Tall Weber Intake Manifolds

[cegerer]

Has anyone replaced their standard manifolds with the tall 'racing' versions? I'd like to know if - making ONLY this change - there was a noticeable difference in low-end torque. Also, any drawbacks to the tall manifolds? -- Curt

[Evans, Marv]

BUMP!
I've got a set of tall intake manifolds I'll be using & am curious about any comments too.

[masraum]

I always thought longer runners were for more high end power?

[RLJ]

When the intake valve closes the condition in the port goes from vacume to pressure. The stack of air that was being sucked into the cylinder is now moving towards a plug, the closed valve, The longer the runner or manifold height the less pulsing at the carb. I think that is the same reason CIS won't work well with hot cams. The CIS air flapper does not like the pulsed air flow.

Randy Jones
1971 911

[Wayne 962]

The tall runners reduce fuel reversion. Here's an excerpt from my new engine rebuilding book:

Another point to mention is that high overlap camshafts have a tendency to cause what is commonly called reversion. On a high-overlap camshaft, the intake valve continues to remain open as the piston passes bottom-dead-center (BDC) and starts to close. At lower RPMs, this can cause the air mixture inside the piston to become slightly pushed out before the valve closes all the way. This mixture gets pushed back up into the injection system, and creates a reversion ‘cloud’ of air and fuel in the intake. At higher RPM, this effect disappears because the momentum of the air-fuel mixture entering into the cylinder is greater, and is not as easily overcome by the pushing back of the piston. A longer duration cam allows the engine to use the full effect of the intake mixture momentum at higher engine speeds, but with the cost of increased reversion at lower RPMs. Reversion causes low power and rough idling – all downsides to running an aggressive camshaft.

Some additional performance options for carburetors include tall, secondary venturis that funnel the air better when being drawn into the engine and gives more mid-range power. Also useful are the tall manifolds that raise the carburetors’ height above the heads. This additional height helps to reduce intake reversion that can often occur with aggressive camshafts with significant intake and exhaust valve overlap.

-Wayne

[Nader]

I've understood (from the Alfa crowd with Weber DCOE side-drafts) that shorter stacks improve high-rpm power, having to do with less work to move a shorter column of air.

[TimT]

I have tall manifolds and the 3/8" insulators on my car, the tall manifolds are supposed to help with top end power, and have no detrimental affect on the bottom end.

Im not able to give back to back comparisons, since I installed the manifolds during a complete engine overhaul, at which time I raised the c/r, installed new cams, twin plugged, etc.

[HawgRyder]

Think of pipe tuning (both intake and exhaust) like tuning a pipe organ.
The shorter the pipe (and smaller diameter) the higher the pitch.
The longer and larger the pipe...the lower the pitch.
If the intake and exhaust are tuned with slightly different power bands, you can spread the power curve.
This does not work with every engine...and the Pcar is a major problem because we don't have enough room to play with the header length and collector size.
I often wondered if anyone has ever run the exhaust forward to come out in front of the rear wheels?
This would be a straighter run of tubing...and easier to control the power band.
Bob

[cegerer]

Well, I think I'm going to install a set and see what happens. I'm running 'Solex' cams and they have a fairly high overlap. The power comes on strong at about 5000RPM. I'm hoping to improve the 4000-5000RPM band.

Are there any issues with carb linkage to deal with? -- Curt

[TimT]

Youll have to make some slight adjutsment to the linkage, its not a big deal though