[elefantrider]

Having the unfortunate experience of blowing my original heater core in my AC car, I am going to a lower pressure radiator cap and running a restrictor instead of a thermostat in all my cars with old heater cores.

What do you think?

Do you think the thermostat or the 15lb radiator cap puts more stress on old heater cores?

[Dick Boneske]

It's the surge of pressure from the water pump that blows the heater core, not the 15# cap. The problem is especially prevalent on the '64 and earlier Pontiacs where the pressure line to the heater was unrestricted. The '65 and later engines had the pressure line to the heater restricted with that fancy little flat coil that is part of the hose nipple on the passenger side cylinder head. If you're having the heater core problem on a '65 and up engine, perhaps the coil is not in place.

With any Pontiac, heater core stress can be lessened by putting a restrictor in the smaller hose to the core. You can use any round stock, plastic, steel, etc., 5/8" in diameter with a 1/4" hole drilled through the center. Your heater will still function just fine, but pressure surges as you rev the engine will be lessened. Just be sure to install it in the smaller hose, not the larger return hose.

Remember, the 15# cap raises the boiling point to over 250 degrees F.

[elefantrider]

My '65 has 2 GM replacement hose nipples, one pressed into each head. They were connected to a "T" which feeds the heater core. This was done to eliminate the usual hotspot on the rear of the driver side head where circulation is poor. I seem to remember the nipples are already restricted inside. Mine are.

[Dick Boneske]

First time I've run across this--running two sources for heater feed. I may be wrong, but you've doubled the peak flow with the arrangement you have, even with the restrictors. If it was mine, I'd add the restrictor in the hose to the core as described above. Just my opinion.

[elefantrider]

Anyone with a hydromechanical background here care to chime in? I believe flow is not what bursts the heater core, excess pressure is.

This mod would not have increased pressure, if anything I'd think it would reduce pressure.

[242177P]

Look at it this way. Picture an unrestricted garden hose, with your hand almost
directly in front. Cranked wide open or not, it's the same pressure. But you can
actually feel the 'weight' of the water against your hand as the flow increases.
Why? It's the force of the collision, not the pressure.

Now picture a heater core without the core, just the tank, with the same garden
hose feeding it now. As flow increases, it slams into that 'wall' in front of it. "For
each and every action there is an equal and opposite reaction". This says it's
trying to pull the soldered neck out of the core.

And then there's thermal shock. The water from that garden hose after a while
seemed colder than it actually was, because it was continuously stripping heat
from the warmer object in front of it. Principle works in either direction, more
consistent temperatures subject the core to less stress.

The heater simply doesn't need the kind of volume wide open throttle brings.

[elefantrider]

Interesting. Thanks both. Many racecars run the dual outlets at the back, and I know from experience that cyl #7 always runs the hottest prior to this mod.
The heater nipples are already both restricted. I am not sure of the size of the restriction but I seem to remember seeing it was rather small. Maybe 1/4" already.

Since I'm no longer racing this car, I might just put a brass freeze plug back in the driver side head, once I change the heater core.

[Bluesugar]

Dick, what thickness of said material is required to be of 5/8'' diameter?..What do you prefer?

[8LUG]

How about this.

[elefantrider]

Looks good. What is the restrictor diameter inside?
I will compare it to the diameter of the nipple pressed into the head.

[Dick Boneske]

I use a piece of round stock 5/8" diameter, about 1" long. Drill a hole 1/4" through the center, put it inside the hose, and you're done. Plastic, brass, or aluminum would be best to prevent corrosion.

[Bluesugar]

Thanks Dick,...how far from firewall would you put the restrictor in the hose?

[Dick Boneske]

You could put it anywhere. I'd put it near the pipe nipple coming out of the intake manifold.

[dld]

I have heard of using a valve in the hose to regulate water to the core. A/C cars use it (heater control valve)

In the summer you can close water to core to lessen the heat in the car completely