I have recently discovered my heat riser is working poorly if at all. I am not sure of its purpose exactly but it is quite rusty. I lubed it and now I can push it open and closed but I am not sure it was doing much on its own. Will the engine even run if it is rusted closed? I am also supposed to have 2 1/2 inch exhaust system for the LT-1 but the riser is only about 2 inches in diameter. Is there supposed to be a larger diameter one for the bigger systems? Should I just push it open and leave it that way until I can find another one?

Thanks in advance
Neil

 

Get rid of it! Mine broke and destroyed #6 & #8 cylinders--burned them up.

 

Uh oh! Do you mean broke open or closed. I presume you mean closed.

Last week I pulled all spark plugs for general tune-up and found #6 and #8 black and oily, all others ok. Why does a broken rise effect those 2 in particular?

Thanks for the reply
Neil

 

The flapper broke off in mine and I didn't know it--it got stuck in the exhaust flange below--I started getting some knocking--no matter what I did it wouldn't go away--I ended up having to replace the motor because of it. #6 & #8 are the closest to it I guess.

 

Before switching to headers, I simply cut of the flapper porton of the valve-free fix.

 

Good thing I removed mine because I was driving with it closed for a few months

 

" I am not sure of its purpose exactly but it is quite rusty. "

They used to sell a special shaft lubricant formulated specifically for heat risers. Normal lubricant just burns off. ACDELCO 12302866 X88A

The purpose of the part is to force hot exhaust gas across the bottom of the carb via a passage in the intake when the engine is cold. This speeds up the warm-up process for a cold engine/carb. It also causes oil to carborize on the hot surface at the bottom of the intake. Makes a huge nasty mess in there, usually under the heat shield "pan" which is designed to keep the oil away from the hot passage. The chunks sporadically fall off into the lifter valley and end up flushing down to the oil pan where they plug up the oil pump intake screen. All in all a crappy idea. and... it causes the crossover passage to get so hot that no engine paint, no matter how resistant to heat, will stay on the manifold at that place. If your paint stays on, then odds are, the passage is clogged or your intake gasket has the passage closed off.

It's good to have in cold climates, but marginal in the temperate zone.

The spring changes length when heated and opens the "flapper" valve. When cold, it defaults to closed. Newer engines have a vacuum control which defaults to open in case of failure.

My Silverado has the passage and a vac unit. I put a small bearing in the hose to block it off, and use intake gaskets without a crossover hole. It only "looks" functional. Truck runs fine with it blocked, but I don't live in the snow belt either.

I believe you can buy a "heat riser eliminator". Or you can probably find some of that heat riser penetrant/lubricant and free it up... assuming the spring is also intact.

As posted.... you can also gut it and put it back on. Probably have to plug the shaft hole as well.

 

A muffler shop welded mine open for $5.00.

 

I used to wire them open with mechanics tie wire.... cheaper yet. But I think those were the Chrysler & Ford designs.

 

but I did it for free at home

 

The purpose of the valve was to allow the engine to warm up faster on cold days. You can either remove the flap, wire shut etc. or do as I replace it with a riser spacer. When I had a new exhaust put on my car, they removed the riser valve and replaced with the other unit; the same exact thing except without any provisions for the flapper in it.

 

The LT1 used the same valve as other motors. I actually measured mine and it was 1 7/8 ID. Get rid of that thing at once. They do not flow at all. I never really understood the 2 1/2 system when it starts out at 2 inch necks down through the heat riser then goes to 2 1/2 inch. I ran the stock system for 1 summer and it never wanted to rev much past 6000 rpm and it was low on power up there. I put on headers and noticed an immediate improvement. If you want the motor to breathe above 5000 rpm put on a set of headers and get a good 2 1/2 system.