Compression calculation - 12G202 on otherwise standard 850

[abri]

I have read what it entails to calculate compression ratio and have heard of various calculators available to do this. I can't seem to get the Mk1 Performance one to work (from links that came up in threads found while searching, they just point to dead ends).

I want to know exactly how much to skim off a standard 12G202 head to get the same 8.3:1 ratio as is the standard for a stock standard mid 1963 Morris 850 on which I plan to put it. The head is currently at its original thickness of 2.750" and has had no chamber work done, so the chamber volume should be 26.1cc.

My first question is, can anyone point me to a calculator that will allow me to calculate what I need to know?

Secondly, I have read about the issues with high compression and the fact that the cam does not have a bearing at the back. What is a safe compression ratio to aim for if I use the standard cam. Can I go to 8.5:1?

Thanks

[Vegard]

The compression ratio doesn't affect the cam bearings one blind bit. That's valve springs/camshaft.

However I'd raise the compression from 8.3:1. That's nothing.

I'm sorry there is no calculator you could trust. Take the old head off, measure the distance from the piston at TDC to the block. Then we could sort something out. Never assume!

[cookie1071]

I recall method for working out compression ratio is documented in Vizard's book Tuning the A series engine...

[graham in aus]

Yeah, Vizard Yellow book is good, I did this for Ausmini......

http://www.ausmini.com/forums/viewtopic ... 25&t=53856

Hope it helps...

[mk1]

This link should work;

http://mk1-performance-conversions.co.u ... r_calc.xls

Its very clunky, but does work OK.

Simply replace the red numbers with the ones suitable for your calculation either from the tables to the left of from your own knowledge.

M

[abri]

Thanks a lot Mark, that's exactly what I was looking for. So I need to make sure of land volume and piston volume (or get standard readings on the web) and then I'll have all the variables to get what I need for the head.

Thanks a lot Graham as well for a most useful link.

Would a standard 63 850 be able to handle 9:1 compression (assuming I can safely skim enough off the head to get it there)?

[ianh1968]

I have an old DOS program that I use to calculate things.
I have also compiled this as a Linux program.

If you tell it a ratio, it will calculate the UNSWEPT volume
required to achieve this. It works for any size and number
of cylinders so it can be used for pretty much any engine.

If you tell it an unswept volume, it will calculate the CR.
Similarly, use it to calculate how much the CR will change
by a reduction in chamber size.

Say you want to increase the CR from 8.3 to 9.
Run the calculation twice, ie once with 8.3 and then
again with 9.0 . Subtract the volumes from each other.
This will tell you how much volume you need to lose.

If you are changing nothing apart from the head volume,
you do not even need to even think about ring-land,
dish and gasket volumes. You are simply working out
"before" and "after" and taking the required figure off
the head surface.

Eg:
Swap a 24.5cc head on an 850 to 26.1cc,
then increase the CR to 9.0

We "know" the 848 with 24.5cc is 8.3:1 ...
Total unswept volume works out at 29.0cc

We want 9:1, on 848cc this calculates as 26.5cc.

So to go from 8.3 to 9.0, we need to lose 2.5cc.

The 1100 head is PLUS 1.6cc on the 850 head,
so we need to reduce this by 4.1cc (2.5cc+1.6cc)
to go from 8.3 to 9.0

If anyone wants these programs, let me know.

As regards the CR, 9:1 sounds reasonable, but with a
short/standard cam and an exhaust heated inlet it may
be as high as you would want to go.

So far as working out how much to machine off, this is what I do:

I get a piece of 5mm squared paper and trace round the shape
of the chamber, normally by tapping round the edge with a jeweller's
hammer.

Alternatively, you could use a wax crayon like you would do with
a "bark rubbing"...

For each row of squares, write down the number of whole squares
and make a best guestimate of the part-squares. Each whole square
will be 0.25cm so you can easily work out the area of the combustion
chamber shape.

Once you know the area, just work out the thickness to take off.
A certain amount of "finger-in-the-air" needs to be applied here as
the sides of the chamber will not be vertical, but should be close
enough to come up with a reasonable accurate figure.

Attached is one I did with an ally head I got from MiniSpares.

I always check the capacities before and after with a burette and
flat piece of perspex, but if most of the parts are fairly standard this
would not necessarily be worth bothering with.

CR-Calc.jpg

[ricardo]

I also made a calculator excel sheet a while back. It's easy to use to calculate a compression or find how much ccs you need to measure in the head for the wanted compression. However, I'm with Vegard, don't assume anything and measure the parts.

The sheet isn't very well tested as I only used it myself once or twice, but from the comparisons I made it seems OK.

https://dl.dropboxusercontent.com/u/21540254/CR_v1.xlsx

Measure well the deck of the head to see how much it can be skimmed. Some heads have more meat than others and you need to skim a bit to raise the compression to a decent level.

[ianh1968]

I'm with Vegard, don't assume anything and measure the parts.

Generally speaking, I would wholeheartedly agree with both of you. However, abri has
stated that this is a bog standard engine, apart from the pending head-swap. On this basis
we could probably assume that things will work out OK without getting too technical...

Some heads have more meat than others and you need to skim a bit
to raise the compression to a decent level.

A very valid point, it is possible to break through into the oil gallery that transfers the oil
from the block to the rocker assembly. There is a fix for this which involves braising up
the original channel and cutting a slot across the face of the head from the original start
point across to the nearest head stud hole. The oil then passes up round the outside of
the stud and emerges through another small slot to come out in the base of the pillar
where it then goes vertically up to the shaft.

The head would need to be re-skimmed to flatten out the braising, afterwards.

The ally heads from MiniSpares are made like this in the first place, probably because it
is easier to use the slot method than drill and bung various oil galleries...

We probably don't need to get too complicated on this:
We know that the original engine is 848cc, 24.5cc and 8.3:1.
(Or if it isn't, it's probably close enough...)

The TOTAL UNSWEPT VOLUME to make 8.3:1 from 848cc works out at 29cc.
Some of this will be in the head, some in the gasket, some in the ring land and
maybe in a dish, but none of this matters because everything apart from the head
will be exactly the same on the modified engine...

To get 9:1 on 848cc, we need a TOTAL UNSWEPT VOLUME of 26.5cc, again, it
doesn't matter where this resides because we are only comparing the TOTAL
UNSWEPT VOLUME, before and after.

This total figure for 9:1 is 2.5cc less than it is for 8.3:1.

As every thing else is going to stay the same and the thing being
modified is the head, so we need a head with 22cc.

In this case, this will be either an 850 head with 2.5cc removed,
or an 1100 head with 4.1cc removed.

Sticking the 1100 head straight on without raising the CR will give 7.9:1.

Even if the exact figures are not quite 24.5cc and 8.3 on abri's engine,
the figures suggest that taking 2.5cc/4.1cc off should increase the CR
by 0.7:1, or thereabouts...

Good Luck!

[abri]

Thanks a ton guys, especially Ian. I was thinking along the lines you describe with everything remaining exactly standard (the motor is untouched, has about 45k miles on it and runs like a dream without a puff of smoke). I will obviously check that the pistons are dished as original, which would mean everything is in fact standard. I also thought the manufacturing dimensions/clearances etc would not be so far off that it would merit re-measuring the ring land volume etc, although it would be easy to measure the height from the top of the pistons to the top of the cylinders.

I have read that the 202 heads have quite a lot of metal that can be machined off, but there is the Vizard suggestion of how to measure the remaining "meat" on the head (although I'd have to read up a bit more on exactly where that is measured).

@Ian: To make sure I understand your calculation correctly....

If the unswept volume on a standard 848cc is 29cc as you say and the original head had 24.5 cc chambers, the ring land volume plus piston to deck volume plus piston dish volume, plus compressed gasket volume must add up to 4.5cc. This volume of 4.5cc is composed of gasket (1.4cc), ring land volume (0.6cc) (volumes taken from Mark's calculator), piston crown to deck cylinder volume and piston dish. This means the piston dish + piston to deck is close to 2.5cc (please correct me if I'm wrong).

The 202's chamber volume is 26.1cc.

So adding the 26.1cc on to the standard 850 block, pistons etc. we'll end up with 26.1 + 4.5cc (30.6cc).

So, if, to get 9:1 we require 26.5cc chamber volume, then we need to remove 4.1cc (30.6 minus 26.5) from the 22G202 head's chamber.

I'll check the 202 head's chambers with a burette. The head is also unmolested (in fact may be NOS). I was also considering reducing the valve shrouding because the 202's chambers look small and deep.

If the figures above are all reasonable (and I will measure some), at least now I know the final chamber volume needs to be 26.5cc, so I may be able to address the shrouding to the extent that the skimming can allow.

Thanks a lot for the help guys. Learning and therefore appreciating this most useful input.

[ianh1968]

to get 9:1 we require 26.5cc chamber volume

Yes, apart from I prefer the term "Total Unswept Volume" as this makes
it quite clear that what we are talking about is EVERYTHING volume-wise
above/round the piston that is not included in the engine cc calculation...

Of the 26.5cc, 4.5cc will be fixed, the gasket, ring land etc, and the
remaining 22cc will be how big your actual chambers will need to be.

Measure well the deck of the head to see how much it can be skimmed.

Forgot to mention...

Poke the depth measuring tail of a vernier caliper down the oil feed hole for the rocker
pillar on the top face of the head.

Subtract this amount from the total measured thickness and this will tell you
APPROXIMATELY how far from the surface the oil channel in the head is.

It may not be very horizontal, so be wary - I'd make sure that there was at least
40thou (1mm) thickness, but seek other opinions before sending the head to the
machine shop.

If the head breaks through, use the fix detailed in my last post.
(Or get another head)


The other thing about having a thin deck is the head's ability to hold a gasket.
Too thin and this could get compromised.

[abri]

Yes, apart from I prefer the term "Total Unswept Volume" as this makes
it quite clear that what we are talking about is EVERYTHING volume-wise
above/round the piston that is not included in the engine cc calculation...

Point taken, your term is actually accurate.

That method of measuring the remaining metal on the face of the head is what I was referring to. Will definitely do that.

I have a spare 202 head in case it all goes pear shaped.