2 vs 4

die996 · Post by **die996** » Fri Jan 20, 2012 4:03 pm

DJ99X wrote:(about rotary engine bikes) I'd say they'd be too heavy

Yeah, that's why the Norton 588 smoked all the competition (basically 750cc V4 and 1000cc V2) with a lot less displacement...

Axle McOctane · Post by **Axle McOctane** » Fri Jan 20, 2012 4:33 pm

Shadow wrote:
jlv wrote:
DJ99X wrote:So is it still burning 125cc of mixture? I know two-strokes have a compression ratio of about 8.5:1 whilst the new four strokes are more like 13.5:1. But are the two strokes actually getting the full 8.5:1, or is that just a comparison of the volume at the lowest point to the volume at the highest point (i.e not the true amount of compression)
Since the exhaust port is about 1/2 the height of the stroke, probably around 62.5cc multiplied by whatever boost you get from the pipe. So around 75cc if I remembered the 1.2 times pressure correctly. That would be burnt mixture. If you're counting what actually flows through the motor it would be higher.
Going with jlv's numbers it would take a 208cc 2-stroke to burn 125cc of mixture during one power stroke.

I like where this is going, 210cc DI 2-strokes against 250fs.
I'm pretty sure that would crush the 4-stroke in both power and efficiency.

SHWIING

On The Pipe · Post by **On The Pipe** » Fri Jan 20, 2012 8:37 pm

Rotarys are good but they get horrible gas mileage and have low torque. Yeah Vital is nothing but a bunch of trolls i just surf the site sometimes to see whats going on it ridiculous the stuff that happens over there. Im not sure how you could tell by effiency there probably is some complex equation to tell though. I would say the 4 strokes are more efficent at putting all that power to the ground.

Im still waiting for direct injection KTM hopefully 2013?

haydugjr · Post by **haydugjr** » Fri Jan 20, 2012 9:21 pm

die996 wrote:
DJ99X wrote:(about rotary engine bikes) I'd say they'd be too heavy
Yeah, that's why the Norton 588 smoked all the competition (basically 750cc V4 and 1000cc V2) with a lot less displacement...

I'm willing to bet if a rotary bike came around, it'd be a repeat of the Cannondale fiasco. Can't believe they closed down :'(

Post by **jlv** » Sat Jan 21, 2012 12:26 am

Shadow wrote:Going with jlv's numbers it would take a 208cc 2-stroke to burn 125cc of mixture during one power stroke.

I like where this is going, 210cc DI 2-strokes against 250fs. :mrgreen:
I'm pretty sure that would crush the 4-stroke in both power and efficiency.

If I had to choose between direct injection or a CVT transmission I'd take the CVT. All DI gets you is less emissions and better efficiency which isn't important for MX. I'd be really curious how much HP a 125 would make if it only had to run at a fixed RPM.

DJ99X · Post by **DJ99X** » Sat Jan 21, 2012 10:07 am

On The Pipe wrote:Rotarys are good but they get horrible gas mileage and have low torque. Yeah Vital is nothing but a bunch of trolls i just surf the site sometimes to see whats going on it ridiculous the stuff that happens over there. Im not sure how you could tell by effiency there probably is some complex equation to tell though. I would say the 4 strokes are more efficent at putting all that power to the ground.

Im still waiting for direct injection KTM hopefully 2013?

My assumption was that a 125 and 250f burnt the same amount of fuel/air mixture at any RPM, and as the 250F torque curve is higher at pretty much every RPM, it is the more efficient engine. Although, now that JLV has stated that 2strokes dont actually combust the full capacity, my theory goes out the window.

Ballsfalsky · Post by **Ballsfalsky** » Mon Jan 23, 2012 10:58 am

Isn't MX, or any other form of dirt biking about fun? Who cares about what bike is more efficient. 2 strokes will always be more fun because of their run&gun power, their handling characteristics, and their lower maintenance costs. Some will disagree, but that's OK.

Also, the way it SHOULD be looked at, is power to weight. A 250 2 stroke weighs roughly the same (give or take 4 or 5 lbs) as a 250f.

For 2009, the YZ250f made 33.37HP @ 11,900RPM, and 17.43lb-ft @ 8,700RPM and weighed roughly 226lbs (full of fuel). P:W = 1HP:6.8lbs & 1ft-lb:15.7lbs

For 2009, the YZ250 made 41.3HP @ 8,400RPM, and 28lb-ft @ 7,600RPM and weighs roughly 229lbs (full of fuel). P:W = 1HP:5.5lbs & 1lb-ft:8.1lbs

From what I see there, the 2 stroke needs to rev 3,500RPM less to make 8HP more, and needs to rev 900RPM lower to make 10.5lb-ft more. So in reality, 2 strokes produce both more torque and HP per CC, but do it less efficiently.

DJ99X · Post by **DJ99X** » Mon Jan 23, 2012 11:29 am

Completely missed the point of this thread. I don't give a hoot about what people think is better. We're talking technical here.

Also, as we've established, it is unfair to compare the two types of engines based on displacement. Of course a 250cc two stroke will make more power. It fires twice as often

Axle McOctane · Post by **Axle McOctane** » Mon Jan 23, 2012 3:22 pm

Someone ough to come up with an 8 stroke. That way, it would be fair to race a 500 against a cr125. Now lemme get my umbrella

ShackAttack12 · Post by **ShackAttack12** » Mon Jan 23, 2012 3:36 pm

jlv wrote:
Shadow wrote:Going with jlv's numbers it would take a 208cc 2-stroke to burn 125cc of mixture during one power stroke.

I like where this is going, 210cc DI 2-strokes against 250fs.
I'm pretty sure that would crush the 4-stroke in both power and efficiency.
If I had to choose between direct injection or a CVT transmission I'd take the CVT. All DI gets you is less emissions and better efficiency which isn't important for MX. I'd be really curious how much HP a 125 would make if it only had to run at a fixed RPM.

I can't quite understand why you think efficiency is unimportant. Doesn't efficiency = power (for the most part)? IE: Efficiency is a measure of how well an engine can turn fuel into power. A more efficient engine could be lighter and make the same amount of power.

A Direct Injected 2-stroke engine would be better than a 4-stroke in motocross competition, hands down. Just based on the numbers that were discussed earlier, 2-strokes waste about 30% or so of their potential because some fuel in unburned and passed through the exhaust. Imagine a 2-stroke system that completely eliminates unburned fuel. Instead, all (or a lot more) of the fuel will be converted to power.

I remember reading specifically about DI 2-stroke marine applications and their claims of lighter weight to make the same amount of power.

Think about it. A 2-stroke engine only needs 1 revolution of the crankshaft to complete a power cycle. A 4-stroke needs 2 revolutions. In a 4-stroke, for 1 revolution, there is no power stroke, and thus the piston is actually decelerating until the next power stroke due to friction and compression.

die996 · Post by **die996** » Mon Jan 23, 2012 3:42 pm

Le Mans will have its first fully electric-powered entry this year with over 800hp and less than 1100kg, can't wait till they come to motocross

Racing electric bikes would be awesome for the riders and bad for the crowd. Just imagine you riding and really hearing the crowd cheering (only sound would be dirt ripping, shocks, air and crowd)... or the stadium getting awkwardly silent, LoL

Axle McOctane · Post by **Axle McOctane** » Mon Jan 23, 2012 3:58 pm

The day mx goes all electric is the day I hang up my boots. I would prefer the sound of a diesel tractor four-stroke a million times over to a quiet electric buzz. I guess you could really yell and talk shit to your competitors though.

Kawasakis · Post by **Kawasakis** » Mon Jan 23, 2012 4:01 pm

And again die.. I bet that like 90% of dirt bike racers won't agree with you on this one again.

die996 · Post by **die996** » Mon Jan 23, 2012 4:05 pm

Axle McOctane wrote:The day mx goes all electric is the day I hang up my boots. I would prefer the sound of a diesel tractor four-stroke a million times over to a quiet electric buzz. I guess you could really yell and talk shit to your competitors though.

That's the good thing, it'd be a complete revolution for riders when there's a huge crowd. Most of the riders can't imagine how crazy things get when there's some good battle for the lead

Accept it, we'll get electric in about 5 years. Racing is the biggest and best laboratory you can have to develop some technology. 2nd era (2000s) 4-strokes MX bikes were developed in a racing enviroment

rideblue56 · Post by **rideblue56** » Mon Jan 23, 2012 4:14 pm

Kawasakis wrote:And again die.. I bet that like 90% of dirt bike racers won't agree with you on this one again.

I agree with him. Its the future. Once the battery technology catches up with the idea, it'll be an obvious choice. 100% efficient motors, no emissions, cost to "fill up" will be in the cents range rather than a $10.00 range..

We may not like it, but thats how it will be.

Axle McOctane wrote:The day mx goes all electric is the day I hang up my boots. I would prefer the sound of a diesel tractor four-stroke a million times over to a quiet electric buzz. I guess you could really yell and talk shit to your competitors though.

Youd really give up motocross because of the changing technology? Thats pretty sad, bro.

2 vs 4

Re: 2 vs 4

Re: 2 vs 4

Re: 2 vs 4

Re: 2 vs 4

Re: 2 vs 4

Re: 2 vs 4

Re: 2 vs 4

Re: 2 vs 4

Re: 2 vs 4

Re: 2 vs 4

Re: 2 vs 4

Re: 2 vs 4

Re: 2 vs 4

Re: 2 vs 4

Re: 2 vs 4