Carb (bore) size.. What is best? (and intake tract length)
Posted: Wed Jun 06, 2018 10:11 pm
Ok, this will probably be a more theoretical thread...
(assuming anyone replies.. lol)
But could be helpful to some on a practical level.
So, which is better for a given combo? Larger or smaller?
What does "better" mean, for that matter?
Well, none other than the great tuner/columnist Kevin Cameron taught me years ago
(not to name drop or anything like that)
"The larger the carb bore, (within a reasonable range of course) the better the low end".
That is to say: the power curve *when on the pipe/in the powerband* is fatter down low and
drops off faster on top.
Vice versa too.
IE: Smaller bore carb more top end, more peaky.
Of course, this assumes ALL else is equal.... within the "system".
This seems counter-intuitive..
But only in terms of "four stroke think"..
Four strokes do the opposite.... bigger bore carb = more top end power.
It's all a matter of two stroke theory.
BTW... please feel free to chime in and correct my thinking on this if I get something wrong..
(but gently plz... I'm sensitive.....)
This is, after all, meant to be a question/discussion and such..
During the overlap period... (as I'm sure everyone here knows) IE: when both ports are briefly
open to the atmosphere, resonant tuning (waves bouncing back and forth etc.) cause a nice charge of fresh mixture to be drawn into the combustion area, resulting in a denser charge when the plug lights... etc etc..
It's why expansion chambers add power... over, say, a straight pipe.
Well.... a larger tube resonates at a lower frequency..
Lower frequency = lower RPM.
Think of trombones...
A bass trombone has a larger bore than a tenor (the kind usually used), so it's range is lower.
(for a given slide position)
So, I'm wondering....
On the practical level, since I've not had opportunity to experiment, who has found this to be true with carb bore sizes, and what did you find? What is a "reasonable" range of bores.... particularly with these engines?
Ah Hah! you say.... because the other obvious aspect of the trombone occurred to you while thinking about the above?
You pull the slide out to get lower tones!
Yup...
ALL else being equal.... a longer tube (of the same bore) will resonate at a lower frequency!
This brings to MY mind.. something I've been wanting to experiment with for many years...
but never seem to find the time..
Lengthening the intake tract..
Velocity stacks for example.
The longer ones....
(it also makes the argument for reed block spacers)
Back in the day, I was often told that adding long stacks (where practical) not only help with the lower end of the power band, but also helped cure flat spots!
By keeping velocity up, at slight expense of top end HP.
(you'd probably only notice on a drag bike where absolute max HP is king)
The theory basically being...
A vibrating column of air resonates... a longer intake tract and/or larger carb bore would cause
it to do so at a lower frequency, hence lower RPM, making more power at the lower end of the curve.
Helping cure flat spots would be a very helpful side effect.
A reason for finding a way to integrate stacks all by itself, for me.
Keeping the velocity (of the vibrating column of air) up might help cure jetting out flat spot woes
It's almost certainly also why the stock airbox works well.
It's very possible to put velocity stacks inside of K&N pods, and likely others.
Perhaps one could eliminate the clunky/clumsy stock airbox, and the odd Y-Boot doohicks with greater success this way.
Thoughts on this would be MOST welcome... since this all has been banging around in my head for ages..
Practical experience would be GREAT!
TIA
Greg
(assuming anyone replies.. lol)
But could be helpful to some on a practical level.
So, which is better for a given combo? Larger or smaller?
What does "better" mean, for that matter?
Well, none other than the great tuner/columnist Kevin Cameron taught me years ago
(not to name drop or anything like that)
"The larger the carb bore, (within a reasonable range of course) the better the low end".
That is to say: the power curve *when on the pipe/in the powerband* is fatter down low and
drops off faster on top.
Vice versa too.
IE: Smaller bore carb more top end, more peaky.
Of course, this assumes ALL else is equal.... within the "system".
This seems counter-intuitive..
But only in terms of "four stroke think"..
Four strokes do the opposite.... bigger bore carb = more top end power.
It's all a matter of two stroke theory.
BTW... please feel free to chime in and correct my thinking on this if I get something wrong..
(but gently plz... I'm sensitive.....)
This is, after all, meant to be a question/discussion and such..
During the overlap period... (as I'm sure everyone here knows) IE: when both ports are briefly
open to the atmosphere, resonant tuning (waves bouncing back and forth etc.) cause a nice charge of fresh mixture to be drawn into the combustion area, resulting in a denser charge when the plug lights... etc etc..
It's why expansion chambers add power... over, say, a straight pipe.
Well.... a larger tube resonates at a lower frequency..
Lower frequency = lower RPM.
Think of trombones...
A bass trombone has a larger bore than a tenor (the kind usually used), so it's range is lower.
(for a given slide position)
So, I'm wondering....
On the practical level, since I've not had opportunity to experiment, who has found this to be true with carb bore sizes, and what did you find? What is a "reasonable" range of bores.... particularly with these engines?
Ah Hah! you say.... because the other obvious aspect of the trombone occurred to you while thinking about the above?
You pull the slide out to get lower tones!
Yup...
ALL else being equal.... a longer tube (of the same bore) will resonate at a lower frequency!
This brings to MY mind.. something I've been wanting to experiment with for many years...
but never seem to find the time..
Lengthening the intake tract..
Velocity stacks for example.
The longer ones....
(it also makes the argument for reed block spacers)
Back in the day, I was often told that adding long stacks (where practical) not only help with the lower end of the power band, but also helped cure flat spots!
By keeping velocity up, at slight expense of top end HP.
(you'd probably only notice on a drag bike where absolute max HP is king)
The theory basically being...
A vibrating column of air resonates... a longer intake tract and/or larger carb bore would cause
it to do so at a lower frequency, hence lower RPM, making more power at the lower end of the curve.
Helping cure flat spots would be a very helpful side effect.
A reason for finding a way to integrate stacks all by itself, for me.
Keeping the velocity (of the vibrating column of air) up might help cure jetting out flat spot woes
It's almost certainly also why the stock airbox works well.
It's very possible to put velocity stacks inside of K&N pods, and likely others.
Perhaps one could eliminate the clunky/clumsy stock airbox, and the odd Y-Boot doohicks with greater success this way.
Thoughts on this would be MOST welcome... since this all has been banging around in my head for ages..
Practical experience would be GREAT!
TIA
Greg