Boxerworks.com

I have seen a lot of "modified" boxers with filter pods mounted directly onto the carburetor intakes and doing away with the stock air box and intake tubes. While this looks neat, is there really any benefit to performance? I have learned here that there is nearly always an effect to the way the motor performs when changing systems from stock. Sometimes good, sometimes not so good. Is this just for looks or does removing the intake tubes have a desired result?

I've heard a number of things:

- the stock air and filter system does not really limit air getting to the motor, so if that's the reason for using some other system, it's probably not really helping.
- intake tuning is quite an art. The idea behind the intake track is to ensure there's no standing waves of air that sit right in front of the intake valves. If that happens, then the engine can be starved for air. By slapping a set of pods on, vastly changing the length of the tube feeding air, who knows what's happening.
- I would be concerned more about the filtration capability of these alternate systems. Some of these systems require oiling and the quality of filtration changes over time. The last thing I would want to do is allow more dirt and larger dirt into the engine.
- there is something positive about getting cooler air to the engine. In the late '70s, the factory drilled the rear of the clamshell airboxes. This allowed somewhat cooler air to reach the filter. In reality, it just made for more engine noise.
- I suspect there would be some general affect, maybe changing some range of performance or torque, such as mid-range. If one were really pushing the bike and monitoring the performance, say on a dyno, you might be able to quantify any differences. For the typical rider, I can't imagine the change would be that noticeable.

Kurt in S.A.

Those things are for looks alone, as they don't filter as well. Those things should have "insecure" written all over.

I have two R75/5 engines, one is in a /2 frame and I have run pod foam and K&N filters on it.



The other is a stock /5 with stock airbox. When I swap the carbs from the conversion on to the /5 I have to run a richer mixture. That is because the stock airbox allows more airflow than either the foam pod filters in the pic above or the same size K&Ns.

Duane Ausherman wrote:Those things are for looks alone, as they don't filter as well. Those things should have "insecure" written all over.

there must be some advantage to running velocity stacks, race bikes use them. I'm just curious. I like my drilled air box and the clean lines associated with it.

Race bikes are all about decreasing the weight and running at WOT all the time. Also race bikes are rebuilt on a regular basis, so they don't really care about extreme filtration during a race. I ran across this link that discusses various filter systems:

http://forums.nicoclub.com/debunking-th ... 80100.html

It offers a number of discussion points and tradeoffs.

Kurt in S.A.

Duane Ausherman wrote:Those things are for looks alone, as they don't filter as well. Those things should have "insecure" written all over.

Not true. Just not true.
"Those things" will not be for "looks alone" –
The system is proven effective and will be designed efficiently to produce certain results.
To say otherwise presumes a narrowly defined and personal agenda to the exclusion of all other.

Even as the conclusion might prove to be valid ...

Back on task ...
Individual filters and short intake tracts
will be THE optimum if traditional design for performance –
but on a track bike.
This will be a bike that runs almost exclusively at higher RPMs,
without thought of being able to idle at a stop-light
or any consideration of acceptable performance in many of the situations
that real-world road traffic might present.
And then, all of this with the tacit understanding that a track engine will likely be rebuilt every 250-500 miles.

Will these circumstances be appropriate to your (daily) riding agenda ??

A truism to know about engines:
Any engine design will have only so much to give.
This "given" may be redefined within certain parameters.
However, what you might do with what is available can only be a barter -
If something might be gained, something will certainly be lost.
There will be no such thing as a "free lunch".

My "narrow" agenda is to use and ride it as BMW intended. Not for performance at some particular rpm, but over a range. I also consider long term reliability to be a given.

Everyone who doesn't want clean air in the engine and is willing to do top end rebuilds often, please respond here.

Duane Ausherman wrote:My "narrow" agenda is to use and ride it as BMW intended. Not for performance at some particular rpm, but over a range. I also consider long term reliability to be a given.

Everyone who doesn't want clean air in the engine and is willing to do top end rebuilds often, please respond here.

Any agenda or "intention" may well be personally defined – and appropriate results can be attained.
No one agenda or "intention", whether it be the factory directive – or your directive,
can be accepted as the "one and only", eh ?

To the point –
I do not believe that we must all subscribe to any particular agenda ...
My reason to respond will be to counter a popular misconception
which will have owner / builders seeking to re-define factory performance characteristics
without any understanding of the necessary performance "costs" that must be paid
in order to achieve those re-defined results.
A misconception that patently ignores the reality of mucking about within a finite system !
I will not propose to say, "Everyone who doesn't want ...'
But I will advise, "Understand all of what you may be asking to have"

Chuey responds: My Cafe Racer has K&N tapered pods. I don't really think much of K&N filters but some respected people think they're great. I've made my Cafe Racer first and foremost to look a certain way that I thought would be "cool looking". Fact is, it is by a pretty good margin, the most fun bike I've ridden and I attribute that to the riding position and some other things like that but not specifically to the engine.

That "cool look" for me, dictated an underseat exhaust which may have cost me some horsies and the way I modified the battery position and braced the frame dictated pod air filters. I don't like the idea of dumping the crankcase fumes into the atmosphere so I made a system that (I think) replicates the stock system of running it back through the carbs. I suspect that at some point, the suction of intake helps evacuate the crankcase pressure.

The engine in my Cafe Racer is basically stock, other than the modifications mentioned above. It runs just great! I like that it is not temperamental. I'm hoping I'm not giving it a diet of dust.

The next modification to that bike will be to add a bicycle part to the throttle cable. Mountain bike brakes called V-brakes require a little more cable be pulled. For people who, for one reason or another, use a non V-brake lever with a V-brake, there is an eccentric pulley that increases the pull just enough to make the brakes work well.

A few weeks ago, I rode a really twisty road and feel the ride was limited by the amount of rotation the throttle required. What I plan to do is use a "Travel Agent" (bicycle parts need clever names, don't they?) to enable me to increase the throttle opening to a level that will work better in the twisty sections. It will be tricky to adapt the part to my one into two throttle cable. Of course, it will only be the single cable I'll attach it to. I'll post how that turns out.

I hope I didn't ramble too much. I did digress but not with the intention of hijacking this thread.

Chuey