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12.8-1 vs 13.5-1 compression

By tribalbc,
in General Dirt Bike Discussion

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Grand Master

tribalbc

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I have an 07 WR450. I want to raise the compression for more bottom end. But I don't want to run race gas, too hard to get were I live.

My stock compression is 12.3-1.

Will I see much of a gain with 12.8-1 and know I'm safe with pump fuel ?

I know lot's of people are fine running 13.5-1 with pump fuel but is it worth the gamble :busted:

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Grand Master

the white stallion

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Run 13.5.1 If you want.Big differance over 12.3.You won't need race gas.I'm running a 13.5 and 14.1 in the 250's.No race gas.

The only question that ponders me is how you are looking for more low end on a 450.Maybe because it's a WR.Like said above you can change the gearing to achieve that.If you haven't done any others mods to the WR I would start there first.Open up the pipe(new one is better on this model)/re jet/Open up the air box and get rid of the throttle stop.

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Grand Master

tribalbc

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Run 13.5.1 If you want.Big differance over 12.3.You won't need race gas.I'm running a 13.5 and 14.1 in the 250's.No race gas.

The only question that ponders me is how you are looking for more low end on a 450.Maybe because it's a WR.Like said above you can change the gearing to achieve that.If you haven't done any others mods to the WR I would start there first.Open up the pipe(new one is better on this model)/re jet/Open up the air box and get rid of the throttle stop.

Bike has all the free mods, YZ full exhaust, airbox opened, rejetted, etc. I've also already added 2 teeth to the rear sprocket.

I race Hare Scrambles A class and we have lots of gnarly tech stuff where I like the low end grunt.

My piston is due for replacement anyways so I figured I might as well bump up compression.

I guess I'll take a chance on the 13.5-1, seems everyone figures it will be fine on pump.

No one with any thoughts on the 12.8-1 ???

Thanks guys :busted:

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notevenclose

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Bike has all the free mods, YZ full exhaust, airbox opened, rejetted, etc. I've also already added 2 teeth to the rear sprocket.

I race Hare Scrambles A class and we have lots of gnarly tech stuff where I like the low end grunt.

My piston is due for replacement anyways so I figured I might as well bump up compression.

I guess I'll take a chance on the 13.5-1, seems everyone figures it will be fine on pump.

No one with any thoughts on the 12.8-1 ???

Thanks guys :busted:

You could always cut it to 13.5:1 and retard the ignition if it is pinging on pump gas at first.

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Mentor

DieselSJ

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doulbe that an then we are talking diesels run at 25 to 30 CR

Really? Cummins run at about 17.5:1, my little Detroit is at 21:1 (and that is WAYYY high because it was not designed for turbocharging - most guys drop them to 18:1 when they turbo them). I'd really like to see a diesel that is running anything higher than about 22:1.

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Grand Master

the white stallion

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Bike has all the free mods, YZ full exhaust, airbox opened, rejetted, etc. I've also already added 2 teeth to the rear sprocket.

I race Hare Scrambles A class and we have lots of gnarly tech stuff where I like the low end grunt.

My piston is due for replacement anyways so I figured I might as well bump up compression.

I guess I'll take a chance on the 13.5-1, seems everyone figures it will be fine on pump.

No one with any thoughts on the 12.8-1 ???

Thanks guys :busted:

No one with any thoughts on the 12.8-1 ???

Go big or go home!?

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Grand Master

JSanfilippo

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The only question that ponders me is how you are looking for more low end on a 450.Maybe because it's a WR.

The cams are a big reason. The 5 valve head is a contributing factor to a certain degree. I think thats why Yamaha went with a 4 valve head for the 2010 YZ 450f.

I'm interested to see how tribalbc likes the HP piston. Have you thought about doing the cams too?

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Grand Master

the white stallion

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The cams are a big reason. The 5 valve head is a contributing factor to a certain degree. I think thats why Yamaha went with a 4 valve head for the 2010 YZ 450f.

I'm interested to see how tribalbc likes the HP piston. Have you thought about doing the cams too?

cams too?:busted:

That would be the iceing on the cake! It would make everything come together like a pear in a apple pie factory..................Wait never mind.:banana:

Wrong anology.:banana:

The cams are quite a bit retarded??

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Grand Master

tribalbc

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The cams are a big reason. The 5 valve head is a contributing factor to a certain degree. I think thats why Yamaha went with a 4 valve head for the 2010 YZ 450f.

I'm interested to see how tribalbc likes the HP piston. Have you thought about doing the cams too?

Yeah I'm in the process of getting a set of YZ cams from a fellow TTer right now.

I'll probably rebuild in OCT and give a report then :busted:

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