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Measure The Diameter of the Exhaust Throat - What's Your Problem?

Mystery Cam
Dominic O'Loughlin; Preston, Victoria, Australia: I love your column, it's the second most educational one in the magazine (next to Krass & Bernie, of course!). Can you please tell me the simplest and easiest way to determine unknown cam specs on my preloved V-8? All I know (and all the previous owners knew) is that it has a hydraulic cam and I've had it chassis-dyno-tested at 220 rwhp. The engine is a GM Holden V-8 originally 308 ci stroked to 355 inches. It is currently out of the car. I suspect the stock heads are holding back its true potential. I want to port them to suit the cam installed. Cheers from Australia!

Jeff Smith: It's great to hear that Car Craft has found its way Down Under, Dominic. While I don't know very much about the Holden V-8 engines, it really doesn't matter because the procedure for determining the cam timing with the cam in the engine is a relatively easy thing to do on any four-stroke engine. What you will need is a lifter, a degree wheel, a fabricated pointer, a piston stop, and a dial indicator to determine when the cam lobe opens and closes. The best way to do this is to access the lifters as opposed to attempting to read lifter movement off the pushrod.

The first thing to do is remove all the spark plugs and come up with a way to rotate the engine in both directions. The crank bolt may work, but what can often happen is the bolt loosens when turning the engine backward, the degree wheel spins, and you have to start all over again. Next, you will need enough room to mount a degree wheel on the front of the engine. That usually requires removing the crank pulleys to mount the degree wheel directly to the harmonic balancer along with a coat-hanger wire pointer. Rotate the engine by hand until No. 1 piston is roughly at top dead center (TDC). Line up the pointer to the TDC mark on the degree wheel. Now you'll need to turn the engine backward about half a turn and install a spark plug-style piston stop in the No. 1 spark plug hole. Slowly rotate the engine until the piston hits the piston stop and record the number of degrees on the wheel before TDC (BTDC). Next, rotate the engine backward until the piston hits the stop and record the degrees after top dead center (ATDC). To establish true TDC, the numbers need to be the same on either side of TDC at the stops. This step is absolutely essential to accurate cam readings.

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