I'm looking at the piston options for a 95" build and I'm seeing flat top and I know what they are but, then I see things like 10cc dome and 8.75 pop up. I dont know what a pop up type piston is. Is there a difference between dome and pop up? What would the advantage or disadvantage of a pop up be? Also I did a search and really could find a answer to the difference. I saw somewhere say dome and pop up were the same but, that doesn't make sense to me as looking in the same manufacture they list all 3 types.

Thanks

 

They generally mean the same thing as far as increasing compression goes. The difference is a dome will be thicker on the top so it can be machined to adjust to the desired compression whereas a pop up will have the same shape on the underside of the piston & can't be machined.

 

figure out the complete build before even worrying about pistons, (id go 98" over 95"),figure what cams fit your riding style then match compression to them.your builder should be able to tell what pistons are needed

 

That's good advice and I will be sure to do that however, now I'm still wanting to understand this pop up type of piston. I've been looking and have seen a few pictures and on most of them I can't see any type of dome or anything. The pictures I've seen look like flat top so I'm just confused now. Any one have some show and tell pictures?

 

Before this topic goes down a non-understandable road, do you know what compression ratio is and how it relates to engine power output?

 

Yes I understand compression ratio. For a basic description it's the volume from bdc to tdc that is of course very basic and just a static ratio. And cams and elevation and that will figure in to the corrected ratio.

I'm just trying to get a better idea why a company like s&s for example would list some pistons as dome and others as pop up if they're the same. I agree this conversation could go down the rabbit hole very quickly.

 

I'd bore to 98".
We configure dome volumes as needed here.
Our Wiseco/J&E forgings, as per their Engineering Dept, are reduced 1cc per every 2.76 grams of weight removal, starting at 8.5 cc's.
Scott

 

Probably best to ask S&S why they list some as pop-up and some as dome; don't think you will find that answer here. WFO Larry has answered the dome vs pop-up question but it really doesn't matter what you call the piston. What matters is how the dome/pop up volume affects compression. Kirby has laid out how to arrive at the piston choice via the development of a build plan to deliver the target corrected CR and CCP. As you appear to know, static CR is just a number; the motor does not operate under static CR but under corrected CR. So, if your target corrected CR is 9.8 and or your target CCP is 195psi, build a plan to hit those targets.

 

pop-up and dome mean the same thing. It's two descriptive terms to imply there is increased piston volume above the crown of the piston to assume combustion chamber space and in turn increase compression ratio.

I think the term "dome" is from years gone by where the dome shape was hemispherical and the dome on the piston looked like an actual dome. Now that combustion chambers come in all types of bathtub shapes the piston is left with this funky looking plateau over the piston crown that doesn't look at all like a dome. They needed to call it something, so pop-up was used.

 

Thank you