Intake manifold exhaust crossover? - Team Camaro Tech
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post #1 of 9 (permalink) Old Apr 9th, 04, 06:13 AM Thread Starter
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I have a 396 in my 69 Camaro, it has the 375 Hp cam and oval port heads. The intake manifold is a Weiand and a 750 Dp Holley. Hooker Headers as well.

My problem I have is that when cruising on summer nights it gets very hot in the carburator and I get vapor locks. The engine starts to behave bad, smoking black and needs to open the throttle to be able to bread.

I have had thoughts on putting in a return line to the gas tank so it will not get to much fuel.

Now to the subject:
I started to think about the exhaust crossover that runs under the carb to preheat the manifold. It must create very much heat that spreads to the carb and starting these problems. When buying gaskets there is a blocking plate that makes one of the holes smaller for the crossover.

But what if I blocked it completely?

That must solve this problem, wouldn't it?

What are the consequenses? Will it run poorer when cold? I don't have a choke.


Jan

Jan Suhr
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'69 Camaro Z-28, X77, Cortez Silver, LS1 / T56, Still has the original BU rear end

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post #2 of 9 (permalink) Old Apr 9th, 04, 06:34 AM
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The Exhaust Heat Crossover allows the spent exhaust gasses to help heat the base of the manifold and carb. to assist in the engine cold run characteristics when an engine is started from cold-soak.
In milder temperatures it has no noticable function and in higher heat areas it can contribute to vapor lock and other hot-start problems.
It is normal here in sunny 'Kalifornea' to completely block off these passages or run manifolds that don't have any passages at all with no problems.
I have both blocked the passages in the manifold and run "blocking" type manifold gaskets on street vehicles with no problems.
The "old School" way of blocking these passages was with small pieces of tin-foil - rolled into balls and packed in with a drift (blunt ended punch) until they were filled solid. This sounds 'hoakey' but it works well and none of the material ever fell out of came loose in my experiences.

Sorry so long - but I wouldn't worry about running blocking gaskets on a street driven vehicle at all - if temps remained above freezing during my cruise...

John

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post #3 of 9 (permalink) Old Apr 9th, 04, 07:33 AM
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You can also use an insulated gasket or spacer. An insulated gasket is 1/8 to 1/4 inch thick and should help. I'm talking about a carb base gasket. Saves the trouble of changing manifold gaskets and easier.

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post #4 of 9 (permalink) Old Apr 9th, 04, 09:01 AM Thread Starter
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I have the engine apart at the moment and I have now fabricated a blockplate from sheet metal.

Will consider a heatinsulator on the carb too.

It gets very hot under the hood when standing still or cruising. i hope this will solve some of the problem, the engine itself never runs hot.

Thanks


Jan

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post #5 of 9 (permalink) Old Apr 9th, 04, 02:39 PM
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When blocking off the intake crossover heat passages, you MUST be sure that your heat riser valve on the passenger side exhaust manifold is either free and functional, wired open if it isn't, or is replaced by an open spacer. If that valve is stuck shut or only partially open, there's nowhere for the right bank exhaust gases to go If you have headers (and thus no heat riser valve), there is no issue.

Also note that if you have an intake-mounted divorced choke thermostat coil and you block the crossover passage, it will take a LONG time for the choke to open.

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post #6 of 9 (permalink) Old Apr 10th, 04, 05:39 AM
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I have run into this before. This is something you can do on a car that has an aluminum intake and no heat riser valve.

Cut out a piece of stainless steel "shim stock" with tin snips. It needs to be very thin and roughly 1-3/4" x 2-1/4" in the flat. You can make it from the flat part of one of your wife's stailess steel mixing bowls. Do it when she is still at home if you are a real man Bend a little 90 bend to form a 1/4" lip that will prevent it from falling into the engine. You can drain the engine coolant from the block, loosten the intake bolts and slip or tap it in between the block and intake, then snug the intake bolts back again. Fill the cooling system and fire up the engine to check for leaks. You only need to do the drivers side. This way the divorced choke coil will still function if you ever have need for it now or in the future.

The cast iron intake engines will take way too long to reach operating temp this way so I would not reccomend the mod for those applications. Not many of us here on this forum have those but I thought I would mention it. On an aluminum intake engine it will still take a little longer to warm up but only about 15-20 minutes or so.

-Mark.
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post #7 of 9 (permalink) Old Apr 10th, 04, 06:34 AM
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I guess it goes without saying, make sure your ZL-2 hood is functioning to take advantage of the cooler temps outside of the engine compt. Nice Car! [img]graemlins/beers.gif[/img]

Rob

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post #8 of 9 (permalink) Old Apr 10th, 04, 07:01 AM
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Quote:
Originally posted by JohnZ:
When blocking off the intake crossover heat passages, you MUST be sure that your heat riser valve on the passenger side exhaust manifold is either free and functional, wired open if it isn't, or is replaced by an open spacer.
Removing the Heat Rizer if using the stock Exhaust Manifolds is a good point JohnZ.
As he is running 'Hooker headers' I didn't include it - but, it should have been included so anyone searching archives would know.
Thanks;

John

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post #9 of 9 (permalink) Old Apr 14th, 04, 08:28 AM Thread Starter
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Quote:
Originally posted by olympic69:
I guess it goes without saying, make sure your ZL-2 hood is functioning to take advantage of the cooler temps outside of the engine compt. Nice Car! [img]graemlins/beers.gif[/img]

Rob
Thanks!

Well I have the 3 inch open filter aircleaner and I think that it creates a pocket around the carb that doesn't let the hot air out off around the carb. It sort of gets trapped there.

We'll see how the blocking plates handles this before I make any further steps.

BTW I will soon have some new pics of the coming ProTouring look of this car. Stay tuned!


Jan

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'69 Camaro Z-28, X77, Cortez Silver, LS1 / T56, Still has the original BU rear end

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