Big bore kit or not?

[jbelfiore]

I have a 2004 YFZ 450 and siince I am out for 6 more months with the broken wrist I was going to get a new piston put in the quad. I was talking to a local shop and they said I should consider putting in the Cylinder Works big bore kit which they say is better than the Athena. They said they would port and polish, put in titanium valves, and the big bore kit. My question is is spending $2300 for this really going to give me a huge power difference or should I just leave it as it is? I am all about more power, but for that kind of money I want to notice a big difference. I have a lot of performance parts on it already. I mainly do the Whiplash series but also do some MX racing. I know it is a matter of opinion,but wanted to hear from some people whether or not this would be worth the money?

[Kramar119]

Wow....Cylinder Works....Athena....STOP! I'd call and talk with Ken at Kenz Cycle Tech before you do anything. Tell him exactly what you want to do with your quad, and he will build you a motor to fit your exact needs. Ken knows more about the YFZ then anyone I have ever talked to. Those huge big bore kits aren't for racing desert or MX....they are for drag racing. I ran a Kenz Cycle Tech built YFZ motor in the ITP Quadcross and WORCS for 2005 and 2006, and NEVER had to do anything once it was built except replace the piston every 6 months. Once you put the newer crank in your '04, you will be at 449cc (instead of 439cc), and with some head work, a higher compression piston and cam, you will be set.

[jbelfiore]

My YFZ has the 06 crank in it already. In terms of Cylinder works versus Athena, I guess the only reason the shop said this is b/c the kit comes with a much better and stronger piston??? Not sure as I am clueless when it comes to big bore kits.

[Jeremy]

Worth the money? Probably not. I'm sure you have more than enough power for the racing you do.

[06YFZ45racer]

LISTEN to Kramar! He knows his stuff and wouldnt send you to a bad place!

DO NOT do a big bore kit for MX or dessert racing. And i think you ALREADY have titanium valves installed. Im not sure but i KNOW that my '06 YFZ has titanium valves stock!

[knoxjmk]

Don't do the big bore kit for desert racing. The Stock motor has lots of potential without it and big bore kits are extremely undependable. Contact Kenz or Trinity Racing out of socal and do the port and polish and make the motor flow better. Go for balance and heavier crank and blueprint if your going to spend the cash. FLOW & BALANCE = MAJOR Horsepower gains! You'll get greater top end, low end and every other end and still have a great dependable motor. Of course if you do that I'll have to since I also race Whiplash...

[joedrives2fast]

Big bore kits are Good for duning. Thats about it. If they port the heads too much you loose all your intake velocity which equals loss of low end torque. Large intake ports give you more "intake Volume" which on a larger displacement engine will mean more horsepower but it's the torque that gets you up the hill. If I were to do any head work to my bike I'll just "clean up" any casting marks on the intake side with nothing finer than an 80 Grit flapper wheel and polish up the exhaust side. Some studies with cars show that too much polishing on the intake side causes fuel to almost "stick" to the intake runners where the 80 grit scratches kind of keep the fuel atomized.

[dunerinaz]

In my opinion, $2300 sounds WAY high for what you're getting. A typical (good) port job will be in the $500-$600 range, the cylinder works cylinder alone is around $250, and valves would probably be around $200 or so. I wouldn't get the cylinder works big bore kit. I would buy a CP piston seperately and gaskets. Do the work yourself or have a friend help and you're in the ballpark of $1100-$1300.

But like everyone has already said, I woulds stay stock bore and get your head ported (I would stay away from Trinity) and a pump gas piston and you would have more than enough power for desert racing. Doesn't Morgan at RE do yfz heads? They do great work.

[azhunter]

Big bore kits are Good for duning. Thats about it. If they port the heads too much you loose all your intake velocity which equals loss of low end torque. Large intake ports give you more "intake Volume" which on a larger displacement engine will mean more horsepower but it's the torque that gets you up the hill. If I were to do any head work to my bike I'll just "clean up" any casting marks on the intake side with nothing finer than an 80 Grit flapper wheel and polish up the exhaust side. Some studies with cars show that too much polishing on the intake side causes fuel to almost "stick" to the intake runners where the 80 grit scratches kind of keep the fuel atomized.

Couldn't have said it better myself. I have also heard when you go to "big bore" to start to run into cooling issues. Which, in my experience, in hare scrambles and Whiplash type stuff, can be a major issue. I'd try to find a shop that does extrude honing. IMO