Cam Timing Question

[Blown & Injected]

When I line up the dots, it sets up on the Intake Lobe Centerline on the cam card, but my intake valve opening and closing is 2* retarded. Advance the cam 2* and the results are repeated as expected: the intake valve open and closing is now right on but the centerline is off.

If it were 1* or less I would feel fine with it but this seems like something is off. The lift I measure is also .003" short.

I am checking with the heads off and have re-set the dial indicator several times in different places to try to eliminate any deflection errors. Got the same result every time.

I am thinking about putting the heads on and checking off the valve retainer, maybe that will change some things?
What's going on here?
Is there something I can do to check my work?

Thanks for the help.

[F-BIRD'88]

Go by the .050" lift events.
Be sure to turn the crank in the normal direction of running rotation. Thru 2 complete valve cycles.

[jred]

what cam and do you have cam card?? using standard type chain and gears I wouldn't doubt that it off a little bit,, if you want it right on you will probby have to drill the stock dowel pin hole out and use a degree bushing to make it right on..

[Blown & Injected]

Comp Cams ground the cam and yes, I have the cam card. Using a Rollmaster timing set with +6, +4, +2, 0, -2, -4, -6 keys.

The card states the Intake opening and closing at .050 but I was not going two full valve cycles in the normal crank rotation, the chain is so tight it does not seem to matter but will try it again tomorrow. Wondering if the very tight chain is causing some trouble???

Gettin' too late to mess with this.

Thanks for the help.

[cjperformance]

Comp Cams ground the cam and yes, I have the cam card. Using a Rollmaster timing set with +6, +4, +2, 0, -2, -4, -6 keys.

The card states the Intake opening and closing at .050 but I was not going two full valve cycles in the normal crank rotation, the chain is so tight it does not seem to matter but will try it again tomorrow. Wondering if the very tight chain is causing some trouble???

Gettin' too late to mess with this.

Thanks for the help.

Dont get too hung up on 2 deg on a shelf cam. If you don't plan on testing it at several different points to find the best cam position I would suggest to set it up with 2 to 3 deg advance on the IVC timing you actually want. Once the chain has some run time it will be right on where you want it.

[F-BIRD'88]

The "centerline" shown on your cam card is the math result of the .050 events center, not the point of max lift.

Use only the .050 events to determine the centers from that. Why? Because the cam lobes are not a symetric shape, and the point of max lift is not on center.

The cam phasing is correct, when the .050 events match the cam card.

[fastblackracing]

I always measure the cam at several different points...... .050" open and close while noting
the durations, also the .200" duration if you have the info and yours being a comp you can find the
.200 duration by looking up the lobe numbers or I can do it for you.

I also Check the ICL by going .100" on each side of the nose and you can do the math to see if the @.050
numbers produce the proper ICL.

Is this a roller cam? I have seen cams that were ground with lobes that were designed for a different
cam diameter family that will then produce different timing numbers on the cam that the different
lobes were used on......meaning a very small buick cam core that a larger cam core lobe was used on
like a sbc xfi lobe that was designed at 224* duration but ends up being a few degrees different on the
small buick cam core as the lobe has now been shrunken to fit the smaller cam.

I also check the exhaust lobe the same way and then do a "count down to overlap" meaning
I check the intake lift approaching TDC as it is increasing in lift and note the lift in 2* increments from say
20 degrees BTDC to 20 degrees ATDC....you can see how the valve is opening vs. piston position,
and then do the same with the exhaust valve as it is closing during this same time span.....

This way you can see where the valve overlap triangle is located in relation to piston position.

Then do it again on the cylinder that is 180 degrees later in the firing order.....to see where it falls.....
this method tho time consuming will give you the best idea of where the events really are, and it
will give you a much better understanding of cam events once you grasp everything.

sorry for the long reply but if you take the time to learn this you will be better off.

I have seen comps stuff be off 10 degrees, though the last 10 years have been much better, also if its
a roller you really need to use the roller lifter as I bought one of those nifty little tools that goes
in the lifter bore and is held in with o-rings and found it was giving me 2-3 degrees of error as the
end of the plunger that rides on the lobe was not the right radius and the tool would also cock slightly
in the bore.