4 stroke killin' 2 stroke idea

[brentry]

Another way on a 125 would be to have more stroke than bore. To get the transfer timed area with lower tranfers, to keep the torque up for mx, And big radius tunnel tranfers ,wide as the bore can handle, I use to build/dyno 2strokes Alot of them, and used eng mod 2t. 1000s of sims and dyno pulls,built pipes from scratch. Did it all.
Newer ktms are decent. But theres more to go

That would require a completly new cylinder as transfer timing adv's with more stroke. I did sim up this idea. Think it made like 46hp in a believeable sim, with big torque and on pump gas
I still like idea variable transfer timing

[leahymtsps]

Tom
[/quote]
[The Puch 250cc twin-carb motocross bikes of the mid '70s had one carb feeding the standard piston-port entry with the second carb feeding a rotary valve from the case under the cylinder. Harry Everts won the 1975 250cc Grand Prix Motocross championship on the factory bike, with a limited number of production versions made for 1976.]

Actually the factory had both piston port and reed valve cyl. versions. Local Puch dist. here was across the road from 1 of our practice
tracks and got to try 1 when they were testing it. A week or 2 later Puch took them all back, but I don't remember why.


[-stroke vs 2-stroke is more than HP and torque; it's the power delivery of the 4-stroke where the biggest difference shows up, especially on tracks with less than ideal traction. On a hard-packed track where the 2-stroke requires much more throttle control to keep from breaking loose, the 4-stroke's smoother delivery lets the rider get on the gas harder with far less worry about hooking up.]
[/quote]

And what part of that description is not about the torque curve??

Not that it matters but FWIW I also owned a dealership for 20 plus yrs. pre, during and post 4 strokes. And while they were - are techno marvels,
they caused more people to leave the sport with high failure rates and excessive maintenance costs than anything else in the last 50 yrs.

Tom

[BradH]

Torque curve and power delivery not the same thing. Remember when Moose Racing got its start and was offering bolt-on flywheel weights to help 2-strokes be more tractable? Those wouldn't have made any difference to the measured torque on an engine dyno.

FWIW, I worked in a Yamaha & Suzuki dealership for seven years starting in 1983, was an active amateur motocross racer from about 1980 through the mid-90s, and continued to ride offroad for some years afterward. Although I never owned one of the "next generation" 4-stroke bikes, I did have opportunities to ride them. My comments about how they behave differently on traction-limited terrain are from my personal experience, not something that I read in a magazine.

And, yes, the bikes have evolved way past the point where Joe Dirtbike can handle the majority of the maintenance by himself, with a corresponding increase in both initial purchase price and cost of ownership.

[brentry]

If you want to figure out what is easiest to ride just / the starting rpm by the ending rpm. Obviously something that makes a ton of power in a 3000 spread isnt that easy to ride.

[brentry]

So yes. The dyno will tell you.

[BradH]

How can dyno testing at WOT tell you anything about throttle response, or how it behaves when rolling on the throttle vs just turning it wide open, or how the engine behaves under varying conditions under which it would be more prone to lose traction under acceleration?

Do you know or remember anything about how Honda tried different firing orders and crankshaft configurations in the V4 500cc 2-stroke Grand Prix engines in the late '80s and into the '90s? They used one version referred to as The Big Bang, which essentially had all 4 cylinders firing at the same time. Why, because that configuration reduced the frequency of the power pulses and made the bike easier to ride. They referred to the other configuration where 2 of 4 cylinders were paired to fire at the same time as The Screamer. Eventually The Big Bang approach was eliminated by rules changes.

I believe that shares some similarities to power delivery differences between single-cylinder 2-strokes vs single-cylinder 4-strokes. The 4-stroke transfers the power to the rear tire in a less abrupt manner, making it easier to stay under control during acceleration under "less than ideal" traction conditions. I don't know how much can be done to tune or refine a 2-stroke to mimic this; for a single-cylinder engine, it's pretty much the nature of the beast.

[Steve.k]

In the mtns of western canada the lighter two stroke is king.
https://www.ski-doo.com/en/technologies ... gines.html

[PSA]

How can dyno testing at WOT tell you anything about throttle response, or how it behaves when rolling on the throttle vs just turning it wide open, or how the engine behaves under varying conditions under which it would be more prone to lose traction under acceleration?

Do you know or remember anything about how Honda tried different firing orders and crankshaft configurations in the V4 500cc 2-stroke Grand Prix engines in the late '80s and into the '90s? They used one version referred to as The Big Bang, which essentially had all 4 cylinders firing at the same time. Why, because that configuration reduced the frequency of the power pulses and made the bike easier to ride. They referred to the other configuration where 2 of 4 cylinders were paired to fire at the same time as The Screamer. Eventually The Big Bang approach was eliminated by rules changes.

I believe that shares some similarities to power delivery differences between single-cylinder 2-strokes vs single-cylinder 4-strokes. The 4-stroke transfers the power to the rear tire in a less abrupt manner, making it easier to stay under control during acceleration under "less than ideal" traction conditions. I don't know how much can be done to tune or refine a 2-stroke to mimic this; for a single-cylinder engine, it's pretty much the nature of the beast.

The big bang engines had two cylinders firing at the same time (or close), and I believe they all used it until the 2 strokes were gone. They were uncontrollable without it, well for all but Doohan that used it just to get in to the heads of the rest that couldn't ride them.

[BradH]

History of the NSR500 evolution with explanation of The Screamer vs The Big Bang configurations. The Big Bang was not the version that only fired 2 cylinders at the same time: https://www.boxrepsol.com/en/technology ... evolution/

[brentry]

Won motos at LL
And lot holeshots so obviously it works

[PSA]

History of the NSR500 evolution with explanation of The Screamer vs The Big Bang configurations. The Big Bang was not the version that only fired 2 cylinders at the same time: https://www.boxrepsol.com/en/technology ... evolution/

That makes no sense; "not the version that only", but English is my 4th language out of 3.

They call them "Big Bang" because two cylinders are firing at the same time (or close to the same time). If you would do it to an inline 4 cylinder 4 stroke, it would be 1&2 then 3&4 that fire at the same time. Screamer is 1342 or 1243 on the same engine.
https://en.m.wikipedia.org/wiki/Big-bang_firing_order

[BradH]

Hmmm... you'd think Repsol would be more accurate about their own technical history. All 4 on one firing sequence didn't make sense to me, either. Thanks for the link.

[BradH]

Won motos at LL
And lot holeshots so obviously it works

Congrats.

I don't think you and I are talking about the same thing. If you think we are, then we're definitely not on the same page.

That's life. Carry on...

[levisnteeshirt]

What about a twin carb 125 cc 2 stroke to pounce on the 250cc 4 strokes

1 carb feeds the case via a rotary valve that is smaller to pick up low speed response ( probably a EFI deal now but smaller ) that opens first in the operation of the throttle , then a 2nd carb or EFI opens feeding a typical reed valve for a larger air flow for higher speeds

In addition to the typical governor controlled or electrically operated exhaust power valve , add a rotary aimed transfer port exit to control flow of intake air away from the exhaust to enhance low speed torque and smooth out the power

There has to be a way to make 2 strokes competive again

Puch built twin carb bikes in the mid 70s, KTM has built carb + efi bikes for years. It's not about HP it's about the torque curve.

Tom

The Puch 250cc twin-carb motocross bikes of the mid '70s had one carb feeding the standard piston-port entry with the second carb feeding a rotary valve from the case under the cylinder. Harry Everts won the 1975 250cc Grand Prix Motocross championship on the factory bike, with a limited number of production versions made for 1976.

4-stroke vs 2-stroke is more than HP and torque; it's the power delivery of the 4-stroke where the biggest difference shows up, especially on tracks with less than ideal traction. On a hard-packed track where the 2-stroke requires much more throttle control to keep from breaking loose, the 4-stroke's smoother delivery lets the rider get on the gas harder with far less worry about hooking up.

I didn't know this , good info

[levisnteeshirt]

One thing about 2 stroke moto x is that it is hard, thats what made it good. The riders now dont look past 15 ,, Tee ball is free to watch if i want to watch kids,, same thing with whats going on in nascar and NHRA ,, if a 18 yo girl can beat a seasoned vet, something is wrong ,, IMO