Modern Vespa

Hello i recently bought a 300cc vespa gts and would like to modify it. However during my research, I noticed that there are 2 kits:
_Polini cylinder 294cc without cylinder head (it works with the original cylinder head).
_Malossi 282cc cylinder with v4 Head cylinder head.

I ask myself the questions, is it possible to buy the Polini 294cc cylinder and then put the malossi v4 head cylinder head on it, is it compatible?

Because a friend advised me to take the Malossi 282cc kit which for him will be more powerful than the Polini 294cc because the Malossi kit has the v4 head cylinder head. So I would like to do a mix of the two

thank you for those who can help me.

welcome to the forum.

yes the malossi head can be used with the polini cylinder.

the malossi head itself is the biggest power upgrade. cc and compression are minor by comparison. the malossi cylinder is only 4cc upgrade. polini 16cc

there is a thread on af-1 racing forums . few years back the owner wanted to turbocharge his 300. he started with the malossi head on stock 300 cylinder. with other tweaks to fuel/exhaust/airbox he got 6 whp .

polini kit alone 2hp from the one dyno i could find. keep in mind the 16cc is not all of the gain.
malossi kit alone typically gain 6hp

baseline 15.9hp post #147 22.3hp
https://www.apriliaforum.com/forums/showthread.php?243793-Project-Vespa-300-SUPER-(should-end-up-living-up-that-name)/page10

Last edited by jerryd on Fri, 13 Aug 2021 23:20:45 +0000; edited 1 time

What year gts do you have?

The head changed significantly with the hpe gts. Although the Malossi head is definitely better than the older GTS head I'm not sure that it's better the HPE GTS head ...

I think he has a nexus 300. early bike.

head gasket is same part number for hpe and early 300.

not only that I found the hpe head and malossi head use same size valves. 22.2/25mm.

interesting as scooter west did a dyno test.

stock 300
hpe 300
stock with malossi kit.

they produced hp in that order. the hpe gained x over the stock 300. the malosasi kitted gained another x over the hpe.

important about that video (for those too lazy to watch it fully), the malossi tested was "FULL" malossi meaning not only did it replace the cylinder + head but also had several other changes such as forcemaster + o2 emulator + exhaust + variator

and as Robot says: it would be interesting to go 'full malossi on an hpe'

They say it's a Vespa 300 GTS, not the Nexus 300 in their profile.

Thank you all for your answers. I apologize for the difference when I answer you I am from French polynesia and there is jet lag. Other than her, you had given me a lot of information. I think I looked at this configuration suddenly:
_Culasse malossi
_cylinder polini 294cc
_full variation malossi
_malossi clutch
_force master 2
_ exhaust akrapovic or malossi
_camshaft polini

There you go, I'm not an expert, but if you have any advice or suggestions for parts, such as the injector, I don't know if it will have to be changed or my fuel pump , then all your suggestions will be welcome.

Between Malossi and Polini, the experiences I have seen when people mix them are not good. There was a time when Polini had a big bore kit for the LC Leader engine, and a lot of people bought them, because they were cheaper than the Malossi cylinder, then used them with the Malossi head. I never heard of one doing well. One person on this forum had the cylinder crack in half.
I've had, and I know others who have had, bad experiences using the Polini Cam with the Malossi head and cylinder kit. Broken or bent valves, etc.
Both companies do a lot of R and D with their own products. I doubt they spend a lot of time mixing and matching each other's engine parts. Your best bet for longevity is probably to stick with one.

Absolutely right! Don't mix and match unless you want to enjoy a troubled future. Malossi everytime for me. The one I did for a customer has been completely trouble free and he has put close on 18,000 miles on it in the last 2 years. Polini, great company but less tested over here in the numbers Malossi has been.

Be good, try to combine power with duration ...
I approve of Motovista's vision

engines are just air pumps. they really arent that complicated. air in, air out. squish, bang, blow. there is absolutely no reason these parts woudnt work together to make the engine more efficient at maximizing airflow through the motor.

if there are reports of failures its likely due to improper fuel ratio and nothing else. dont blame the parts. when the tuner is the blame. metallurgy and machining these day is so good engines can be built these days to be reliable at 75hp per 250cc. we arent approaching half that goal. these engines are childs play in the hp world of making hp. where these engines are lacking is in the ecu department. they come tuned dangerously lean right out of the box. thats a necessary evil of being emission compliant. 14.7 ratio is the perfect ratio for emissions. best power is achieved at much richer numbers. 12.5 is a good number to shoot for. high compression and the numbers go even way lower. turbocharging is effectively increasing compression ratio. forcing more air into same space is the same thing as making space smaller mechanically. turbo engines need rich air fuel to live. if you put an a/f gauge on a bone stock 1990-1997 4g63t it will read 14.7 at idle. 13.4 light part throttle and then go to 9.8/1 at wide open throttle.

porco nero built a supercharged, nitrous powered 300 gts that makes 50hp. he knows what he is doing. he has drag race videos online to prove the results.

if there is a place in a mechanical field where lack of tuning knowledge exists it with scooters. and that is just because 99 % of the scooters ever made go through life stock. its rare to see people modifying them for more hp. so little knowledge exists.

Jerry, what are you talking about? Engines are a lot more than just "air pumps" and can certainly fail for other reasons than bad fuel-air mixture. That's not helpful. Have you done this conversion? Do you have first hand experience with either kit, let alone mixing and matching the two?

Someone is asking a question about a very expensive modification to their scoot, where failure can be dangerous and costly as well. Seems to me it warrants caution on the the advice side

my best recommendation is dont modify your scooter at all.

Jerry, you then miss out on so much. Brakes that you know can stop you on a penny. Suspension that can handle the potholes and give you a smoother ride and much more confidant cornering. Smooth power uptake that lets you overtake with ease. A Vespa is a lot of compromises.

I recently took an HPE model for a test ride, and was horrified by it after riding my modified 2015 and 2018 models. Just no comparison. I changed my mind about upgrading one of my scooters. Might get the white one resprayed instead!

I followed your advice. I put a new spark plug into my old fish tank pump, and...
well my gold fish wasn't that happy... It blowed...

So did you wind up trying this? If so, how did it go?

my understanding (please correct me if I am wrong) is that all pre-HPE GTS300's share same head (part number) as the GTS125, which means that either 125cc version was letting much more air than needed through that head, or it was the bottleneck for the 300cc version.

also, have heard that Malossi version head has the bigger ports and valves thus is better performance optimised for the 300cc sized engine

on HPE do not have any experinece, except that my Sis Snookie is having one and that damn think is fast!