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Old November 20th, 2008, 04:52 AM   #1
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Question Adjusting runner length. ?'s and feedback

Sooo I talked briefly with kkim about adjusting runner length and he pointed out the Duck was also considering this?

I'm not looking to make MORE power, but I'm fine tuning my power-band. I was mainly thinking about making the intake runners a shy longer and seeing if I could raise the torque (and likely kill a little horsepower in the process). Has anyone played with this on the '08+ 250r's?
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Old November 20th, 2008, 10:13 AM   #2
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Hi Vex!

Haven't heard of anyone doing this yet on the 2008's, but would be curious to know how it works out. Runner length is always a compromise to balance intake needs at low revs, mid revs, and max revs. Some Yamaha sportbikes now have electronically controlled the runner length by opening them up to shorter length once the bike reaches certain revs. Helps high end power without losing torque down low. So I'd imagine the theory's sound, if you want to move the power band a bit lower in the rev range.
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Old November 20th, 2008, 01:40 PM   #3
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Some background... Vex, Duck and myself have removed the stock airbox and replaced it with pods or stand alone filters connected directly to the carb throats. I love the switch and the improvement to the bike's ability to rev up top has brought, but it came at the expense of some midrange grunt. We have fine tuned the jetting as much as commonly accepted and would welcome any input from someone knowledgeable with tuning intakes runner lengths to perhaps gain some midrange back.

If there is some improvement to be gained, I'm all ears.

Don't get us wrong, the power the bike makes now is much improved from a stock, off the showroom floor bike... I'm just looking to rearrange the power curve a bit to get the lump back in the middle, if possible.

They say you can't have it all, but oh man, is it fun trying!

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Old November 20th, 2008, 04:47 PM   #4
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I already did it! Back in March!



They seem to work, too! The bike ran SUPER lean (especially in the recent cooler dry weather) and required a 120 jet to stabilize it. I notice that at 100% throttle it makes a little less power than 90% throttle. It may be still a little lean.
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Old November 20th, 2008, 09:58 PM   #5
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I already did it! Back in March!



They seem to work, too! The bike ran SUPER lean (especially in the recent cooler dry weather) and required a 120 jet to stabilize it. I notice that at 100% throttle it makes a little less power than 90% throttle. It may be still a little lean.
isn't that a sign that the bike is rich if the power flattens out/goes dead up top?

Care to share a bit more than just that pic? materials used, runner length determination, fitting the tube to the carb and filter... little details like that?

come on, man... inquiring minds want to know.
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Old November 20th, 2008, 10:58 PM   #6
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isn't that a sign that the bike is rich if the power flattens out/goes dead up top?

Care to share a bit more than just that pic? materials used, runner length determination, fitting the tube to the carb and filter... little details like that?

come on, man... inquiring minds want to know.
Nah, it doesnt go dead or even bog, it just makes a little less and you have to ease into max throttle (not to be confused with high RPMS) like in a lean condition. It is more evident when its colder outside in the mornings (35 to 45F) even after the bike as warmed up. As I've increaased the jet size from super lean until "good" I still notice that little niggle, but there is no obnoxious hesitation or slapping-around of the carb slides like when the seasons began to change here. I will go just one size BIGGER and I feel the problem may be resolved. I'll keep you updated on that. I do have another set of jets sitting right here on my desk, ready to be drilled! Weee!

The runner tubes/stacks are just PVC pipe, nothing fancy. They are the same diamater of the outside of the carbs.. It is a thin wall though, the thick wall had a significantly smaller inside diameter than the carbs. I did further grind out the carb side of the tubes to a taper to match a little better to the carb inside diameter size. I cut them out at the maximum length that I could still fit the pods on the end in that confined area. Then I attached them using rubber pipe couplers of the appropriate size.

There was no science or magical voodoo anything. Just measured to make them fit and cut. I painted them black because PVC is ugly, and I put the UNI stickers on them to make them look like some kinda official aftermarket addon, lol.

The big idea behid all of this is to contain the "fuel fog" that the carbs spit back without crushing it flat inside of a short stack or low filter. You'll get a better mixture that way. Also the tubes will resonate similar to the airbox, probably not as well, but BETTER than just plain pod filters alone. I learned this trick from an old VW engine hotrodding book.

I did this on a moped of mine too and also noticed some SERIOUS lean conditions afterwards. Being 2-stroke without reeds the fuel fog is even BIGGER on that thing. The frame used to be wet with oil and gas until I put a nice stack on it.

I hope that satisfies your questions.
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Old November 20th, 2008, 11:14 PM   #7
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So, I guess you did this at the same time you did the pods, so you have no before/after impressions to share? how about you pull off the tubes, connect the pods directly to the carbs and see what diff it makes?

thanks for the ideas.
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Old November 20th, 2008, 11:20 PM   #8
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So, I guess you did this at the same time you did the pods, so you have no before/after impressions to share? how about you pull off the tubes, connect the pods directly to the carbs and see what diff it makes?

thanks for the ideas.
Before: Quiet

After: Noisy... its been so long since I had ridden it at factory specs though, I really don't remenber how it rode before. But it seemed better once I got the jetting *ok* for the HOT SUMMER months with the POD filters but the running at low RPMS seemed little weaker than before. BUT, there were much more obvious changes noticed when I adjusted the mixture screws and added the washers under the needles! Suddenly low RPMs livened up!

When I pull it apart in the future I will try mounting the pods directly to the carbs, but I can assure you, its going to run RICH RICH RICH!
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Old November 20th, 2008, 11:57 PM   #9
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That's a good way to stabilize the air going INTO the carburetors, but I was referring to the intake runners (ie. the pipe between the head and the carburetor). Similar to putting a carburetor spacer on a car engine. I was going to perhaps run to Home Depot and get rubber pipe couplers and experiement with different lengths of pipe. It should yield a LOT more of an impact as a little runner length on these bikes would probably go a long way.
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Old November 21st, 2008, 12:20 AM   #10
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ahh... okay. I guess I misunderstood your original question. Very interested to hear what you find out, though.
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Old November 21st, 2008, 06:16 PM   #11
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Doing a little math with an engine tune friend today on this topic, I think the optimal intake boot length for our engines, would be about 7 inches long from the head of the intake valve back to the output of the carb.. but same diameter as factory. From what I was told, the exact diameter doesn't matter much unless you wind up restricting the flow to a lot smaller than what your carbs can flow or make them so big that the turbulence inside of them is weakened. Keep them about the same size.

That should tune it for the maximum harmonic at 12,000RPM. Where AreaP says the peak power is with their exhaust.

If you're looking for power a little lower down the RPM scale, then try these, all of which are for the same diameter as factory.

8.4inches long for 10,000 RPM (factory peak HP)

10.5inches long for 8,000 RPM

14in long for 6,000 RPM

If you do this, you will probably need different throttle cables, yours will wind up too short!

Now don't commit to this without talking to someone else on the topic, we could be waaaaaaaaaaay off but he's a rather bright engine tuner. He might be correct!

The formula is stupid simple though:

84000 / RPM = length in inches

Anyone else's input?
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Old November 21st, 2008, 08:53 PM   #12
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Quote:
Originally Posted by TheDuck View Post
Doing a little math with an engine tune friend today on this topic, I think the optimal intake boot length for our engines, would be about 7 inches long from the head of the intake valve back to the output of the carb.. but same diameter as factory. From what I was told, the exact diameter doesn't matter much unless you wind up restricting the flow to a lot smaller than what your carbs can flow or make them so big that the turbulence inside of them is weakened. Keep them about the same size.

That should tune it for the maximum harmonic at 12,000RPM. Where AreaP says the peak power is with their exhaust.

If you're looking for power a little lower down the RPM scale, then try these, all of which are for the same diameter as factory.

8.4inches long for 10,000 RPM (factory peak HP)

10.5inches long for 8,000 RPM

14in long for 6,000 RPM

If you do this, you will probably need different throttle cables, yours will wind up too short!

Now don't commit to this without talking to someone else on the topic, we could be waaaaaaaaaaay off but he's a rather bright engine tuner. He might be correct!

The formula is stupid simple though:

84000 / RPM = length in inches

Anyone else's input?
Exxxxcelent I didn't have the formula and was simply going to add sections of pipe until I got the right 'feel'. I'll try out the 7" though first off . I'll have to really take a look at the cables, I knew the choke cable would be a tight one though ... Thanks your Duckness!
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Old November 21st, 2008, 08:55 PM   #13
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Quote:
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Exxxxcelent I didn't have the formula and was simply going to add sections of pipe until I got the right 'feel'. I'll try out the 7" though first off . I'll have to really take a look at the cables, I knew the choke cable would be a tight one though ... Thanks your Duckness!
Find out and share what the measurement is from the intake valve head to the intake port on the outside of the head. Then share!

I'd be even more impressed if you made a telescopic runner that adjusted itself based on engine RPM.
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Old November 21st, 2008, 09:15 PM   #14
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I'd be even more impressed if you made a telescopic runner that adjusted itself based on engine RPM.
so would every motorcycle engine manufacturer out there.
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Old November 21st, 2008, 10:44 PM   #15
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LOL Well Alex pointed out that Yamaha was done it If I were to invest that much time and energy into making something like that, I'm sorry to say it probably wouldn't be with the 250

My favorite runner idea was in the early 90's ZR1 Corvettes. They had 2 intake runners per cylinder. One of the intake runners had a butterfly on it which only opened up at like 3,000 RPM. So you would literally go from a long runner intake design at low RPM's to a huge tunnel-ram type manifold at higher RPM's...

... One difference though is it was a TPI fuel injection set-up, so the intake runners were dry.
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Old November 21st, 2008, 10:50 PM   #16
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LOL Well Alex pointed out that Yamaha has done it
Haven't seen much technical information on their setup, but the following information was in most of the press releases for the new R6:

Quote:
The bigger news is the addition of Yamaha's Chip Controlled Intake (YCC-I) system that electronically varies the intake tract length depending on engine rpm; longer intake stacks boost low- and midrange power, while shorter ones boost high-rpm performance. Yamaha claims the benefits are more pronounced on the higher-revving R6 than on the similarly equipped R1.
(from here)
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Old November 22nd, 2008, 12:42 AM   #17
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My favorite runner idea was in the early 90's ZR1 Corvettes. They had 2 intake runners per cylinder. One of the intake runners had a butterfly on it which only opened up at like 3,000 RPM. So you would literally go from a long runner intake design at low RPM's to a huge tunnel-ram type manifold at higher RPM's..
The Acura RSX base model had it too, the Type-S didn't though.

I don't suppose it would be that hard to build such a telescopic design. But surely on 250 it would just be a labor of love. The gains would'nt be worth the time and parts invested. Plus points to anyone that does it just because!
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Old February 11th, 2009, 10:09 PM   #18
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Welp, talking with my buddy again about the intake runners, I misunderstood him the first time. He said that the head, intake boot, carb, and any velocity stack or other tube all add up for the total runner length. Roughly measured the factory from the inlet on the carb to the intake valve is about 7.5 inches, or tuned for about 10% flow increase peaking at 11,200RPM. This also calculates to a 2nd harmonic at about a 7% flow increase peaking at about 9,500RPM, you can actually see this small boost on most ex250 dyno charts.

My intake stacks that I whipped up are 6 inches long, giving a total intake runner length of about 13.5 inches, give or take, which RETUNED the intake for max 10% flow increase at about 6,200RPM. Aparently there is a second harmonic someplace slightly above idle, I never noticed it, who runs full throttle at that RPM?

All that started to make sense when I attempted to run the bike with the 120 main jets with the intake stacks OFF.

At wide open throttle the bike was a little rich at about idle to 5k, then went really flat and even BOGGED down having difficulty increasing revs and made some smoke from about 5k-8k indicating a rich condition. The top end was a little sluggish, guessing also a little rich.

I put 115 kawi main jets in there without the intake runners attaching the pods directly to the carbs, and the bike at wide open throttle actually had MUCH BETTER bottom end from idle to about 5k than with 120 kawi jets and 6" intake stacks. HOWEVER, the mid range from about 5k-8k was noticably weaker than with the stacks and 120s. The top end was a little better than before It required a different riding/shifting style to make the bike feel snappy.

Anyways, it also started to make sense why I needed to play with the jetting so much when the weather changed, and why the midrange 6k-8k seemed sooooooooo much more peaky than top end. It actually felt like a turbo kicking in, or like a 2-stroke bike when it gets on the pipe, after I get past that hump, the bike would go flat again.

So the moral of the story:

I decreased jet size and am running the bike without the stacks on it now.

Running with these stacks they are a pain in the ass to keep tuned. I would recommend them highly on an engine that ran at the same RPM always, like a generator, lawnmower, aircraft, boat, or something similar. Or even for a drag bike running full bore all the time!

The stacks definately worked, but I expected their volume to help replace the resonance the airbox had. I never considered their harmonic length would be such a contributing factor.

But I still might experiment with some shorties on there about 1-2" just for a laugh.

Shame I didn't have a real dyno to tune this on, I would have loved to have seen the radical difference in the power curve with these 2 different setups.

Now, back on the plan to make the intake runner length adjustable based on RPM....

Its fun playin' in the garage.
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Old February 11th, 2009, 10:15 PM   #19
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How about just using the stacks from the stock airbox that led to the carbs? Isn't that about the right length? I've been meaning to rip them out of the airbox and stick them between the carbs and filter, but have never gotten around to it.
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Old February 14th, 2009, 09:39 PM   #20
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Thanks for the feedback on that Duck I've had to hold off on doing damned near ANY work on the bike as of late so I haven't been able to tweak my runner lengths. Good observations though!
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Old February 14th, 2009, 10:01 PM   #21
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How about just using the stacks from the stock airbox that led to the carbs? Isn't that about the right length? I've been meaning to rip them out of the airbox and stick them between the carbs and filter, but have never gotten around to it.
Give it a shot, but without the airbox they might be kinda longish.
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Old February 14th, 2009, 10:19 PM   #22
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okay... I'm confused (as usual). By runners, are talking between the carbs and engine or between the carbs and filter/s?
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Old February 14th, 2009, 10:36 PM   #23
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okay... I'm confused (as usual). By runners, are talking between the carbs and engine or between the carbs and filter/s?
Well, the "runner length" is the length of all the above, INCLUDING the intake port in the head up to the head of the intake valve.
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Old February 14th, 2009, 10:54 PM   #24
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so which is it that you're playing with? I thought you were looking for a 1-2" runner between the carbs and filter after you had taken off the tubes you had previously.
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Old February 14th, 2009, 11:01 PM   #25
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so which is it that you're playing with? I thought you were looking for a 1-2" runner between the carbs and filter after you had taken off the tubes you had previously.
Nope, I'm not looking for any. I have ones that are 6" long, I can just cut them to 2" and test. Its just PVC pipe anyways, I have extra in my toolbox, lol.
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Old February 16th, 2009, 11:06 PM   #26
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okay, added the runners from the stock airbox yesterday that went from the carb to the filter. Don't know if it will change the power response any, but it was free and it makes for a much neater air filter install. Been meaning to try this for awhile now and finally got around to doing it.

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Old February 21st, 2009, 01:45 AM   #27
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Oh TeeeJay- you might want to try the runners added to the 0990 air filter. It could be all in my head, but the bike seems to have gained some of the midrange we lost when the airbox was removed. It even sounded more like when the stock airbox was in there with the snorkel removed. Acceleration from a cruising speed seemed more lively.

Try it and let me know what you think. I only had it around on the streets and didn't test in the mountains.
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Old February 21st, 2009, 02:11 PM   #28
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I might get to that in this upcoming week Thanks for the feedback. I need to wire up a power tap for my GPS unit and whilst there I was going to put the 110 jets in... It looks like maybe I'll have to make a day of jetting again
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Old February 23rd, 2009, 01:20 AM   #29
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sounds like a perfect time to add the runners to test.

pretty smitten w/ that gps, eh?
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Old February 23rd, 2009, 10:57 AM   #30
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Pardon the digression, but how often do you guys pull the foam filters off your air pods to re-oil them? Is there a set suggested maintenence schedule or do you just do it when they look dirty?
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Old February 23rd, 2009, 11:22 AM   #31
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I've not cleaned mine since installation. My experience w/ K&N filters have said to clean them when they look dirty or start to get caked. Mine still looks relatively clean on the majority of the filter. It would all depend on how dirty/dusty conditions are in your area and how often you ride.
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Old February 23rd, 2009, 05:51 PM   #32
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I've not cleaned mine since installation. My experience w/ K&N filters have said to clean them when they look dirty or start to get caked. Mine still looks relatively clean on the majority of the filter. It would all depend on how dirty/dusty conditions are in your area and how often you ride.
Okay. Then I'll wait for your "Removing and cleaning your air pod filters DIY"
Just wondering because those cans of K&N air filter oil spray ain't cheap.
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Old February 23rd, 2009, 08:32 PM   #33
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Quote:
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okay, added the runners from the stock airbox yesterday that went from the carb to the filter. Don't know if it will change the power response any, but it was free and it makes for a much neater air filter install. Been meaning to try this for awhile now and finally got around to doing it.

I'm trying to tell from this pic how you connected the pod to the factory runner. Did you cut the factory runner or is the rest of it shoved into the filter?
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Old February 23rd, 2009, 08:34 PM   #34
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the rest of it is shoved into the filter.
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Old April 14th, 2010, 04:28 AM   #35
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I'm going to bring this from the dead..anybody had any experiment with adjustable intake runners or velocity stacks since the discussion that happened here?
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Old February 15th, 2011, 10:23 PM   #36
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consensus on intake runner length?

So did a consensus emerge on whether it was better to remove or keep the oem airbox runners when removing the airbox and replacing with a standalone air filter (such as the k&n r0990)?

And is anybody aware of and can recommend a book that covers the topic at an "educated layman's" level or deeper? I looked online but the "articles" are rather brief on the topic.
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Old February 16th, 2011, 03:20 AM   #37
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I have been thinking about this.I am ONLY tuning for top end .I use 32mm CR carbs with velocity stacks.I also have nitrous jets in my stock intake manifolds.things are a bit tight. I was thinking of using first gen manifolds they are longer then the new ones. I will see how that effects the power band when I do.
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Old February 16th, 2011, 10:54 AM   #38
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Originally Posted by Racer x View Post
I have been thinking about this.I am ONLY tuning for top end .I use 32mm CR carbs with velocity stacks.I also have nitrous jets in my stock intake manifolds.things are a bit tight. I was thinking of using first gen manifolds they are longer then the new ones. I will see how that effects the power band when I do.
nitrous? that's sounds fiendishly evil what sort of setup do you have for this? for how long can you run with nitrous boost? how much engine wear does it cause? why nitrous though? seems impractical for street and illegal for racing. but maybe not for drag racing?

What 32mm CR are you using? How does it compare to the Sudco "special" CR's? If I recall they come in odd mm sizes such as 31, 33, etc. They recommend their 31mm for the ninja 250.
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Old February 17th, 2011, 01:00 PM   #39
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Originally Posted by afbarr View Post
So did a consensus emerge on whether it was better to remove or keep the oem airbox runners when removing the airbox and replacing with a standalone air filter (such as the k&n r0990)?

And is anybody aware of and can recommend a book that covers the topic at an "educated layman's" level or deeper? I looked online but the "articles" are rather brief on the topic.
The general idea of tuning your intake runner length isn't terribly complicated. To effectively do it you have to actually know a couple of variables (which involves some math ).

The airbox on the Too-Fiddies is terribly restrictive and it's done this way to create a pulse wave in the airbox. At low RPM's you get a slight bit of pressure in front of the carburetors so you can get more laminar airlow at low RPM's. At high RPM's the effect is lost and you're left with a restrictive ass airbox.

So by removing the airbox you actually lose a bit of torque at low RPM's. Back to runners:

Intake runners act like a shotgun to smooth airflow. Like most things on an engine it's a two edged sword. If you have small diameter runners and they are long airflow through will become super laminar (smooth) and you will get more air/fuel into that particular cylinder. What happens at high RPM's though? At high RPM's it's like sucking soda through a coffee stir stick. The soda will come out at a high velocity, but not a lot of it (and you have to really suck to get soda through.

Using larger diameter and/or shorter runners will allow for a ton of flow through and at high RPM's airflow will be relatively laminar, but at low RPM's air is going to be super turbulent. By keeping the intake snorkels IN the air filter you increase your runner length which helps smooth the airflow. So you regain some of your lost power at low RPMs.

There are various clever examples of getting the best of both world with intake runners. The new R1's for instance have runners which are telescoping. So at low RPM's the runners are long and at high RPM's they shorten. Pretty cool concept.

So yeah, as for the double edged sword: What are you trying to do? Racer X needs maximum horsepower (so high RPM) and hence would need a large diameter or short intake runner. If you want a little more "umpff" getting out of the hole for stop-light to stop-light riding, you might extend your runners.
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Old February 17th, 2011, 01:17 PM   #40
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What? No dyno numbers?

Seriously though, I'd like to see some figures based on the results thus far.
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