working out CR from head alone?
Discussion
Hiya.. i got the MG cylinder head. i can see the head has double springs, maybe skimmed, smooothened chamber and enlarged valves in/out "tubes", bigger chambner
i know my car has 9.4.1 CR as standard. But i want to know what compression ratio of this new head.
How do you work it out off the engine.
Merry xmas
jay
To get an accurate figure you have to take the head off first, as the c.r is dependant on the piston dish volume and the distance the pistons sit down the bores at top dead centre as well as the combustion chamber volume.
However, you can make a rough approximation by measuring the combustion chamber volume. To do this, go and buy a 5cc shringe from a chemists, put a plug into one of the chambers, set the head up level using a spirit level, and fill one chamber with water using the calibrated shringe. You'll find it takes about 20+cc.
Work out the swept volume of one cylinder, assuming you know that overbore size. For example, at +0.040", the swept vol of one cylinder will be 1310/4 = 327.5cc.
Allow 8cc for piston dish and, this is a guess, about 5cc for gasket volume plus piston to deck level volume.
The calculation is then simple:
Add the swept volume to the total unswept volume to give the TOTAL VOLUME.
Now divide the TOTAL VOLUME by the TOTAL UNSWEPT VOLUME. This is the compression ratio.
Remember, however, that without accurate measurements this is really just a 'best guess'. The good thing is that with an 'A-Series' engine the actual numerical comp ratio is not that critical and for good engine breathing, gas velocities, burn levels, etc, anything from about 10.4 up to 11.0 gives good performance.
One of the most difficult things to measure is how much to skim off a head when the ideal combustion chamber volume has been calculated and the gas-flowing has been done. The meniscus of the measured fluid used to establish the skim depth required prevents very accurate measurement and, whatever anyone may say, it's usually an estimate based on experience and an 'eye' for measurements.
I hope this helps and is not too muddled. Contact me if you need any more info.
Peter
However, you can make a rough approximation by measuring the combustion chamber volume. To do this, go and buy a 5cc shringe from a chemists, put a plug into one of the chambers, set the head up level using a spirit level, and fill one chamber with water using the calibrated shringe. You'll find it takes about 20+cc.
Work out the swept volume of one cylinder, assuming you know that overbore size. For example, at +0.040", the swept vol of one cylinder will be 1310/4 = 327.5cc.
Allow 8cc for piston dish and, this is a guess, about 5cc for gasket volume plus piston to deck level volume.
The calculation is then simple:
Add the swept volume to the total unswept volume to give the TOTAL VOLUME.
Now divide the TOTAL VOLUME by the TOTAL UNSWEPT VOLUME. This is the compression ratio.
Remember, however, that without accurate measurements this is really just a 'best guess'. The good thing is that with an 'A-Series' engine the actual numerical comp ratio is not that critical and for good engine breathing, gas velocities, burn levels, etc, anything from about 10.4 up to 11.0 gives good performance.
One of the most difficult things to measure is how much to skim off a head when the ideal combustion chamber volume has been calculated and the gas-flowing has been done. The meniscus of the measured fluid used to establish the skim depth required prevents very accurate measurement and, whatever anyone may say, it's usually an estimate based on experience and an 'eye' for measurements.
I hope this helps and is not too muddled. Contact me if you need any more info.
Peter
If you can find out the volume of the chamber in the standard head you could work backwards from the known CR to find out the standard value for what Peter has labelled TOTAL UNSWEPT VOLUME. You could then take away the standard chamber volume, add on the volume for the new head and calculate CR using that.
Vizard's A-series book might include figures for volume of the chamber in standard heads.
Vizard's A-series book might include figures for volume of the chamber in standard heads.
Andrew Noakes said:
If you can find out the volume of the chamber in the standard head you could work backwards from the known CR to find out the standard value for what Peter has labelled TOTAL UNSWEPT VOLUME. You could then take away the standard chamber volume, add on the volume for the new head and calculate CR using that.
Vizard's A-series book might include figures for volume of the chamber in standard heads.
Andrew, the problem is that the so-called standard heads often seem to have different combustion chamber volumes. What you say is absolutely correct in theory, but BMC/Rover's production tolerances, plus the wide variety of vehicles all using the same basic head castings with different c.r.'s, valve sizes, etc, make it more difficult in practice. For the time to take a head off and measure properly I think that's the best way to do it every time.
Absolutely. Just trying to get a closer approximation without going to the trouble of getting the head off.
You'd also have the problem of establishing exactly which 'standard' head you were looking at - there were zillions of different ones, and there's no guarantee the one on the engine is the 'correct' head for that car.
You'd also have the problem of establishing exactly which 'standard' head you were looking at - there were zillions of different ones, and there's no guarantee the one on the engine is the 'correct' head for that car.
Yes, I always call the standard 12G940 a 21cc chamber, although it can vary a bit.
My view is to always take the head off and measure it properly, that you you KNOW what it is rather than guessing.
I recently had a look at an engine someone was proposing to put in a works replica Mk. 1 Cooper 'S'.
It has Hepolite 21251 pistons, the low comp ones, which were sitting 0.8mm down from the block deck at TDC. The head was supposedly gas flowed and had chambers of unknown volume. A rough guess put the C.R. at about 8.5:1, not the 10.5 he wanted. I have recommended him to take out the rods and pistons, change to 21253 pistons, take off the piston rings and do a trial build then have the block machined to bring the pistons flush with the deck. After that we'll consider the ideal chamber volume and grind out or mill off material to get the correct volume. With the cam properly timed in he will gain, I reckon, about 8 bhp over what he would have had.
This engine had, supposedly, been fully rebuilt by an 'expert' (does 'expert' mean 'a former drip?).
Sometimes I want to weep for the less experienced Mini owner who trusts his pride & joy to these 'experts' and just gets a big bill and a poor quality job.
Maybe you get what you pay for, so never ask a real expert "how cheaply can you build me an engine?". The answer to that should be "I can't".
My view is to always take the head off and measure it properly, that you you KNOW what it is rather than guessing.
I recently had a look at an engine someone was proposing to put in a works replica Mk. 1 Cooper 'S'.
It has Hepolite 21251 pistons, the low comp ones, which were sitting 0.8mm down from the block deck at TDC. The head was supposedly gas flowed and had chambers of unknown volume. A rough guess put the C.R. at about 8.5:1, not the 10.5 he wanted. I have recommended him to take out the rods and pistons, change to 21253 pistons, take off the piston rings and do a trial build then have the block machined to bring the pistons flush with the deck. After that we'll consider the ideal chamber volume and grind out or mill off material to get the correct volume. With the cam properly timed in he will gain, I reckon, about 8 bhp over what he would have had.
This engine had, supposedly, been fully rebuilt by an 'expert' (does 'expert' mean 'a former drip?).
Sometimes I want to weep for the less experienced Mini owner who trusts his pride & joy to these 'experts' and just gets a big bill and a poor quality job.
Maybe you get what you pay for, so never ask a real expert "how cheaply can you build me an engine?". The answer to that should be "I can't".
cooperman said:
With the cam properly timed in he will gain, I reckon, about 8 bhp over what he would have had.
Easily 8bhp I'd have thought. The A series responds very well to higher compression IME, especialy with a good cam. Helps fuel economy as well.
cooperman said:
This engine had, supposedly, been fully rebuilt by an 'expert' (does 'expert' mean 'a former drip?).
My uncle used to say "ex being a 'has been', spurt being a drip under pressure"
Gassing Station | Classic Minis | Top of Page | What's New | My Stuff