Hierarchy of Two Stroke Hop Ups

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?

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

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

newmann wrote:

All depends on what you are after and what you are willing to spend. Finding the right tuner/builder for you favorite flavor of bike can be...

All depends on what you are after and what you are willing to spend. Finding the right tuner/builder for you favorite flavor of bike can be equally important. I've got a small fortune tied up in a 1974 CR250 Honda engine. Matched and ported from the custom air box, carb, intake, cylinder, cases to the exhaust port. Head is modded, custom billet slide in the carb, early 73 billet crank with a long rod kit, PVL electronic ignition and a pipe built to match the engine specs. Recently installed a custom built complete transmission in it from England...$$$

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

newmann wrote:

All depends on what you are after and what you are willing to spend. Finding the right tuner/builder for you favorite flavor of bike can be...

All depends on what you are after and what you are willing to spend. Finding the right tuner/builder for you favorite flavor of bike can be equally important. I've got a small fortune tied up in a 1974 CR250 Honda engine. Matched and ported from the custom air box, carb, intake, cylinder, cases to the exhaust port. Head is modded, custom billet slide in the carb, early 73 billet crank with a long rod kit, PVL electronic ignition and a pipe built to match the engine specs. Recently installed a custom built complete transmission in it from England...$$$

Elsie88 wrote:

Dang newmann, that trans is bad ass! Who makes it? Phil Denton Engineering? How much did it cost ya?

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

ProKawi24 wrote:

I feel like I just had sex after reading that post lol. It would be a DREAM to work in a shop building and testing engines...

I feel like I just had sex after reading that post lol. It would be a DREAM to work in a shop building and testing engines now that I can't ride anymore. Thanks for the info dude, very very interesting to read.

newmann wrote:

All depends on what you are after and what you are willing to spend. Finding the right tuner/builder for you favorite flavor of bike can be...

All depends on what you are after and what you are willing to spend. Finding the right tuner/builder for you favorite flavor of bike can be equally important. I've got a small fortune tied up in a 1974 CR250 Honda engine. Matched and ported from the custom air box, carb, intake, cylinder, cases to the exhaust port. Head is modded, custom billet slide in the carb, early 73 billet crank with a long rod kit, PVL electronic ignition and a pipe built to match the engine specs. Recently installed a custom built complete transmission in it from England...$$$

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.

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.

newmann wrote:

As the story goes, crank forging weren’t ready when the first CR 250’s were nearing production in early ‘73 so Honda had a bunch of cranks...

As the story goes, crank forging weren’t ready when the first CR 250’s were nearing production in early ‘73 so Honda had a bunch of cranks machined from billet. I haven’t weighed them to compare and really don’t know if the offer any advantage but they are rare and somewhat desirable. Here is one on the right.




Of course the young Mr. Harris’s eyes are drawn to the $10,000.00 polished and plugged HRC RC125 crank on the left. Those crank halves may even be two piece, hollow and laser welded. Rumors and stuff. Got a little surface rust since the flood, may need to clean them up a bit.

The guy that did the port work on my 74 made a batch of full circle billet cranks for the early Elsinore's. Unfortunately I never ended up with one. I do believe Andrew Short’s 74 CR250 has one in it....the lucky dog.

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

fullysicmate wrote:

I had a 2002 Cr250 that had porting done buy a local dealers mechanic, this guy had previously built rgv250 motors for road racing. He was...

I had a 2002 Cr250 that had porting done buy a local dealers mechanic, this guy had previously built rgv250 motors for road racing. He was an alcoholic, looked completely dodgy, I know he matched the cases but not sure what else (no big bore), I took it to a big event which involves barrel racing on a beach, up to 750M between barrels. I beat bikes running downpipes, big bores and methanol in the "250" 2 stroke class by half a lap after 4 laps. I had people asking what was done to the bike, looking all over it, it had a standard pipe, looked completely stock from the outside. Whatever he did was magic, if I knew where that bike was I would pay good dollars to get it back hoping it still had the same engine. I am looking for a 2002-2007 to do up and leave in the shed now, I love those bikes.

fullysicmate wrote:

I had a 2002 Cr250 that had porting done buy a local dealers mechanic, this guy had previously built rgv250 motors for road racing. He was...

I had a 2002 Cr250 that had porting done buy a local dealers mechanic, this guy had previously built rgv250 motors for road racing. He was an alcoholic, looked completely dodgy, I know he matched the cases but not sure what else (no big bore), I took it to a big event which involves barrel racing on a beach, up to 750M between barrels. I beat bikes running downpipes, big bores and methanol in the "250" 2 stroke class by half a lap after 4 laps. I had people asking what was done to the bike, looking all over it, it had a standard pipe, looked completely stock from the outside. Whatever he did was magic, if I knew where that bike was I would pay good dollars to get it back hoping it still had the same engine. I am looking for a 2002-2007 to do up and leave in the shed now, I love those bikes.

H4L wrote:

A good tuner can extract the hidden power the CR2 case reed engine has due to the design. I still own an 03 model with PC...

A good tuner can extract the hidden power the CR2 case reed engine has due to the design. I still own an 03 model with PC cyl. / head mods & was amazed that the power output felt close to a modern 18 CRF450r recently purchased. That's impressive considering the new 450 is making close to 58 hp atrw.

doghouse wrote:

Those Honda motors have insane amounts of power waiting to be extracted. I have an 03' I want Chad Watts to massage a bit.