I got a used exhaust manifold off ebay and (as described) the heat riser was frozen up, so I knocked the metal flapper thing off, left the frozen metal rod in and installed it. My question is, what exctaly is the purpose of the heat riser and do you think my slant would run better with a manifold that has one working properly? I've did a search on ebay and other parts places and there abundantly available.

yes it will run better. It helps when the engine is cold. You'll be glad you did. Mine started running much better when I unfroze the heat riser with Kroil penetrating oil.

MJ

The porpuse of the heat risor is to divert exhaust gas to the bottem of the intake manifold to heat it up, to aid in warming up the engine. I don't have a heat risor in my manifold (removed flap valve and rod, plugged holes with bolts), and don't have any driveability problems. However, I kept the heat stove around the exhaust manifold and the tube going to the air cleaner, to draw in warm air when the engine is cold. This topic is up to much debate, so buckle up.....(hi Dan).

Knocking-out the flap and installing the manifold without it is halfåssed. It will cost you gas mileage, performance and hot driveability, since you'll always have full exhaust heat blasting the intake manifold. The heat riser is there for a good reason, and it should be kept in good working order.

There's no debate on the topic itself. A heat riser does one undebatable thing, it works one undebatable way. If it's stuck "off", there'll undebatably be no manifold heat on a cold engine when it's desireable. If it's stuck "on" or the valve is removed, there'll undebatably be full-time manifold heat even on a hot engine when it's undesireable.

The only debate is whether to do it right or do it wrong -- and even that's not a debate, from where I sit. If a job's worth doing, it's worth doing right.

(Next time you might want to ask questions like this before you proceed. It's a lot easier to change course when you're asking "Here's what I'm about to do, is this OK?" than it is when you're asking "Here's what I've done, is this OK?".)

100% agreement from me on the attached statements.

My opinion diverges from Dan's when it comes to de-activation. You can easily get by with this in warmer climates, and some would argue you're better off. On a V8, the valve can be eliminated altogether (been told by a leading mopar guru that '74 Chrysler products had no valve from the factory - not sure if he was refering to all motors or just V8's). On the slant six, it works a little differently. When the valve is in the "warmed up" no heat position, it deflects hot exhaust from blasting the bottom of the intake manifold. Take out the flap, and the intake manifold will be exposed directly to hot exhaust gasses at all times, which is very undesireable. So if you decide to defeat, best bet is wire or otherwise affix the valve in the "warmed up" position.

D/W

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If it ain't broke, fix it!

Uh, wrong. If the valve is removed, the intake will become quite cool to the touch. The valve is designed to force the exhaust gas to the underside of the intake. When it is opened, the exhaust will freely flow out the manifold. At that point the only heat that is transfer from the exhaust manifold to the intake is by transfer thru the metal contact.

It's the base of the intake under the carb we're concerned with. I gaurantee you there are screaming hot, turbulent gasses heating the underside of the intake with the flap removed that would otherwise be deflected by the valve for the most part. Taking the flap out seems retarded to me.

D/W

_________________

If it ain't broke, fix it!

To quote a famous prognosticator: "Uh, wrong."

The intake never "becomes quite cool to the touch" when the engine is running -- it only ever gets warmer. It'll get warmer faster or slower, depending on ambient temperature and operational state of the heat riser, but it will never "become quite cool to the touch".
On a V8 engine, yes, the closed heat riser forces exhaust through the crossover passage in the heads and intake manifold. Not on a slant-6, though. The slant-6 heat riser never "closes". When it's in the heat-on position, it acts as a deflector to increase the flow of hot exhaust past the intake manifold hot spot. When it's in the heat-off position, it does fully open, and almost entirely blocks exhaust access to the intake manifold hot spot.

If the valve plate is missing, there is still plenty of hot exhaust flow past the intake manifold hot spot, since without the valve plate the exhaust manifold's central collector is completely open to the intake hot spot. The exhaust gas molecules aren't smart enough to think to itself "Hey, the engine's probably warm enough, so, y'know what, we'd better move downward into the exhaust pipe and completely stay away from that intake manifold hot spot up there".

That is why the heat riser valve is present, to herd those unthinking exhaust gas molecules this way or that way, depending on temperature.

Aw, no fair! You're only sayin' that 'cause it's true!

I'll let you guys hash this out. In the meantime, I'll enjoy my fine running slant with out the heat riser.

Thanks Dan for the good ass chewing and as always the veteran description and operation of the slant. In my defence, I had already knocked out the metal flapper before I became a member here. Now that I have a better understanding of the heat riser and its purpose, I'll search for a manifold with a working riser. I'll post any differences in performance with the riser verses without as soon as I make the change. Thanks for the responces, ole_blue

This sounds like an interesting experiment for a thermographer. Maybe I can arrange to take some time-lapse thermal images images of normal operation. I could wire mine open for another test, but I dont' have a manifold without the diverter.
BTW, I'm in the "Keep the flapper, and keep it free" camp until I have a good reason to change it. My slant manifolds have the mechanism intact.
I would like to compare flow with the diverter open, closed, sans spring, and absent. Anyone got a suitable flow bench?

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"When you find a big kettle of crazy, it's best not to stir it." - Pointy-haired Boss

1964 Valiant V200, 225/Pushbutton 904
BBD, CAI, HEI, LBP, AC, AM/FM/USB, EIEIO

...already seems like a no-brainer to me, but if you've got the time and the science, why not?

D/W

_________________

If it ain't broke, fix it!

Hey, any excuse to play with the big camera and the funny-looking pictures!

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"When you find a big kettle of crazy, it's best not to stir it." - Pointy-haired Boss

1964 Valiant V200, 225/Pushbutton 904
BBD, CAI, HEI, LBP, AC, AM/FM/USB, EIEIO

oooh technology gadgets==fun! hehehe. Get some more pics just for us! I've often wanted to use one of those cameras to see how inefficient the walls of our house are in the dead of winter =)

MJ