Heat riser - necessary? 1994 TBI 350

[Southwest ORV]

Hello friends,
I have always wanted to know, is the heat riser really necessary on these TBI motors? For example, you see lots of folks flip the lids on their air cleaner housings, which would completely negate the heat riser. I even drove my '95 TBI 305 GMC for a year with the lid flipped and it ran fine year round, which tells me the heat riser is perhaps nice...it helps a little on cold starts...but that's it?
Do folks ever delete them just to clean up the engine bay?
I'm getting ready to do some tune-up and clean up on my 1994 Z71 and I'd love to know prior to getting my hands dirty.
Thanks.

 

[highwaystar]

I deleted it on my ‘94 tbi 5.7L, no issues. No more flapper in air cleaner snorkel.

 

[Southwest ORV]

I deleted it on my ‘94 tbi 5.7L, no issues. No more flapper in air cleaner snorkel.

Thanks! There's one helpful report from a fellow '94 owner.

 

[RichLo]

yea, just be sure to remove or modify the flapper so your pulling air from the colder fender instead of the engine bay

 

[Trlrider]

mine was deleted the day I purchased the truck and the flapper removed.

 

[Schurkey]

1. The heated air intake is NOT a "heat riser". TBI trucks don't use a heat riser valve. Other vehicles have a flapper valve on one bank's exhaust manifold, that forces exhaust gas to pass through the exhaust crossover passage in the intake manifold. The valve can be controlled by vacuum or by a bimetallic spring similar to a choke coil. This is also called an "EFE Valve", Early Fuel Evaporation.

2. There's probably no issue with defeating the heated-air intake valve in the air cleaner snorkel at least when the temperature is above ~50 degrees F. However, throttle-blade icing is a genuine concern when the temp drops below 45 degrees, on a high-humidity day. The most-common time for throttle icing would be about 37--42 degrees F., and raining.

Most severe throttle icing I ever encountered was going "uphill" on I-25 north of Las Vegas, New Mexico, March or April, driving a Honda CVCC Civic. At first, I couldn't hold speed in 5th gear at WFO throttle. Downshifted to 4th, then third...pretty soon I was WFO in first gear at about ten miles an hour. Engine stalled, I pulled over to see what was wrong. Popped the hood, saw nothing obvious. Got in, started the engine...ran great. Took me awhile to figure out that the ice melted from engine heat while I was stopped. All that time running WFO meant the engine had no vacuum to close the vacuum-operated heated-air flapper.

As the temperature drops below freezing, throttle icing is less of a problem because the air won't hold a lot of humidity. Less moisture in the air--less moisture to freeze on the throttle plates. But then you run into issues with poor fuel vaporization resulting in poor cylinder-to-cylinder fuel distribution and poor power/poor fuel economy. The intake on a TBI is heated with engine coolant and "some" exhaust gas movement--but heating the incoming air helps in genuinely cold weather.

On MY truck, I just remove the short flexible hose between the heat stove on the exhaust manifold and the air cleaner during the warmer months. Come fall, I reattach the hose. This doesn't block the warm underhood air from getting pulled into the air cleaner, but at least it's not pulling "HOT" air from the exhaust manifold. Simple and easy.

 

[Southwest ORV]

1. The heated air intake is NOT a "heat riser". TBI trucks don't use a heat riser valve. Other vehicles have a flapper valve on one bank's exhaust manifold, that forces exhaust gas to pass through the exhaust crossover passage in the intake manifold. The valve can be controlled by vacuum or by a bimetallic spring similar to a choke coil. This is also called an "EFE Valve", Early Fuel Evaporation.

2. There's probably no issue with defeating the heated-air intake valve in the air cleaner snorkel at least when the temperature is above ~50 degrees F. However, throttle-blade icing is a genuine concern when the temp drops below 45 degrees, on a high-humidity day. The most-common time for throttle icing would be about 37--42 degrees F., and raining.

Most severe throttle icing I ever encountered was going "uphill" on I-25 north of Las Vegas, New Mexico, March or April, driving a Honda CVCC Civic. At first, I couldn't hold speed in 5th gear at WFO throttle. Downshifted to 4th, then third...pretty soon I was WFO in first gear at about ten miles an hour. Engine stalled, I pulled over to see what was wrong. Popped the hood, saw nothing obvious. Got in, started the engine...ran great. Took me awhile to figure out that the ice melted from engine heat while I was stopped. All that time running WFO meant the engine had no vacuum to close the vacuum-operated heated-air flapper.

As the temperature drops below freezing, throttle icing is less of a problem because the air won't hold a lot of humidity. Less moisture in the air--less moisture to freeze on the throttle plates. But then you run into issues with poor fuel vaporization resulting in poor cylinder-to-cylinder fuel distribution and poor power/poor fuel economy. The intake on a TBI is heated with engine coolant and "some" exhaust gas movement--but heating the incoming air helps in genuinely cold weather.

On MY truck, I just remove the short flexible hose between the heat stove on the exhaust manifold and the air cleaner during the warmer months. Come fall, I reattach the hose. This doesn't block the warm underhood air from getting pulled into the air cleaner, but at least it's not pulling "HOT" air from the exhaust manifold. Simple and easy.

Awesome! Thank you so much for this reply. This is very helpful.

 

[LVJJJ]

Nope, its not. I got rid of the air intake box bolted to the inner fender, got rid of the 3" hose, got rid of the flapper totally, removed everything out of the air cleaner housing, installed a 4" aluminum duct from the fender to the air cleaner, straight flow no obstructions. Always starts good even in freezing weather. Also cut a similar hole in the air cleaner housing on the other side and ran a 4" hose from the driver's side fender to the air cleaner. Have two air intakes now. Used a hole saw to cut a 4" hole into inner fender to route the hose to.

Need cold air, lots of it to make power.

 

[Schurkey]

Nope, its not. I got rid of the air intake box bolted to the inner fender, got rid of the 3" hose, got rid of the flapper totally, removed everything out of the air cleaner housing, installed a 4" aluminum duct from the fender to the air cleaner, straight flow no obstructions. Always starts good even in freezing weather. Also cut a similar hole in the air cleaner housing on the other side and ran a 4" hose from the driver's side fender to the air cleaner. Have two air intakes now. Used a hole saw to cut a 4" hole into inner fender to route the hose to.

Need cold air, lots of it to make power.

Clearly, somebody didn't bother to connect a restriction indicator to the air cleaner.

Twin 4" hoses is crazy. Probably not harmful, but it's gonna look hoopty and won't be of much actual benefit.

 

[Queso Del Rio]

Yep, the hose is mostly wasted real estate in the engine bay. Cool air is always good, unless the temp. outside is close to freezing and the dewpoint is within 7 degrees of it, then you could get some icing. Believe it or not, flipping the lid on your filter housing does next to nothing. I've seen the 305 on the dyno. with all kinds of different bolt-on air and fuel configurations just to see what she would do in regards to hp and torque. The best that could be had (give or take a couple of ponies and pounds) was a 750 carb. and a stock air cleaner.