EFE, traditional Heat riser & thermac informational

[SirRobyn0]

Ok so lets discuss what is the same about the EFE & the traditional heat riser. They are both located in the passenger side between the exhaust manifold and the down pipe. When the engine is cold the valve shuts which diverts the exhaust though the intake manifold cross over into the drivers side manifold. The purpose is to get fuel to vaporize better and faster. This improves cold engine emissions and drivability. This has nothing to do with the EGR valve, other than the EGR happens to draw exhaust from the same chamber.

The traditional heat riser simply has a thermostatic spring on the outside of the valve. When the engine starts cold the valve is shut, as the exhaust manifold gets hot the spring relaxes and the valve open.

In the EFE system, is essentially a traditional heat riser, but instead of having a thermostatic spring it's vacuum operated. So starting at the heat riser there is a piece of metal linkage to the vacuum diaphragm which is mounted around maybe a foot from the heat riser, it has a bracket to the passenger side head to keep it in place. This was done so the diaphragm wouldn't be damaged by the heat from the exhaust manifold. From there a vacuum line runs to the TVS in the manifold. The TVS is screwed into the manifold and reaches into coolant. There is a vacuum line from there that runs to manifold vacuum.

So obviously the traditional heat riser operates by exhaust manifold temperature, where as the EFE operates based on coolant temperature with is obviously a much better indicator of engine temp, than the exhaust is. My assumption is that this change was made so the intake would warm to a more predictable temperature even if the truck was being operated in extreme heat or extreme cold.

The EFE or traditional heat riser, will help the engine to run better during the warm up period, and the choke will come off faster. Without the EFE or traditional heat riser, the engine may tend to run a little rougher or hesitate during the warmup period. This can be compensated for by keeping the choke on longer and reducing the amount the choke pull off, pulls off, but obviously this will lead to a reduction in fuel economy, and increased amount of unburned fuel in the combustion chamber, and for longer periods, in there washing down the cylinder walls which may lead to increased engine wear, and last but not least an increase in emissions.

The thermac system is located on air snorkel for the air cleaner. There is a flapper valve inside the air snorkel which is vacuum operated. From the bottom of the air cleaner is a round tube that looks something like drier vent, but much smaller, which extends to a metal housing or surround on the passenger side intake. NO exhaust gas is brought into the engine from here, what is happening is hot air is being brought in from around the manifold. On the vacuum side, there is a vacuum line attached to the thermac which runs to a temperature sensor located on the air filter housing, generally speaking it is usually on the inside of the air filter, but not always. And another vacuum line off the temp sensor which leads to manifold vacuum.

Now here is where people go wrong with the thermac. They expect that when the engine warms up the valve will open. That's not how it works. The temp sensor is set to maintain an incoming air temperature of about 90F. So your thermac needs to have the plastic air tube from the snorkel to the cold air intake at the core support to work properly. If it's cold out the valve may only open part way even when the engine is fully warmed up. This was done to somewhat stabilize incoming air temps which again helps with fuel vaporization when it's cold out, but also helps to keep incoming air temps stable, which meant that the factory could set the carb and timing for a known incoming air temperature. Most modern cars have a intake air temperature sensor, so that the PCM can make real time adjustments for air temps, this, the thermac system is the next best thing.

But fear not, if you hammer down on the gas and the volume of air needed is to great for the thermac to handle, the valve will open up all the way.

with stabilized air intake temps of 90F from the thermac system, comes the ability to run just a little leaner, and have more stable smoother idle as well as an improvement in idle speed consistency in varying outside temps. The thermac will also help the engine during the warm up period by delivering the warmest air it can to the carburetor.

So why not remove the EFE or the traditional heat riser. Because the engine will run better during the warm up period, and the choke will come off faster. If you remove the EFE or traditional heat riser, the engine may tend to run a little rougher or hesitate during the warmup period. This can be compensated for by keeping the choke on longer and reducing the amount the choke pull off, pulls off, but obviously this will lead to a reduction in fuel economy, and increased amount of unburned fuel in the combustion chamber, and for longer periods, in there washing down the cylinder walls which may lead to increased engine wear, and last but not least an increase in emissions.


So why not remove the thermac system (often done with the installation of an aftermarket air filter housing or the tube from the exhaust manifold is damaged and not repaired / replaced). Because with stabilized air intake temps of 90F comes the ability to run just a little leaner, and have more stable smoother idle as well as an improvement in idle speed consistency in varying outside temps. The thermac will also help the engine during the warm up period by delivering the warmest air it can to the carburetor

I'm sure someone will come along and

 

[bucket]

Heat risers typically aren't found around here anymore, not because people wanted to get rid of them, but because they were stuck closed and/or rattled.

 

[SirRobyn0]

@bucket yes. That and anymore it's not like O'rielly's has them in stock, so they must be ordered.

 

[rich weyand]

Yeah, everybody does the two chrome pie plates thing with air cleaners and then wonders why the thing runs like garbage in the winter. That's a southern California thing, people. Most of the country should be running a thermac.

BTW, they do the same thing on planes, where it's called carb heat. It prevents carb icing, which is bad.

I put the thermac back on my '78. I got it from a junkyard and cleaned it up with evaporust and Rustoleum gloss black. I had to fab up an intake from the headers, using a stainless-steel downturned exhaust tip and some flexible metal hose. Then I knocked out the opening in the radiator bulkhead for cold-air induction, and used the stock cold-air induction tube. I put a chrome dome on the thermac air cleaner.

Oh, baby. It took maybe three minutes when starting in the cold (below zero F) for the intake air to heat up, headers getting hot quick. The idle stabilized, and all my warm-weather torque and horsepower was right there. Used an AFR meter to nail down that mixture, and ran lean and mean year-round.

And it looked good.

You can see the header connection in the first photo. I had a machine shop run a 1/2" end mill about a half-inch deep, three cuts, across the back side of that stainless-steel downturned exhaust tip and attached it to the header pipe with screw clamps.

The cold-air induction hose was an aftermarket copy of the original part, so it was new. The second pic shows my make-do dryer vent tube till the correct one came in. I also used a new Temperature vacuum valve in the thermac rather than the old one.

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[59840Surfer]

Ok so lets discuss what is the same about the EFE & the traditional heat riser. They are both located in the passenger side between the exhaust manifold and the down pipe. When the engine is cold the valve shuts which diverts the exhaust though the intake manifold cross over into the drivers side manifold. The purpose is to get fuel to vaporize better and faster. This improves cold engine emissions and drivability. This has nothing to do with the EGR valve, other than the EGR happens to draw exhaust from the same chamber.

The traditional heat riser simply has a thermostatic spring on the outside of the valve. When the engine starts cold the valve is shut, as the exhaust manifold gets hot the spring relaxes and the valve open.

In the EFE system, is essentially a traditional heat riser, but instead of having a thermostatic spring it's vacuum operated. So starting at the heat riser there is a piece of metal linkage to the vacuum diaphragm which is mounted around maybe a foot from the heat riser, it has a bracket to the passenger side head to keep it in place. This was done so the diaphragm wouldn't be damaged by the heat from the exhaust manifold. From there a vacuum line runs to the TVS in the manifold. The TVS is screwed into the manifold and reaches into coolant. There is a vacuum line from there that runs to manifold vacuum.

So obviously the traditional heat riser operates by exhaust manifold temperature, where as the EFE operates based on coolant temperature with is obviously a much better indicator of engine temp, than the exhaust is. My assumption is that this change was made so the intake would warm to a more predictable temperature even if the truck was being operated in extreme heat or extreme cold.

The EFE or traditional heat riser, will help the engine to run better during the warm up period, and the choke will come off faster. Without the EFE or traditional heat riser, the engine may tend to run a little rougher or hesitate during the warmup period. This can be compensated for by keeping the choke on longer and reducing the amount the choke pull off, pulls off, but obviously this will lead to a reduction in fuel economy, and increased amount of unburned fuel in the combustion chamber, and for longer periods, in there washing down the cylinder walls which may lead to increased engine wear, and last but not least an increase in emissions.

The thermac system is located on air snorkel for the air cleaner. There is a flapper valve inside the air snorkel which is vacuum operated. From the bottom of the air cleaner is a round tube that looks something like drier vent, but much smaller, which extends to a metal housing or surround on the passenger side intake. NO exhaust gas is brought into the engine from here, what is happening is hot air is being brought in from around the manifold. On the vacuum side, there is a vacuum line attached to the thermac which runs to a temperature sensor located on the air filter housing, generally speaking it is usually on the inside of the air filter, but not always. And another vacuum line off the temp sensor which leads to manifold vacuum.

Now here is where people go wrong with the thermac. They expect that when the engine warms up the valve will open. That's not how it works. The temp sensor is set to maintain an incoming air temperature of about 90F. So your thermac needs to have the plastic air tube from the snorkel to the cold air intake at the core support to work properly. If it's cold out the valve may only open part way even when the engine is fully warmed up. This was done to somewhat stabilize incoming air temps which again helps with fuel vaporization when it's cold out, but also helps to keep incoming air temps stable, which meant that the factory could set the carb and timing for a known incoming air temperature. Most modern cars have a intake air temperature sensor, so that the PCM can make real time adjustments for air temps, this, the thermac system is the next best thing.

But fear not, if you hammer down on the gas and the volume of air needed is to great for the thermac to handle, the valve will open up all the way.

with stabilized air intake temps of 90F from the thermac system, comes the ability to run just a little leaner, and have more stable smoother idle as well as an improvement in idle speed consistency in varying outside temps. The thermac will also help the engine during the warm up period by delivering the warmest air it can to the carburetor.

So why not remove the EFE or the traditional heat riser. Because the engine will run better during the warm up period, and the choke will come off faster. If you remove the EFE or traditional heat riser, the engine may tend to run a little rougher or hesitate during the warmup period. This can be compensated for by keeping the choke on longer and reducing the amount the choke pull off, pulls off, but obviously this will lead to a reduction in fuel economy, and increased amount of unburned fuel in the combustion chamber, and for longer periods, in there washing down the cylinder walls which may lead to increased engine wear, and last but not least an increase in emissions.


So why not remove the thermac system (often done with the installation of an aftermarket air filter housing or the tube from the exhaust manifold is damaged and not repaired / replaced). Because with stabilized air intake temps of 90F comes the ability to run just a little leaner, and have more stable smoother idle as well as an improvement in idle speed consistency in varying outside temps. The thermac will also help the engine during the warm up period by delivering the warmest air it can to the carburetor

I'm sure someone will come along and

You've put up a lot to digest, and what I've read so far is truthful and honest. Bravo.

Up front and right here - I am not in a bashing mood nor do I want to create (much) debate, as I find your article pretty damned good - so far.

I DO wonder, however, if you factored-in the advent of the electric choke --- and I apologize for having not read your copious post yet ... not totally anyway.

The old choke stove device did indeed, depend on the runner under the carburetor base to be heated by exhaust flowing underneath the casting - and that took a while to register on the bi-metalic element.
---> At this point, I'd like to give credit to the engineers to KNOW of- and plan for- this delay happening and to mitigate it by allowing the spring to be more rapidly affected by whatever heat was coming st it, perhaps faster. IDK for sure. It is an interesting thought though.

But by changing over to an electrically heated element --- alà: a toaster --- the time to heat soak the element could be better controlled - although this was speculative if the driver had the ignition ON and the engine not running as that would still heat the choke element --- making it more-or-less useless as designed.

(And) again, I plead that I have not read more than a small portion of your interesting post --- but the "older style" heat riser --- the one with the coil outside the riser itself and mounted on the passenger's side of the lower header, was exposed to airflow under the hood - especially so as the vehicle got to roadspeed - and that might've afforded a certain amount of cooling and a re-loading of the coil to the bypass-mode, forcing more exhaust flow through the crossover again.

I know the Chrysler 383/LA series truck engines had a lot of cracks and casting failures under the carburetors from the spring re-tightening and forcing a lot of the exhaust through the heads and generally destroying the intake gaskets or burning away the manifold runner.
> FWIW - I don't remember if Mopar had a default weight on the heat riser to make it open if the spring failed or not; I know GM employed such a safety device however: I digress.
> The Mopar bi-metallic spring would be in the cooler ram-airstream and it would retighten, forcing the exhaust gasses to flow heavily through the intake manifold runner. Chrysler never seemed to figure this out and stayed with this bad design for a number of years.
> Mopar, did, later on, in a renewed design of the venerable 383/LA engines, use under-manifold heated coolant flow instead of exhaust gasses to preheat the intake septum and floor.
>>> Note To Self ---> Never mix LA heads with the non-LA versions! B/T - Dood It! Bad results!
> EGR was not, at that time, applicable to the GVWR of most 383/LA engine applications on the over 5001 lb GVWR-rated vehicles.
> On those under 5001 lbs GVWR vehicles WITH EGR - it was tapped into what I considered a "dead-end" exhaust passageway, cast under the intake well where the exhaust flow was not controlled except by the size of the two ports that were neither regulated nor controlled in any way; they just "flowed" pretty much all the TCP-ON time, not affecting the Exhaust Gasses flow by/from intake manifold vacuum to any measurable extent.
> This "dead-end" exhaust channel was only opened to flow when the EGR was actually in operation, and as such, did not significantly add to the running manifold temperature as an effective EFE device.
> Exhaust gas flow was, however, ultimately controlled by a TCP/VDV circuit which would have precluded it's effectiveness as an EFE device anyway.

It's been fun reminiscing with you concerning these bygone days ... and YEAH --- I destroyed a 383 Dodge truck once by mixing a non-LA head and an LA head on the same engine. They were otherwise, indistinguishable to me and even though it was the machine shop that mixed the two, it ultimately was my fault for not looking hard at them.

I promise to go back later on and fully read your post.


/

 

[59840Surfer]

OK --- I have a little time to slay, so I have something to say about the TAC system (aka: Thermac).

I worked. Very well.

On some of the GM big block motorhomes and local-delivery type trucks, they used the TAC and a TVA (ne:VIDS) ignition vacuum advance/retard as the ONLY emission controls.

The exhaust was quite clean with just that minimalist system -- no A.I.R, no EGR, no '***' (I forget the alphabet on the system that allowed vacuum advance only in High Gear and coolant above 160°F --- help if you know).

 

[SirRobyn0]

You've put up a lot to digest, and what I've read so far is truthful and honest. Bravo.

Up front and right here - I am not in a bashing mood nor do I want to create (much) debate, as I find your article pretty damned good - so far.

I DO wonder, however, if you factored-in the advent of the electric choke --- and I apologize for having not read your copious post yet ... not totally anyway.

The old choke stove device did indeed, depend on the runner under the carburetor base to be heated by exhaust flowing underneath the casting - and that took a while to register on the bi-metalic element.
---> At this point, I'd like to give credit to the engineers to KNOW of- and plan for- this delay happening and to mitigate it by allowing the spring to be more rapidly affected by whatever heat was coming st it, perhaps faster. IDK for sure. It is an interesting thought though.

But by changing over to an electrically heated element --- alà: a toaster --- the time to heat soak the element could be better controlled - although this was speculative if the driver had the ignition ON and the engine not running as that would still heat the choke element --- making it more-or-less useless as designed.

(And) again, I plead that I have not read more than a small portion of your interesting post --- but the "older style" heat riser --- the one with the coil outside the riser itself and mounted on the passenger's side of the lower header, was exposed to airflow under the hood - especially so as the vehicle got to roadspeed - and that might've afforded a certain amount of cooling and a re-loading of the coil to the bypass-mode, forcing more exhaust flow through the crossover again.

I know the Chrysler 383/LA series truck engines had a lot of cracks and casting failures under the carburetors from the spring re-tightening and forcing a lot of the exhaust through the heads and generally destroying the intake gaskets or burning away the manifold runner.
> FWIW - I don't remember if Mopar had a default weight on the heat riser to make it open if the spring failed or not; I know GM employed such a safety device however: I digress.
> The Mopar bi-metallic spring would be in the cooler ram-airstream and it would retighten, forcing the exhaust gasses to flow heavily through the intake manifold runner. Chrysler never seemed to figure this out and stayed with this bad design for a number of years.
> Mopar, did, later on, in a renewed design of the venerable 383/LA engines, use under-manifold heated coolant flow instead of exhaust gasses to preheat the intake septum and floor.
>>> Note To Self ---> Never mix LA heads with the non-LA versions! B/T - Dood It! Bad results!
> EGR was not, at that time, applicable to the GVWR of most 383/LA engine applications on the over 5001 lb GVWR-rated vehicles.
> On those under 5001 lbs GVWR vehicles WITH EGR - it was tapped into what I considered a "dead-end" exhaust passageway, cast under the intake well where the exhaust flow was not controlled except by the size of the two ports that were neither regulated nor controlled in any way; they just "flowed" pretty much all the TCP-ON time, not affecting the Exhaust Gasses flow by/from intake manifold vacuum to any measurable extent.
> This "dead-end" exhaust channel was only opened to flow when the EGR was actually in operation, and as such, did not significantly add to the running manifold temperature as an effective EFE device.
> Exhaust gas flow was, however, ultimately controlled by a TCP/VDV circuit which would have precluded it's effectiveness as an EFE device anyway.

It's been fun reminiscing with you concerning these bygone days ... and YEAH --- I destroyed a 383 Dodge truck once by mixing a non-LA head and an LA head on the same engine. They were otherwise, indistinguishable to me and even though it was the machine shop that mixed the two, it ultimately was my fault for not looking hard at them.

I promise to go back later on and fully read your post.


/

So I think the only thing I have to say about the choke thing, is that there is a crossover and heat riser / EFE on trucks even with an electric choke. The biggest difference is just the electric control on the choke.

My article is really specific to the square body, but certainly the principals apply to many different makes and models.

 

[SirRobyn0]

Yeah, everybody does the two chrome pie plates thing with air cleaners and then wonders why the thing runs like garbage in the winter. That's a southern California thing, people. Most of the country should be running a thermac.

BTW, they do the same thing on planes, where it's called carb heat. It prevents carb icing, which is bad.

I put the thermac back on my '78. I got it from a junkyard and cleaned it up with evaporust and Rustoleum gloss black. I had to fab up an intake from the headers, using a stainless-steel downturned exhaust tip and some flexible metal hose. Then I knocked out the opening in the radiator bulkhead for cold-air induction, and used the stock cold-air induction tube. I put a chrome dome on the thermac air cleaner.

Oh, baby. It took maybe three minutes when starting in the cold (below zero F) for the intake air to heat up, headers getting hot quick. The idle stabilized, and all my warm-weather torque and horsepower was right there. Used an AFR meter to nail down that mixture, and ran lean and mean year-round.

And it looked good.

You can see the header connection in the first photo. I had a machine shop run a 1/2" end mill about a half-inch deep, three cuts, across the back side of that stainless-steel downturned exhaust tip and attached it to the header pipe with screw clamps.

The cold-air induction hose was an aftermarket copy of the original part, so it was new. The second pic shows my make-do dryer vent tube till the correct one came in. I also used a new Temperature vacuum valve in the thermac rather than the old one.

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Nice work, and it payed off for you with a better running vehicle. If I had to make the choice I would choose a working thermac over a the heat riser, for basically the reasons you state. The thermac can be a VERY good thing in freezing whether whether the engine is warmed up or not.

 

[59840Surfer]

Nice work, and it payed off for you with a better running vehicle. If I had to make the choice I would choose a working thermac over a the heat riser, for basically the reasons you state. The thermac can be a VERY good thing in freezing whether whether the engine is warmed up or not.

I won't say it's right, the way I am running my K5 --- I removed the Thermac for stupid reasons --- but I have an Edelbrock Q-Jet and an electric choke at least and an abiding belief that a good choke is worth moving to Tecate, Mexico from Montana, Anywhere.

As an aside --- I still have an A.I.R Injection system running. <ahem> That is all.

 

[Geoduck]

I agree with you SirRobyn.
From what I've read a stock EFE setup is effective and causes no performance issues if in proper working condition.
I'm missing the cold air snorkel other than that my truck is bone stock and seems to work.
I will say this. All the vacuum lines steel tubing and other clutter drives me nuts. Eventually I'll replace or rebuild the worn out 305 without all the stock smog gear.

 

[SirRobyn0]

I agree with you SirRobyn.
From what I've read a stock EFE setup is effective and causes no performance issues if in proper working condition.
I'm missing the cold air snorkel other than that my truck is bone stock and seems to work.
I will say this. All the vacuum lines steel tubing and other clutter drives me nuts. Eventually I'll replace or rebuild the worn out 305 without all the stock smog gear.

That's my plan with the 305, when it's done I'll likely install a stock or near stock 350, but I'll still run the EFE and the thermac.