YFZ vs YZ compression

[sledhead999]

I was looking at the yamaha website, I noticed that the YFZ has 11.4 compression ratio and the YZ450F dirtbike has 12.3 CR. Where does the extra come from for the bike??? Is it the base gasket, head gasket, piston, cylinder, head????

[Faded]

The piston.

[Woody_YZF]

All it would take is a higher compression piston for the YFZ for it to match.

[TUK101]

Plus I am sure that the ignition mapping is different to give the YZ more advanced timing. The porting is probably a bit different as well. A 4 wheeler is toting around a lot more weight over its 2 wheeled cousin and woudnt be able to withstand the more aggressive timing and higher compression reliably enough to let them go that way off from the showroom floor. Not to mention that a 4 wheeler has the potential to get a lot more traction too, which in turn puts more stress on the engine. An atv engine generally relies as much on torque as it does hp to get the job done.

[Woody_YZF]

You'd want to advance the exhaust cam one tooth also to get the same timing as the bike (referred to as the "cam mod").

[yz250r]

does that do anything noticeable? im guessing this will probly void the warranty? 6 months is not that long to wait to do it though

[Woody_YZF]

It doesn't void the warranty if you have them do it. It is a noticeable difference, but I put mine back to stock because it made my quad tough to start and it cost me a few races.

[yz250r]

what do you mean by tough to start? any effect on reliability?

[Woody_YZF]

It would crank over and over before it would start. I messed with the jetting until I just got sick of it and finally changed it back. The only effect on reliability was that it was hard on the starter and the battery.

[TUK101]

Originally Posted by Woody_YZF

It would crank over and over before it would start. I messed with the jetting until I just got sick of it and finally changed it back. The only effect on reliability was that it was hard on the starter and the battery.

It would also be harder in the bottom end and the rest of the engine bearings. Because the engine would be "hitting" harder. Like I said in my above post, Yamaha relaxed the timing and compression for a reason. If they thought that the extra performance was worth it they would have sold them off from the showroom floor that way. On the dirt bike you have a longer throw on the kick start lever which is why the bike will start with the extra compression. Even then, sometimes the bike is hard to start too. Thats just one of the many reasons why I still prefer a 2 stroke.

[2fun]

I did the cam mod to my YFZ and it didn't seem to affect starting at all??

I did however notice a bit of gain in the mid range and top end though

BTW, mine is an 04' and I believe the compression on it is 11.9:1??? (If I remember correctly that is)

[yz250r]

does the yfz retard ign timing during starting? if so this shouldnt affect starting

[2fun]

Originally Posted by yz250r

does the yfz retard ign timing during starting? if so this shouldnt affect starting

Hmm.. I'm not sure. When doing the cam mad you are retarding the exhaust cam, and for some, it can affect how easily it starts due to the position of the decompression pin if I remember correctly??

[yz250r]

oic, wasnt thinkin about the decompression valve

[kenyz]

Do the mod. Mine starts fine all the time.

[yz250r]

i def will be doing that, does anyone know the hp/torque numbers on a stock yfz? weve got a chassis dyno here at school, so i might throw mine on there just for fun

[2fun]

I've heard mixed results......

PLUS, depending on what dyno you use and how it is calibrated, two identical YFZs could read totally different!

I think somewhere around 35-39 HP stock and mid to upper 40's with pipe, intake and cam mod??

Don't remember the torque #'s, but its less than the HP #'s

[fofitty]

Ok first off Do the cam mod it has no effect on starting. I have done over 20 mods and never had a single problem. second the YZ does have a differnt psiton also the YZ has a smaller wrist pin 18mm YFZ has a 20mm pin. You can buy a WR450 cranks shaft and conneecting rod and put in a YFZ there about 12 ounces lighter. WR also has a 18mm wrist pin so you have to buy a WR piston after market or stock.
I run a 05 YFZ crank with a hotrods connecting rod. 14:1 venom piston cam in the mod postion.

As for stock HP look at teh exhaust and the intake they are restricted not by performance people at Yamaha but buy there lawyers Quad lawsuits are slam dunks if they machine are crazy fast out of the crate. So slow them down. Not very many people get hurt on totaly stock YFZ's and if you modify it then liability shifts from OEM to you.

[Woody_YZF]

Originally Posted by fofitty

Ok first off Do the cam mod it has no effect on starting.

I guess I imagined my problems?? I jetted that thing every way I knew how. With that said I know many people who have had NO issues. It's definitely worth a try.

[fofitty]

Woody have you check your shims lately?