400ex piston compression question

[NTPRacing#19]

i need a new piston and rings and i want a piston that is higher compression than stock but nothin to high that i have to run strictly race fuel.

also what do the numbers or the ratio stand for? ive never understood that

9.1:1 what u r running now...10.8:1 highest u can run on pump gas...11:1 requires a mixture of pump gas and race gas....12:1 stricktley race gas

[chad502ex]

Originally posted by XxHonda_RacerxX
10:1 what u r running now...10.8:1 highest u can run on pump gas...11:1 requires a mixture of pump gas and race gas....12:1 stricktley race gas

9.1:1 stock LOL!

Chad502EX.com

oopsie...my bad lol

[F-16Guy]

I think compression ratio means this:
11:1 -- volume of the cylinder at BDC is 11 times greater than the volume at TDC. For example; if the volume of the cylinder was 11 cu.in. at BDC, that air would be compressed to a volume of 1 cu.in. at TDC. Someone correct me if I'm wrong.

[Ryan]

Originally posted by XxHonda_RacerxX
10:1 what u r running now...10.8:1 highest u can run on pump gas...11:1 requires a mixture of pump gas and race gas....12:1 stricktley race gas

I disagree.

You can run Pump gas "93" with the 11:1 ratio. Any higher then that, you should be mixing 50/50 "93/AV race gas."

I've heard of people running pump gas in Cannondale motors :eek2: ........ probably didn't run so good though.

[trx400ex]

Originally posted by Ryan
I disagree.

You can run Pump gas "93" with the 11:1 ratio. Any higher then that, you should be mixing 50/50 "93/AV race gas."

I've heard of people running pump gas in Cannondale motors :eek2: ........ probably didn't run so good though.

I disagree.

I actually have an "11:1" motor and i have to run race gas(110 Octane). Where i live is pretty cool compared to further south and west so i would imagine just about everyone would need to run race gas with this motor, UNLESS you dont ride hard. I can get by with pump gas if i ride it for 5 minutes and let it cool back down, otherwise it will knock like a mofo. I could probably get by with a 50/50 but what fun would that be? I also heard not to run AV fuel unless you have an airplane.

Originally posted by Ryan
I disagree.

You can run Pump gas "93" with the 11:1 ratio. Any higher then that, you should be mixing 50/50 "93/AV race gas."

I've heard of people running pump gas in Cannondale motors :eek2: ........ probably didn't run so good though.

ya u can dur...they are watercooled...all watercooled bikes can run i think 12:1 on pump gas...they all come stock with 11:1...y do u think a Z can beat a 400...thats the only reason

[Ryan]

Originally posted by XxHonda_RacerxX
ya u can dur...they are watercooled...all watercooled bikes can run i think 12:1 on pump gas...they all come stock with 11:1...y do u think a Z can beat a 400...thats the only reason

ohhh ok.... I see. Sorry about that.

[One_Bad_400]

depends on the way the piston is designed, for example in my Z I run 93 octane and it has 11.3:1 compression.

F-16guy: that's what I've always thought compression to be too, but then how just by changing a piston that is the same size will the compression be bumped up?

[F-16Guy]

[i]F-16guy: that's what I've always thought compression to be too, but then how just by changing a piston that is the same size will the compression be bumped up? [/B]
The dome on top of a high compression piston decreases the volume of the combustion chamber when the piston reaches TDC.

[NTPRacing#19]

yea a higher compression piston has a rasied dome with valve cutouts in the top of it, if you like at a stock compression piston you will see 4 cresent moon shapes and if you look at a higer compression piston it whould have 4 half moon shapes

[bmw500hp]

This phrase is quote Mark Baldwin " the 400ex motor has a problem with detonation, at 11:1, race gas of over 102 octane should be ran".

I asked him if it was a heat issue and he then re-inforced the fact that the issue was "detonation".

I ask him about a mix, and he said "No"

Take it as you will.

[NTPRacing#19]

so whats the next comprssion piston i can get thats higher than stock?

[remlapr]

I think most people would agree that the safe play is 10.5:1.

As far as what compression is - I agree with f-16 guy, but I believe this is the correct way to think about it: 11.0:0 would mean at TDC the air inside the chamber has been compressed to 11x normal atmospheric pressure. At BDC the air would be at 1x atmospheric pressure - same as the air around you.

[kgbg]

You need to realize that cam shafts and cyl. head height deal with compression as much as a piston. I have a 13.5 to 1 compression piston (its a flat top). My head is decked and so is the jug. Alot of compression has to do with "squish" of the compression chamber, alot has to do with the piston it self, but the cam is a big factor as well. With the 13.5 to 1 piston, and the cam and "squish" I have on my motor, I am running about 11.8 to 1 compression in te chamber. Which needs about 102 octane to keep from detonating, as long as the out side temp and barometric pressure is nothing too wacky.

[proquad_20]

11/1 on mine runs just fine on pump gas, pretty fast too

i ran pump gas in my 416 with 11:1 for a while...but i now run a mix 50/50 (93/110)....my bike seemed to run fine on pump gas...but also its a wiseco piston and is not a true 11:1 compression..so that could also be a factor also

[F-16Guy]

Originally posted by nacsracer27
i ran pump gas in my 416 with 11:1 for a while...but i now run a mix 50/50 (93/110)....my bike seemed to run fine on pump gas...but also its a wiseco piston and is not a true 11:1 compression..so that could also be a factor also

My 87mm Wiseco was almost exactly 11:1 with ex gaskets. The CR problem with Wiseco pertains to their 89mm (440) pistons. They are flat top, but should be domed. The cylinder needs to be machined to zero deck height to achieve close to 11:1 on that piston. The stock bore, 86mm, and 87mm pistons are all domed, and produce a true 11:1.

Originally posted by F-16Guy
My 87mm Wiseco was almost exactly 11:1 with ex gaskets. The CR problem with Wiseco pertains to their 89mm (440) pistons. They are flat top, but should be domed. The cylinder needs to be machined to zero deck height to achieve close to 11:1 on that piston. The stock bore, 86mm, and 87mm pistons are all domed, and produce a true 11:1. my 86mm piston and 8 mm piston both have a flat top...they are domed but the top is flat..here is a pic comparing the 86mm to stock

[F-16Guy]

That raised area is considered a dome.