-
IIRC polishing used to be recommended as part of engine tuning, but has been up for debate for some decades - at least since I was an undergraduate in the '70s.
The argument for polishing used to be couched in terms of pressure drop across the manifold, and a highly polished surface will theoretically give a higher flow rate.
The argument against a highly polished finish is that turbulent flow gives better mixing of fuel and air - a matt abraded surface finish should be sufficient to disturb laminar flow in most cases.
My suspicion is that with older manifolds, the early stages of a polishing process will remove gross casting core marks, which in turn allows a higher flow rate than the 'as cast' finish, but going on to a high polish will be time consuming and may even be counterproductive in terms of overall efficiency. More recent castings are likely to have a 'better' surface finish to begin with?
I'd take time to confirm that everything lines up correctly, there are no steps or edges introduced by gaskets, no obvious (internal) surface defects, and then put it all together.
You are reading a single comment by @MrE and its replies.
Click here to read the full conversation.
MrE
Thanks, but no need to now.
Just got a pair today! The clue was scrutinising obscure parts fiches for the polizia version, which turned out to have the same engine, but had added replacement versions of these parts for a much longer period. That gave me two other parts numbers to search for.
Say hello from a fellow Guzzi rider though.
You still have the old BMW?
Talking about inlet manifolds: Do I polish them inside before mounting? Or doesn't it matter?