Which of the two has a smoother ( polished job ) on the inside of the M8 intake manifold? Trying to decide which one to get for my 19' FLHXS. Thanks.

 

I don't know about the SE but the S&S is not polished, I know because I have a new one.

 

My SE 64mm isn't either

 

Thanks fella's. Reckon don't make much difference then, Dennis Kirk has them on sale right now the S&S. Ok, just ordered it with free shipping.

 

Saw that sale too on DK so had to get one too.

 

If I can find the data I will post it. One independent test showed that the SE 55mm manifold outflows the S&S 55mm in unported form. The S&S manifold, once ported, was on par with the SE. In person, the SE manifold is of nicer quality with less casting flash.

 

Found it. Was posted by Alpha Performance Engines in 2021:

 

Technically, you don't want a polished intake port. The big issue is the bend where the manifold split to the port. A rough surface there helps to keep the flow laminar and stuck to the inside edge. If polished, the flow can break away from the inside turn, cause turbulence and actually kill flow. I've seen a few times when testing different throttle bodies and carb manifolds.

 

I'm not sure this is the correct reason for not polishing the intake. I don't think we have laminar flow, which is good. I haven't run the Reynolds number, but I'd guess we are for sure not laminar. The rough surface insures turbulent flow. Back in the day, the old hot rodders discovered this.

 

Feel free to run the numbers.

I not sure how else to explain it.. It likely depends on how rough the surface is. I do know the I can quiet turbulent flow on an inside corner by applying a rough surface say with a 80 grit flap wheel in aluminum.. Port quiets down and flow goes up.. I used to polish ports. Don't anymore.

The way I understand it is that the surface slows the flow around the bend and creates a boundary layer that keeps the flow from breaking away from the surface..

I'm not an expert by any means. I have spent a lot of time on a flow bench and discussed this stuff with a few of the pros.