68 GT-500: exhaust manifold heat riser question

[Radojko]

Hello all,

I have an original exhaust manifold heat riser/spacer for the passenger-side manifold. It has the thermostatic "butterfly" valve. Is this "correct" for a 68 GT-500? If so, how does it work, and what is it's interaction with the "stove choke" mechanism?

Many thanks,
Bob

[Coralsnake]

I dont think that is correct. No butterfly on the GT500

[Bob Gaines]

If you mean this part pictured in the assemblyline manual then it has a heat sensitive spring that opens the butterfly when it gets hot. The only relation to the choke furnace is that hot air causes the spring in the exhaust flapper to open just like hot air causes the spring in the automatic choke o the side of the carb to open the flap/choke plate.

[68blk500c]

Yes, uses butterfly/flapper type for 2" H-pipe.  Round counter weight installed toward the engine.  Factory were cast with C7Z marking.  Service could also have C6O, or blank.  390GT origin.

[Radojko]

Ok, I get it... sorta.

Can someone then enlighten me: what purpose does this flapper mechanism serve? If it's normally in the closed position (I.e., when the engine is cold), to me that would appear to block exhaust expulsion on one side of the engine. That can't be a good thing, can it??

Also, I conducted an experiment to see what happens when heat is applied to the coiled spring element. It CONTRACTED when heated, but the hook RECEDED AWAY from the mechanism that is supposed to force the flap open. So I'm confused... even though it sounds like I won't need it in my application. I'd still like to understand it, though. 😁

[68blk500c]

If working properly, it briefly restricts exhaust flow, but doesn't block it.  The purpose is for faster heating of the manifold where the carb choke tube is located so the carb choke reacts faster--in theory anyway. 

[KR500]

No it's function is not to open the choke faster. It's function is to heat the intake manifold. When the engine is cold  fuel is not atomized very well and the fuel sticks to the cold manifold. The exhaust heat riser valve backs the right side exhaust stream up the intake manifold and out the left side exhaust manifold. This heats the intake manifold up and atomizes the fuel much better providing better cold drivability and fewer emissions. As the exhaust heats the bi-metallic thermostatic spring on the valve it looses tension and opens up ( just like the choke stat cap ) as you don't want the intake to hot as it will cause detonation, overheating and vapor lock. A lot of vehicles of this era accomplish this by heating the intake air by using a heat shield on the exhaust manifold, and a tube to the snorkel with a thermostatic valve. The 68 GT500 has an open air cleaner so that type of system would not work on this engine.
Rodney

[Coralsnake]

Did 1968s use it? I dint r3call seeing one on original Shelbys before.

[KR500]

No I don't think 68 GT 500's used them. I believe 67 GT 500's did though.

[gt350hr]

  The GT500 ( PI engine) had a spacer without a butterfly similar to the one with the butterfly.  I have never seen "all '' of them though.
       Randy

[mr68shelby]

I am looking at the 68 Mustang Chassis Assembly Manual. If I am reading it correctly, the notes for the valve show Canadian Market only for 390-4V. For the 427-4V engine, it shows Canadian and Domestic Market. The 428 engine is not referred to in this diagram. Page 59 in the manual.

The 428-4V-G.T. is listed under 68 1/2 Mustang and it shows the spacer being used instead of the valve. Page 60.

Mike

[gt350hr]

  The Mustang Chassis manual would not show a "specialty engine" like the '68 GT500 engine.
    Randy

[Bob Gaines]

No I don't think 68 GT 500's used them. I believe 67 GT 500's did though.

FYI, yes 67 GT500 used them (heat riser spacer) but a different style with the mechanism on the opposite side compared to the design in the 68 chassis book.The 68 version is not interchangeable with the 67 version .

[shelbydoug]

Those are the things that are always rusted inoperable?

[Bob Gaines]

Those are the things that are always rusted inoperable?

Yes those are them . As a alternative you can take the flapper out but leave the outside mechanism intact so that they will never rust shut but still look the part of a assemblyline operational unit. If the look isn't a concern then gut it , strip it but don't forget to plug the holes remaining in the spacer after removing the mechanism with some high temp silicone to eliminate exhaust leaks.