Air box help

[TerraRoot]

Hi all been lurking here for a couple of weeks, i love the sight of old and totally unsuitable bikes racing
I'm building up my first bike (although it's had three frame and countless engine swaps, it's still the same bike ) got some spare time on my hands these days and though i'd give it the attention i never did.






Don't see any rxs's racing over there so i suspect that it's wasn't imported?

Back to the point anyway what type of airbox should i try and fit? as you can see the frame is in the way. scooter air pod thing? what do you guys run on your bikes?

[SHELRACING]

You don't need an air filter, not for bucket racing anyway. I see you have a micron pipe. Just re-jet the carb to suit your pipe without the filter.

[koba]

If you really wanted to build one it could be made to fit, the volume is what it is about more than the shape. A carefully designed airbox should give a performance advantage but following SHELRACINGS advice with or without a wee filter is sooo much easier!

[TerraRoot]

Thanks for the point about the volume rather then the shape being important koda, think i'l make it out of something interesting.
Theres no bucket racing in Ireland shelracing im all by myself, have no fear about the jetting, i live quite close to a dyno and will have it done right.

[F5 Dave]

if you are making one easiest option is some polystyrene & water soluble plaster over that (check that can remove/insert into frame). Then cover with fibreglass, sand & paint black. Then cut a hole to fit a suitable rubber piece (from an old GS750 airbox or whatever fits the carb) so it just fits in. Then cut a hole at the top & dig out the plaster & styrene. Can fit a filter in this hole if plan it right & out of sight.

[SS90]

You don't need an air filter, not for bucket racing anyway. I see you have a micron pipe. Just re-jet the carb to suit your pipe without the filter.

Personally, I agree!

What are you expecting/wanting to gain from having an air box?

Is it because you want to run an air cleaner, or are you hoping to make a little more power with the inclusion of an air box.?

In something like a small capacity two stroke (used for road racing), I don't really see the need for an air filter ( you will certainly make more power with out one), and personally, (Motocross bikes, of course need a filter) I can't imagine an RX125 ever making enough speed for any sort of "ram air" system to be of use!

[TerraRoot]

Every little helps when your engine is tiny, I am hoping that a nice cold supply of calm air would be better for the engine then turbulent air straight off the cooling fins. have been looking at gokart box's for inspiration
like this one
http://cgi.ebay.ie/GO-KART-2009-AIRB...3286.m63.l1177

[SS90]

Every little helps when your engine is tiny, I am hoping that a nice cold supply of calm air would be better for the engine then turbulent air straight off the cooling fins. have been looking at gokart box's for inspiration
like this one
http://cgi.ebay.ie/GO-KART-2009-AIRB...3286.m63.l1177

OK, cool!

It will certainly make your overall engine noise lower (reduced induction noise), but I am not 100% certain you will notice a power gain compared to an open carb (with a bell mouth)

[koba]

Every little helps when your engine is tiny, I am hoping that a nice cold supply of calm air would be better for the engine then turbulent air straight off the cooling fins. have been looking at gokart box's for inspiration
like this one
http://cgi.ebay.ie/GO-KART-2009-AIRB...3286.m63.l1177

If you build it right is should give you some gain, if you build it wrong it may take away more than you gain.

There is a fair bit of info out there about it, heres a good start:

http://motorcycleinfo.calsci.com/Airboxes.html

[TerraRoot]

yeh maths! once i get some feedback on the size of the kart airbox (if it fits!) I will go to the dyno and get a base map for the new 171 engine will do two runs with/without the box, at the rate i work this will probably take about a month
I have seen in the past airbox's with reflection plates to refect the pulse back into the engine increaseing the VE, I don't see any mention of that in the above equations? is this idea bollox?


Gotta shorten forks and sort proper tyres soon

[SS90]

yeh maths! once i get some feedback on the size of the kart airbox (if it fits!) I will go to the dyno and get a base map for the new 171 engine will do two runs with/without the box, at the rate i work this will probably take about a month
I have seen in the past airbox's with reflection plates to refect the pulse back into the engine increaseing the VE, I don't see any mention of that in the above equations? is this idea bollox?

I am of the opinion that the Kart air boxes are designed to operate efficiently at more or less one RPM range (the air box you showed the link to is for Yamaha KT100 engines, as as you are probably aware they are a "fixed drive" (no gear box), and operate at around about 14,000 RPM (or so) for most of the race (OK, not the whole time, but it doesn't vary much from that)

Whereas, you have a geared engine, which the requirements for an air box change dramatically, I personally think that if you are able to test run an air box from a KT100 on the dyno, it will give no advantage. Which MAY stop you developing the idea further (as it may give the (FALSE) impression that
the idea does not work, when it's not the concept at fault, it's the shape of the "test subject" (It's more than likely unsuitable for the purpose!)

Perhaps it would be prudent to find out what sort of airbox shape would be suitable...... BEFORE testing the Kart one (as if you find the shape is totally incorrect for the purpose, it would be (sadly) a waste of good time (maybe)

I commend you on your thinking, but my experience is that the only (real) advantage with having an air box on a two stroke is when you have enough speed to generate a "pressurised" air box, (unfortunately this bike won't be achieving those speeds (short of Zues's lightening bolts) whereas a 4 stroke

engine (with a CV carb) really requires a "still air box" to carburate correctly!

Please don't think I am knocking your idea, I really commend it, it's just that I foresee I lot of work for questionable gains!

[xwhatsit]

I commend you on your thinking, but my experience is that the only (real) advantage with having an air box on a two stroke is when you have enough speed to generate a "pressurised" air box, (unfortunately this bike won't be achieving those speeds (short of Zues's lightening bolts) whereas a 4 stroke

engine (with a CV carb) really requires a "still air box" to carburate correctly!

Please don't think I am knocking your idea, I really commend it, it's just that I foresee I lot of work for questionable gains!

That's interesting. So what's your opinion on a small 4 stroke with a slide carb?

[SS90]

That's interesting. So what's your opinion on a small 4 stroke with a slide carb?

Well, personally, I am of the opinion that because of the speeds you are going on a bucket, you have less problems with "bad air", and with something like a conventional slide carb you are unlikely to need an air box.

If it was something like an in line 4 600cc (carbed) engine, ideally it will take it's air from above the radiator (or thereabouts, where the headlight would be is the highest pressure point), and in this scenario, sometimes a badly designed front guard can have an effect, and often an "air dam" is required.

But, as I said, a bucket is (in my opinion) highly unlikely to be hindered by these effects (due to it's comparitively low speed,and in most cases lack of fairing/pressurised air box) and as such, unless you have a CV carb, an air box is unlikely to be of any benefit.

[F5 Dave]

I am of the opinion that the Kart air boxes are designed to operate efficiently at more or less one RPM range (the air box you showed the link to is for Yamaha KT100 engines, as as you are probably aware they are a "fixed drive" (no gear box), and operate at around about 14,000 RPM (or so) for most of the race (OK, not the whole time, but it doesn't vary much from that). . .

um, you may wish to rescind that comment.

Kart engines sound like they scream, in some cases they really do as they have no gears they only get faster by revving out. But once you have set the final gear ratio they have to operate over a far wider rev range than a conventional engine. For if they had to operate at 14,000 all the time they would have to have a constant speed the whole track.

Oops.

[SS90]

um, you may wish to rescind that comment.

Kart engines sound like they scream, in some cases they really do as they have no gears they only get faster by revving out. But once you have set the final gear ratio they have to operate over a far wider rev range than a conventional engine. For if they had to operate at 14,000 all the time they would have to have a constant speed the whole track.

Oops.

Yea, I guess you would be more accurate (It was perhaps incorrect for me to state that they ran at 14,000 most of the time, but they do run at those speeds for much longer than a geared engine, and I don't think a fast driver racing a KT100 would dip much below 11,000 RPM, even then not for long), and as such, they do have a different set of requirements for intake volumes etc, compared to a two stroke bucket!

I may well be incorrect here, but my understanding for why KT100's run air boxes now is because of noise reduction requirements (opposed to power gains), and any gains made in power by "better air boxes/modifications" is returning back to the power had before with no air box.

But, the point I was trying to make was that the airbox shown is suited to operation at very high RPM, I believe making it unsuitable for the intended purpose