Re: Gasoline distillation curves
Another point. The heat riser valve does not regulate inlet air temperature. There is no feedback loop for this function. It's a simple, crudely contolled device whose sole purpose is to create a "hot spot" at the base of the inlet manifold plenum (and in some cases the base of the carburetor where the idle/off idle discharge ports reside) in order to promote rapid fuel vaporization during engine warmup.
It has to provide decent drive-off characterisitcs after a cold start at 20 below, but is probably not necessary in Phoenix during the summer, except for, maybe, the first minute or two after a cold start. Precise control of air A/F ratio was not important in the pre-emission era, so no attempt was made to do so. Carburetors were set up rich enough to avoid lean stumble due to cylinder to cylinder fuel distribution variation in order that the leanest cylinder didn't misfire.
Heat riser valves degrade with time and miles due to thermal cycles, corrosion, and deposit build-up. The shaft can become sticky or jammed. I've observed heat riser valves that are closed on a fully warmed up engine in warm weather at idle, but if you blip the throttle, it opens up. This pumps way too much heat into the manifold and carburetor at idle and low load. I don't have one handy to inspect, but I think the shaft is offset from the center, so high exhaust flow will tend to open it up. If someone has one handy to inspect, maybe you can verify.
The damned things are a PIA (notwithstanding the fact that they tend to rust out the right hand muffler prematurely), and many owners have found that wiring them open or replacing with the FI spacer improves overall engine behavior and drivability, except, maybe, for the first few minutes after a cold state since we rarely cold start and drive our vintage Corvettes in below freezing weather.
In the emission control era a more consistent A/F ratio was required in order to meet emission targets. One solution was to install a heat stove on the exhaust manifold with a duct to a thermostatically controlled valve on the air cleaner snorkel. This maintained inlet air temperature at about 100F (except for a WOT override because the EPA emission certification test does not require the use of WOT, so WOT emissions were not an issue). With a regulated inlet air temperature, air density is closer to constant regardless of underhood or ambient temperature, which maintains the A/F ratio in a narrower range during normal idle and part throttle operation. The heat riser also heats the air from the carburetor and through the inlet manifold, and the temperature increase is greatest at low air flows, but no attempt was made to control air temperature beyond the carburetor entrance; however, the emission control engineers knew that "hotter is better" for fuel distribution equality, and the more equal the distribution, the lower the emissions.
Control of the heat riser valve became a bit more sophisticated in the seventies. It's closed during cold start and warmup, but once coolant temperature reaches 180F a vacuum actuator opens it up fully and keeps it open at and above 180F. Since these engines have 195F thermostats, the valve is kept open all the time at normal operating temperature. The system is called EFE - early fuel evaporization.
Duke
Another point. The heat riser valve does not regulate inlet air temperature. There is no feedback loop for this function. It's a simple, crudely contolled device whose sole purpose is to create a "hot spot" at the base of the inlet manifold plenum (and in some cases the base of the carburetor where the idle/off idle discharge ports reside) in order to promote rapid fuel vaporization during engine warmup.
It has to provide decent drive-off characterisitcs after a cold start at 20 below, but is probably not necessary in Phoenix during the summer, except for, maybe, the first minute or two after a cold start. Precise control of air A/F ratio was not important in the pre-emission era, so no attempt was made to do so. Carburetors were set up rich enough to avoid lean stumble due to cylinder to cylinder fuel distribution variation in order that the leanest cylinder didn't misfire.
Heat riser valves degrade with time and miles due to thermal cycles, corrosion, and deposit build-up. The shaft can become sticky or jammed. I've observed heat riser valves that are closed on a fully warmed up engine in warm weather at idle, but if you blip the throttle, it opens up. This pumps way too much heat into the manifold and carburetor at idle and low load. I don't have one handy to inspect, but I think the shaft is offset from the center, so high exhaust flow will tend to open it up. If someone has one handy to inspect, maybe you can verify.
The damned things are a PIA (notwithstanding the fact that they tend to rust out the right hand muffler prematurely), and many owners have found that wiring them open or replacing with the FI spacer improves overall engine behavior and drivability, except, maybe, for the first few minutes after a cold state since we rarely cold start and drive our vintage Corvettes in below freezing weather.
In the emission control era a more consistent A/F ratio was required in order to meet emission targets. One solution was to install a heat stove on the exhaust manifold with a duct to a thermostatically controlled valve on the air cleaner snorkel. This maintained inlet air temperature at about 100F (except for a WOT override because the EPA emission certification test does not require the use of WOT, so WOT emissions were not an issue). With a regulated inlet air temperature, air density is closer to constant regardless of underhood or ambient temperature, which maintains the A/F ratio in a narrower range during normal idle and part throttle operation. The heat riser also heats the air from the carburetor and through the inlet manifold, and the temperature increase is greatest at low air flows, but no attempt was made to control air temperature beyond the carburetor entrance; however, the emission control engineers knew that "hotter is better" for fuel distribution equality, and the more equal the distribution, the lower the emissions.
Control of the heat riser valve became a bit more sophisticated in the seventies. It's closed during cold start and warmup, but once coolant temperature reaches 180F a vacuum actuator opens it up fully and keeps it open at and above 180F. Since these engines have 195F thermostats, the valve is kept open all the time at normal operating temperature. The system is called EFE - early fuel evaporization.
Duke
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