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Old 02-23-2008, 11:07 AM
geneo geneo is offline
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Cool ? what is this heat cross over on intake?

i keep hearing people talk about this heat cross over on a intake manifold, they said to plug it off, i have a 70 challenger 383, just purchased a rpm edel intake does this have cross over that need to be blocked off, and also what does this cross over do, { what does it all mean im so confused]
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Old 02-23-2008, 11:40 AM
crisser crisser is offline
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The heat crossover is used to warm the bottom of the intake below the carb. It`s purpose was to help speed up atomization of the fuel while the engine is cold. If not, the fuel would puddle in a cold intake. It`s good to use on a street engine especially in a cold climate. On a race engine, you can block it to keep the air/fuel mixture cooler and denser for more power, and the center cylinders that the crossover draws from will flow better because all the exhaust has to leave the header as opposed to some of it flowing back into the crossover. There, it clashes with the other side trying to crossover itself. So, you can see, on a race engine, it`s good to block off. On a street engine where you aren`t looking for every HP, then it`s good to leave open for good cold weather performance. Down here, it`s warm most of the time so you can block it off even on the street.
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Old 02-23-2008, 11:43 AM
jelsr jelsr is offline
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It's the small passage in the center that lets exhaust pass under the floor to preheat the intake charge for driveability when cold.
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Old 02-23-2008, 11:43 AM
geneo geneo is offline
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right on thanks. i live in wisconsin so ill just leave alone. thank you.........
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Old 02-23-2008, 11:43 AM
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rumblefish360 rumblefish360 is offline
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This is a picture of a small block head, but the same idea is still there. Notice in the center of the intake ports, between them, there is a little port window just under a half moon shape.

This little port has a passage to one of the exhaust ports. The idea is to let hot exhaust up this port to warm up the intake and carb. During cold seasons, this aids in warm up times. Shortening them.

Blocking this off via gasket or tin plates sandwiched between the gasket and intake block the heat from rising making the intake run cooler for a cooler denser air and fuel charge.

Blocking the passage is not a great idea if you run the car everyday through the winter. Warm up times will increase and mileage will suffer. As if it is not bad enuff with a big block, it will only magnify with the cross over blocked.
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Old 02-23-2008, 11:54 AM
geneo geneo is offline
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cool appreciate picture, kinda clarifies it now, dont run in winter but do run in spring and fall does get cold in this tundra, always noticed car warmed up nice. thanlks
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Old 02-23-2008, 01:41 PM
440roadrunner 440roadrunner is offline
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Blocking heat crossovers on street cars is generally a really poor idea. The problems you could have do NOT depend solely on temperature, but on a combo of temp and humidity. This is because of the "refrigeration effect."

You all know what refrigeration effect is. That's why air tools run cold--you run high pressure gas through a small orifice, and the gas (air) cools as it expands

The same thing happens to incoming air through the carb venturi, aided by the vaporizing gasoline at that point. If any of you have ever seen a Corvair or VW dune buggy engine with an aftermarket intake, you'll see this. These can actually ice up on a hot day!!!

This is exactly what causes aircraft "carb icing" and why they have heat devices to combat the problem.

Putting headers on a 6 cylinder inline engine can cause similar drivability problems. Because the typical 6 intake is "hanging out there" with little warming from the engine, headers on these types of engines--with no carb heat--WILL give you grief!!

By the way, the intake crossover is why most engines--my sixpack did not--have a thermostatic butterfly in one exhaust manifold. When cold, this butterfly partly blocks one exhaust, forcing exhaust gas up and across through the manifold.
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Old 02-23-2008, 04:11 PM
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nhdriver nhdriver is offline
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Sometimes the crossover becomes clogged w/ carbon & closses itself off, becoming ineffective.
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Old 02-23-2008, 05:15 PM
440roadrunner 440roadrunner is offline
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Very true, nhdriver


On a side note, the first few years (71? 72?) that Mopar applied EGR (exhaust gas recirculation) all they did on the V8s was to either drill holes or install orifices in the bottom of the intake, in the bottom of the carb plenum. This allowed exhaust to leak into the intake for "EGR."

I'm sure you can see that this makes it easy to defeat this, uh, "feature."
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Old 02-23-2008, 07:16 PM
valiant64 valiant64 is offline
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I'll have to disagree on headers on inline 6 cyl engines having poor driveability when cold. I have no problem starting my slant 6 w/dual carbs & no choke during cold weather. Granted, the coldest it gets here in the winter is around 25 degrees, but it will start & idle on its own with no problem.
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Old 02-24-2008, 04:51 PM
69Sixpackbee 69Sixpackbee is offline
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Unless you drive in sub-freezing weather block it off.
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Old 02-24-2008, 06:07 PM
440roadrunner 440roadrunner is offline
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I see you live in S Cal. As a guy who's driven from the north latitudes in weather down to -20, to the dry heat of AZ and the humidity of GA, I can tell you that this is very poor advice.

The crossover does a lot more than aid cold startup. On a street car it also affects general driveability during warmup, as well as fuel mileage. And for those of us who had to put up with--in an earlier life--with emmisons checks, it might just flunk you there, too

On a cold, wet day, say, in the low '40's rainy, humid, a cold carburetor box can cause real poor driveability
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Old 02-26-2008, 10:26 AM
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rumblefish360 rumblefish360 is offline
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The Advice of blocking the hot air cross over is good for strip rides. Street strip rides could go ethier way, but would bennifit from having it open. It is not a big deal to have it open.

On the EGR valve, a little inert gas never hurt anything and aids in more timing advance if needed. The amount is very little through the valve and theres no performance hurt in running it.

Better off not having it, but at the same time, your not looseing 10 hp with it hooked up. When you go to W.O.T., the EGR valve closes off.
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Old 02-26-2008, 02:11 PM
440roadrunner 440roadrunner is offline
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If you are referring to my earlier comment about the first few years of EGR in the manifold floor, there WAS no control valve. They were simply holes or orifices drilled down through the manifold floor into the crossover. This means that at high RPM--and WOT there was still EGR
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Old 03-02-2008, 09:14 AM
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rumblefish360 rumblefish360 is offline
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Thanks 440. I didn't know that. I guess a plug will fix that.
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Old 03-02-2008, 10:50 AM
N.R.F.D. N.R.F.D. is offline
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the type of egr that 440 is talking about is call floor jet egr. how many of you guy up north run with w2s. unless you have the manifold with water heat its the same as blocked passages and i dont consider them as real race heads so ask thoses guys what they think of cold weather hot humid weather my m1 on my 383 just has a very low idle when its cold and a fast idle when its hot so if you can live with that then plug them. its deffinetly faster with them blocked if your carb is tuned for it. modern fuel injected engines dont have heat cross overs but it is a little different though the computor take care of the idle and the jetting
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