Heat riser flex duct tube w/ gutted heat riser and electric choke

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77Chief77
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Heat riser flex duct tube w/ gutted heat riser and electric choke

Post by 77Chief77 » Mon Sep 07, 2020 7:18 am

My exhaust manifold heat riser is gutted and I’m also running an edelbrock electric choke. Would I lose performance or fuel economy by removing that aluminum flex duct tube? My duct tube is pretty trashed and not even really seating correctly on both ends as it is. I’m just wondering if I need to even replace it at this point. Btw, I’m in a no smog state due to age of vehicle.
Last edited by 77Chief77 on Mon Sep 07, 2020 11:13 am, edited 1 time in total.
1977 Chief S 360 V8 4bbl Automatic quadra trac

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tgreese
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Re: Heat riser flex duct tube w/ gutted heat riser an electric choke

Post by tgreese » Mon Sep 07, 2020 7:31 am

Depends on where you are living. The hot air duct is about cold weather drivability, not performance or fuel economy.
Tim Reese
Maine beekeeper's truck: '77 J10 LWB, 258/T15/D20/3.54 bone stock, low options (delete radio), PS/PDB, hubcaps.
Browless and proud: '82 J20 360/T18/NP208/3.73, Destination A/Ts, 7600 GVWR
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77Chief77
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Re: Heat riser flex duct tube w/ gutted heat riser an electric choke

Post by 77Chief77 » Mon Sep 07, 2020 7:47 am

tgreese wrote:
Mon Sep 07, 2020 7:31 am
Depends on where you are living. The hot air duct is about cold weather drivability, not performance or fuel economy.
I forgot to add my location but I’m in central TX so on average it stays warm.

Ultimately I’d like to clean things up and run a simple edelbrock air cleaner. I’m just not sure what I’d need to do with the vacuum hose that connects to the thermostatic air cleaner. Do those edelbrock intake housings have a port for that hose?
1977 Chief S 360 V8 4bbl Automatic quadra trac


candymancan
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Re: Heat riser flex duct tube w/ gutted heat riser and electric choke

Post by candymancan » Tue Sep 08, 2020 4:44 am

I temoved all the emission stuff on my 90. The air pump. Blocked the broken egr not spending 100$ to fix that worthless thing. Removed the factory air hat and falling apart tubing. Got rid if like 200 yards of vacuum lines.

Jeep runs the same as it did with it all on from what i can tell. Gas mileage is 11mpg last i checked. So i say if you dont have emission testing remove it. Va doesnt do emissions on 25 years or older. State inspection requires the original emission junk to be on the engine. But most of the state inspectors ive been too were 5 years old when the Jeep was out and have no idea what it came with lol. All they look for is a cat.
1998 Jeep Grand Cherokee 5.9L Limited 197k
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tgreese
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Re: Heat riser flex duct tube w/ gutted heat riser and electric choke

Post by tgreese » Tue Sep 08, 2020 5:40 am

The air duct is only secondarily about emissions. When the temperatures are low, there are two problems in the carburetor. First, if the air is laden with moisture, the drop in pressure at the venturi causes a drop in temperature which can cause carburetor icing. Adding hot air from the exhaust manifold stove helps to prevent this. Additionally, gasoline does not stay atomized very well when the temperature is cold. The gasoline will condense into liquid on the inside of the cold intake manifold. Adding warm air at the air cleaner mitigates this. The exhaust manifold damper ("heat riser") also helps deal with this, by heating the intake manifold with exhaust gas so the atomized gasoline won't condense.

Mostly this is about being able to get in the Jeep and drive away under all conditions, without a lengthy warm-up period. Emissions are secondarily affected because you need less choke to keep the engine running. You also eliminate the several minutes of idling that's needed to warm up the engine to where it won't cough and stall when you try to drive. (Most people around here still idle their cars in their driveway to warm the interior and melt frost on the windows - I know I do when it's really cold. Some locales have laws against this.)
Tim Reese
Maine beekeeper's truck: '77 J10 LWB, 258/T15/D20/3.54 bone stock, low options (delete radio), PS/PDB, hubcaps.
Browless and proud: '82 J20 360/T18/NP208/3.73, Destination A/Ts, 7600 GVWR
Copper Polly: '75 CJ-6, 304/T15, PS, BFG KM2s, soft top
GTI without the badges: '95 VW Golf Sport 2000cc 2D
Dual Everything: '15 FCA Jeep Cherokee KL Trailhawk, ECO Green


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77Chief77
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Re: Heat riser flex duct tube w/ gutted heat riser and electric choke

Post by 77Chief77 » Tue Sep 08, 2020 7:54 am

I did some research and it's actually pretty cool how the factory air cleaner housing works. It basically lets cold air in when its hot and hot air in when it's cold.

I wonder if my edelbrock AVS2 carb cares about any of these factory temperature enhancements... I don't think edelbrock optimized their modern carburetors to work with all the factory stuff, maybe they did... :roll:
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carnuck
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Re: Heat riser flex duct tube w/ gutted heat riser and electric choke

Post by carnuck » Tue Sep 08, 2020 11:28 am

77Chief77 wrote:
Tue Sep 08, 2020 7:54 am
I did some research and it's actually pretty cool how the factory air cleaner housing works. It basically lets cold air in when its hot and hot air in when it's cold.

I wonder if my edelbrock AVS2 carb cares about any of these factory temperature enhancements... I don't think edelbrock optimized their modern carburetors to work with all the factory stuff, maybe they did... :roll:
If you’re in an area that bobbles near the freezing mark with heavy moisture in the air, like Seattle, carb icing is a real thing.
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candymancan
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Re: Heat riser flex duct tube w/ gutted heat riser and electric choke

Post by candymancan » Tue Sep 08, 2020 1:05 pm

Dont forget about the passenger side exhaust the Y pipe has a flapper thing inside the pipe that stays closed keep hot air backed up to warm the engine up quicker. The spring on mine is broken so i tied it so it stays open
1998 Jeep Grand Cherokee 5.9L Limited 197k
1998 Jeep Grand Cherokee 4.0 I6 laredo 299k
1990 Jeep Grand Wagoneer 138k


Topic author
77Chief77
Posts: 51
Joined: Fri Mar 20, 2020 4:18 pm

Re: Heat riser flex duct tube w/ gutted heat riser and electric choke

Post by 77Chief77 » Wed Sep 09, 2020 9:32 am

candymancan wrote:
Tue Sep 08, 2020 1:05 pm
Dont forget about the passenger side exhaust the Y pipe has a flapper thing inside the pipe that stays closed keep hot air backed up to warm the engine up quicker. The spring on mine is broken so i tied it so it stays open
Mines gutted and welded the hinge pin hole shut. I'm TX so I don't need all those heat assist components. I think I'll keep the stove pipe that gets residual heat from the exhaust manifold and stock air cleaner for the vacuumed Thermal Air Cleaner flapper. I don't believe you need that heat riser in the y pipe to make it all work but I'm sure it gets hotter with it.
1977 Chief S 360 V8 4bbl Automatic quadra trac

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