125cc gp vs 125cc mx?

[Bioflex]

Given both road racing gp and mx 125's run the same engine, does anyone have any info in terms of what hp each makes, as well as the rpm of the redline ?
I imagine the gp bike would be tuned as the ultimate top end screamer but would like to know the difference between the two.

[ford832]

Set up more for upper mid to top which is  125 anyway but mostly they are geared wayyyyyy higher.Did you ever watch them try to get off the start line-they almost have to pedal their feet. :)They aren't far removed from your typical 125 mx.


http://www.google.ca/url?sa=t&source=web&cd=2&ved=0CB4QtwIwAQ&url=http%3A%2F%2Fwww.topspeed.com%2Fmotorcycles%2Fmotorcycle-reviews%2Faprilia%2F2010-aprilia-rs-125-ar76549.html&rct=j&q=125%20gp%20motorcycle%20technology&ei=JtIFTvaaD6bd0QHX9fjlCw&usg=AFQjCNGCbezs_jXYnzNtfngkR-DzJOe8LA&sig2=B--8lUeGI_39izgwAQwtpA&cad=rja

Yes please.

[msmola2002]

I've got a 125GP bike laid up back in oz. It is an older model though

What I can tell you about mine, is that it is a 91 model honda, based on the cr but no kicker setup, and no power valve, also a 6 spd trans.

stock, the cr based ones like mine make around 28hp rwhp on a dynojet, mine has been tickled to about 34.5, peak power at 12.5 and revs to about 14. the NX honduhs (mine is the NF variant with cantilever rear) also have the powerjet carb which buys a few more top end hp before it falls off the cliff, maybe 1000rpm

Gearing makes a big difference, very tall. In first i can do like 40odd mph, so is a bitch to get off the line
for reference:
RS125 Oran Park
Super clutch clip required.


the later NXhondas  (95-) don;t use the MX based engine and they make more powahs for a variety of reasons - the pipe does not have a big bend as soon as it leaves the engine for one - the cylinder is canted further forward, the power jet carb.
The TZs are a pretty good package, they rock a balance shaft, power valve, dry clutch (the fcc dry clutch for the rs is rocking horse shit and tend to go for about 1k) and a few other bits and pieces.

For more info and tech nerdery, paging the 2 stroke institute... He could write a book.

[msmola2002]

oh and the power valves on the gp stuff when equipped (honda 2fiddy and tz) is not vacuum operated as per mx.  They tend to run a servo which drives the valve - and run by a computer. There are a few aftermarket options for tweaking this, and when i do migrate my fat arse back to oz my nf4 will be getting fitted with a power valves cylider driven by a laptop programmable cdi.

Other differences, the nf has an MX based high voltage ignition stator, the nxs run a 12v system as they tend to run more electrickery, the nf is just the coil, in the nfs you get 12v to run the power jet, often a deto counter and other toys.

Jetting is more on a knife edge than for an MXer, I have a weather station you haul out to the track, measure the temp, baro pressure and humidity to calculate RAD (relative air density) and jet accordingly - have a nice excel spreadsheet that allows you to select you theoretical jet based on a known good jet at a known RAD.

these things are jetted real fine and will not make power too rich, and seize too lean (duh) but there is far less wiggle room than for an MXer

[msmola2002]

They aren't far removed from your typical 125 mx.

the big boy international stuff - respectfully disagree, and the honda customer spec stuff has not run an MX based engine since 94.

The world champ 125s run stuff like disc valves, and rock billet CNC cases. like saying a kids tricycle is like a racing bicycle.

And the RS125 you linked is not a GP bike. It is a GP replica, flogged to learner riders -  the same as my RGV250 in teh garage back in oz is a GP replica, that makes 50hp with a tailwind and weighs 125kg vs teh honda RS which even in pleb spec makes 90hp and weighs 100kg. Apples and oranges!

[2T Institute]

Given both road racing gp and mx 125's run the same engine, does anyone have any info in terms of what hp each makes, as well as the rpm of the redline ?
I imagine the gp bike would be tuned as the ultimate top end screamer but would like to know the difference between the two.

Not really Ben, they are the same bore/stroke but ends there. I hear stories of 40odd HP MX 125's but have yet to see the dyno graph. GP 125's are not peaky at all they are very forgiving, they have peak power at about 12,700. The gold standard for many years has been the Aprilia RSW and followed by the RSA, developed by Jan Thiel. Which is a 54 X54.5mm 125 rotary valve engine, has a 120mm long rod connected to a 107g piston with a 0.8mm ring.42mm magnesium flatslide Delorto carb with S-DAS 2 electronic control.Fully programmable ignition that controls ignition/power valve and power jet. The S-DAS system links ,ignition to power valve to TPS to gear position to fueling to detonation(the 250 to lean angle), so in an instant if the detonation counter has hits, ignition is retarded fuel is added, if the dets stop, timing restored and fuel reduced......all in the blink of an eye.
Pipes are universaly titanium construction.
Safe to say a decent 125 GP engine in a MX frame would lap the best mx engines, unfortunately this is the last year for 125GP at a world championship level. You will not find a better tuned 2T engine
HP


 

[spainYz250]

last week watching moto gp. the guys who were commenting said that a 125 oficial GP has 2 cilinders and they gave 60cv o more. but they were in their limits in termes of evolution. there was a estudy that showed that the teams that tried to improve the power, they really made it worse because it was imposible to improve them.

[spainYz250]

also they spoke about the jettings in these bikes. they were speaking about that next to the top revs the carburetor would let more air inside and in proportion less gasoline so i would run a bit more dry but by that way the got a few more horse power. they also were speaking that these carbs have powerjet

[2T Institute]

Jan Thiel said he still had things he wanted to try at the time of his retirement from Aprilia Corse. 2T engines are by no means at the end of their evolution.

[ford832]

They aren't far removed from your typical 125 mx.

the big boy international stuff - respectfully disagree, and the honda customer spec stuff has not run an MX based engine since 94.

The world champ 125s run stuff like disc valves, and rock billet CNC cases. like saying a kids tricycle is like a racing bicycle.

And the RS125 you linked is not a GP bike. It is a GP replica, flogged to learner riders -  the same as my RGV250 in teh garage back in oz is a GP replica, that makes 50hp with a tailwind and weighs 125kg vs teh honda RS which even in pleb spec makes 90hp and weighs 100kg. Apples and oranges!

More details please msmola.I still don't see that much of a difference.PJ's and tps's are nothing new on mx bikes,nor are servo controlled pv's.Can Ams ran a rotary valve.Impressive engineering and,as in all racing stuff,more adjustable than something you'd buy off the floor and of course are set up for something completely different with different power requirements.I just don't see much design difference in the engine.
I know the RS link is a street bike but I just think it's cool-and would like to have one.Back when I bought my first RZ,the sales brochure had the RZ on one side and the TZ250 on the other.I would have rather the TZ but as you couldn't plate it,and it was ridiculously expensive,I bought the one I could ride 

[SachsGS]

Are the RSA/RSW Rotax based?

[2T Institute]

Are the RSA/RSW Rotax based?

You can trace the DNA back to the Rotax 128 (125) and Rotax 256 (tandem twin 250) which is what Aprilia began racing with, in the early 90's Rotax released the 258 Vtwin, that became the basis for the 250.
The 125 took a different path, both the 125 and 250 engine development was taken from Rotax and brought in house to Aprilia Corse(under Jan Witteveen), output steadily rose. The 125 retained the same layout as the orignal 128 engine (side disc/carb) with the cylinder and electronics constantly updated.
Meanwhile a small Spanish company (Derbi) was starting to make in roads on the 125GP scene(under Harald Bartol) after Bartol left and went to KTM did Derbi draught Jan Thiel in. A clever Derbi engineer designed a rotay valve engine with the disc/carb behind the cylinder about the same time a young Jorge Lorenzo hit the GP scene. The Derbi went on to win the 125cc championship.
Not long after Piaggo (Aprilia parent company) bought Derbi and merged the race departments, Aprilia were mighty miffed that a very small outfit could out do  a race dept with 100 staff.
The the Aprilia cylinder got grafted onto the Derbi bottom end and became the Aprilia RSA. 
So VERY loosely based on the Rotax

The Aprilia RSW/RSA both 125 and 250 is the absolute pinnicle of current 2T engine development.

[Bioflex]

Many thanks for the detailed responses guys.
The fact they moved away from the MX engines many years ago and the development since that point is impressive. Really makes you wonder how one of these engines would go slotted in an MX bike.

It would have been very interesting to see a "full house" Mx bike on a dyno in comparsion to these, from the sounds of things the MX bike would have struggled in comparison.

Lozza, apologies for not getting you that motor already. 6 weeks ago my wife decided to bring back a new "machine" one like I have never seen before. It requires massive amounts of maintenance, needs a refuel after the shortest of spells, and needs several good cleans a day.
Once we've got it running a bit more regularly should have a bit more time!

[VintageBlueSmoke]

Rotax=Can-Am 

[2T Institute]

Here is what a 2005 RSW250 makes on the dyno, a private bike 7 years behind in development