Bonneville America Forums Forums Technical Forums Performance Mods Polaris Bellmouth

Just before I parked my bike with a faulty stator, I took 'er on a 100 mile ride with the Polaris bellmouth and new K&N installed. On that ride I had a lot more sound (in a pleasant way) coming from under the seat. WOW!! What a difference... like driving a 440/Holly 4-barrel without the air cleaner (not quite but along those lines). So she's getting a lot more air than before (stock) which leads to the question... is it a good idea to re-jet after this simple mod? I'm running TORs for pipes but otherwise no performance upgrades at all.

What does Dinqua's charts say, been to long since I looked up mine, does he have any thing in there with the bellmouth added either Polaris or Hyde?

Hi Keith,

Short answer:

I took my Speedy to BIM in Vancouver to ask the same question. They didn't think that changing to K&N and / or the belmouth would make much difference at all.

Long answer:

However, seeing as I was getting a dent in my tank fixed, had just got my stock pipes back from Chevy605, had picked up some Thruxton needles plus needed to send the bike in for a service anyway I asked them to rejet while it was there.

I bought the bike last September, it was 4 years old with 2200km on it so they also suggested stripping and cleaning the carbs too as the tank was off anyway for a week while it was getting pulled and painted.

When I got it back they had set it up with the stock main and pilot jets (120/42) back in there but installed the Thruxton needles with 1 shim (I never asked about shim thickness or how many turns out)

I thought the combination of pipes, filter and belmouth would mean re-jetting for sure but it appears not when taking the needles and shim into account. When I entered my config in to Pats jetting calculator it actually suggested the exact same setup as they gave me. 120/42/Thruxton/1 shim. That's the same result the calculator gives without entering any option for having a belmouth or not.

That was 4 months ago now and I've put 2000km on it and it's running great. Even get an extra 25km out of a tank round the city before I have to switch to reserve.

The point of my post is that the dealer set it up exactly how Pat's calc suggested (maybe they use it too) which makes me trust its suggestions and they were confident the K&N + belmouth over the stock filter and snorkel wouldn't make much of a difference to A/F mixture

Actually, I'm on crack. Now I've gone back and checked there is an option for snorkel vs no snorkel in the calculator .... I guess it does make a difference

Also, now I'm remembering what actually happened while I type what the dealer said was he didn't think there would be a big difference over belmouth vs no snorkel at all

Ya know, I'd forgotten about the little calculator. Just ran it and yes, it does suggest slightly bigger mains. I would have expected larger mains just going by the sound coming from under the seat. The difference is really amazing (I like it)!!

Another related question is... why is the bellmouth a better thing than just pulling the snorkel (or is it)?

It's makes the opening more aerodynamic. Like pouring water into a funnel.

They are also called velocity stacks, as seen here on a 12 cylinder Ferrari GTO.
(Please to be not stepping in my drool puddle thank you)



See also, Venturi
http://en.wikipedia.org/wiki/Venturi_effect

When the air gets squeezed down through the smaller opening, the pressure drops, but the velocity increases.

True "tuned exhaust" uses the same principle. The engineer creates a low pressure zone in the exhaust pipe that helps move exhaust out faster than natural aspiration can.
I recall some years ago a wood cutter's competition where all the competitors were using tuned exhaust on their 2 cycle chain saws. The saws were 2 feet long, and the exhaust pipes were 3' long. They had the wicks turned up to around 10 or 12,000 RPM. Very loud, very fast, very cool.

A more modern competition saw with tuned exhaust
http://barkhamstedhistory.org/images/Mike%20and%20super%20saw.jpg

That makes sense. Thank you!