Hey. I hear a lot on plus and minus on piston domes. I would like someone to explain what a (example) 10:25:1 dome piston is when calculating compression. is it a plus dome or minus dome. what cc would this be. thanks.

 

Depends on the manufacturer. Some list it as a + meaning they're adding to the size of the dome while others list it as - meaning they are subtracting from the combustion chamber volume. When figuring compression on an online calculator for example you have to see how they make adjustments for chamber size & do it the same way or your #'s will be way off.

 

As long as you know the volume of the Dome (or reverse dome) it's obvious once doing the calculations if it has to be plus or minus. If the comp goes down but it was meant to go up, change the plus to a minus or viceversa

 

I'd only give business to piston manufacturers who don't assume my combustion chamber is a certain size. Wiseco shows a dome volume, which I can work with. Most of the others I've seen do the same. Who does it the backwards-*** way?

 

They all assume it's a certain size to give you a compression #. The difference is + or -. Nobody does it assbackwards. It's just a calculation. To know what your compression really is you would have to measure everything so every piston manufacturer that lists a compression # for their piston is basing it on the assumption that everything measures up to stock dimensions.

 

You said "assume" twice, which shines a glaring light on my point. Piston manufacturers shouldn't assume anything about my engine.

If I'm building an engine, I'm measuring everything.

 

GREAT QUESTION-
A person assembling an engine without a simple CC beaker is just guessing.
This is NOT the place to guess anything, unless you are a novice and rebuilding a lawn mower engine.
Next, a piston dome is a way of cheating (legally)
The dome gives you more real estate
Force times area equals more .
A domed piston gives you more force even if all else remains the same.
Albeit most do this to raise the CR a point or 3
Even if you know what the manufacturer says the piston dome value is -
No one knows what the value of the combustion chamber is unless you measure it.
Now we are getting to some nuts and bolts.
Do we sink a valve to gain?
Or replace all seats to get it back?
(or a 100 other nuances)
Then consider you squish measurement.
That is free power that most people let go right on by.
Of course, in there the head gasket has to have a known value
Side point -
Cometic will make you most any thickness of head gasket or base gasket you want.
No extra charge.
Right now.
Custom work from quality people, no extra charge .
The made me 4 each - 0.004 Evo base gaskets a couple months ago

 

Then you're going to adjust the dome volume to get the compression you want & not just install a piston that's advertised at the compression you want. Same thing.

 

So what do you use to measure dome volume?

 

How to you measure a volume accurately with a CC beaker?