Turbo manifold design.

[Bernd Felsche]

Jörgen Karlsson tapped away at the keyboard with:

> The stock manifold has separate flanges, but since the manifold is pretty
> strong it does not flex anyway.

It should. That's how you cope with the different expansion.
The stiffer the manifold, the greater the stress on the manifold
attachment. The manifold should only be strong enough to contain the
exhaust gases. :-)

With an allow head and steel manifold, you automatically have to
cope with different rates of expansion. If each runner is
individually-flanged, then the stress is reduced because the runner
flexes individually. Only a small amount of stress results in each
flange because it's fixed in at least two places.

> My thinking was that if I used one large flange it would not get as hot as
> the manifold itself. Maybe it would be 250ºC, thats a big improvement over
> the 700+ºC temperature of the tubing.

More, smaller flanges is better because of the reason described
above. If you put all the runners onto the same flange, then they
work in parallel, trying to keep the flange from expanding under
the applied heat in addition to the flange stretching from the bolts
of the flange...

If you can't do individual flanges, then reduce the flange cross-section
between ports to increase flexibility. That will reduce the stress
on the bolts, as well as the head as the strain (stretch) is
accomodated in the more flexible portions of the flange.

Avoid sharp edges where you reduce the flange section and ensure that 
the gasket is adequately supported.

> Unfortunally the flange will only be in contact with the head at
> the ports, if the head was flat like that of a Honda VTEC engine
> the flange would have the same temperature as the head and all the
> stress would be on the tubes and welds of the manifold itself.

> I think that one problem are that when the manifold slides against
> the head the nut does not slide against the manifold, the stud
> gets bent back and forth everytime the temperature changes. But
> sometimes when the temperature gets real high the nut might slides
> and gets stuck in an other position, then when the manifold cools
> down the stud is breaks. I am pretty sure that this is what
> happens.

That's typical of an "over-constrained" system.

-- 
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