Manifold heat riser valve?

[Alfa]

I am fitting a new exhaust manifold to my 1935 Desoto Airflow SE 6. I have a bare casting  and am busy drilling and tapping holes and the like.

 

The manifold heat riser valve is secured into the old manifold by welding to the spindle in 3 places.  It will be tricky to swap over and may necessitate a new valve. The problem is I need to get the car running very quickly, having agreed to drive some friends to their wedding in September.

 

My question is, how important is the heat riser valve for driving in moderate conditions? Could I get away with leaving it out? even if only temporarily?

 

I have heard of problems when the valve is stuck in one position and heating the base of the inlet manifold, but if there were no valve present, I think this should be less of a problem. Does anyone have any experience of this they can share?

 

Thank you

Adam..  

[MCHinson]

I would leave it out. On Buicks of the same era, the exhaust manifold valve is best removed for use with modern fuels. Unless you plan to do a lot of driving in sub-freezing conditions, I can think of no reason to install the butterfly in an exhaust manifold valve. Buick reproduction exhaust manifold valve bodies being sold do not have the butterfly valve in the valve body.  

[5632]

Correct.  Do not use it.

[m-mman]

Agreed. The heat riser valves were put on cars because people expected their brand new car to fire up quickly, run without stalling, and produce heat within moments of starting during a bitterly cold northern Minnesota winter morning. 

 

Nobody drives their collector car that way. They just cause problems during summer driving when most collector cars are on the road. 

It might be of minimal value for a Christmas parade but in that situation just warm up the car before you ever take off. 

[Bloo]

I disagree. I wish I had a dollar for every time I solved a driveability issue by fixing a heat riser that someone thought the car didn't need.

 

If the car was built in the 20s, there might be a very good reason to remove some heat. Some of those systems really had the heat overdone. The gas was heavily laced with kerosene then because the popularity of cars was exploding and they couldn't really produce enough of the lighter fractions. There were problems getting gas to vaporize at all, and most of the cars had updraft carbs. Not true in the mid 30s.

 

If an automatic heat riser, of the type used commonly from the mid 30s on is causing trouble in hot weather, it is either stuck, broken, or someone left the plate out. It should be OPEN when hot. That is the whole idea.

 

The heat riser usually used on inline engines is not a restriction. It is a plate that, when open, redirects the hot exhaust down the pipe, rather than up into the passage under the carburetor. If you leave the plate out, you will have more heat under the carb, not less. Why? Because that plate that directs the exhaust down is not there. Will it be enough extra heat to make the carburetor boil when it wouldn't have otherwise? I don't know, but it is a pretty strong possibility.

 

A hotspot under the carburetor is not just for Minnesota. EVERY even remotely modern car with a carburetor or TBI has one. Why? Because gasoline has a tendency to fall out of the air right underneath the carburetor. Sometimes coolant is used for the heat rather than exhaust, but it is always there.

 

 

Edited August 7, 2019 by Bloo (see edit history)

[MCHinson]

Bloo, 

I don't know how the DeSoto one works but from his discription, I think it is similar to the Buick ones. On the Buick ones, the butterfly valve is in the exhaust stream and deflects hot exhaust up to the carburetor. It is totally unnecessary with modern fuels. There is already plenty of heat there without redirecting any more there. Buicks typically have a tall gasket installed under the carburetor these days to help thermally insulate the carburetor from the exhaust manifold. The butterfly valve is not needed in the exhaust manifold valve body on a Buick from the 30's. If you have fixed a bunch of drivability problems, by replacing such an exhaust valve,  I would wager they were not on a mid to late 30's Buick within the last 10 years or so. I will stand by my advice, assuming the DeSoto exhaust valve is similar to the Buick design.

[Alfa]

Thanks everyone!!

The De Soto valve is similar to the Buick one, I think. It is a flap rather like a choke flap in a carburettor, that pivots in the valve body, controlled by a bi-metallic spring. When cold, the spring ensures that the valve is held such that the exhaust stream from the manifold is directed upwards to hit the underside of the cast iron inlet manifold. The gas then does a U turn to head down the exhaust pipe. When warm, and the bi-metallic spring reacts very quickly if you get a blow lamp near it (!), the valve is rotated so that the hot exhaust is deflected downwards to the exhaust pipe directly and shielded away from the base of the inlet manifold.

 

I very much appreciate all the advice here. I think I will try it without the valve, but in the knowledge that I can remove the manifold and drill and install if need be.

 

Adam..

[Mark Shaw]

9 hours ago, Bloo said:

If the car was built in the 20s, there might be a very good reason to remove some heat. Some of those systems really had the heat overdone. The gas was heavily laced with kerosene then because the popularity of cars was exploding and they couldn't really produce enough of the lighter fractions. There were problems getting gas to vaporize at all, and most of the cars had updraft carbs.

Bloo is correct.  However, many early 30's cars still used heat risers for the same reasons.