Manifold heat cross over physics and benifits

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krazykuda said:
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.
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This is actually incorrect, the temperature of the burned gases is directly related to how hot they are before they start burning. A hotter intake charge will produce hotter end temps during/after combustion; this is why one of the best ways to reduce detonation is to cool the intake charge because the burn temps are reduced to where the unburned gases in the chamber won't combust on their own before the flame front moves through them. The reason colder intakes make more power is simply because colder=denser and you get more fuel/air in each of the cylinders in each intake stroke.

Heat crossover is purely for vaporization of fuel in cold weather upon startup. Without it you will make more power but it will take longer to warm up and drive depending on how cold it is outside, and during that time there will be raw fuel going into your cylinders which we all know wears them out and dilutes the oil. Not much more to it than that...
 
Rice Nuker said:
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.
plenum tuning more economically due to manufacturing advantages.
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I thought that this thread was trying to analyze how an engine will run with or without a heat crossover passage operable. What is the purpose of it and how will the engine be affected if it was blocked off. To make a proper comparison, you have to isolate and change only one variable at a time or your rusults will not show how that particular change affected the engine.


Trying to compare a 1975 engine to a 2001 engine is trying to compare apples to oranges. Too many variables to isolate how changing just one will affect the process. And why are you choosing 75 model year that had lots of emissions problems to affect the engines and how they ran. Try a 68 - 71 era engine and isolate the heat crossover and explain the affects the engine.

There was a couple of statements worded so that someone may get the wrong idea and I was just trying to make it clearer.
 
Well.... I hate to say it but I am thinking about EGR now. :happy2:

Thanks for the contribution Mope!

MOPEkidD-3 said:
This is actually incorrect, the temperature of the burned gases is directly related to how hot they are before they start burning. A hotter intake charge will produce hotter end temps during/after combustion; this is why one of the best ways to reduce detonation is to cool the intake charge because the burn temps are reduced to where the unburned gases in the chamber won't combust on their own before the flame front moves through them. The reason colder intakes make more power is simply because colder=denser and you get more fuel/air in each of the cylinders in each intake stroke.
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Hey, its all good here krazy :)

I reckon in my recent pondering statement I was just speaking primarily to my belief that was also mentioned by RustyRatRod regarding plastic MPFI intake vs heated crossover old style intake and the potential difference in density of charge.

Good point. Use of scientific method is usually pretty far away from most discussions.

krazykuda said:
I thought that this thread was trying to analyze how an engine will run with or without a heat crossover passage operable. What is the purpose of it and how will the engine be affected if it was blocked off. To make a proper comparison, you have to isolate and change only one variable at a time or your rusults will not show how that particular change affected the engine.


Trying to compare a 1975 engine to a 2001 engine is trying to compare apples to oranges. Too many variables to isolate how changing just one will affect the process. And why are you choosing 75 model year that had lots of emissions problems to affect the engines and how they ran. Try a 68 - 71 era engine and isolate the heat crossover and explain the affects the engine.

There was a couple of statements worded so that someone may get the wrong idea and I was just trying to make it clearer.
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I think you hit the nail directly on the head right here.

How do we get the best of both worlds?

TBI and MPFI are sweet as pie but expensive and challenging as hell to most. Lots of economical TBI don't flow much over 300 - 325 HP. Obviously TBI still has some wet manifold issues but they are much less.

How the heck do we get a cold start and driveability, longevity of engine rings and cold air charge, all wile running a carburetor?

Maybe a heating element plate molded into the floor of the intake under the carb of some sorts? With a thermostatic operation based on engine temp / coolant temp?

Hmmm. Yessss its all becoming :violent1:now.. Air gap intake with a plate in the floor which gets toasty for 10 minutes or so, then kicks off.

I used to work with these a bit in an R&D de-icing system.



340-4spd said:
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).
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I'm running an Air-Gap manifold on my 360 in my '70 Duster, it was the only choice with Magnum heads which don't even have a heat crossover. My solution was to get a factory dual-snorkel intake from the early '70s with the vacuum-operated heat riser and fab up some plumbing around one of the header tubes. With that system working I can start up the car cold in below-freezing temps and be on the road in less than 10 minutes.

I actually can't imagine having a heat crossover in summer, even being an Air-Gap manifold the carb gets really hot and boils the gas when creeping in traffic on a hot day (90*+). I need to add a carb spacer but there's no room for it with my stock hood and high-rise manifold.
 
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