New project bike.... build a CRF400R

The inspiration for this project is to make a mid size thumper, for those of us that feel a 250cc is not quite enough some times (and/or do not like the High Rpm nature of that bike) and also feel that a 450cc is just a little too much some times.  We also tested and rejected the KTM 350 due to suspension and handling being less than a CRF.  I love the thought of a mid-sized CRF350-400cc, I think that will be "just right"

 

Then I recalled the L.A. Sleeve project bike in Dirt Rider mag a few years back, they built a CRF400X and it felt much lighter than a 450, but had way better low RPM torque than any 250 could ever dream of producing.  Sounds great to me!

 

a fellow TT member built one, found it here: http://www.thumpertalk.com/topic/716138-meet-my-crfxr400x-a/?hl=%2Bcrf400x#entry7291345   He agreed with Dirt Rider, it was a nice bike.

 

I race the A class in an area dominated by 250cc 2 strokes, lots of first and 2nd gear very tight and technical trails, races are 3 to 4.5 hours long, very tough!  The bike has to be Light as possible (like a CRF450R) and very smooth and friendly torque is the hot set up here.   very few people race a 450cc MX bike here.  I find a 250cc 4 stroke is fun, but too high RPM and a little underpowered at times.  The 450 is a bit too much most of the time in the tight woods where I race.  I have been waiting for Honda to make a 350, no sign of it yet.  I do not care for the suspension and handling of the KTM 350.

 

so...... called L.A. Sleeve.  They say they can sleeve a cylinder to 400cc and give me a custom piston and gasket kit for around $500 (bit cheaper than a big bore for a CRF250). I expect the CRF400 to "feel" quite a bit lighter, and the power to be very friendly for tight woods and solid competition to a 250c 2 stroke.

 

 

I would appreciate any engine builders to chime in here with opinions on the best approach to making a very civilized easy to start and race 450cc MX bike. Something comparable to a 250cc 2 stoke.

 

In 2009 I took a 09 CRF450R, added a 17oz FWW, auto clutch, G2 throttle with 400 cam, Eddie remapped for smooth and easy to start, it worked pretty good, the 250cc 2 stroke was still a better race bike, the 450cc still felt heavier with the gyro effect of that big piston.

 

In 2008 I built an Athena 280cc CRF.  it worked pretty good, still not quite enough.  Instead of overstressing a 250cc why not understress a 450cc motor? would it not be even more reliable, less like to boil over etc etc.

 

Thinking of trying again....  the Choices:

(1) sleeve a CRF450 down to 400cc and add an auto tune FI controller to ensure crisp response. (will it really be that simple, or should other components also be addressed/changed/modified, like the head)

 

or

 

(2) Find a softer Cam profile, Hotcams or a TRX Cam?  get a thicker custom base gasket by Cometic to drop compression, or maybe use a lower compression piston from a 2002? Or ???

      add a 17 oz FWW?  G2 throttle? check in with Eddie to see if he has even softer new maps to make it as friendly as humanly possible? 

 

Any comments from the experienced engine builders on the best approach here will be greatly appreciated. 

Edited by stroker

I dont have anything on front of me.... but wouldnt closing the bore up that much cause some problems with the valves opening? It seems like it wouldn't even be possible without major headwork.

Short stroke would be way more better :)

If you're using the same crank you won't impact gyro at all. You're better off make a larger-displacement bike out of a 250-4 than making a smaller-displacement bike out of a 450-4 if you're looking to reduce mass and rotating gyro.

Going to a shorter stroke on the 450 will kill your torque and low-end and turn the bike into a high-revving screamer, the opposite of what I think you're looking for.

Edited by Eddie8v

I think building a -01-07 cr250r with a big more low-mid porting, exhaust, lectron carb, 18inch rear wheel, flywheel weight and revalve would be miles and miles ahead of wgat your thinking up.

I agree. I have a Service Honda 265AF 2-stroker and it's untouchable in the woods with its very light weight and gobs of low, mid and top-end power:

IMG_00000147_zpsfc4cbf68.jpg

Edited by Eddie8v

Oh no, not this argument again.

 

Just get a G2 'throttle tamer' throttle tube and be done with it.

That service Honda CR is awesome!  I have a minty 2003 CR250 that I restored and have been racing for 3 years, it is going to be VERY hard to beat that bike in my area.  I will be keeping it for sure!!

 

I ride in California in the Winter, so I fully appreciate how people that live in open areas have trouble understanding what it is like riding in a rainforest in the rocky mountains, but when my buddies from California come up here to ride, they cannot believe how hard it is to ride for 3 or 4 hours in 1st and 2nd gear through deep mud and crossing big logs in trails that truely are handlebar width, try it and you will quickly understand that horsepower is not a requirement to go really fast in my riding area!!!

 

My sole motivation is that I just started feeling like building up a new and unique project bike.  The 450 MX bike is such a rare bird at our races, it really is too much bike for tight slow trails we have. The 350 KTM is getting somewhat popular here, I thought a 350-400cc Honda would be a very cool project! The Dirt Rider article sounded really cool, and it is not too expensive (under $500)

 

I am just trying to figure out the best approach.  I really appreciate the advice/opinions!

I don't think you will get what your'e looking for by sleeving down a 450 to 400.  Most of the rotational mass is from the crank and valve train.  50cc smaller piston is not going to make much difference.  Plenty of simpler things to make the bike easier to ride, if that's what you are trying to do.

 

- flywheel weight 15oz.

-Rekluse clutch

-higher gearing

 

These modifications would work on a 2 stroke also.  I have ridden the KTM 350.  No way a CRF 400 is going to be as nimble. Good luck and let us know how it works out.  

I don't think you will get what your'e looking for by sleeving down a 450 to 400.  Most of the rotational mass is from the crank and valve train.  50cc smaller piston is not going to make much difference.  Plenty of simpler things to make the bike easier to ride, if that's what you are trying to do.

 

- flywheel weight 15oz.

-Rekluse clutch

-higher gearing

 

These modifications would work on a 2 stroke also.  I have ridden the KTM 350.  No way a CRF 400 is going to be as nimble. Good luck and let us know how it works out.  

No, it's absolutely going to make a difference: about 4 less hp, and a slower revving motor.

I just don't get why people like to mess with Honda's proven motor design. 

A G2 throttle, Steel clutch basket, flywheel weight, longer header, enduro muffler, would all achieve the same needs, without the side effects.

Just change your gearing on a 450 save money and time!

In search of softer power delivery, I have had the cam reprofiled, softer mapping and g force softer cam in the throttle. I would like the power to be a bit softer as most of my riding is is in tight woods 1st and 2nd gear, what would happen if I fitted 2 base gaskets? I know this would lower the compression, but would it be too much?

sounds like we ride similar type terrain and are looking for the same characteristics.  What cam are you running (supplier) and can you offer some comments on the changes you noted with that cam?  Thanks!

The 2002 CRF450R has a 11.5:1 compression piston and a TRX450 has a 2 ring piston and 12:1 compression same bore/stroke of 96 x 62.1 as the newest CRF450R.

Does anyone know if either of these pistons will fit a 2013/2014 CRF450R? Wondering if the valve pockets are the correct size and depth.

Also any engine experts know if Hallam76's suggestion of using a thicker base gasket is a viable way to drop the compression. Not sure how much vertical lift we can get away with using the stock cam chain etc But if that will work, that may be the cheapest and easiest solution of all.

The 2002 CRF450R has a 11.5:1 compression piston and a TRX450 has a 2 ring piston and 12:1 compression same bore/stroke of 96 x 62.1 as the newest CRF450R.

Does anyone know if either of these pistons will fit a 2013/2014 CRF450R? Wondering if the valve pockets are the correct size and depth.

Also any engine experts know if Hallam76's suggestion of using a thicker base gasket is a viable way to drop the compression. Not sure how much vertical lift we can get away with using the stock cam chain etc But if that will work, that may be the cheapest and easiest solution of all.

I wouldn't try it but it would take three to four base gaskets to drop the compression one full number. Exp. 12.1 to 11.1 with that being said before you try to tame a beast that's already known for running hot I'd invest in a 250 before taking those chances and and spending that much effort.

That's three to four EXTRA base gaskets btw.

That's three to four EXTRA base gaskets btw.

 

Ya, that pesky cam chain will love that. It'd be easier to just mill .010 or .020 off the dome of the piston.

 

If it were me, I'd up scale a 250F. Longer stroke and a big bore will do exactly what you want it to do. But for the type of riding you do, Id still keep the compression down to 10/11:1

 

Or better yet, stick a 250F in a CRF150 chassis. Done. Go flick that around all damn day.

Milling the piston may be an option.  I called J & E piston today.  They said they can make me a piston with pretty much any compression ratio I want.  That may prove to be the cheapest option, and easiest to "undo" and go back to stock if I don't like it.  Any suggetsions on how low a compression ratio would be acceptable and stil work well? 10:1 ?? 

 

I really appreciate the feedback guys, I know I am going in the opposite direction of most people on here, but I live in a unique place with unusually tight low speed technical trails mixed with crazy steep and long hill climbs.  Hence the popularity of the 2 strokes here.

Edited by stroker

In 2011 Dirt Rider did a test of a YZF450 that L.A. Sleeve dropped to a 400.

http://www.dirtrider.com/tests/adventure-bikes/141_0705_yamaha_yz400f_dirtbike/

They said it was very close to a 250cc two stroke MX bike but with a longer pull and much more torque and friendlier to ride!!

 

That is the true objective here (CRF250R is not quite enough sometimes and it tends to be more high RPM power, only goes 60 mph on fire road connector sections before it sounds like it is over-revving.  CRF450R tends to be lower RPM power, just too much power and too violent, plus runs too hot for extreme offroad 1st & 2nd gear enduro).

Edited by stroker

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