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Author Topic: "Hot Slot" equipped cast iron intake manifold modifications..  (Read 9294 times)
Camaroon
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« on: February 03, 2012, 12:52:55 AM »

It has been frequently mentioned herein to plug the two "hot slot" holes in intake manifolds used on Q-Jet cars with two 7/16 steel freeze plugs, as a fire prevention measure. Other enthusiast websites have similar recommendations to perform this measure, with the added suggestion to also modify the exhaust manifold heat riser valve by removing the round metal plate from the shaft inside the heat riser.
Is it advisable or nessessary to remove the plate from inside the heat riser when plugging the hot slot holes? 
Perhaps JohnZ can chime in on this with his thoughts.
As always, Thanks!
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Mike S
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« Reply #1 on: February 03, 2012, 09:25:35 AM »

  I tapped and installed allen plugs in the hot slots on both of my 67 396 Camaro's. I left the riser valve alone though. That opens fairly quick anyways and I don't see how that would contribute to performance issues. The stock exhaust manifolds are the large performance reduction culprits (you can work around that a little by using a cam with a slightly longer exhaust profile). I did install a restrictor in the intake manifold gasket crossover opening to limit the amount of flow under the intake mainly to reduce paint discoloration and flaking at the ends but still retaining the functionality of the choke coil.
 If you intend to drive the car in cool weather or show the car I would recommend to leave the exhaust valve in place.
I don't think any of us use the cars today the way they were intended due to their antique status and value, including performance concerns, so aside from the hot slot plugs, I would recommend to leave the rest intact.
 
Mike
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67 LOS SS/RS L35 Hardtop - Original w/UOIT
67 NOR SS/RS L35 Convertible - Restored
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« Reply #2 on: February 03, 2012, 09:47:10 AM »

 I bought this cast iron intake to replace alum. edelbrock as I wanted to have it more original. It came from the seller with the holes welded shut.  Just FYI

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Click is Jim , central Minn.  Moderator at Team Camaro www.camaros.net
Camaroon
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« Reply #3 on: February 03, 2012, 10:15:40 AM »

Thank you 67-Z27 ! Great picture, too!
I tend to agree with you about leaving the heat riser intact.
An added reason to leave the heat riser to operate normaly is the mechanical heat-operated Rochester choke mechanism mounted on the intake manifold. Perhaps leaving the heat riser
intact helps this choke to function as intended?
More input from others on this topic will be appreciated! 
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Mike S
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« Reply #4 on: February 03, 2012, 10:28:30 AM »

 I found this informative article (attachment) about heat risers you may find of interest.
It's in a Word format.

Mike
« Last Edit: February 03, 2012, 11:02:51 AM by 67-Z27 » Logged

67 LOS SS/RS L35 Hardtop - Original w/UOIT
67 NOR SS/RS L35 Convertible - Restored
Stingr69
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« Reply #5 on: February 03, 2012, 11:22:04 AM »

If you block the passage and keep the heat riser valve and it shuts closed as it should - where does the exhaust from the passenger side go?Huh

My 2cents - Put a heat riser spacer in there like they used on FI cars that eliminates the flapper. You can then block the passage holes.

Aluminum intakes can run well without the exhaust riser heat but cast iron will take a LONG time to heat up.

On my 302 I have the FI spacer that eliminates the valve function and the aluminum intake with only the drivers side manifold passage blocked. The choke stove on the manifold still gets enough heat to work. and the car warms up fast. I needed to block one side just because the heat was getting to be too much on the carb for todays gas. New fuel seems to percolate much more than the good old stuff did.

-Mark.
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« Reply #6 on: February 03, 2012, 11:25:53 AM »

 Here is photo of my setup after its all done... the heat riser works the choke nicely in our cooler climate here in Minnesota.



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JohnZ
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« Reply #7 on: February 03, 2012, 11:31:40 AM »

It has been frequently mentioned herein to plug the two "hot slot" holes in intake manifolds used on Q-Jet cars with two 7/16 steel freeze plugs, as a fire prevention measure. Other enthusiast websites have similar recommendations to perform this measure, with the added suggestion to also modify the exhaust manifold heat riser valve by removing the round metal plate from the shaft inside the heat riser.
Is it advisable or nessessary to remove the plate from inside the heat riser when plugging the hot slot holes?  
Perhaps JohnZ can chime in on this with his thoughts.
As always, Thanks!

There are two different "heat" circuits on intake manifolds with the "hot-slot" channel at the front of the carb pad; both are controlled by the heat riser valve. The primary circuit directs hot exhaust gases from the right bank through the heat riser crossover passage in the intake manifold to the left side head, where it merges into the exhaust ports and exits through the left side exhaust manifold. This crossover passage goes UNDER the floor of the intake plenum, warming it, which warms the incoming fuel/air charge for improved cold driveability so the fuel droplets don't condense on the cold walls of the manifold and intake ports; it also gradually warms the intake manifold casting, which in turn eventually warms the carburetor itself to prevent "carburetor icing".

On cars with the "hot-slot" channel, the holes at the ends of the channel are cast/drilled into the crossover passage that goes below the manifold plenum floor; the idea was to get hot exhaust gases to the carburetor baseplate right away to prevent carburetor icing, which is why those cars also use a stainless steel baffle between the carb baseplate and the carb gasket, to protect the aluminum carb baseplate from direct exposure to the hot gases. This design resulted in a number of damaged carburetors and carburetor fires (and a huge safety recall), and was abandoned after 1969.

Plugging the holes at the ends of the "hot-slot" prevents overheating and damaging the carburetor, but it leaves the crossover passage functional; this is important for proper function of the remote choke thermostat coil mounted in the well in the intake manifold on the passenger side; if that well doesn't get hot from the gases going through the crossover passage, the choke won't fully open.

In most cases, the best compromise is to plug the "hot-slot" holes (to protect the carburetor), and either remove the flapper valve in the heat riser or just wire it in the fully open position; that will still allow some exhaust gases to heat the choke well so the remote choke thermostat works.

Some guys choose to plug the ends of the crossover passage entirely, with gaskets with stainless inserts; that stops all flow of exhaust gases and keeps the intake manifold (and carburetor) cooler, but it also means the choke won't work any more (won't open fully due to no heat in the choke well); if you choose to do that, you MUST either remove the heat riser, gut it, or wire it open, or the exhaust gases from the right bank won't have anywhere to go when the heat riser is closed.
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'69 Z/28
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KevinW
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« Reply #8 on: February 03, 2012, 04:00:44 PM »

I plugged my carb crossover ports with the freeze plugs, have the restrictor plates in the intake gaskets (small hole left) and took the flapper off on my 69 SS350 engine.  No issues with cold start (std manifold choke coil) or any other issue, but the paint on the manifold still chars.  Anyway to get around that? Smiley
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JohnZ
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« Reply #9 on: February 04, 2012, 11:01:21 AM »

the paint on the manifold still chars.  Anyway to get around that? Smiley

Nope. That occurred on all factory painted iron intakes. The only way to avoid it is to block the crossover passage openings completely, which will screw up the choke operation.
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'69 Z/28
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Camaroon
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« Reply #10 on: February 05, 2012, 07:05:02 PM »

OK, thanks guys, I think I'm getting the picture now as to how to proceed, I'm in Wisconsin, and will only use the car during warmer weather, and since it rarely gets "Arizona hot" around here, I don't have to worry about ambient outsire air temperature extremes. The only remaining issue is regarding the stainless steel baffle plate that was originally factory installed between the carb baseplate and the carb gasket. (Which I don't have)  I believe there was a gasket installed on both sides of this plate, and this plate was installed to eliminate vacuum leaks between the hot slot and the manifold intake ports.  Is this baffle plate no longer needed if both of the hot slot holes are blocked? I plan to drive 7/16 inch steel freeze plugs into the hot slot holes.
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jeff68
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« Reply #11 on: February 05, 2012, 09:04:05 PM »

There was only 1 gasket used.  Intake manifold, then gasket, then heat shield, then carb.
I would keep the heat shield, but don't know if it's actually required for some other reason other than protecting the base of the carb from direct contact with exhaust gases in the hot slot.
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68 L30 / M20 Convertible
Ash Gold
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« Reply #12 on: February 08, 2012, 02:30:50 PM »

OK, thanks for the update regarding just one gasket used at the factory for a Rochester 4 bbl and cast iron itake setup on the '69 SS350, with the "hot slotted" gasket against the intake, and the stainless steel baffle plate against the carb.
I also found the stainless steel baffle online. So, here's my plan: Plug the intake manifold hot slot at both ends. Use an intake manifold gasket that has the center port opening to allow exhaust gases to rise and pass thru the intake manifold to allow the intake manifold's well mounted thermatic choke to get warm so that it'll open in a resonable time. Use the stainless steel baffle under the carb, and I may put a gasket on both sides of the baffle to ensure a good seal and to keep the carb a bit cooler. Use a working heat riser on the exhaust manifold. And, afterwards, I can test the setup, watching for proper choke and heat riser operation, and if I determine the exhaust manifold heat riser action is not needed, I can easily wire it open.
A special thanks to Dave in PA for the heat riser fact and fiction article, and to others who have repeatedly mentioned the hot slot problems and plugging fix. It would be cool if someone has the original GM hot slot manifold recall document link or pdf, etc., as I don't think we've discussed what GM's repair  procedure was back then when the recall was active. Wonder what a Chevy dealer would say if I showed up with my '69 and demanded they perform the recall ??    Grin  Shocked     
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jeff68
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« Reply #13 on: February 12, 2012, 07:46:39 PM »

...and I may put a gasket on both sides of the baffle to ensure a good seal and to keep the carb a bit cooler.
Isn't that going to raise the carburetor in relation to the choke, which will affect the operation of the choke?
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68 L30 / M20 Convertible
Ash Gold
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« Reply #14 on: February 13, 2012, 08:02:57 AM »

If you have the right part numbers the height should not be an issue. The "tin plate" goes on top of the manifold then the fiber gasket then the carb. Once upon a time you could buy one gasket that sandwiched the plate and the fiber in one - maybe at Acme Auto Parts.
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1969 Z-11 350/300 with 4 Speed
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