Might seem like a stupid question, but can you work out the Compression ratio of an engine with the head on? I think i know the answer but......

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Jay: 0431404607

I don't see why not.

You know what atmospheric air pressure is, and you can measure the pressure in the cylinder at full compression. Crudely, your compression ratio should be one divided by the other.

It's probably not, though

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Simon

The adventures of an owner builder in the Tallarook Ranges

atmospheric pressure

so if TDC is say 147 psi on a comp gauge and Atmospheric presure is 14.7psi, you have 10:1 comp ratio?

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*Reasonable pricing
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Jay: 0431404607

static and dynamic compression are different though, 'cause of things like inertia of the intake slug. I don't think you can do it without the head off.

As suggested by Phil Irving I think, in his book `tuning for speed'.
Easy to do and more accurate then with the head off- if you know the cc of the motor, let's say 1000cc.
1. park the Mini uphill.
2. remove all the spark plugs.
3. turn motor until #1 piston is at TDC on compression stroke.
4. using a big syringe, or measuring cylinder, measure as you fill the cylinder with fluid (I suggest ATF or motor oil) until it's at bottom of plughole, rock engine a little until it's at max height, then top up to 1/2 way up the plughole.
5. calculation- let's say it took 280cc total.
displacement of 1 cylinder = 1000/4 = 250cc.

so total chamber vol CV= 280 - 250 = 30cc.
CR = (displacement + CV)/CV
C/R therefore = 280cc/30cc = 9.33:1.

To get the ATF out put rags over the front of the car and crank the starter...

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

Haven't got a head in front of me to check, but the picture in my head says that would have to be one steep hill to get the sparky hole as the topmost point?
I guess it depends on chamber shape.....

NO you don't need the plug hole vertical, even a 5° incline will do it.
the plug hole intersects the top corner of the chamber.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

No I know not vertical....just my memory was telling me the hole wasn't quite at the roof of the chamber....and it wouldn't take much to through the calculation off.

I think i would rather take the cylinder head off and check it. Its a mini not really a big deal to take head off