Bottom end grunt comparison

Im looking at getting another bike soon, i currently have a stock 02 yz426f. And am looking at getting something lighter, im looking at an 08 yz450f, im just wondering how the bottom end grunt is on the 08 compared to the 426, cause i love bottom end snap, and i ride alot of sand so i need that bottom end power to be able to pull hard out of corners, i love my 426 power, but wish it was lighter

For a stock '08 to have the grunt that a 426 does, it would need:

 

  1. A different pipe.  Any pipe. Good pipe.  Cheap pipe.  Used pipe.  Stock pipe off an earlier year. Anything but the stock pipe.
  2. The CDI box from an '06 YZ450F, or an aftermarket unit that can set the same advance curve.
  3. The exhaust cam from an '06.

...pretty much in that order.  But compared to what you would have to do to the 426 to make it handle like the '08.... Piece of Cake.

Grey, what are the engine differences between the 06 - 09 engines. Each year what did they changed, cams, cdi maps, pistons, transmission.. Why are the older bikes like the 426 and 03 - 05 450's such grunty bikes?

There were 3 different cam sets and 3 different CDI timing maps used; '06, '07, and '08-'09.  Each was slightly more biased to ward more power at higher speeds.

So 06, 07, 08 all have different cams, just one cam or both. And which year cam was the torquiest down low? What about the maps, which one is the most responsive?

Both cams, and I already said that the '06 is the torquiest of the lot.  You can, however , use only the '06 exhaust cam in an '08 and get good results that way.

 

If you're looking for complete uncontrollable brutality, use the cams and CDI from an '03-'04 (but not the CDI from an '05, it was a slug by comparison).  This is possible, but I don't think you'll like it as much as the '06 set up.  I race my '06 in the desert, and I don't ever remember wishing I had more bottom end.

So will the 03-04 cams go in no problem? Is the timing procedure the same? And you can also use the 03-04 cdi box on an 08? And yeah i love a bruttle snappy bottom end hit

Also grey, what about 426 cdi, is the 426 cdi compatible and is it more agressive than the 03-04 cdi map?

Also grey, what about 426 cdi, is the 426 cdi compatible and is it more agressive than the 03-04 cdi map?

 

No.

So the 03-04 maps are the gnarliest?

So the 03-04 maps are the gnarliest?

if you want bottom end grunt your on the wrong side of the stroke spectrum, two strokes are where bottom end is king, four strokes are all about the top end

if you want bottom end grunt your on the wrong side of the stroke spectrum, two strokes are where bottom end is king, four strokes are all about the top end

 

That's about as backwards as anything I ever heard.

Im not trying to argue but look at a dyno and look at where four strokes make there power and where two strokes make there power. Plus my 09 yz450f made about 33 ft pounds of torque and my yz250 made about 36 ft pounds in stock form. Four strokes make there power from about 4 grand up to 10 give or take. Two strokes make there power from about 2000 all the way to about 7500 when they sign off. If youve watched any enduro or crazy woods riding two strokes are favored because they are about twice as hard to stall and will go from an a stand still up a slimy hillclimb with no hesitation. A four stroke will just stall and leave you kicking. Four strokes are preffered when low rpms arent necessary, like mx, and dessert and you can get away with them in harescrambles also.

If you think low RPM tractability isn't necessary in desert racing, you haven't done any of that lately, either. There's all kinds of twisty, rocky fun out there in the badlands, trust me.

 

The tendency for modern MX four-strokes to stall has nothing at all to do with the question of whether they produce torque at low RPM; that's a rotating mass issue.  The YZ450 in particular has a magneto flywheel smaller than its gas cap that weighs almost nothing.  The WR's, XR's, and EXC's of the world don't really exhibit that kind of behavior.  Stall tendencies don't have much to do with torque output, either, frankly.

 

It's a fundamental of reciprocating engines that the longer the stroke in relation to the bore, the lower RPM range the engine will efficiently operate within.  Two-stroke singles are necessarily built "under square", meaning that the stroke is longer than the bore diameter, because they are so heavily dependent on crankcase compression for their operation.  Absent a supercharger driven by a power source separate from the engine, they have to be set up this way if they are to have any useable low end power at all.  That hampers their ability to rev, but helps them at low speeds.  Notice that a typical YZ250 produces in excess of 25 hp from about 3600 to 7000, where the power drops off hard.  Also notice that at low speeds, two-stroke dynamic compression is very low compared to a 4T due to the exhaust port being open until around 90 degrees BTDC.  That reduces the resistance that the compression stroke puts on the engine.  Now add that they fire on every rotation instead of having to spin over twice without power at low speeds while loaded, and you have less stall tendency.

 

By contrast, four-strokes can't produce a level of horsepower that's competitive with two-strokes in the MX world unless they do it by building RPM.  Since crankcase compression isn't needed, and dynamic compression is actually better at low speeds than high, the stroke can be shortened, and shortened radically, without sacrificing good low end pull.  Whereas two-strokes are built under square, modern racing 4T's are "over square"; the bore diameter is a larger dimension than the stroke, often by a very large ratio. This gives the ability to run into a very high rev range.  Because a four-stroke also depends a great deal less on harmonic tuning, that range is also very wide by comparison,  A good stock '06 YZ450 makes 25 hp at 4200 RPM, and stays above that number until 11k, twice as long as the YZ250.  And even though it wasn't built with low speed power in mind, it produces over 20 ft pounds of torque from 3700 to the rev limit.

 

Four-strokes originally built for off-road rather than for MX don't stall so readily.  They have less aggressive cam timing and more rotating mass, and can pull like tractors all day long.  The main reason people like two-strokes over 4 in off sticky off-road conditions is their weight.  They're usually a little lighter and less top heavy, which makes them easier to manhandle through miles of slop.

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