Manifold heat cross over physics and benifits

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Rice Nuker

Let the Coal Roll!
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Hi,

I have been reading alot on the purpose for heat cross over in the manifold.
Throttle response and fuel economy both are affected by this feature quite a bit on a mild or stock engine.

Anyone want to discuss fuel atomization, evaporation, manifold wetting vs. manifold pressure, manifold temps etc?

One of the first times I really decided to agree with the nesessity for manifold heat (in a street car) was when slant six dan posted a video which looked like a 50s cartoon.

I have read recently about manifold wetting and the variables involved and how they contribute to poor throttle response and of course fuel economy.

Has anyone tried to moderate the exhaust transfer thru the manifold? I know the heat valve in an exhaust manifold encourages the transfer when the engine is cold although none of my mopars have this feature any more.

My concern is turning off the heat when the engine is running at high speeds.

Heat soaking is a concern too since hot manifolds warm up the carb substantially after the car stops.

Here is some neat info that leads info accelerator pump rates vs. manifold wetting which leads into manifold heat:

http://yarchive.net/car/carb_tune.html

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I think it depends to a certain extent upon the engine

Mopar/ Chev/ Ford small blocks have a "natural" heat, at least once the engine is warm, because the bottom of the intake is heated by oil splash, and probably "some" heating from the front coolant passage.

But slants, after being converted to headers, have NO heat, especially "way out there" on the end of the manifold, and B/ RB type designs, with elevated manifolds, don't get as much, either.

It also is going to depend on your local climate.

But you must know that it's not JUST the "heat," it's the COOLING. A carburetor is really, a type of refrigeration unit. the fuel and low pressure in the venturies cause a cooling effect. I once saw (forgotten) either a VW or Corvair dune buggy with an aftermarket tube manifold and Holley 2 BBL. In 90* heat, it was generating ICE on the outside of the manifold tubes

I believe what you are hinting at is the "answer," though, and that would be having "some way" of shutting the heat OFF or throttling the heat "down" when warm.

I'm considering (small block) experimenting with the Ford type coolant "heated" carb base. Turns out the Ferd ones are not all that adaptable because of the casting of the device. I've acquired a couple of 'em, but both need some work.

Really, the "stop gap" has become by default, using various thicknesses of insulator gaskets under the carb.
 
67Dart273 ,

Yes, I understand the pressure differential and venturi cooling effect reasonably well and have seen carburetors that seemed ice cold to the touch on a few vehicles. :) Pretty neat.

One of our cars has an essentially stock 440 w headers, I put a performer and an Edel 800 avs on it. During that, I blocked off the heat crossover at the intake gaskets. I am going to pull the manifold soon and just leave the crossover open (there is no egr setup either).

I am pretty sure the mild differentials of pressure in the dual 2.5 exhaust with H pipe will encourage some hot gas transfer thru the crossover. Interested to see how this affects the car up on cold mornings and throttle response (throttle response now is quite great).

Why are you looking into the ford heat coolant plate under the carb (heat exchanger)? Do you have cold weather drivability issues? I reckon it would warm the manifold and carb by conduction but nothing like a heat crossover which heats the floor of the manifold where you need it most to get the fuel which is falling out of suspension back in motion. Or are you looking at it as a cooling / heat sink when the manifold is too hot? Just curious as it is a cool idea :)

,
 
No. Don't want to discuss. I just block them all off like I always have and they run fine. No need to discuss that.
 
Posted somewhere else on this website but here it is again.

[ame="https://www.youtube.com/watch?v=fKbSloJ5o7o"]Down the Gasoline Trail (1935) - YouTube[/ame]

Very informative to the nature of heated manifold floor and if you combine this info with the concept of manifold wetting delay, a person will see why running a big acc pump shot is a compensation for lack of fuel atomization. From this a person can further surmise that they are wasting fuel and wearing out their engine faster by utilizing less than efficient atomization.
 
Sooner or later, I will pull my drive train to paint my car. When I do I will repaint the engine especially because the heat riser passage has burned the paint off the intake. At that time I will disable the valve and block off the passage to the intake. Most of us don't drive our cars in the cold weather and most of us start our cars up and warm up the engine before we take off for a drive. The heat riser system really isn't needed under these conditions. tmm
 
I'm not sure how relevant that movie really is. One point, is that engines such as many conventional sixes, slants, Fords, Chevies, etc, manifold heat is a real need because the intake is "out in the breeze," not heated by being cuddled in the middle of the engine, splashed on the bottom by hot oil, and in many, with coolant through part of the thing.

Many sixes with headers don't run all that well in fairly warm weather, as the carb heat is removed, and I can still remember the VW? Corvair? dune buggy with ICE on the bottom of the manifold on a fairly warm day!!!
 
I can see where 6's need heat. V-8's not so much.
 
im not sure what you guys are smoking but slants get more heat... you seem to think that a 200* engine is warmer than 550*-1000* exhaust temps?

and besides that heat rises, even with my headers (before coating), my hood vents, and just putting around/or 1/4 passes i cant even touch my carb sitting on a 2" phenolic spacer! so this BS needs to stop with a slant having a hard time keeping the intake warm...

i ran mine with it hooked up to the manifold and it ran like crap, over heated, hard starting, usual HEAT issues...
 

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Got a valid point there. Air Gap manifolds work too.
 
Got a valid point there. Air Gap manifolds work too.

now let me be clear, on a wet intake there does need to be factors to keep the fuel in the charge, whether its rough castings causing turbulence, plenum design and runner design, and a bunch of other factors BESIDES heat. SL6Dan is a Sharp Cookie and anyone who thinks differently needs to start researching his posts BUT he also like EGR which in the last thread maybe a couple of months ago was proven to have more major disadvantages than gains.

My point is every type is going to have +'s and -'s, the number one issues with slant six guys in stock form?? anybody vapor lock, fuel boiling in the carb, pinging, all heat related issues (in these cases).
 
The relevance is when you start any engine and it is 75 F, 30 F, 0 F or even at running temp. The other relevance is the rate of atomization and the quality of atomization.

Which warms the intake quicker, manifold heat crossover or oil spash? Every time you start and wait 10 - 20 minutes for a hot manifold or wait 5, that is the difference between having to run a richer mixture for longer or for less time.

Cold start rich mixture needed to ensure enough fuel is vaporized and therefore efficiently combustible is dependent to a reasonable extent how warm the floor of the intake is. Intake heats up quite quickly in the area under the carb with crossover heat.
Already discussed is the cooling effect of running fuel and air thru the venturi.

The cooling or an already cool floor of the manifold leads to poor atomization and requires a richer mixture and heavier acc pump squirt to wet the manifold to the point where there is sufficient to surface area of fuel raising the vapor pressure to the point of vaporization. Only then do you have efficient power produced.

This is why you will need richer mixture and more acc pump volume with a cold manifold. The manifold has to be wetter in a cold manifold or you will get lean mixture behaviors. This raw fuel which is necessary for proper F/A ratio during combustion (vaporized fuel mixed with air is the part that makes power) then just runs thru the engine raw, wears the rings and partially vaporizes as it moves from the intake valve to the tail pipe.

Running a cool manifold makes a bit more power. This is why everyone hates exhaust crossovers. That and the cooked oil on the bottom of the intake. The reason cold manifold helps power is only because of the concentration of oxygen per cubic foot of air volume and the cool charge's resistance to expansion prior to explosion helps the charge fit in a smaller place. The best of both worlds is properly vaporized fuel in a dense cool air charge. Hence MPFI.

MPFI eliminates the need for manifold wetting almost entirely as it is not responsible for maintaining a wet environment conducive to fuel vaporization. It doesn't need to maintain even a wet environment in the intake port because the fuel is essentially vapor when it leaves the injector. So, accelerator "Pump" effect or volume is also all but eliminated as well because you don't have to pre-saturate the walls of the intake in order to ensure adequate FA ratio when the throttle blades open.
 
Logically speaking creating or encouraging porosity or derivations in the texture of a wet intake manifold does two things. One, yes it creates turbulence which may lead to fuel being agitated off the walls, into suspension which hopefully leads to atomization and secondly it doubles or triples the surface area compared to mirror smooth. This surface area is where the wetting effect gets optimized due to exposure.


Yep, pinging and carburetors boiling fuel out is a major situation.

now let me be clear, on a wet intake there does need to be factors to keep the fuel in the charge, whether its rough castings causing turbulence, plenum design and runner design, and a bunch of other factors BESIDES heat. SL6Dan is a Sharp Cookie and anyone who thinks differently needs to start researching his posts BUT he also like EGR which in the last thread maybe a couple of months ago was proven to have more major disadvantages than gains.

My point is every type is going to have +'s and -'s, the number one issues with slant six guys in stock form?? anybody vapor lock, fuel boiling in the carb, pinging, all heat related issues (in these cases).
 
I think I would take more fuel consumption (from a cold manifold) over less power and more boiling carburetor action.

Down side is drastically accelerated engine wear and poor cold weather driving to boot.
 
carb manifolds vs efi manifolds are too different beast. most plastic manifolds I see are efi. I think for a carb you need heat to keep the fuel atomized. I like to keep the exhaust crossover and use a 195 thermostat if I intend to get maximum mpgs with the motor. If you want power drop in a 160 thermostat and block off the crossover.

I speculate a heated carb spacer would end up hurting in the summer but maybe helping slightly in the winter. Might be worthwhile if you had a on off valve so you could turn it off in the summer. Not sure if it would really be worthwhile for someone in the states (besides alaska).

Air gaps, honestly they probably are horrible for mpgs. Great for performance though. If anyone has heard of an air gap getting good mpgs prove me wrong.
 
As compared to a heated manifold, I seriously doubt anyone could.

The carb spacer with coolant served both purposes. One it heated for cold weather but the other benefit was it was a heat sink as a buffer for heat rising from manifold after engine stopped. That is my guess.

If anyone has heard of an air gap getting good mpgs prove me wrong.
 
Then you should not post in this thread where the subject is the discussion of manifold heat.

I wasn't saying it to be a dick. I simply gave my opinion on what I usually do. I don't want to discuss the other simply because I don't know that much about it. Blocking off the crossover IS related to the thread subject. I simply posted what I know rather than what I didn't. I am also willing to bet both my nuts that we don't have a fuel atomization expert here on this site, either. Just trying to keep it simple. You had no need to be an ***.
 
I am interested in discussing it. I wonder if the heat cross-over was designed for worst-case conditions, like Alaska or Minnesota. Did the Mexican market get them? Not sure they even got cabin heaters there. In the 2 Mopar V-8 intakes I have pulled, the cross-overs were plugged solid with carbon, so no cross-overing was happening. I prefer mileage to power, so I try to keep mine flowing.
 
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