Author Topic: Algebriac Formula for Calculating Compression  (Read 601 times)

Offline ghwallice

  • Hero Member
  • *****
  • Posts: 505
Algebriac Formula for Calculating Compression
« on: March 16, 2019, 08:57 »
If I use a rod on my Zanes that is 0.5mm shorter, or a head gasket that is 0.4mm thicker or a piston with different compression height (piston shapes aside) , is there an algebraic formula I can use to determine compression ratio?

Regards, Greg
Kubota B7300
'54 Royal Enfield Meteor 700 (basket)
Some Laverda's and Guzzi's and Ducati's, my turbo KTM  & her KTM
Glass door beer fridge

Online Vince

  • Hero Member
  • *****
  • Posts: 7743
Re: Algebriac Formula for Calculating Compression
« Reply #1 on: March 16, 2019, 09:26 »
Actual compression ratio is the ratio between the swept volume, bore and stroke, compared to the combustion chamber volume. The later is found by adding fluid via the spark plug hole and watch it as it comes out of a measured beaker, or burett.I learned this after thinking wrongly that all it took was finding out what pistons are used. 10 as to 1 piston = 10 x atmospheric pressure. Not the right way, same as compression testing with a gauge, valve timing effects this. Seems proper engine building is more that just assembling parts in the correct order.

Offline Piranha Brother 2

  • Hero Member
  • *****
  • Posts: 1629
  • It was once a 1973 750 SF1
Re: Algebriac Formula for Calculating Compression
« Reply #2 on: March 16, 2019, 09:57 »
Big mistake many make is failing to add the buretted cc volume to the swept volume before doing the division. Gives quite a different reading.
I would have thought adding or subtracting the volume calc for + or - 2mm to swept volume would be pretty straightforward. Once you have an accurate figure for your  cc (combustion chamber).
What ... leave it standard??!!

Offline Dellortoman

  • Hero Member
  • *****
  • Posts: 11991
  • My wolf mate 6/5/1996 - 3/1/2011
Re: Algebriac Formula for Calculating Compression
« Reply #3 on: March 16, 2019, 10:08 »
is there an algebraic formula I can use to determine compression ratio?

Aha, some engineering calculations. This is my forte  :D

The answer is yes. But only if you know what the CR was before you put the thicker gasket or shorter rod in. Otherwise you need to go back to first principles and measure the combustion chamber volume as Vince described.

To make a formula, we need to set a few variables, so let's say:

CR1 = the original compression ratio
CR2 = the compression ratio after you've added the gasket (or shortened the rod)
D = cylinder displacement in cc  (ie. 1/3 of engine displacement if it's a triple cylinder engine, half if it's a twin)
π = Pi = 3.14159
b = cylinder bore in mm
d = the distance in mm by which you increase the cylinder head-piston clearance (the 0.5 or 0.4mm you're talking about).
      Note, it doesn't matter whether the extra combustion chamber space is achieved by shortening the rod or using a thicker head or base gasket.
      The piston stroke and engine displacement remain the same.

Then the formula will be (I hope this comes out looking ok with denominators in the right place)


CR2 =                      1                 
                 1       +    π x b2 x d
                 CR1          4000 x D

I can go through the algebraic steps I used to derive the above formula if you're interested.


PS. I just added a little Excel sheet to do the calculation for you.
« Last Edit: March 16, 2019, 10:23 by Dellortoman »
Location: Tasmania, Approx 4253S 14723E

Offline Tippie

  • Hero Member
  • *****
  • Posts: 2118
Re: Algebriac Formula for Calculating Compression
« Reply #4 on: March 16, 2019, 10:48 »
"Big mistake many make is failing to add the buretted cc volume to the swept volume before doing the division."
Common mistake too.
SF2 17483 (race)
SF2 17424 (road)
An Australian living near Oslo in Norway

Offline sfcpiet

  • Administrator
  • Hero Member
  • *****
  • Posts: 7813
  • NRW, Germany
Re: Algebriac Formula for Calculating Compression
« Reply #5 on: March 16, 2019, 12:00 »
Compression ratios can vary wildy between engines, even if identical pistons are fitted.  The stated CR for pistons is only a nominal size.  Actual CR can only be determined by measuring combustion chamber volume. 

Different gasket thicknesses, rod lengths and piston heights are simple volume calculations that can be added/subtracted from known quantities.

Chamber volume can be drastically influenced by a proper or not-so-proper valve job.  How often has the head/barrel been skimmed?  These are all unknowns that can throw an equation out the window.  If actual measuring is not done, everything else is only guessing...

piet
180s feel quick, 120s are...      If it ain't broke, fix it till it is.

"A motorcycle is a bicycle with a pandemonium attachment and is designed for the especial use of mechanical geniuses, daredevils and lunatics"  Georg Fitch 1916

Offline ghwallice

  • Hero Member
  • *****
  • Posts: 505
Re: Algebriac Formula for Calculating Compression
« Reply #6 on: March 16, 2019, 21:55 »
Aha, some engineering calculations. This is my forte  :D


Thanks for the formula Cam! I'll give it a go. OEM Compression was stated at 10.5:1. b is the bore diameter? not 1/2 the bore diameter? (r squared)

Then if I get to it, as the heads are off, I might do the swept volume of the head with oil. One would think with the head off, that I would have to calculate the volume of the thickness of the gasket (say 1.6mm*pi*r2) and the volume of the shorter rod (.5mm*pi*r2), and add to the volume of the oil in the head to get a final swept volume. (then of course, adjust for dish or dome of the piston if required)
Kubota B7300
'54 Royal Enfield Meteor 700 (basket)
Some Laverda's and Guzzi's and Ducati's, my turbo KTM  & her KTM
Glass door beer fridge

Offline Dellortoman

  • Hero Member
  • *****
  • Posts: 11991
  • My wolf mate 6/5/1996 - 3/1/2011
Re: Algebriac Formula for Calculating Compression
« Reply #7 on: March 16, 2019, 23:52 »
Bore is diameter, not radius.

It's not the swept volume that you measure with oil. It's the combustion chamber volume. Swept volume is determined by bore & stroke.

You can't get an accurate measurement of combustion chamber volume with the head off because the piston crown projects into the space. You need to do it with the engine assembled and the crank at TDC on the compression stroke, also with the engine leaning over so the spark plug hole is uppermost so as to avoid air pockets. Then fill the cylinder with oil from a burette. Don't take all day to do it because the oil can seep away via the ring gaps if you leave it long enough. Ideally you should only fill to the bottom of the spark plug thread. But it's hard to see exactly when the oil level hits the bottom of the plug hole, so it's easier to fill to the top of the plug thread and subtract the volume of the spark plug hole (2.62 cc)
Location: Tasmania, Approx 4253S 14723E

Offline Tippie

  • Hero Member
  • *****
  • Posts: 2118
Re: Algebriac Formula for Calculating Compression
« Reply #8 on: March 17, 2019, 07:42 »
I was taught to fill to half way up spark plug hole, but the different volume in there is pretty neglible. I use 50/50 mix of engine oil and kerosene, after 5 minutes or so it hadnt seeped down at all. As I dont have a burette I used a measuring cylinder, started at a given round figure volume and subtracted what was missing after filling the comb chamber.
SF2 17483 (race)
SF2 17424 (road)
An Australian living near Oslo in Norway

Offline ghwallice

  • Hero Member
  • *****
  • Posts: 505
Re: Algebriac Formula for Calculating Compression
« Reply #9 on: March 17, 2019, 09:43 »
The underlying assumption here is that the motors are still together. These are Zanes I'm working with. 8 motors, not all mine, 8 broken cranks, 8 engines stripped.

But, on a positive note, after success, then failure, I am on the road to success again!

Don't try, cant win.
Kubota B7300
'54 Royal Enfield Meteor 700 (basket)
Some Laverda's and Guzzi's and Ducati's, my turbo KTM  & her KTM
Glass door beer fridge

Offline Davo

  • Hero Member
  • *****
  • Posts: 8424
  • West of the Rabbit Proof Fence
Re: Algebriac Formula for Calculating Compression
« Reply #10 on: March 17, 2019, 11:18 »
Actual compression ratio is the ratio between the swept volume, bore and stroke, compared to the combustion chamber volume.

Been thinking about this. Shouldn't the formula be combustion chamber volume/total volume (being swept volume + combustion chamber volume) as the gases within the combustion chamber are compressed to the same degree. Probably only a tiny difference I know, but...

Waiting for Cam to pile on and show me why I am wrong.
1976 3CL Redaxed
Triumph Sprint
Moto Guzzi Le Mans III 850 Stucchi
Honda V Twin Fastrak hustler

Offline AndyW

  • Hero Member
  • *****
  • Posts: 3108
  • Adelaide
Re: Algebriac Formula for Calculating Compression
« Reply #11 on: March 17, 2019, 11:48 »
Yep

(Swept +  CC volume) / CC volume
'71 TR6 Tiger, '73 X-75, '73 3C, '74 3C, '75 SF2, '75 850T3 Cal, '75 T160V, '78 Mirage, 78 SSD Darmah '80 SD Darmah, '82 TR7/T, '83 Tenere, '85 R80g/s, 2017 MV F3 (Ago edition)

Online Vince

  • Hero Member
  • *****
  • Posts: 7743
Re: Algebriac Formula for Calculating Compression
« Reply #12 on: March 17, 2019, 12:14 »
Yep, it was pointed out a couple of times, seems its a common error. Its also important to try to equalise the combustion shape and volume between cylinders. Blueprinting is far from simple, I used to read the articles in Performance bike by John Robinson. He interviewed a bloke whose name I forget who built engines for Honda Britten, Tony Something. The detail he went to was jaw-dropping, at an hourly rate it must cost a huge bomb
« Last Edit: March 17, 2019, 12:20 by Vince »

Offline Tippie

  • Hero Member
  • *****
  • Posts: 2118
Re: Algebriac Formula for Calculating Compression
« Reply #13 on: March 17, 2019, 15:22 »
The underlying assumption here is that the motors are still together. These are Zanes I'm working with. 8 motors, not all mine, 8 broken cranks, 8 engines stripped.

But, on a positive note, after success, then failure, I am on the road to success again!

Don't try, cant win.
For checking this you dont even need to fit the cams, as long as the head is tensioned down. Pistons and rings, fit barrel and head, then measure. Do it when you get the bottom ends together. It is most definitely a valuable part of reassembly if you want the best from a motor; cr, squish (and valve/piston clearance), and dialling in the cams are well worth it. CCing the comb chamber is far the easiest to check among this lot, and maybe not too hard to adjust (depending on those valve and squish clearances).
SF2 17483 (race)
SF2 17424 (road)
An Australian living near Oslo in Norway

Offline maurice turner

  • Hero Member
  • *****
  • Posts: 2927
Re: Algebriac Formula for Calculating Compression
« Reply #14 on: March 17, 2019, 21:42 »
I like your never give up attitude Greg !!
81 Jota*7604*