Cross the God of Speed at your own peril

[PeterBurgess]

Time and again folk polish the inlet manifolds and, even worse, mess about with the main jet bridge in an SU. The polished inlet manifold hinders fuel/air homogeneity and the main jet receives minimal pressure drop to lift fuel. Very hard to fuel!
The MG Midget today had an HIF44 single carb and Titan inlet manifold. It popped and banged and was not responsive to fuel or timing, it should have been somewhere near on the BDL needle, it was too weak even though the needle had been modded to increase fuel!
On inspection the carb had been 'flowed' and each side of the bridge had been metal fillered. Greta for increase in air flow but kills the signal the jet relies on!
I attach a pic of the bridge in the carb.

[PeterBurgess]

Part 2
I attach a pic of the carb with the filler chiselled out.

[PeterBurgess]

Part 3
Inlet manifold before

[PeterBurgess]

And after

[PeterBurgess]

The engine ran crisper and sweeter straight away and the misfires went. We ended up with a BGB needle as a std BDL was a little weak especially for cruising.

Here is a graph of wheel power before and after, a massive difference!
Mess with the God of Speed and take the consequences

[Jhonno]

Nice improvements there! Must drive like a new car..

[feef]

I've often wondered about over-polishing areas that end up creating laminar flow where there was previously turbulent, as my assumption is that the turbulence helps mixing?

[PeterBurgess]

I understand flow in our engines is fully developed or turbulent already and gas speeds are too high for laminar. The effect of rough manifold walls ( inlet port finish seems to make little difference) is to encourage a roughness to air flow in the boundary layer region twixt no flow on the manifold walls and flow in the manifold, this helps keep the fuel in suspension, especuially around turns.
I like to be able to feel the roughness of the port by putting my thumbnail onto the wall and dragging my thumb back out of the port or manifold, if I cannot feel the ridges I feel it is too smooth.

Johnno....owner grinning like a Cheshire cat when he road tested it

[GreenV8S]

PeterBurgess said:

this helps keep the fuel in suspension, especuially around turns

As well as helping scour fuel off the walls, I suspect it would also encourage the boundary layer to stay attached where there is an adverse pressure gradient.

[Jhonno]

PeterBurgess said:

I understand flow in our engines is fully developed or turbulent already and gas speeds are too high for laminar. The effect of rough manifold walls ( inlet port finish seems to make little difference) is to encourage a roughness to air flow in the boundary layer region twixt no flow on the manifold walls and flow in the manifold, this helps keep the fuel in suspension, especuially around turns.
I like to be able to feel the roughness of the port by putting my thumbnail onto the wall and dragging my thumb back out of the port or manifold, if I cannot feel the ridges I feel it is too smooth.

Johnno....owner grinning like a Cheshire cat when he road tested it

I can imagine he was..

Funny how sound theory doesn't always work as you imagine sometimes!