Backwards Engine YZ250F Dominance

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5/2/2021 8:08 AM
Edited Date/Time: 5/2/2021 8:42 AM

Bike has won 7 of 16 250SX titles (per matthes) and won 5 of 7 250 National titles since they turned the cylinder around in 2014. Not bad.


Pretty close to PC’s level of dominance from 2004-2012

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Alright Lunger, Let's Do It

5/2/2021 8:40 AM

I honestly thought the others would follow. I hope the engineers got a nice bonus when they decided to flip it around. Its really obvious outdoors.

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If I'm healthy enough to complain. I shouldn't be complaining.

5/2/2021 8:51 AM

Numbers speak by themselves, they almost average 2 titles per year since 2014.

Outdoor: Jmart '14 & 15, Webb '16, Plessinger '18, Ferrandis '20

Indoor: Webb '15 & '16, Plessinger '18, Ferrandis '19 & '20, JCooper '21, Nichols '21

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5/2/2021 8:58 AM

I think the reversed intake/exhaust pays dividends in the 250f class. The HP numbers and torque that I've heard whose bikes put out are insane. I've heard ~54hp and 22 lb-ft of torque. For a 210lb MX bike, it seems to work very well. What's interesting is that it doesn't seem to be as big of an advantage in the Yami 450. I won't debate if it's a disadvantage or not in the blue 450 but it just doesn't seems to be as big of an advantage. The Star Yamaha 250f's have been noticeably faster than the competition for a while.

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5/2/2021 9:45 AM

Not an advantage on the 450 because they already have more power than is needed. On the 250 though more power is better.

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5/2/2021 10:00 AM

It's not the backwards design itself that makes it good. It is the intake airflow not having to squeeze by the shock and get a straight shot at the valves. Let's see if Honda's new 250F has the same downdraft setup as their 450?

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5/2/2021 10:06 AM
Edited Date/Time: 5/2/2021 10:21 AM

Johnny Depp wrote:

It's not the backwards design itself that makes it good. It is the intake airflow not having to squeeze by the shock and get a ...more

They moved the intake to a downdraft above the shock (a la the 450) in 2018.

It came with a significant HP increase, and the bike is much more powerful in terms of numbers than the Yamaha. But, seat of the pants, everyone has always complained that the 2018+ CRF250 is weak on bottom, the opposite of the YZ250F engine characteristic.

So, as often has been the case in the little class, it's not about how much power, but the TYPE of power.

Photo

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5/2/2021 10:30 AM

Dirty Points wrote:

They moved the intake to a downdraft above the shock (a la the 450) in 2018.

It came with a significant HP increase, and the ...more

When was the last time you saw a pro 250 rider using any amount of power below 6k?

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Die Antwoord

5/2/2021 10:32 AM
Edited Date/Time: 5/2/2021 10:52 AM

Johnny Depp wrote:

It's not the backwards design itself that makes it good. It is the intake airflow not having to squeeze by the shock and get a ...more

Dirty Points wrote:

They moved the intake to a downdraft above the shock (a la the 450) in 2018.

It came with a significant HP increase, and the ...more

gt80rider wrote:

When was the last time you saw a pro 250 rider using any amount of power below 6k?

Every single SX race.

Of course I hear the bikes peak against the rev limiter, but I literally hear every sound from throttle-stop-to-stop. It is probably pretty challenging to come up with an engine package for those guys, and I would bet proportionately little time is spent chasing peak hp.

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5/2/2021 11:20 AM

Dirty Points wrote:

They moved the intake to a downdraft above the shock (a la the 450) in 2018.

It came with a significant HP increase, and the ...more

gt80rider wrote:

When was the last time you saw a pro 250 rider using any amount of power below 6k?

Dirty Points wrote:

Every single SX race.

Of course I hear the bikes peak against the rev limiter, but I literally hear every sound from ...more

you are comparing stock bikes here...i bet you they push ALOT more out of it on the factory bikes due to the design.

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5/2/2021 11:28 AM
Edited Date/Time: 5/2/2021 11:29 AM

gt80rider wrote:

When was the last time you saw a pro 250 rider using any amount of power below 6k?

Dirty Points wrote:

Every single SX race.

Of course I hear the bikes peak against the rev limiter, but I literally hear every sound from ...more

Hrzk75 wrote:

you are comparing stock bikes here...i bet you they push ALOT more out of it on the factory bikes due to the design.

I am sure they do. But the base line is where it is.

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5/2/2021 11:36 AM

HRC Hondas seems to have pretty good power this year.

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5/2/2021 3:39 PM

Johnny Depp wrote:

It's not the backwards design itself that makes it good. It is the intake airflow not having to squeeze by the shock and get a ...more

Dirty Points wrote:

They moved the intake to a downdraft above the shock (a la the 450) in 2018.

It came with a significant HP increase, and the ...more

Thanks for that. I'd imagine the dual exhaust will be gone.

With so many choices in 4t engine design, 5 valves vs. 4, short vs. long stroke, SOHV vs. DOHC it's interesting to see where it ends up. It seems Yamaha made the decision to drop 5 valve and go for more bottom end and it has paid off. I can't believe no one has followed them yet.

Rumor is Beta is working on a 250f, and it will be interesting to see what direction they go. Their company's direction in engineering, from the Owner himself, is rideability. Beta engines are often overlooked because they feel slow, as is common in engines with smooth low end torque. They hook up though, and have deceptive speed, and the less you are dancing with the clutch and shifter the faster you can go by pulling taller gearing.

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5/2/2021 4:07 PM

Amazing how this bike is universally loved and then the 450 version is the big pig when they are virtually identical ergonomics.

So strange and anyone complaining here can't ride SX at the top level to talk from a point of authority.

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5/2/2021 4:48 PM

DynoDan22 wrote:

I think the reversed intake/exhaust pays dividends in the 250f class. The HP numbers and torque that I've heard whose bikes ...more

Per the Vital 250 shootout, the Yamaha 250 weighs 235 pounds. I wish we could see 210 pound 4-strokes. Just saying, in the name of accuracy...

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5/2/2021 5:06 PM

DynoDan22 wrote:

I think the reversed intake/exhaust pays dividends in the 250f class. The HP numbers and torque that I've heard whose bikes ...more

Do you know at what rpm those hp/tq figures are made?

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5/2/2021 8:53 PM

DynoDan22 wrote:

I think the reversed intake/exhaust pays dividends in the 250f class. The HP numbers and torque that I've heard whose bikes ...more

SilverSpurs wrote:

Do you know at what rpm those hp/tq figures are made?

To make 54 hp with a 250F i would say around 15,500 RPM's or higher. It would take around 18.5 ft lbs of torque at that RPM to put it around 54 HP which is pretty impressive. 22 ft lbs would be the max torque at a lower RPM.

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5/2/2021 9:39 PM

DynoDan22 wrote:

I think the reversed intake/exhaust pays dividends in the 250f class. The HP numbers and torque that I've heard whose bikes ...more

SilverSpurs wrote:

Do you know at what rpm those hp/tq figures are made?

NoMuff2Tuff wrote:

To make 54 hp with a 250F i would say around 15,500 RPM's or higher. It would take around 18.5 ft lbs of torque at that RPM to ...more

22 lb/ft is a very high figure at any rpm for a 250. Z👍 The cylinder head obviously breathes very well if they can hold the torque right up with the limitations on inlet valve size the restriction on bore size implies while still having the 6000 to 16000 range.
Moto GP 1000cc Yamaha 4's with finger followers and PVRS -ie no springs- but similar bore size and implied valve diameter are reputedly good for 260 to 280 HP so 65 to 70 peak HP at around 17500 rpm per 250 cc cylinder -ie but with less range.

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5/2/2021 9:44 PM

smoothies862 wrote:

I honestly thought the others would follow. I hope the engineers got a nice bonus when they decided to flip it around. Its ...more

You mean after Cannondale and Husaberg did different things with their engines right?

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5/3/2021 8:14 AM

I think like most racing success, it's a combo of the bike, team and rider. If you have a good bike and team then it's likely that the better riders will gravitate towards that. Basically why Yamaha wins in the 250s and KTM/Hus/GG shares similar success in the 450s.

As far as the Yamaha configuration, it certainly works great for 250s. I've had a couple now and they rev fast, produce great power and obviously the pro teams have been able to make monsters out of them. Any quirks that people have about the Yam chassis aren't that big of a deal on the smaller bike. The weight is lower as is the inertia. And the bike doesn't feel as big/wide as the 450 does. You can still toss the 250F around, which is a little tougher on the 450.

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5/3/2021 8:20 AM
Edited Date/Time: 5/3/2021 8:52 AM

Dirty Points wrote:

Every single SX race.

Of course I hear the bikes peak against the rev limiter, but I literally hear every sound from ...more

Preach. Peak HP matters most in forum bragging and magazine shootouts. I'm sure these guys spend plenty of time making sure the right amount of horsepower is in the right place to get around a track as efficiently as possible. Does more good than the highest horsepower in the wrong place I would assume.

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5/3/2021 10:37 AM

gt80rider wrote:

When was the last time you saw a pro 250 rider using any amount of power below 6k?

Dirty Points wrote:

Every single SX race.

Of course I hear the bikes peak against the rev limiter, but I literally hear every sound from ...more

jamiechapman418 wrote:

Preach. Peak HP matters most in forum bragging and magazine shootouts. I'm sure these guys spend plenty of time making sure ...more

This.

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Braaapin' aint easy.

5/3/2021 5:39 PM

Dirty Points wrote:

Every single SX race.

Of course I hear the bikes peak against the rev limiter, but I literally hear every sound from ...more

jamiechapman418 wrote:

Preach. Peak HP matters most in forum bragging and magazine shootouts. I'm sure these guys spend plenty of time making sure ...more

Falcon wrote:

This.

🤔So does motherhood.

That is the entire point of the discussion: they have a good peak number and a lot under the curve

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5/3/2021 5:55 PM

It does make more power and that means something in the 250 class. Probably makes to much with the 450 and seems to hurt handling. Yamaha is still the dominate selling competition brand behind the Austrian bikes and by a large measure. They are still selling a lot of 2 strokes and 450 sales for them are really good.

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5/3/2021 6:28 PM

We all know the YZ250Fs are making great power.

What’s more impressive is that they have developed a chassis, suspension, and mapping package that lets them put all that power to the ground in a way that still makes the bike very rideable for a variety of different riders. To me, this is way more impressive than peak HP/TQ numbers from the dyno.

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5/3/2021 6:56 PM

mxtech1 wrote:

We all know the YZ250Fs are making great power.

What’s more impressive is that they have developed a chassis, suspension, ...more

bingo... the whole is greater than the sum of its parts

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5/4/2021 2:52 AM
Edited Date/Time: 5/4/2021 3:21 AM

Hondas Moto3 engine of the NSF250R and RW (and whatever designations they've gone through in the last near decade of Moto3) , is a reversed, tilted back cylinder and head design, in use since that class started. Photo Photo

Said to be developed in 2011, whereas the YZF450 came out in 2010, but you can be your arse that Honda had that reverse cylinder in 'development' well before 2011, with their machinations for 'all 4t stroke, all the time ' racing classes. Yams 250F Reverse cyl /head, came out in 2014, I think.

I'd say Honda were very pissed that Yamaha brought out their version before they themselves put it into use in a vastly larger sales market that the 250 and 450 Offroad and MX market is over the Moto3 side of things.

Honda would, I think, never want to be seen as following another manufacturers engine 'idea', especially Yamaha. I really do think it was a design they wanted to be 'theirs'. They got thoroughly gazzumped by Yamaha, who've adopted it as their USP on the YZFs and WRs . cool

Of course, there's been myriad reverse cylinder and head engines in bikes, both 2t and 4t for decades. TMs Moto3 engine (I think only used so far in Italian domestic Moto3?) is a reversed engine, but heck, I think most of their Kart Engines are 'reversos'. Well, many Kart engines from other manufacturers are 'reversos'.

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5/4/2021 4:04 AM

Everyone focuses on the downdraft intake in this design and laughs at the reverse/ tilt of the cylinder. The reverse/ tilt is ten times more to do with the numbers than the intake tract. The cylinder is offset from center with the crank, it is tilted i think, 12 degrees? May be wrong on exact number, but, there is a purpose and that is to give the piston/rod combo a free moving rotation through the stroke without the sidewall push every other engine and prior Yamaha engines have. That, is why the reverse/ tilt engine performs the way it does. Cylinder sidewall friction is all but eliminated. All those ponies and torque numbers other manufacturers lose to friction, Yamaha is showing you how to get them back

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5/4/2021 4:41 AM

mxtech1 wrote:

We all know the YZ250Fs are making great power.

What’s more impressive is that they have developed a chassis, suspension, ...more

Good point.

What if:
1. another mfg decided to go this way?
2. Yamaha produced a 350 that would smoke KTM's version?

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5/4/2021 5:35 AM

mister2dt wrote:

Everyone focuses on the downdraft intake in this design and laughs at the reverse/ tilt of the cylinder. The reverse/ tilt is ...more

So is that why those bikes have felt like they rev so free/fast since they have gone that route?

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