Boit

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After doing a ton of research and reading, I've decided that I would like to concentrate my energy and focus on maximizing the cylinder head on my '00 YZ426 instead of doing any displacement increases. I've emailed and talked to a few specialty shops about this and am leaning toward one in particular. What concerns me at this point is the possibility of losing the crisp throttle response that I've spent so much time and energy to attain. I have an incredibly powerful low-end snappy response that I don't want to lose or diminish one iota. Is it possible to retain this attribute. . . . and/or even enhance it while still adding to the mid and mid-top? I'm an engine-sensitive rider who avoids the rev limiter by about 1200 RPM so I'm not looking for a high revver. If I could keep or add to the off-idle power response plus adding in the mid, I would be in a higher Heaven. I use C-12 fuel and enjoy doing the jetting for maximum return. Any advice, ideas, opinions, experience...etc. would be appreciated.
 
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Rich Rohrich

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Boit - What were you planning on doing to the cylinder head?
 

Boit

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Rich; First, I had in mind to have the head flow benched and ported. I was hoping for some advice and guidance on other areas such as cam options and is this worth pursuing? Are slightly larger valves and anti-reversion cuts worth consideration? To get better flow numbers, do I have to sacrifice my marvelous low-end burst for a stronger mid? That sort of thing.
There is a fellow racer who is 6'9" and about 320 pounds who holeshots regularly on his 426. He's very secretive about his engine work but whatever it is, it works! If he has upped his displacemnt, more power to him(pun intended), but I'm not interested in going that route. I want to concentrate on the head area. I'm not expecting a huge horsepower gain, but a modest improvement with what is already there. Is this attainable?
 

Rich Rohrich

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The cylinder head can be made a little better but nothing earth shattering. The guys who are charging $700 plus for headwork are to put it bluntly, jerking people over.
I believe there is a lot to be had by using adjustable cam sprockets like the Falicon pieces and different base cam timing. I have some ideas on what should be done but so far I haven't seen anyone that is producing a cam that fits my needs. Even with a different grind, without a lot of dyno time you are rolling the dice. This engine responds incredibly well to an increase in displacement, an increase in compression, tightening the squish clearance, and retarding the intake cam timing. I'm afraid if you go the porting route alone you'll be disappointed.
 

Boit

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Thanks. Your info and opinion gave me something to plan around. The Falicon adaptors are something I am going to when I decide on a definitive plan. I'm not fast enough, or brave enough, to take advantage of an increase in displacement.
 

Rich Rohrich

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The mods I outlined above are generally part of a properly done big bore. The changes tend to clip the high end (above 10K) power a bit in favor of a much more broad torque curve. The peak torque increase isn't as important as the increase in the breadth of the torque curve. The low-mid rpm torque trough is pretty much eliminated especially when using oversized headpipes. All things considered it's the best mod I can think of for the average rider/racer. Flat trackers are about the only group that notice the difference in overrev power.

If you aren't comfortable with a displacement increase then by all means save your money and pass on the headwork. I've spent a lot of time with this cylinder head on the flow bench and dollar for dollar high priced headwork is a bad deal in this case.

A good interim approach would be to use an oxygenated race fuel like Phillips B35 or one of the VP MR series fuels. Lots of response and good top end power is just waiting. Probably the best bang for the buck for guys like you who are comfortable with jetting the FCR.

Let us know what approach you decide on.
 

Boit

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The advice on the fuels is my next step. I'm sure I can get the MR2 but not so sure about the Phillips. Am I correct to expect to go richer on the MR2 when I switch from C-12? It will be fun and beneficial to my learning experience to compare the differences. I'm all giddy now.:)
 

Rich Rohrich

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Originally posted by Boit
Am I correct to expect to go richer on the MR2 when I switch from C-12?

Yeah you'll definitely have to go richer, but given the heat and humidity you are probably experiencing this time of year it may just even out :)
 

sfc crash

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hey rich,wouldn't performance mods for a four stroke yzf motor be the same as a car, ie, port matching the heads to intake/exhaust manifolds, pollish the exhaust port(some in the auto world argue that leaving intake "rough" helps to atomize the fuel mix) increase vavle dia with a three angle valve seat, increase vavle duration( either with a cam and or rocker ratio) ,run richer on the carb, advance the spark timing in conjunction with higher octane fuel, and low flow air filter and exhaust? i fear i might have just shown my total ignorance of the subject, but i'm picking up my wr426 this weekend and had planned on those mods.
:think
 

Rich Rohrich

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If you aren't satisfied with the stock performance of the WR (pretty unlikely) there is very little that you need to do.
Everyone starts by cutting the throttle stop to YZF specs so the throttle opens all the way.
If you are still looking for more you can retard the exhaust cam 1 tooth on the cam gear to set your WR to YZF cam timing, uncork the exhaust by using a Vortip exhaust insert or something similar, get the jetting sorted to your liking, fill it with good gas and leave it alone. The rest of the stuff you've described is essentially a waste of money and time on this engine for the vast majority of riders.

The specifics:

- Polishing the exhaust is just BS eye candy on ANY engine
- The ports, manifold junctions and valves/seats in WR/YZF cylinder head are remarkably good stock so unless you are taking it apart for some other reason don't bother.
- The cam acts directly on the valve bucket so there is no rocker. The lift is more than adequate stock, and the profile is VERY GOOD.
- The ignition timing is fixed unless you pop for a $400+ programmable ignition, and unless you have a lot of experience testing curves it's pretty hard to improve on the OEM setup.
-The stock air filter flows the same as a TwinAir on my flowbench but doesn't seem to be as rugged so the TwinAir IS a worthwhile investment from a durability standpoint
- Once you change to a less restrictive exhaust insert the OEM pipe and silencer is hard to beat.

Be happy, you bought an incredibly powerful well engineered motorcycle that is difficult to improve on with ancient hot rod techniques :)

One last thing. Make sure you lock the doors of your truck when you park at Chicago Cycle. You never know what kind of creepy freaks might be wandering around in that neighborhood :eek:
 

JasonJ

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Jun 15, 2001
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Here Here Rich!
Sfc, congrads on your decision to go with the big WR, you Will NOt be sorry unless your less than 5' 8 and can not bench press your own weight!
As far as old school hop ups, Yamaha knows all about all that apparently, they did all that good stuff before even tooling the production line! Dude, 5 Titanium valves, 12.5 :1 compression on (good) pumped gas, The only real mods you need are the throttle stop, exhaust insert and DONT FORGET THE GRAY WIRE! This is a High performance racing bike that is cheaper brand new than cheapest car you can get in the states! (dont quote me on that)
Sarge, strap this bike on once you get it unwrapped and jetted and see if you still think it needs anything. It is a great tribute to the design of a bike when the most popular aftermarket purchase for a bike is stickers and seat covers :)
 

Rich Rohrich

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They sell a TON of pipes for these bikes but I fear the majority are bought by complete boobs who hurt the performance more than they help it. The good part is if you have a large enough surplus of power (these tend to) you can screw the bike up with stupid mods and still have a great bike. This phenomena is often described as the Z1 factor :)
 

sfc crash

Human Blowtorch
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ancient hot rod mods! wtf! listen, i'm working on something right now that'll revoullutionize the motorsports industry, it's a tire with, get this, steel belts!ha! i kinda figured i was showing my donkey on the last one.but hey. anyway, as far as the stock bike being fast enough, i'm not good enough to ride a rt100 let alone a wr426, but being all grown up i get too 'cause my paper route money has built up!:D . oh yeh, i always lock up in chi, i used to live on chicago and wollcot. thank god i'm a country boy,now!
 

Boit

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I was hoping to spark some discussion about this cylinder head. Thanks. Also, to drift off topic a little and to respond to the exhaust issue, I would like to add my opinion based on my experience. The first thing I noticed on my 426 was that it seemed to be very soft on the low-end compared to my Stroker KLX 338. Yes, it had a fairly sudden burst of power once it reached the low-mid, but I expected more on the low from a 426cc machine. After installing a certain aftermarket headpipe/silencer combo, the off-idle power came alive like an angry wasp and tamed the transition into the mid range. As a side note, to make the most of this engine, I DID begin using C-12 fuel and jetted crisp to attain the throttle response that I demand. The stock exhaust allows the engine to rev very well, but not all riders want to hit the rev limiter corner to corner. I prefer to shift when my seat-of-the-pants dyno says that power is beginning to wan. The low-end burst that I now have allows me to feel confident that I can drive hard from the inside line and accelerate enough to clear a double that the 2-strokes need a better run at. This is my strength but at the sacrifice of another weakness. Doug Dubach is notorious for hitting the rev limiter. Well, he's light years faster than I will ever be. I race/ride for the sheer pleasure and fun of it. I've been riding since I was 12 and am now 48 and it's more fun than ever. It's fun to not only ride, but to piddle with my machine. I don't drink, smoke, gamble, do drugs, chase women..etc. I live for dirt bikes and I make no apologies for that.

Oh, I DO wear a size 11 Aunt Marilyn style butt-hat.
 

Rich Rohrich

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Originally posted by Boit
Changed my mind. Don't want to be a pest.

It's not being a pest unless you plan on asking "what color grips should I buy?" :silly:

On the subject of pipes, it's not surprising that you were able to find one that gave better off idle response than the stocker. There is a pronounced dip in the torque curve at low rpm where the stock pipe and intake are out of tune. Using a pipe tuned to a lower torque peak via a smaller pipe ID or longer primary header length will shift the bulk of the torque curve area to a lower rpm at the expense of breathing at higher rpm. Fixed length exhaust systems can only be tuned over a fairly narrow speed range, so it's a balancing act to find the sweet spot. For MX use Yamaha seems to have hit it fairly well, for other purposes maybe not. Many of the aftermarket pipe companies seem to lack the most rudimentary knowledge of how engines work as a system and just hang well constructed but poorly engineered pipes on these bikes. With this approach it's not surprising that there are so few riders seeing real improvements from pipes.
 

sfc crash

Human Blowtorch
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hey boit, be a pest a smidge longer. what type of pipe combo are you talking about with the low end torque? i will be riding primarily wood/ravines and figure it'd be better not to have to kick up the rpms in that riding environ. thanks, any thoughts? buehler, buehler? anyone?...
 

JasonJ

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Jun 15, 2001
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Ok Rich, It sounds like you may have put some of these pipes to the dyno. In your humble very educated opinion, Whats the best pipe for these bikes. I am looking for a good pipe for my 99 WR. I am running the stocker with the baffle out. Power for me is fine, I would like to be able to whack the throttle at some of the lower mid range speeds and get very positive lift out of the front wheel. I am more concerned with loosing weight up high on the bike. I saw a picture of the $350 GTYR carbon fiber pipe, it looks very nice and light and all, but whats the scoop on these pipes? I cant belive I dont see any dyno numbers or curves on these things, just post on, dose anyone ride this pipe or that one, do you like it and so on. The manufactures list questionable round HP figures with scetchy weight and almost no Db ratings. Are we dirt bike riders that uneducated about what really counts that we just buy what looks purddy? I am not having any issues with the noise of the stock WR pipe with no baffle in the stock tip, it just is not that loud.
 

Rich Rohrich

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Stock pipe with YZ timing :) Anyone who uncorks the stock pipe and runs around with the WR cam timing is just kidding themselves.

Put some REAL gas that will vaporize in the bike and do a search on the excellent posts from James Dean on needle mods to the FCR for increased response. Everything you are asking for is there already, it just needs some minor tuning.
 

JasonJ

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Jun 15, 2001
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10-4 Rich, I will have to try the YZ cam timing when I check the valves. I am hesitent to lose the super smooth botom end traction on tight slippery rocks and mossy downed trees, but I am all for giving it a try. I know my carb settings are not spot on yet, its a slow process due to the fact that I have to drive for an hour to get to good legal riding. I just took my GPS up the the riding area and to my surprise found it is at 1700 Ft as opposed to the 230 ft where I keep my bike. So I suppose I can throw those settings out the door. Do you know where that altutude guide is for setting these things is at?
Anyway, My main reasons for getting a pipe will be weight loss. I will be hoping for a mild power increase as well, any favorites that you KNOW are respectable? I just do not want to be a pipe buying boob :) .
 

DualSportr

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Aug 22, 2000
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As far as the porting on a YZF goes, Powroll built the engines for the Four Stroke National winner last year.

In testing on a flow bench it was found that:
a: changing the size or design of the ports is not very helpful, plus there's not much room to work without breaking into a water jacket.

b: a very tiny percentage of increase in flow can be obtained by just cleaning up the small imperfections and doing a good three angle valve grind. It will give slightly better throttle response and a bit more top end.

c: riders who like more pull off the bottom and better midrange hit like the cams to be re-degreed. Powroll can do this for about $50 for anyone who's interested. This is a drop-in modification, and is recommended for owners who don't want to mess with a vernier sprocket.

d: Like Rich said, the stock exhaust really does work. After exhaustive (pun intended) testing on the dyno, Powroll found no aftermarket system that worked as well overall as the stock system on the YZF. Some did give little peaks that weren't available with the stock pipe, but most also had valleys that didn't occur with the stock pipe. Testing was done using a hydro dyno, which allows the user to put a specified load on the engine to really find flaws in an engine's performance.

Bang for your buck - For those riders who feel that too much is barely adequate, the best bang for your buck with a 426 is a stroker.

They pull incredibly hard off the bottom, gain in midrange and are a little less revy than the stocker.

Most riders, including the pros, find the 440 stroker engine too much hp for regular tracks, although Craig Decker used it to win a couple of 4Stroke National races at high elevations or on sandy tracks last year.

Powroll also sells many strokers to quad racers who are utilizing the 426 engine in a quad chassis - the increased torque and smoother power delivery work well for the quad chassis.

One warning - for those of you who are messing with your cams - Make Sure to follow Yamaha torque specs for the cam holder bolts -- It's very easy to over-tighten these and pinch the camshaft. We've even seen "race" mechanics do this.

Hope this helps some of you save a few $$ :)
 

SFO

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Bang for buck?
I saw that the stroker setup was like 1200$.
Correct me if I am wrong...
That is alot of cake, man.
Does that include a Pankl Ti rod, or a squished one?
What happens to the rod ratio?
Where did you guys run into water passages porting this head, on the short turn of the intakes?
How much do you charge for retiming the cams? What if my deck height is cut to set the squish up?
 

JasonJ

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Jun 15, 2001
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Interesting thread, I am surprised on how good the stock Yamaha exhaust is. I think I will go with the GTYR rig to loose weight and I know the pipe was made to work with the bike, and wont hurt it. Any dino stats on it?
I think that its cool that SQUISH is a technical term :)
 

J_dem_Bones

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Jun 23, 2001
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I have no problem with my stock exhaust except that my header covers the oil filter cover!:think Is there a header that I can buy that will keep the same powerband and not cover my oil filter cover?????
I had problems with what I thought was the bottom end of the powerband, but what I changed was my rear sprocket! I went up a tooth to a 50t. That helped alot!:D
 

DualSportr

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Aug 22, 2000
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SFO,

BTW, it's not man, it's girl.

The price for the stroker kit includes a new Yamie crank, so if you wanted to send yours in for the mods, it saves about $300 (maybe more, can't remember exact $). It comes with all the parts, including a piston kit, gaskets, etc.

The kit uses the stock length rod, and a shorter piston, so the angle isn't increased to the point it becomes a problem.

We did try a shrunk rod stroker on the 400, it was hard on pistons, like you mentioned, too much forward thrust with that ratio.

Reliability has been really good with this setup. We have a few that have been running in the quads, and they've looked great after 3 months of pro racing (torn down just for regular maintenance).

As far as adjusting the cams to work with a shaved head -- the only way to do that right is with a vernier sprocket. Even then it's an iffy deal because the timing chain has more slack.

As far as the way we ported the YZF heads, it's classified -- we still use it for our race engines, every little bit counts there :p

Jason, squish is a very technical term! It just sounds goofy. Your piston has a squish band. Squish can be used to describe the act of creating compression in a cylinder. It can also be used to describe that weird sound some forks make when they're compressed -- or is that squoosh? :confused:
 


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