Hierarchy of Two Stroke Hop Ups

AK74 wrote:

@Derek: are you referring to Jan Witteveen at Aprilia? I interviewed him not longer after he went into semi-retirement, and he was an absolute gold mine...

@Derek: are you referring to Jan Witteveen at Aprilia? I interviewed him not longer after he went into semi-retirement, and he was an absolute gold mine of knowledge. He was the engineering brain behind Cagiva's two MX world titles before going to Aprilia, where he built absolute rocketships. His bike collection is stunning too.

OldYZRider1 wrote:

Wasn't Witteveen with a Chinese team to develop a 125cc to race the 125 MotoGP series with several years ago? At least that's what I thought...

Wasn't Witteveen with a Chinese team to develop a 125cc to race the 125 MotoGP series with several years ago? At least that's what I thought I read somewhere. I don't know if they ever got a machine together and then the format was changed to Moto3. I thought at the time we might see some high performance two stroke engine offerings out of China. Of course in today's world it'd probably be a small market to fill.

AK74 wrote:

@Derek: are you referring to Jan Witteveen at Aprilia? I interviewed him not longer after he went into semi-retirement, and he was an absolute gold mine...

@Derek: are you referring to Jan Witteveen at Aprilia? I interviewed him not longer after he went into semi-retirement, and he was an absolute gold mine of knowledge. He was the engineering brain behind Cagiva's two MX world titles before going to Aprilia, where he built absolute rocketships. His bike collection is stunning too.

vikingBoy wrote:

I think he is referring to Jan Thiel, the mastermind behind the RSW/RSA 125. There is a reason that 2 strokes never took off as road...

I think he is referring to Jan Thiel, the mastermind behind the RSW/RSA 125.

There is a reason that 2 strokes never took off as road going vehicles in the 70’s. The (4 stroke) engineers slaighted to develop the 2 stroke car admitted after years of research and millions dumped into the project, that they couldn’t figure out these 2 strokes. The simplicity of the engine is only on the surface, the complexity of the gas exchange process is the black art, as Derek was referring to. If these researchers and engineers new enough about the complexity back then, maybe things would be different now. Honda shit canned the 2 stroke right when their leading engineer had a breakthrough (the info is out there if you search), but Honda new that profits were in the 4 stroke, and for whatever reason Mr. Honda hated 2 strokes. There still is a bunch of shade tree mechanics and home hobbiests that are developing the 2 stroke and trying to take it to the next step. Imagine if the 2 stroke had the money and resources poured into it that the 4 stroke had gotten all these years...

TJMX947 wrote:

When I was a kid (mid 90s) I drooled over the factory bikes. I've read Mitch Payton talking about lightened pistons in 125s, and MXA tested...

When I was a kid (mid 90s) I drooled over the factory bikes. I've read Mitch Payton talking about lightened pistons in 125s, and MXA tested a YZ125 that Chad Watts built which required a rebuild every 4 hours.

My question - What goes into one of these motors, and what would the order of operations be?

Seems like most guys stop at exhaust and the normal port/polish & head mods. Where do carb mods, balancing, blue printing, case matching all fit in? How different were 125s from 250s?

Derek Harris wrote:

Two strokes are a thinking mans motor - and not a challenge of parts so much as four strokes are. The carb going larger/tapered/smaller whatever only...

Two strokes are a thinking mans motor - and not a challenge of parts so much as four strokes are.

The carb going larger/tapered/smaller whatever only works if it fits the "package" - sometimes increasing the qty of air in the crank cases will HURT power...sometimes it requires a completely different pipe or port combo etc.

Balancing is a "myth". No single cylinder engine can be truly balanced - so you pick a balance FACTOR. A rule of thumb is 50 percent - and you move this around to suit the harmonics of the chassis - or feel the rider may experience. A crank SHOULD be TRUED extremely well - this is more noticeable to vibration. Crank stiffness is really important - many failures in bearings are due to the halves wanting to bow/bend. The crank pin "straightness" is really important too - if it's slightly tapered or off it causes failure.
Case stiffness again is ultra important. Old worn cases loose power due to flex.

Special inertia changes that DONT change the balance factor have been done over the years with interesting results. The crank does not spin a constant "rpm" each revolution. This was measured in a neat sae paper by honda at some point.
Increasing the crank inertia holds the crank rpm more consistent - and generally is worth HP on the top end/over rev. Unfortunately in racing avenues where the engine must change RPM relatively quickly, often the higher inertia cranks were slower in lap times. This is true of some rod length combos too - often in recent times (karting) long rods make more power (with a slew of other changes). Sadly they also can loose out on lap times because of the added inertia.

Case matching as in the bearing pockets being true is critical. Other than that - it's mostly all bullshit. Same with "case porting". Sometimes on specific packages epoxy in the cases can help - but not always.

The single most important part is the pipe - closely followed by the SCAVENGING layout (not "port layout" as commonly thought) It makes all the HP. Without a pipe - a 125 will produce around 15-16 hp. Pipes and cylinder scavenging have evolved hand in hand over the years. For example - suck harder with a pipe - but the cylinder isn't right and lets it get sucked right out the exhaust port - no gain.
Cylinder improvement to trap that charge - gain.
Matching the pipe to the ignition curve - and to the compression ratio - is where the real nitty gritty lies.

In mx - since most people are limited to off the shelf pipes - you end up with a narrow window of ignition curve, compression, and porting combos. These combos best match the PIPE - but are the "best overall way to go".

If you closely follow 2 stroke tech over it's history - port sizes, timings/widths, stagger etc have been largely settled YEARS ago - as far back as the early 80's. You can only make the hole so big before the ring falls out. But scavenging technology CONTINUES to improve year by year - little by little.
In 1996 yamaha destroyed the mx market with it's new motor. The porting technology - upward angles A transfer (main) of about 25-30 degrees), lightly upward angled B transfer (secondary) of about 5-10, and there directional angles entering the bore - coupled with their short side turn radius and port tunnel size, were an offshoot of some specialized flow testing they did that won them the GP titles the following year.

The yz 125 power band/curve was chased ever since for years - and amazingly a 2018 yz125 hasn't changed much in the port arrangement from a 1996 model.

the same basic port idea is still close to state of the art - although the karting world and moto GP 125s took it to the next level. The aprilia rsa125 made over 55 hp at the sprocket - and was unstoppable (originally was a derbi) for years.

Honda's best effort was around 46 hp - with ktm in the middle near 50.

If you saw the rsa internals - all published if you look hard enough - and compared it to an mx engine - the casual observer wouldn't see much difference between it and a ktm 125/150.

But the dyno sees a HUGE difference. It's not polishing - or balancing - or "tricks" or coatings or magic that makes things go - just really smart people working really hard with big budgets and excellent resource to make and try things.

The man behind the aprilia dominated gp type racing from the 70's all the way to his retirement in mid 2000's....and single handedly took 125s from around 40 hp to 55. Amazing fellow.

Now days the italian kart guys are making almost 50 hp out of 125cc motors restricted by rules to a 30mm carb, straight line ignition (read shitty). These engines are offshoots of the aprilia - and in unrestricted form with large carbs and proper ignitions go about the same 55 hp.

In mx - the ktm125 is about as close in base design as you can get to a really good base point. With a proper pipe, ignition curve, and time and effort it could go 45 hp pretty easily.

The 250's were never chasing power. Ironically - because the two stroke IS limited to a power width dictated by the pipe resonance window - manufacturers were always chasing feel and delivery over power. Back then - 45-50 hp was considered "enough", but it was because more power resulted in a peakier delivery that was harder to ride good laps on - mostly for sx.
Now days the 450 is so wide and easy to tune with EFI/gearing that we can hit huge power numbers but still be easy to ride.

I'd say a best effort 250 two stroke that was rideable with all modern technology could hit around 55-57 hp and be SX capable - and 60 hp for outdoors would be doable. Super kart 250's are putting out 65 on production cr250 cylinders and ktm cylinders - but with pipes so large they wouldn't ever fit a dirtbike

bluestreak04 wrote:

Fake News, Derek. I have ridden one of your bikes and I'm still fat.

southwick_mx wrote:

I´d like to add some more details accounting for some of the huge power in GP125s as compared to mx125s. GP engines are built to be...

I´d like to add some more details accounting for some of the huge power in GP125s as compared to mx125s.

GP engines are built to be rebuilt after every single race meeting. This means, among many other things, the use of 0,7/0,6mm piston rings, drastically reducing the ring/cylinder friction. Gp engines also use both detonation counters and EGT sensors to help in getting the jetting/programmable ignitions just a hair from disaster PERFECT. The Aprilia 125 GP cases were water cooled, and, as mentioned above disc valves were used instead of a reed valve. As engines are rebuilt after very few KMs of running (and of course VERY accurate tolerances in crankcases, crankshafts,conrods etc, squish clearances of as little as 0,6mm can be used. Pretty evil race gas helped in setting up these packages, of course.

The 250cc v-twin GP engines made around 110hp using the same basic setup in a doubled up design at astronomic costs...

As in any race engine, 2 or 4 stroke, the type of gas used dictates the setup.

Bring it on! I love this! /Lasse

Derek Harris wrote:

Two strokes are a thinking mans motor - and not a challenge of parts so much as four strokes are. The carb going larger/tapered/smaller whatever only...

Two strokes are a thinking mans motor - and not a challenge of parts so much as four strokes are.

The carb going larger/tapered/smaller whatever only works if it fits the "package" - sometimes increasing the qty of air in the crank cases will HURT power...sometimes it requires a completely different pipe or port combo etc.

Balancing is a "myth". No single cylinder engine can be truly balanced - so you pick a balance FACTOR. A rule of thumb is 50 percent - and you move this around to suit the harmonics of the chassis - or feel the rider may experience. A crank SHOULD be TRUED extremely well - this is more noticeable to vibration. Crank stiffness is really important - many failures in bearings are due to the halves wanting to bow/bend. The crank pin "straightness" is really important too - if it's slightly tapered or off it causes failure.
Case stiffness again is ultra important. Old worn cases loose power due to flex.

Special inertia changes that DONT change the balance factor have been done over the years with interesting results. The crank does not spin a constant "rpm" each revolution. This was measured in a neat sae paper by honda at some point.
Increasing the crank inertia holds the crank rpm more consistent - and generally is worth HP on the top end/over rev. Unfortunately in racing avenues where the engine must change RPM relatively quickly, often the higher inertia cranks were slower in lap times. This is true of some rod length combos too - often in recent times (karting) long rods make more power (with a slew of other changes). Sadly they also can loose out on lap times because of the added inertia.

Case matching as in the bearing pockets being true is critical. Other than that - it's mostly all bullshit. Same with "case porting". Sometimes on specific packages epoxy in the cases can help - but not always.

The single most important part is the pipe - closely followed by the SCAVENGING layout (not "port layout" as commonly thought) It makes all the HP. Without a pipe - a 125 will produce around 15-16 hp. Pipes and cylinder scavenging have evolved hand in hand over the years. For example - suck harder with a pipe - but the cylinder isn't right and lets it get sucked right out the exhaust port - no gain.
Cylinder improvement to trap that charge - gain.
Matching the pipe to the ignition curve - and to the compression ratio - is where the real nitty gritty lies.

In mx - since most people are limited to off the shelf pipes - you end up with a narrow window of ignition curve, compression, and porting combos. These combos best match the PIPE - but are the "best overall way to go".

If you closely follow 2 stroke tech over it's history - port sizes, timings/widths, stagger etc have been largely settled YEARS ago - as far back as the early 80's. You can only make the hole so big before the ring falls out. But scavenging technology CONTINUES to improve year by year - little by little.
In 1996 yamaha destroyed the mx market with it's new motor. The porting technology - upward angles A transfer (main) of about 25-30 degrees), lightly upward angled B transfer (secondary) of about 5-10, and there directional angles entering the bore - coupled with their short side turn radius and port tunnel size, were an offshoot of some specialized flow testing they did that won them the GP titles the following year.

The yz 125 power band/curve was chased ever since for years - and amazingly a 2018 yz125 hasn't changed much in the port arrangement from a 1996 model.

the same basic port idea is still close to state of the art - although the karting world and moto GP 125s took it to the next level. The aprilia rsa125 made over 55 hp at the sprocket - and was unstoppable (originally was a derbi) for years.

Honda's best effort was around 46 hp - with ktm in the middle near 50.

If you saw the rsa internals - all published if you look hard enough - and compared it to an mx engine - the casual observer wouldn't see much difference between it and a ktm 125/150.

But the dyno sees a HUGE difference. It's not polishing - or balancing - or "tricks" or coatings or magic that makes things go - just really smart people working really hard with big budgets and excellent resource to make and try things.

The man behind the aprilia dominated gp type racing from the 70's all the way to his retirement in mid 2000's....and single handedly took 125s from around 40 hp to 55. Amazing fellow.

Now days the italian kart guys are making almost 50 hp out of 125cc motors restricted by rules to a 30mm carb, straight line ignition (read shitty). These engines are offshoots of the aprilia - and in unrestricted form with large carbs and proper ignitions go about the same 55 hp.

In mx - the ktm125 is about as close in base design as you can get to a really good base point. With a proper pipe, ignition curve, and time and effort it could go 45 hp pretty easily.

The 250's were never chasing power. Ironically - because the two stroke IS limited to a power width dictated by the pipe resonance window - manufacturers were always chasing feel and delivery over power. Back then - 45-50 hp was considered "enough", but it was because more power resulted in a peakier delivery that was harder to ride good laps on - mostly for sx.
Now days the 450 is so wide and easy to tune with EFI/gearing that we can hit huge power numbers but still be easy to ride.

I'd say a best effort 250 two stroke that was rideable with all modern technology could hit around 55-57 hp and be SX capable - and 60 hp for outdoors would be doable. Super kart 250's are putting out 65 on production cr250 cylinders and ktm cylinders - but with pipes so large they wouldn't ever fit a dirtbike

erik_94COBRA wrote:

Do you have any more info on this Honda breakthrough? First I have read about this.

Derek Harris wrote:

Two strokes are a thinking mans motor - and not a challenge of parts so much as four strokes are. The carb going larger/tapered/smaller whatever only...

Two strokes are a thinking mans motor - and not a challenge of parts so much as four strokes are.

The carb going larger/tapered/smaller whatever only works if it fits the "package" - sometimes increasing the qty of air in the crank cases will HURT power...sometimes it requires a completely different pipe or port combo etc.

Balancing is a "myth". No single cylinder engine can be truly balanced - so you pick a balance FACTOR. A rule of thumb is 50 percent - and you move this around to suit the harmonics of the chassis - or feel the rider may experience. A crank SHOULD be TRUED extremely well - this is more noticeable to vibration. Crank stiffness is really important - many failures in bearings are due to the halves wanting to bow/bend. The crank pin "straightness" is really important too - if it's slightly tapered or off it causes failure.
Case stiffness again is ultra important. Old worn cases loose power due to flex.

Special inertia changes that DONT change the balance factor have been done over the years with interesting results. The crank does not spin a constant "rpm" each revolution. This was measured in a neat sae paper by honda at some point.
Increasing the crank inertia holds the crank rpm more consistent - and generally is worth HP on the top end/over rev. Unfortunately in racing avenues where the engine must change RPM relatively quickly, often the higher inertia cranks were slower in lap times. This is true of some rod length combos too - often in recent times (karting) long rods make more power (with a slew of other changes). Sadly they also can loose out on lap times because of the added inertia.

Case matching as in the bearing pockets being true is critical. Other than that - it's mostly all bullshit. Same with "case porting". Sometimes on specific packages epoxy in the cases can help - but not always.

The single most important part is the pipe - closely followed by the SCAVENGING layout (not "port layout" as commonly thought) It makes all the HP. Without a pipe - a 125 will produce around 15-16 hp. Pipes and cylinder scavenging have evolved hand in hand over the years. For example - suck harder with a pipe - but the cylinder isn't right and lets it get sucked right out the exhaust port - no gain.
Cylinder improvement to trap that charge - gain.
Matching the pipe to the ignition curve - and to the compression ratio - is where the real nitty gritty lies.

In mx - since most people are limited to off the shelf pipes - you end up with a narrow window of ignition curve, compression, and porting combos. These combos best match the PIPE - but are the "best overall way to go".

If you closely follow 2 stroke tech over it's history - port sizes, timings/widths, stagger etc have been largely settled YEARS ago - as far back as the early 80's. You can only make the hole so big before the ring falls out. But scavenging technology CONTINUES to improve year by year - little by little.
In 1996 yamaha destroyed the mx market with it's new motor. The porting technology - upward angles A transfer (main) of about 25-30 degrees), lightly upward angled B transfer (secondary) of about 5-10, and there directional angles entering the bore - coupled with their short side turn radius and port tunnel size, were an offshoot of some specialized flow testing they did that won them the GP titles the following year.

The yz 125 power band/curve was chased ever since for years - and amazingly a 2018 yz125 hasn't changed much in the port arrangement from a 1996 model.

the same basic port idea is still close to state of the art - although the karting world and moto GP 125s took it to the next level. The aprilia rsa125 made over 55 hp at the sprocket - and was unstoppable (originally was a derbi) for years.

Honda's best effort was around 46 hp - with ktm in the middle near 50.

If you saw the rsa internals - all published if you look hard enough - and compared it to an mx engine - the casual observer wouldn't see much difference between it and a ktm 125/150.

But the dyno sees a HUGE difference. It's not polishing - or balancing - or "tricks" or coatings or magic that makes things go - just really smart people working really hard with big budgets and excellent resource to make and try things.

The man behind the aprilia dominated gp type racing from the 70's all the way to his retirement in mid 2000's....and single handedly took 125s from around 40 hp to 55. Amazing fellow.

Now days the italian kart guys are making almost 50 hp out of 125cc motors restricted by rules to a 30mm carb, straight line ignition (read shitty). These engines are offshoots of the aprilia - and in unrestricted form with large carbs and proper ignitions go about the same 55 hp.

In mx - the ktm125 is about as close in base design as you can get to a really good base point. With a proper pipe, ignition curve, and time and effort it could go 45 hp pretty easily.

The 250's were never chasing power. Ironically - because the two stroke IS limited to a power width dictated by the pipe resonance window - manufacturers were always chasing feel and delivery over power. Back then - 45-50 hp was considered "enough", but it was because more power resulted in a peakier delivery that was harder to ride good laps on - mostly for sx.
Now days the 450 is so wide and easy to tune with EFI/gearing that we can hit huge power numbers but still be easy to ride.

I'd say a best effort 250 two stroke that was rideable with all modern technology could hit around 55-57 hp and be SX capable - and 60 hp for outdoors would be doable. Super kart 250's are putting out 65 on production cr250 cylinders and ktm cylinders - but with pipes so large they wouldn't ever fit a dirtbike

jeffro503 wrote:

Jesus! Thanks for all that info Derek! That took some time.

erik_94COBRA wrote:

Do you have any more info on this Honda breakthrough? First I have read about this.

Ebs wrote:

Honda had some interesting stuff going on into the late 90's. Look up the Honda EXP-2 race bike they ran in Dakar, and the CRM250 AR...

Honda had some interesting stuff going on into the late 90's. Look up the Honda EXP-2 race bike they ran in Dakar, and the CRM250 AR production bike.

Some reading on the engine tech: http://dwolsten.tripod.com/articles/jan97.html

make1go wrote:

I can only geuss graham bells work is not rated here.??

Micahdogg wrote:

[quote]A crank SHOULD be TRUED extremely well - this is more noticeable to vibration. Crank stiffness is really important - many failures in bearings are due...

A crank SHOULD be TRUED extremely well - this is more noticeable to vibration. Crank stiffness is really important - many failures in bearings are due to the halves wanting to bow/bend. The crank pin "straightness" is really important too - if it's slightly tapered or off it causes failure.

Increasing the crank inertia holds the crank rpm more consistent - and generally is worth HP on the top end/over rev.

Really interesting stuff! I wonder how do you check for crank pin straightness?

Micahdogg wrote:

[quote]A crank SHOULD be TRUED extremely well - this is more noticeable to vibration. Crank stiffness is really important - many failures in bearings are due...

A crank SHOULD be TRUED extremely well - this is more noticeable to vibration. Crank stiffness is really important - many failures in bearings are due to the halves wanting to bow/bend. The crank pin "straightness" is really important too - if it's slightly tapered or off it causes failure.

Increasing the crank inertia holds the crank rpm more consistent - and generally is worth HP on the top end/over rev.

Really interesting stuff! I wonder how do you check for crank pin straightness?

Katoomey wrote:

v-blocks and a dial indicator...like everything else.

Micahdogg wrote:

[quote]A crank SHOULD be TRUED extremely well - this is more noticeable to vibration. Crank stiffness is really important - many failures in bearings are due...

A crank SHOULD be TRUED extremely well - this is more noticeable to vibration. Crank stiffness is really important - many failures in bearings are due to the halves wanting to bow/bend. The crank pin "straightness" is really important too - if it's slightly tapered or off it causes failure.

Increasing the crank inertia holds the crank rpm more consistent - and generally is worth HP on the top end/over rev.

Really interesting stuff! I wonder how do you check for crank pin straightness?

Katoomey wrote:

v-blocks and a dial indicator...like everything else.

newmann wrote:

https://www.cookseycrank.com/crank-rebuild

Katoomey wrote:

v-blocks and a dial indicator...like everything else.

newmann wrote:

https://www.cookseycrank.com/crank-rebuild

BenG wrote:

Andrew Cooksey seems like a good dude, has some history here in Texas. I just had him true my crank.

Ebs wrote:

Honda had some interesting stuff going on into the late 90's. Look up the Honda EXP-2 race bike they ran in Dakar, and the CRM250 AR...

Honda had some interesting stuff going on into the late 90's. Look up the Honda EXP-2 race bike they ran in Dakar, and the CRM250 AR production bike.

Some reading on the engine tech: http://dwolsten.tripod.com/articles/jan97.html

newmann wrote:

https://www.cookseycrank.com/crank-rebuild