Can I use 46mm PMO manifolds with 40mm webers??

[matty74]

Can I use 46mm PMO manifolds with 40mm webers??

If so what impact on performance?

[Steve@Rennsport]

As long as the bottom matches your ports, yes,...

[SCrescue]

Hi Matt,
I've been thinking about experimenting with this on my freshly rebuilt 3.0 motor. I have a new set of 46mm PMO manifolds (46mm in, 41mm out) that I received in trade for some other parts, and my motor has '78/'79 heads with large intake ports (39mm), so it will be a 6mm fall off going from the carbs into the manifold and a minimal 2mm step going from the manifolds into the heads. I'm sure both conditions would show a significant disruptiion to flow in a flow bench test, but I suspect in actual driving conditions the performance loss will be negligible in use on a street motor (as I plan to use it).

I know this is not an optimum setup from a performance perspective, but I expect that the carbs can be tuned to run well regardless. I've considered trading for the correct manifolds, or selling the ones I have and buying 40mm manifolds, but I figured why not try these and see how well they work first. I've also considered that I may want to switch to 46mm carbs in the future, so that's another incentive to hold onto the mainfolds.

Mike...

[matty74]

I do have 78 SC heads which are 39mm I think?

I thought the PMO 46mm manifolds were 43mm at the head

[SCrescue]

Matt,
The set I have are marked 46/41 (hand written with a marker on the upper gasket) as received. I did not buy these directly from PMO, but I would guess that you can buy specific manifold configurations directly from them. Not sure if there is a standard cinfiguration. The step I can see going into the head is very minimal, especially with the intake gasget in place, so I'm confident that any flow disruption will be minimal as well. I could probably eliminate it entirely by slightly increasing the chamfer at the head inlets, but I doubt it will be necessary for my application.

If I were looking to extract every possible bit of power and performance from my combination to support a racing goal, I would do things differently, but again, that's not my goal.

Mike...

[RWebb]

get rid of ANY steps for best performance

turbulence created is NOT a linear function of the step size!