Hi folks,

I bought a set of AFR heads a few months back, and was told the heads had heat risers. I don't know what the heck the heat riser looks like, as up until now all the 3rd gen engines I've had experience with have had pre-smog engines swapped in, so I didn't know. Well I gave them a good lookover tonight and it turns out they don't have the heat risers. Now in my county we don't have emissions testing, but they recent instate visual emissions checks. Meaning nothing actually has to work, it just has to look right. So my question is, do I have a chance of making it look right? I'm using a Millican converted LT1 intake (and I'm going to run distributor-less ignition so that I can mount the egr valve).

I've heard that the TPI setups on 'vettes don't use the heat riser but take the exhaust gasses from the headers? Is this the way the LT1s work, in which case I'll hook it up properly. However if not, can I get it to look like it would work. They're not going to yank my intake or anything, just give it a once over to make sure all the equipment is present and hooked up.

Thanks for any information!

Adam

 

The heat riser in the head is an exhaust port in the center of the head on the intake side. It isn't visible when the intake is on.

 

Excellent. FWIW, I did some searching and it looks like the LT1 feeds the EGR off of the header so I don't think I need the heat riser to have a working EGR (right?)

 

You shouldn't. The heat riser was used for three purposes that i'm aware of, to warm the intake faster, supply exhaust to the EGR valve, and to heat a hot air choke. None of these appear to be applicable to you.

 

Thanks for the help.:lala:

 

Thats all part of the EFE system (early fuel evaporation). The ports in the heads you reference are the exhaust crossover ports. The heat riser is more or less a valve, either vacuum controlled or solenoid/ECM controlled, which diverts the exhaust gas through that crossover during the warm up period after a cold start. This warms the intake and allows the engine to allow for better fuel evap (lean of mixture as possible, immediatly following the cold start). Since a good portion of HC emissions are created at the early startup due to the larger amount of fuel consumed, the EFE system was installed to help reduce the HC output as soon as possible to drop the emissions. That's the basics of it. If all you have is visual, then they MAY look for the EFE valve, hard to say as i don't know how thourough they inspect.

 

3liter914-6, What is the AFR part number on your heads? I just ordered a set of heads from AFR. I am also converting to a LT1 intake. I don't want an EGR port in the center of my cylinder heads(intake side).

To all other's out there, is it correct to say that the heat riser is in fact the center port on the intake side of cylinder heads???

 

"Exhaust crossover" as Stekman says is probably more correct.

 

A little history lesson (I'll stick to SBC's for the sake of brevity):

"Heat riser" was the name applied to the valve located on the passenger side exhaust manifold outlet that forced exhaust from that bank, through a passage in the center of the head intake mating surface, through a passage in the intake manifold under the plenum, and out the other bank through an identical passage in the head (hence the "crossover" part) while the engine was cold. This was done to improve driveability, and existed on the very first SBC in 1955. The valve was operated by a bi-metal thermostat spring built into the "heat riser" valve, which closed the valve when "cold" by moving the butterfly closed, and a weight would pull the valve open when the engine warmed up. In the 70's, the spring was replaced with a vacuum servo in most applications, operated by a thermovacuum switch in the intake manifold coolant passage. In addition, the choke was operated via warm air heated by a tube that passed through the passenger side exhaust manifold (open to the air below the manifold), through a tube that went up to the choke, pulled through the choke thermostat by a small vacuum "leak" passage built into the carb. This was later replaced by a tube built into the intake manifold crossover passage, with the air inlet going up to the carb airhorn so the incoming air was filtered.

Somewhere along the line the factory started referring to the "heat riser" or "crossover" system as "EFE", or "early fuel evaporation".

As emissions standards started coming into play, the EGR valve tapped off of this crossover passage to get the needed exhaust gases - handy, it was already there, so they used it. This continued through the carb years and into the EFI years.

Hotrodders would often block off this "crossover" passage and eliminate the "heat riser" valve, which had the dual benefits of allowing a cooler, denser intake charge, and smoother exhaust flow path. The factory knew this, and as EFI became more sophisticated, the need for EFE was reduced, and since performance increased without it, in some applications it was eliminated (such as L98 Vettes with aluminum heads and Vortec engines). This took away their EGR gas source, so they went to the exhaust manifold for an external path to supply that.

So, hopefully that helps explain where some of those terms came from, why the systems existed, and gives some clues what needs to be done when parts from different era systems are mixed.

Class dismissed.

 

Very good explination. It all makes sense now.