Which one of the two would have higher compression? compression height and recess size in mm

Hi, a would give a higher compression as less volume of air is contained in the recess. If you need a quick comparison without considering the taper, the volume in the recess is pi x radius x radius x height.

Jon

[Edited on 21/9/17 by jonny007]

If compression ratio is 11.5:1

Is it possible piston A can raise the compression to 12.3:1?

Also: In the formula why isn't compression height taken into consideration because if you take that into account the pistons should be identical.

The compression height of piston B is 0.35mm more therefore this negates the difference in the recess depth. You understand?

[Edited on 21/9/17 by bikecarbfred]

[Edited on 21/9/17 by bikecarbfred]

Not sure you can determine that without knowing length of swept vol

can 0.1mm extra recess depth which if you using your calculation is a difference of 5% more volume if you take compression height of the piston also into account.

Is this alot?

0.1mm less depth in a piston make a setup more dangerous if compression is already 11.5:1??

[Edited on 21/9/17 by bikecarbfred]

Sorry yes I'd misinterpreted the dimension

Compression height makes no difference to compression ratio. However increasing it might lead to piston to head contact depending on existing deck height and clearance.

Changing compression height makes no difference to swept volume, it just moves the upper and lower extremes of the top of the piston movement within the bore, therefore the CR doesn't change.

The volume of the piston crown will determine the compression ratio if all else remains the same. So piston A will have the higher CR of the 2.

Do we assume the same stroke (i.e. crank and con rod or vertical travel) in both cases?

Yes : same stroke 86.9mm

You need to start with the volume of the piston bowls This is best done with liquid using a measuring pipette
Add this to the volume above the piston crown at tdc This will be different in each case as the piston heights differ.

To determine the volume above each piston crown at tdc you will need to add three values
1.volume contained within the cylinder head if any (measuring pipette again)
2.volume above the block deck due to head gasket thickness (pi x r x r x h) h is head gasket thickness
3.volume remaining in the bore above the piston crown (pi x r x r x h) h is from the top of piston to the block deck
In each case add this value to the piston bowl volume, the case with the lowest compression volume (given piston at tdc) will give you the highest compression ratio as the swept volume will be constant.

Of course make sure that neither piston will strike the head or the valves

I wouldn't question your measurements but if you don't know the history of your engine or these parts could piston a be a piston b that's been machined to suit a decked block or a skimmed head?

Some of the previous posts are incorrect
Reducing the compressed volume must increase the compression ratio as your pushing the same swept volume into a smaller space


[Edited on 22/9/17 by SCAR]

Thanks.

The reason I am going through this exercise is because the piston for my engine is no longer available to you have to use either one of the two alternative pistons. BCB Golf that are interchangeable but I do not want to upset the compression ratio as there are two types.

KS (BCB) pIston:
Recess depth: 2,7mm
Bowel diameter: 64mm
Compression height: The same


Yenman (BCB) piston:
Recess depth: 2,35mm
Bowel diameter: 64.60mm:
Compression height: The same



I will now try and calculate volume; 3.142 x radius x radius x depth. Can somebody please check my answer.

KS piston:
3.142 x 32 x 32 x 2.7 = 8687

YENMAK piston:
3.142 x 32.3 x 32.3x2.35 = 7703

Conclusion: Yenmak piston gives high compression ratio

[Edited on 24/9/17 by bikecarbfred]

In that case the yenmak piston will give you a slightly higher compression ratio as you will be squashing the same swept volume into a smaller space. This is providing the piston height is the same in both cases as piston height affects compressed volume
You cannot get to the actual CR without knowing the other values mentioned in my previous post.

Right. You said Yenmak would give slightly higher compression ratio.

I know that for my engine setup with the original pistons that are no longer available gave a compression ratio of 11.5:1.
All I know is that those pistons had the same compression height of that of the two alternative pistons available that I have just listed above post.

What is impossible to find on the internet is the bowel depth of the original pistons that are no longer available.

All I was told in regards to the alternative pistons available was that one is for high compression setup and the other is a low compression setup.

http://www.csgnetwork.com/compcalc.html
cr calculator should help

Think I am going to have to go with the one with the biggest bowel depth. Because one or the other gives a CR of 11.5:1 (This is proven) and that is very high CR so best to take the biggest volume.