Piston/Deck height measuring confusion !!

[hoodeng]

I think the confusion has come from a piston drop cc test done with the piston pushed down the bore a set distance with the piston not on the rod and crank, versus a mock up with the piston rod and crank assembled setting the height, this is what Bill was referring to.

My comment was from people doing a bench cylinder/piston/head mock up without bottom end assembly.

Cheers.

[AA Performance]

I'm not sure how you did it but Madbill is on the money. When you bring the piston to TDC, put a dial indicator on the crown of the piston and bring it down a set amount, say 10mm. Then pour it and see what you get. Then work out the volume of the bore diameter by the amount you moved the piston down the bore. Subtract one from the other and that will give you the dome, deck area all in one hit.

[Eagle1903]

I'm not sure how you did it but Madbill is on the money. When you bring the piston to TDC, put a dial indicator on the crown of the piston and bring it down a set amount, say 10mm. Then pour it and see what you get. Then work out the volume of the bore diameter by the amount you moved the piston down the bore. Subtract one from the other and that will give you the dome, deck area all in one hit.

Hi,
In the method A, I did this exactly as you described above. With the crank, rod and piston assembled in the block I put the piston down 8.24 mm ( not 10 mm ) and calculate the dome volume with the fluid and burett. I think I did not make a mistake there. But since the piston’s crown edge ( deck height ) is 2.20 mm down the bore at TDC, the whole piston is actually down the bore 2.20 mm is a plus volume. And only when I add that volume to gasket volume and chamber volume minus the measured the dome volume, I am reaching to a sensible CR figure which I have checked it with the method B ( filling the fluid from the plug and directly measuring the total compressed volume ). Meaning that, the piston below the block face is playing a role in the CR calculation. If the piston/crown edge was at 0 deck height at TDC then I wouldn’t add the volume of that - 2.20 mm volume.
So, in this particular case if I were to use any CR calculator ( instead of pen and paper ) I would have to enter the - 2.20 mm value into the deck height value. In my very first post, my confusion was; I was not able to determine if the red area or blue area was representing the deck height because normally the blue area should be extended to the edge of the piston and would represent the piston deck height.
In this engine the measured/calculated dome volume is 14.57 cc which should be subtracted from the gasket+chamber volume to find the compressed volume but the piston is not coming up to the block face at TDC. So there is an additional volume of 2.20 mm height. Because if I am correct, any area/volume above the crown at TDC is a part of the compressed volume just like the gasket volume and chamber volume.

[MadBill]

...In this engine the measured/calculated dome volume is 14.57 cc which should be subtracted from the gasket+chamber volume to find the compressed volume but the piston is not coming up to the block face at TDC.

Yes. by moving the piston down from a zero reference of TDC, the "additional volume" of the deck height is captured within the 14.57 cc net dome volume and so does not have to be separately considered.

.So there is an additional volume of 2.20 mm height. Because if I am correct, any area/volume above the crown at TDC is a part of the compressed volume just like the gasket volume and chamber volume.

To repeat: with this measurement method deck height is irrelevant. Ignore it; don't give it another thought.

[Eagle1903]

...In this engine the measured/calculated dome volume is 14.57 cc which should be subtracted from the gasket+chamber volume to find the compressed volume but the piston is not coming up to the block face at TDC.

Yes. by moving the piston down from a zero reference of TDC, the "additional volume" of the deck height is captured within the 14.57 cc net dome volume and so does not have to be separately considered.

.So there is an additional volume of 2.20 mm height. Because if I am correct, any area/volume above the crown at TDC is a part of the compressed volume just like the gasket volume and chamber volume.

To repeat: with this measurement method deck height is irrelevant. Ignore it; don't give it another thought.

Hi MadBill,
I understood you. Then, I have to find my mistake since method A produced a different compressed volume than the method B ( pure filling via the spark plug ) by a large margin. I have not yet filled with kerosene, may be there were large bubbles trapped In the closed chamber because of the ATF. ( as large as 12 cc ). I will also calculate the dome again by placing the piston down 10mm from TDC.

[Nut124]

Eagle, take a look at this picture and see if it makes sense. This is how i do it.



Compressed volume equals:

Head volume (fluid)
plus Gasket volume (calc)
plus Deck HT volume (calc: Pi x R^2 x DHT)
minus dome volume (fluid and calc)

Make sure you use all the same units. The deck HT and the dome vol calc must use the same reference point on the piston.

This is assuming that the dome volume is positive as opposed to negative.

What do you think?

[MadBill]

<Edit: written before seeing above post>

The only reason you should have to add the "2.2 mm x bore" volume to get a sensible CR number using my suggested method would be if when you "...put the piston down 8.24 mm. ", your zero reference was the block gasket surface (zero deck) rather than some convenient outer diameter horizontal surface on the piston top, with the piston at TDC,( 2.2 mm down the bore).

Here's a analogy: Suppose your plastic sealing plate was an inch or more thick, with a machined 100 cc dish, deep enough to clear the piston dome. To find the 'dome volume' (To be precise, the algebraic sum of dome, valve relief, deck height and any other piston feature volumes combined.) you would turn the engine to exactly TDC, seal the plate into position and fill its dish with the measuring liquid. Subtracting the resulting number from 100 gives you the 'dome volume', apparently 14.57 cc, which you subtract from the chamber plus gasket volume total to get compression volume.

To recap,the "dome volume" that results from this method consists of several elements: the negative volume of the actual raised dome, the deck height positive volume and possibly some small positive volume from the valve reliefs.

The combustion volume then consists of chamber volume plus gasket volume minus net dome volume.

[Eagle1903]

Eagle, take a look at this picture and see if it makes sense. This is how i do it.



Compressed volume equals:

Head volume (fluid)
plus Gasket volume (calc)
plus Deck HT volume (calc: Pi x R^2 x DHT)
minus dome volume (fluid and calc)

Make sure you use all the same units. The deck HT and the dome vol calc must use the same reference point on the piston.

This is assuming that the dome volume is positive as opposed to negative.

What do you think?

Hi Nut124,
This is exactly what I did and ended up almost exactly with my pure filling total compressed volume ( my method B ). Like me, you also calculate the Deck HT volume and add gasket volume and head volume and subtract the dome volume to find the “ compressed volume “ This method produces the very same result with pure filling method. ( filling the whole sealed/built assembly from spark plug hole )
Above MadBill has a different opinion. But when the piston at TDC, there is a still an additional volume above the piston which must be the part of compressed volume. Because the piston crown edge is not level/flush with the block face at TDC and that previously calculated dome ( dome volume ) has 2.20 mm travel to block deck. If the crown edge was flush with the block deck then there would be 0 deck height and the CR would be way higher as I found wrongly ( in my opinion) via my method A.

[Nut124]

Not sure MadBill has a different opinion. He has a slightly different method.

He may be saying move the piston down from TDC a known distance for the dome fluid test and use this move distance in the dome formula. This may then end up including the deck HT volume in the "dome volume". This is harder to visualize.

[Eagle1903]

Not sure MadBill has a different opinion. He has a slightly different method.

He may be saying move the piston down from TDC a known distance for the dome fluid test and use this move distance in the dome formula. This may then end up including the deck HT volume in the "dome volume". This is harder to visualize.

Hmm, it is quite possible that I couldn’t visualize what MadBill meant !!! I certainly did the fluid test correct but then made a mistake somewhere. I can kind of visualize now also what MadBill is saying. However if I do this in two steps like you did; I was able to calculate total compressed volume easily. And this compressed volume verified via the pure filling method.

[Eagle1903]

<Edit: written before seeing above post>

The only reason you should have to add the "2.2 mm x bore" volume to get a sensible CR number using my suggested method would be if when you "...put the piston down 8.24 mm. ", your zero reference was the block gasket surface (zero deck) rather than some convenient outer diameter horizontal surface on the piston top, with the piston at TDC,( 2.2 mm down the bore).

Here's a analogy: Suppose your plastic sealing plate was an inch or more thick, with a machined 100 cc dish, deep enough to clear the piston dome. To find the 'dome volume' (To be precise, the algebraic sum of dome, valve relief, deck height and any other piston feature volumes combined.) you would turn the engine to exactly TDC, seal the plate into position and fill its dish with the measuring liquid. Subtracting the resulting number from 100 gives you the 'dome volume', apparently 14.57 cc, which you subtract from the chamber plus gasket volume total to get compression volume.

To recap,the "dome volume" that results from this method consists of several elements: the negative volume of the actual raised dome, the deck height positive volume and possibly some small positive volume from the valve reliefs.

The combustion volume then consists of chamber volume plus gasket volume minus net dome volume.

I think I found my interpretation mistake. My zero reference was the block gasket surface, zero deck.

[MadBill]

Not sure MadBill has a different opinion. He has a slightly different method.

He may be saying move the piston down from TDC a known distance for the dome fluid test and use this move distance in the dome formula. This may then end up including the deck HT volume in the "dome volume". This is harder to visualize.

Correct!
The method I mentioned is more accurate as it eliminates one measurement and calculation and automatically compensates for varying cylinder-to-cylinder deck heights.

[MadBill]

...
I think I found my interpretation mistake. My zero reference was the block gasket surface, zero deck.

Yes, well I did mention TDC at least five times...

[gmrocket]

Eagle, take a look at this picture and see if it makes sense. This is how i do it.



Compressed volume equals:

Head volume (fluid)
plus Gasket volume (calc)
plus Deck HT volume (calc: Pi x R^2 x DHT)
minus dome volume (fluid and calc)

Make sure you use all the same units. The deck HT and the dome vol calc must use the same reference point on the piston.

This is assuming that the dome volume is positive as opposed to negative.

What do you think?

Hi Nut124,
This is exactly what I did and ended up almost exactly with my pure filling total compressed volume ( my method B ). Like me, you also calculate the Deck HT volume and add gasket volume and head volume and subtract the dome volume to find the “ compressed volume “ This method produces the very same result with pure filling method. ( filling the whole sealed/built assembly from spark plug hole )
Above MadBill has a different opinion. But when the piston at TDC, there is a still an additional volume above the piston which must be the part of compressed volume. Because the piston crown edge is not level/flush with the block face at TDC and that previously calculated dome ( dome volume ) has 2.20 mm travel to block deck. If the crown edge was flush with the block deck then there would be 0 deck height and the CR would be way higher as I found wrongly ( in my opinion) via my method A.

It doesn’t matter if it’s flush with the deck or not, the deck height it can be anywhere.

if you think the deck height matters , why wouldn’t deep valve reliefs matter then if they are cut deep below the deck height and take away from the dome volume above the deck?


Any deck clearance is the same as deep valve reliefs cut in a dome...it would take away from the dome above deck volume and reduce comp ratio

That’s what’s great about the piston down method, it’s accurate and takes into account anything like that

[Eagle1903]

Dear Lizardracing, PRH, Turbocamino, MadBill, Gmrocket, Hoodeng, Nut124, AAperformance,

Many many thanks for all your help, guidance and patience !!!

I must admit I was slow in understanding the important point of using the TDC, although calculating above the crown volume and adding it to the other compressed volumes and finally understanding the method A working was a treat. However, all this led to an rather odd feeling. I normally work on simple I4 engines and built 5-6 engines per year. I have noticed that I have been using CR calculator too long which made me forgot the basics and using pen and paper and visualizing the things. When I did come across with this somehow machined complex crown piston I am simply confused about the piston deck height which I should use in the software. Then, you know the rest.

Again, many many thanks to each and everyone of you !!!