125cc - how much power could they make without regulation

[Rusty569]

To start have owned and ridden a wide range of bikes, but also have a little 125 scooter for commuting on and frankly despite the bullying from my colleagues I think it’s a great little tool and can’t be beaten for cheap urban transport. (145 calculated tank to tank mpg!)

But reading a recent review for the xsr 125 it mentioned that it made bang on the Lerner legal limit. Ignoring 2 strokes like an off-road ktm 125 sx which makes 37.5bhp how much power could a 125cc 4 stroke make RELIABLY do you think?

I know the market would be tiny but are there any example of small capacity bikes which make relatively high power?

Given that most cutting edge 1000cc sports bikes around the 200bhp mark now that’s 0.2bhp per cc, so a 125 could make around 25bhp by that logic but I know it’s not that simple.

[Krikkit]

I bet 30hp could be had reliably enough for most, maybe a 20k mile rebuild interval. Just no money in it with the lack of market.

[Decky_Q]

Cagiva Mito/ Aprillia RS125 Both made near 40bhp in the 90s so something similar but new would be a good bet, learner legal is 13bhp. A restrictor can be fitted to the mito and rs to make them learner legal.

[Rusty569]

Decky_Q said:

Cagiva Mito/ Aprillia RS125 Both made near 40bhp in the 90s so something similar but new would be a good bet, learner legal is 13bhp. A restrictor can be fitted to the mito and rs to make them learner legal.

Both of these are 2 strokes though.

[pitlane]

Krikkit said:

I bet 30hp could be had reliably enough for most, maybe a 20k mile rebuild interval. Just no money in it with the lack of market.

No way anyone is getting 30bhp out of a 125cc 4T with 20K rebuilds. You would be hard pushed to get that at 2K rebuilds without some fairly trick parts inside!

[Carlososos]

The best old bike I’ve owned was Honda varadero 125cc facelift. It was the perfect bike. The only reason I changed to a pan was power. If it had 25bhp I wouldn’t have got rid.

[CoreyDog]

Carlososos said:

The best old bike I’ve owned was Honda varadero 125cc facelift. It was the perfect bike. The only reason I changed to a pan was power. If it had 25bhp I wouldn’t have got rid.

Loved my old Varadero 125, really regret selling it.

Been a V Twin, you’d assume it would be prime for more power though mine used to sit at 65 quite comfortably and I even saw 75 on GPS a couple of times, just took a while to get there!

I’d have another in a heartbeat.

[Gareth9702]

The answer must depend on the form of induction. With a turbocharger there is every possibility of 30hp and longevity because the revs can be kept low. Without a turbocharger the power could only be made by running at very high revs. That is expensive to engineer and will lead to a short life. Neither option makes any commercial sense.

[CrgT16]

I always wanted one of those 2stroke aprilias! In go kart the 125cc 2T are around the 45hp.
4T would be tricky to get over 25hp I think with normal aspiration.

Key is minimise weight then you don’t need that much power. On a small bike if you are a lardy guy shedding 1-2 stone of your own weight will be a massive performance increase. If you are already thin then it dies then matter I guess.

Interested to hear other more knowledgeable facts about these little engines.

[Speed addicted]

Gareth9702 said:

The answer must depend on the form of induction. With a turbocharger there is every possibility of 30hp and longevity because the revs can be kept low. Without a turbocharger the power could only be made by running at very high revs. That is expensive to engineer and will lead to a short life. Neither option makes any commercial sense.

That's the thing I suppose, no-one makes fast 125s because it's far easier to make a bigger engine than a highly strung 125 that now uses more fuel and needs serviced every 5 mins, thus knackering it's cheap transport USP.

[Derventio]

My old 1980 unrestricted Honda CB125T squeezed a whopping 17bhp from it's little parallel twin four-stroke lump.

At the time, my mates all took the Mick as they all buzzed around on their restricted TZR/NSF/NSR/RGs. Right up until the point where I cruised past them at 80+mph.

Sadly, after the fifth top end rebuild, I succumbed and got a 2 stroke.

I got my dad to buy a brand new, full fat 22bhp AR125, then bought it off him the day after. Ahem.

[Ambleton]

I'm fairly sure the UK learner limit is 15bhp.

Our scooter - a Piaggio Medley 125 is 11kw, which is 14.7bhp.

Does 69mph with me on it and a long enough run up but would likely do well over 70 with my wife on it whose circa 60kgs and 5ft5. Alas she doesn't go much over 45 as she only rides it through and around town but it's handy away from lights, junctions and roundabouts.

ETA- I notice the XSR mentioned by the OP is also 11kw

Edited by Ambleton on Sunday 14th August 09:21

[PT1984]

The A1 licence or CBT with provisional lets you ride up to 11kw which is 14.7hp. This is what the current XSR, Duke, CB125r, and MT output. Reliably with excellent MPG. And let’s face it, a lot of them without proper maintenance.

We will only ever see the output of a 125 increase if the licence allows.

Given a power commander, air filter and exhaust see modest gains in a Duke, a reliable 18hp should be possible.

I have a 2018 CB125r which has a modest 13.1hp and it sometimes surprises me at its pace on a country lane.
I would love to try the full fat 14.7hp version

PS, at 38 I’m not staying on L plates. I started the DAS but have had to stop due to an upcoming operation. I won’t be on two wheels until next year.

Edited by PT1984 on Sunday 14th August 11:21

[black-k1]

It's an interesting question but you'd have to ask why? The cost of parts, the requirement for maintenance, the likely loss of ridability and the loss of economy ( more power comes from more fue) means you'd just be better going for more cc's

[Tyrell Corp]

If a modern supersport 600cc is good for 120 bhp reliably, no reason why a 125 couldn't do 30...in fact a 125/4 motor would have tiny reciprocating mass so could rev much higher. Honda did a lot of this multi cylinder small race motors in the 60's.

This 5 cylinder 125 doing 30 bhp at 18000rpm, I suspect 60 years later and they could move that power and rpm quite a lot higher with a modern design.

https://www.motorcyclespecs.co.za/model/Racers/hon...

Limiting factor with bikes is the very poor drag co-efficient, much higher than a car, why so often a superbike with a much greater power to weight ratio is nailing it to the max to keep up with a supercar at full tilt.

[ofcorsa]

Decky_Q said:

Cagiva Mito/ Aprillia RS125 Both made near 40bhp in the 90s so something similar but new would be a good bet, learner legal is 13bhp. A restrictor can be fitted to the mito and rs to make them learner legal.

Had an indicated 90+Mph on my 2000 RS125. It was supposedly learner legal but looking back that seems unlikely.

[S2r]

Decky_Q said:

Cagiva Mito/ Aprillia RS125 Both made near 40bhp in the 90s so something similar but new would be a good bet, learner legal is 13bhp. A restrictor can be fitted to the mito and rs to make them learner legal.

Derestricted the Mito will make 33 (Cagiva) BHP so probably a bit less in the real world, it was restricted by having a massive chamber on the exhaust (that screwed up everything) and different airbox lid amongst other things. To get anywhere near 40hp would take a lot of work.

Anyway back on topic, a Honda RC149 was supposedly making 34hp back in the 1960s, a 5 cylinder, 8 speed 125 doing 20,000 rpm so with modern technology I'd think 40hp + would be possible.

Quite how long it'd last though is another matter, I seem to recall reading that the conrods used to stretch on the RC149 and would only last a race or 2 !

[slopes]

I'm fairly sure the right minds could make a 125cc four stroke make silly horsepower. Years ago, Bill Hunter was racing in the Superbike magazine Ultimate Streetbike drag racing events and he went to Williams Engineering for a tour and got talking to some of their engineers.
Oh yeah, we could make you an engine producing x amount of horsepower - which at the time was more than his turbo charged GSX engine was making - with ceramic this and fancy bits and bobs.
How much?
He never did reveal how much they told him but it was way more than he could afford or wanted to pay but that was in the 80's, so there must be some engineers in F1 now who could make a small engine make silly horsepower. maybe even in MotoGP too.

[Rushjob]

Derventio said:

My old 1980 unrestricted Honda CB125T squeezed a whopping 17bhp from it's little parallel twin four-stroke lump.

At the time, my mates all took the Mick as they all buzzed around on their restricted TZR/NSF/NSR/RGs. Right up until the point where I cruised past them at 80+mph.

Sadly, after the fifth top end rebuild, I succumbed and got a 2 stroke.

I got my dad to buy a brand new, full fat 22bhp AR125, then bought it off him the day after. Ahem.

My first bike was a CB125T as well. It used to rev all the way up to Nippon Seiki!

[Derventio]

Rushjob said:

Derventio said:

My old 1980 unrestricted Honda CB125T squeezed a whopping 17bhp from it's little parallel twin four-stroke lump.

At the time, my mates all took the Mick as they all buzzed around on their restricted TZR/NSF/NSR/RGs. Right up until the point where I cruised past them at 80+mph.

Sadly, after the fifth top end rebuild, I succumbed and got a 2 stroke.

I got my dad to buy a brand new, full fat 22bhp AR125, then bought it off him the day after. Ahem.

My first bike was a CB125T as well. It used to rev all the way up to Nippon Seiki!

I fitted a set of Dunstalls on mine so it actually sounded good for a tiddly 125. Not like these incredibly annoying singles cylinders that bomb around our with stupidly loud drainpipe exhausts.