How to measure cam?

[logician]

Hi,

Question: How does one measure the valve openning and closing angles, and is it possible to do this with the cam in the engine?

While I have my 930 engine out, I thought I'd check the cam to verify that it is stock. I'm using a dial gauge to measure the valve lift, and rotating the crank to see when the valves open and close. I get approximatly the right intake openning and closing angles when I consider the angles to be the point where the valve has lifted 1mm (.04") from the seat -- I'm measuring the lift at the valve spring seat.

I've read on the internet that the standard lift to use is .05", but that gives an open duration that is too long. I'm guessing that perhaps for metric cars, the standard lift value might be 1mm. I couldn't find a definitive procedure on the internet. I think the relevant parameters are:

1) What is the lift amount to use to measure the valve open
and close points, .05", 1mm, other?
2) Where should valve lift be measured, valve, rocker (which
side of rocker), or cam lobe?
3) Should all lash be removed from the valve for this
measurement (assuming the measurement is done at
the valve)?

Does anyone know the answers to these questions? Any information would be greatly appreciated.

Thanks!
-Juan

[HawgRyder]

First...if you consider the difference between 1mm (.040") and .050" ...the .050 numbers should be slightly smaller.
This is because the cam has to lift a little more before you start to take the numbers and you finish sooner as the cam comes down through .050"
If you align the degree wheel (bolted to the front pulley) so that 0 deg on the wheel aligns with the zero mark on the pulley....then rotate the engine in the direction of normal travel...and take the reading with a dial indicator pushing straight down on the rocker (on the cam side of the rocker) ... carefully read off the start and finish numbers for both the intake and exhaust.
While reading the intake..take note of the place where the exhaust starts in relation to the intake and vice versa (overlap number).
The amount of lift is the direct reading of the cam lift multiplied by the rocker arm ratio....or take the number directly off the valve end of the rocker with the dial indicator guage.
Bob

[logician]

Hi Bob,

Thanks for your reply:

> First...if you consider the difference between 1mm (.040")
> and .050" ...the .050 numbers should be slightly smaller.

Yes, you are right. I stated my results incorrectly. Here are my measurements:

Factory spec:
open: 3 ATDC
close: 37 ABDC

Measured using .050" lift
open: 8 ATDC
close: 36 ABDC

Measured using .040" (1mm) lift
open: 5 ATDC
close: 39 ABDC

Indeed, the measurement using .050 lift has a shorter duration than the measurement taken using .040 lift. At any rate, I still need to know the following parameters in order to make the measurement precisely:

1) What lift value shoud be used when measuring cam angles as quoted by factory, .040", .050" or something else?

2) Where should the measurment be taken: lobe, rocker (cam or valve side), or valve?

3) If the measurement is done on the rocker or valve, should all lash be removed?

-Juan

[HawgRyder]

As you pointed out....the metric number .040" or 1mm would probably be more correct for a P-car cam.
To get the duration for each lobe you add 180 to the 2 numbers.....so your duration comes out as 5+39+180= 224
I don't know if that is the exhaust or the intake...but it sounds about right .
The overlap is harder to calculate...but if you have 2 dial indicators on at the same time you might get it.
The measurement should be at the valve end if you are using the .040" lift figure (remember...you are judging the valve lift).
The lash should not matter because you are pushing down on the rocker and removing the gap in doing so.
The last (and hardest) measurement.... the timing of the cam.
If you know the overlap...you divide it in half to figure out whether you are advanced or retarded on the cam timing.
If the cam is retarded by 2 - 4 deg it will enhance the top RPM range.
If it is advanced by about the same....more torque at low end.
This is judged against top dead centre of the piston.
These are just generalities..not carved in stone.
Also...P-cars may not allow advance or retard (valves hit pistons) !!
Lots of things to contemplate.
Good luck
Bob

[jluetjen]

FYI - According to my shop manual and spec book, the factory referenced the measurements for their cams at 1mm of lift.