calculating compression ratio

[Ryder]

Ok I'm looking at lowering the compression ratio on the R6 as part of a turbo mod. Have looked around on the internet for calculators for compression ratio, but they all seem to need data I don't have, and I don't have the engine apart to measure it.

I'm wondering if there is another way to do it? Because I know what the compression ratio already is, is there a problem with just working backwards? ie

600cc divided by 4 equals 150cc per cylinder
Bore = 65.5mm
Stroke = 44.5mm
CR = 12.4:1

So if you worked backward,

150 / 12.4 = 12.096cc

^^ the total volume of the combustion chamber


therefore, if you were to increase the volume of the combustion chamber with a thicker headgasket, you could put that back in and work out the new compression ratio. So for a 1mm thicker gasket;

area = pi x R (in cm) squared
= 3.1416 x 3.275 sq
= 10.289 cm2
Vol = 10.289 x extra height of thicker head gasket in cm
= 10.289 x 0.1
= 1.029

So new combustion chamber volume assuming a headgasket 1mm thicker would be 13.125cc
Therefore new compression ratio would be

150cc / 13.125cc = 11.42:1


However thats just my thoughts, and it may be totally wrong - I've never needed to play around with CR on an engine before, so this is a learning curve for me.
This is just what I would assume to be true, unless I've missed something...


Could someone please check over my theory and maths?

[Desmo]

I always end up in trouble when I work backwards.
I always like to take exact figures and start a new equation from there.
Here's a good CR calculator too.
Engine Compression Ratio (CR) Calculator

[g0zer]

there might be a problem with working backwards

the total volume of the combustion chamber = (volume remaining inside the bore at TDC) + (volume of squish inside the head)

the shorter the stroke and the greater the size of the head dome the more your math will be out

ie:

your calc is x = pi*(r^2)*h

the reality is x = ((pi*r^2)*h) + y

you could measure exactly the dome volume.

remove the head and seal up the valves where they seat with a smear of vaseline

use a burette (see picture) to measure accurately the volume of the combustion dome, do this for all cylinders and make sure they are identical.

then make sure all TDC piston positions are identical.

then set compression ratio.

and you have blueprinted your combustion chamber

[Goofy]

What the hell are you planning Graham? and will it be ready for August?

let us know if you want us to scout around for a turbo

[g0zer]

what is the manufacturers specified compression ratio, capacity, bore and stroke?

with all the above information as specified by the manufacture you can back calculate the volume of the squish

[Mockery]

I think you've missed the fact that engine capacity is measured by the swept volume, not including the clearance volume at the top.

see here: http://www.pumaracing.co.uk/CAPACITY.htm

It shows how to get a desired compression ratio; their maths seems okay, but could perhaps be better explained.
Anyway, I think you might be able to leave the clearanceVol undefined and solve for the required increase; will try later when I have some time.

[Ryder]

Thanks for the link Desmo.

there might be a problem with working backwards

the total volume of the combustion chamber = (volume remaining inside the bore at TDC) + (volume of squish inside the head)

the shorter the stroke and the greater the size of the head dome the more your math will be out

ie:

your calc is x = pi*(r^2)*h

the reality is x = ((pi*r^2)*h) + y

you could measure exactly the dome volume.

remove the head and seal up the valves where they seat with a smear of vaseline

use a burette (see picture) to measure accurately the volume of the combustion dome, do this for all cylinders and make sure they are identical.

then make sure all TDC piston positions are identical.

then set compression ratio.

and you have blueprinted your combustion chamber

Yeah you might be right. I was hoping to avoid taking the engine apart untill I worked out what thickness I needed for the gasket - it seems if I want to be accurate I wont have a choice.

Then again, I may not need to be all that accurate - provided it's around the 10:1 ballpark, it may be good enough... I'm aiming to run about 10-12psi, and I've heard of people running turbo's at about 8psi on stock 600cc compression ratios... hmm...

What the hell are you planning Graham? and will it be ready for August?

let us know if you want us to scout around for a turbo

Lol, no unfortunately. However I already have a turbo!
Subaru WRX TD04 Turbo - eBay Other Performance Parts, Performance Parts, Car Parts, Accessories, Cars, Bikes, Boats. (end time 30-Jun-09 12:25:24 AEST)
It will be here on tuesday. I spent today buying stainless steel for the new headers and working out the plans for them - all done, they are cut to size and I'll weld them up at work later next week!!

[g0zer]

I think you've missed the fact that engine capacity is measured by the swept volume, not including the clearance volume at the top.

see here: Puma Race Engines Technical Guide - How to calculate engine capacity and compression ratio

It shows how to get a desired compression ratio; their maths seems okay, but could perhaps be better explained.
Anyway, I think you might be able to leave the clearanceVol undefined and solve for the required increase; will try later when I have some time.

that article says.. its saying exactly what i am saying i think except their drawing shows the combustion dome geometry identical to the bore which is not true in reality

ie:

compression ratio is defined as (Swept Volume + Clearance Volume) / Clearance Volume

you cannot ignore what they call clearance volume.. its part of their equation for compression ratio :p

[Desmo]

Yeah you might be right. I was hoping to avoid taking the engine apart untill I worked out what thickness I needed for the gasket - it seems if I want to be accurate I wont have a choice.

Pull the head off, then you can measure true swept volume, bore diameter (NOT piston diameter) and the combustion chamber volume.
Easiest way to measure the chamber volume is to get a bit of perspex with a hole in it and secure it to the head over the chamber with some grease.
Then use a pipette to measure your volume.
I know it's a root around, but at least you know your figures will be spot on.

[Ryder]

what is the manufacturers specified compression ratio, capacity, bore and stroke?

with all the above information as specified by the manufacture you can back calculate the volume of the squish

CR = 12.4:1
capacity = 599cc
Bore = 65.5mm
Stroke = 45.5mm

Thats what I tried to do in the first post, but as you pointed out it may not be all that accurate

[Ryder]

Easiest way to measure the chamber volume is to get a bit of perspex with a hole in it and secure it to the head over the chamber with some grease.
Then use a pipette to measure your volume.

Cheers mate, thats a good idea

I would have to measure the volume of the cutouts in the piston top as well and the depth the piston is below/above the top of the block, and the thickness of the stock head gasket too. should be fun

[g0zer]

hmm

based on those numbers.. the swept volume is greater than 599cc

which backcalculates to give a negative squish volume

[Desmo]

Domed piston?

[g0zer]

nah just motor explosion

[GD Engineering (Gavin)]

A couple of things to consider........ manufacturers quoted comp ratios are notoriously misleading. If manufacturing/casting/machining/assembly tolerances were perfectly tight, ie. no deviation, there would NEVER be a need to pursue ANY avenue of blueprinting. Ever ridden 2 identical bikes with similar km's etc and wondered why one motor seems so much stronger than the other? A part of the explanation will include the dissimilarity in, and the number and range of tolerance variation between the 2 engines in a number of critical areas INCLUDING comp ratio. In the late 90's and early 00's, we put a good number of R1 motors in speedway sidecars...... we measured the comp ratios before pulling each motor down and typically found a +/-2 compression points from highest to lowest against the manufacturers spec. This is not a Yam only thing, all motors will give you a similar variation, older motors more again as manufacturing processes were equally less finite. Measure YOUR motor empirically or your result will be totally inaccurate. Use the formulae listed and the inherent error found in certain steps can either : a) cumulatively send you on a wild goose chase or b) inadvertently get you close to the mark by the sum of over-stating some and under-stating other measurements because of this inherent error.
Another point .... comp ratio is very easily calculated using this:
BORE DIAMETER SQUARED(in cm) x STROKE (cm) x .7854(constant)/ UNSWEPT VOLUME.
This will be accurate for ONE cylinder only. Why? Again, variations in the volume of unswept space between cylinders of the same motor and the reason why you must know what that volume is before calculating an individual comp ratio. Once this is known, a simple calculation will yield the change in volume needed for the necessary comp ratio change. This will usually be expressed as a single ordinance of vertical displacement UNLESS piston compression height and/or crown footprint is changed in which case there are other things to consider.
Another thing to ponder is that the piston crown, with respect to shape and spatial abberation , has NOTHING to do with the CR if the same piston is to be re-used after the CR adjustment . The same applies to the head IF the face isn't machined or the chamber volume changed....... make sense.
Therefore, all you need to know is the unswept volume to calculate the CR changes with respect to any deck height changes. Careful location of TDC will yield this measurement with a bit of well thought improvisation. As a point of interest, modern 4 valve multis will work well with stock or modestly lowered comp and boost pressure ratios of .35-.6 depending on a number of factors that I wont go into. The beauty of this is that relatively elevated comp ratio enhances off-boost throttle response and transient throttle spool-up, someting to consider with a mid-displacement bike motor. Any idea what particular turbo you will use?......... another important choice. One last thing, try to avoid really thick headgaskets to lower the CR, they don't tend to last long due to yield complications. There are a plethora of other considerations to make before you start...... do a bit of research before you roll the dice mate

[Desmo]

/thread.

Awesome advice mate.

[Mockery]

that article says.. its saying exactly what i am saying i think except their drawing shows the combustion dome geometry identical to the bore which is not true in reality

ie: you cannot ignore what they call clearance volume.. its part of their equation for compression ratio :p

Yeah, you're right sorry... I read it a bit fast.

Anyway, if you take the stated bore, stroke and CR as gospel (which you shouldn't) then you get a squish/clearance of 13.45cc.

cV = sweptVol/(CR-1)

If you wanted to reduce the compression to, say, 10.4, that's a clearance volume of 16.31cc.

Take the difference between those, then divide by the area, would give a theoretical ~0.85mm of head lift needed.

***BUT as stated so excellently by Gavin, the CR numbers are not to be trusted. Measure your unswept volume empirically. I just felt like doing maths to keep my hand in...

[Ryder]

Thanks for that,

Using that formula gives me the same calculated unswept volume as I worked out in post #1 (12.09 cm3). Which is good news, because it sounds right - I found out the combustion chamber volume in an online manual. It lists cyclinder head volume as 10.3-10.9 cm3, which of course doesn't take into account the volume added from the thickness of the original head gasket, or the height the piston is below or above the block. So when you consider that, it even sounds right.
Though, like you say, that is only calculated.

Another thing to ponder is that the piston crown, with respect to shape and spatial abberation , has NOTHING to do with the CR if the same piston is to be re-used after the CR adjustment .

Fair enough. But I'd still have to account it anyway for an accurate compression ratio calculation of the stock motor.

Just a question, what would you consider to be too thick a headgasket? The plan at the moment is to machine it out of pure copper, I can use the mills and lathes at work for this.

Lastly, this project probably doesn't need to be built to theoretically ideal specifications. As you said, you can get away with a relatively high CR on turbo bikes, and I have heard of people running stock 600's on 8-9 psi. I would like to run about 10-12 as a goal (Depending of course on how she goes during tuning), and to do this I think it best if the CR is lowered, even if it is only slightly.

My thoughts on how much to lower it at the moment are, provided that it is in the ballpark of 10-11:1, it would be fine. The reasons for that are that my goals are fairly flexable, my budget is fairly low, and provided I can run around 10-12psi safely I will be happy.
It seems pointless to me to build exactly to a calculated ideal, because by the time I get there I'd probably find out the calculated ideal isn't actually ideal at all.

What are your thoughts on how thick a headgasket is too thick, and what kind of CR (ballpark) will be suitable for the ammount of compressed air I want to put in?

[Ryder]

Yeah, you're right sorry... I read it a bit fast.

Anyway, if you take the stated bore, stroke and CR as gospel (which you shouldn't) then you get a squish/clearance of 13.45cc.

cV = sweptVol/(CR-1)

If you wanted to reduce the compression to, say, 10.4, that's a clearance volume of 16.31cc.

Take the difference between those, then divide by the area, would give a theoretical ~0.85mm of head lift needed.

***BUT as stated so excellently by Gavin, the CR numbers are not to be trusted. Measure your unswept volume empirically. I just felt like doing maths to keep my hand in...

Thanks mockery, but that formula doesn't seem quite right to me? I don't have a lot of time ATM, will look again when I get home.

Cheers guys

[Mockery]

our numbers are a bit different because you took 600/4 = 150cc per cylinder;
whereas if you use the bore & stroke to calculate the swept volume it says ~153.3cc per cylinder.
Gozer alluded to that already, a few posts up.
I checked the formula again, it works out okay. It's just rearranged from: CR = (cV+sV)/cV