Calculating Compression Ratio

[Mark]

Here is a guide to calculating the CR of your engine and is useful to those who are fitting low comp pistons or thicker head gaskets.

To start with the formula for working it out is

(Swept Volume (SV)+Compression Volume (CV))/Compression Volume (CV)



Swept volume is the volume the piston moves and is calculated by:

3.142((Cylinder Bore^2)/4)*Cylinder Stroke


Compression Volume is the volume in the cylinder when the piston is at TDC and is calculated by:

Head Volume+Head Gasket Volume+Piston Bowl Volume

Head Volume and Piston Bowl Volume need to be measured as there is no real way to calculate it. Head Gasket Volume is calculated by

3.142((Head Gasket Bore^2)/4)*Head Gasket Thickness

I will now work through an example using a SR20DET fitted with a 88mm Diameter 1.6mm Head gasket :thumbs:

Swept Volume = 3.142((8.6*8.6)/4)*8.6 = 499.62cc

Compression Volume

Head Volume = 47cc
Piston Volume = 11cc
Head Gasket Volume = 3.142(8.8*8.8)/4)*0.16 = 9.73cc

Total Compression Volume = 47+11+9.73 = 67.73



Compression Volume = (499.62+67.73)/67.73 = 8.38:1


Simple :wack:

[Jezz_S13]

I did mine properly with burettes and stuff, it's 7.8:1 if you care.

[Mark]

Yer sorry forgot to add thats how you measure the head and piston volumes :thumbs:

[Jezz_S13]

I know. :D

Tedious it is too, esp matching the chambers.

[nene]

I did mine properly with burettes and stuff, it's 7.8:1 if you care.
Presumably you do this with the head and block seperated?

What steps do you or can you take to ensure they're as close as possible between cylinders?

[SteveCarter200]

Presumably you do this with the head and block seperated?

What steps do you or can you take to ensure they're as close as possible between cylinders?

Remove material from the head chamber.Or add it but thats the difficult way.:)

[bren]

What does ^ mean?

[Kev]

its a symbol to represent that the next number is a higher plane, in this case, the 2, means its squared, if it was a 3 it would be cubed:)

[Jezz_S13]

There are some other, less instrusive ways of calculating the CR, though at the same time are probably a bit less accurate, though how many decimal places do you really need?

Two that spring to mind are...

1. Measured filling of the comb chamber to find the combustion volume and substituting that into the above equation with new figures for either piston dish or gasket thickness.
2. Transposing the above equation to give chamber volume based on the book figure for CR, then substituting that back into the equation with the new piston dish or gasket thickness.

I'll write something in more detail in a bit... :)

[Petrol]

CR & Balancing comments…
A fully balanced engine runs smoother and revs better. There are two things that need to be done to fully balance an engine – weight and compression. I built a Mini screamer engine some time ago and will try to explain….
Weight…
It’s important that the pistons / rods / rings all weigh the same. Every piston and rod will weigh slightly differently. This will cause an imbalance. Shot peened & polished rods can make this imbalance worse so it’s important to weigh everything and balance them up. This will mean removing weight by filing and polishing until everything weighs the same.

Compression balancing.

This is not easy and on a forced induction car I do have my doubts about it. To balance the cylinder’s though you need to measure them. This involves building the bottom end up and measuring the volume of each cylinder. To do this you will need a burette to fill each cylinder to measure the exact volume. This has to be done a few times to take an average. Once you have an average of each cylinder it’s time to compensate in the head to balance the CR’s up. The volume in the cylinder head can be measured in the same way. The best way to measure the head volume is to use a piece of Perspex with some grease on it ….

Am I boring you guys or should I continue?

[TomM]

What does ^ mean?...to the power of...


its a symbol to represent that the next number is a higher plane, in this case, the 2, means its squared, if it was a 3 it would be cubed:)Succinctly put, Kev ;)

[Kev]

thanks tom, for some reason i just couldnt get that phrase in my head at all.
so i figured an explanation would do instead:)

[Jezz_S13]

Am I boring you guys or should I continue?

No not boring feel free to carry on.
Saves me doing it. :)

I have to say I didn't volume balance the bottom end of my engine, I'm not sure how you'd go about correcting it if you found it to be wrong, perhaps grind the crank true or build the journal up with weld and then grind, maybe even reface the block.

I expect it would be prudent to clock all of the journals up to ensure that they are all at exactly the same offset.

[Petrol]

Can't seem to edit my post but to continue..

The best way to measure the head volume is to use a piece of Perspex with some grease on it to create a seal. White spirit is the best fluid to use as it’s very “searching” and will also prove the valve seals.
The CR of each cylinder can then be calculated. Correction can be made by machining out the head chambers. This will result in each head chamber being a different volume but the CR of each cylinder will be exactly the same. You need the patience of a saint to do this, it’s a case of machining tiny amounts out of the head and re measuring until you have the exact volume.

I have driven 2 NA cars that have had this done and it does make a difference. Not sure how effective it is on a turbo though.

[Vez]

If you want to see the difference of said precission balancing, this can be seen of the company Alpina of BMW tuning fame, all of there engines go through this process once recieved from BMW, with quite impressive results, with upto 15% additional power and tourque from more for more or less the same identicle engine.

You can see them doing some of the processes on there website:

http://212.227.48.10/

Click "English" after entering the site, then click "Alpina" then "Philosophy"

[driftking]

Correction can be made by machining out the head chambers.

Of course, not altering the shape of the combustion chambers, if they are shaped.:)

[skyshack]

Compression balancing can only reduce the compression to match that of the lowest cylinder. If it's too low with thinnest gasket then head or block has to be skimmed to bring to back up.

Check pistons all come to same height in bore.
This chap used a depth mic to measure it.
http://www.vwtrendsweb.com/tech/0303vwt_compression/
others use a dial indicator
http://hotrod.com/techarticles/54258/
However you do measure it it has to be repeatable (not by averaging). If they don't match then you need to investigate why. Twisted crank? Mixed set of pistons? Bent rod? Unequal rod lengths due to tolerance on length? If all OK then you can match piston top to deck height to less than the tolerance on rod length (0.1mm) by skimming piston. If rules don't allow some blueprinters will take a large number of rods and select a set of rods that are within 0.01mm on length - very expensive as someone has to pay for all the spare rods they collect. Any set of forged rods bought at the same time should be well matched on length, if not they have sold you a bastard set made up from different batches send them back.


CR & Balancing comments…
A fully balanced engine runs smoother and revs better. There are two things that need to be done to fully balance an engine – weight and compression. I built a Mini screamer engine some time ago and will try to explain….
Weight…
It’s important that the pistons / rods / rings all weigh the same. Every piston and rod will weigh slightly differently. This will cause an imbalance. Shot peened & polished rods can make this imbalance worse so it’s important to weigh everything and balance them up. This will mean removing weight by filing and polishing until everything weighs the same.Separate topic... the rods need end balancing too. The weight of the part of the rods that mainly goes up and down should match. The weight of the big ends and part of the rods that mainly goes round should match.

Changing the mass of the rod and piston by fitting forged items will change the force they create and thus shape that the crank flexes to when run. The 4 balance weights are to control crank flex and not for balance as measured by crank balancing machines. Jun's web site shows how a single crank throw distorts but they don't give the full details for an inline 4. You will have to imagine the next throw bent the other way, 3rd bent as the 2nd and 4th again like 1st. This makes the middle (3) and two end journals (1 and 5) run parallel in the bearings but offset in the direction off the crank throw while 2 and 3 remain skewed as shown.
http://www.junauto.co.jp/products/cylinderblock-part/crankshaft/index.html?en
http://www.junauto.co.jp/products/cylinderblock-part/crankshaft/crank-fig1.gif
Jun claims that "full counter weight" can correct the flex and make the journals run true in the bearings, they are totally utterly completely wrong, it can't, not ever, no way. Counterweights (also called balance or bob) can only correct fully for the rotating crank throw and part of big end that goes round. The reciprocating part of the rod, all the piston with pin and circlips can not be fully balanced, it causes crank flex twice every rev as it hammers up and down. If it is 100% balanced then the same out of balance force is produced horizontally instead of vertically, this would cause the same crank flex as leaving it unbalanced but at mid stroke instead of TDC and BDC, the hammer action would be side to side. The usual balance factor for the reciprocating forces is 50%, so half the force is countered at TDC and BDC, producing the same half force horizontally at mid stroke. What was an up and down force like a jack hammer will become a steady force, 1/2 the size, rotating with the crank. This causes a steady flex of the crank that doesn't vary with every rev but only with engine speed (force = mrw^2, m=mass kg, r=radius meters, w=radians/sec or 2 x pi x rpm / 60) as it's accelerated though the gears. The reduction in frequency of crank flex is vital to avoid fatigue problems, especially at high rpm when size of force is large and frequency is high. If the mass of reciprocating parts is reduced then the balance factor will not be 50% but maybe 55% or 60%, which produces a slightly larger force horizontially than vertically. This causes a slight variation every rev in the force causing flex, the crank will have sightly more deflection at mid stroke and slightly less at TDC/BDC than it would have with 50% balance. Even this slight variation in flex with every revolution can cause an earlier failure due to fatigue, the balance weights have to be reduced to bring the balance factor back to 50%.

Most balancing firms say that 4's don't need anything other than rods and pistons matching. Finding someone who can get it right by weighing all the bits and taking a drill to the couterweights is not too likely in the UK. I doubt Lotus or Ricardo are going to be interested in a one off.

Makers can't decide the best way to split the counterweight between the crank throws. Nissan have used all 3 common variations, FJ had 4 big and 4 small counterweights, big, small, small, big, big, small, small, big, the bigger ones take all the reciprocating loads and shared the rotating loads with the small ones, CA lost the 4 small counterweights and only has the 4 big ones same balance with less total mass (the mass at crank ends is more effective at bending the shaft straight against the reciprocating force causing flex and counters the rotating part too), while SR is 8 same size counterweights (requires more total mass in balance weights to bend shaft straight but gives more compact balance weights and may be better for torisonal vibration - an even more complex topic). Newer Dodge SRT-4 has same arrangement as FJ, I've not seen KA24 or QR25 to see what Nissan do on similar long stroke motors.

A few motorcycles (usually singles, parallel twins and V twins) have balance factors other than 50% to give a different vibration (typically more fore/aft with a reduction of the up/down component) that the frame and cycle parts don't transmit so well the the rider. This requires the crank to be designed to take the high frequency of flex.

Mods maybe an idea to edit this out to a new topic. Not sure if Petrol agrees.

[DAVE610]

This is all well and good for calculating static compression ratio, but when the engine is running, compression is dynamic, and down to cam specification..

ie, a 8:1 engine running a low lift short duration cam will have a higher compression ration than a 9.5:1 running a long duration race cam.

Building an engine, and choosing a compression ratio, without taking into account cam timing is only half the story..

[Chris_H42]

Compresion balancing? Never heard of that one!

Bottom ends need to be true, the deck has to be flat and in line with the main journals, often its not so compresion varies from one end to the other, this is good for drag V8's as the CR is staggered on them. If the deck and mains are parrallel that ensures the piston rises to the same point.

You then need to make sure the bore is centred over the crank, Not offset to the side as this will cause not only excessive drag but kill the bores and piston. I have built a few engines and some which were dry linered when gettign centred the liner was machined into and through, a rover V8 was that far off about 1 a third was missing and on the opposite side there was nearly 2mm of material left!

Then the bore has to be parrallel to the crank (and thus deck), this again ensures minimal drag and long life. As a result these 3 things and assuimg the bores are all bored the same and the pistons are the same then it will all be identical.

Cranks need to be blanced both dynamically and static. Static balance is when you put the cranks main journals on say a knife edge and spin it, it should spin then stop. Not spin then stop rock back a bit then forward before finally stopping.

Dynamic balancing is balancing as its rotating. This ensures the crank itself isn't pumping out funny harmonics and vibration.

Rods and pistons like already said should be polished up then matched, you should also end to end balance them. End to end balancing is when 2 rods are bolted to a common rod via their big ends, the rods should be horizontal, if one is heavier it will drop that side down, this means material should be removed from the heavy rod to bring it back up level. The little end area is the end to concentrate on as its the end dealing with the forces (further out from the crank the forces and harmonics increase), thing is you can't/don't want to weaken the little end.

The figures for compresion ratio measure static CR only, only any use as a ballpark tbh. Dynaimc CR is the actual CR when the engines running and constantly variable. Simple example full throttle 10:1, half throttle 5:1, throw in cam timing, forced induction and so oon it can vary greatly.

[sx_DEVIL]

What would be a simple way to calculate this with an existing built engine with known, bore, stroke and head gasket thickness? Just for a rough idea?

[liquidsmoke]

you forgot to work deck height into it. if the piston protrudes from the block face etc.

[shogun]

Also use a good highly accurate scale to measure the weight difference of the head (with and without filled chambers). With the delta weight you easily find out how much to adjust in each chamber. It's a wet issue