Q&A / Tech

Mailbag: A Case Study in Cam Size and Idle Quality

You’ve got questions. We’ve got the answers—the Summit Racing tech department tackles your automotive-related conundrums. This week, we’re discussing cam size and idle quality.

Q: I am modifying a 1990 Mustang LX 5.0L. I installed a B303 Ford Racing roller cam. Other modifications include a Flowmaster Force II exhaustunderdrive crank pulley, 180-degree thermostat, K&N air filter, Hypertech Power Module, GT-40 style intake manifold, 1.6-ratio roller rockers, and springs. I used retainers on all valves and did not shim any of the springs. I also installed a 155 liter-per-hour fuel pump and Crane adjustable fuel pressure regulator, set at 42 pounds.After all the parts were installed, the car idled roughly and balked and jerked when run under 2,000 rpm. In the upper rpm ranges, the car ran very well and seemed to rev very smoothly. It also smoked, mostly out of the right tailpipe. I later learned that the lower intake manifold was warped and was pulling oil out of the lifter galley. I replaced the manifold and also removed the cam.

Some people have told me this cam isn’t good for my setup; others say the cam is mild and shouldn’t greatly affect idle quality or low-end performance. Is the bucking and jerking to be expected from this cam? If the cam is advanced two to four degrees, would this help? Will I lose any top-end power? Should I have shimmed the valve springs?

A: The B303 camshaft is very marginal for use with the stock computer, and the Hypertech Power Module you installed is mostly effective at full throttle. Your cam has caused a reduction in engine vacuum, which your car’s computer interprets as a load or acceleration. That causes it to advance spark timing and fuel delivery at idle. This is likely why your vehicle has run so poorly below 2,000 rpm.

If you want a smoother idle, you’ll want to go with a slightly smaller camshaft. Crane makes a CompuCam camshaft specifically designed to work with your vehicle’s computer without causing fits or surging.

Advancing the camshaft two to four degrees, as you suggested, will increase the cam’s low-rpm power—but at the expense of the upper-rpm power. Typically, a two-degree change equals a 250 rpm change. Four degrees equates to 500 rpm. Many aftermarket camshaft makers advance their cams four to five degrees from factory spec already, so we would leave yours alone or you will be really advanced. But, never assume anything! Always degree an aftermarket camshaft properly to avoid catastrophe!

To check your valve springs, bring the cam up to full lift and check the distance between the spring coils with a paper clip or a 30 thousandths feeler gauge. Also, check the distance between the retainer and the valve guide with the same feeler gauge. Keep in mind that hydraulic lifters tend to bleed down a bit when the engine is not running; therefore, this should only be considered a quick check. If these clearances end up being really tight, seek the assistance of a properly equipped machine shop!

If you installed the B303 camshaft without converting to adjustable rocker arms, that is a big problem. You need to do this to compensate for the increased cam lift and duration. Crane has a kit to do this—part number CRN-36655-16. It has adjustable bolt and pushrod guideplates to let you adjust the rockers like a small block Chevy. Just set the rockers to zero lash then add a half-turn.

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2 Comments

  1. Noel O'Kell says:

    Very interesting article, the part about the low vacuum at idle (caused by overlap) and the response of the cars computer should be a wak-up call to many street modifiers.

  2. What about adding a vacuum pump?

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