Long intakes with linked carbs

[arcticXS]

Extended intakes between head and carbs is available for VM, DellOrto and other non-linked carbs. But for the stock BS34/38, EX500 carbs, Mikuni TM/RS 34/36, and other linked carbs, the frame tube makes extending the intakes difficult.
Unless you make them long enough to put the carbs behind the tube, that is. Has anyone tried just that?
I am pretty sure the stock intake length may be too short for optimal intake resonance in the rpm band most of us use.
Extending the intake length with velocity stacks would obviously be an option, but then the diameter would be larger/ gas velocity lower. And I assume higher gas velocity combined with a longer intake would give the "slug" og air and fuel more mass and momentum. At least that seems intuitive to me.

 

[Max Midnight]

According to Phil Irving in Tuning for Speed, good results are obtained with a length of 90in. divided by the revs in thousands (the r.p.m. at which you want to get the maximum effect). The measurement is taken from the head of the valve to the mouth of the inlet.

for 6000 r.p.m - 90/(6000/1000) = 15

He does make the point that changes in cross sectional area such as a long intake bell mouth or any diversion from a plain straight inlet will affect the result.

 

[jaybar6]

Extended intakes between head and carbs is available for VM, DellOrto and other non-linked carbs. But for the stock BS34/38, EX500 carbs, Mikuni TM/RS 34/36, and other linked carbs, the frame tube makes extending the intakes difficult.
Unless you make them long enough to put the carbs behind the tube, that is. Has anyone tried just that?
I am pretty sure the stock intake length may be too short for optimal intake resonance in the rpm band most of us use.
Extending the intake length with velocity stacks would obviously be an option, but then the diameter would be larger/ gas velocity lower. And I assume higher gas velocity combined with a longer intake would give the "slug" og air and fuel more mass and momentum. At least that seems intuitive to me.

Hi arctic, This is interesting which is why I wanted to move the battery to a different place in my article: https://www.xs650.com/threads/putti...-of-the-swinging-arm.67118/page-2#post-856954
I could then experiment with different intake lengths.

 

[jaybar6]

According to Phil Irving in Tuning for Speed, good results are obtained with a length of 90in. divided by the revs in thousands (the r.p.m. at which you want to get the maximum effect). The measurement is taken from the head of the valve to the mouth of the inlet.

for 6000 r.p.m - 90/(6000/1000) = 15

He does make the point that changes in cross sectional area such as a long intake bell mouth or any diversion from a plain straight inlet will affect the result.

This is interesting, so using this equation I put it in xl and:

the length from valve to intake of standard air-box is approx: 13 inches, this corresponds with the max revs of 7000rpm!

 

[jaybar6]

no wonder xs's with pods on, run rough. they are tuned to 9000rpm!

 

[AUTOMAN]

Just where my 2 stroke triple peaks with that set up.

9000rpm but a 2 stroke...........
Who is "phil"

 

[AUTOMAN]

On the Viper engine, we tuned the intake to peak at 3600 and 5400 rpm.
1992-2003

 

[jaybar6]

using the intake runner calculator: https://lengthcalculators.com/intake-runner-length-calculator/ for maximum effect at 4000rpm, I would need a length of 17.17 inches.

 

[Its been a long time]

using the intake runner calculator: https://lengthcalculators.com/intake-runner-length-calculator/ for maximum effect at 4000rpm, I would need a length of 17.17 inches. View attachment 336472

My take would be that given how modest the motor tune is, the upside of shock wave tuning on the 650 motor is going to be quite subtle. I wouldn't try to persuade anyone from not doing so. But I wouldn't expect night and day results from such optimisations. There's more low hanging fruit to harvest I would say before putting much effort in to that. Goes hand in hand with exhaust system tuning, getting both inlet and exhaust systems optimised together.

Of course, on a full race motor etc it's a different story.

The other aspect is that there's going to be harmonics in the inlet tract so there might be a subtle beneficial effect at half those calculated inlet tract lengths.

Lots to go at it you have time on your hands though

 

[jaybar6]

using the intake runner calculator: https://lengthcalculators.com/intake-runner-length-calculator/ for maximum effect at 4000rpm, I would need a length of 17.17 inches. View attachment 336472

Using photoship, I mocked-up a solution of increasing the length of the intake to 17 inches, the correct length for maximun tuning at 4000rpm. where I ride at most of the time.

 

[AUTOMAN]

With that airbox in the way, all desired effects are negated.
With a throttle in the way(part throttle), most desired effects are negated.

cliff

 

[Raymond]

With that airbox in the way, all desired effects are negated.
With a throttle in the way(part throttle), most desired effects are negated.

cliff

Interesting discussion but if the benefits are only at full throttle, then IMHO it's not something for most of us here to worry about.

Or maybe I'm an exception and most of you ride around at full throttle all the time?

 

[bazz]

Does any of this make any difference with CV carbs?
I thought the CV liked a big airbox with still air?

 

[jaybar6]

Of course I will ignore all you "naysayers" and do my mod! And I predict, there will be an increase in torque at 4000rpm, with a wide torque band. So wide I will have to increase the gearing so it is cruising at 80mph at 4000rpm. So torquey that one could run fifth gear down to 10mph and pull away up in speed. with a re-jetting because of the increased efficiency get 80 mpg!

 

[Its been a long time]

Of course I will ignore all you "naysayers" and do my mod! And I predict, there will be an increase in torque at 4000rpm, with a wide torque band. So wide I will have to increase the gearing so it is cruising at 80mph at 4000rpm. So torquey that one could run fifth gear down to 10mph and pull away up in speed. with a re-jetting because of the increased efficiency get 80 mpg!

......and then you'll wake up.....

 

[Max Midnight]

Of course I will ignore all you "naysayers" and do my mod! And I predict, there will be an increase in torque at 4000rpm, with a wide torque band. So wide I will have to increase the gearing so it is cruising at 80mph at 4000rpm. So torquey that one could run fifth gear down to 10mph and pull away up in speed. with a re-jetting because of the increased efficiency get 80 mpg!

I trust you will dyno the bike before the mod?

 

[AUTOMAN]

More torque at 4000?
Does it not rev faster 4K?

c