Is it OK to block the intake heat crossover?

[Donald H.]

This is a 66 L79 aluminum intake. So it has the choke coil that I assume get heat from the crossover (since it sits right on top of it) to bring the fast idle down.

Is there any issue if I use a gasket set that has the heat crossover port blocked? I assume it would hold the fast idle cam up longer without the heat, but is there any other issues?

By the way, there is no heat damper and my understanding is that the heat damper is at least partially what forces the exhaust gas back up and through the heat crossover.


thanks,

Don

[Richard G.]

Re: Is it OK to block the intake heat crossover?

Don;
I see this procedure used (blocking the crossover) mostly on intakes that are painted as they want to keep the paint from burning.
However with it plugged it might make it difficult to pass a PV test, if that is a concern.

The choke may not come off at all, with the passage blocked. You may have to test it to know for sure.
What is the reason for the block off, better performance?

If I was me and I was convinced it needed it blocked I would disable the choke so it never activated.
A lot of reasons I wouldn't want the motor running on the choke for extended periods.
You can start the motor using the accelerator pump and keep it running by foot till it warms up.
Not very bothersome especially if you live in a warmer climate.

You can also purchase blocking gaskets that have a small hole it them, this allows the choke to work and cuts down on the heat at the manifold. Maybe a set up with the holes would get you what you need and still allow the choke to function, even if it is slower.


Rick

[Donald H.]

Re: Is it OK to block the intake heat crossover?

A friend gave me the intake and it was in somewhat sad shape. It had a bolt broken off in the thermostat housing that he had tried to center drill and remove with an easy out, and he broke the easy out. Also a heater hose nipple was frozen in the intake. I got both of these out used better science, i.e. chemistry.

The intake also has two small holes in the heat crossover area (picture attached). I have ground these out some and made a couple of plugs that I hope to get welded in.

The exhaust gases have really done a job on the inside of the heat crossover tunnel. Something has really corroded this area. It's probably OK to use for what I plan and I will probably go ahead and not block off the crossover, but I was just wondering how big an issue it would be if I did.

Thanks,

Don

[Mark E.]

Re: Is it OK to block the intake heat crossover?

The two most effective yet inexpensive mods I know about for our engines are:
- Remapping the ignition advance- centrifugal and vacuum (this includes disabling TCS)
- Replacing the heat riser with a spacer and blocking the heat crossover in the intake manifold

The latter improves exhaust flow and enables a cooler, denser intake air charge. For late '60s engines with Q-Jets, blocking the heat crossover can also prevent carburetor damage due to an exhaust leak at the base plate.

Each of these mods achieved significant seat-of-the-pants difference for my cars. More significant than headers, louder mufflers, aftermarket intakes. And much less expensive and intrusive.

The downside? None that I know about. The choke continued to work fine in all of my cases.

[Donald H.]

Re: Is it OK to block the intake heat crossover?

Mark, did you block the crossover with a intake gasket that doesn't have the hole for the crossover?

I have read some threads where people block at the base of the carb with the cast iron intakes, but the aluminum L79 intake (casting 3890490) does not have the hot air slot under the carb.

Thanks,

Don

[Mark E.]

Re: Is it OK to block the intake heat crossover?

Back in the day, gaskets came with two separate sheet metal plugs you can choose to install over the crossover passage. They work fine, but I don't see them anymore.

Felpro sells intake gaskets with the crossover passage blocked by the gasket itself. I've never used those, but assume they work since Felpro sells them. Maybe someone else has direct experience with these...

I've only seen an exposed crossover passage at the carburetor flange with early (pre- '70) spread bore manifolds. Maybe some square flange manifolds have this "feature", but I haven't noticed one.

[Duke W.]

Re: Is it OK to block the intake heat crossover?

Just block the driver's side. This will allow some heat to reach the choke coil, but will considerably reduce the amount of heat transferred to the carb. You said there is no "heat damper". I assume you mean what is called the heat riser valve that has either been gutter or replaced with the FI spacer.

If the heat riser passage is blocked, the valve MUST be either wired open, gutted, or replaced with the FI spacer. Failure to do so will result in no exit for the exhaust gas on the RH side when the engine is cold.

Duke

[Mark E.]

Re: Is it OK to block the intake heat crossover?

Just block the driver's side. This will allow some heat to reach the choke coil, but will considerably reduce the amount of heat transferred to the carb. You said there is no "heat damper". I assume you mean what is called the heat riser valve that has either been gutter or replaced with the FI spacer.

If the heat riser passage is blocked, the valve MUST be either wired open, gutted, or replaced with the FI spacer. Failure to do so will result in no exit for the exhaust gas on the RH side when the engine is cold.

Duke

It's more work, but I prefer using a spacer. Less restrictive, and less opportunity for rattles and exhaust leaks.

Maybe it's because I'm a California/Texas guy, but I haven't had issues with the choke when the crossover is completely blocked (both sides). The manifold seems to warm up enough from the heads and coolant passage to enable choke operation. As a bonus, the manifold doesn't discolor.

[Donald H.]

Re: Is it OK to block the intake heat crossover?

I think I may just take a medium approach. Pictured is a Fel Pro 1256 gasket. It has a metal shield with a smaller hole for crossover.

And yes, the heat riser was replace with a spacer, and no, I'm not concerned about NCRS judging.

Any negatives to this?

Don

[David B.]

Re: Is it OK to block the intake heat crossover?

See Volume 11 #4 Spring 1985 "A BAFFLING OPTION FOR THE 1956 CORVETTE' also reference 6th Edition of the 56-57 JM page 296 taken from the same. The article I wrote at the time researching Inlet Manifold Baffle part #3731247 was amusing as well as enlightening. Your question brings back fond memories. The GM Engineer who worked on this after Sebring was Frank Burrell

[Carl N.]

Re: Is it OK to block the intake heat crossover?

Been running them blocked on L78 for over 40 years along with flapper removed from exhaust manifold with zero issues.

[Mark E.]

Re: Is it OK to block the intake heat crossover?

I think I may just take a medium approach. Pictured is a Fel Pro 1256 gasket. It has a metal shield with a smaller hole for crossover.

And yes, the heat riser was replace with a spacer, and no, I'm not concerned about NCRS judging.

Any negatives to this?

Don

Blocking the crossover completely has no downside, yet if only partially blocked you forgo the benefit of a cooler intake (denser air charge, no discoloration).

[Mike M.]

Re: Is it OK to block the intake heat crossover?

enjoyed re-reading your article '85 restorer. thanks, mike

[David M.]

Re: Is it OK to block the intake heat crossover?

This is a 66 L79 aluminum intake. So it has the choke coil that I assume get heat from the crossover (since it sits right on top of it) to bring the fast idle down.

Is there any issue if I use a gasket set that has the heat crossover port blocked? I assume it would hold the fast idle cam up longer without the heat, but is there any other issues?

By the way, there is no heat damper and my understanding is that the heat damper is at least partially what forces the exhaust gas back up and through the heat crossover.


thanks,

Don

https://www.forums.ncrs.org/showthre...in-hot-weather

See the thread above scroll down a bit to #9 to see my comments on the heat riser block off.
If its something you are interested in doing it can be done so its for the most part undetectable. The aluminum intake will benefit too as they can discolor from the heat as well. I suggest using the plugs. Either weld it into the intake(aluminum plugs) or weld them into the heads. Done correctly with the intent of reversing the modification some day no harm will come to the modified components.
The thin gasket or metal inserts in the gasket kits dont last and can burn through in short order.

A Q jet it can be converted to an electric choke. I was able to tweak the divorced choke on mine to get it to work.
With the right tweaks it will work.

Ive preformed this mod on a nut and bolt resto with positive results. No more burnt paint, hot start issues and better throttle response. If some one really looks they will notice the absence of the stove flap in the exhaust manifold. It runs a bit cold blooded but only really cold days, 50*F and below.

If you are not going for PV or flight judging go for it. If you dont want to harm original numbers parts purchase another intake and have it modified.

[Gene M.]

Re: Is it OK to block the intake heat crossover?

Mark, did you block the crossover with a intake gasket that doesn't have the hole for the crossover?

I have read some threads where people block at the base of the carb with the cast iron intakes, but the aluminum L79 intake (casting 3890490) does not have the hot air slot under the carb.

Thanks,

Don

WHAT? That would make the heat riser valve a total block off on the right bank.