Airbox setup

[wolfgangh]

I am in the final stage of completing the airbox on my YZR500 Antena 3 replica. It will consist of 2 elements on each side of the engine with a connecting bridge under the radiator. The complete airbox looks like an "H" from above. Air intake will be through ducts in the front of the side panels (like the original YZR design). Carbs will sit inside the airbox.

Now I am unsure how to deal with the potential Ram-Air effect at high speed. It will potentially pressurize the airbox and I assume this would cause the mixture to become lean.

How can I keep the AFR constant at a given throttle position, independant from speed? Some guys advise me that I only had to connect the tank vent to the airbox to have same pressure in the tank and inside the airbox which would automatically adjust float level accordingly. This would not allow for a breather vent on the tank though and I would have to deal with petrol spitting into the airbox at breaking. YAMAHA solved that with a catch box inside the frame and a solenoid in the hose between tank and airbox which was activated by the front brake switch and shut the hose during braking, stopping petrol flow into the airbox.

To make it easier, I would prefer to get rid of any pressurizing at all. What if I added additional "outlets" e.g. in the connecting bridge below the radiator, to let the excessive air escape and keep airbox pressure close to ambient pressure?

Those of you with any experience with pressurized airboxes please share your ideas with me.

best regards


Wolfgang

[Jeff B]

I thought about doing something like this a number of years ago. There didn't seem to be anyone with ram air 2-stroke experience back then. Unfortunately you may be on your own with this one.

[Gary Papesh]

I don't have any direct experience in pressurizing an RZ 500, however I am familiar with "blow through" supercharging. I used to have a car with a Paxton supercharger on it. It is basically a belt driven turbocharger. The carb was located in an airbox that had a linkage shaft to run the throttle.
Since the float bowl was under the same amount of pressure as the air inlet, the venturi effect still works, but at a higher pressure. You would have to fully enclose all 4 carbs to a common plenum that the ram air is ducted into. I don't know what the pressure differential would be ,between atmospheric pressure, and pressure in the plenum at top speed. However I suspect that if you were to measure this, the pressure of fuel flowing at atmospheric pressure, would be enough to feed the engine. Since the venturi effect in the bore lowers the pressure in the fuel bowl and allows fuel to flow through the jet and into the airstream, the fuel bowl pressure is lower and should allow the fuel to gravity feed into the bowl , even though the air box pressure is higher than atmospheric.
I hope this helps.
Sincerely, Gary P.

[vma500]

can a fuelpump sort this out ?
if i remember correctly some TZs had fuelpumps !

extra pressure was only about 50 mbar

[78]

Hi Wolfgangh ,
when you presurize the carbs inside the airbox it forces the fuel through the jets too . The ram effect only starts when you have a positive air pressure inside the air box , this starts at 160 km/h . The fuel tank needs to be presurized too to feed fuel to the carbs . The guys normally install richer pilot jets for race use .
The solenoid will open when the front brake is applied to dump excess air pressure when you shuttoff at the end of a long straight for example . This is were the bigger pilot jets are needed .
There is very little info on ram air that i have found , the rgv 23 came with " ram " air .

I am installing a kit onto my nsr250 , i will report back once i have more info .

[wolfgangh]

Hi Gray & 78,
so this would imply that I leave the carb overflow hoses inside the airbox and would not be able then to recognize flloding carbs?

I would reálly prefer to avoid that pressurization and have a "normal" carb setup, so do you think it would help if I just design a second opening (or a flap) outside the plenum where any extra air forced through the Front ducts could escape and ensure "normal" = not pressurized conditions inside the plenum?

What exactly is the definition of the plenum? is this everything after the airfilters?

[Jeff B]

[quote="wolfgangh"]I would reálly prefer to avoid that pressurization and have a "normal" carb setup, so do you think it would help if I just design a second opening (or a flap) outside the plenum where any extra air forced through the Front ducts could escape and ensure "normal" = not pressurized conditions inside the plenum?

quote]

What's your goal with this setup? The YZR500 look? Feed the engine cool air?

[Gary Papesh]

Hi Wolfgangh , what I meant when I said "plenum", is an area inside the airbox which is fed from both ram air ducts. In your case it would be after the filters. I don't know what the pressure drop across the filters is. Did the YZR use filters? Would there be any ram air effect left, after going through the filter? I suppose you could run the overflow tubes out of the airbox, and into the ram air ducts. That way they would see the same pressure as the plenum. And when you see fuel pouring out of the duct, you will know when your float valve is stuck
One thing is certain , you will have quite a job building an air box which will include ram air ducts, and some sort of enclosure around the carbs, which will be able to have the throttle cables,and choke cable, and fuel line go in , allow the overflow tube to go out, and seal up the whole thing so it does not leak. And while you are at it, make it easy to take apart and put back together again .
I have seen your work , I'm sure you are up to the challenge.
But is the result worth the effort?
Sincerely, Gary P.

[wolfgangh]

Jeff B,
your are correct with your assumption. It is for getting cool air, having still air for the carbs and the genuine YZR500 look. I am a maniac.

Gary,
the airbox is almost completed, it took me 1 year in total to work out the design and build the plugs. It includes modified mounting of my PWK28s, they have a flange welded on like RG500 carbs so that I have easy access to the mounting screws from the outside (after removing a lid on the airbox) instead of fiddling with the clamps on the rubberboots which requires removal of the tank.
I have no idea yet if it will work, but in 1-2 months I will see.
There is almost no source of information available on that subject, so it will be trial and error.

[Jeff B]

Back when I was looking into ram air, I thought about using an altitude compensating device of some sort. For example, Ski-doo had a system about 10 years ago called DPM. I had a snowmobile with it. I'm not sure how it worked, but the vm mikuni's vent tubes were plugged into a manifold. My thought was to jet the bike in on a dyno with full ram air boost so with less speed (and less air pressure) the altitude compensation device would automatically adjust the fuelling. Just like if you were driving up a mountain.

Disclaimer: This was my theory, but I have no idea if it will actually work!!!!

[Jeff B]

Here's another device I was looking into:

http://www.holtzmaneng.com/

[wolfgangh]

Since pictures tell more than thousands words, here is how my airbox will look like:

[Jeff B]

Wow! That's cool!

[Gary Papesh]

Yes Wolfgangh , you ARE a maniac!!! Keeep up the excellent work

[Raresed84]

wow wow...great job...it's awsome what u did...