L72 has heat riser on it. Why? Thought that this engine did not come with heat riser. Does anyone know if this L72 is supposed to have the heat riser?
Riser is on there, but no pipes or tubes lead from the exhaust manifold to anywhere.

Also, what bad things might be happening to my engine and performance due to the heat riser being on there? Overheat? Performance loss? Bad fuel mileage? Manifold or valve damage?

Should I pull remove it?

Thanks,
Mark

 

Carb cars have a heat riser to hasten the heating of the intake manifold so that the rich fuel mixture will vaporize. Whether or not there is a choke tube is not relevant. So far as I know, only the FI cars had a spacer instead of the heat riser. John Hinckley (JohnZ) wrote an excellent article on heat risers in the latest issue of Corvette Enthusiast.

 

OK..thanks. But how does the heat riser do this? Does it slow down the escape of exhaust gases? Remember, on my car there are not pipes or anything that would deliver any heat to the intake manifold. All that would happen is a narrowing of the exhaust pipe just south of the exhaust manifold.

 

Well, I can't say as I've ever looked specifically at a L72 intake manifold, but most manifolds have a passage in the center which goes under the carb. Heat and exhaust gas is routed from the right bank, through the manifold, and out the left bank. These valves are supposed to open thermostatically, but a lot of them stick closed. A lot of folks wire them in the open position. The down side of this is that you get a lot of wet, rich fuel in your cylinders during warm up. This condition has its own set of ptoblems.

 

Essentially all carbureted engines have heat risers. The L-88 and ZL-1 are exceptions, but these engine were intended strickly for racing, not street use. The L-72 was designed as a high performance engine for street use and was expected to start and run properly at 20 below, just like the base 327/300. When a carbureted engine is started from cold, especially in very cold weather, the fuel has poor vaporization characteristics and can puddle under the carb.

By closing when cold, the heat riser valve forces all the exhaust gas from the right bank through the passage in the inlet manifold to the left bank. This creates a "hot spot" on the base of the manifold under the carb. that rapidly evaporates the fuel. Without a heat riser, the mixture to the cylinders can be lean and unevenly distributed, which can cause very poor cold engine operation, especially in very cold weather.

As it heats, thermostatic spring on the heat riser slowly rotates the valve allowing exhaust gas to escape in greater quantity out the RH exhaust bank, and as the engine warms, less exhaust gas is forced through the inlet manifold passage.

Vintage and modern port FI cars do not need heat risers for two reasons. First, fuel distribution is inherently even because the injectors are flow matched, and, second, injection pressure causes good fuel atomization and only slight cold enrichment, handled by a mechanical or electronic cold enrichment system, is required.

On a carbureted car, good cold engine operation is provided by a combination of choke and heat riser.

If you only drive your Corvette in mild weather you can usually dispense with the heat riser by blocking it open or replacing it with a FI spacer. Heat riser valves tend to get sticky and sometimes freeze closed, which can cause overheating of the carb manifold and rough running due to fuel percolation, and, even if it operates properly the RH exhaust bank is slow to heat up, which generates condensate that tends to rust out the RH muffler lot sooner the the LH muffler.

Duke

 

Yep, L72's came with them. I gutted mine, plugged the holes, and opened it up significantly inside with a die grinder, with a head porting burr actually. Can you image it backpressure it causes at high RPM's?..... even in the open position?

If you're interested in performance consider headers. The factory BB exhaust systems really chokes a L-72...... If that's out of the question, I'd open the "heat riser" and get a pair of good, stock looking, low backpressure mufflers (or if you have side pipes, a pair of 2.5" CORE pipes...... stockers are 1 7/8" CORE).

 

Ok.
Thanks to everyone. I am going to try to wire the riser open and see what happens.

Mark

 

check your choke operation after you disable the heat riser valve. If your choke spring coil operates from the heat of the cross over, your choke may open later due to not heating as soon as if the heat riser valve was in use

 

Unless you operate your car in actual cold weather, adjust the choke so that it opens as rapidly as possible. Except in really cold weather, a choke really isn't necessary for starting. Don't keep your engine running the rich mixture any longer than is actually necessary.

 

My L72 does not have heat risers...or any evidence of them. If they came factory, I suspect they were removed some time before I became caretaker. The manidolds are the correctly numbered ones. It was a southwest car, so probably didn't need them anyway. I use a mech secondary, no choke BG carb, and haven't had any problems whatsoever in the Denver region. :D She'll have headers on within the next month or so anyway.....they are hanging in the garage at present. :eek: :yesnod:

:cheers:

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Jeff; Charter CCC
Ô66. Modified L72
ÒThe Silver BeastÓ
Keep on :cheers: :seeya :flag :steering: