Chris Im going it go get 1 more drink and re-read that and see if I can understand it better [img]i/expressions/face-icon-small-wink.gif[/img] Thanks for the confusing info LOL

 

The compressibility of the air becomes a factor when the air enters the intake port around the intake valve. The intake port/valve forms a constriction, much like the throat of a nozzle. Because air is compressible, it can only be pushed through a constriction so fast. Regardless of how much pressure you apply, the maximum velocity possible through the throat of a nozzle is a velocity equal to the speed of sound (Mach=1.0).

For convenience, the ratio of the typical velocity to the intake sonic velocity is called the inlet Mach index. From the science of fluid mechanics we know that the controlling velocity in a compressible flow system is usually the intake valve opening. For a given cylinder and valve design, the inlet Mach index is proportional to the piston speed. This seems reasonable, that the fuel/air charge flows in faster when the piston moves down faster. Of course, at some point the constriction of the valve opening starts to limit this. When the inlet Mach index exceeds 0.5 (intake velocity equal to half the speed of sound), the volumetric efficiency falls rapidly with increasing speed. Therefore, engines typically are designed such that the inlet Mach index does not exceed 0.5 at the highest rated speed.

The design of the intake manifold also affects the amount of momentum built up in the flow of the fuel/air charge. The momentum of the fuel/air charge is the sum of the effect of standing waves built up from previous intake strokes (remember any tube will have a resonant frequency, just like you hear blowing over the top of a coke bottle) and the effect of the transient wave caused by the current intake stroke. While the standing waves contribute to the overall effect, there are no sudden changes in the volumetric efficiency when the RPM of the engine is an even multiple of the natural frequency of the intake manifold.
Long, skinny intake manifold pipes give high volumetric efficiencies at low piston speeds because high momentum (lots of velocity) is built up in the pipe during the intake stroke. At high piston speeds, the small diameter of the pipe causes a constriction and the volumetric efficiency falls.

Basically, it comes down to the intake manifold should be designed according to the engine requirements. If you need high torque at slow piston speeds, use long, skinny intake pipes. For high torque at intermediate piston speeds, use long, fat intake pipes. For high torque over a wide range of piston speeds (i.e. a flat torque curve), use shorter intake pipes.

Depending on the engine configuration, the testing of the new RW intake may have marginal gains and thus be misleading. I hope that the BOM can test the intake on a low hp motor and on a high hp motor. Good to know where its application is best suited.

 

hey that is very cool reading indeed, and on the science level. my very first sentence in my last post should be corrected - moving air thru an intake is not "identical" to moving air thru any other fluid power system, as the reciprocation of the piston and the resonance of the intake tube you mention with the standing waves from previous cycles being something that would not necesarily be present in other industrial systems that work much differently than an engine's intake.


so given all this info, and recognizing that an ideal intake tube would be designed for a specific motor and not just any general application, I wonder if RWR used this kind of approach in its design, and if so then what specific engine setup is it designed for? Did they make it to work best with their stage 3 head, or a simple bolt on for a stock motor? Yes it would be cool to see dyno charts for both.


I remember reading an article a long long time ago about an interview with Bomb about the DS's airbox design, and the conversation included stuff along the lines of what oldmanracing is talking about. after a few years Bomb finally realized that their intake boot was oscilating too much at higher rpm's, especially on modded bikes, and finally came out with the heavier duty boot with more rigid ribs for stiffness and support. I have this new improved boot on my old '01. Given what oldmanracing is talking about, that high rpm oscilation of the old weak and ripply intake boot was doing more harm than I thought to the flow, aside from the obvious variance of throat diameter during oscilation.

 

I have some testing done.

The bike tested was my 725 trail bike. 12.5 to 1,trail port and valves,A1A2 cams,inframe drag pipe and TM48.

03 stock tube 6000rpm 62hp 55tq 7550rpm 67.98hp 47tq 8750rpm 58hp 36tq
RW air tube 56hp 49tq 64.35hp 44tq 52hp 32tq
Dir air filter 58hp 53tq 69.99hp 49tq 66hp 40tq

The RW's new tube was disapointing. It is a very nice piece, but the hp's not there. The direct mount is still being under rated. You see it makes more peak hp and holds it out higher longer. The stock 03 tube give a hp and tq advantage up to 6750rpm the the direct mount takes over.

The odd thing about the whole test was I could get a good afr curve on both the direct and stock filters. The RW's was crazy. It would go rich ,lean ,rich by about +3 whole points. ?????


I plan on doing the same test again on a stage 2 with a stock head ,TM45 ,trail cams old style 11.5 to 1 and stock mufflerw/ end cap .

I think the smaller valves will like the tubes better.We will see if this helps the RW tube any.

J Ross

 

Nice write up Jeff. Thanks for the info.

 

Thanks for sharing the data. I thought that just maybe the RW would resonate well at high rpm and be a good fit for bigger motors. Resonance and inertia have to be in balance.
With a motor with smaller valves, the reversion may be less and allow the inertia in the RW pipe to stay constant. Main reason that fuel ratio readings were not also constant.

With data we are not trying to find fault with RW intake pipe, but learning how, where and other needed information to make any possible improvements.

Thanks BOM and look forward to your next test session.
OMR

 

thanks jeff!

 

The good thing about this data is that a person can look at the numbers and make a determination about what type of filter setup they need based on the RPMs they are pulling. It would seem that its no longer true that the Direct Mount filter is giving up horsepower in all RPM ranges when comparted to the stock air tube. It actually will be better for consistantly high RPM motors to use the direct mount filter over the stock air tube at RPMs over 7550. However one must also consider the power curve. On most stock DS's, the power curve has already topped out at 7550. However, there are ways to broaden that curve [img]i/expressions/face-icon-small-wink.gif[/img]

 

Something else to think about. . .

What altitude were the tests done and what altitude do you ride at.

I have no data to support it but, if snorkles run out of air at the higher RPM's at lower elevations, they are going to be WAY starved at higher altitude where the molecules aren't so close together.

I'm still a big fan of the direct mount for the dunes. I can see the snorkle for the trail though.

 

So what you are saying is that the snorkle can pack in the molecules better than the direct mount filter - Good at elevations where the molecules arent so packed?