Heat riser crossover?

ford38v8

ford38v8

Well-known member
The heat riser holes on the manifold deck are sometimes blocked with pennies, which provide both positive and negative results.

Has anyone a junker manifold that can be disected in the name of a Better Ford Idea? :D

I know the early manifolds had a heat riser directly up to the base of the carburetor, while later manifolds had no such hole at the carb base.

I believe that if those risers were modified to directly crossover, the goal being to reduce heat transfer, the result would be beneficial given the volatility of the fuel we have today.

I'd like to hear ideas on the subject, and if there actually are any junk manifolds out there waiting to make that ultimate sacrifice? :D :D :D
 
is your goal to slow down the heat flow or to eliminate it. I have used pieces of thin SS shim stock placed between the manifold and gasket to stop this flow of heat, but you could drill a small hole in it to slow down this heat.
 
The idea is to warm up the manifold itself, so the gas doesn't condense from vapor into puddles. Warming the manifold sooner is a good idea regardless of the volatility of the fuel used. When pennies block the manifold risers it takes a longer time for the engine to warm up enough to idle smoothly.

Your concern is about evaporation inside the carb, which I think is better solved with an insulating base between the carb and manifold.

Keep your carb cold and your manifold hot!
 
Mike, that makes sense, but I'd still like to see inside a manifold to see if an internal mod could benefit.
GM reported a short while back that the insulating block wasn't as effective as hoped for.
If the internal cavity is one large open chamber, perhaps an insulating filler could be used at the top for an additional reduction in heat transfer to the carb. No magic bullet, but a heat reduction here and there...
 
You have to remember that the carb is a self cooling object. What I mean is that the venturi action cools the carb down, regardless of weather and engine heat. Carb icing is a serious problem on aircraft (and snowmobiles) even when outside temperatures are above freezing. Place your hand on the side of a carburetor while the engine is running and feel how damn cold it gets.

Hot carbs and fuel evaporation result from "heat soak" after you turn the engine off which is why an insulating base plate works so well.
 
ford38v8 said:
Mike, that makes sense, but I'd still like to see inside a manifold to see if an internal mod could benefit.
GM reported a short while back that the insulating block wasn't as effective as hoped for.
If the internal cavity is one large open chamber, perhaps an insulating filler could be used at the top for an additional reduction in heat transfer to the carb. No magic bullet, but a heat reduction here and there...
Click to expand...

What I reported a while back is that gadgiteer@aol.com provides a vented carb insulator that works great to cool the base of the carb which prevents the carb from vapor lock and boiling the gas out after shut down. What I said was the main vapor lock problem is the new gas boiling in the fuel pump, dropping the fuel pressure as it gets hot and when in the 135 degree area shuts the fuel off completely. The pump won't pump fuel until it cools or is cooled by placing a bag of ice or pouring cold water on the pump. This is a hot weather problem. G.M.
 
Thanks, George, and yes, I do remember and agree that heat soak after shutdown is the biggest culprit.
Mike, carb icing on aircraft, I can see, but with a Ford V8? Are there snowballs in hell? :D
Is anyone averse to a few additional minutes to warm up to avert carb boil later? Not just on shutdown either, it happens at stop lights too.
 
I've run with welch plugs in the holes, in three different configurations: Completely blocked, 1/4" holes in the plugs, 5/16" holes in the plugs, and no plugs. Even in the summer (80+ deg) it took either 10 minutes of idling, or 3 miles driving, to get to where it wouldn't stumble off the line, with the plugs in (either hole size). If I ran it a couple minutes, shut it off for a few minutes, then restarted, it was greatly improved. In the end I took out the plugs, and have not regretted it.

It is shocking how cold the carb will be just idling in 85+ deg. weather. If there was any moisture in the air here, I'd bet it would be frosty.
 
ford38v8 said:
Thanks, George, and yes, I do remember and agree that heat soak after shutdown is the biggest culprit.
Mike, carb icing on aircraft, I can see, but with a Ford V8? Are there snowballs in hell? :D
Is anyone averse to a few additional minutes to warm up to avert carb boil later? Not just on shutdown either, it happens at stop lights too.
Click to expand...


Carb icing can happen on any carb, it's just a bit more serious on a plane (you think?). It's not about low air temps, it's about humidity condensing on the cold carb.

I'm a snowmobiler and my sled has factory installed carb warmers which are recommended to be used in above-freezing temps when humidity is high.

Anyway, sorry for the digression. I was just trying to explain that carburetors are like little refrigerators when running.
 
I live in SW Florida where it's hot most of the year. I took a lot of temperatures with the laser thermometer over a long period of time almost every day under the same conditions. A hot carb don't seem to have to much effect while driving and you can get mislead into where you think the hot gas problem is. Under some weather conditions which I think humidity plays a part you can feel the venturi's of the carb get cool. But when the base of the carb gets 135 degrees this heat over comes the cooling action. The real vapor lock problem as I found is in the fuel pump. All you need to find and see the problem is a laser pointed thermometer and a low pressure gauge in the fuel line between the pump and carb. In 85 degrees and above look at the fuel pressure and pump temperature when the engine is cold started. The pump pressure is 4 lbs and the pump temperature 80. As the temperature of the pump rises the fuel pressure drops this don't take very long. When the pump temperature gets above 125 the pressure gets lower. As the pressure gets below 2 lbs the pressure fluxuates. At idle you still get enough fuel at 2 lbs but at 45 MPH there is not enough fuel and the engine gets a little rough. This is due to the time the fuel stays in the fuel pump at slow speeds or idle and gets hotter. At speeds above 55 MPH the fuel flowing through the pump from the tank cools the pump and the pressure stays up. This will be noticed on a trip on highways above 55 the engine runs good but when you slow down through town and hit a few stop signs or traffic lights is when the engine start to stall and gets worse until it won't start. At this point at first I replaced the fuel pump a number of times and later cooled it with cold water before it would run. I set my pump pressure to 4 lbs knowing the pressure drops from heat. If set at 3 lbs cold the problem would come on sooner. All of the guess's and "I thinks" don't show or solve the real problem the simple tools and a visual look at the problem shows what it is but don't fix it. I don't like electric pumps but they will over come the problem when it occurs. In hot areas I think the real solution is a return line after the fuel pump back into the fuel fill pipe to keep the fuel flowing through the pump cooling it. This return line want's to have an ID of between .100 and .125 or a drilled orfice in the fitting of the line of the ID mentioned above. Some more modern cars run the fuel line through a small tubular unit about 1 1/4" OD and 4 "s long with Freon from the air conditioner running through it to cool the fuel. G.M.
 
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GM do you mean the bypass pipe comes off the pressure side of the pump and runs back to the fuel tank to enter above the fuel level with the pressure drop on the return line being enough to provide fuel pressure at the carb?
 
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Here is an image that is over on the Hamb . The guy said he was driving in 40deg weather for a while . Granted it has no hood , a blower etc etc but it is an example of iced carbs .

picture.php
 
Bolts said:
GM do you mean the bypass pipe comes off the pressure side of the pump and runs back to the fuel tank to enter above the fuel level with the pressure drop on the return line being enough to provide fuel pressure at the carb?
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If you remove the fuel line at the carb and crank the engine you should get a full 3/16" diameter stream that shoots out about 12's. This is more full than the carb will ever use. Ford determined the "line size" at 3 to 4 lbs pressure needed to be .098 if I recall correctly. They tried up to .108 and down to .090 or somewhere in that range. The small diameter didn't provide enough fuel at high speed and the large diameter opening caused the bowl in the carb to flood over the top. The .098 was the size settled on for the Stromberg carb. Through a given size hole only a certain amount of liquid will flow AT CERTAIN PRESSURE. Increase the size of the hole OR the pressure and more fluid flows. At 60 MPH lets say an old Ford burns 4 gallons going 60 miles. At this rate the needle valve is operating in small motions keeping the fuel at a fairly even level in the bowl. If the pressure was way down to 1 lb the carb would most likely be starved of gas at higher RPMs. At 5 lbs the needle valve would probly still hold the float level even though more fuel is available. Much above 5 lbs and the carb would be over flowing from the bowl and run real rich. I'm using the 5 lbs as a guess and it could be a higher or slightly lower number. To answer your question there is more than twice the amount of gas available to the carb than required. You could run dual carbs and maybe 3 carbs from a stock fuel pump under normal driving conditions on a stock engine. That's where I guesstimate a return hole of from .100 to .125 will be in the area that will work. With the test set up still on my 39 I have a "T" fitting installed ready to test a return line. Will post the results after I try it. I don't have to wait for real hot weather to see if the return effects the engine running only need hot weather to see how it relieves the vapor lock. G.M.
 
G.M., you might try an oil filter restrictor, .060", for convenience. That's about the size of return restrictor a Chrysler I had used on its carb fuel system.
 
George, regarding the return line diameter, a friend used the early fuel gage line for his return. I don't know what that diameter is, but he was running a stock fuel pump, and that setup did solve his vapor lock problem.
 
ford38v8 said:
George, regarding the return line diameter, a friend used the early fuel gage line for his return. I don't know what that diameter is, but he was running a stock fuel pump, and that setup did solve his vapor lock problem.
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I figured it would solve the problem from my trips on I-95 on days 100 degrees or better. The engine ran good over 55 MPH when the fuel was flowing about full blast into the carb but slowing below 55 MPH the fuel was in the pump to long and started showing minor vapor lock. I think the oil filter restriction may be just a little to small, as far as I'm concerned the oil restriction hole is to small for the oil filter. I opened mine up 130,000 miles ago but don't remember the size. One thing I wonder is when the fuel tank is low with a hot highway and hot tank recirculating the hot fuel may loose it's effect at some point. Filling the tank with new cool fuel from an underground tank will take care of that. G.M.
 
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