68PK21 440.6bbl
Old Man with a Hat
Why it's important to run your heat riser with a carburetor on the street vs EFI no heat riser.
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Uncle Tony's Garage - EFI vs Carburetor & Heat Riser
- Thread starter 68PK21 440.6bbl
- Start date
Uncle Tony! 
He doesn't directly mention it, but I assume you also remove the riser valve gate from the exhaust manifold and plug the shaft holes. ?
Or just block it open. Wire or otherwise. Blocking off the ports in the intake are very important. Hot exhaust flows through the base of the intake even when the heat riser valve is open. Some intake gaskets block the passage and many guys block it with a piece of sheet stainless steel.
He forgot to mention ONE way to get rid of the heat riser issue, which is to use some aftermarket aluminum cyl heads, with no heat riser passages. It would double the price of the conversion, but that is one way to do it.
I agree with the cooling effect of the carb venturi situation on the charge temperature. Which is one reason some engines used a "Charge Temp Sensor" in the earlier 1980s. One day while fooling around with my '80 Newport 360 2bbl, I noticed that the manifold runners were actually COLD rather than at engine temp. I never had noticed that on any engine before, but that LA engine certainly was that way.
The comments about the heat riser flow "being there" is accurate. When we think about exhaust flow, we perceive it is a smooth flow as we observe at the end of the tail pipe. Yet at the cyl head end of the system, there is a massive amount of pressure spikes, positive and negative. I became aware of that when the ball stud end of a vacuum-actuated heat riser valve disengaged from the actuator rod. The heat riser valve was clanging loudly, a sound I had never heard before. It was slamming open and closed due to the positive and negative pressure pulses. As it was nearing the time for a new short block with OEM alloy cyl heads (with no heat riser passages), a spacer was made from an old manual heat riser valve, to go in its place.
With the new short block sitting under the existing intake manifold and carb, I had to tweak the electric choke setting to get good drivability while engine head migrated into the intake manifold. Mainly idle speed and off-idle response issues, which is where the better spark properties of the NGK V-Power and later Iridium plugs came into play.
IF you think about the many variables involved AND the science behind them, it all makes sense. We knew from dyno runs at Nick's Garage YT videos, that TBI EFI made the same power as a carburetor would.
Uncle Tony makes, inadvertently, a case for HP cast iron cyl heads with heat riser passages in them. At one time, it was claimed those passages hurt exh flow out of those ports, as a reason to plug the heat riser passages. WELL after it was a "hot rodder trick" for the masses.
I had always suspected the real reason the OEM EFI heads had no heat riser passages was the fact the EFI was compensating for everything, with their oxygen sensors and such. With better atomization from the TBI injectors. Never had suspected the "fuel under pressure" issues, even at 55psi, much less the GDI pressures of much more.
Another neat Uncle Tony explanation of things,
CBODY67
I agree with the cooling effect of the carb venturi situation on the charge temperature. Which is one reason some engines used a "Charge Temp Sensor" in the earlier 1980s. One day while fooling around with my '80 Newport 360 2bbl, I noticed that the manifold runners were actually COLD rather than at engine temp. I never had noticed that on any engine before, but that LA engine certainly was that way.
The comments about the heat riser flow "being there" is accurate. When we think about exhaust flow, we perceive it is a smooth flow as we observe at the end of the tail pipe. Yet at the cyl head end of the system, there is a massive amount of pressure spikes, positive and negative. I became aware of that when the ball stud end of a vacuum-actuated heat riser valve disengaged from the actuator rod. The heat riser valve was clanging loudly, a sound I had never heard before. It was slamming open and closed due to the positive and negative pressure pulses. As it was nearing the time for a new short block with OEM alloy cyl heads (with no heat riser passages), a spacer was made from an old manual heat riser valve, to go in its place.
With the new short block sitting under the existing intake manifold and carb, I had to tweak the electric choke setting to get good drivability while engine head migrated into the intake manifold. Mainly idle speed and off-idle response issues, which is where the better spark properties of the NGK V-Power and later Iridium plugs came into play.
IF you think about the many variables involved AND the science behind them, it all makes sense. We knew from dyno runs at Nick's Garage YT videos, that TBI EFI made the same power as a carburetor would.
Uncle Tony makes, inadvertently, a case for HP cast iron cyl heads with heat riser passages in them. At one time, it was claimed those passages hurt exh flow out of those ports, as a reason to plug the heat riser passages. WELL after it was a "hot rodder trick" for the masses.
I had always suspected the real reason the OEM EFI heads had no heat riser passages was the fact the EFI was compensating for everything, with their oxygen sensors and such. With better atomization from the TBI injectors. Never had suspected the "fuel under pressure" issues, even at 55psi, much less the GDI pressures of much more.
Another neat Uncle Tony explanation of things,
CBODY67
Luke Pokrajac
Active Member
Seems to be a good argument for keeping a carburetor on our vintage engines. The AVS2 650 I have on my 383 really does work quite well...very snappy performance at all RPM's/load levels and great gas mileage. However, summertime vapor lock issue is present sometimes on 90+ South Carolina summer days with 10% alcohol gas. I'll take the trade-off....simpler system...no electric fuel pump, etc.
Vapor lock while running or hard start after a hot soak?
Luke Pokrajac
Active Member
Hard start after hot soak...it's never vapor locked while in operation.
I have never had a problem with heat passage delete. When I worked in the dealer parts dept way way back early 80s, the cop cars intake gasket heat passage was blocked.
The top gasket.
The top gasket.
Last edited:
USSMOPAR
Active Member
Uncle Tony does not own a modern digital air fuel meter of any sort....if he did he would not be so ignorant
73Coupe
Senior Member
They offer block-off valley pan gaskets for the RB engines, and I used one in the '73 440 when I did the intake. I modified it by punching a 1/2" hole on the choke side only. With the exhaust heat riser valve removed, it ran fine, just took longer to warm up. The paint stayed nice on the intake manifold side opposite the choke, too. I thought I'd get less fuel evaporation from the thermoquad, but it did not make any difference that I noticed. If I were to do it again, would probably keep it stock. I did like the engine paint preservation aspect, though, lol.
Thanks for the info. You have a fuel problem and not a electrical/starter issue it sounds. The fuel is evaporating in the carb because of the heat. I recommend a small low pressure pump near the tank operated with a fused momentary switch. The next time it acts up, take the air cleaner off and see if you have plenty of accelerator pump squirt.
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