Purpose of Heat Riser

[Dan Grunwald]

I was cleaning the starter terminals on my 67 427/435 and I noticed that the heat riser spring had come off the pin so it was always open(I think). The car seems to be running rich and starting a little hard lately. I put the spring back on the pin so it looks like it will now close off the exhaust on the pass side untill it heats up. The car has side exhaust. Can anyone give me an idea of the purpose of this valve? And why is only put on one side. It would seem to give uneven back pressure and it just seems weird to pot obstructions in the way of a smooth exhaust flow.
Thanks, Daniel Grunwald

[Jack H.]

Re: Purpose of Heat Riser

Valve is there to substantially block exhaust flow on the RH bank as speed up engine warm up process. Emissions are higher until engine has achieved steady state operating temperature. You'll find items like carb's fresh air tube that feeds the choke mechanism, Etc. work off the engine's RH side. So, forcing some heat retention after initial start-up helps to speed the process of thermal soak.

Spring is a bi-metal component that expands as it heats up. The expansion forces open the internal butterfly valve in the heat riser allowing the engine to free flow exhaust. If there's no spring tension, the butterfly valve can wander at random being forced either open or closed based on exhaust gas flow. You do NOT want this to happen!

[John H.]

Re: Purpose of Heat Riser

It forces the exhaust gases from the right bank through the manifold crossover passage and out the left bank manifold; in so doing, it heats the floor of the intake plenum, improving vaporization of the cold (and rich) initial air-fuel mixture, and it provides heat under the remote choke coil to gradually open the choke as the engine warms up.

If the bimetallic coil spring comes off (and the shaft isn't stuck), the butterfly will go full open, as the butterfly is offset on the shaft; if the shaft is stuck with the butterfly closed (as many are), you'll get erratic idle, vapor lock, fuel percolation in the float bowl, etc., and the passenger side exhaust system will corrode much faster than the driver's side due to lack of heat and airflow to carry off internal condensation. Unless you drive the car in very cold weather, just wire it open (and remove the wire for judging), or replace the valve with a spacer if judging isn't an issue.

[Duke W.]

Re: Purpose of Heat Riser

Upon cold starting the heat riser should normally be closed. This forces exhaust gas through the manifold cross over to heat the "hot spot" on the manifold under the carburetor. This improves fuel evaporization during initial warmup, which means the mixture can be leaner, and initial drive-off driveability is better. Without the heat riser, the engine could stumble in very cold weather, but its value is much less in warm weather.

As exhaust gas heat expands the bimetallic spring, the heat riser should open, but risers often stick closed, which can result in very poor operation when the engine is fully warmed up, so many owners wire them open, or remove them and replace them with the FI spacer, which is just a semi-finished heat riser housing without the valve and spring.

The heat riser should be checked for proper operation frequently - as least with every oil change, When cold, it should be closed but the valve should open with light finger pressure, and when the engine is warm the valve should be fully open. The position of the counterweight indicates the valve position, and an open valve is indicated by a counterweight in the "down" position on most applications.

Another way to check is to feel exhaust flow just after start-up. The RH side should have little flow, but when warmed up the flow should be equal. Even normal heat riser valve action will tend to corrode the RH exhaust system rapidly because the low flow when the engine is cold causes a lot of condensation, especially if trip length is less than twenty to thirty minutes at low speed.

Vintage FI Corvettes did not require a heat riser because they have port injection and the same applies to modern EFI cars. Heat risers, and their associated problems have gone the way of the carburetor.

Duke

[Daniel Grunwald]

Re: Purpose of Heat Riser

Thank you again gentlemen. Is there anything you guys don't know?