centerline of 110 degrees.

Most cams do NOT deliver that way (on purpose) and have 4 degrees of advance ground in, so a cam with a 110 LSA is usually designed to be installed at 106 ICL, which is certainly NOT straight up.

People seem to use "the zero position on the lower timing gear" as "straight up" and that is incorrect.

So sorry for being anal, but the term has a very real meaning and that is LSA=Installed ICL/ECL, so the last comment to be very accurate should read:

If the correct cam has been selected for a particular application, installing per the manufacturers desired opening and closing events at .050" lifter rise on the spec card is the best starting point.

That may or may not be at the zero position of the timing gear, is probably NOT straight up, and should be checked (degreed)

One last comment, for the most part (aside from intake valve / piston clearance) having a cam too far advanced is far less detrimental than having one too far retarded. The reason is that cylinder fill and vacuum is affected worse by a retarded cam due to piston position in the stroke. So I would never say haphazardly advance a cam, but it goes to show the importance of degreeing.

[FORD428CJ]

Totally agree with ya Ross. We used center line method & duration at .050" lift method on the Poor Mans 427 when we degree the cam on it. Its 3 degrees advanced. We also used a dial indicator to find TDC for the crank (with piston and rod installed) Here is some more good info.


http://www.cranecams.com/?show=article&id=3


http://www.carcraft.com/howto/116_0701_ ... index.html

[Dragon]

That is good info but out of the expertise and the monetary range of most people here.

I never install my cam but straight up and then I change my rpm band if I don't like it I advance the cam or retard the cam accordingly. Baja Torque Master cams come in at 1200. If my engine does I put my crank advanced 4 degrees for 1000 or 8 degrees for 800 rpm power band for the off idle torque I like.

[DuckRyder]

I agree Ross,

In addition, you really need a degree wheel and dial indicator to determine if it is even installed as designed, they can be ground several degrees off of the cam card which is already (as noted) advanced in nearly all cases.

Simply sticking the cam in with the timing marks aligned is not a guarantee of optimum performance; in fact, it is nearly irresponsible on a performance motor.

It is my opinion that anyone contemplating the building of a performance engine needs to consider the time and money required to understand these principles and acquire the tools and knowledge to apply them as a part of the building budget.

We have posted several good discussions in the past and there has been a lot of excellent discussion on the FE Forum as well. In addition, comp cams has some good technical information on their website that is easy to understand.

http://www.compcams.com/Technical/TimingTutorial/

[My427stang]

Thanks Robert

For everyone, remember, that it is not only the cam that can cause the cam to appear to be ground incorrectly either.

Loose chain, retards slightly
Tolerances in the upper gear, lower gear
Crank twist, machining differences in the crank itself

All of these together can add up and make an engine run very different than you expect.

Matter of fact, an engine builder I respect greatly once told me that he also checks valve timing AT THE VALVE, as once you get the cam in place, all the next level of moving parts add more variables.

Interesting stuff. Now I dont recommend everyone goes crazy chasing a degree of difference, but if you understand what can happen, you'll appreciate why the big guys measure everything

[Dragon]

I always measured at the valve rocker and not at the lifter. It was easier to get the dial indicator to mount.

[FORD428CJ]

I have always measured mine at the lifter. But Its doesn't mean its wrong! LOL I think the other way (at the valve) there are too many variables and I like to be as dead on as possible. I don't want to fight the whole thing. That's just me. More then one way to skin a cat. Ether way, it doesn't matter. JMHO

[Dragon]

True. My way was brought on by not being able to keep the dial indicator clamped to the rounded inside surfaces. At the rocker end the mag base set on the rail for the valve cover.

[My427stang]

I do mine at the lifter as well, then just verify rocker ratio at the valve.

However, I suppose if you used intake centerline method, instead of .050 events, it "should" be more precise at the valve, assuming of course slow moving valve operation is the same as at speed