The Vespa GTS has been around for the best part of a year now and it has already established itself as the leading seller in its 126cc-350cc market sector, outselling its nearest rival by around three times. I reported in TAG 47 on the dilemma I faced when the engine on mine failed totally, but we have moved on from then and no further engine faults have surfaced at this point.
The use of the scooter during the early winter months started to take its toll on the silencer, as can be seen from the picture of it on this page. So it couldn't have been timed better when I was contacted by Paul Melici of PM Tuning Racing Products. The GTS has quite a lively performance anyway, but there is always someone who wants that little bit extra power and PM Tuning were looking to develop aftermarket goodies. So I agreed to loan them my bike over the remaining winter period so they could do just that.

A sound investment?

If you are looking for that little extra, then one of the first things that gets changed is the exhaust. Given PM’s reputation in this area, it came as no surprise that this is where they would start. The result of this was the development of the PM68 X-Tech performance pipe kit. The eagle-eyed amongst you may have already seen it mentioned and pictured in the previous TAG, but now that we have one ‘in the flesh’ so to speak, we can tell you a little bit about it.
The PM68 isn't the first performance pipe available for the GTS, but as far as we are aware (unless someone tells us any different), it is the first one to be solely developed specifically for use on this Quaser-engined Vespa (unlike some others that have been altered and adapted from the existing Granturismo 200cc version). This involved tests of various prototypes and different setups in order to provide the optimum system for everyday use. One area that caused problems was the soundproof wadding which quickly broke down under the intense temperatures flowing through the pipe. This was solved via a combination of a stainless steel wool, ceramic matting, and a unique ‘spiral’ design being incorporated into the muffler, which increased the gas velocity and sound absorbing effect of the spent gas.

The graph seen here charts the performance of the original exhaust compared to the new PM68. No other alterations have been made to the standard Piaggio engine setup. It shows quite clearly the improvements and extra BHP gained across the power band by fitting just the pipe alone.

Smooth operator TFI

Once the exhaust setup had been sorted, PM then moved onto the fuel injection system. The standard fuel injection setup errs on the lean side due to latest strict emissions legislation, leaving lots of room for improvement. To this end PM have developed with the help of former Dynojet founder Mark Dobeck their own ‘plug-n-play’ fuel injection module . The base unit is not exclusively used on the GTS, but as they had the bike at their disposal, they took the opportunity to develop a variety of settings that can be used to re map the injection system to suit individuals needs. The TFI unit is available pre programmed to optimize the PM68 fueling requirements with easily adjustable function should a different setup be required (E.G. using it with the standard exhaust system or other after market tuning parts) and can also be done at the side of the road if needed. Full instructions are available from PM’s website www.pmtuning.co.uk if required.
The unit can also be disengaged at an instance and the original Piaggio setting reinstated by simply swapping the plugs over. It couldn't’t be simpler. And because the settings can be remapped quickly to suit different engine sizes and specifications, it is not a hard job to swap the unit from one bike to another, whether it is the same make of model or not. In other words, you can take the unit with you when you change your ride.

What's it all about

It adjusts your fuel requirements without tricking sensors or modifying the factory settings, which can limit the ECU’s effectiveness at compensating for changing atmospheric conditions.
There are several ways to modify your stock electronic fuel injection, but come at a price in complexity of use, operating limitations, and cost.

Here are some examples:

OE Computer Re-Calibrations change the factory engine management computer’s PROM data to recalibrate fuel and spark delivery. The recalibration may or may not be as good as stock - much less better, but whatever you get, once you drive away with new PROM data, you’re stuck.

Sensor Manipulation Boxes basically lie to your bike’s computer about engine sensor values in a way that influences it to change fuel delivery in a pre-planned way. Unfortunately, this expensive scheme breaks down when your engine encounters temperature and altitude conditions that require the enrichment you were depending on to enhance performance and ridability.

Auxiliary “Interceptor” Computers monitor and extend or interrupt signals to and from an OE computer in order to influence or modify injection or spark timing. They are expensive: and may require modifications to your EFI harness. They are typically not trivial to program properly, and have the potential to initiate a “fight” with your stock computer by triggering hostile active counter measures.

TFI is different in its method of adjustment by dealing directly with the fuel injection signal after it leaves the factory ECU. It adjusts your fuel requirements without tricking sensors or modifying the factory settings, which can limit the ECU’s effectiveness at compensating for changing altitude or weather conditions

Why PM's TFI

The Fuel Injection Module Anyone Can Use And Anyone Can Afford

The TFI was designed and developed to provide an affordable solution to increasing the “fun factor” or ridability of your scooter. With today’s pollution regulations, many if not all of today’s scooters are setup extremely lean to meet these regulations. Unfortunately they are not always the best setup for performance or ridability. Therefore, adjusting the air fuel ration on these machines is essential to achieve optimum performance and ridability.

The TFI is not an interceptor and it does not alter factory air/fuel computer tales or pulse-width calculations. It taps into the injector-driver circuitry of a stock scooter's EFI and selectively appends extra pulse-width voltage keeping injector's open the exact incremental time required for precise fuel enrichment. The scooter's on-board computer and wiring harness remain intact, with no wires needing to be cut. The stock air-fuel Map remains intact, as does its correlation to the engine’s volumetric efficiency (breathing) curve.The TFi is simple to adjust. Four adjustable dials (pods) and circuitry in the control unit (1) define the breakpoint between cruise and “main-jet” RPM, (2) the length of time and magnitude for auxiliary transitional enrichment, (3) and the incremental enrichment in tenths of milliseconds for the three operating ranges.

Key benefits

• Increases Horsepower
• Improves drivability
• Eliminates the need for expensive Factory Re-Mapping
• Improves Throttle Response
• Eliminates Backfire
• Completely Adjustable/Tuneable
• Can be re programmed to suit alternative machines with same bosch type connectors

The dyno graph shows the difference achieved between the standard injection system (green line) and PM’s remapped system (red line). Both of these dyno runs were done with the new PM68 already fitted, so the BHP increase shown is ‘extra’ to the increase achieved from fitting the pipe alone. (Note: During the development period of these performance parts PM Tuning purchased a new Dyna-Pro eddy brake dyno unit, hence the different dyno presentation styles).

Doing the sums

So what is all of this going to achieve? Well, first let me reiterate that the information shown here has been done without making any other tuning changes to the standard engine. This particular setup is aimed at smoothing out the power curve and providing an increase in acceleration across the power band, whilst retaining full engine reliability. The Piaggio fitted rev limiter still cuts in at the same point and the top speed remains much the same – you just get there much quicker. In a future issue, we hope to compare the acceleration of a completely standard GTS against the improvements made by this one, so watch this space as they say.
The PM 68 is priced at £249.95+vat and the TFI unit costs £179.50+vat Both available from PM Tuning or their appointed agents.

Customers' feed back about the TFI unit

From Alan in New York USA - " took the fuel module off, The scoot felt dead without it. What a difference in power that thing makes, it's like day & night. You'll never know it till you ride with it for a week & then take it off"

Polini variator test in Scootering magazine

Download a pdf of the GTS 250 variator test and check out how well the Polini variator did.

Scootering Magazine GTS 250 variator test

The red line represents the PM68 with TFI injection module TFID001 dyno optimized for correct air/fuel ratio configured on a wind drag simulation cycle based on PM Tuning's new Dyna Pro eddy brake dyno.

Green line setting as above represents stock GTS250ie with PM68 X-tech power pipe