Compression truth! Lipstick on a pig?

[Kenny]

Kenny, have you ever measured a stock piston Yamaha claims @ 9.2/1

Yes. The factory piston has a 9cc dish, it sits .050" below deck and when combined with a head gasket .028" thick, comes up to EXACTLY 9.2:1 I will give you an example of piston differences. The first piston is a Wiseco. Notice how the valve reliefs negate some of the dome, which obviously doesn't fit the shape of the chamber. This piston sits .050" in the hole just like stock.


Notice my 11:1 piston (on the right), even though the valve reliefs are much larger than the Wiseco, the corrected deck height requires much less dome for MORE compression

[Colorado700R]

Good talking to you again Kenny 

Per the manual the stock 9.2-1 equates to a standard sea-level pressure of 450 kPa (4.5 kg/cm2, 64.0 psi), so I think we could use that as a baseline when we get your pistons installed to create a PSI graph so the average joe can test their PSI and compare results and approximate their "true" compression ratio.

Aaron

[Colorado700R]

Like we discussed, the cam, valve lash, and piston ring conditions may very the results, but at a minimum cylinder pressure should be a helper guide fuel octane considerations correct?

[Kenny]

Here's a calculator
http://rapidshare.com/files/145622362/KDS_Racing_compression_calculator.xls

[preddy08]

Kenny, so does that mean us guys with the "14:1" pistons could get away with a half and half mix of race gas since they are only 12.8:1?

[exentix]

wow  my freakin brain hurts now...

lol i think i get some of this but not really 

[Kenny]

Kenny, so does that mean us guys with the "14:1" pistons could get away with a half and half mix of race gas since they are only 12.8:1?

Not necessarily.... Look what happens as you tighten up the quench .010" and shave the head .040". This is a 12.5:1 example:

[socalrappy700]

Ok, now I'm really lost.

[NaturalRaptor]

Great info Kenny!      I feel like Kelly Bundy where I need to forget something in order to learn something new!  

[Peelz]

Great info Kenny!      I feel like Kelly Bundy where I need to forget something in order to learn soemthing new!   

  great analogy I still feel like "the guy in scanners when his head blew up"

[Kenny]

 I don't think I have done a very good job of turning this into digestible bites. I promise it isn't as hard as I make it look. Just hang with me, and as soon as I can get my sick ass off the couch and think clearly, I will get you guys some "good" understandable info

[1FST690]

Other than the obvious difference being 4 valves vs 5, are the pin heights and ring lands the same between a 660 and 700? Would it be posible to take a 700 uncut piston and make a 5 valve 660 piston out of it? I have a friend that owns Issco Man. which does nothing but CNC out airplane parts all day? I really need a 15+:1 piston. Do you have any sudgestions?

[bamaquad]

I don't think I have done a very good job of turning this into digestible bites. I promise it isn't as hard as I make it look. Just hang with me, and as soon as I can get my sick ass off the couch and think clearly, I will get you guys some "good" understandable info

We want more little neat parts to put on to go along with the info.

[VelociRaptor]

Actually the Wiseco dome fits better than any piston Ive used, dont have to clearance the chamber sides to keep the dome from hitting when milling the cylinder.

[Kenny]

Actually the Wiseco dome fits better than any piston Ive used, dont have to clearance the chamber sides to keep the dome from hitting when milling the cylinder.

I think the Wiseco piston is a good budget piston, but the dome sucks. I haven't had any issues at all when milling the cylinder for correct deck height. But it is definitely preferable to have a piston that is designed correctly from the start. The price of milling the cylinder and flycutting for valve clearance make my piston seem a lot less expensive. Especially when you consider the Wiseco is made from 4032 material, and my piston is 2618 which is far stronger. I don't think the Wiseco is gonna stand up to squeezing things real tight. You really need to pay close attention to the wall clearance when you are running a motor hard.