Modern Vespa

This is why I was considering doing holes . . . i figure if i want to reverse, i can epoxy some plugs in there.

That attitude is not helpful. In the interest of science, I am curious about the "price" of such magic power increase. I think it is a very legitimate question and has nothing to do with worrying about economy or giving up fun. The alternative is not to cruise everywhere at half throttle but to maintain your personal riding style and find out how the engine is converting the "free-er" lung into more power.

It's going to use more fuel. The ECU will throw more in to compensate for the extra air, how much I don't know. I'm ordering a pod filter tomorrow for £12 as I can't bring myself to hack the air box to bits.

In fairness to Mike, if he doesn't track fuel consumption, that's not something that he should be called out on--especially since he has no clue about the pre-mod consumption.

Energy is better spent on finding someone who currently tracks fuel economy and had made the mod after establishing a fuel consumption pattern.

I would volunteer for this, but i currently only have about 1250 miles on my GTV, and (if you click on the banner below), you will see that my MPG is steadily increasing with each fill up, and thus i really haven't established a baseline at this point.

Quite simply, the effect will be the same as if you could open your throttle a little bit further. You would get more performance by sucking in more air and corresponding more fuel. So fuel comsumption would go up in proportion to the performance.

I am retired, and only ride my Vespa for fun. I am not dependent on it every day, as I guess many others are. So I use a more expensive petrol and don't count the pennies. But I am delighted that my GTS costs half in fuel and oil what my PX used to cost me.

If you try my mod, I am sure you won't have any reason to back it out. I backed my mod out (by glueing in a shaped piece of plastic) when I thought it was too noisy. But then I found the noise was a blown exhaust bush. So the bit of plastic came out again.

Mike

Hi, someone can tell me the tube diameter, where you connect the cone filter for the GTS300???

I would like to try an RamAir or K&N filter

Thanks

Last edited by MiguelAngelo on Wed, 31 Mar 2010 17:39:39 +0000; edited 1 time

Now that I think about it, this mod simply gives you the potential to accelerate quicker and hold higher speeds by supplying air more efficiently when that air is needed.

Having said that, it seems to me that you could achieve the same pre-mod fuel efficiency by accelerating in the same manner as you did pre-mod, you just would not twist as far to achieve the same rate of acceleration, but you now have the potential to get more power if you do request it. Is that right?

Looking at a parts diagram, I found out that a complete air box is $77 (+ shipping). So, if you screw it up or don't like it, the cost to restore to stock is a paltry $77.

In other words, I'm cutting the fucker open tonight.

Giving some thought to the initial intake to the airbox and ways to open it up a bit while also smoothing direction in the direction of the filter I'm trying the following:

So you've obviously done it then? Come on give us your considered opinion Patrick!

I am sure this has made my scoot better. I made the mods in the least drastic way possible, so that I can reverse them easily with some flat plastic and epoxy. I'm too tight to fork out £50 on a new airbox.

I think that's the best approach so far.

How did it go then?

Family obligations.
Plan to do it after work tomorrow or Saturday morning.

AHA! The old NACA scoop shape... only backwards!

Here's the template. Not sure the NACA duct is really applicable here as we are going from a confined stream into a plenum but there's no room for a bell mouth.

I have an extra airbox so I figured I'd give it a try. Will try and swap it out this weekend on the GT.

Attached is the pattern, I'm sure others can cut it into the plastic better than I did.

GT-naca.pdf
Description:		Download
Filename:	GT-naca.pdf
Filesize:	8.61 KB

Thank you.
I will be using your design as well.
Hope to do it tomorrow then will report back.

I'm also eagerly awaiting your opinion, Stinky.

If I can get one or two more responses approving my mod, I shall advertise it on GTSOwners forum and the Aussie sites. And maybe nag PM Tuning - Paul said he would try it on a dyno when he had time.

Mike

I've cracked the book (Design and Simulation of Fourstroke Engines) open again to the section on noise reduction of the intake.

The two main features to be messed with are box volume and intake silencer pipe length. The bigger the box volume the more torque and lower the noise. The intake silencer pipe length has to be tuned to give a positive pressure in the box at the point when the intake is needs it.

On the GT/GTS there are two plenums connected by the U shaped opening (or short rubber pipe in the case of the GT). The more you open the wall containing the U the more it's turned into a single box of larger volume. The extreme of this would be to remove the wall and replace it with a metal bar to retain the filter. From there it's a mater of tuning the intake silencer pipe length. The intake silencer length does not have a simple longer or shorter is better and it's easy to end up with less power and more noise. A dyno or simulation is very much needed to get it right though 1/4 mile times could probably do it as well. The variator reacts to torque though so this would make the 1/4 mile trick less useful. A good tach may be a good indicator of torque though.

Just thinking out loud here.

Now I'm thinking out loud . . . what is to be said of those who have bypassed the box all together and have used a separate air filter?

That is like having a VERY large intake box at a constant 1atm.

Would that not be a case of less power/more noise? In other words, does the act of slapping an open filter on the intake actually result in a noisy result that ends up providing less power than a more nuanced approach with a finely tuned air box would provide, especially if you have something more than a straight pipe on the exhaust side?

I openly admit that I know nothing about the science of pushing air through an engine, but i find it very interesting, thus the questions.

Put it this way - I wouldn't do it. The OEM airbox design is obviously 'engineered' - for best all-round performance no doubt. Plus it helps with crankcase breathing - the two are not isolated. Tinkering with the airbox itself could well give some gains at some revs/loads - but removing it altogether seems bound to be disappointing.

The reason an air box can reduce power is that the intake is sucking in a 250cc volume of air from the airbox. If the box is too small then the pressure drops in the air box and the mass of air in that 250cc is reduced. Given the size of the earth's atmosphere removing 250cc from it is not going to change the pressure, but doing the same to a 1000cc air box will. To counter this the intake pipe to the air box can be tuned to provide a positive pulse at the start of the intake stroke. The air box will still drop below 1 atm at the end of the intake stroke but if it's done right you can get almost the same average pressure during intake.

I know this is off the subject but i thought maybe one of u guys would have a idea.. I after doing the airfilter mod.I keep hearing a popcorn noise only once in awhile it sounds like its in the back of the motor. Im not sure what it is its it kinda sounds like my rollers are loose or something.. if anyone has any idea.. let me know..

"Crankcase breathing", Jim? Aren't you thinking of two-strokes? But I presume you are talkiing about getting a back-pressure wave at the inlet valve at the right moment for goiod cylinder filling.

According to my reading, the size of the box should be 130% the size of the cylinder for optimum charging. Haven't measured the Vespa airbox volume.

I am a bit sceptical about the "design" of the Vespa airbox. Was it designed for a GT125, a GTS250 or a GTS300? I think basically the same box is used on all three, with a large range of engine capacities. So it can hardly be tuned to the engine.

Mike

Big thumbs up on this. A no-brainer.

I did the U to V mod as well as the NACA cutout on the intake. No pix needed--it looks just like everybody else's. If you attempt this, make sure to get all of the loose plastic bits out before you reassemble.

It's not significantly louder (or really any louder for that matter). However, it feels much more torquey and seems to pull more responsively, especially when you give it throttle at speed. I'm exaggerating when I say this, but I was cruising at about 25 mph or so and gunned it, and it felt like it wanted to pick up the front end . . .

It seems to pull through corners a little more confidently as well when you roll on the throttle.

Peppier. It's much peppier. It doesn't necessarily feel quicker off the line, but it seems to hit the meatier part of the torque curve a bit sooner.

I may give it a week or so and then hack at that divider a little bit more.

Mike, I can't thank you enough for taking the initial plunge. Big karma for you.

Many thanks for that positive report, Stinky. you made my day!

I can just hear the shears going snip-snip everywhere this weekend!

"Mike's Mod" will now become world famous!

I'm with you on this. They pulled it out of the parts bin and slapped it on because it fit in the frame.

Good for you, glad you're happy, and thanks for following through. TU for you.

The stanley knife comes out this weekend then

Well it's done. I've cut the U to a Vee but not quite as wide as Mikes. I've cut the vee as per Oops photo in the inlet.

Will go out for a blast later and see what it's like!

Been out for a short ride. I concur with Mike's findings. Seems quicker through the whole rev range. It just seems to rev more freely. Got up to 80 on a short bit of "A" road, pretty sure I've only managed 70ish on this bit of road before, although this wasn't exactly a scientific test.

While reviewing this thread, i noticed a second "nipple" at the bottom of maleezer's airbox cover.

Mine doesn't have the circled one, only the one toward the rear of the airbox.

Am I missing a nipple?

I did notice that my filter was a bit dirty toward the bottom by this opening . . .


mine doesn't have this nipple

The ECU can compensate for small changes but all of these mods are just leaning out the fuel mixture and undue stress on the ECU's ability to keep up.

To make all of these airbox /exhaust / head mods work someone would really need to make a power commander or a device where you could run it on the dyno and tune the map accordingly. Making a whole new map.

Without the ability to significantly change the fuel map these mods are kinda pointless from a performance standpoint.

More Air needs more fuel.

More fuel needs more air.

just my 2 cents.

However I would be more willing to try these mods on a 200 as you can mix and match jets to get a better mix, although a bigger carb would soon be in order to allow it to flow more air.

The mechanic at the local Vespa garage told me the closed loop ECU will compensate for the extra air. The system runs lean any way, but it won't run any "more" lean.

We will see how we get on.

hopefully the top mod will get a little more air in especially as its using a less gummed up part of the filter.



second mod Ive just extended the u cut downwards. keeping the factory standard way of clamping the filter but with a little more surface area. the way the rubber intake pipe is situated I dont think it will make a huge difference to air flow.





looks a little rough as my focus diy dremmel decided to burn itself out

also reset the ECU. testing tommorow.

There's been a fair bit of input on this on the GTS OC forum, with some slightly different takes, worth looking if your going to do this mod. CLICKY

I'm still mulling this over but I've done a little research and the GT125 & 200 do not use the same air filter assembly as the GTS250 or 300

GTS 250/300



GT200

As far as I can see the u shaped apperture in the 250/300 airbox was designed to have that central pipe in the 200 diagram fit into it. Thats why its the shape it is.

The fact there isnt a central pipe in the 250/300 airbox leads me to think that piaggio in thier wisdom left it out, presumably to allow a larger and smoother airflow into the box for the larger engines.

makes sense to me.

If piaggio had specifically designed the airbox for the 250/300 why cast it with the u shaped apperture? why not cast it with a hole lower and close the u up across the top so the filter is being held even more securely?.

because they used the 200 airbox design, just took the central pipe out and it worked well enough.

Its a massive global conspiracy to rob us of power!

Its just like you guys pretending you landed on the moon or Elvis didnt shoot JFK .

I can't speak to the GT125 but the ONLY difference between the GT200 and GTS250 airbox is the length of the tube from the carb/throttle body and the addition of a rubber tube filling the U space on the GT200.

The parts diagrams do not reflect what is actually delivered on the bikes.

I was referring to the pic earlier from the GT200 as the diagram doesnt seem to show internal detail.





thanks oopsclunkthud