I have a 2007 YFZ450 that I flat-track race on clay(ovals). I've done lots of suspension work along with tons of trinkets so that my quad runs very well, however I need a bit more stomp for some of the bigger tracks I'm planning to start running at. Currently I have a sparks racing high comp. piston, pro-design intake, full Sparks Big-Core exhaust, TPS bypass & GYTR high-rev box for my power mods. I'm contemplating a sparks version(higher compression) of the athena 480 big bore, or perhaps a 3mm stroker crank which would bump me to approx 470. -- I'm curious as to the pro's & cons of these 2 options, and if anyone has done either of them. I'm looking for more power all the way thru, not just top or bottom-end. I'm leaning toward the big-bore and changing the GYTR rev-box to the DynaTek, but I'd appreciate any feedback anyone might have on my situation... or any feedback in general about how to squeeze all the power I can out of my YFZ (without making it crazy unstable or hard to dial-in). Thanks.

 

I would reccomend headwork first! Sparks,Kenz, Nmotion, etc your pick. Be carefull of the bigger strokers with the rod angles putting alot of side load on the piston. Save the overbore for when you need it. Its actually cheaper to replace the cylinder with an oem from yamaha than to have it bored and replated.

 

Thanks for the advice. Does boring the carb to 41 or 43mm help much too? And anyone have any thoughts on the rev boxes... which one(s) perform best for race applications.

 

does your tps help with the low rpm hesitation? I have heard there are two ways to smooth out the off idle hesitation one is with a $250 carb power bowl, the other is the $20 tps. big $ difference i am trying to figure out the performance difference. also, boring your carb wouldn't be neccessary unless you increase displacement of your motor. thanks for any insight.

 

I have a 400ex that I bought a 4mm stroker for. I built it to ride in the woods and it is great for that. I seems to rev a little slower than it did and that seemed to hurt the flat tracking ability. It took longer to climb back up out of the corners. Unless you have bottom-less pockets I would stay away from the stroker.

As scholle said, I would go with porting and a cam. You would be amazed at what that $500 can do. If done right you keep all reliability, also.

 

i have the sparks built 460 national motor on mine with the JE 13.5:1 piston, it is a freakin beast!!! so heres what you need to do... i'd say go with the sparks motor work myself, Definatly get it port and polished you will get amazing gains from a simple port and polish, the carb bore to 41.5 will add about 2-3hp and give you more top end, if you don't have it done already then do the Cam Mod!! it will open that Yfz up a whole lot!! i have a vortex ignition on mine and i love it its bad ***! all of my mods are in my signiture this thing has tons of low, mid, and top end power, i kill modded 700 raptors and everything else i have raced...

 

just to add a little something to anyone in there with a yfz that hasn't done it, no matter what year yfz you have do the Cam Mod!! its easy and its free, it does not void your warrenty either! if you need instructions on it they are on yfzcentral.com

 

Yes I agree start with the cam and head work first. If you dont need to and you want longer life out of your cylinder just do them and see if you are happy or not.

If you decide to go with internal motor work, I would say for the flat tracking I would go for the overbore as it will rev faster than a stroker motor. A stroker will give you a little more low end torque but you want that 450 to rev like it should.

 

definitely stroking the motor will slow the reving ability. fatter shorter stroke = faster revving motor. a longer power stroke will = more torque. the 450 motor is such a good design because of its lightweight piston and ability to rev VERY fast. top end motor work would make the motor shine a lot brighter than stroking it. you wouldn't want to strip the 450 of its greatest atribute by stroking.

 

All of these posts are giving you VERY good advice.
Here is the way I see it. I race cars, not quads, but it's all the same really.

If I'm on a tight track with tight turns and two long straight aways I want all the torque I can get to get me off of the turn. It's a drag race to the next turn. The more torque the better to get me off of the turn. The best way to get that torque is with a big arm. (Stroked crank) It will give the leverage I need to get off of the turn.

Now, if I'm on a long oval with no real turn I want all the HP I can get. I don't want a big arm to slow the rev's down because I'm on the power band as is. I just want it to rev and rev and rev...............I don't need torque to get off of any turn.......I jsut need HP.

That being said, I really agree with SCHOLLE and others that said do the head work first. I also agree with giesdog, the best thing about a 450 (just like a small block chevy) is the ability to wind it up. Don't ruin that by putting a big arm in.