Degreeing cam

[Texasdoc]

OK, what am I doing wrong? Brent, feel free to chime in here.

Trying to degree my new custom Comp cam. These are the steps I've taken.

1. Install a pointer and verify crank and piston 1 is at TDC. I have a dial caliper. I have verified TDC by visualizing the #1 piston is at the top of the stroke (head off), and then using a dial caliper to measure .050 travel before TDC and .050 travel back down after TDC. Both readings are the same on the cam dial before and after the TDC mark. So I know the motor is at TDC and the cam indicator dial is accurate.

2. Insert a sold lifter into intake #1's bore. I know this isn't a roller lifter, but it should still work. Set the dial indicator onto the top of the solid lifter. Rotate the crank until the lifter rises .050. This is 20 deg BTDC. Keep rotating the crank until the lifter reaches full lift, then goes back down. Stop when the dial gauge reads .050 before it gets back to zero. This reads 250 ATDC.

3. Add 20 + 250 = 270. Divide by 2 = 135. This should be the installed centerline, right? My cam card says: Valve timing at .050 should be intake open at 9 BTDC, close at 41 ABDC (221 ATDC). Cam installed 106 degree intake centerline.

OK, so what am I doing wrong? The cam is installed straight up. Why would I be off by 11 degrees on the opening, 29 degrees on the closing, and 29 degrees off on the intake centerline?

Any suggestions?

[bobcowan]

Use a roller lifter, your measurements are more accurate. Since the lifter is not compressing a spring, it doesn't matter. Use two lifters and a dog bone. The big issue is making sure the lifter drops back into the bore to maintain contact with the cam. I just push it down with my finger.

[blykins]

I'd say it's your tools and the way you have found TDC. You need a piston stop and check TDC with that and the degree wheel.

Use a roller lifter...

Make sure you're using the correct set of numbers on the degree wheel if it has two sets. One set is for the ICL method, the other for the valve event method.

To check it with ICL method, you go to full lift on the lifter, zero the indicator, back up about .100", roll it in normal direction until you get to .050" then write down the degree wheel setting, then keep rolling until it hits zero then goes back to .050". Record that number, add to first, divide by 2, and that's the ICL.

Do the same thing for the ECL.

Add ICL and ECL together, divide by 2 and verify LSA.

Check lobe lift on both intake and exhaust.

[Barnsnake]

The roller lifter contacts the cam lobe at a different point than the flat tappet through most of the cam cycle. The flat tappet will give you accurate data on the base circle and at max lift only. Everything else will be out of time.

[eschaider]

You guys are missing it.

Cam timing figures at 0.050 Valve Lift are measured at the valve not at the cam. If you measure at the cam you have to factor for the rocker ratio. Additionally if you use a different rocker ratio than the cam specs were developed with, you need to translate all the cam phasing specs to reflect the rocker ratio you are actually using.

If you choose to degree at the cam instead of the valve anyhow, then you have to use the centerline approach where you come within say 0.100" of max lift and record the crank position both before and after the max lift point. The cam's current centerline phasing will be exactly between those two figures.

Ed

[blykins]

Actually, those numbers are given at tappet lift. Some cam cards can be deceiving by saying "Valve timing", but if you looked at a Crane cam card, they word it more correctly as "Cam timing".

You have to make sure you use the correct range of numbers on the degree wheel if you use the valve event method. This method is actually more accurate as a lot of cams today do not have symmetrical lobes. If they were perfectly symmetrical as far as lobe design, then the centerline method would be accurate as you would have the same amount of lobe on either side of the centerline.

I use a lot of Bullet cams and a lot of their lobes are asymmetrical, so there's only one way to accurately degree the cam in.

There's a lot of room for error when degreeing a cam. Some common mistakes are:

1. Not finding true TDC.
2. Mounting the dial indicator so that it goes out of travel, or mounting it so that the lifter or dial indicator plunger doesn't auto-return easily.
3. Using the wrong set of numbers on the degree wheel.

You should be able to find TDC, tear everything down, do it again, and get exactly the same number. You should also be able to repeat every step that you make, and also double check yourself at the end by degreeing the exhaust lobe as well, then checking the LSA against your results.

Lift numbers are often given in both lobe lift and valve lift. If you get the valve lift numbers, then you have to divide by the rocker ratio that's given on the cam card, then multiply by the actual rocker ratio that you're using.

[Gaz64]

Originally Posted by eschaider

You guys are missing it.

Cam timing figures at 0.050 Valve Lift are measured at the valve not at the cam. If you measure at the cam you have to factor for the rocker ratio. Additionally if you use a different rocker ratio than the cam specs were developed with, you need to translate all the cam phasing specs to reflect the rocker ratio you are actually using.

If you choose to degree at the cam instead of the valve anyhow, then you have to use the centerline approach where you come within say 0.100" of max lift and record the crank position both before and after the max lift point. The cam's current centerline phasing will be exactly between those two figures.

Ed

.050 figures are lifter rise, not valve lift, never has been valve lift.

[Wbulk]

Bobcowan and Jim are correct, but Jim was more articulate on the reason. I made the same mistake last year after degreeing in many cams over the years. I was helping a friend with his Shelby big bore aluminum block and the cam measured way off. I checked and re-checked everything three times and could not figure it out. This was a Thursday night so somewhat embarrassed I said let's try it again Saturday. When I was laying in bed that night it dawned on me. I have special flat tappet lifters for Fords and Chevys I made 30 years ago that are 6" tall so I can put the dial indicator right on top without using a pushrod. I used one of these in his engine. Saturday we put in some solid rollers and it checked out within a degree.