Things I Hate About 80's GM Tech: The Heat Riser

Owning an older car for more than a decade gives you a window into the minds of the people who designed and built the thing. Parts break, and when you fix them, you see the handy work that went into the original part. You see the purpose and execution of the design. Some of these things make you think “Oh, I see what Mr. Engineer did there! Clever girl.” Others make you think, “Meh, must of done it to save costs.” But every so often, you come across something that just makes you think: “WTF were they thinking!”

In my second post on this topic, I’d like to discuss a second mystery within the exhaust system of my ‘82 Trans Am: The Heat Riser

Fact Checking

Before I begin my rant, let’s go over some facts about carburetors:

1. Carburetors were originally purely mechanical instruments for injecting a fuel/air mixture into an internal combustion engine

2. GM did use a Quadrajet Carburetor (Designated E4M-E) which had a computer adjust the fuel settings while in closed-loop mode

3. Carburetors don’t like to function the same when they are cold versus when they are warm

4. A “Choke” can be used to adjust the settings of the butterfly valves to change to Air/Fuel Ratio when cold

5. On the E4M-E Q-Jet, an electronic choke was fitted

6. The faster the carburetor heats up, the more efficient the carburetor will function

7. GM used other methods, along with the choke, to warm the engine and carburetor

Opinion Checking

And to counteract these facts, let’s hear from some of my opinions:

1. Electronic Q-Jets are an excellent example of a poorly executed great idea

2. I have yet to get my E4M-E to run correctly

3. I haven’t learned enough about my E4M-E to make it run correctly

4. I’ll probably switch to TBI before I find enough time to learn enough about my E4M-E to make it run correctly

5. I firmly believe GM also switched to TBI before they learned enough about the E4M-E to make it run correctly

Method #1: The Stove Pipe

Before you all get confused, there is a difference between the “stove pipe” and a “heat riser”.

The stove pipe is a tube that connects a metal jacket/heat shield (circled in the picture above) to a valve in the factory air box (Not pictured in the image above). The purpose of the stove pipe is to funnel hot air into the air intake of the E4M-E. The valve would shut once the engine warmed up, and ambient temperature air would be funneled in.

So, essentially a “hot-air” intake until the carburetor warms up. That doesn’t sound too bad, right? So you’re low on power (by like 5hp) until the motor warms up. Well, you’ll be low on power anyway until the motor warms up, so who cares? The Stove Pipe doesn’t interact with the exhaust flow at all, it just takes the ambient air around the factor manifolds and funnels it into the carburetor. No worries, just warmth.

This is why the Stove Pipe is not the topic of today’s discussion. No, the topic of today’s discussion is far, faaarrrrrr worse than this. Something, only the genius engineer named Bob Boberson could have thought of. Probably with the help of Betsy. Power duo, them. Probably got promotions because of this.

Method #2: Heat Riser

Pictured is an emissions hose diagram for a 1986 carburetor 305. You see some normal things like a Charcoal Canister, EGR, Temp sensors, alphabet soup and the lot. Really normal emissions equipment for the mid 80’s. But in the bottom left of the diagram is something mysterious. Some off. Something……….. sinister.

A Heat Riser

This is what the diagram is referring to. Just like the valve above the stove pipe, the heat riser will close at cold temperatures and open at warm ones.

And Another one

Now, onto what I expect your train of thought is:

“Oh, I see! Closed when cold, open when warm. That’s pretty nifty, what’s so wrong with that? I don’t get what this Knight guy is on about. I mean, he already said the intake valve was fine, it makes the car run better when cold. This must do the same thing! This is a good thing! Bob Boberson and Betsy did a fine job! I mean they have an intake valve to warm the carb, so it makes sense they have a……….. Wait, Knight didn’t say where this Heat Riser valve goes………… In the diagram it seems pretty far away from everything……. and that second picture is all rusty, and the valve looks like it has some carbon deposits on it………. I wonder where it goes………… I mean, the carbon deposits means there was some exhaust flow………no………no no no……… they didn’t……. no, they couldn’t have…………..NO!……….They couldn’t have done that!!!!! ……… they wouldn’t!!!!!!”

They Would

Yes, they did. They put a closing valve IN the flow of the exhaust. On the passenger side of the engine, the heat riser valve connects the Y-pipe to the factory manifolds. Think about this for a second. They put a valve into the direct flow of the exhaust. Do they not care about backpressure?

Now, I’m sure you’re wondering as to why in the world GM decided there needed to be a valve in the exhaust; what purpose could it fulfill? The function of this valve was to keep the warm exhaust air in the manifolds for a longer period of time when the engine is cold. This would warm the manifolds faster, which would warm the engine faster, which would warm the carburetor faster. Now a huge problem with this is the exhaust itself builds carbon up on the valve, clogging it, and preventing it from closing or opening all the way. So not only would it not fulfill its purpose after a while, it would forever provide uneven back pressure as the Driver’s side did not have a heat riser! This valve was only located on the passenger’s side of the Q-Jet cars. The TBI and TPI equipped cars did not have a heat riser; they had a spacer in its place.

Another gem from the design books of 80’s GM that shows they only wanted to kill off the any performance hidden within these cars. I’m no expert at heat transfer, but I am seriously curious as to how efficient this method would be for warming the engine as to compared with having no valve.

Come on GM, you can do better

So as you can see, I’ve killed off the factory airbox as well as the stock manifolds. The stove pipe and heat riser have also died. I do have an issue of rough running when the motor is cold, but after 5 min of poor driving, everything evens out.

This means I’m being led to believe a VALVE in the exhaust supposedly warms the engine faster, but “faster” is less than 5 MINUTES! For an effect to occur in less than a 5 minute window, they designed as system which ruins performance for the other 99.9% of driving the car. Why would they ever think the ends would justify the means! Really GM?! Really?!?!?!? Maybe there is a greater difference in colder, sub freezing climates , but for the rest of the time, its pointless.

Luckily, it’s no big deal to take the part out, gut the valve, and put it back as a spacer solving the problem. Or, just replace the stock setup with shorties and a proper Y-pipe (not that stupid T-pipe I previously discussed). The surprising part was finding this thing in the first place. Imagine you decide to replace the factory manifold with headers, and you find a valve attached to the back of the manifolds. My only thought was WTF GM? Can’t see how this saves costs, and I definitely don’t think its clever.

A Peak Into The Future

And with that, I can calm from my rants on the exhaust system. I’ve got nothing else to complain about…. for now. What comes next though…… it’s a good one. From the exhaust, I will move onto……………

Let me know what you guys think! Is it interesting enough to want to know more, or are these posts just taking up digital space on the Carthrottle community. Is there anything I’m not detailing enough? Anything I’m detailing too much?? Any obvious jokes I could have made, but sadly missed?!?