Why 400cc?

[BASSic]

I was just wondering why Yamaha chose to produce a 400cc bike when they came out with the YZ400. It is my understanding that AMA rules were changed to allow 450cc bikes to compete, so why didn't they just start out with a 450?

Also, was there any significance to using a displacement of 426cc in 2000? 426 is such a strange number - historically I thought most motorcycles manufacturers just round off the number to something even.

Pardon my ignorance, I'm just trying to learn the history of my beloved toy.

[cypher2]

they didn't change too 450 straight away.

initially i belive that the ruling, in europe at least, was that 4st up to 400 could compete against 250 2st.

they were all sorts of reasons for the intro of the 4st, some legitimate some bull, the 450 displacment came about when they realised that the 400 4st still didnt really have an advantage over the 250 2st.

so hey presto they upped the capacity.

why yam did a 426? think it was the biggest they could make the cylinder with out re-engineering it until until they sorted the 450, but thats only a guess.

[grayracer513]

The YZ400 was the brain child of a single Yamaha engineer. He actually had a great deal of difficulty selling the idea to the company, but when he finally did, they all agreed that the 4 stroke would need a displacement handicap. The engineer was told that getting the AMA and FIM to allow this would be tricky, and to make the bike no larger than it would need to be to be competitive. The engineer thought that would be 400cc, so that's what they built.

The AMA, in its normal display of wisdom and deep consideration of the subject matter at hand at any moment, decided that would be OK, and that 4-strokes as large as 550cc could be run in the 250 class, and 250cc in the 125 class. Now, you have to understand that while the Yamaha engineer's decision was based on careful calculation, The AMA's decision was based on nothing in particular at all. In fact, they honestly felt that a four-stroke could never win anything regardless of how big it got, so they figured it wouldn't matter anyway. After Doug Henry's performance in the 1997 season, they quickly retreated to 450cc.

[KAS]

I'm not sure why Yamaha settled on "426", but I do know that a 420cc piston kit was a popular mod to the original 400 in order to boost low-end. So I'm sure thats what Yamaha was aiming for by the displacement increase. Again, why they called it a 426 instead of a 430, I don't know. Maybe they thought "426" sounded cool.

Honda came right out of the gate with a 450 in '02, so naturally Yamaha stepped up the following year.

[grayracer513]

426 was an even 3mm overbore on the original dimensions of 92 x 60 mm. It allowed the bike to grow in displacement without a completely new set of crankcases being necessary.

[BASSic]

Thanks for the informative replies, that's what I was looking for. The Wikipedia entry for the YZ450F (at least what it says at the moment I'm typing this) states that "In 1996, the AMA changed racing rules to allow 450 cc four strokes to compete in the same racing class as 250 cc two strokes." which had me wondering why Yamaha didn't just go with a 450 off the bat. I didn't realize it was just an engineer's educated guess to decide what would be competitive, then hope the AMA would allow it.

After Doug Henry's performance in the 1997 season, they quickly retreated to 450cc.

Now I'm left wondering why the AMA only scaled back to a 450cc limitation considering how competitive the 400cc engine was. ?

[MX4Life]

I remember back in 1979 Yamaha sold a "426 OW kit" for the YZ400F two stroke, or maybe they loved the old 426 Hemi.?

[BKRacing]

I heard that what Doug Henry race was accually a 426 the whole time(since they had the year of developement per the AMA) and the version sold to the public was 400cc's. Not sure where I heard it but I did.

[yz454]

Ok it's time to rock the boat ,right now you are not riding the latest tech. All of this engine designs are were up and running as early as 1912 to 1919.The only diff is metals we have now.The ama had rule in the 70s that let 350 4t run with the 250ts ,honda had very trick xl350 based works bikes that were faster than any thing else in the trans am series but they started building and selling the cr s and stop racing them.But this were I upset the motor dummies .the 2s and 4st get even on power at about 300 cc any thing above that and the 4st gains the upper hand by a lot, below 300 its's 2st ,but it's not because of power out put The 4st has to over come the friction which doesn't change much from an 80 to 500.The 2st is at the end of it's design,what you see is what you get,All the new tech out there in the future will help the 4st a lot more than the 2st.There is about 8 more 4st engine designs that are up and running now, some with out cams some with out valves and no cams, rotary heads with ports and so on .And yes I knew the 450 thing was coming in 98 we built the first 400 out to a 450 and yamaha tested it.That bike is still and raced with the same parts cams and steel valves and all . Thats all of the time I got now,This will get the little minds talking.

[grayracer513]

...now,This will get the little minds talking.

So then you are trolling?

It's true on a simplistic level of concept that DOHC engines were in existence in the early 20th century, but so what? The "latest technology" includes metallurgy, as well as the complete set of refinements, big or small, that bought the 4 stroke cycle engine to its current state, so yes, regardless of you contention to the contrary, the current crop of MX 450's really is the latest technology available for the purpose. These bikes would have been impossible to build as little as 30 years ago.

As far as 4T vs. 2T, the concept represented in your argument is basically correct in that each engine type has a range of sizes through which that design functions better in terms of power output than others, and that the 2T is a better choice for the smallest engines. However, I disagree with the specifics. For one thing, some of the best builders in the country have struggled for years to create 450cc thumpers that put out 60-65 hp. I had my CR500 at nearly 70 with of the shelf parts and very little effort.

You also ignore the fundamental differences in the two designs. The newest advances in both engines, short of those at the F1 level, will allow both to exceed 100% volumetric efficiency by a fair margin. However, the 2T fires every single time it goes around, while the 4T fires only once every other. This is the primary shortcoming that it has vs. the 2T, and the only approach available to 4T builders to counter this is to rev the engine higher. To that end, we have todays radically oversquare (big bore/short stroke) engines with slipper pistons and advanced valve trains.

[beanmop]

Thats all of the time I got now,This will get the little minds talking.

For somebody with such a large all encompassing mind, you'd think grammar would be a concern.

[grayracer513]

To be fair, one of the most successful and respectable engine guys I know is also somewhat handicapped with respect to written communication. It just isn't his talent. But you see what sort of assumptions that leads to.

[yz454]

Like I said simple motors for simple minds.I would be a shamed to be the dyno guy that would claim 70hp out of a cr500.I have been lucky over the years to ride some of the best cr500s on the ama circuit in the day.That said and done there is very few 4st builders out there that built 4st all there life they were 2st tuners.You can spend all your life just in ports and valves and will not know it all.Every little thing you work on can be more or less power.You got rod length to bores to stroke ,put It this way there is an endless combo. I started building 4st in the mid 60s ,had my 2st that I raced in the 70s 80s built them like 4st as much as possible.Wide power, ran very well and didn't break, but love them. I was sponsored to ride them, in the open class was a highly modified xl350 in a light frame. In 1980 I had cr250 loved it, wish I still had it,but I would go home and ride my c and j xr 250 it was 10 pounds lighter and handeled better made more power but you could no shift it to save your live under power.Buy the way I spend to much time in the shop and on the flow bench to worry about spelling try getting off the computer and see how motors work.Buy now got to go port a 660 head I all ready got 8hrs and more to go.

[gbalias]

hey gray, still got that CR? ill gladly take it. hahaha.

i wanna build one for supermoto. one day, one day.

[grayracer513]

Not any more. Sold it. It was a gas though, in a deeply twisted sort of way.

I've seen CR500's at sumo races a couple of times, but they never did well. They were fast, but the thumpers have a wider power range and the handling thing killed them. "Can't slide 'em" was how one of the guys put it.

[The Italian Stallion]

I remember they claimed the RC500 works bikes of 85-86 made close to 70hp.My 85 CR500 made 60hp bone stock. You tell me it was not possible to get another 10 out of this engine?

[02WR426Cali]

I'm not sure why Yamaha settled on "426", but I do know that a 420cc piston kit was a popular mod to the original 400 in order to boost low-end. So I'm sure thats what Yamaha was aiming for by the displacement increase. Again, why they called it a 426 instead of a 430, I don't know. Maybe they thought "426" sounded cool.

Honda came right out of the gate with a 450 in '02, so naturally Yamaha stepped up the following year.

they called it a 426 because that is actually what it is. For once a manufacturer named it what it is rather than rounding up like most other bikes. I like the idea:excuseme:

[BASSic]

they called it a 426 because that is actually what it is. For once a manufacturer named it what it is rather than rounding up like most other bikes. I like the idea:excuseme:

Me too. Now can someone recommend a good 2-cycle oil for my YZ 249?

[grayracer513]

There may have been a bit more to "426" over say, 430 than just total candor. After all, they don't call my bike a 459. But that's a lot better than calling a 516 a 530, don't you think?

[kxracer987]

Ok it's time to rock the boat ,right now you are not riding the latest tech. All of this engine designs are were up and running as early as 1912 to 1919.The only diff is metals we have now.The ama had rule in the 70s that let 350 4t run with the 250ts ,honda had very trick xl350 based works bikes that were faster than any thing else in the trans am series but they started building and selling the cr s and stop racing them.But this were I upset the motor dummies .the 2s and 4st get even on power at about 300 cc any thing above that and the 4st gains the upper hand by a lot, below 300 its's 2st ,but it's not because of power out put The 4st has to over come the friction which doesn't change much from an 80 to 500.The 2st is at the end of it's design,what you see is what you get,All the new tech out there in the future will help the 4st a lot more than the 2st.There is about 8 more 4st engine designs that are up and running now, some with out cams some with out valves and no cams, rotary heads with ports and so on .And yes I knew the 450 thing was coming in 98 we built the first 400 out to a 450 and yamaha tested it.That bike is still and raced with the same parts cams and steel valves and all . Thats all of the time I got now,This will get the little minds talking.

No valves:thinking: How would it get gas? Oh, ports in the cylinder. Well, you would want to keep oil on the crank and piston. Why not just put some oil in the gas, that will make it simple. Oh wait, you just designed a 2-stroke.?

In all reality though, if it had no valves or ports, it couldn't get gas. If it had ports, it wouldn't need 4 cycles to make power, only 2, because it wouldn't have valves to open. YOU MAKE NO SENSE.