HP vs. Torque

16 replies to this topic
  • Thumper33

Posted September 18, 2001 - 02:21 PM


I'm starting to look into different mods and getting more power, but I don't want to make this into a "all power, no torqe" machine. My questions are about which of these mods do you think will adversely effect the low end on the bike. I guess I should say my setup is stock except I've taken out the stock exhaust insert and done a few free mods seen in my sig. I desperately need to quiet the beast down a bit since it's loud as hell, therefor some kind of exhaust mod is a high priority. Might as well do what suits my riding style.

Here is a list of what I'm looking at doing, and my understanding of what happens to the power curve:

1.) Exhaust Modification/Replacemnt: I know exhaust can have a HUGE effect on how an engine produces power and torque. My understanding is that first off, the factory exhaust if very very heavy. I want to lighten my bike up, and especially since exhaust is high weight, it seems like a no-brainer. I've been told that any aftermarket exhaust complete system that I go to will give me lots of power increase, but actually hurt my low end torque. If this is true, can I just replace the muffler and get power, but not lose much torque?

2.) YZ Timing: My understading here is that the factory just retarded our timing to make them breath cleaner, so changing the timing will not adversely effect anything. Am I wrong? Is there a down side to YZ timing?

3.) BK Mod: I'm not sure exactly why the bk mod works, so I haven't done it yet. I don't like doing modifications that I don't understand what they do. I understand that it controls how long the accelerator pump squirts fuel in each pulse, but could someone touch on why this helps get more power and what it would do to your torque.

I appreciate any responses.

2000 WR400F - Throttle stop shortened, Grey Wire Cut, Filter Cover Removed, Panoram Computer; MNevitt@OffroadToyStore.com

[This message has been edited by Thumper33 (edited September 22, 2001).]

[This message has been edited by Thumper33 (edited September 22, 2001).]

  • funktree

Posted September 18, 2001 - 03:03 PM


if you get one of the tunable exhaust systems you can make your own powerband to a certain extent. I played around with different numbers of discs on my white brothers E sereies and you can really change the power delivery. The white bros pipe saved me 3 pounds according to the scale in my fishing tackle box, i would suspect its actually a little more than that. My bike is canadian though and came with the worst exhaust ever put on a dirt bike. I highly recommend the YZ timing as well, gave me a big boost in power, especially in the midrange without losing anything anywhere else. it made the bike more ridable I thought.

  • funktree

Posted September 18, 2001 - 03:07 PM


I just noticed the part about the loudness, my silencer isnt the quietest out there at all. My stock pipe didnt have a baffle to remove so I havent heard what that sounds like. Any quiet pipe will probably give you more lowend power, so you might be able to have your cake and eat it too.

  • Thumper33

Posted September 18, 2001 - 06:22 PM


I've been looking seriously between two recomendations from BajaDesigns: Big Gun Exhaust or FMF PowerBomb header and IV Squared Exhaust. I'm leaning toward the Big Gun setup because of the ceramic coating. Being an engineer, I've always been a fan of higher technology. Keep the heat in that pipe :) Anyone use this setup, or know if it's worth it. It's very expensive!!!! I also like that they advertise gains in low mid and high. Also, anyone know a cheap place to buy these?

[This message has been edited by Thumper33 (edited September 18, 2001).]

[This message has been edited by Thumper33 (edited September 18, 2001).]

  • Hick

Posted September 18, 2001 - 08:37 PM


3.) BK Mod:
All it does is limit the stroke of the mechanism that operates the accelerator pump. The carb body has a flange in the casting that was obviously designed to house a spring-loaded adjusting screw that butts the flat edge of the plastic pump actuating arm that was obviously designed to accept just such an adjusting screw.

So I wouldn’t say this at all alters the power characteristics of the bike other than clearing up what is, IMO, an irritating carb glitch on the YZ/WR thumper.

1.) Exhaust:
This isn’t my territory, but on a YZ the freer flowing stuff robs the top end, like the PC T4 did to my ’00. Louder than many airplanes, and as big around as a beer can, it helped the low end and mid but mellowed the over-rev. The FMF “Q” silencer that I have on my ’01, which is obviously more restrictive than the stock, straight-through YZ can, LOVES to rev. It sounds like a gigantic bait-fishing reel with a killer whale on the other end stripping the line off it.. Like a KTM would sound, if it could even fathom spinning at 11,500.

2.) YZ Timing:
Again I’m out of my league but I would say one downside to YZ timing is they are a bit less tractable and seem to stall easier. You have more low-end torque IMO w/ YZ timing, but it isn’t quite as smooth as the WR.

Hope this helps.

  • Taffy

Posted September 18, 2001 - 11:24 PM


i have a 99WR

powerbomb header gave me a slight increase in torque

BK mod is there to clean up the carburation and if that means more power? fine!

YZ timing
increase in power from bottom to top all the way. listen, it's noticeable everywhere but higher up it blows your mind the first time and everyone says "it gave me more top end" people often comment on the mod after the first ride when, sometimes they haven't so much as touched the carb. IMO there is no draw back at all to this mod.

funktree doesn't like the std euro/canuck/aussie pipe fair enough. it's yamaha's and their YZ pipe can't be beaten by a single aftermarket pipe manufacturer-it's that good. so i think that given the incredibly low DB's-it's a superb pipe.

i live nr Newmarket, suffolk and you may have heard that it's famous for it's horse racing. no problem going past stables on the throttle, horses in the street etc.

i "indexed" the cams and gained a lot more torque so if you're clever enough you should look at this.

i'm told the vortex ignition cleans up the bottom end and aids 'releasing' the power. mitch is selling me his and i will be able to tell you in mid winter.

boring the engine to 420cc or 445cc will aid torque GAURENTEED.

someone is about to experiment with a 37mm carb off a WR250 on their DR400 and the numbers will be available we all hope soon.

like you, i've been looking for either "something for nothing (no loss elsewhere)" or an increase in torque.

don't forget your white bros air filter, work, work and work again at your carburation because quite honestly the jetting is all over the show with this model. i went to the DR site last night and in about 20 days there have been two threads. look at this side and we have about a dozen!

i'm also getting a CF airbox so we'll see what that does as well.


  • tbronco

Posted September 19, 2001 - 02:01 AM


In my experience, the WR stalled much easier before going to YZ timing.

If you don't tune the jetting, you can't make a comparison. Period.

Also, be careful. The WR's timing was not retarded, it's advanced. If you advance it from WR spec, you'll be riding an oversized XR200. And it will stall very easily.

In 3 years I haven't been tempted by aftermarket exhausts. I have contemplated a YZ silencer though. But hey, I figured it would be like training with ankle weights to ride with the stock exhuast.

98 WR400 Dualsport!

[This message has been edited by tbronco (edited September 19, 2001).]

  • Ynahg

Posted September 19, 2001 - 02:48 AM



What do you exaclty mean whith indexing the cams ?

And are you sure you don't lose any tractability when going to YZ ?


'99 WR400 F, YZ exhaust, throttle stop trimmed, WR timing, airbox cover removed

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  • Taffy

Posted September 19, 2001 - 03:27 AM



indexing the cams
in england i would simply call that getting the valve timing correct. there is a company in clearwater, florida that makes variable timing camwheels. the idea is that the camgear is split into an outer wheel which pinch screws to an inner wheel which presses on your cams.

after much checking you alter the cam position so that it opens at the perfect moment.

the company are called falicon crankshafts. if you go into search and put Taffy in it will come up with the full story from beginning to end.

the figures are ones that i made up after studying the valve duration and lift etc. one or two boys poo-poo'd it and one or two tried to do the job without success.

then a works mechanic called BK told everyone that you should do it over on the YZ side. after that of course they all pulled their fox boots off as a sign...ooops! there i go again (life of brian eat your heart out!).

anyway there are figures on the YZ side that are good. YZ's are peakier than i need and because i'm about 3D (degree's) out in the CORRECT direction, far from confirming that i don't know what i'm doing they actually confirm that my hunch was spot on!

it is a significant difference. anyway don't ask me to repeat everything. go find it. read up-even bring it back up!

it has too much power but it's still tractable. it lugs, digs, rips and does it. listen if i'm trying to ride out of 30cm (for you mr belgiun!) of mud and gloop i wish i had an XR200 with a misfire! other than that i want what i have.


  • funktree

Posted September 19, 2001 - 05:40 AM


the stock yz pipe is great, the stock wr pipe is junk. My white bros made it a whole new bike.I actually tried to buy a stock yz pipe from you guys but couldnt find one quick enough so I went with the WB

  • Terry_in_Reno

Posted September 19, 2001 - 09:09 PM


The FMF power bomb does have a benifit in low end torque. I changed to the YZ timing and noticed a shift upward in the power band more than an increase. The power bomb brought back some of the low end without effecting the mid to upper band.

The FMF power core IV was just slightly quieter at low RPM, but about the same as the unplugged stocker on the upper end noise.

I found the BK mod really helped get the adjustability needed to get the carberation dialed in. For YZ timing and pipe changes it is essential to jet the bike accordingly. Their is much to be said for the performance hiding in carberation.

My 2 cents.

  • Ron_in_SoCal

Posted September 20, 2001 - 11:46 AM


Anyone try Stroker's Torque Cam? Check it out Here. You may also consider a flywheel weight, makes one heck of a difference down low.

  • Taffy

Posted September 20, 2001 - 12:40 PM


i asked them about these about 10 months ago and they didn't know anything about them, they'd never had any and no valve lift or duration figures were available. i think they've been advertised for nealy 1.5 years!

maybe they've done something at last.

ynahg are you ok on that?

i just checked the vaults for old threads and found them. if someone cleverer than me could do a 'click-on' i'd appreciate it!

they're called
more midrange
more torque

funktree, calm down! ok so you don't like the pipe. i happen to have tinitus in my left ear due to 13 years of roadracing a duke on an open pipe. i value silence over performance.

happy times everyone!


[This message has been edited by Taffy (edited September 20, 2001).]

  • Ron_in_SoCal

Posted September 20, 2001 - 01:36 PM


I am checking into the specs for the cam and will let you know what I find out…

  • MN_Kevin

Posted September 20, 2001 - 03:55 PM


Are you saying that Stroker knew nothing about their own cams?

  • Taffy

Posted September 20, 2001 - 09:58 PM


absolutely, kevin.

i did it by e-mail because a call from over the pond would be expensive. that was their answer. no good to man nor beast. i think i mentioned it on a post at the time.

these bikes have produced more bhp than any other 400 for four years after it was developed, stroker would have done well to improve on it.

sometimes people do this kind of thing to guage public interest. they ask "their" cam grinder "can you improve on standard?" and when the answer is an obvious "yep" they go off to guage public response.

all cams can be tuned by doing what is known as a base grind. this is very cheap and worth a coupla bhp.

basically all cams are designed so that they run quietly. the cam grinder takes away this quietening ramp and takes metal off the bit of the cam that doesn't (i'll repeat that-DOESN'T) OPEN THE CAM. this is called the cam base.

yamaha put enough hardening in their cams for the cam grinder to take .5mm off. "aah!" i hear you say, "how does that help produce more power?". well you simply start using .5mm thicker shimms to take up the slack and then when the cam lifts the valve it is no longer say 8.25mm lift but 8.75mm lift.

this only works if the restriction is the valve. i know, i know; the valve must get in the way? no it doesn't!!!

the port might already be to small-so it needed porting first.

the port may have too sharp a bend.

even the shape of the back of the valve is important.

generally if the cam doesn't work opening further it should react to being open longer.


  • Ynahg

Posted September 20, 2001 - 10:46 PM


Thanks a lot Taffy



'99 WR400 F, YZ exhaust, throttle stop trimmed, WR timing, airbox cover removed


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