Just some general "carb porn"

Polished surfaces actually slow down the flow of air/fuel flow.
 
Polished surfaces actually slow down the flow of air/fuel flow.
Well, not really. Surface roughness plays a minor part in fluid flow resistance, so generally, smoother is better. (With the exception of golf balls, but that is another story)
However, I have read that polished surfaces can cause fuel droplets to collect, instead of staying dispersed in the intake air and being available for combustion. So many tuners recommend a satin, or even glass bead blasted intake ports for that reason.
But in this case, there will be no fuel droplets in the air, and the main point of the exercise is not surface finish, but rather the carb bell shape and dimensions.
 
Yes really, be it fuel droplets or just straight air flow, a polished surface will hinder the flow. As you have already pointed out, a satin finish, like that in glass bead finishes, improves flow, be it air or air fuel mixture. The shape of that bell mouth is also not optimal, the outside edge of the bell mouth is square, or sharp, rounding off that edge will improve the air flow.
 
Yes, see below overview of various velocity stack shapes and their influence on flow. But it is still possible that the customer that ordered the carb had an air box that complemented the carb bell. In any case, I am pretty confident it will flow more air than a stock, unmodified VM28.
 

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Yes, see below overview of various velocity stack shapes and their influence on flow. But it is still possible that the customer that ordered the carb had an air box that complemented the carb bell. In any case, I am pretty confident it will flow more air than a stock, unmodified VM28.
chart saved thnx!
 
Good on you! But it is still possible that the customer had an airbox that didn't compliment the bellmouth. Many things are possible, only the actual has any truth to it!
 
Good on you! But it is still possible that the customer had an airbox that didn't compliment the bellmouth. Many things are possible, only the actual has any truth to it!
Anyway, this sort of takes us back to the discussion when you made those pancake air filters. Your filters would work well with this carb I believe!
 
The high performance of that fairly flat shape one unexpected takeaway.
Straight taper better than a curved bell another?
 
The affect of velocity stacks has been well studied: they work. I recall seeing a similar chart in a Mikuni manual somewhere (they actually sell stacks too). "Seat of the pants" experience is that, aside from being more "open", they must pull more fuel out of the carb too as I had to drop two main jet sizes vs pods - increased efficiency??

20220916_143400 (2).jpg
 
The high performance of that fairly flat shape one unexpected takeaway.
Straight taper better than a curved bell another?
I think there are two different things at play when discussing velocity stacks and bellmouths. Outright flow/ flow resistance, and pressure waves/ tuned lengths, just like with exhaust pipes.
I once read a writeup where that flat type bellmouth gave a noticeable power increase. IIRC, it was on an older I-4 sports car engine, maybe Triumph or MG. In that case it was a single downdraft carb on a conventional intake manifold, with a large, round traditional air filter. So the intake tract length was mainly in the manifold, and the bellmouth improved flow from air filter to carb.
Most of the time, when modifying motorcycles, we are limited by the space available, and the types of filters, velocity stacks, etc. that are available from aftermarket suppliers. But if you look into the airbox of fairly modern, high output bikes, you will see that the manufacturer has paid a lot of attention to both bellmouths and intake lengths. Yamaha R6 comes to mind.
 
In my old Datsun 510, with an inline 1600cc, 4 cylinder, I ran dual Weber 45mm side draft carbs. They had velocity stacks inside the air cleaner assembly. I used to run occasionally with the air cleaners off because it looked so cool. Did it run better? Dunno but it certainly increased the cool factor many fold and sounded great too.
 
Hmmm speaking of drag leading to increased flow.

Anyone ever try a Kline/Fogleman foil on their bell mouth/velocity stack???

In terms of air flow at slow speeds they work well (not sure the speed at which air is flowing into a carb)

I have adapted the idea and applied it to surfboard fins with success.

Another thought in terms of a carb and flow, air and water (gas) do not flow the same, optimal flow for one isn’t always optimal for the other.

What works on paper, doesn’t always work in reality. What works in a straight line, doesn’t always work in a turn.
 
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Now if you were to stick a velicity stack over the bellmouth of the Allens performance carb, it would give twice the performance boost, right? Two is much better than one, right?
 
Ha, although you are having a laugh Yoshimura think its worthwhile. I think the idea is to have the benefits of both short and long velocity stacks. Making a set of these to try out is on my very long to do list
http://shop.yoshimura-jp.com/en/product/syousai.php?id=1384
Now if you were to add a third and a fourth stack, the performance increase would be incredible. I'm on the lathe today, I'll make six of the buggers
 
some double stacks
double velocity stacksa.jpg double velocity stacks.jpg double velocity stacksb.jpg

From a Yamaha WZF 600
intakea.jpg 20201117_152918.jpg
note how the mixture, float chamber and slide chamber ports have been isolated from the intake pulses. I suspect the XS650 would benefit from a stack that does this. perhaps even more than that 600cc 4cyl.
 
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