Both my '71 454 and '72 350 flow heat when fully warmed up and driving say on the freeway. But I don't have the heater blower on. Is this normal? If so, where does this heat come from. It's more than just radiated heat coming from the engine compartment into the cabin. It's actually heated air that's flowing in. Not fast, but enough that things get a bit toasty under the dash. How can I block it off?

Steve

Steve,

The air is coming from your heater core. Even though you don't have the heat on, the force of the highway air is pushing air through the core.

Your best bet is to bypass the core completely.

Also, the shift boot is famous for tearing, and a great source of heat. This gathers air heated by your exhaust.

I hope this helps. Good luck

Alan

Gribble123 said: Steve, The air is coming from your heater core. Even though you don't have the heat on, the force of the highway air is pushing air through the core. Your best bet is to bypass the core completely.

and if you have astro vent the blower is always on..

that heater core bypass works wonders

Even with the heater bypassed, you will still get a lot of heat thru the floor. There's a lot of hot cast iron up front. There has been other posts here about that. Some one said there is a foil backed insulation available at Home Depot to put under the carpet

Hmm...so bypass the heater core. Ok. Then of course no heater right? Probably fine, since the radiate heat from the engine and exhaust still come into the interior a la shifter and floors. Ok, guess I'll try this. So the air is flowing by the heater core and then through some kind of vents under the dash? Couldn't I just block these vents? Haven't looked under the dash yet.

Steve

no you install a bypass valve in the heater hoses.. turn the handle and heater works again..

Also, you don't want to block those vents. You need verntilation when the windows are up. My '81, and I assume older models as well, has the blower fan on very low ALWAYS pushing a little fresh air into the cockpit. Even with the blower switch off, the fan still runs slowly. If it didn't, with the very small interior volume of our cars, the air would get stale in a hurry, plus the fresh air reduces the effect of any exhaust fumes that may find their way into the interior, both those created by the car and the driver!

I have the same thing 70 vette 350 4 speed. Where can you buy the bypass valve.

Bypass valves are available at most auto parts stores. You will need to know the ID of the hoses going to/from the heater core (I don't think the OD of the hoses is an issue) to get the right one.

Another source of flowing air could be an open hole or slot in the firewall. Missing or failing grommets or seals are usually the culprit.

There are a bunch of other threads on this topic so a little browsing will answer some specifics.

Years ago, this was called, "Astro Ventilation", by GM. Started around 1968 or a couple of years earlier. It was always on. On some of the cars, the words, astro ventilation, was scripted above the side dash vents. The ventilation was advertised as coming in thru the floor heater vent and flowing out of the car, but I never saw where it flowed out at. As far as heat coming out of the floor, I would assume something is getting bypassed somewhere.