Eternal Modding Optimism

[Wheelman-111]

Greetings:

Performance "enhancements" are not always effective. There are 4 possible outcomes to the mods I've performed on various machines over the years, which I have called Categories:

4: The machine starts, sputters, catches fire and burns down the garage.
3: The machine doesn't start.
2: The machine starts, runs execrably, but I convince myself that the change is somehow "better".
1: The machine starts and actually does run with improved performance.

I raise this topic because I recently pulled off the stock airbox from Flash III in favor of a UNI pod filter that is so highly regarded on this forum. The engine setup is otherwise stock. This was a Category 1.5 mod.

First, the intake noise is louder. A lot louder. Now I'm not some sort of Noise *, but if something makes more of a ruckus, it darned well better be worth it in terms of performance gain.

The UNI swap was not. Stock Honda jet was 78. I tried it with the 82 (ran hot) the 85 (still warm but not as seamless in the midrange and nothing gained on top) and the 88 (temps back to "normal" but minus 2-3 MPH on top speed). No combination of jets I tried worked quite as well as the stock airbox. I suspect the box acts as it should; a "resonance chamber" akin to the function of a well-tuned pipe. Honda's engineers made some real progress between the Aero airbox and the later Elites'. The stock box is back on, along with an 82, prepared for stock block test results next week.

Maybe I'll try this again once the pipe arrives and the 49mm Nikasil takes up residence atop the cases. The airbox inlet may not be big enough to supply the increased displacement/RPM demand. Until then I'll stick with Honda's design. It works better.

[Bear45-70]

This is the kind of thing people need to know. Another thing is make one change at a time. I once made 3 changes on the race boat at same time. Did some sponson work, raised the engine (removed some gear case out of the water reduce drag) and changed the prop to one more suited to the higher engine setting. Lost 3 mph so lowered the engine and put the old prop back on and the sponson work was worth 1 mph on the radar gun. 3 months later was reminded of the 1 change rule and swapped the prop back, I picked up 2 mph raised the engine and lost 5 mph. So ran the new prop for the 2 mph gain plus the 1 from the sponson work. In stock racing 3 mph is a mountain of improvement. So unless you are doing a proven change to a setup identical to yours, one change at a time.

[SpyderMike]

i had made a CT mani work w/ the stock carb and the only difference i noticed was it took 3-4 kicks to start -vs- the 1 that it normally took it will be getting replaced w/ the stock one this afternoon and placed on a shelf until a BBK and bigger carb warrant its use

[Bear45-70]

i had made a CT mani work w/ the stock carb and the only difference i noticed was it took 3-4 kicks to start -vs- the 1 that it normally took it will be getting replaced w/ the stock one this afternoon and placed on a shelf until a BBK and bigger carb warrant its use

Yeah, I knew a guy that thought because the big time Pro Stock guys ran tunnel rams with dual 4 barrels that it was what he needed on his mildly modded street machine. The car was a major dog until 5000 RPM whenit ran great right up to 6000 RPM where the cam quit. Things have to be matched up to work properly. In most cases a big intake and carb hurt more than help unless you have the cc's, port size, port timing and exhaust to handle it.

[mr pibbs]

Greetings:

Performance "enhancements" are not always effective. There are 4 possible outcomes to the mods I've performed on various machines over the years, which I have called Categories:

4: The machine starts, sputters, catches fire and burns down the garage.
3: The machine doesn't start.
2: The machine starts, runs execrably, but I convince myself that the change is somehow "better".
1: The machine starts and actually does run with improved performance.

What did you do when #4 happened?

[Wheelman-111]

Greetings:

Mr. Pibbs axt:

what did you do when #4 happened?

Which time?

[Lunytune]

What did you do when #4 happened?

I'm glad I have a concrete slab and metal building. But I do have to wear a gas mask when I bust one off inside, no ventilation.

I remember several years ago watching a rural house burn down while the family waited on the fire truck. Later, the guy donated land for the VFD to build a firehouse next to his house.

[noiseguy]

I keep a fire extinguisher on hand, and put it within reach when I'm working on fuel systems.

If things are really iffy, I push the bike outside. Had a friend almost torch his garage working on a dirty XS750, carb fire. He managed to smother it before it ignited the tank.

Back to topic; I don't care for pod filters for that exact reason. They need to be tuned with the rest of the system; frankly for a broad tuned scooter you're better off with an airbox, IMHO.

That said, my '87 Elite's been running one with few problems (poor starting); I'm betting it will run even better with the proper airbox installed. It has the long intake, though, which probably helps. Never could get the Sprees to run with these.

[Wheelman-111]

Greetings:

Now that Flash III's all grown up, I'm re-thinking the airbox thing. The higher rate of intake volume seems to have pushed the performance "window" out of reach of the tuned-for-50 airbox. Regardless of which of several jets I have tried, there's a bog as soon as the RPMs exceed 6k, and the off-idle response is no longer sweet. Conversely, the UNI Pod now seems no longer to have the low-midrange issues it had @ 50cc, and certainly accommodates much higher RPMs than could be achieved with the airbox.

[Bear45-70]

I still think someone needs to try the pod filter on the end of the air box to carb snorkel (the pipe between the air box and the carb. This would maintain a closer to tuned length while still giving you the better breathing ability.

On my race motors there were two carbs originally available, ones without the velocity stack on the front of the carb and the ones with. The ones with even though identical in every way to the without carb were worth about 20 horsepower. The velocity stacks were only about a 1/2" long. Eventually the velocity stack was available as a retro fit for all carbs.

[noiseguy]

Wheelman, sounds like you've pushed your tuned RPMs up, which would require a change of some sort.

I agree on the velocity stacks being a better option than flat pod filters. *Good* pod filters have integral velocity stacks inside the mouth; I've never seen one in the 1.25" size needed for Honda 50 apps, though a KZ750 pod filter set that I looked at had them. The flat internal pod filters should theoretically be peaky, whereas the stacks would give a better effective range. I don't have experience with tuning these, though.

I've also yet to find a good source for small (or heck, any size) velocity stacks. Seems everyone makes them from scratch?

[scooterwerx]

i made one for my aero, definitely improved bottom end

[Lunytune]

I agree on the velocity stacks being a better option than flat pod filters. *Good* pod filters have integral velocity stacks inside the mouth; I've never seen one in the 1.25" size needed for Honda 50 apps, though a KZ750 pod filter set that I looked at had them. The flat internal pod filters should theoretically be peaky, whereas the stacks would give a better effective range. I don't have experience with tuning these, though.

It's time for you gurus to explain what it is you are doing, and a little bit of the theory behind a velocity stack. Sounds like you are saying that it is more than just having an "opening" for air, but that design or length of the "stack" makes a difference? I have noticed that the intake on factory airbox is faced forward and does seem to have a "design" to it, not just an opening for air. I drilled holes in the "side" of the cover but can't say it really helped.

[Wheelman-111]

Greetings:

Black Arts are these. The Dark Side are they. Omm!

Columns of moving air have momentum and inertia. Pressure waves within a pulsed column move at predictable velocities. The length of the intake runner affects the timing of the arrival of such a pressure wave, just as the length of an exhaust header and expansion chamber design enhance performance on the backside. The goal is to stuff as much fresh mixture into the combustion chamber as possible, just before the ports close, to get maximal output.

All of the Big 4 SuperSports 600s now have some system to vary the intake runner lengths. Ideally they'd be continuously variable but evidently it was cheaper to design the systems to "shift" at a given RPM under ECM control. The device changes the effective length of the "velocity stacks" theoretically to permit both better low-end torque and maximal wallop on top. It's an awfully big expense to incur if it doesn't help maintain parity in this most competitive of motorcycle categories. Still cheaper than the variable valve timing found only in Honda's VFR models.

Nobody has explored the science as applied to our underachieving little 50s, 72s and 120s, unfortunately. That said, there's probably a little more performance to be had than the Pod-type filters alone can offer.

My first Arreche filter had a 32mm flange, too big for the 28mm Arreche bellmouth. When I "adapted" filter to Flash I, I used a Urethane hose that filled the gap. I deliberately left it about 4cm too long, allowing it to protruded into the filter for a good bit. Did it help? Not a clue, but that carb worked in the low end better, if anything, than the stock Keihin. The Forum member who has a Craftsman filter plugged onto a foot-long "snorkel" may be onto something...

[Lunytune]

The Gipper had Voodoo Economics and Wheelman has Voodoo carburation. Just what I thought.

My Banshee AF16E 50mm overbore ran its strongest without anything, just raw intake. Putting the airbox didn't seem to effect it much but the filter and cover did. But I've got to have some kind of filtering system. So what does that mean? Eh Kildare?