911R Tall Manifolds - implications

[gilbert911]

What would be the implications on/of ;
a) fitting
b) running with

The tall Weber induction manifolds as fitted to the 911r (pic attached).
I would like to mount them with my Weber 40's - although I'm sure the original fitment was with 46's
my engine is 2.4....>2.5L

Thanks in advance

[andyjboy]

Chris - I don't know that the tall factory manifolds were made for 40 IDA/IDS carbs or with smaller port sizes.

All of those that I have had (& I have a pair here now) have had a bore of 46mm at the top & 38mm at the bottom.

OliveR (SG Racing) in Belgium produces repros of the Factory tall manifolds in aluminium (rather than magnesium) so maybe able to offer some with a top ID of 40mm & a smaller (36mm ?) bottom ID.

The other option would be the 98mm PMO manifolds - you could paint them black to disguise their appearance...

[Paul Schooley]

I get what you're asking. My stock manifolds have a broken mounting ear and I have a tall set of PMO's to use. I'm also wondering if changing to the tall ones affects jetting or carb settings on my Webers as well. I think the broken mounting ear is the cause of my backfiring when I let off the gas pedal so I need to get around to using those tall ones myself.

[304065]

The last thing you want is an intake manifold with a bottom port diameter bigger than the port in the head, e.g. an original 911R tall manifold designed for a Weber 46 with a 38mm intake port sitting on top of a 911S head (for example) with a 36mm port. The narrower diameter of the head creates a lip all around which seriously inhibits flow and causes reversion back up the carb. The other way around, with a smaller manifold and a bigger hole in the head is what you want on the intake side. Not so for the exhaust, where a larger diameter primary tube (such as a 1-5/8" header, which 41.3mm) situated downstream of the exhaust port can prevent reversion of the exhaust pulses back into the port.

Also, air cleaner clearance is a problem, which wasn't an issue for the 911R which used watershields not to keep the rain out (if you've ever seen a set there's a slim chance of that happening) but to bounce the pressure wave back into the carb. Works great if you happen to own the car company and can tear the engine down for a rebuild after every race, not so good for those of us who tear it down every few SEASONS (or heaven forbid, venture onto public roads).

[BAPhotowerks]

I love that photo! Great info in here too...

[Paul Schooley]

I think one of the most important questions besides the height has gone unanswered here. Does changing to a taller manifold require rejetting of the carbs to match up?

[Steve@Rennsport]

I think one of the most important questions besides the height has gone unanswered here. Does changing to a taller manifold require rejetting of the carbs to match up?

Nossir. The tall manifolds were a measure to help contain intake reversion; a byproduct of using 906 cams.

[Paul Schooley]

Thanks Steve.