Two vs. Four

[BNaylor]

Originally Posted by NaKeDZX

I 100% agree with everything he said. I was just offering an explanation of why they see fit to give the 4s a displacement advantage.

I'd still take a 4 stroke anyday. I prefer the way they make power. Like you said, different strokes....

Four strokes are good bikes. I had a Yamaha YZF 426 but the maintenance costs were too high when it came time for re-doing the top end.

The most simple explanation of the 4s displacement advantage is they need it to be competitive. For example: we had a racer that got caught with an overbore kit on his YZF 250 and got bumped up to the 250 cc
class from 125cc class. Needless to say he did not not fair too well on the track against the 250cc two strokes, often getting lapped.

As to the CRF 450s and YZF 450s, they are not stock when competing against two strokes with AMA legal mods such as porting, Pro Circuit or FMF exhaust, V Force Reeds, etc which doesn't cost an arm & leg in dinero.

A box stock Honda CRF 450 or Yamaha YZF 450 cannot beat a modified two stroke 250 on a AMA type track. For proof just check out the Supercross racing results....although they are factory works bikes, the two strokes dominate. When the regular season starts they will switch over to the thumpers because they shine on hilly tracks with long strait aways but the two-strokes always surprise everyone. Lets see how James Stuart will do this year riding a Kawasaki works KX 250 against all the thumpers. Reed and Carmichael will switch over to their works thumpers.

[emkay4597]

in addition to all that, if your working on your bike at home and truck it up the hills your going to have to second guess carb jetting for altitude, if its a significant difference.... i think a four stroke would be more forgiving in this respect.

[emkay4597]

Originally Posted by crwth2

i'll respond on the drag strip case you cant get any power with a two stroke on the drag strip due to weight of the vehicles on the strip now you take a lighter 2 stroke bore it out and then place a four stroke next to it and yeah your gonna smoke it if they are the same (or close to the same) size and lets be real how many two stroke motorcycles out there ten years or older hasnt been bored out due to the fact that two stroke engines wear out pretty quick if you realy ride them hard

well, what about drag bikes? they all seem to be 4's ?

[emkay4597]

Originally Posted by NaKeDZX

The reason they find it fair to compare and race double displacement 4 strokes against 2 strokes is that the 2 stroke fires on every crank rotation instead of every other. So a 250 2 stroke makes "500cc's worth" of power in the same number of crank rotations as a 450 4 stroke. It's all in how they make the power.

not only that but what you gain in rpms you lose in stroke and this would be a fair arguement if rpms were limitless so you realy need to compare hp torque @ redline i'm a fan of both realy, i even think diesels are cool... theres an aircraft 2 stroke diesel motor that makes 350 hp and weighs 350 pounds, (i've heard) It runs on jp1. because of worldwide availibility... i'll try to find a link for this motor... also what about akt bikes? don't they have some crazy ass motor?

[emkay4597]

ok, check this out...?
http://tec.avl.com/wo/webobsession.s...wMDMxMDY3.html
a diesel two stroke motorcycle?

[BNaylor]

Originally Posted by emkay4597

in addition to all that, if your working on your bike at home and truck it up the hills your going to have to second guess carb jetting for altitude, if its a significant difference.... i think a four stroke would be more forgiving in this respect.

Thats true. Two strokes are more sensitive to humidity, temperature and altitude versus four strokes.

It's just something a two stroke owner has to consider and deal with. Doesn't hurt to keep jetting kits handy. I can swap out a needle, pilot or main jet in under 15 minutes and the manufacturer's shop manual has various charts to help you make an educated decision on jetting.