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4 stroke killin' 2 stroke idea

General engine tech -- Drag Racing to Circle Track

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levisnteeshirt
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4 stroke killin' 2 stroke idea

Post by 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
6.50camaro
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Re: 4 stroke killin' 2 stroke idea

Post by 6.50camaro »

Let them compete size per size . I thought it was BS back when it started , letting 250's run in the 125cc class and 450's in the 250 clad . Dan
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Re: 4 stroke killin' 2 stroke idea

Post by hoodeng »

The biggest killer of loop scavenge two strokes was not their competitiveness which was very high, it was their emissions in the end. A four stroke could never get the HP/Litre of a two stroke, hence the 125 two stroke 250 four stroke etc formula for parity. If you could get two strokes back on track there would be an emissions bun fight.

The king of marine diesel is still two stroke piston port engines by Wartsila-Sulzer and piston port/ex valve Hanshin engines.
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Re: 4 stroke killin' 2 stroke idea

Post by levisnteeshirt »

Closed course racing isnt effected by EPA BS
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Re: 4 stroke killin' 2 stroke idea

Post by leahymtsps »

levisnteeshirt wrote: Wed Oct 07, 2020 11:17 pm 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
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Re: 4 stroke killin' 2 stroke idea

Post by leahymtsps »

levisnteeshirt wrote: Thu Oct 08, 2020 6:08 am Closed course racing isnt effected by EPA BS
Actually the EPA did play an important part at the PRO level along with the AMA and manufacturers to
tweak the rules in the 4 strokes favor.

Tom
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Re: 4 stroke killin' 2 stroke idea

Post by BradH »

leahymtsps wrote: Thu Oct 08, 2020 6:24 am
levisnteeshirt wrote: Wed Oct 07, 2020 11:17 pm 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.
Last edited by BradH on Thu Oct 08, 2020 8:40 am, edited 1 time in total.
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Re: 4 stroke killin' 2 stroke idea

Post by BradH »

leahymtsps wrote: Thu Oct 08, 2020 6:28 am
levisnteeshirt wrote: Thu Oct 08, 2020 6:08 am Closed course racing isnt effected by EPA BS
Actually the EPA did play an important part at the PRO level along with the AMA and manufacturers to
tweak the rules in the 4 strokes favor.

Tom
A friend of mine who was a long-term employee in the offroad division of one of the big motorcycle manufacturers mentioned years ago that the EPA was involved in the move away from 2-strokes.
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Re: 4 stroke killin' 2 stroke idea

Post by 6.50camaro »

Emissions was only the Vail the over reaching EPA used to try and kill the whole off-road industry . It was not the few pro racers it was after it was the hundreds of thousands weekend desert riders woods riders trailer riders that they were after for going into there precise nature and enjoying it with something powered by an ICE . The AMA bowed down because they were afraid of the EPA out right banning all motorcycles off-road . The Japanese bike manufacturers met the challenge and developed lighter higher hp 4 stokes but they could never compete head to head with a 2stroke of the same displacement . So the AMA give them the edge to make i them competitive with an advantage and keep the EPA happy . Because who's going to race a bike that is now under powered .
Just my warpped view on it . Dan
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Re: 4 stroke killin' 2 stroke idea

Post by sbcharlie »

watch thishttps://youtu.be/P2hgK-wZ1XA
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Re: 4 stroke killin' 2 stroke idea

Post by n2omike »

BradH wrote: Thu Oct 08, 2020 8:34 am
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.
THIS

One of my best friends has a son who raced Professional MX. The son raced the stadium supercross you see on tv, and dad built the engines. They made it to the main a couple times, but it turns into a political mess after that, and it's basically just a competition between factory teams that use parts and technology nobody else has access to... even things like tires nobody else can get their hands on.

That aside... The tracks are INSANELY technical. Extremely tight, crazy whoops, big jumps right out of the corners, etc... and very, very technical. What it comes down to, is a 4-stroke bike is much easier to ride in these tight arenas. A 2-stroke has a much narrower power band, and shifting/gear selection is critical. The 4-strokes have a much broader power band with a LOT more over rev. There are a lot of places a 2-stroke will need shifted, and a 4-stroke will not.

Plus, nowadays they are indeed allowed to compete straight up. A 250cc 2-stroke can compete with a 250cc 4-stroke. Several years ago, my friend was making 48 rear wheel horsepower with a 250 4-stroke in MX trim. I believe power peak was around 14k rpm. Factory teams are using traction control, active suspension controls, etc that others have no access to. The factory teams don't even always go through tech, where others go through with a fine tooth comb.

For a couple examples... My friend and his son were burning up out of the hole, not able to get any traction out of the gate, while the factory guys were doing fine. He was able to secure two 'last years' rear factory tires, but they would not give him any fronts. With the factory tire, he was able to blast out of the hole with the rest of them. Was never able to get any more of those tires. I don't know if they were a special radial, or what... but they WORKED. Factory bikes also didn't have to sound test, and were able to use a larger exhaust silencer. Things become very political and it's all about sponsors and advertising for the factory bikes. Plus, the courses are pretty much designed to kill/maim people... all under the guise of entertainment.

But, the 2-stroke vs. 4-stroke is as stated. 4-strokes are easier to ride, and a lot more forgiving. Smoother power delivery, less shifting, more over-rev... while still having plenty of power.
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Re: 4 stroke killin' 2 stroke idea

Post by brentry »

Needs variable transfer timing.
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Re: 4 stroke killin' 2 stroke idea

Post by rebelyell »

circa 67-68 a karting champ w/ shop at Garden Grove CA had karts w/ twin MAC 101; each having two (Tillotson IIRC) pumpers. One piston-port and the other on a reed valve stuffer in case. Heck there may've been total 4 carbs; two and two. Also, IIRC, that champ's name was Jerry. I was chief floor-sweeper & gopher w/ a kart made of left-overs & cast off West Bends. In addition to MACs, shop also began selling/servicing Parilla-Tecno karts and Montessa & Bultaco bikes. I wasn't there long; military.
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Re: 4 stroke killin' 2 stroke idea

Post by David Redszus »

Currently, two cylinder, two strokes, 440cc, are running in snowmobile circle track Champ class.
They produce well over 125 hp.
And very soon, fuel injected engines will increase the power even more with much better drivability.
And with emissions equal to or better than a four stroke (without a cat).

The engine torque curve is determined not just by porting but by the shape of the tuned exhaust pipe.
There are trail sleds and there are drag sleds...and almost anything in between.
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Re: 4 stroke killin' 2 stroke idea

Post by sbcharlie »

Look up Thailand drag races. 150 cc bikes turn 9 seconds quarter mile
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