What does a heat riser do?
Thread Starter
Cruising
Joined: Sep 2014
Posts: 11
Likes: 0
From: Warner NH
What does a heat riser do?
I am putting my 71 350 together. My son-in-law and I are trying to figure out what the heat riser below the exhaust manifold is for. I had one on my 65 Dodge 413 and all it did back then (that I could tell) was make funny noise. My son in law told me it robs horsepower and we should get rid of it. I told him I would check with the experts.
thank you.
Yzordderrex
thank you.
Yzordderrex
Melting Slicks
Joined: Nov 2002
Posts: 2,884
Likes: 260
From: Jamison Pa
St. Jude Donor '11-'25
Heat riser
I am putting my 71 350 together. My son-in-law and I are trying to figure out what the heat riser below the exhaust manifold is for. I had one on my 65 Dodge 413 and all it did back then (that I could tell) was make funny noise. My son in law told me it robs horsepower and we should get rid of it. I told him I would check with the experts.
thank you.
Yzordderrex
thank you.
Yzordderrex
The closed valve forces hot exhaust gases under the intake manifold to make cold weather drivability easier.
When the engine reaches operating tempature, there is not a need for the hot gases under the intake. The spring on the heat riser allows the flapper valve to open.
Bob K.
Team Owner
Joined: Jul 2006
Posts: 22,169
Likes: 4,172
From: Kannapolis NC
I disconnected the spring on mine so it stays open all the time and I see no difference in cold weather driveability. In fact, it has to get pretty warm to open fully anyway. If you start your car and let it idle with a heat riser, you'll find that almost all the exhaust comes out the driver's side and almost none out the passenger side. Try it, see how long it takes to open, it probably never will at idle. You can either disconnect the spring so it drops down, or remove the plate, or replace it with a spacer that is commonly sold by auto parts stores and all Corvette vendors.
Safety Car
Joined: Apr 2006
Posts: 4,057
Likes: 421
From: BC
The only problem with removing or disabling it is if you are still running a divorced style hot air choke. The hot air through the intake manifold from the exhaust is what warms the choke as the passage is right below it and allows the choke to release faster than with no exhaust heat. You might have to lengthen the choke rod a bit.
Last edited by CanadaGrant; Feb 7, 2016 at 03:40 PM.
Drifting
Joined: May 2012
Posts: 1,353
Likes: 44
From: Calgary Alberta
I like to keep my cars as close to factory look and function as I can, but when I replaced the exhaust on my 1970, I replaced the valve with a spacer (and put the valve in a box). It's not necessary on a vehicle that is only driven in warm weather. My stock divorced choke mechanism works normally.
Something like this is a good option (depending on the size of your downpipes):
https://willcoxcorvette.com/corvette...nch-w-hp-62-74
Something like this is a good option (depending on the size of your downpipes):
https://willcoxcorvette.com/corvette...nch-w-hp-62-74
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,117
From: Crossville TN
If you want it to look original, just cut the valve off the shaft inside the heat riser. It will look the same from the outside, but not freeze up and do damage to the engine (if you don't notice it is staying closed).
Drifting
Joined: May 2012
Posts: 1,353
Likes: 44
From: Calgary Alberta
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Brett Foote
Team Owner
Joined: Sep 2006
Posts: 31,237
Likes: 4,330
From: Westminster Maryland
Hi drs,
Here's the inside of the heat riser with the flapper in "cold start" position and in "normal operating temperature" position.
I'm not sure how the flapper "robs power" if the heat riser is operating properly.
Regards,
Alan
Cold Start
Operating Temperature (In this picture the flapper isn't open quite all the way because I really couldn't get the camera to see the flapper when it's fully open)
Here's the inside of the heat riser with the flapper in "cold start" position and in "normal operating temperature" position.
I'm not sure how the flapper "robs power" if the heat riser is operating properly.
Regards,
Alan
Cold Start
Operating Temperature (In this picture the flapper isn't open quite all the way because I really couldn't get the camera to see the flapper when it's fully open)
Last edited by Alan 71; Mar 3, 2016 at 08:12 AM.