Thread: New 350sbc do i have a blow head gasket? Or something worse..

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Electric fans will always turn the fan at most effiecnt speed for airflow--crank driven fans aren't fast enough at idle and too much above cruise speed. An alternator will only put a load on crank when battery or accessories in use require power.

This guy has a wiring problem--------running his fan load thru ignition switch on terminal lets the rotataing fan create a reverse charge circuit that fires the ignition

Originally Posted by jerry clayton

crank driven fans aren't fast enough at idle and too much above cruise speed.

Use a steel 18", 7-blade fan with 2 1/4" to 2 1/2" pitch with a thermostatically-controlled fan clutch and full shroud. In my opinion, that's about as good as it's going to get. Lay the fan down on your bench and measure from the bench to the fan blade leading edge to find the pitch. If you really want to move some air, use a steel fan from an Oldsmobile diesel (1978 to 1985). You should be able to find them in old boneyards. (3 inch pitch).

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"running his fan load thru ignition switch on terminal lets the rotataing fan create a reverse charge circuit that fires the ignition"

That's interesting, I wouldn't have thought of that.

If that were the case, then the symptom wouldn't have gone away when he changed gas.

He should wire the fan on a relay, sourcing power from the distribution node or block in his harness. Fused, of course.

Originally Posted by jerry clayton

running his fan load thru ignition switch on terminal lets the rotataing fan create a reverse charge circuit that fires the ignition

While this can happen when the car is stopped in extremely rare cases, the current produced by the fan rotation depletes rapidly when there is no power source (wind or other) to continue their rotation. And as this generating effects current is applied to the coil to run the car slows down so does the firing of the coil bringing it to a stop (usually quickly). Further, the load applied to the fans by the coil's demand acts as a brake to the fans as the load applied across a generator requires horse power thus lessoning their rotational speed faster.

Afterrun is usually caused by a hot spot in the cylinder, be it carbon or metal surfaces themselves, the heated area fires the fuel charge as it enters the cylinder causing a very rough and uneven continuation of the motor. It is a sign of other problems that need to be addressed and corrected.

And agreed, the guy needs to fix his electrical system as well.