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Old 09-16-2002, 03:29 AM
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Post exhaust crossover

I might be on the wrong train of thought here, so I thought I would ask.

"Only dumb question, is the question not asked"

Well, I will be using the smog heads off of my 350 with an edelbrock performer intake (perhaps later invest in better heads). Since I don't have to undergo any emissions inspections, I opted to dump all the unnecessary junk from under the hood. Now the heads and intake both have the exhaust crossover port. I know some aftermarket heads don't have this crossover. Can I block off this crossover at the head? My logic here is that it would have to let the intake manifold run a little cooler since some of the exhaust isn't flowing thru under the carb. Is blocking the port off a realistic idea or do I need to get the thought out of my head? Thanks in advance for any suggestions.

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Old 09-16-2002, 03:39 AM
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That used to be a common modification, we would use heavy aluminum foil and pack it in as tightly as possible. It helps keep the intake charge cooler and denser ( actually the same thing). The by product is that if you use it as a daily driver warm up is extended from not having the pre-heat. Most aluminum heads heat up quick enouh that they probably aren't as sensitive to it and since they are performance heads it makes the casting a little simpler.
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Old 09-16-2002, 03:40 AM
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[quote]My logic here is that it would have to let the intake manifold run a little cooler since some of the exhaust isn't flowing thru under the carb.<hr></blockquote>

You logic is correct. Just ask for intake manifold gaskets that have the block off feature.
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Old 09-16-2002, 06:35 AM
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Just remember, if you block off the heat to the intake manifold, don't use a carburetor that has a heat stove type of choke.
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Old 09-16-2002, 07:19 AM
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I understand that for performance reasons you want as mutch quanity of fuel into the the cylinders. But that exhaust going under the intake is there for a reason. I am one of those people who want my car to get desent gas milige since my car is going to be an everyday driver. The exhaust heats up the intake manifold so when the fuel and air drop downinto the intake and it vaporizes a lot more. A better vaporized fuel is more effeceint, so for street use with a very mild cam it would give you more power and better fuel effeincy. To me this passage is a must, that is why I didnt go with the Vortec heads that lack this passage. The reason for using it, is cause cold air and gas is more dense. SO more gas goes in, less gas is burned effeincy but there is a lot more gas in it so in-turn you get more power. I would guess that exhaust passage-way adds over 2-4 miles per-gallon. It also would help in the winter with warming the car up
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Old 09-16-2002, 09:52 AM
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The purpose of the exhaust heat crossover is cold weather driveability. Believe me, after the engine has warmed up, there is more than enough temperature in it to atomize fuel. Aluminum intakes dissipate heat more readily than does cast iron, so if you live in a cold climate and have an aluminum intake, I'd use the crossover, and even if it's cast iron. If it's all out horsepower you want, then you want to reduce the air intake temperature as much as possible, this equals more horsepower period, why do you think intercoolers work so well on turbo applications? They lower the intake temperature.Warm intake fuel air mixtures atomize well and helps throttle respose and driveability, while colder air makes more power.
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Old 09-16-2002, 12:42 PM
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[quote]The purpose of the exhaust heat crossover is cold weather driveability.<hr></blockquote>

CORRECT!

That is why it is used in conjunction with a heat riser. Once the riser opens, there is very little exhaust gas circulated under the plenum, but enough to warm the incoming charge..

The reason you need it on a street engine is that with a carburetor, you have a wet intake manifold, i.e. gasoline and air are introduced as a mixture. The trouble is, with carburetion, especially with the choke operable (heavy gasoline introduction), the fuel mixture will separate once it changes direction at the runners and gasoline will puddle on the plenum floor, giving you those lean backfires that drive you crazy for the first few miles until the intake manifold absorbs heat.

If you will notice on EFI (not TBI), there is usually no crossover as the fuel (gasoline) is introduced downstream from the throttle valve (dry intake manifold).

So for a mildy modified street engine, you will have better performancee with a cool plenum rather than a heated plenum. It will use more fuel until the plenum absorbs engine heat...but just go easy on it until that happens and the electric choke fully opens.
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Old 09-16-2002, 01:38 PM
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thanks, i just attributed that occasional back fire (when i pull out on the highway and punch it on the way to work) as just the engine not being warmed up enough, or loaded up from idling to warm it up, now, i know, THE REST OF THE STORY!
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Old 09-23-2008, 11:21 AM
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everyone here has valid points but just to give an example of one extreme to another. I had a chevy style manifold on my big block mopar which seals to the ends of the block with silicone. Where a mopar intake is in the air like a edelbrock rpm manifold. One hot days if you shut it off hot then tried to start it 15min later it would be hard to start and flood for a min once started then it was ok. This is refered to as a heat soak. basically boiling the fuel in the carb. So I switched to a traditional mopar intake but single plane and block the crossover. Now I can drive for an hour then start the car with the touch of a key and the carb never gets over 100 deg. Great for horsepower execpt one flaw. Lots of jets lots of gas huge accell pumps. Bigest flaw on a good launch the car would have a huge lean bog off the line. Someone here said it the fuel drops out of the air and puddles on the floor of the intake. So I went from one extreme to another. Removed blockage from the crossover and found a happy medium for driveability I would not block this off. Unless you are having a heated carb problem.
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Old 09-23-2008, 12:26 PM
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I was stupid..

I have put in spacer with phenolic gasket..and Holley heat shield. Helps but I still get heat soak..10 minutes after (or so) shut off..
If I remove the top fo the air cleaner..I have no problem. If I use the cheap Edelbrock foam/mesh top aircleaner..also no problem.

I won't use the foam..too much dust get through..and the foam is flamable..mine caught fire on a backfire ! Tuning the motor when I first put it in..plug wire shorted to manifold..flamed the foam.

When I did my motor I left the crossover open. I live in Tucson..we rarely ever need any warm up time..and my car IS NOT a daily driver.

To get my distributor out..to remove manifold..will require removing the hood..not something I want to do...thin aluminum..hand formed and way more $$ than I ever want to deal with it damaged.

Soo..

I remember reading that it's possible to...loosening the manifold center bolts..sliding in a piece of roof flashing (or wha would be better ?)to block the crossover.

I also used to use the aluminum foil with the manifold off..(I don't think they made blocked gaskets then).

Would have been so simple to use the blocked gasket..but too late now..so..

Anyone have success blocking without removing manifold ?

Last edited by Docc; 09-23-2008 at 12:33 PM.
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Old 09-23-2008, 12:59 PM
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I wouldn't loosen the manifold to put shim stock between the head and manifold to block the heat - there are water passages that if you break the seal may cause more problems then any heat. Yep, I used to do it on my old Ford flatheads, but there were no water passages in that manifold. The best bet is a Performer RPM manifold which has no heat/EGR mount provisions. The down side of the RPM manifold is a bit more height at the carb pad, 4.05" vs 4.72" (~11/16")

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Old 09-23-2008, 01:02 PM
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Quote:
Originally Posted by Irelands child
I wouldn't loosen the manifold to put shim stock between the head and manifold to block the heat - there are water passages that if you break the seal may cause more problems then any heat. Yep, I used to do it on my old Ford flatheads, but there were no water passages in that manifold. The best bet is a Performer RPM manifold which has no heat/EGR mount provisions. The down side of the RPM manifold is a bit more height at the carb pad, 4.05" vs 4.72" (~11/16")

Dave W
I was going to drain the coolant..as it's probably time for that anyway.
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Old 09-23-2008, 01:52 PM
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Originally Posted by Docc
I was going to drain the coolant..as it's probably time for that anyway.
Draining the coolant wont be much help as the manifold gasket will start to separate at the water passages - remember how much remnant material you had to clean off either the head or manifold or both - assuming this was a used engine when you initially disassembled it for rebuild. Had it never been run, you MIGHT have been able to get away with doing it that way. It, IMO, isn't worth the risk of an engine full of water and possibly "hydraulicing" it to destruction.

But with the above said - it's your car/engine.....

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Old 09-23-2008, 02:35 PM
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5000 miles on new GM crate.

Might have to figure a way to get distributor out without removing hood. Maybe a different manifold..

we'll see..
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