Copression ratio calculations
Discussion
Evening all.
Just doing some CR calculations and I need a few figures.
How thick can I expect the head gasket to be when it is clamped down?
Anyone know the volume of the dish in a standard MG Metro piston?
What would I be looking at for the ring-land volume? Or put another way, what is the difference in diameters between piston and bore?
The consensus view will dominate on this, so I'm hoping for an answer from each of you
.
Thanks in advance,
Charlie
Just doing some CR calculations and I need a few figures.
How thick can I expect the head gasket to be when it is clamped down?
Anyone know the volume of the dish in a standard MG Metro piston?
What would I be looking at for the ring-land volume? Or put another way, what is the difference in diameters between piston and bore?
The consensus view will dominate on this, so I'm hoping for an answer from each of you
.Thanks in advance,
Charlie
From memory 3.5cc to 4cc is about right for head gasket, ring land is about 0.5cc, pistons are normally about 6cc on high comp and 8cc on low comp, and the "lost volume" above the piston is about another 4 or 5cc's unless your block has been "decked".
The best way is to measure it of course, I ended up visiting a local vets and buying a 60cc horse syringe, which was a bit of a bizare purchase and only graduated in single cc's, but easier to get hold of and a lot less fragile than a burette !!
The best way is to measure it of course, I ended up visiting a local vets and buying a 60cc horse syringe, which was a bit of a bizare purchase and only graduated in single cc's, but easier to get hold of and a lot less fragile than a burette !!
From memory the MG Metro Piston crown height was increased to bring up the compression ratio.
Best suggestion as stated is to measure everything and contact the gasket manufacturer (e.g. Payen ) for compressed volumes. Again from memory the popular AF460 Payen H/gasket is 2.8cc compressed.
Best suggestion as stated is to measure everything and contact the gasket manufacturer (e.g. Payen ) for compressed volumes. Again from memory the popular AF460 Payen H/gasket is 2.8cc compressed.
I always allow 3 cc for head gasket volume with the BK450, which now seems to be the gasket of choice. The older copper-faced gaskets were slightly more at 4 cc. As for ring land volume it's 0.7 cc on a 1275 motor.
You have to measure the actual piston dish volume, as it varies so much for different piston part numbers. The Hepolite 21253 which is the most popular 1275 replacement piston, is 8.4 cc. A damn good piston it is too for most high performance work, although for use at up to 7000+ I think the Karl Schmidt are the best, but very expensive (if you can find any now).
Don't forget to add in the volume between piston crown and block deck. As a guide, on a standard 1275 unit, for each 0.010" the piston sits down the block at TDC, you will have an extra 1.0 cc combustion volume.
Of course, that all becomes a bit academic when you put the correct amount of liquid in the combustion chamber and then try to measure how far down from the head face the liquid surface is. No matter what liquid you use, you still have to 'guess' a bit as the meniscus 'climbs' up to the vernier probe.
A tip learned from bitter experience:
If you build up the head and, after gas flowing the chambers, you check each chamber volume and find a discrepancy, before opening out the smaller chambers a bit to equalise the volumes, take out the new valves and temporarily fit old ones in case the die grinder hits the valves. I know it takes a bit longer, but it's better than hitting a new valve with the grinder and having to replace it and re-lap the seat.
You have to measure the actual piston dish volume, as it varies so much for different piston part numbers. The Hepolite 21253 which is the most popular 1275 replacement piston, is 8.4 cc. A damn good piston it is too for most high performance work, although for use at up to 7000+ I think the Karl Schmidt are the best, but very expensive (if you can find any now).
Don't forget to add in the volume between piston crown and block deck. As a guide, on a standard 1275 unit, for each 0.010" the piston sits down the block at TDC, you will have an extra 1.0 cc combustion volume.
Of course, that all becomes a bit academic when you put the correct amount of liquid in the combustion chamber and then try to measure how far down from the head face the liquid surface is. No matter what liquid you use, you still have to 'guess' a bit as the meniscus 'climbs' up to the vernier probe.
A tip learned from bitter experience:
If you build up the head and, after gas flowing the chambers, you check each chamber volume and find a discrepancy, before opening out the smaller chambers a bit to equalise the volumes, take out the new valves and temporarily fit old ones in case the die grinder hits the valves. I know it takes a bit longer, but it's better than hitting a new valve with the grinder and having to replace it and re-lap the seat.
The best way is to measure it of course, I ended up visiting a local vets and buying a 60cc horse syringe, which was a bit of a bizare purchase and only graduated in single cc's, but easier to get hold of and a lot less fragile than a burette !!
[/quote]
Boots sell 10cc syringes for a couple of quid which are easy to read and work well. By the way, they don't like petrol which does the seal very quickly. I use water usually as I always end up spilling it onto the bench!
Thanks everyone.
I live on a pig farm so syringes are pretty easy to come by.
I'll measure the deck volumes at the weekend and give you all marks out of 10 for accuracy. Should take account of land, deck and dish volumes - I'll just have to check it doesn't leak down too quickly.
Toodle-oo.
I live on a pig farm so syringes are pretty easy to come by.
I'll measure the deck volumes at the weekend and give you all marks out of 10 for accuracy. Should take account of land, deck and dish volumes - I'll just have to check it doesn't leak down too quickly.
Toodle-oo.
Plug in place.
Always handy to have an old one or two around for things just like this, or so you can cut down the length of the threads with a hacksaw to create a handy tool for cleaning out paint or other such gumph.
Another tip I have recently discovered the hard way. Don't use water. I did and a bit made its way down one of my iron valve guides. Stuck fast. Haven't tried the copper mallet yet though.
Always handy to have an old one or two around for things just like this, or so you can cut down the length of the threads with a hacksaw to create a handy tool for cleaning out paint or other such gumph.
Another tip I have recently discovered the hard way. Don't use water. I did and a bit made its way down one of my iron valve guides. Stuck fast. Haven't tried the copper mallet yet though.
Cooperman, re. measuring the chamber volumes I take it you mean fitting the valves/springs, putting the head on its side, filling the chamber with liquid to see if the chambers are the same volume?
How does this work for the central exhaust? (inlets being shared getting both the same volume I can get, and I can see how you would be after equal outer exhaust chambers). Is there a figure you work to for the central exhaust?
Would white spirit/meths be a suitable liquid for doing this? (Can get my hands on this and probably cheaper than the liquid gold known as petrol!)
(Got a cylinder head by the way, so expect lots more questions like this to follow! )
How does this work for the central exhaust? (inlets being shared getting both the same volume I can get, and I can see how you would be after equal outer exhaust chambers). Is there a figure you work to for the central exhaust?
Would white spirit/meths be a suitable liquid for doing this? (Can get my hands on this and probably cheaper than the liquid gold known as petrol!)
(Got a cylinder head by the way, so expect lots more questions like this to follow!
)Edited by Cooper1999 on Monday 17th March 13:23
It's only the actual combustion chambers you need to measure and equalise, not the inlet or exhaust tracts themselves.
Fit the valves complete with springs, fit some old plugs and check for leakage from the valve seats. One good way of doing this is to lay the head face upwards, fill each chamber with liquid and blow compressed air up the inlet and exhaust tracts, looking for small bubbles around the valve seats. If you get any bubbles, lap the valves in a bit more.
Then again place the head with the face upwards, check that it's absolutely level with a spirit level and measure the combustion chamber size using a syringe and suitable liquid. As I said, I usually use water as it doesn't harm the syringe seal.
Fit the valves complete with springs, fit some old plugs and check for leakage from the valve seats. One good way of doing this is to lay the head face upwards, fill each chamber with liquid and blow compressed air up the inlet and exhaust tracts, looking for small bubbles around the valve seats. If you get any bubbles, lap the valves in a bit more.
Then again place the head with the face upwards, check that it's absolutely level with a spirit level and measure the combustion chamber size using a syringe and suitable liquid. As I said, I usually use water as it doesn't harm the syringe seal.
It always amazes me that some engine builders don't even do a proper comp ratio measurement and calculation. I've just been asked to assemble a complete engine & box for which the block has been 'machined and assembled'. It turns out that the block has not been decked to match the height of the piston tops and the head has not been measured and skimmed to suit the requirements. In fact, the guy has no idea of what the C.R. is or what he wants it to be. He didn't even appreciate that going out to 1380 cc would up the standard compression, dependant on piston top to deck height being maintained.
I guess that's why some engine builders are much less expensive than others. They fit 'super cams', big valve heads, super exhausts, but don't get the basics right like very accurate cam timing, comp ratios, port matching, etc., then wonder why their super-tuned engines don't give the anticipated performance.
Hey-ho, you get what you pay for in this world!
I guess that's why some engine builders are much less expensive than others. They fit 'super cams', big valve heads, super exhausts, but don't get the basics right like very accurate cam timing, comp ratios, port matching, etc., then wonder why their super-tuned engines don't give the anticipated performance.
Hey-ho, you get what you pay for in this world!
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