Sign in to follow this  
Followers 0
BOULMAN

Big Bore Kit

26 posts in this topic

I am considering putting a big bore kit in my 426. Is there a big difference when going to one of them? I am wanting to get my 426 closer to a cr500 (since I am selling it). Best kit? Reliability? It there a big difference when compared to a well tuned bike?

Also, is there a difference between the big bore for the 426 vs 400 (my buddy has a 400 and doesn't want to get left in the dust)

thanks

jb

Share this post


Link to post
Share on other sites

big bore kit for the 426 is 444cc and the 400 kit is 420cc I believe. Someone correct me if I'm wrong. JE or Wiseco , it's just personal preference. I prefer JE. A 444cc WR is still going to have less outright power than a cr500. Maybe you should be looking at a KTM 625?

Share this post


Link to post
Share on other sites

My 426 is dual sported as well as my buddies 400. I love the bike and I am likng it more as I tweek it. I was just curious how much it added and if the kits (400 vs 426) closed the gap between the two.

thanks

jb

Share this post


Link to post
Share on other sites

Unless your a pro you really might not notice much of a difference. At least I did not with mine. I would recommend if you do go with the over bore to NOT use the high compression piston.

Share this post


Link to post
Share on other sites

To get the most permormance from the big bore kits it really helps to get a set of cams that can deliver more mixture to the motor thus more energy. The old saying goes, there is no substitute for displacement.

I'm just pulling my motor apart to put in a wiseco 97mm piston and hot cams (intake and exhaust ). I'll post my thoughts of the BIG BORE when my baby fires up again and gets broken in.

I didn't by a kit from anyone, I got a lager wiseco piston and will just be sending out my cyclinder to get bored and nikaciled (sp?).

For kits Thumper Racing and RPM's are the guys to use, they've been around for a long time. Shoot them an email, they answered all my question.

Share this post


Link to post
Share on other sites
Unless your a pro you really might not notice much of a difference. At least I did not with mine. I would recommend if you do go with the over bore to NOT use the high compression piston.

please explain?? aside from racegas why wouldnt you want to use one??

Share this post


Link to post
Share on other sites

Starting the bike with the high-comp piston is more difficult than with the 12.5:1. It is more stress on the engine, by way of more pressure in the combustion chamber, and I have heard that the power gain is not that noticable. I could see that if you totally tweeked your engine that a high comp pistion would be a neccessity. my $0.02 :)

Share this post


Link to post
Share on other sites

my buddy does the big bore kits.. Lukesracing.com He does good work at a fair price.. He owns a 99 wr400 and has the 444 kit, I personally haven't ridden it, but we raced a few times.. I had a older cr 500 (1989) and he either beat me or we tied.. I think it is a different kind of power though.. With that old CR it was "on" or "off" so I never won the drag racing because of traction, the 400 always hooked up better.. I don't think you can beat a cr 500 for the power, but it isn't useable all the time like it is on the 4 stroke..

By the way, his name is Mike...

Chris

Share this post


Link to post
Share on other sites
Here's some dyno runs of a 97mm big bore WR426 running the stock cams. I can assure you that if you build the engine properly you WILL feel the difference :)

http://www.four-stroke.com/Dyno_runs/SFO_index.htm

couldn't help but notice that the vortex lost peak hp and torque in its power mode? looks like it really does its job in traction mode,but is that worth a 2-3 hp drop?

Share this post


Link to post
Share on other sites
couldn't help but notice that the vortex lost peak hp and torque in its power mode? looks like it really does its job in traction mode,but is that worth a 2-3 hp drop?

Nope, it's really not worth it. From these dyno tests and rider evaluations on three different bikes (with different pipes and varied basic spec) we found it's a waste of money unless you buy the programming kit and tune the curve on the dyno to your specific setup. Even then I don't think the cost of the Vortex unit ever justifys the cost on the YZF400/426 engines. It might prove more effective on other engines, but on the YZF4xx we found it to be a poor investment.

It is nicely made though.

Share this post


Link to post
Share on other sites

If I take a stock wr400 and a stock wr 426 and put the big bore kit in each will the power be "equalized" between the two.

Also, it appears that the higher compression setups are for me due to hard starting and such. Is the 12.5 to 1 not considered high compression?

This fall I want to do a big bore on a wr400 and a wr426, so i want to get all of my info before i do them both.

thanks

jb

Share this post


Link to post
Share on other sites
If I take a stock wr400 and a stock wr 426 and put the big bore kit in each will the power be "equalized" between the two.

If you use a 426 cylinder and a 426 rod in the WR400 engine you can run the 97mm/444cc piston (a 5mm increase) otherwise you are limited to a 94mm bore in the 400 which will give you a 417cc engine. Standard big bore for the 426 is 97mm/444cc (a 2mm increase). The bigger engine will give you more torque at a lower rpm, all else being equal.

Share this post


Link to post
Share on other sites

Does the 400 (bottom end) seem to have any problems when you go with the 444 kit? I guess you would have to split the case, any idea (ball park is fine) what it costs to split the case and purchase the rod?

Have any of you guy run the 417 and then gone to a 444 kit?

thanks for the info

I only get so much fun money from the wife and a I want to spend it wisely!!

Share this post


Link to post
Share on other sites
Does the 400 (bottom end) seem to have any problems when you go with the 444 kit? I guess you would have to split the case, any idea (ball park is fine) what it costs to split the case and purchase the rod?

Have any of you guy run the 417 and then gone to a 444 kit?

On my personal 98 YZF4 I rode it as a 400 for one season, a 417 for one season and then rode it as a 444 from then on. It's great fun as a 417 and a torque monster as a 444. The only real issue with the 400 is the rod. Going to the 426 rod eliminates that as an issue.

If you have a lot of hours on your engine it's not a good idea to go to a big bore without replacing the rod. The bigger pistons are heavier and will stress an old rod beyond it's limits at high rpm.

Either big bore setup is an excellent value in terms of fun per dollar spent.

Share this post


Link to post
Share on other sites

Sorry for all of the questions. Is the bike a lot harder to start? Right now I can start it with tennis shoes. With my cr500 I can kick a hole in my shoes, I have to use a boot.

thanks for all of the info.

jarrod

Share this post


Link to post
Share on other sites
Sorry for all of the questions. Is the bike a lot harder to start? Right now I can start it with tennis shoes. With my cr500 I can kick a hole in my shoes, I have to use a boot.

thanks for all of the info.

jarrod

I have my 444 setup at 14:1 compression and I can still start it wearing gym shoes. I haven't found it to be any different regardless of the bore size.

Share this post


Link to post
Share on other sites

Last question I promise :) . Can I get away with using 91 octane or will it not run good without 100+ octane or avgas? Being dual sported I often have to fill up at the gas stations and I really haven't seen any good octane boosters.

thanks

jb

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!


Register a new account

Sign in

Already have an account? Sign in here.


Sign In Now
Sign in to follow this  
Followers 0