hey,i got some questions.

[JKWGA400EX]

i would really like to get a stock bore high compression piston,and i got some questions.what brand should i get(JE or Wiesco?),what compression,and do u have to run race gas in a 13:1 piston,i there alot less reliability if it has higher compression?my style off riding is,take it out for a spin every now and then,drag racing a bit,sand,but nothing like racing a series or somethin.what would yall recomend?
prices and estimates on power gain would be nice.




thanks,
jared

[d3ktrix]

The highest comp you can run on pump gas is 12:1

If you are just a recreational rider I wouldn't wana spend the $8 a gallon on race gas :P

[ghott]

I've been told that the highest compression you can run on 93 Octane pump gas is 12.5:1. Anything larger requires higher octane fuel which comes with a hefty price as trix stated previously.

[Pappy]

Actually, the shelf 12-1 or 13-1 pistons can run well on pump gas, but care needs to be taken on the cam chosen. Ive seen 14-1 engines run on pump gas:eek2: it has to do with dynamic and static compression. I will leave that to the gear heads:p


A 12.5-1 piston and crf cam would make a good back yard ripper to play on:p

[GPracer2500]

Pappy hit it.

There's no set-in-stone guidelines for octane requirement based static compression. Intake valve duration, ignition timing, air density (of your typical riding locations), bore size, jetting, and even characteristics of the mixture turbulance within the combustion chamber all contribute to what octane an engine requires. Switch the discussion from liquid cooled to air cooled and things change all over again. Most limit the discussion to static compression ratios cause that's what's easy. But if you do that then you wind up with a pretty wide range of possible static ratios that could possibly run deto-free on premium pump...

Measuring a cylinder's compression in PSI is a fairly accurate measure of dynamic compression in a 4 stroke. It's a far better guage for what octane requirement to expect from an engine. But even that doesn't encompass ALL of the contributing factors.

I have a pretty firm grasp on what an EX requires regarding octane because I have one and have experimented with various fuels, elevations, and engine configurations. I can't provide any real-world guidence on a 450r cause I don't have one (damn--I need a 450).

A guy at 5000ft with running a 14:1 piston with a radical cam and slightly rich jetting might not be detonating with premium pump. A different guy at 500 feet, with a stock cam, jetted for best power (right at the edge of lean), and a 12.5:1 piston might be detonating under high load conditions using the same fuel.

On an EX, 11:1 is the common wisdom threashold for pump vs. race. Nonetheless, I've gotten some deto with 10.5:1 and lots of deto with 10.8:1 because of my particular running conditions and tune....

Bottom line: My best advice is to talk to some engine builders and/or keep compiling a "consensus" of what people have and what their experince has been. You know you're getting solid info when people include details; the cam is an important one. Elevation and ignition (stock CDI?, modified stock CDI?, aftemarket?) are pretty significant too.

[JKWGA400EX]

hey thanks for all the info.i have a hrc cam in it already,so u think a 13:1 piston would be good?or should i go with the 12:5:1?is there a big difference in power,and relibility?how much more power should i expect for a setup like that?will i noticee a big difference?

keep the info comin.

-jared

[JKWGA400EX]

anyone?????

[Flodies400ex]

I dont no but I want to know because I want to do that piston change to soon so poeple help us out lol

[GPracer2500]

As far as I'm concerned, upgrading the 04/05's 10.5:1 piston to at least the same spec as the 06's 12:1 is mandatory.

12.5:1 or 13:1?? Hmmm. Dunno about which I'd choose. Prolly not that big a performance difference between the two. Like was discussed above, a cam change could easily make up that difference as far as PSI is concerned.

I'd be inclined to go 12.5:1 myself. But I'd choose that mostly because I ride dunes a lot and nothing seems to bring on deto like dragging up huge sand hills and racing around giant bowls. Plus the highest pump I can get is 91 around my local. If I could get 93 that might be enough to push me over the edge and go for 13:1. Even if I did pick up some deto at the dunes it wouldn't be the end of the world for me. But if for some reason I was getting deto from more than the most demanding of conditions I'd wish I hadn't gone that high....

Here's another way to look at it: Maybe compare piston weights. I mean, if your really on the fence about what piston to choose, then seems like whichever choice was closest to stock would run the smoothest. The counterbalancer is setup for the stock piston weight so....

Never really thought of that until just now. I'm not sure how valid a point it is. Could be that any A/M piston is going to be so far from stock weight that it's not worth considering. Just a thought [shrug]...

[JKWGA400EX]

sooooo,if i were to buy a 12:5:1 piston what brand is best(what would u get)?u think that would be a powerful setup?12:5:1 piston,hrc cam,fmf power bomb header w/ titanium 4 pipe,k&n with jets,so how does that sound?would that give me good power?the next thing to do would be getting a different rev box,or what do yall think?by the way i like planning ahead if u have'nt noticed,lol.


-jared

[GPracer2500]

Originally posted by JKWGA400EX
sooooo,if i were to buy a 12:5:1 piston what brand is best(what would u get)?u think that would be a powerful setup?12:5:1 piston,hrc cam,fmf power bomb header w/ titanium 4 pipe,k&n with jets,so how does that sound?would that give me good power?the next thing to do would be getting a different rev box,or what do yall think?by the way i like planning ahead if u have'nt noticed,lol.


-jared

Sounds good to me. If that turns out to not be enough than consider having the head ported.

[JKWGA400EX]

hey thanks a bunch GPracer.changing topic slightly,what does it mean having ure stock cdi modified?what do they do to it,were can i get it done,how expensive and all that good stuff?also whats the average price on a port job?


thanks again,
-jared

[GPracer2500]

Originally posted by JKWGA400EX
hey thanks a bunch GPracer.changing topic slightly,what does it mean having ure stock cdi modified?what do they do to it,were can i get it done,how expensive and all that good stuff?also whats the average price on a port job?


thanks again,
-jared

The only reason I mentioned "modified stock CDI" is because on some bikes/quads there is a wire that can be cut or a new wiring harness that can be used that alters the rev limit and ignition advance of the stock CDI. The 06 450r is an example--I know it changes the rev limit, I'm unclear on whether it changes the spark advance.

Porting costs? Not sure exactly. Prolly $300 and up.