Triumphrat

Echoance · 08-03-2007

Hi, guys. I've done a search and came up with nothing so: in lay-terms, can anyone tell me the theory behind bellmouths? I see a lot of the Bonnie owners have them but do they have the same affect on the cruisers, which have a different airbox setup? One thing that puzzles me is how they can work, as smoothing out the air and then forcing it through a filter, which is going to disturb it again, makes no sense. Cheers, neill

Shorty · 08-03-2007

A Bellmouth (on jet engines anyway) provides a smoother / less turbulent airflow to the engine. I'm not an engineer, but I don't think it really increases the performance any other than maybe increasing the efficiency of the motor.
I used to have a polaris bellmouth installed when I still had the airbox, and I don't think it really made much of a difference (other than a cool sound with the throttle open).
Smooth airflow going into a filter and airbox assembly negates the bellmouth principle in my opinion since the smoothed air bounces around inside the filter and through the restrictor making it turbulent again. Now, bellmouths at the carbs might be a different story and maybe could result in better efficiency performance of the carbs due to the smoothed out airflow mixing with the fuel.

Shorty

mag10 · 08-03-2007

it is all about making the airflow more laminar.

Your point is well made about 'how much effect could the bellmouth have feeding into a filter', The passing thru the filter will disturb the airflow, the thought is that the efficiency of the air flow into and thru the filter will be improved.
A lot of the cruisers have the bottom of the airbox cut out to increase airflow to the filter. I have bellmouths (polaris) top and bottom of the filter because they were very cheap and have no down side.

What I think really made the difference is the is bell mouths (velocity stacks)that set in the throats of the carbs but now reside in a box of filtered air.
So I get the airflow I want without the risk of running open stacks on the street.

I don't know if this really answers your question but it may provide an alternative instead.

speedblastr · 08-03-2007

Mag10, Did you screen the bottom bellmouth to keep out debris?Just wondering,that would be a good sized oval. Not trying to be nosy, well maybe a little, with that extra air (intake) >> what kind of adjustments did you have to make,jets,needles,turns out? I'm still in contemplation about Freak or airbox and yes I wonder about "street legal" myself if that is your meaning.

StaggerLee · 08-03-2007

The bellmouth shape creates a venturi effect on the air. If ya really want a detailed explanation read up on Bernoulli's equation

So what difference does that make? It creates higher air flow (sort of like a ram air effect) to your filter

Hope that helps

speedblastr · 08-04-2007

Already know this , interested in bottom of airbox cutaway for secondary bellmouth.

road_dog · 08-04-2007

Quote:

Originally Posted by StaggerLee

The bellmouth shape creates a venturi effect on the air. So what difference does that make? It creates higher air flow (sort of like a ram air effect) to your filter

Hard to explain via keyboard, but yea... that's the jest of it.

Another short explanation is that it creates a pressure gradient force to the air entering the airbox. Therefore, adding a second bellmouth to the same airbox will defeat the whole purpose.

StaggerLee · 08-04-2007

Quote:

Originally Posted by road_dog

Hard to explain via keyboard, but yea... that's the jest of it.

Another short explanation is that it creates a pressure gradient force to the air entering the airbox. Therefore, adding a second bellmouth to the same airbox will defeat the whole purpose.

Mmmmmm I know we are both trying to keep it simple here but there is an important detail to hold onto ...

The Bellmouth will increase mass air flow (both volume and speed) but NOT pressure (in fact pressure drops). The overall effect however is to increase the amount of air going into the filter / airbox.

However any stuffing around with the airbox / filter can have unintended consequences and a suck it and see approach is best (pun intended).

road_dog · 08-04-2007

Quote:

Originally Posted by StaggerLee

Mmmmmm I know we are both trying to keep it simple here but there is an important detail to hold onto ...

The Bellmouth will increase mass air flow (both volume and speed) but NOT pressure (in fact pressure drops). The overall effect however is to increase the amount of air going into the filter / airbox.

Right you are... that's what I was trying to explain. Since pressure gradient force accelerates air from high pressure, to low pressure.

But I should have said (or in this case typed)... "However, I think that adding a second bellmouth to the same airbox will defeat the whole purpose". And not "Therefore, adding a second bellmouth...".

I do know this much... I run a NH Bellmouth, and believe in my own mind it's helping my performance. But since I installed it at the same time I installed Staintunes, K&N filter, re-jetted and removed the AI system... it's hard to say for sure.

mag10 · 08-05-2007

Quote:

Originally Posted by speedblastr

Mag10, Did you screen the bottom bellmouth to keep out debris?Just wondering,that would be a good sized oval. Not trying to be nosy, well maybe a little, with that extra air (intake) >> what kind of adjustments did you have to make,jets,needles,turns out? I'm still in contemplation about Freak or airbox and yes I wonder about "street legal" myself if that is your meaning.

the bottom bellmouth opens right above the swing arm joint, not directly open to the road. i have not had any issues (so far) with either dirt or water getting sucked up.
As for adjustments, UNi filter with bottom cut out, 130 mains, 45 pilots, SM neeedles w/2 shims, drilled out slides (optional).
By street useable I meant that velocity stacks are generally open with out filters and can allow dirt directing into the engine, okay on the track not for everyday riding.
If you search the achives there were a number of threads on the cutout air box last year (with pix) and part numbers.