How to make a 125cc quicker?

[oliverpetrie]

Just bought a 125cc Derbi Cross City bike, and i've read on quite a few forums that there are a few things which you can do to bump up the MPH by a few e.g. new exhaust.

Was just wondering is there anything I can do to get as much power out of a 125cc as possible?

[arry]

Take test, buy bigger bike. /Thread

[The Shaggy D.A.]

Weight and wind resistance is your enemy. What's your budget?

https://confrariadasmotos.files.wordpress.com/2010/09/rs1251.jpg
____________________
Chances are quite high you are not in my Monkeysphere, and I don't care about you. Don't take it personally.
Currently : Royal Enfield 350 Meteor
Previously : CB100N > CB250RS > XJ900F > GT550 > GPZ750R/1000RX > AJS M16 > R100RT > Bullet 500 > CB500 > LS650P > Bullet Electra X & YBR125 > Bullet 350 "Superstar" & YBR125 Custom > Royal Enfield Classic 500 Despatch Limited Edition (28 of 200) & CB Two-Fifty Nighthawk > ER5

[MarJay]

[Kickstart]

Hi

Various things can be done to an engine to increase power, and more power can give you a better top speed.

Exhaust system can help. But the exhaust is a carefully designed pipe of a particular length and bore. Just using a larger exhaust could easily lose you power. A well designed silencer will quieten the bike while losing a negligible amount of power (and badly designed silencer will destroy power).

Air filter, largely depends on area. Larger area means less restriction. But after a point there will be no gain.

Air boxes can be restrictive, but can also be critical for power. Again easy to lose power.

Carb size is important. But a larger carb won't work as well at low revs, and beyond a point won't gain you anything at high revs. So easy to lose power low down and gain nothing up top.

Ignition timing can be fiddled with, along with the compression ratio to take advantage of higher octane fuel (higher octane fuel on its own will give you nothing at all). But on a normally aspirated engine the gains are minimal. And if you do this and then get caught short without access to high octane fuel you risk major engine damage if you use normal unleaded.

Cam / cam timing / valves can be adjusted / changed to move power around a bit. But by this stage you are talking about a lot of money.

Realistically without a big bore kit (at which point it is no longer a 125) the gains on a well designed engine will be minimal. And gaining (say at the best) 10% more power will only increase the top speed a small amount.

With old 2 stroke 125s significant gains in power were easy as the bikes were heavily restricted to comply with UK learner laws. With the 4 strokes they can barely make enough power to reach the learner power limits anyway.

The killer for top speed for any bike is aerodynamics (they are dire for it), and any kind of bike with an off road style riding position is especially bad. But not much you can do about it.

All the best

Katy
____________________
Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[Snowdonia Rider]

Lose weight, seriously. Crouch down too. Other than that, big bike tests as already stated
____________________
I want your clothes, your boots and your motorcycle.
Suzuki GP125 Suzuki GSX600F Suzuki SV650S KTM EXC250F SkyTeam Bongo 125
My YouTube channel: https://www.youtube.com/c/SnowdoniaRider

[CyrilSwan]

https://i1160.photobucket.com/albums/q496/Karl_JB/Mobile%20Uploads/seriousShit_zpsfiqqnwd6.jpg
____________________
Current: Honda 2015 MSX125.
Previous: Honda 2012 CBF125.

[nitrosurf]

I think the most poignant response is the BTTF3 reference above. Take it from someone who has done the CBT 3 times (insert shame face here), just enjoy the Derbi for what it is and line up your tests. I had a TZR125 Gen1 at one point that was UK spec. Spent ages installing the powervalve controller etc to get more speed, the damn thing was off the road more than I used it, it was illegal on full power anyway and ultimately still just a 125. If I'd have ploughed the money into DAS instead I would probably have been on my tenth bike by now that exceeded anything a 25hp 125 could ever dream of attaining.

125 tuning = flogging a three legged dead horse. If it was a project in the garage for fun whilst you had a ZZRGSXRCBRR600 to ride daily fair enough. Otherwise, don't invite the wallet/heartache!

[G]

Open the throttle earlier.

That way you'll always have another HP or two!

[stevo as b4]

Is this for fun and without any real limited budget?
Or is it that you can only ride a 125 currently, you possibly bought the wrong one, and you want to get as much speed out of it desperately for next to no cash as you don't have any?

Sure four stroke 125cc tuning is hard work for any even moderate gains, but it makes no sense to spend £££'s and more on a bike for a few bhp and possibly make it far less reliable at the same time, unless you don't want the cash for anything bigger, or are keeping the bike until it falls to bits, or have some decent engine building experience.

Personally I wouldn't bother trying to get anything more out of the air cooled Honda based 2v motor, it's possible but not very good value. The only Derbi I would consider tuning is the 15bhp liquid cooled motor, and that's because the Base line is higher, and you can get a 170-180cc big bore kit for them too.

More cc despite the illegallites of it would always be my first change, and then I'd look at head mods, cams, carbs and pipes. I remember performance bikes YZF125R project, where they took a std bike 12.11bhp r/w and fitted all the Malossi tuning parts to it, then had the fueling re-mapped on a dyno, and just got to 18bhp. It needed higher gearing to go any faster though, but it was just about an 85mph bike afterwards. That's on a fully faired sports 125.

I'm in the process of fitting some tuning bits to a 125, I'm only doing it for fun though and will be about £1000 into it soon. Im not expecting anything though, another 5bhp would be nice but is a big ask. And I'm also keeping a 200cc tuned cylinder and head spare, in case i either blow it up or I'm really disappointed with the end result.

[Teflon-Mike]

Power is Force x Speed... force is 'drag' mostly wind resistance, which increases with speed; hence only needs about 3bh to go 30mph, but needs about 9bhp to go 60, and about 27bhp to go 90,and about 81bhp to go 120mph:- ie, you have to tipple the horse-power for every extra 30mph you want to go.

That's where power is 'used'. Where its 'made' the equation is this; Power = Cylinder Displacement x Cylinder Pressure x Crank-Revs.

Which gives lie to the 'lore' there's no replacement for displacement.. there is.. cylinder pressure or crank revs! BUT, looking at that explains WHY the 'lore' holds.

To make more cylinder pressure, you need to get more 'charge', air and fuel, the bulk of which is air, into the cylinder each 'rev'... bulk is 'air', so all comes down to shifting air.

But, with a fixed cylinder displacement, at 'best' that's all the air you can get in the pot each 'bang'... and the trouble is, just like shoving a bike and rider through 'free-air' wind resistance getting that much greater, the faster you go, same s happening inside the engine, drying to drag same air through the ports and passages into the cylinder.. so as rev's increase, so the amount of 'fill' you get in the cylinder tends to start to drop.

Max cylinder filling, is denoted by maximum 'torque' on the power chart, cos that's when you get most combustion pressure, and that max torque, tends to be some fair few rpm lower in the rev range than max power... gap between the two, power is increasing from the less well filled cylinder baking more 'bangs' in the same time, rather than bigger bangs.. until the motor runs out of breath completely, and the cylinder filling becomes so short that extra revs dont make up the dfference, and power starts to drop off.

'Tuning' for power? Well, if you open up the induction tract, inlet ports and valves, change the cam, to better optimise air-flow for higher revs, you might push the better cylinder filling higher up the rev-range, and get more 'peak' power, but usually with similar loss of low and mid-range power..

Its all swings and round-a-bouts, compromises and optimisations.

BUT, real world....

Rough reckoner for a 4-stroke motorbike engine is about 1bhp per 10cc, or 100bhp per litre. THAT is around about the natural 'optimum' for how much power a 4-stroke engine might make, and be 'practical'.

125cc 4-stroke, that males 12.5bhp then is about right.... you could get more, but, hances are you will loose more than you gain to get it, and to get it will be expensive and difficult and NOT somethng you can do quickly and easily n an afternoon with a halfords socket set and something from the Moto-Fiz catalogue!

We are talking properly re-engineered mechanics, with the whole engine taken to bits and 'worked' to get the most from each component and make them all work together... startng with the weakest like, which will normally be the cylinder head , valves and cam shaft profile.

So how much power do you want? You got around 12bhp, and 65mph.. if you hope to get anywhere near 90mph, you will need at least double that power, about 25bhp... could you get that engine to rev twce as high, 20,000 rpm? AND can you get it to fill the cylinder at those revs? Unlikely... possible, but unlikely.... certainly from an air-cooled 2 valve motor.....

Yamaha R1, makes about 150bhp per litre, with fives valves, water coolig and a hell of a lot of clever technology to make the valves light eough to work at higher rpm.. and it has the advantage of scale, and shear 'excess' where optimising everythig for 'the max' has left the low and mid-range weak, to keep the bike rideable.... but accept that 'niggle', and scale down.... 1/8th litre, suggests you MIGHT with all the best and latest technology chucked at the job... be able to get about 19bhp!

Err... yeah... be an absolutely astounding and fantastic achievement in shedology to do it.... and make a 125cc 4-stroke... ABOUT as 'fast as a Honda CB Two-Fifty comuter! An 80 'ish' mph motorcycle! It ent going to get you close to the 25bhp you need to scare 90, let alone the 30+ you need to crack the ton. You will still have a rather pitifully 'slow' motorcycle, for all your endeavours.

And THAT is if you achieve the amazing! Real-World, the 'best' 125 four strokes struggle to geuinely better 70mph, and with a dirtbike based machine you are starting from behind...

It is an excersse in futility to even try; 'performance' exhausts and air-filters or spark-plugs, are likely to do absolutely NOTHING to genuinely make the thing 'faster'.. because they aren't tackling the fundementals you need to find more power... all they are likely to do, is make the bike loud, run weak, and possibly damage itself!

Spending money, to make the bike less valuable; spending time, to make it less reliable, and MAKE potential problems.. its a fools errand!!

And I am a chap with two 125 4-stroke engines on the bench being played with for JUST that reason!

If the bike dont do what you want 'as is'.. then you got the wrong bike... is the simple answer.. flog it on and buy one that does do the job you want.. and if thats to go faster, and significantly faster, odds o that will have to be b'gger... 'cos there ent no replacement for displacement, you know?
____________________
My Webby'Tef's-tQ, loads of stuff about my bikes, my Land-Rovers, and the stuff I do with them!
Current Bikes:'Honda VF1000F' ;'CB750F2N' ;'CB125TD ( 6 3 of em!)'; 'Montesa Cota 248'. Learner FAQ's:= 'U want to Ride a Motorbike! Where Do U start?'

[The Shaggy D.A.]

...and that's just the tip of the icesperg...
____________________
Chances are quite high you are not in my Monkeysphere, and I don't care about you. Don't take it personally.
Currently : Royal Enfield 350 Meteor
Previously : CB100N > CB250RS > XJ900F > GT550 > GPZ750R/1000RX > AJS M16 > R100RT > Bullet 500 > CB500 > LS650P > Bullet Electra X & YBR125 > Bullet 350 "Superstar" & YBR125 Custom > Royal Enfield Classic 500 Despatch Limited Edition (28 of 200) & CB Two-Fifty Nighthawk > ER5

[Hong Kong Phooey]

Unless you're already a riding god, how about spending the cash on rider training?

Track day or roadcraft type training pays off no matter what cc bike you're sitting atop of.
____________________
'81 CG125, '97 FZS600 : '99 CBR600F4, '09 KTM RC8

[andyscooter]

pink powerband

bcf I am disappoint
____________________
gilera runner vxr200 (chavped)
if its spelt wrong its my fat fingers and daft auto correct on my tablet

[recman]

[MCN]

Grind 25-34 thou off your bob-weights, Open up your doodah to the next size up and add a bigger spring under the saddle.
____________________
Disclaimer: The comments above may be predicted text and not necessarily the opinion of MCN.

[recman]

[Ariel Badger]

75 PSI front and back, strip the bike of all excess weight, port head, profile cams, up compression ratio and re-jet and refuel to suit.
Or....... Pass your test and get a bigger bike.
____________________
Bikers make great organ donors, get 115 on your licence today.

[recman]

[SteeVeeDee]

Bear in mind that a bike is a balanced system (or should be). Improve power and you may well have to also improve brakes and suspension too. 125's are by definition low power and that is for a reason. In the bad old days learners could ride 250's (or unlimited if they had a sidecar or something pretending to be one for legal reasons - google 'sidewinder'). There was also no CBT. End result was a lot of dead or maimed kids. Stick to the bike as standard until you get used to it and pass your test. Then work your way up your desired type of bikes and refrain from jumping on some rocketship model and you'll get to enjoy a long fun life on bikes.

[Chuffin Nora]

There are three questions going on here.
Well, two and an observation.

1: How to make a 125cc quicker? --- Speed-up the manufacturing process.
2: [How] to bump up the MPH (by a few) --- Your location is, erm, Manchestershire. Go South, Young Man!
The closer you get to the equator, the faster you'll be going…but sideways.
3: Is there anything I can do to get as much power out of a 125cc as possible? --- Yes.

HTH


*Edit*
Yeah, this sideways malarkey --- you could hence change your name to Clint!
Clint Eastward!
gerrit?
____________________
There's a fine line between integrity

[Laamapalmu]

Does it have the Chinese 156FMI-2A engine, or CG125? There's a carving next to the oil drainage bolt. From aliexpress you can get an 150cc cylinder set for around 200-250€. Bigger cylinders won't fit because the crankcase isn't wide enough for bigger cylinder. Exhaust and carb modding is one. The exhaust in Derbi Terra is terrible, it restricts the exhaust gasses so much. My bike had the same one and it's just "limiting" it. It becomes so hot that you burn yourself on it, and there's a pipe that does random turns. Changed air filter, main jet and exhaust muffler and I got so much better performance, and with different muffler that's straight pipe, it's not hot to touch anymore.

When you are trying to get more power and maybe a bigger cylinder save money for clutch just in case, unless you are going straight to 200-250cc engine. They are around 350-500€ depends on postage.

About the engine, if it's 156FMI, it has centrifugal oil filter. Remember to clean that also when you change the oil and use the correct screwdriver because the cover screws are really tight and the heads tend to loose the screw mounting.
I have Baja, they both have the same engine. Current mods are 149cc Bore/cylinder head. 26mm carb with "aftermarket" air filter, 120 main jet and aftermarket muffler. Now I need new clutch plates because the old ones mixed up with the oil somehow
Bigger carburetor is useless because the intake port is only 26mm at least in my engine, and bigger one would probably just create an vortex that could make it worse. Spark plug sidegapping and indexing, might sound stupid but it actually made difference for me, reacts to throttle better.

When you get more Torque/Power you can get a bigger front sprocket and then should be getting better top speed.

Here's the manual for the engine:
https://www.pdfmotomanual.com/index.php?r=site/getdownload&unique=099e073606b17da26a3fef3ad17e8565

The engine has camshaft with one ball bearing and a ring on the other end. I haven't found any performance camshaft for the engine. New ones cost about 20-30€