Intake Mods to Air Box Trumpets

[koko64]

I have shortened the air box trumpets that clamp to the carb mouths and which go into the air box.

I am testing this at present and hope to do a dyno comparison with the stock trumpet length.

The length of the shortened trumpets when combined to the alloy adaptors for the FCRs is closer to that of the blue velocity stacks supplied with the FCR 41s. It has been suggested to me that these blue stacks provide the best peak power. I am testing this mod-n to see if I can get the mid range/torque benefits of a larger air box, keep the filtration, and the peak power benefits of shorter stacks.
I am familiar with this mod with some Hondas with shorter than stock rubber stacks in modified air boxes. There was actually a HRC kit that was available for racing for some Formula 400 bikes. The rubber stacks were flush with the bottom of the air box and 'radiused' down to the carb from there. This placed the tip of the trumpets further away from the filter element and deeper into the air box.

The length of the modified rubber stacks is within a few mm of the blue stacks, the diameter is 5mm larger and about 17-20mm shorter than the stock rubbers. With the alloy adapters it tapers down nicely to the carbs. The mouth radius is more open like the Blueys. We will see if it's better, worse or no different. There is no Wasp or TPO kit for the carb Monster so I am playing around with the stock parts. A friend has the blue stacks going into his air box so we should be able to do some comparisons sometime. I will be working on his carbs soon so I will see how the blue stacks were fitted to his air box and if they seal properly.

Some of the theory behind the mod relates to having the trumpets/velocity stacks further away from the filter element and trying to trick the motor to behave like it has a larger air box, shortening the overall intake length to assist holding the torque peak for longer to help the peak power and tuning the resonance through the intake tract. The inlet ports and manifolds of this carbed 900 have been ported, it has an open air box and the bike is running hi comp pistons.

So far I have noticed no mid range dips in power in street riding, but the dyno will tell with a back to back comparison.
If I have robbed the bottom end or midrange to add to the top I have so far felt no difference while street riding.

It's easy to theorize all you like so I have decided to test it out.

[Drunken Monkey]

Cool, let us know how it goes 

Just don't forget to get a good O2 reading, as the change you're making may be sufficient to affect the fueling...

[koko64]

I am continuing the street ride testing and have so far not detected any nasty flat spots in the power curve. On the street I am using less throttle for good progress before the gear change point. I believe this is due to the jetting change which needed to happen anyway as it was a bit lean when asking for power between 1/3rd and 3/4 throttle. So the improvement is down to that. It may now be a little rich on the transition from slow jet, slow air jet to needle root diameter, clip position and beginning of taper so I opened the slow air jet a quarter of a turn which has helped offset that. I could get a needle with a more aggressive taper and lower the needle a notch or two. We'll see.

My view is that the motor wants quite a bit of fuel due to the type of porting that was done to the inlet side. It flows alot more air and I am making an educated guess around the intake mods working well with that for holding top end power. It still has long manifolds which have been 'ported', so it should still have the long manifold grunt. It's whether the shorter, larger, more aggressively tapered trumpets help hold onto the peak torque for a little longer. The guessing is mine and the education is others'. Yeah I know, they're still old two valve heads.

Acceleration tests (on a controlled closed course of course), are not giving rich plugs and Istill hit 220kms before the fuel light comes on with a variety of street riding. Chocolate brown plug rings, a bee's dick on the rich side.

I will set up the A/F ratio monitor for some testing. I got a K&N one from Chris Kelly (like a m did I think). To be honest I have usually been quite accurate with test ride tuning, so the K&N A/F monitor is really just a rough measuring stick to stop me second guessing myself (which can drive you nuts when you were right the first time).

I am confident regarding jetting but it's easy to make a mistake with this sort of mod. It can take away as much as it gives. I have to organise back to back dyno runs to eliminate the jetting as a factor.

How's the dyno going Brad?

[greenmonster]

You`re nuts in a interesting way.  
Thx f doing & sharing this!  

Does the A/Fmonitor have any logging possibility?
You`ll go nuts, drive off the road etc otherwise.
Tried that, it`s really idle, cruising & WOT you can check.

Some of the theory behind the mod relates to having the trumpets/velocity stacks further away from the filter element and trying to trick the motor to behave like it has a larger air box, shortening the overall intake length to assist holding the torque peak for longer to help the peak power and tuning the resonance through the intake tract. The inlet ports and manifolds of this carbed 900 have been ported, it has an open air box and the bike is running hi comp pistons.

Guess F1 has some answers here.

Read that cars used a pulse plate f similar efforts, Ford GT40 Le mans had one f e.

Taper-boring the carb bodies and trimming the throttle shafts appears to do the trick. The power curve gains and smoothes out, just like when using 39mm Keihins.

http://www.visi.com/~moperfserv/carbs.htm

Anyone seen or has a pic of this mod?
Wondered f a long time now what he did in that mod.

[koko64]

You`re nuts in a interesting way.  
Thx f doing & sharing this!  

Yeah a bit But not as whacky as the guy you quote below. He seems quite brilliant.

Does the A/Fmonitor have any logging possibility?

You`ll go nuts, drive off the road etc otherwise.
Tried that, it`s really idle, cruising & WOT you can check.

. Yeah, I figured that, that's why I only want to use it as a rough yardstick. I am looking for a data logging system at present


Guess F1 has some answers here.

Read that cars used a pulse plate f similar efforts, Ford GT40 Le mans had one f e.

http://www.visi.com/~moperfserv/carbs.htm

Anyone seen or has a pic of this mod?
Wondered f a long time now what he did in that mod.

It sounds quite labour intensive and he doesn't do it any more.

[koko64]

Hey greenmonster.

I am always looking for a cheap bang for buck mod of a standard part to get a few more ponies. You see something and wonder if with a bit of care and attention, you can improve it. I really respect Doug Lofgren's idea of not just replacing stuff but tuning existing parts whenever possible. I must say I gave up on the stock carbs however. I offered to ship my stock carbs to him for him to weave his magic but he is too busy doing other stuff now.

A strange thing is that there are competing theories at work which can cancel out performance gains, so I really need to get hold of a dyno and a sympathetic operator and test out some variations to the inlet mods.

[ungeheuer]

Interesting...  You're off on a long and winding road with this one T (and I'm not talking about riding either)  .

I really need to get hold of a dyno and a sympathetic operator and test out some variations to the inlet mods.

When you find both of the above..... I'm in too! - See if we can get us a package deal  .  Sounds like you think BB is looking like getting access the Dyno goodies again sometime??

[koko64]

He has one, but he needs to finish off his dyno room. Buying a dyno is one thing but building the room for the noise and exhaust is another.
He is so busy working he hasn't had time.

I would test 1. Short stock stacks with the adaptors
                 2. Stock stacks with the adaptors
                 3. The blue FCR stacks (if I can seal them to the air box since they don't use the adaptors)
                 4. K&N Pods with the adaptors

I have two mates who would pitch in with the dyno time and I would set up all three bikes with the best configuration. It would be good to actually really know which intake set up gives the best result re power, drivability, etc. We could settle some arguments for good (or about five minutes anyway).

THIS MUST BE DONE

[koko64]

Minor update.

A mate in the industry can get a good deal on dyno testing for the sake of R&D. I reckon we may be able to do this testing sooner than I thought. Looking forward to discussing this on Monday. Maybe we can include some FI intake testing. I wanna see 100 .

I will be working on my mates M900S this weekend and will investigate whether his blue FCR stacks are set up to seal properly to his air box. That's one option I won't bother testing if it's not up to road riding spec. His bike is from WA so maybe it has a Vee Two set up I haven't seen that actually works to correctly seal the carbs to the air box. If that's the case then it could be the favoured theoretical set up. We'll see.

I have made some minor adjustments to my jetting to test which will tell me if I need the next richer needle (taper wise). That would be going from EMT to FMT for those FCR guys out there. I have been alternating between needle pos-n #6 and #7, but have dropped back to #6 with a bigger main jet. Patrick Burns ain't joking about the effect the main jet has on part throttle positions with a richer emulsion going past the same orifice/needle. FCR's are 'linear' carbs in tuning logic and that's why I love 'em (not like CV carbs with multiple personality disorder)!

So I will have the results sooner rather than later.

[koko64]

Update:
worked on a mates 1998 M900S. It has FCR 41s which I serviced and tuned. Interestingly it was running the blue FCR racing style velocity stacks into the stock airbox which I thought would be great to assess. I figured that the blue stacks would give good performance and possibly be the best option out of the four set ups I am looking to dyno test.

Disappointingly, the blue stacks were cut down to fit the stock rubber stacks that sit in the air box. There was no sturdy seal to the blue stacks, they were a press fit and had been trimmed of their outer diameter to fit the stock rubber stacks/boots. There was no clamping arrangement to keep things air tight and dirt free. The seal was 'OK'. Also the main air jet and slow air jet were exposed to the elements as they sit under the airbox at the base of the carb. They were pretty crusty and you can't get to them without pulling the airbox. With the alloy airbox adaptors sold with the correct kit you can access these jets by removing the air filter and can tune the bike running or with a dyno or EGA.

The bike now runs an open airbox with K&N filter. I set the float which was out and this was bad since the previous owner had an invoice saying the float levels had been set. The IMS were butchered and needed alot of TLC to operate. I jetted the bike for an open airbox and a set of Leo Vince pipes. I changed main jets, slow jets and adjusted the needle pos-ns. The slow air jet was adjusted with the carbs off the bike, but require a tweak when we get some alloy airbox adaptors. We separated the airbox from the battery box for ease of service (needle adjustment and  throttle synch).

Exact fit coils were fitted and they fitted exactly. My mate says there is a very noticeable improvement in the running of the bike with them. They are half the size and weight of Dyna coils and this matters when space is at a premium.

The bike is very responsive and smooth, and it's acceleration is excellent for a stocker. But the way the blue stacks are used and have been hacked is not the best for street use. I wont test this set up. I will test pods, short stacks and stock length stacks. Unfiltered stacks probably give the best power, for a while..

But we will test set ups for street bikes.