Cam Lobe Separation Spec...HELP!

[GRX]

I thought it was measure angle from the CENTER of exhaust lobe to CENTER of intake lobe? I must be wrong because I layed out my cam specs from Crower in AutoCAD and it doesn't come out to be LC 114*. HELP!! Intake: Duration: 260* Exhaust: Duration: 270* ________________________________________ _________________________ Specs at .050” tappet lift: Intake: Duration: 209* Exhaust: Duration: 217* Intake: Opens at: -5.5 ATDC Closes at: 34.5 ABDC Exhaust: Opens at: 46.5 BBDC Closes at: -9.5 BTDC ________________________________________ _________________________ Lobe Lift: Intake: .330” x 1.5 rocker= .495” Exhaust: .343” x 1.5 rocker= .515” Lobe sep angle: 114* (I hope because I need to pass SMOG have a good idle too: Supercharged app) [Modified by GRX, 3:22 PM 4/11/2003]

[SWCDuke]

Could be the lobes are asymmetrical like the OEM LT-1 cam. Chevrolet specs out "points of maximum lift" and most guys just compute "lobe separation angle" from this data without understanding that some cams are asymmetrical, so the the points or maximum lift DO NOT correspond to the center point or the lobe. Not all cam lobes are symmetrical!!!

That's why simplified representations such as lobe center angle and duration and .050" lift that get thrown around here quite a bit don't mean much, particularly if you are comparing cams with similar numbers. The only way to understand a cam in detail and compare with others in detail is to do what you have done - get a detailed representation of the lift-crank angle relationship. Then take the first and second temporal derivatives to determine the velocity and acceleration profiles. In short - reverse engineer the cam, so you know exactly what you're dealing with. If you want to pay to have it analysed by a "cam doctor" you get a nice computer generated printout of the data.

Once you have the lift crank angle diagram, and if you understand your actual rocker geometry you can pick the actual timing points required to represent the cam accurately in a simulation program. Though these programs don't allow you to input the entire lift-crank angle diagram, if you can pick out the actual .006"/.006" valve lift points, the programs will provide reasonable representation of valve action, which is critical to achieving accurate simulation results. For an asymmetrical cam like the LT-1 cam, I also have a file for the .006"/.010" valve lift points to compensate for the fact that the simulation program assumes the lobe is symmetrical. The .006"/.006" file makes a bit more top end power at the expense of a bit of top end torque. Vice versa for the .006"/010" file, which is believe is closer to actual performance.

Duke

[CFI-EFI]

Again, with appologies to SWDuke, whose dissertation I have no quarrel with, a little simple math bears out the cam card. For the sake of a "test" of the numbers, assume the cam is not asymetrical.

Specs at .050” tappet lift:

Intake: Duration: 209*
Exhaust: Duration: 217*

Intake:
Opens at: -5.5 ATDC
Closes at: 34.5 ABDC
Exhaust:
Opens at: 46.5 BBDC
Closes at: -9.5 BTDC

Lobe sep angle: 114*

The intake duration is 209*. That would put the theoritical centerline with a symetrical lobe, 104.5*s past the opening point, or 110.0*s ATDC. (5.5 + 104.5 = 110.0)
The exhaust duration is 217*. This puts the centerline at 108.5*s past opening, or 62*s ABDC. (108.5 - 46.5 = 62.0)
Two points to remember, here. First: 62* ABDC is the same position as 118* BTDC. If you are at TDC, you go counterclockwise 118* to get to the exhaust centerline. Also from TDC you go 110*, clockwise to get to the intake centerline. 118* + 110* = 228* The second thing to remember is that because the cam travels at half crank speed the lobe centerline separation angles are measured in CAM degrees. 228 / 2 = 114. SWCDuke's engineering and lab work, notwithstanding, it appears that the specs on your cam card "add up".
By the way, those specs should produce respectable sniffer numbers. Good luck, and...

RACE ON!!!

[Vetterodder]

The only Crower I could find that matched your total and .050 numbers is the 00464 hyrdraulic roller. It has a 110º lobe separation and is ground with 4º advance. Installed straight up would put the intake centerline at 106º, exhaust at 114º. A cam with 114º lobe separation installed 4º advanced would have it's centerlines at 110º/118º

[GRX]

Thanks guys! Vetterodder, the cam is a custom grind made from that 00464 Crower p/n. :thumbs: