The Stovebolt Site Forums The Shop Area Engine Shop GMC exhaust manifold flapper

This military exhaust manifold does not have a thermal spring operated flapper.

The flapper is set in the position as shown in the photo … or maybe it is designed to be in this position and manually adjusted after the manifold warms up. ?? The flapper and shaft is not rusted in place it does move but not in the full range.

My question: is this flapper design manually opened as the manifold is heated? I am familiar with the spring design but I am unfamiliar with this flap design.

That definitely looks like a "set it and forget it" flapper. That's based on the setscrew and lock nut in the first pic. It looks like the divots are there for positioning the flapper for various conditions. The military probably wasn't too concerned about fuel economy.

Found a military site but it asks for a log in but may have your answer there

Thanks, men. I am going to set it on the old woodstove and see what happens. Set it and forget it makes sense. Why track down missing flapper springs while under fire.

I've never seen a flapper that didn't have a spring. (Cadillac, Olds, Chevy).

Some are operated thermostatically like the Chevy while others are opened by the exhaust stream and closed by the spring.

I think you're probably missing parts. It probably had a weight on it that held it shut - maybe with the help of a spring - and it opens under exhaust stream pressure.

Upon closer inspection there are three presets or half drilled holes in the manifold side: closed, half open/closed (depending on your view of the world) and full open flapper.

Applied heat to the set screw, turned it out, and replaced the rusted in nut.

Not sure I see the performance benefits to this set and forget flapper other than one less part in the field. Otto, the flapper locks in once the set screw is in place. I dont believe this military design used a spring.

This is another great mystery that will haunt me until I exhaust my last breath.

My 302, ( which most are military versions) is exactly the same. I don’t see the benefit of restricting the exhaust in the full closed (#3) position as pictured above. My unit is also in the “closed” position. In testing my motor for idle/fuel mixture, it is running very rich. If that flapper is opened, I’m sure the motor will breathe better and lean out for the better?
Why have better breathing exhaust if it is restricted at the collector?
I think heat will be the only way to remove the nut and back out the set screw to reposition the flapper position.

Yep Pd, red hot and it comes out like butter. Everything is a compromise with those limiting settings. I will probably shelve this manifold and use the traditional spring flapper on the 270.

If you change your setting - report back if you do a performance test.

From all of the pics, it looks like it functions exactly like what is on my AD 216s and 235s, except that instead relying on a bimetallic spring to automatically change the flapper from the "heat the intake" to "divert heat away from the intake" it is adjusted manually.
There is no restriction of the exhaust going on at all. All the flapper does in my AD trucks and in the OP's truck is direct hot air to the manifold, or direct hot air away from the manifold according to what the engine requires to operate smoothly.
When the engine is not up to operating temperature, the flapper needs to direct hot air toward the intake to heat up the intake runner so that raw fuel does not fall out of suspension from the air/fuel mixture.
When the engine approaches full operating temperature, the flapper needs to direct hot air away from the intake manifold to prevent fuel/air mixture from overheating causing poor performance.
What is the middle position for? That is for operating in extreme cold conditions.
I am pretty sure that in January in northern Minnesota an AD engine with a bimetallic spring, the spring will hold the flapper in the middle, half open position.
The concept is very similar to choosing between having a manual choke or an automatic choke.

Thanks for the explanation and insight. Makes sense. Working under the hood with a wrench and screwdriver to adjust the flapper away from the intake would be a challenge on a hot exhaust manifold.

But yes, if the original military 302 was used in a very cold climate that would make sense.

After giving this some more thought - i came to the conclusion that in my next reincarnation- i would definitely not want to come back as an exhaust flapper. It is not something that I would aspire to be.

Thanks, Carl.

This is in the full open position and the closed position. Toward the block is closed and toward the fender is fully open . I'm almost sure this will be fine for sunny Kalifornia

Do you need to loosen that lock nut and set screw to move the lever, or is that lock nut designed so that you can adjust how far the end of that screw protrudes so that it will click into one of the 3 holes by moving the lever?
If the latter is true, I would recommend that you turn the lever to at least the middle position on a warm day, and at the "open" position on a cold day until the intake warms up a bit, then move the lever to to the "closed". As far as how long should you wait to turn the lever to the fully closed position, pay attention to how the truck responds to a solid step on the gas under load. Turn it to the closed position as soon as the cold engine bog goes away.

Moving the locknut out also moves the set screw out from the drilled "dimple" on the manifold. Between the two of us that has the setting preconfigured to "closed" (far right setting), Should we keep it there?

Other than mileage, does opening the flapper to the open position in warm climate driving alter the drivability or torque differences between the three settings ? Thanks

The engine will run best in temperatures above freezing once it reaches operating temperature with the flapper closed.
Before the engine reaches operating temperature in any climate (unless you live where the ambient air temperature is 180 degrees) it will run best with the flapper closed.
In frigid sub-zero climate, the engine will likely run best in the middle position.

It would be handy if you could change the setting on your flapper without any tools when you start the engine first thing in the day.
Then you could easily set the flapper to the open position which diverts heat to the intake, thus preventing raw, unburned fuel from leaking past your rings, scrubbing the oil from the cylinders, causing premature wear, and then contaminating the crankcase oil causing premature wear to the bearings.
Once the engine reached operating temperature, you could just move the lever to closed and roll on.

Short answer to your question is leave it in the closed position except for cold starts.

I find it puzzling that they would make that setup to require tools to change the position of the flapper.
What is the lever made of? If it was spring steel, you could back off the set screw to where the spring steel lever would allow the the lever to move by hand, but still fall into the detents for the 3 positions and stay put.