Topic: Dome displacement

[bwaller]

How do piston manufacturers calculate net dome displacement? I understand nowadays they are designed using computer programs and the displacement is likely one available factor.

What about days gone by, before computers, how did they come up with the advertised compression ratio? Can someone here explain how to calculate the net displacement of a piston dome. I can (and have many times) physically measure by liquid displacement, but I'm curious if it's possible mathematically and whether, or not, a dummy like me can do it?

 

[Old Scrambler]

http://www.monolithic.org/calculators/dome-calculator

I don't try to subtract the dome area of the piston unless fill-capacity is needed to calculate flow efficiency - which is only maximized at WOT

[754]

Can you use a ring stuck to the piston with grease, then a glass plate above that.. Filled up with liquid, like used to cc heads.
 After you do that, stick the ring on a flat surface, and repeat.
 Deduct piston measurement from full cylinder measurement.

[MRieck]

How do piston manufacturers calculate net dome displacement? I understand nowadays they are designed using computer programs and the displacement is likely one available factor.

What about days gone by, before computers, how did they come up with the advertised compression ratio? Can someone here explain how to calculate the net displacement of a piston dome. I can (and have many times) physically measure by liquid displacement, but I'm curious if it's possible mathematically and whether, or not, a dummy like me can do it?

With computers Brent. You do need to know chamber volume etc. Sorry.....I'm a dick. 

[scottly]

The liquid displacement method is a hell of a lot easier to do than the math, especially when you try to add in the valve reliefs and odd shaped crowns.  

[turboguzzi]

hei brent

Been into engineering long enough to remember pre-cad.....

 In similar situations i dealt with when i was into aeronautic engineering, you'd simplify the shape into easy to calculate volumes like cones, cylinders, balls, etc. and make approximations.  Think about measuring a flat surface by breaking it into several big and small rectangles and you'll get the idea.

I built in the past the heads of a G50, CB500/4 and the GPZ in CAD, but even then it's all approximate unless you send the heads for 3D scanning. Compared to measuring with a pipette, i got within 5% of the actual volume.

[PeWe]

Make molds with clay that can be filled with proper liquid? Just stamp clay with the piston making a print....

[bwaller]

When I had these pistons made the manufacturer told me the net dome displacement was +2.39cc's. I sent them back for final touches (deepened reliefs 1mm, plus some tracing) and then was told the ndd was -0.495. I had trouble understanding this but they're the experts.   

At that point four years ago I measured by liquid displacement twice and came up with nearly equal figures of 3.2cc's....WTF? Last week I tried again with the same results. All the clearances are very tight, piston/head/valves but there is some room top center for more dome height and I'd like to increase compression some. I need to know exactly what this original piston value is so I can redesign the dome.

One manufacturer tells me he can tell me what the dome of my original piston displaces, the other says no, just tell him what dome displacement I want & they'll build them accordingly. I guess I'll send one to the first guy and see what he finds. 

[Old Scrambler]

Just for consideration..............tall piston domes may impede swirl and the flame..........Lots of aftermarket pistons for British and early Hondas were too tall and lost mid-range power.

Could you gain a few thou at the base-gasket?

[bwaller]

Thanks Dennis. Because the chamber is so small (13.4cc) these can be flat top for the reason you mention.. . I can still add some height and keep it flat. It likes 28 degrees ignition advance so the result is obvious. They're at 0.030" piston/head now so I can't change gasket thickness either.

[Old Scrambler]

An old-school trick was to weld the top of the dome and grind (mill) to make each chamber equal. Depending on the plug location, you may want to mill that added material at a slant.........something like incorporating a wedge-head into a hemi

I would be VERY careful with that MJR head.

Another consideration is your ignition.........I don't know what it is..........but if it can be programmed............maybe a few more degrees in the mid-range would help........and then retard for high-rpms..............just thinking out-loud here 

[70CB750]

How do piston manufacturers calculate net dome displacement? I understand nowadays they are designed using computer programs and the displacement is likely one available factor.

What about days gone by, before computers, how did they come up with the advertised compression ratio? Can someone here explain how to calculate the net displacement of a piston dome. I can (and have many times) physically measure by liquid displacement, but I'm curious if it's possible mathematically and whether, or not, a dummy like me can do it?

Integrals existed before computers 

More info for instance here:

http://www.wyzant.com/resources/lessons/math/calculus/integration/finding_volume

[754]

Sounds faster to cc it than to do all the old math..

[bwaller]

I think you're right Frank. Truth is I have and it didn't agree with what I was told.....therein lies my problem.

OK thanks guys, on to bigger & better things.

[70CB750]

Sounds faster to cc it than to do all the old math..

I don't doubt that and it is not like I would be able to do it now, but they sure drilled me on it in college 

[754]

The problem i see with the math , a few calculation, easy to make a mistake, and how do you measure a partial valve cutout with a radius in the corner.
 As usual..leaving nothing to doubt or chance...usually builds a good motor..

[bwaller]

I'm sending a piston to Wiseco for an accurate measurement of the dome.

[Big Jay]

Easy these days with a CMM maching

[bwaller]

Thanks Big Jay, I hope so. I need an accurate number to start with.

[faux fiddy]

Make molds with clay that can be filled with proper liquid? Just stamp clay with the piston making a print....

This sounds like it would work, if you knew stack height of everything else.

I know people use a small ball of clay to find just the clearance under heads they are working on by putting it in and bolting things down and rolling the motor over once. If it was all conforming  shapes you could use area and thickness of the squeezed out clay to get an estimate.

[Tintop]

Here's a whole pile of race calculators, something for everyone - http://www.wallaceracing.com/Calculators.htm

[Old Scrambler]

Thanks TT

I need a better header