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.
fury fan
Senior Member
2 things about switching to EFI that nobody really talks about:
#1 - Using a system that can control ignition timing - this is crucial to getting the throttle response/snappiness we desire, esp when the engine is cold. If UTG is correct, and the fuel behavior is truly different, then we should expect the timing needs to differ also. If those aren't adjusted -> non-optimal results.
#2 - Getting a sealed fuel system, wherein the vehicle does not lose fuel to evaporation during heat soaks, or just sitting on a hot summer day. This feature is NOT inherent in the EFI retrofit kits. You need to retrofit a late 70s-80s charcoal canister or a modern EFI car's tank-purge module. (which should be doable with a few solenoids and pressure switches). In fact, a modern EFI retrofit can make that scenario worse, due to returning hot fuel from the engine back to the fuel tank. I've read some stories of guys on Hot Rod Power Tours that had fuel-starvation issues due to the returning fuel causing their fuel tanks to get quite warm. (those scenarios might be due to very-high-HP engines, though)
If the EFI'd fuel mixture is really burning that quickly due to being too much vapor, then ign advance can be reduced, which reduces negative torque in the engine due to burning fuel/pressure increase when the piston is still approaching TDC on the compression stroke. Reducing the time of burn also reduces the amount of time for heat loss to the cooling system. Both of those are an efficiency gain, and therefore a potential HP gain.
I think UTG is ignoring that the pressure drop thru the carb venturi, which reduces temperature, allows more oxygen to enter the engine, and that's what causes the HP difference, not too-much-vapor in the EFI'd system. Fuel is of lesser importance than oxygen. Cooler charge air will always make more power (if it is given fuel to oxidize, of course).
I think it also makes a difference whether one is considering port-EFI or throttle-body EFI. I would anticipate to use a heated intake with TBI and not with port. At least to evaluate it that way first.
#1 - Using a system that can control ignition timing - this is crucial to getting the throttle response/snappiness we desire, esp when the engine is cold. If UTG is correct, and the fuel behavior is truly different, then we should expect the timing needs to differ also. If those aren't adjusted -> non-optimal results.
#2 - Getting a sealed fuel system, wherein the vehicle does not lose fuel to evaporation during heat soaks, or just sitting on a hot summer day. This feature is NOT inherent in the EFI retrofit kits. You need to retrofit a late 70s-80s charcoal canister or a modern EFI car's tank-purge module. (which should be doable with a few solenoids and pressure switches). In fact, a modern EFI retrofit can make that scenario worse, due to returning hot fuel from the engine back to the fuel tank. I've read some stories of guys on Hot Rod Power Tours that had fuel-starvation issues due to the returning fuel causing their fuel tanks to get quite warm. (those scenarios might be due to very-high-HP engines, though)
If the EFI'd fuel mixture is really burning that quickly due to being too much vapor, then ign advance can be reduced, which reduces negative torque in the engine due to burning fuel/pressure increase when the piston is still approaching TDC on the compression stroke. Reducing the time of burn also reduces the amount of time for heat loss to the cooling system. Both of those are an efficiency gain, and therefore a potential HP gain.
I think UTG is ignoring that the pressure drop thru the carb venturi, which reduces temperature, allows more oxygen to enter the engine, and that's what causes the HP difference, not too-much-vapor in the EFI'd system. Fuel is of lesser importance than oxygen. Cooler charge air will always make more power (if it is given fuel to oxidize, of course).
I think it also makes a difference whether one is considering port-EFI or throttle-body EFI. I would anticipate to use a heated intake with TBI and not with port. At least to evaluate it that way first.
fury fan
Senior Member
You didn't specify, but was the crossover passage on that engine plugged with carbon?
Every 318 I've ever had plugged either the intake manifold or the interface area between intake and head.
1 engine plugged it way down in the port near the valve.
Caused the intake to be cool to the touch as you mentioned.
I had no concerns about that as the divorced choke still operated as it should.
The heat crossover is only under the center of the manifold. The particular runner is for#6, which is where the Charge Temperature Sensor is located.
In the middle 1970s, I went into the local Chrysler dealer's service dept to check on something in parts. In one of the stalls was a '67 Belvedere sedan. I noticed the intake manifold was off of it. I asked the service manager about it. He said that the heat crossover was plugged with "coke", and they were having to chisel it out. The reason it came in was that the automatic choke never came off. The car was owned by one of the elderly ladies the dealer sold car to, back then. Seems that he sold many of the more-basic Belvedere V-8s to that local clientele. Their main use was "grocery store, church, friends, and shopping", all within about a 5 mile radius from their house. Wasn't the ONLY one they had to do that on, back then.
fury fan
Senior Member
Ironically, my 318s got pretty substantial highway usage and still plugged the crossover.
And yeah, that crap is near-rock-hard, and a chisel is the best tool. It'll ruin a screwdriver for sure.
And yeah, that crap is near-rock-hard, and a chisel is the best tool. It'll ruin a screwdriver for sure.
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