General Tech Cam measurement.

[rooster]

Hi,

I am in the process of timing the cam in my TR6 engine and I am having trouble determining the point at which the cam starts to open and the point at which it is closed.

I am using a dial gauge on the exhaust and intake lifters to do the measurements. The cam I am using is a stock regrind of a post 72 cam.

I find that it is very difficult to get an approximate start of the lift (or end of close) as it starts off very gradually and then shoots up quickly.

From my research on the net, some videos say that to determine the position at which the cam starts to open, you use the point at which the lifter has gone up 50 thou. However some videos don't mention this at all. I Also find the same for determining the duration of the cam.

So, my question is, which way is correct on a Triumph, from 0.050 lift (easy to measure), or the point at which it just starts to lift (hard to measure)?

Thanks

 

[TR3driver]

If you know it's a symmetrical grind, then just look for the point where both intake and exhaust are open by the same amount. Otherwise, see https://www.tildentechnologies.com/Cams/Tip_DegreeCam.html

Turns out "duration" is a slippery concept. https://www.tildentechnologies.com/Cams/Take_050Dur.html

 

[rooster]

Hi Randal,

If you know it's a symmetrical grind, then just look for the point where both intake and exhaust are open by the same amount.

This is what I'm planning on doing, but I want to find out what the grind on the cam is, and see if it's a pre or post 72 grind. How would Triumph have specified the duration, by using the 0.050 lift?

Turns out "duration" is a slippery concept. https://www.tildentechnologies.com/Cams/Take_050Dur.html

Exactly, and the reason I need help.

Thanks

 

[dklawson]

Rather than focus on what the duration "is", I suggest you turn the engine over in the normal direction of rotation, stopping every 10 degrees to read and record the lift on the intake and exhaust pushrods for cylinder #1. Go all the way around for a full 360 degrees then do it two more times. Put all three sets of data into a program like Excel. Average the three sets of exhaust lift data then average the three sets of intake data.

Take the averaged data and create an X-Y scatter chart of the intake lobe and the exhaust lobe. From the resulting chart you will be able to confirm if the profile is symmetrical. Once you know the grind is symmetrical you can decide if you are comfortable setting the cam timing with the intake/exhaust rocking (i.e. equal lift of 0.050").

 

[rooster]

I'm going to try calling the place that ground the cam today, and see if they can tell me what the grind is supposed to be. IF no luck there, I'm going to "plot" the cam as you guys suggest.

Thanks

 

[CJD]

If it was a re-grind with no specific instructions given, it will be a relatively conservative ramp, symmetrical grind. Aggressive ramps and high lift require attention during installation...and the grinder won't re-grind aggressively unless he is sure the owner needs/wants aggressive. Use the .050" lift points to degree it. You can measure the total lift once it is installed.

Then, when degreeing, you do know you can slide the torque curve? Installing the cam up to about 4 degrees ahead of crank increases the low-end torque at the expense of high end HP. Up to 4 degrees retarded increases high end HP at the cost of low-speed driveability. You don't gain/loose any torque. You just slide where it "comes on".

 

[rooster]

I called the cam regrind place and got the specs. It's pretty much the same as the post 72 US TR6 cam, using the .050 lift. I am going to double check it, and then use the point where the valves are equally open to degree it.

Thanks guys for all your help.