What motor mods?

[Zakrad_u]

I ride In alot of open areas so looking for more all around power. Wanted cams and realized that would cost the same as a big bore so what's the best mods for power? And how do cams v. a big bore stack up

[2grimjim]

Big bore will give the most bang for the buck. The big bore will add power EVERYWHERE. Cams are very selective about what RPM zone you will see the power improvement. Going to a more aggressive cam profile will usually require more compression to reap the full benefit from the cams.

[Zakrad_u]

What's the highest CR I can go on 93? For a big bore high comp. or is compression not as much as a factor as it is in my 400ex

[2grimjim]

What's the highest CR I can go on 93? For a big bore high comp. or is compression not as much as a factor as it is in my 400ex

You can run up to 13.5:1 with 93 octane, but, don't try to run that much compression with a big bore kit. The added load of the heavier piston and higher compression is very hard on the stock 18mm wrist pin. You may see high-compression big bore pistons available but they are usually for the YFZ450 (four wheeler) and will not work on your YZ because they use a 20mm wrist pin. The YFZ will tolerate the hc-big-bore because of the beefed up connecting rod.

Unless you want to go to the trouble of a custom crank from Falicon or figuring out how to install a YFZ connecting rod on a YZ crank I would avoid a hi-compression big-bore kit.

[grayracer513]

Compression primarily adds power in the lower half of the RPM range, although it's effective throughout.

Cams, as stated, will tend to move your power range upward as well as increase it.

Increases of displacement provide a slightly less than linear, slightly less than 1:1 increase across the range. For example, a 478 kit on a 450 is a 6% increase in engine size that will yield about a 4-5% increase in power, mostly in the mid range.

The practical compression limit you ask for depends on what "93" means. If it's the number posted on a gas pump in North America (and it's accurate) you should be able to run 13.75 with it unless you're very close to sea level most of the time. If it's a Research octane number, it's not enough for a stock engine, and if it's a Motor Octane number, it's easily more than you need.

If speed is what you are after, the big bore is probably the best single choice, and the one of the three most likely to be completely compatible with the rest of the unmodified stock stuff. Engine modifications such as cams work much better when installed in concert with supporting modifications to the rest of the engine as a complete set than they do when just added in piecemeal.

[scrubbin627]

The YFZ will tolerate the hc-big-bore because of the beefed up connecting rod.

Unless you want to go to the trouble of a custom crank from Falicon or figuring out how to install a YFZ connecting rod on a YZ crank I would avoid a hi-compression big-bore kit.

Thats interesting. So the quad crank wont work in the dirtbike cases? I always thought they had basically the same motor

[Zakrad_u]

What's the hp gain with a 478 kit?

[grayracer513]

The original YFZ450 was a 439cc engine with a shorter stroke than the 450 and was somewhat plagued by wrist pin problems. The '09 and up YZF450R is different. In either case, though, the quad crank left side axle is longer and heavier to accommodate the starter drive and the heavier flywheel. Won't fit.

There isn't anything particularly inadequate about the OEM YZ450F crank assembly from '04 upward.

[2grimjim]

The original YFZ450 was a 439cc engine with a shorter stroke than the 450 and was somewhat plagued by wrist pin problems. The '09 and up YZF450R is different. In either case, though, the quad crank left side axle is longer and heavier to accommodate the starter drive and the heavier flywheel. Won't fit.

There isn't anything particularly inadequate about the OEM YZ450F crank assembly from '04 upward.

The 04'-'09 YFZ450 (standard and X models, not the R model) was heavily based on the '03-'06 WR450 engine. The most notable difference was the transmission gears being wider and requiring the cases be redesigned to accomidate the new cluster. However, a lot of the remaining parts are identical; the head, everything under the clutch cover and the cover itself, starter drive parts and magneto, cylinder, and crank will physically swap with '03-'06 WR parts. The YFZ crank will work in a WR450 provided you use a YFZ piston and flywheel (the '04-'06 WR flywheel will work as well, but not the'03). ALL YFZ's have use a 20mm wrist pin. The '04-'05 YFZ's did indeed used a reduced stroke crank but the wrist pin was unchanged when the crank was redesigned to yield 449cc in '06. The change in the crank (to increase displacement more than anything else due to the incessant whining by the ATV crowd) wasn't done for any specific reliability issues. Reliability of the YFZ has always been somewhat less than that of the WR despite the 20mm wrist pin and wider transmission, mostly due to the operational conditions in which they are used.

You can use a '06-'09 YFZ connecting rod with a YZ450 crankshaft (any year up to '09) and have a YZ crank with a 20mm wrist pin, of course you will have to use a YFZ piston. The '04-'05 YFZ con rod is slightly longer and will not work. The ability of the YFZ crank to fit in the WR opens the possibility to use a YFZ stroker crank in the WR (I have a stroker WR).

Another change made to the YFZ in '07 is the addition of a piston cooling jet incorporated into the clutch cover. The '07-'09 YFZ clutch cover with cooling jet will bolt on to YZ's from '03-'05 and WR's from '03-'06. If this swap is done with a '03 YZ/WR the '04 and later kickstart shaft needs to be used because the shaft diameter was increased 2mm from '03 to '04.

[scrubbin627]

What's the hp gain with a 478 kit?

Grayracer513 said a 4-5% increase with the 478. So if a stock 09 450 is 52 horsepower and 5% increase would be around 54.6

[2grimjim]

Grayracer513 said a 4-5% increase with the 478. So if a stock 09 450 is 52 horsepower and 5% increase would be around 54.6

That might be a peak number but the gain in power may be something closer to 8 or 9 at some point between 5000 and 9000rpm. What you will notice in seat-of-the-pants testing is the power difference 'under the curve'. Peak HP numbers don't necessarily reflect what you feel when you are riding the machine (unless we are talking about drag racing). For off-road use most riders can use an increase in 'usable' power more than peak power.

Keep in mind how often do you have your bike bouncing off the rev limiter when you are riding. Peak HP numbers usually aren't more than 1000rpm below that.

[grayracer513]

The rev limiter on a YZ450 is set at 11,500. The power peak of most YZ450 models, and the '09 in particular, is 8,500.

[Zakrad_u]

A 2 hp increase doesn't justify 700$ ... Guess i just need to get that banshee I've been thinking about lol

[grayracer513]

If you still have the stock exhaust in place, you can pick up as many as 7 HP at around 5000 RPM by ditching the "shorty" muffler and installing almost any slip-on or complete exhaust system. The OEM can puts a huge dip in the power curve between 4000 and 7000 that makes the bike feel weak through the low and mid range. Changes the feel of things considerably.

[Zakrad_u]

I have a 4.1 slip on and it definately changed it a lot. I was actually suprised because I've never felt a gain like that from an exhaust

[2grimjim]

Another simple bolt on that can make a noticable difference is the Dyna programable CDI for your bike. I added one to my YZ about a year ago. List price is $350 but you can pick one up online new for under $300 (I got mine for $245). The only catch is if you actually want to CHANGE the advance maps, you have to buy a interface cable and software for $100. Well worth the money though. I have 3 different maps I use. It's not quite as slick as the Vortex setup because you can't change maps on-the-fly but it's a lot cheaper and if you're not happy with any of the Vortex maps pre-programmed into the box you have to send it back to them for re-programming (unless you want to by their re-programming kit for $900.....right).

[grayracer513]

I was actually suprised because I've never felt a gain like that from an exhaust

That's a measure of how bad the shorty muffler is. Designs like that were used on cheap custom pipes for street bikes in the '60's.

The CDI is another element of the reduced low end power on the '08/9. An alternative to aftermarket ignitions that used to be cheaper until it got to be so popular is the CDI from an '06 YZ450. Lots more grunt, not much change on top. Only one map though.

[Zakrad_u]

Does the Cdi change timing?

[grayracer513]

Yes. The ignition advance curve is different between the '06 and '09. More so at lower RPM than at the top. That's the ignitions Jim mentioned work too, but they offer more choices, and in some cases, programmability.

[Zakrad_u]

Would that alter compression ratio?