CR maths

[sdp]

My engine is 998cc, 4 cylinder and the combustion chambers in the actual cylinder head are 24cc each.

The stroke is 76.2mm, the bore is 64.59mm, and I believe the compression ratio is 10.3:1 as standard.

...I think

How much lower would the CR be if I replaced the cylinder head with one that has combustion chambers of 28cc?

It's only 4cc difference so I'm wondering if it's really worth skimming the cooper head, especially seeing as the later mini's had CRs of either 8.3:1 or 9.6:1 anyway.

[Rave]

Right...well....it's a good thing the Windows calculator has full scientific functions, eh?

Anyway, if the compression ratio is 10.3, then this is my working out:

Swept volume (per cylinder) is 249.5cc
X is the volume of the combustion chamber _and_ the little bit at the top of the cylinder which is not reached by the piston at TDC
Y is the volume of that little bit

So:

249.5 + X = 10.3
----------
X

10.3X = 249.5 + X

9.3X = 249.5

X = 26.83

So, Y = 26.83 - 24 = 2.83

Now, with the cooper head, X = 28 + 2.83 = 30.83.

CR = 249.5 + 30.83 = 9.093
--------------
30.83

So, the CR would be roughly 9.1:1 by my (probably flawed) calculations.

As for whether it's worth skimming the head, I'd probably say yes: increasing the compression ratio is a pretty easy route to more power; since we have good quality petrol in the UK you might as well optimise your engine to take good advantage of it.

Rich :¬)

Edit: the calculations don't show properly 'cos I can't get the ----- lines and bottom half of fractions to move across properly and line up.

[sdp]

Nice one, thanks. I'll get it skimmed

err... is there an easy way to work out how much needs to come off?

[TiG]

EH?

isn't it a lot simpler than that mate?, 24cc is what it compresses everything into? * 10.3 CR = volume which is 247.2 CC per cylinder?.

*4 for the engine and i actually make it a 988cc bike but nm?.

If i follow your calcs rave, and you've got 26.83cc with the difference between the bore and the stroke volume, you actually come out with a bike with 1105cc????

I'll go with the simpler calculation for the moment, so 247.2cc / 28 = what you get approx a 8.8 compression rate?.

But this is purely maths based.

TiG

[sdp]

According to this that I just found it's 9.1:1 as well with the cooper head.

It's all a bit over my head though. No pun intended.

[Rave]

EH?

isn't it a lot simpler than that mate?, 24cc is what it compresses everything into? * 10.3 CR = volume which is 247.2 CC per cylinder?.

No. I was hoping it would be simpler, then I wouldn't have had to do a load of suspiciously complicated calculations at my desk, luckily my boss didn't seem to notice .

Anyway, work out the swept volume of the cylinder out again, I make it 249.675cc if we're being exact (which the engine itself probably isn't unless it's been blueprinted). The formula is pi * radius squared x stroke. Now:

249.675 + 24 = 11.4
-------------
24

...and an 11.4 compression ratio sounds a bit marginal for use with standard 95 octane petrol.

If i follow your calcs rave, and you've got 26.83cc with the difference between the bore and the stroke volume, you actually come out with a bike with 1105cc????

Eh? I've lost you there. The bore and the stroke give you the swept volume which is the difference between the volume of the cylinder and the head when the cylinder is at top dead centre and bottom dead centre. If you think about how an engine looks with the head off, there's still a little bit of space above the piston when it's at top dead centre; apart from anything else, there are often cutouts in the piston to clear the valves. That small amount of volume must be added to the volume of the head to give the volume you need to work out the compression ratio.

As for how much to skim the head by SDP, I have no idea. If the bottom of the hole where it meets the barrels is circular, or as near as dammit, then you could work out how much of a cylinder you'd need to shave off to reduce the volume by 4cc, but I'd advise you to find someone who's experienced at tuning Minis and ask them.

Edit: After writing the above reply, I went and read SDP's link, which is an interesting read and, if you follow the 'required data' link, it explains things a bit better than I have. It turns out that the 998cc Cooper head is designed to be used with pistons with raised crowns, which would up the compression ratio again.

Rich :¬)

[Crazy Cricket]

And then you get the awkward sods which have the combustion chambers in the pistons

[sdp]

The raised crowns take up 4.5cc, which cancel out the increase in size of the combustion chamber for the cooper.

A lot of people fit these heads to mini's that have dished pistons, and I think this is why there's a bit of a haze over how much needs to come off. The guy I'm buying it off says it's had 8thou off and was fine on his flat-topped 998 like that, but a lot of people are saying 60 thou + needs to come off.

Mine has flat top pistons too... meh I think I really need to wait til it arrives so I can do some rl measurements.