"Lugging" Camshaft

Dear Ron,

I have a question: Is it possible to degree a cam by using a compression tester? My friend says that you hook up a comp. tester to a cylinder and crank the engine. You then advance or retard the cam to see if compression goes up. Is this possible?

If this does work, can you also do it on a dual overhead cammed engine? One cam at a time on the dohc?

Thanks. I have 25 bucks riding on this!!

Dave

Dear Dave,

I am often asked about the advisability of dialing in a given camshaft's angular phasing relationship to the crankshaft (i.e.0.4 degrees advance or retard, etc.) by the use of a compression gauge tester. The scenario usually seems to be presented as follows. Advance the cam 2 degrees at a time until your gauge reading peaks and begins to actually decline. Then go back to the highest gauge reading position. This procedure seems to be popular among a small but devoted group of its followers (most of whom are oval track racers) that adamantly insist it's results are unquestionably correct in every case.

Before passing judgment, let us review why gauge readings increase up to a certain point when you advance the camshaft. Advancing the cam makes all the valve events (intake open/close and exhaust open/close) occur earlier. In particular however, it is the earlier intake valve closing which raises your gauge readings by generating higher cranking compression and higher torque in the cylinder (because of the resulting longer effective compression stroke!

This would seem to imply that all camshafts will perform better when timed in such a manner and for the most part this statement is generally correct, especially in oval track. Let us however, dig a little deeper to discover how so and to what extent.

Timing or "Dialing-in" your camshaft by this method involves cranking the engine over at starter-crank speed (about 200/300 RPM). This, of course, is relatively low considering most oval track engines operate between 2500-6500 RPM. Additionally most oval track cars (especially those on 1/4 - 3/8 mile tracks with good hard biting traction surfaces) respond favorably to higher torque in the low-mid RPM range to get "off the corner" faster. In fact he who gets off first, usually can't be caught because strait-aways on the these short tracks aren't long enough to allow for the cars with better "top end" power to catch up.

No wonder then, why nearly every one of these cars would typically install their camshafts 4,6,or even 8 degrees advanced. But is this the range in which the gauge readings will peak as far as cranking compression goes? Often not, for those readings are taken at a starter-crank speed of 300 RPM.

What's better for increasing compression here is not necessarily what's best for doing so at 3,000 or 4,000 or 5,000 or higher RPM. Relying on starter-crank speed gauge readings to determine your cam's installation position may have you advancing your cam 10,12 or perhaps even 15 degrees or more in some cases. While such an extreme advanced position may be appropriate for a camshaft known to be way too big in duration for a given application, I would hardly recommend it for most other cams whose specs are generally considered to be "in the ball park." In such a case you would almost always be over advancing the cam, and throttling yourself back at higher RPM.

As to the question of a dual overhead cammed engine where the intake and exhaust cams may be positioned independently of each other, only the intake cam need be advanced for this purpose.

Also keep in mind that when you advance any camshaft your intake valves will be off their seats further at T.D.C. (Top Dead Center) decreasing your intake valve to piston clearance about .010" to .015" per cam degree of advance, so be sure you have enough clearance to spare.

In conclusion, while it is possible to position your cam in this manner, beware of any extreme advance indications or "false positives." Use discretion in interpreting the results.

Finally, I absolutely do not recommend this procedure for new camshafts! Why? Because excessive cranking of the engine before it fires up for break-in will wipe off the pre-lube of a new cam and lifter assembly rather quickly and can easily lead to premature cam-lobe failure due to lack of lubrication and the resulting metal to metal contact. If you wish to experiment, wait instead until the camshaft is properly broken in.

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