Manifold heat cross over physics and benifits

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BillGrissom said:
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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your better off getting rid of your mechanical fan and getting electric ones, this way you dont have cold air blowing across the intake allowing it to heat up faster. and if thats not enough get a air cleaner assembly with the hose to the exh manifolds. then once its warm enough you get cool air (if ducted) and no overheating issues...

but i agree, you nuts in the north need that stuff if your crazy enough to drive your car when its snowing lol... i can say ive done it with the motor above with no issues, slick as snot though...
 
The heat crossovers are for cold start and driveability. They heat up the intake charge to keep the fuel vaporized and keep it from condensing on its way to the head and cylinder.

If you want more power for the track, block them off. Heat makes things EXPAND. By keeping the air/fuel mixture cooler (once the engine is warmed up) allows more air and fuel into the cylinder by packing more air and fuel while it is cooler and more 'condensed' . More air and fuel makes more power. If you heat it up, it expands and takes up more room for the same amount of air/fuel than when it is cool. That is why the cars seem to run better when the outside temp drops between 40°F - 60°F. They are packing more air and fuel into the cylinder than when it is hotter and making more power.
 
krazykuda said:
The heat crossovers are for cold start and driveability. They heat up the intake charge to keep the fuel vaporized and keep it from condensing on its way to the head and cylinder.

If you want more power for the track, block them off. Heat makes things EXPAND. By keeping the air/fuel mixture cooler (once the engine is warmed up) allows more air and fuel into the cylinder by packing more air and fuel while it is cooler and more 'condensed' . More air and fuel makes more power. If you heat it up, it expands and takes up more room for the same amount of air/fuel than when it is cool. That is why the cars seem to run better when the outside temp drops between 40°F - 60°F. They are packing more air and fuel into the cylinder than when it is hotter and making more power.
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and better MPG
 
BillGrissom said:
I wonder if the heat cross-over was designed for worst-case conditions, like Alaska or Minnesota.
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Yes, they try to design the vehicles to work in every condition that it may encounter. They are tested in Death Valley, Colorodo Springs, Pikes Peak, Phoenix, and throughout Texas to make sure that they perform under extreme heat. Then to Bimidji, MN and upper Michigan for the cold portion of the testing. Bimidji records the coldest temps in the US.
 
805moparkid said:
and better MPG
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That could be debated. The cooler intake charge will make more power than the heated one, therefore may save fuel due to making the same power at less RPM. The difference may be so minute that it is not worth worrying about...
 
Best of both worlds would suite me fine. I always completely warm up to operating temp b4 driving a carbureted car.

The debate is weighted when a person cold starts and drives to the bank then the car is off for 30 minutes and this is repeated all day.

During less than optimal operating temperatures the carbureted car is required to run considerably richer which is very wasteful on fuel and accelerates engine wear dramatically.
 
Rice Nuker said:
Best of both worlds would suite me fine. I always completely warm up to operating temp b4 driving a carbureted car.

The debate is weighted when a person cold starts and drives to the bank then the car is off for 30 minutes and this is repeated all day.

During less than optimal operating temperatures the carbureted car is required to run considerably richer which is very wasteful on fuel and accelerates engine wear dramatically.
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letting it warm all the way up isn't good for it either, because i bet you not kicking the choke off causing a over rich condition...
 
SirDan said:
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.
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Yes, for two reasons. Back in the day when our cars were made, they did not have the plastic technology that they have today. they could not make a plastic intake that could withstand the underhood temperature range cycles. Today's plastics are better suited for that application.

With EFI, you just place the injector by the valve and spray the fuel directly into the cylinder past the open valve.

With carburetors, the intake air has to carry the fuel vapor mixed with air from the carb to the cylinder.

Once the engine is warmed up, there is enough heat to keep the fuel vaporized.
 
My carb will remove choke incrementally based on temp (really its mainly a timer the electric choke) until it is 90% eliminated, while maintaining the fast idle cam in position if you dont "kick" it off. It is possible that many carburetors will do this if adjusted properly. So, I'm not running an unnecessarily overly rich mixture while warming up.

805moparkid said:
letting it warm all the way up isn't good for it either, because i bet you not kicking the choke off causing a over rich condition...
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Rice Nuker said:
Best of both worlds would suite me fine. I always completely warm up to operating temp b4 driving a carbureted car.

The debate is weighted when a person cold starts and drives to the bank then the car is off for 30 minutes and this is repeated all day.

During less than optimal operating temperatures the carbureted car is required to run considerably richer which is very wasteful on fuel and accelerates engine wear dramatically.
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yes, but driving a cold engine shortly after starting wasts less fuel than waiting for it to completely warm up to operating temperature before driving it. You are wasting all of the fuel during the richest burning time and going nowhere.

I had a 2nd gen Barracuda with a 318 with dual exh, 4bbl electric choke, and electronic ignition. When the choke was hooked up and set properly, I could go out in 0°F (or colder) winter weather, pump the gas pedal twice, crank the engine, wait for the oil pressure gage to come up, then put it in drive and drive away without any stumble (heat crossovers were covered).

Driving a cold engine gets better MPG than waiting for it to completely warm up before driving it. While it warms up, you are getting 0 MPG which really kills your average. If you have a newer car that reads out current and average MPG, try starting it and idling it to running temp without moving and see what that does to your average MPG.

You have to set it up for the conditions that you are going to drive it in....
 
Hi,

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?
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A thorough reading of "Internal Combustion Engines and Air Pollution" by Edward F Obert and "Scientific Design of Exhaust & Intake Systems" by Philip H.Smith and John C. Morrison will answer most of your questions

No easy reads but if your really interest these 2 books are a great start

Hysteric
 
Doesn't the cross over help equalize the exhaust pressure like a H or Y pipe does ?
I know the exhaust is forced right to left before the baffle is fully open but afterwards there wouldn't be a forced direction.
 
Manifold carb heat is like most things, way too much or too little creates problems. Adding ethanol seems to reduce the vapor pressure of fuel, so older designs are too much heat for todays gasoline. Heat risers were designed as a means of heat control, however conduction adds heat, as well as the control of exhaust.

As others suggest port EFI is the solution.

An air cooled engine with remote carb will ice internally, in humid weather near 32 Fdegrees. The same engine with EFI runs without problem.
 
If blocking the crossover is so bad and detrimental, then Fel Pro would not sell the intake valley pan/gasket for the big block with the exhaust crossover blocked off, but they do. I have one in the box on the shelf in the shop. Now, as to the hows and whys, as I said, I am stupid, so I don't know all that. I do know keeping the air/fuel mixture cool on an HP engine is important. Seems to me blocking the exhaust crossover does just that, but again, I digress. I am no scientist.
 
krazykuda said:
That could be debated. The cooler intake charge will make more power than the heated one, therefore may save fuel due to making the same power at less RPM. The difference may be so minute that it is not worth worrying about...
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It was always explained to me that the cooler the charge is, the denser the mixture. That molecules were smaller because they were cooler and that allowed more air and fuel into the cylinders and that's what caused more power from a cooler intake charge. So, taking that theory into account, fuel mileage could suffer from a cooler mixture. If all that BS is true.
 
krazykuda,
Yes, idling for 15 is not economy conducive. I should have mentioned that is not why I do it. I do it to warm up the oil, intake and entire engine.

Having block and oil fully warm ensures optimum lubrication to the top end. Having engine warm improves throttle response and 4 barrel response (my experience). When I hit the highway, I prefer optimum lube and power.
 
Rusty, I reckon that's why MPFI manifolds / systems are the best of both worlds. Amongst a multitude of other benefits, they allow a cool(ish) denser charge with out the vaporization and fuel distribution issues of a wet manifold.

For a carbureted engine it is primarily effecting accelerator pump effect, cold drivability, improved initial fuel economy and decreased engine wear. I totally agree that it ain't for making maximum HP :) although a warm intake may help initial throttle response.

RustyRatRod said:
It was always explained to me that the cooler the charge is, the denser the mixture. That molecules were smaller because they were cooler and that allowed more air and fuel into the cylinders and that's what caused more power from a cooler intake charge. So, taking that theory into account, fuel mileage could suffer from a cooler mixture. If all that BS is true.
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I'm convinced manifold heat is only really important for cold driveability and associated quicker warmup in cold climate. I struggled for years getting my 440 to run right during the summer where it would constantly be suffering the carb boiling and hard starting issues associated with heat and ethanol in the fuel. The thing that finally cured it was new intake manifold gaskets with the crossovers blocked. It's a little longer to idle right until 10-15 minutes or so after startup, but then again it always was.

That was the only difference apart from curing the main problems(s).
 
RustyRatRod said:
It was always explained to me that the cooler the charge is, the denser the mixture. That molecules were smaller because they were cooler and that allowed more air and fuel into the cylinders and that's what caused more power from a cooler intake charge. So, taking that theory into account, fuel mileage could suffer from a cooler mixture. If all that BS is true.
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exactly. with the intake air and fuel being denser, you can pack more in the same amount of space and get more power. Therefore you make the same horsepower at lower RPM compared to the "hotter" mixture. Making more power (or the same horsepower at lower RPM) then takes less RPM to get the same power, hence the fuel saving (slight fuel saving).

Another way to think about it is when you spark (burn) the mixture, it burns at the same final temperature at the end of the burn cycle (no matter where it initially starts burning). Now with the cooler air/fuel mixture is starting at a lower temperature. So by starting at a lower temperature before spark, then ending up at the same temperature when you finish burning, you are getting more heat out of the burn than starting with a "hotter" mixture (by about 20° to 30°). It is the change in the heat output that makes the available power. So you are getting another 20° to 30° of heat out of that charge. It is that heat that is released when burning that causes the expansion of the air fuel mixture on the power cycle to push down on the piston. The more heat that is released, the more power that the engine makes. This is purely a thermodynamic cycle (the otto cycle). The change in temperature and pressure is where the power is coming from.
 
Rice Nuker said:
Rusty, I reckon that's why MPFI manifolds / systems are the best of both worlds. Amongst a multitude of other benefits, they allow a cool(ish) denser charge with out the vaporization and fuel distribution issues of a wet manifold.
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Not really. MPFI manifolds don't have to carry the fuel from the throttle plates into the cylinder. You don't have to keep it suspended in the air mixture through the port. You just spray the fuel past the valve into the cylinder. Since the fuel is not being carried through the ports, it doesn't have the opportunity to condense in the ports because you are only moving dry air.

With a cold engine and cold port passages, sometimes the fuel will condense on the port walls because they cool the mixture to where the fuel condenses on the port walls for a carbureted engine. Since the FI engine is not carrying any fuel, it is not there to condense on the port walls.

At the same temperature, the density of the intake air is the same for both carbureted and fuel injected engines.
 
Hysteric,

Thank you. Ill get both and see how I do. Apparently the second one is full of fluid dynamics and other advanced physics so on the mathematical portion, I will be sucking wind. They are cheap = used on amazon.

Hysteric said:
Hi,



A thorough reading of "Internal Combustion Engines and Air Pollution" by Edward F Obert and "Scientific Design of Exhaust & Intake Systems" by Philip H.Smith and John C. Morrison will answer most of your questions

No easy reads but if your really interest these 2 books are a great start

Hysteric
Click to expand...
 
This is probably why I have never seen any ill effects from blocking the crossover off. Since I live in middle Georgia, it never really gets cold enough to notice much difference in cold weather drivability. Although I can remember one or two winters where the car I had at that time was particularly cold natured. That was likely the reason.
 
Krazy,
We are still going around and around but that is fine with me. I reckon you are disagreeing w me because my posts must not be clear or you have not read them all. I have literally been stating exactly what you stated below, for this entire thread. :) I agree.

The only thing I may beg to differ on is the density of intake charge based on two engines: One a 1975 V8 w all factory systems functioning perfectly and the other a 2001 4.7 Dodge with all systems good (or any modern v8).
Both engines running in a test cell with identical air inlet temp.


If you compare the process of a heated carbureted wet intake manifold to a non heated plastic MPFI manifold you will get similar results (similar objectives achieved totally differently) regarding fuel atomization but the plastic mpfi will yield a slightly cooler charge, due to the lack of heat crossover, oil splash and conduction but still yeild efficient (obviously more efficient) fuel distribution. In addition you get better flow, runner and plenum tuning more economically due to manufacturing advantages.



krazykuda said:
Not really. MPFI manifolds don't have to carry the fuel from the throttle plates into the cylinder. You don't have to keep it suspended in the air mixture through the port. You just spray the fuel past the valve into the cylinder. Since the fuel is not being carried through the ports, it doesn't have the opportunity to condense in the ports because you are only moving dry air.

With a cold engine and cold port passages, sometimes the fuel will condense on the port walls because they cool the mixture to where the fuel condenses on the port walls for a carbureted engine. Since the FI engine is not carrying any fuel, it is not there to condense on the port walls.

At the same temperature, the density of the intake air is the same for both carbureted and fuel injected engines.
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