Cool article in dirt bike magazine

suzuki_dude

suzuki_dude

I thought this was entresting so i thought I'd share it with you guys.

Jimmy Jerrett's
Geico
Honda
CRF450R​

by: Tom Webb

Jimmy Jerrett a multi-time OMA champion and one of the fastest "technical" off-road racers, races a pretty much full blown motocrosser. All of the top off-roaders do. His Honda CRF450R is in no way de-tuned, softened or plugged up. It doesn't have flywheels, auto clutches, or long mufflers that soften the power. The suspension hasn't been revalved to be plusher; in fact, it's stiffer. He runs moto gearing on a hard-edged moto bike in terrain that lacks big triples and whoop sections. He races on slime, roots, rocks and on closed-course loops that get flat hammered. His Geico-sponsored Honda could easily race an outdoor national motocross. Why do the top hare scramble pros prefer the stiffer, faster and more volatile MX-based machines? Simple; they require a specialized machine that barks, is light, and stays up in the stroke when abused in hack ruts and wont bottom when they slam a road crossing tapped in fifth. A hybrid motocross machine fits the bill perfectly.

We had a chance to race test JJ's machine at the OMA Arkansas round and came away impressed and subdued. Frankly, it's too much machine for even an above-average woods rider and most definitly requires the talents of a skilled racer. The OMA events are more tachnical in nature than GNCCs, yet Jerrett races an identical machine. Here's a look at his wagon, his mods, and our thoughts on the fire breather.

it has pics but im typing this out from the magazine so im just gonna tell ya the mods

mods: FMF MegaBomb and 4.1, a Rekluse clutch basket, and standard gearing. IMS tank, low Renthal bars, Cycra hand shields, and Factory Connection suspension.

Conclusion

Transcribed from a voice memo from Tom Webb following the OMA race: "Luckily I got to ride JJ's wagon yesterday, so you would have thought that I'd have been clued in to it's appetite for todays race. Not even close! Heres what i loved: Great suspension. Factoy Connection did a remarkable job of stiffening the stocker, yet they gave it manners that sponge up the trail junk with great zeal. I carry two or three TV sets more than Jerrett in the tonnage department, yet i loved the bike's ability to stay strait, not deflect, swallow the big hit and give me enough cush to not whine too much. The Kenda rubber was excellent in the slimy conditions. JJ runs a Washougal front and a Millville rear, and both gave strong grip and feel and didn't pack up. Heres what hurt me: the clutch is a definite weakness. It lacked feel; the handle would go away, and this seemed to coax the flameouts. I conked, stalled, flamed and gorked an embarrassing number of times. the CRF 450 has what I call an FI stumble off idle, so it lacks a smooth flow of bottom power. When I tried to short-shift and carry higher gears through the woods, the ruts and the goo, it would inadvertently cough--always when i least expected or wanted it to. These are all things that i never feel on a motocross track, and quite honestly, they prove that skill and youth are needed to manhandle the Geico Honda CRF450R in the tight ribbons of goo-laced trail that have been layered with roots shined clean by a hundred knobbies."





 
CDA

CDA

Blury Pic:

2714_phx003.jpg
 
jackson409

jackson409

Ok, so I am not much on these 4jokes. So what is the deal with the expansion chamber on the exhaust. I know why two strokes have them. They serve a purpose but there shouldn't be any unburnt fuel going out the exhaust on these bikes do the 4 strokes having a intake and exhaust valve system. Are they just trying to look cool like a 2st :smirk:

nah really why? :noidea:
 
Y

yamadogyz

jackson409:
Ok, so I am not much on these 4jokes. So what is the deal with the expansion chamber on the exhaust. I know why two strokes have them. They serve a purpose but there shouldn't be any unburnt fuel going out the exhaust on these bikes do the 4 strokes having a intake and exhaust valve system. Are they just trying to look cool like a 2st :smirk:

nah really why? :noidea:
Click to expand...

:noidea: I have my OEM pipe and header, which is marvolous :hail:
 
snowflake

snowflake

jackson409:
Ok, so I am not much on these 4jokes. So what is the deal with the expansion chamber on the exhaust. I know why two strokes have them. They serve a purpose but there shouldn't be any unburnt fuel going out the exhaust on these bikes do the 4 strokes having a intake and exhaust valve system. Are they just trying to look cool like a 2st :smirk:

nah really why? :noidea:
Click to expand...

FMF, the creator of the "Power Bomb" stated a much different purpose for that design some time ago. Four stroke headers can be tuned using pipe length and diameter to sync with the pressure waves produced by the exhaust in such a way as to cause a wave of negative pressure to arrive at the exhaust valve as it closes, thus aiding both the extraction of the last bit of exhaust and the induction of the incoming fuel/air charge. This works, but like any harmonic event, it is most effective within a fairly restricted range of engine speed.

FMF claims that the PB (which is not a simple expansion chamber, but a perforated section of pipe with a chamber around it) acts on the pressure waves within the pipe in such a way as to make the header "appear" to the engine as if it has no defined length, causing it to be less effective at peak efficiency, but more effective over a wider range of RPM. At least in the case of the PB header for an '03 YZ450, this does seem to work.
grayracer513 from tt :)

although the stock wr system sucks, even with the baffle completly removed (in which case the exhaust is heavy, loud, and congestive), I saw a big improvement down low when I put my fmf full system on (powerbomb with powercore IV)
 
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