The heat risers I have in my two L79s ('65 and '67) don't seem to ever open. I have replaced the one in the '67 with a new one, as the old one never opened no matter how hot the engine was, and it acts the same. The engine can be at normal operation temp of 180 degrees or so and when the exhaust is checked at idle, it is still hardly allowing any exhaust out the right side. This isn't how it is supposed to work, is it?
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Big Tanks In the High Mountains of New Mexico -
Re: Heat risers, how are they supposed to work?
Michael,
No, that's not how it's intended to function. My first question would be, is the new one installed correctly so that it can function as designed? Was there any binding in the butterfly prior to installation? Pictures below of a "cold" closed valve and a "warm" open valve.
Cold start...
Operating temperature (almost...not quite fully open)
Leif
'67 Coupe L79, M21, C60, N14, N40, J50, A31, U69, A01, QB1
Top Flight 2017 Lone Star Regional -
Re: Heat risers, how are they supposed to work?
Thanks Leif, the arm, and thus butterfly, moves freely, hot or cold, and it is installed correctly, at least according to the judges last year at the Regional. It just will not move on its own when the engine heats up, it stays closed. I suppose a new one can be defective, but having 2 that are new and not moving have me stumped. I should have mentioned that in the '65, it was restored recently and has a new one too, I replaced the one in the '67, they look the same and neither will open when hot unless I push the arm down with a screw driver. and of course it pops back closed when I take the screw driver away. Maybe the spring is defective if that is what is supposed to open it and keep it open.Big Tanks In the High Mountains of New MexicoComment
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Re: Heat risers, how are they supposed to work?
Mike
you're an engineer, so you can see how the thing is designed to work. When engine cold, the spring is tight and contracted, holding the weight up, and butterfly closed. When engine hot, spring expands and relaxes, allowing weight to fall, opening butterfly. Is it possible to "re-engineer" the spring to make it more relaxed when cold, thereby giving it better chance to allow weight to fall when hot? Or, buy only FI tankers, and don't mess with heat risers.Comment
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Re: Heat risers, how are they supposed to work?
Yes, the fuelies and the 427 (which has one that works) are easier to deal with. I guess I'll just have to order 2 more new ones for the L79s and hope they have springs that work.Big Tanks In the High Mountains of New MexicoComment
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Re: Heat risers, how are they supposed to work?
Most of us in the south just install the FI spacer and call it a day. Don't need the heat riser most of the time........especially around here (SE Louisiana).
Larry
PS: Or for judging, simply remove the butterfly flapper and leave the rest intact.
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Re: Heat risers, how are they supposed to work?
Yes, I am thinking of using the old trick we did as teens, just wire the flapper arm open, and then remove the wire for judging (non-PV of course), probably easier, cheaper, and simpler than trying to tempt fate by buying another new one that doesn't work.Big Tanks In the High Mountains of New MexicoComment
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Re: Heat risers, how are they supposed to work?
Michael,
I wouldn't be so fast thinking the heat riser valve spring is to tight. My 67 300hp engine is the same way, the pressure against the butterfly is much more when the engine is up further in the rpm range. The heat in the plenum is your friend but with ethonal fuel things are different, when the weather is hot just wire it open.Comment
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Re: Heat risers, how are they supposed to work?
The last "NOS" or NORS one I installed I had to "unwind" the spring either half a round or one full round.
It never opened even at full op temp and on shut down you could hear the fuel boiling in the bowls.
No problems since unwinding - it is closed on start up and the counter weight has begun to move down after 100-120 temp on the gauge.James A Groome
1971 LT1 11130 - https://photos.app.goo.gl/zSoFz24JMPXw5Ffi9 - the black LT1
1971 LT1 21783 - 3 STAR Preservation.- https://photos.app.goo.gl/wMRDJgmyDyAwc9Nh8 - Brandshatch Green LT1
My first gen Camaro research http://www.camaros.org/forum/index.p...owposts;u=4337
Posts on Yenko boards... https://www.yenko.net/forum/search.php?searchid=826453Comment
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Re: Heat risers, how are they supposed to work?
Notice that the shaft is offset, so if exhaust velocity is high enough the valve will open. I've observed that at idle even with a fully warmed up engine they can be closed, but a good blip of the throttle will open them up.
Given the above I agree that the best thing to do is wire the damned thing open except for judging .
I might also question whether currently available heat riser valve assemblies are really make to OE spec.
DukeComment
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Re: Heat risers, how are they supposed to work?
Duke,
As Joe Lucia pointed out, the latest GM offerings, prior to their discontinuation, did not have shaft bushings. The NOS example I recently bought and quickly disposed of was a poor quality part. Clearly not the same quality as the earlier production line parts.
GaryComment
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Re: Heat risers, how are they supposed to work?
Yes, mine do open when you accelerate, but at idle, with the A/C on, they are closed and this causes excess heating at low speeds or sitting at stop lights that is not a good thing. I'm wiring them open, done! Thanks all.Big Tanks In the High Mountains of New MexicoComment
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Re: Heat risers, how are they supposed to work?
Hi Michael
This is a repo that was still in the bag. It was for 57-74, by the label.
Before and after putting it in the oven. You have to get it hot to open.
JoeComment
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