Piston/Deck height measuring confusion !!

[Eagle1903]

Dear Friends,

Could you please help me correctly measuring the piston deck height in this kind of complex shaped piston for entering the correct figure in to the typical CR calculator softwares ?

As you can see in the below photos, there are 2 areas ( marked red and blue ) which I could rest the pointer of the DTI gauge when the piston is fitted in the block to measure the piston to block height at TDC.

In this particular example, the red area/surface is 2.20 mm below the block deck and the blue area/surface is 0.40 mm below the block deck. Naturally, entering these 2 different height values produces different CR calculations.

Please note that, if I were to calculate the total “ dome volume “ by placing the piston let’s say 10 or 15 mm below the deck for measuring with fluid filling method; it gives an absolute cc volume of let’s say 5 cc ( which is a minus value since it is a dome not a dish ) which can directly be used In the CR calculation but almost all the calculators asks for the piston deck height to enter. This is why I am confused where to rest the DTI gauge pointer to read where the piston sitting at TDC in relation to the block’s deck.

If there was not the red area and the blue area was extended fully to the outside edge of piston, I would simply rest the DTI pointer and measure the piston to deck height and enter that value ( which is 0.40 mm below ) in the calculator. I guess.

[Eagle1903]

Dear Friends,

Could you please help me correctly measuring the piston deck height in this kind of complex shaped piston for entering the correct figure in to the typical CR calculator softwares ?

As you can see in the below photos, there are 2 areas ( marked red and blue ) which I could rest the needle of the DTI gauge when the piston is fitted in the block to measure the piston to block height at TDC.

In this particular example, the red area/surface is 2.20 mm below the block deck and the blue area/surface is 0.40 mm below the block deck. Naturally, entering these 2 different height values produces different CR calculations.

Please note that, if I were to calculate the “ dish volume “ by placing the piston let’s say 10 or 15 mm below the deck for measuring with fluid filling method;

E0632E54-2C65-4CBA-B0EC-3C36B85EAA98.jpeg

37B1A025-8D4D-49F1-9126-C24FCCF3E571.jpeg

[Eagle1903]

I use the dial gauge ( DTI ) with the magnetic bridge resting on the block deck.

[Lizardracing]

You could also move the piston down .500" and CC the whole area. Then subtract the area of the bore of .500 and the left over is the CC of the whole piston. Using the same Math, figure the remaining stroke lenght and you have the CR. No software needed and deadly accurate.

[PRH]

To me, the red area is where the CH would be measured from.

[Eagle1903]

You could also move the piston down .500" and CC the whole area. Then subtract the area of the bore of .500 and the left over is the CC of the whole piston. Using the same Math, figure the remaining stroke lenght and you have the CR. No software needed and deadly accurate.

Thank you very much. If I understand correct, as I kind of try to describe ( in my 4. paragraph) by putting the piston down the bore ( let’s say .500” ) I will measure the left over CC which will be added to cyl head chamber volume and plus + the headgasket volume will give me the total “ compressed volume “ which then will be used the final calculation.

Cylinder vol. + Compressed vol. / Compressed vol.

And one cylinder volume will be as normally calculated 0.7854 x bore x bore x stroke = cylinder volume which will be used in the above formula.

[Eagle1903]

To me, the red area is where the CH would be measured from.

Thank you very much. Then, once the crown dome calculated and added to gasket volume and chamber volume to find the total compressed volume, deck height will represent the compression height since the crown surface actually starts with the red area in the photo. So, if the calculator is going to be used this height must be entered as the deck height. And if we were to measure the CH of this particular piston, we will have to measure it from the pin center to the red area.

[turbo camino]

The 'deck height' field in the CR calculators is just a shortcut so you don't have to calculate the volume, you input it as a linear distance instead of in CCs. If you're going to do as suggested and put the piston a known distance down and measure the volume, you're directly measuring the volume of the dome, the deck height, crevice volume, etc.

Pick any reference surface you like and use that to move the piston down a known distance. As long as you are measuring in the same spot for the top and bottom it doesn't matter if you're using the true deck of the piston or the top of the dome, .500" is .500".

[Eagle1903]

The 'deck height' field in the CR calculators is just a shortcut so you don't have to calculate the volume, you input it as a linear distance instead of in CCs. If you're going to do as suggested and put the piston a known distance down and measure the volume, you're directly measuring the volume of the dome, the deck height, crevice volume, etc.

Pick any reference surface you like and use that to move the piston down a known distance. As long as you are measuring in the same spot for the top and bottom it doesn't matter if you're using the true deck of the piston or the top of the dome, .500" is .500".

Thank you very much. Now, I think I have a clear picture.
The stroke is 90 mm.
I will put the piston down for example 20 mm down the bore and cc the volume between the crown and the top of the block. Then I will add that volume + gasket volume + head chamber volume. The total will be the compressed volume.
Then I will find the remaining “cylinder volume”which will be calculated with the remaining 90-20 = 70 mm stroke and follow the normal formula : “cylinder volume” + compressed volume / compressed volume
If I seal the crown to cylinder wall by grease then I will miss the crevice, if I don’t seal with grease then the crevice volume will also be included in the compressed volume.
I hope I got it right.

[MadBill]

<Edited to correct math>
You mention deck height, but it appears it is only the compression ratio that you want. In that case per Turbo, do this:
o Piston crevice volume is pretty small; I'd seal it with grease to avoid leakage errors.
o Set the piston far enough down the bore for its highest point to be below the deck, lets say that's 10.00 mm down from TDC. (Far enough that the fluid can flow into the resulting gap but not much more, it will just add potential errors as you re-fill your burette.)
o CC the resulting dome/dish/valve relief/piston travel volume. Call it 'A'
o Calculate the volume of a cylinder 10.00 mm tall by bore diameter. Call it 'B'
o A minus B equals piston dome/dish/relief volume. For a dome this will be a negative number.
o Add this to the gasket and chamber volumes to get compression volume.
o Calculate compression ratio in the usual manner.

[gmrocket]

Your confusing yourself thinking your need to calculate the down distance into the equation?

That .500” down or whatever you choose, it doesn’t matter. All your doing is figuring out the dome volume.

I prefer to use 1” down as it’s a whole number and for beginners it’s easier to figure out.

Find your max TDC point with your dial indicator, then rotate crank until its 1” down the bore, smear a small amount of white grease around the top ring land making sure it seals the areas above the top ring (this ring land volume can be calculated after) Cover the bore with your fixture, fill the void until there is no air...record that volume as V1

Now , on paper, figure out that same volume with a 1” stroke and the bore , then record that volume as V2

Subtract V1 from V2, this is your dome volume used for your calculation...it’s a plus value, valve reliefs or a dish is a negative value

[MadBill]

Per corrected math above, dome volume is negative, subtracted from the combustion volume.

[hoodeng]

What was the manufacturers claimed volume for the crown? Can you wet out a cylinder? Did they have a reason for a two step squish shelf? Who made the pistons? Can you post a photo of the combustion chamber including the head surface?

Your red marked band i would expect be the datum for piston deck height, your blue marked shelf in pistons of this type would be to accommodate a recessed combustion squish shelf.

Cheers.

[gmrocket]

Per corrected math above, dome volume is negative, subtracted from the combustion volume.

Doh,, yup

Too many beers..a negative 12 means no beer left

[Eagle1903]

First of all, many many thanks for all your help.
I would like to share my measurements and the confusing results.

Bore : 84,65 mm
Stroke : 90 mm
Gasket : 1,20 mm
Cylinder volume : 506,5 cc
Gasket volume : 6,85 cc
Cylinder head chamber volume : 41,3 cc

Piston’s red area crown is 2,20 mm is below the deck surface. So may be called 2,20 mm deck height. But that was the confusing part for me in the beginning anyway.

First, in order to calculate the piston dome volume:

I put the piston down the bore 8,24 mm ( from the block surface/deck ) To measure the dome volume by filling the fluid and calculate.

8,24 x 42,325 x 42,325 x 3,14 = 46,373 cc

This volume took 31,80 cc of fluid when filled SO I noted that 46,373 - 31,80 = 14,57 cc ( piston’s absolute dome volume )

Then I did the basic CR calculation as follows;

Cylinder volume + Compressed volume / Compressed volume

Cylinder volume : 506,5 cc

Compressed volume : 41,3 ( head ) + 6,85 ( gasket ) - 14,57 ( dome volume ) = 33,58 cc

506,5 cc + 33,58 cc / 33,58 = 16,08:1 CR

If we call the above calculation method A, then certainly I am missing out something !! Probably the “ deck height “

Then as a method B :

I put the piston at TDC ( RED area deck height is 2,20 mm down the bore ) placed the headgasket, fit the cylinder head and filled it with ATF fluid and measured the fluid with burrett. The total fluid went into engine is now 45 cc. So the new compressed volume is 45 cc.

Doing the math again : 506,5 cc + 45 cc / 45 cc = 12,25 : 1 CR

It seems that 12,25 CR is more likely to be correct since the engine was running with CR although cams were not timed correctly etc versus it probably could have blown with 16,08 CR.

Now, I am wondering where is my mistake in the method A.

Coming back to my original question, which I was trying to determine the “ deck height “ in this rather complex Crown shape in order to enter the correct value in the CR calculation softwares if I were to use a calculator.