Intake tuning, length and where to extend.

[arcticXS]

In the "Ron Wood BMW" thread there was a comment regarding intake length. I believe I have posted on that subject before, but anyway. So here goes:
On a stock framed XS650, with any linked carbs, like the stock BS 34 and 38, Mikuni RS34, Keihin CR/FCR, and the popular Kawasaki 500 flatslide CV carbs, there is no room to move the carbs further back. So any lengthening of the intake tract will be by adding velocity stacks or straight runners between carbs and pod filters. And those will be roughly 55-60 mm in inside diameter.
With separate carbs, like Mikuni VM or TM, various Keihin and clones, and DellOrto, it is also possible to have longer intake tubes between carbs and head, typically with 34 to 36 mm diameter, like these two options:
https://www.heidentuning.com/racing-parts/carburator-manifolds-racing-aluminiun-set-detail.html
https://www.heidentuning.com/racing-parts/carburator-manifolds-racing-aluminiun-set-1-detail.html
Or use velocity stacks between carbs and filters, like on linked carbs.

Now here is the question: Given the smaller diameter when adding length between carbs and head, this will also give a higher gas velocity, and hence more inertia for the incoming charge. Will this be more beneficial to power than adding a velocity stack behind the carbs? 90s carburetted Ducatis have 6-7" long intake run ers between carbs and heads, and only an approx. 1" long rubber adapter/ bellmouth between carb and air box, as shown in drawing on top. (Dunno why it ended up there.....)
Which is very close to that on the Ron Woods Norton, pic. attached below:

 

[RustiePyles]

Adding length the atmospheric side of the carburetors does very little to increase anything, velocity stacks can increase incoming air velocity but don't do much to the air fuel mix velocity. This can only be achieved by moving the carburetor further away from the intake valve, the longer the intake runner the higher the rpm the flow becomes efficient at. So you have to determine at what RPM you want the flow to have the most velocity and be the most efficient at to determine optimal runner length. If you do the actual calculation for optimal runner length you will find that they become ridiculously long for anything over midrange RPM for most motors.

 

[RustiePyles]

So for instance the optimal intake runner dimensions for a stock bore & stroke XS650 with a shell cam with 272D of duration @4000rpm would be
D27.26mm X L842mm (thats a 33" long intake).
The take away is that there is no reasonable amount of additional intake length that a motor of this type would not benefit from from a performance standpoint.

 

[bosco659]

I had these aluminum spacers on my PWK carbs. Eventually removed them and didn’t notice a significant change in performance.

 

[jay760]

Mine are about 5 1/2 inches long, tapered from 36mm carb end to 34mm to match the head

 

[RustiePyles]

View attachment 187346
Mine are about 5 1/2 inches long, tapered from 36mm carb end to 34mm to match the head

Sweet! Drag bike or SCTA?

 

[jay760]

Sweet! Drag bike or SCTA?

Sprint, basically the same as drag apart from the time starts when you cross the line

 

[arcticXS]

Mine are about 5 1/2 inches long, tapered from 36mm carb end to 34mm to match the head

Interesting! So what kind of redline rpm and peak power rpm does your bike have? And what camshaft?

 

[jay760]

Unknown camshaft, not high lift but odd duration,
Never had it dyno'd quite often goes over 9000 rpm, usually crosses the line just above 8000rpm at around 108-110mph, best time of 11.88 with a 200lb rider

 

[arcticXS]

View attachment 187330 View attachment 187332


I had these aluminum spacers on my PWK carbs. Eventually removed them and didn’t notice a significant change in performance.

It may be just me, but the distance from head to carbs looks to be pretty much the same on both setups. And that is what matters.

 

[hovel]

Ok Rustie, I'll bite, what is SCTA ? In the states it is (the) Southern California Timing Association - a far cry from Somerset.
Jay, high 11's at 110 is impressive and so is 9K rpm. I'd guess some serious valve springs along with the odd duration.

 

[RustiePyles]

Ok Rustie, I'll bite, what is SCTA ? In the states it is (the) Southern California Timing Association - a far cry from Somerset.
Jay, high 11's at 110 is impressive and so is 9K rpm. I'd guess some serious valve springs along with the odd duration.

That is the same SCTA (land speed) I was referring to, I did not realized he lived in the UK when I posted that.

 

[jay760]

Ok Rustie, I'll bite, what is SCTA ? In the states it is (the) Southern California Timing Association - a far cry from Somerset.
Jay, high 11's at 110 is impressive and so is 9K rpm. I'd guess some serious valve springs along with the odd duration.

National sprint association in the UK.
Kibblewhite valve springs, xv750 valves, the inlet tracks added 5mph to my terminal speed.

 

[arcticXS]

So for instance the optimal intake runner dimensions for a stock bore & stroke XS650 with a shell cam with 272D of duration @4000rpm would be
D27.26mm X L842mm (thats a 33" long intake).
The take away is that there is no reasonable amount of additional intake length that a motor of this type would not benefit from from a performance standpoint.

Rustie, could you share the formulae you are using?
I have pretty much zero experience with intake tuning, but a decent grasp on basic college physics. So I struggle a bit with the concept of a longer intake pushes the powerband up in the rpm range. Did you calculate for second, third or fourth order pressure wave?
As I see it, there is at least three different positive aspects of a somewhat longer intake tract, and at least one negative (friction). In addition to exploiting intake harmonics (pressure waves), the mass of the moving gas column is increased (so more inertia).And if the carbs are moved away from the head, the fuel mist may have more time to evaporate, resulting in a cooler, denser intake charge.
Please correct me if I am totally lost here...

 

[arcticXS]

Just FYI, I am working on the intake on my XS, and also making some velocity stacks/ UNI filter adapters for a set of 30 mm DellOrto carbs for my buddy's CB 350.
(So he at least has a theoretical chance of keeping up with me on the RD350 )

 

[Team Junk]

(So he at least has a theoretical chance of keeping up with me on the RD350 )

From previous experience it is gonna take more than carb inlet changes to make a CB350 keep a RD i in sight much less keep up.

 

[RustiePyles]

Rustie, could you share the formulae you are using?
I have pretty much zero experience with intake tuning, but a decent grasp on basic college physics. So I struggle a bit with the concept of a longer intake pushes the powerband up in the rpm range. Did you calculate for second, third or fourth order pressure wave?
As I see it, there is at least three different positive aspects of a somewhat longer intake tract, and at least one negative (friction). In addition to exploiting intake harmonics (pressure waves), the mass of the moving gas column is increased (so more inertia).And if the carbs are moved away from the head, the fuel mist may have more time to evaporate, resulting in a cooler, denser intake charge.
Please correct me if I am totally lost here...

I used this calculator,
http://www.exx.se/techinfo/runners/runners.html
I have the actual Helmholtz Resonator formula somewhere, I'll have to dig out my MMI books. Intake length calculations are always done using 4th or 2nd wave, 2nd being half the length of 4th. 4th wave is used for most high rpm calculations, 2nd is used for low rpm optimization. but none of this really matters if you don't first calculate the runner diameter. All of these calculations are from venturi (point of mixture) to the valve face. so assuming that you cannot decrease the existing size of the intake port you now have to base your calculations for length on an already existing diameter. Throw a CV carb into the mix and now your modulating venturi size as well as metering air/fuel flow. And for what its worth, CV carbs are tuned (slide drafts/springs) to work at a certain intake length at a specific reversion rate, if you start moving them around to much the slides start acting weird. So all this intake length alterations is assuming you're using non-CV carbs.
Are you working on a CB350F or CB350T? Best thing you can do for a 350T (I have a CJ360T) Is clean up the port castings, cut the valves to a 3 angle face, and provide sufficient backpressure at the exhaust (exhaust length is WAY more critical than intake length), and put a mega cycle cam in it with some heavier valve springs.

 

[RustiePyles]

BTW the 350/360 twins had super tight valve lash when set to factory spec. You can increase compression (and oil film dispersion on the valve stem face) by increasing valve lash. This reduces effective duration, which at OEM lash is longer than necessary for optimal flow. So by increasing lash you close the valves earlier and increase compression and reduce valve overlap. I don't remember exactly what I set them to but I will look in my notebook this weekend.

 

[Jim]

(So he at least has a theoretical chance of keeping up with me on the RD350 )

 

[arcticXS]

I used this calculator,
http://www.exx.se/techinfo/runners/runners.html
I have the actual Helmholtz Resonator formula somewhere, I'll have to dig out my MMI books. Intake length calculations are always done using 4th or 2nd wave, 2nd being half the length of 4th. 4th wave is used for most high rpm calculations, 2nd is used for low rpm optimization. but none of this really matters if you don't first calculate the runner diameter. All of these calculations are from venturi (point of mixture) to the valve face. so assuming that you cannot decrease the existing size of the intake port you now have to base your calculations for length on an already existing diameter. Throw a CV carb into the mix and now your modulating venturi size as well as metering air/fuel flow. And for what its worth, CV carbs are tuned (slide drafts/springs) to work at a certain intake length at a specific reversion rate, if you start moving them around to much the slides start acting weird. So all this intake length alterations is assuming you're using non-CV carbs.
Are you working on a CB350F or CB350T? Best thing you can do for a 350T (I have a CJ360T) Is clean up the port castings, cut the valves to a 3 angle face, and provide sufficient backpressure at the exhaust (exhaust length is WAY more critical than intake length), and put a mega cycle cam in it with some heavier valve springs.

It is a twin, not sure if the engine came from a CB350 or CL350, or even an SL. The stock CV carbs are going into retirement, and DellOrto PHBH 30 is the new type. These are fairly similar size wise to Mikuni VM28 or VM30 (small body VM30, mostly used on snowmobiles)
I am not too concerned regarding CB vs RD, as the CB is mostly stock engine wise, but lightened a bit, maybe 10 kg or 22 lbs. BUT my buddy is at least 20 kgs lighter than me....
(Although my RD is also lightened quite a bit, has chambers, a single big K&N in the stock rubber "Y-boot", YZ125 fiber reeds, and a Powerdynamo CDI ignition. Plus rearsets of course.