Calculating the compression ratio of an ABY

[gnutz2]

My plan is to use the standard pistons with a reduced CR and keep the squish so i save money not buying new pistons and in the long run i save on fuel consumption.

I got my 50cc burette through the post the other day and i cut up some old polycarbonate i used for the windows and got too it.

I worked out all of my combustion chambers to be 44.7cc`s (+/-0.1cc).

The steel head gasget is 8.18cc`s.

The piston dish works out to 3.75cc`s with a minor innacuracy because the sqush area on standard KS pistons actuall protrudes the block deck height and cannot be calculated using a burette. If the pistons were deck height i could have used a burette.

And cylinder volume 445cc`s

Add these all together and divide them by the top three and it works out to,

8.86:1 commpression ratio

This is much lower than i belived it would be and i have a suspicion tht the head has been skimmed so the figure would have been lower. I calculated that if the dish was taken out to 70mm instead of 60 and it was taken from 1.2mm deep to 1.7mm only loosing .5mm on the crown thickness. This will bring the piston dish to 6.78cc`s and a new CR of 8.46:1.

So i`ve took the pistons to a local engineering firm and hes going to machine out the pocket with a radius`d cutter on a cnc mill, all for £35 so well within my budget, i`ll post pics when i get them

[S2central.net]

That does sound a tad low alright... How did you arrive at 8cc for the head gasket - does that account for any compression when its squeezed into place. Using a figure of 5cc there results in CR = 9.3 which is more like it.

Have a search on this subject - I think Marco and MarkH have some history on this.

Paul

[4WheelDrifter]

Using feeler gauges i worked out the the steel head gasket was 1.55mm thick

8cc sounds about right from your measurements.

I'm not sure how you measured the dish and squish area on the top of the piston. If part of it protrudes above the block deck, you need to subtract that from the chamber volume. Obviously you need to add any volume below the deck.

[Jamo]

The people to ask about this are Pisobiker and Jeffer.

Jeffer did the same and posted his results in the 3B rebuild thread he has on the forum/

[twoqu]

It is possible to measure the piston cc but tricky and messy! Bit late now if you are having them machined. You would have to grease the piston/rings enough to prevent any leakage, place down the bore a specified amount and then cc it. The observed amount of fluid needed to fill the cylinder is then subtracted from the area of the perfect cylinder bore diameter and height(distance from piston to top of bore). If that makes sense!
Its certaintly useful to have actual measurements. From what I have seen @45cc is common for the combustion chambers.

[gnutz2]

That does sound a tad low alright... How did you arrive at 8cc for the head gasket - does that account for any compression when its squeezed into place. Using a figure of 5cc there results in CR = 9.3 which is more like it.

Have a search on this subject - I think Marco and MarkH have some history on this.

Paul

The calculations i use to work out cc`s is (pie x diameter squared x depth) divide by 4000 this seems quite standard in most books.

If the head gasget was 5cc`s this would calculate to a head gasget of under 1mm thick. A 1.5mm gasget is 8`cc`s

The only very negligeble innacuracy in my calculations is the piston dish but in the worst case scenario it could only bump the CR up to about 8.89:1 or there abouts.

[gnutz2]

I used the same gasget thet was used for honing the block and torque`d it down on the block to gap the rings, at the same time i hsed feeler guages to measure the distance between the block and torque plate which was 1.55mm



if you look at the metal elring gasget it breaks down into 4 layers.



1st layer 0.2mm
2nd " 1.0mm
3rd " 0.1mm
4th " 0,2mm

So at the very minimum the gasget is 1.5mm because the gasget mechanically stops the head from getting any closer

[Jamo]

Side issue dude you have an I5 torque plate?

[gnutz2]

Yes got one made?

[LHD_Kid]

what 'fluid' did you use when measuring the volume of the cylinders?

[gnutz2]

Methalated spirit, but you have to be quick cos it evaporstes like fook.

[4WheelDrifter]

It is possible to measure the piston cc but tricky and messy! Bit late now if you are having them machined. You would have to grease the piston/rings enough to prevent any leakage, place down the bore a specified amount and then cc it. The observed amount of fluid needed to fill the cylinder is then subtracted from the area of the perfect cylinder bore diameter and height(distance from piston to top of bore). If that makes sense!
Its certaintly useful to have actual measurements. From what I have seen @45cc is common for the combustion chambers.

As twoqu says. Drop the piston down by a set amount from TDC, then measure the volume as you did the head chamber. Then subtract the cylinder volume you have displaced, for example a 10mm drop from TDC on a 81mm bore would be 51.53cc.

HTH

[gnutz2]

It` did cross my mind to do it like this but i could`nt keep the piston straight, but like he says if you install the rings first this will keep it straight.

I used maths to calculate the dish because it`s such a simple design. The 60mm x 1.2mm dish is an easy -3.5cc`s then there`s the valve cutouts which i put a few drops out of the burette in them and worked out about -0.4cc`s and the squish in front of these which is only 0.5mm above deck, so alltogather it`s going to be about -3.5 to -4cc`s.

When looking through past posts in general people agree thet the steel head gasgets are around 1.6mm compressed so me 1.55 seems in the ball park.

Using these figures still leaves the ABY in the 8.8:1 CR bracket.

[4WheelDrifter]

IIRC it's been said that the ABY/ADU CR with the metal HG is ~9.0:1, and the figure of 9.3:1 comes from the original paper HG. I can't see that they can both have the same CR anyway.

[gnutz2]

Got my pistons back with a 70x1.7 dish in them using the same centre as the original dish and reducing the crown thickness by 0.5mm, this increases tha total nedative volume to -6.75cc`s and brings the CR down to 8.45:1 with a 1.55mm head gasget and keeps most of the squish as standard.



I just need to deburr the outside of the piston where the dish got close to the edge and eliminate any possible hot spots.