intake manifold exhaust crossover questions - Team Camaro Tech
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Old Nov 10th, 05, 10:45 AM Thread Starter
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Duke
 
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Question intake manifold exhaust crossover questions

After reading some other posts, I am questioning some advice I once recieved regarding the exhaust gas crossover in the intake mainifold (1969 350 300HP, standard exhaust mainfolds). I was told it was a good idea to block off the crossover because there was trouble with burning/melting the carburetor base in the factory design. What is the real story, and what negatives would there be to blocking off the channel in the intake manifold?

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Old Nov 10th, 05, 11:01 AM
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Re: intake manifold exhaust crossover questions

Your 69 intake manifold sends exhaust heat directly to the carb. There's a thin piece of stainless sandwiched between two carb gaskets. This was designed so that cold engines would run well in cold weather.

If you don't use this stainless piece the fuel in the carb actually boil. Also, if you don't use the correct q-jet gaskets for a 69 you'll a have vacuum leak. The correct gastket has a 1/4 inch channel about five inches long in front of the primaries. This channel is where the carb is heated. Don't try universal q-jet gaskets,..won't work.

I think Rick's still has the stainless piece you need, and Fel-Pro still has the correct gaskets (you'll need two).

As a side note,..this configuration led to a huge carburetor recalli in 1969,..turns out, repeated heating of pot-metal with 1200 degress of heat will cause problems in the q-jet. Duh.

Or,..you can block the EGR port between the head and intake by purchasing an intake gasket set that has the port blocked off. This is what most do.

Good luck!

Last edited by Straight-line-69; Nov 10th, 05 at 08:15 PM.
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Old Nov 10th, 05, 01:29 PM
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Re: intake manifold exhaust crossover questions

You can also drill and tap the two holes in the Intake Manifold, that lead up to the channel, from the top and plug them with a set of 'Pipe plugs'.
This will seal off the exhaust gasses and prevent them from getting to the carb base, but still allow some heat to get to the manifold base. This can increase the drivablity in cooler weather if you need it. If cold weather operation is not important, you can also install the cross-over blocking gaskets to help keep the manifold base cooler with these plugs.

Since you have "standard exhaust mainfolds" be sure to remove the Exhaust Manifold Heat Valve , at the right (passenger) side exhaust manifold if you block off the passages.

John

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Old Nov 10th, 05, 02:28 PM
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Re: intake manifold exhaust crossover questions

There was a post a while back, I think from JohnZ about using 7/16 freeze plugs in the intake holes under the carb. That is what I did as well as using the gasket restrictors and taking the flapper out of the original exhaust manifold valve. Only have 1 season on it, but no problems so far. No real cold start issues either.

Kevin


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Old Nov 10th, 05, 04:24 PM
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Re: intake manifold exhaust crossover questions

Here is the post:

https://www.camaros.net/cgi-bin/forum...c;f=9;t=009937

https://www.camaros.net/cgi-bin/forum...;f=17;t=000870

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Old Nov 10th, 05, 04:26 PM
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Re: intake manifold exhaust crossover questions

As quoted by JohnZ:

Quote:
No paper gasket - the heat-resistant thick gasket goes on first, then the metal baffle without the slot in it, then the carb. That slot in the intake lets hot exhaust crossover gases heat the carb base without contacting it directly, and has ruined (and rendered unrebuildable) more Q-Jets than you can count. A better long-term solution is to use 7/16" steel cap plugs (or freeze plugs, depending on what your parts guy calls them) and drive them into the holes at the end of the slots in the intake to keep exhaust gases out of that slot entirely; GM eliminated that slot entirely after 1969 due to all the carb failures it caused.

--------------------
JohnZ
CRG
Quote:
The extreme localized heat from the exhaust gases warps the baseplate. It's also not coincidental that the intake slot was eliminated following the biggest carburetor recall in GM history when the bowl plugs started coming out of the bottom of Q-Jets and causing engine fires.

--------------------
JohnZ
CRG

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Old Nov 16th, 05, 09:19 AM Thread Starter
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Re: intake manifold exhaust crossover questions

Thanks to all of you for quick responses. This site never fails to help us folks in need.

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