Carb and Manifold Cold

[Vicky]

I installed a Burns downdraft Manifold and a Holley 94 carb on our Model A.

I seems to run fine, but the intake and bottom of the carb are "cold" to touch.

Could this be caused by a vaccum leak? or any suggestions?

Thanks,

Bob

[Jim/GA]

How cold is cold? Is there a coating of frost on the outside of the manifold and carb base?

I suspect it is normal (caused by the fuel evaporating).

[Flop]

on my 47 chevy the carbs i put on are freezing in the winter ....sometimes even with frost on the out side and thats even with manifold heat .

[jkeesey]

Hasnt the rule of thumb always been the colder your in coming fuel/air the more power?

[Vicky]

The manifold is very cold, with condensation turning to frost. It is big contrast between the header and intake temperatures.

I thought it may be fron the Venturi effect, but wanted to make sure it is not a problem.

In general, the cool intake (and fuel mixture) temps are fine with me.

[sphanna]

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It's the venturi effect. Lower pressure in venturi cools the ambient air.

[columbiA]

The DD manifold doesnt have the heat plate that bolts to side of ex manifold,as a result the carb & manifold run cooler & will ice up & result in a poor idle or stalling when the temp is around 0* C.

[BRENT in 10-uh-C]

Originally Posted by jkeesey

Hasnt the rule of thumb always been the colder your in coming fuel/air the more power?

No.

[RockHillWill]

Brent; Im not sure that I agree with you on this, maybe I misunderstand, but I was also thinking that cooler air providrd more power.

When I had a dyno for the larger motors, I could see a rise from 690 HP to over 701 or even 703 by the putting ice cubes around the intake manifold. Of course, this only lasted until the ice melted, but it led to 'interesting' under hood sheet metal 'devices' and the use of the phenolic spacers under the carb. I felt that the cooler air is more 'condensed/dense' thereby having more oxygen content and that seemed to be backed up by the fact that i could get an even further increase in the horsepower by installing at least one size larger jets to accomodate the increase in ozygen.

Just a thought.

[LukeDahlinger]

Cold fuel does provide better performance, whereas warm fuel provides better economy but with a loss of performance. This has been my experience with all of my carburated cars.

[George Miller]

Originally Posted by RockHillWill

Brent; Im not sure that I agree with you on this, maybe I misunderstand, but I was also thinking that cooler air providrd more power.

When I had a dyno for the larger motors, I could see a rise from 690 HP to over 701 or even 703 by the putting ice cubes around the intake manifold. Of course, this only lasted until the ice melted, but it led to 'interesting' under hood sheet metal 'devices' and the use of the phenolic spacers under the carb. I felt that the cooler air is more 'condensed/dense' thereby having more oxygen content and that seemed to be backed up by the fact that i could get an even further increase in the horsepower by installing at least one size larger jets to accomodate the increase in ozygen.

Just a thought.

I agree to a point, but I think there is a limit. I would think when it gets to cold. The fuel would not break up as well and hold it in suspension as well.

[Walt Dupont--Me.]

Here's a cute little story. I ran a McCulic super chager on my channled 40 coupe for about 10yr's. I never ran a hood. I was on a little trip one day in late fall, on the turn pike cruising 60mph. Outside temp about 32 degrees. It started to snow, I let up in the throttle and the throttle was stuck. I could see my carb about in front of my windshild, It was all frosted around the bottom. The further I went the slower the engine died down, when I rolled off the pike about a mlle from my house I was going 20MPH. I pulled into my drive and pushed the button for the garage door opener and drove into the garage. Got out and watched the engine at about 800rpm. As the engine warmed up the rpm keped riseing, I had to jump in the car and turn the key off. The throttle plated in the carb had iced up and left a very small hole for the air to get through. In a couple of min. it all melted and the throttle was fine. That's what happens when there is no heat the carb. Walt

[RockHillWill]

Good afternoon George, I agree with you 'to a point' as well. It's been a LONG time since I screwed around with this, but my memory at this time makes me want to offer this:

The fuels ability to atomize is more dependant on the measure of the fuels latent heat of evaporation and its ability to vaporize as opposed directly to the rate of change in temperature, and as I recall as we could cool the air down and increase the air density to the point where we could add fuel as I described earlier. The cooler air would have a minor effect on the evaporation rate of the fuel mixture but the high compression ratios (and their ability to create rapid heat) seemed to be such that it was rare that we had to adjust timing. At Daytona and Talladega, if we had to run long cautions and the cylinder temperature would cool down, we would sometimes make a late quick stop and add a small amount of Benzene to increase the vaporization rate until we could get back up to speed. There were a lot of blown motors right after extended cautiuons untill we figured some of this out.

Not positive I spelled all the names right, and my old mind has slowed down, but this is the jist of what I recall.

[ford3]

you want cold air going into the carb, cold air is compacted and carries more volume, but once into the manifold you want the air to heat up and expand giving more volume going into the engine, more air volume in the cylinder the more power generated

[ctlikon0712]

Maybe by saying "No" Brent meant that it isn't cooler air but denser air. Cooler air is denser and so all else being equal you can pack more of it in the cylinder and get more bang or power out..... You want the fuel itself to be warmer. The easier it atomizes the more power and the better Air Fuel mixture you will get.

[Will N]

I'm not familiar with the design of the Burns downdraft manifold. Does it make contact with the exhuast manifold the way that the standard Model A manifold does? This standard design feature was intended to heat the incoming air/fuel charge to promote better vaporization, and to radiate heat to the body of the carb in order to prevent icing. More modern approaches were the butterfly valve heat risers that directed heat at the intake manifold until operating temperature was reached, then the butterfly closed. Then came the ducted air cleaners that drew warm air from around the exaustpipe into the throat of the carb until operating temp was reached. Without some way to warm the charge and or carb until operating temp is reached, you run the risk of icing up and stalling with your setup, particularly on humid days.

[RockHillWill]

I am neither arguing nor instructing.

I just offer one perspective at this point, but my opinion is that atomization is a function of the latent heat of evaporation and the fuels ability to vaporize, more than the temperature. I am also of the opinion that the best approach is to cool the air, making it dense to accumulate additional oxygen, allowing an increase in jet size, then formulate the fuel in a manner that maximizes its ability to vaporize, then at the last minute rapidly increase the temperature to expedite the burn rate by manipulating the timing to take advantage of the compression ratio and the combustion chamber profile. The hardest thing for me was to design/pick the combustion chamber design / piston contour that was most efficient at maintaining an increase in combustion pressure relative to changing piston position throughout the duration of the flame travel.

Never did get it figured out completely! I sacrificed many pistons, several heads and a block or two to the Gods of Speed.

[Vic in E-TN]

Two things to consider.
1. Venturi effect. The velocity through the venturi increases inversly proportionally to the area ratio. The pressure drop in the venturi is a square function of the velocity ratio. The temperature change is not significant. You are trying to suck fuel through the jet. That is why you have a venturi. The lower pressure is directionally helpful to fuel vaporization.
2. You need heat to vaporize the fuel. The vaporization of the fuel is what is causing the the cold manifold. If you do not get enough heat, you do not get good vaporization. The heavier (higher molecular weight) components tend to be the last components to vaporize. If you do not get good vaporization you do not get good fuel distribution and this will lead to some cylinders putting out more power than others. Think multi-point fuel injection instead of throttle body fuel injection as one of the factors in recent increases in fuel economy.

I really wish that I could have worked for a car manufacturer instead of just designing oil refineries to make more gasoline and causing traffic congestion and global warming.
Just kidding.
Vic

[Vicky]

Thanks to everyone for the helpful information. I now understand the need to more heat to make the fuel mixture burn more efficiently. I thought I understood how the fuel system worked, but now know a lot more.

Thinking back, I did notice the cool manifold after the engine had idled for a longer period of time (while I was checking fuel pressure and checking for leaks, etc.).

I will monitor the performance when I have more time on the set up to make sure I get it right.

Just to clarify, the Burns Manifold has a surface to mount to the exhaust manifold like a stock intake, but this set up uses a header and there is no contact between the header and intake.

Thanks again for the helpfuL information.

Bob

[Ron Lachniet]

I noticed this fall that my two front spark plugs are always sooty and my rear two are clean. Could that be caused by warmer air in the back cylinders or is something else going on here. My engine was completely rebuilt this past spring and it just started doing this recently.