Club 80-90 Forums

Never was very good at maths or physics.

And bless Susan Boyle - singer and mathmetician. Wouldn't think it to look at her.

Thanks for your time and patience guys, and sorry for hijacking OP thread.

AdrianC wrote:

jamesandtheopenroad wrote:With that in mind, aren't the compression ratio and compression pressure two different things?

Not unless Boyle's Law just got repealed.

P1 x V1 = P2 x V2
Pressure 1 x Volume 1 = Pressure 2 x Volume 2

P1 = 1 bar (atmospheric pressure)
V1 = 8.6 thingies
V2 = 1 thingy (the actual volumes don't matter, it's only their ratio that does matter)

1 x 8.6 = P2 x 1

therefore P2 = 8.6 bar

Thinking about this, it's not quite that simple. If we forget about the heat of combustion, there's still heat being generated by the compression, raising the pressure further, so it's more of a question of the combined gas laws than straight Boyle. Boyle's law assumes that the temperature remains constant.

http://en.wikipedia.org/wiki/Combined_gas_law

It's the reason why the barrels on air compressors have cooling fins, after all. Or why a metal-bodied bike tyre pump is hot to the touch after you've inflated a flat tyre. Ain't just friction.

AdrianC wrote: Thinking about this, it's not quite that simple. If we forget about the heat of combustion, there's still heat being generated by the compression, raising the pressure further, so it's more of a question of the combined gas laws than straight Boyle. Boyle's law assumes that the temperature remains constant.

It's the reason why the barrels on air compressors have cooling fins, after all. Or why a metal-bodied bike tyre pump is hot to the touch after you've inflated a flat tyre. Ain't just friction.

That's obviously where the discrepencies lie....the increase in pressure due to heat...makes sense.
Martin

Hello,


I think you may find that:

PV= n x r xT

may be pertinent

Where r is the 'gas constant'

and T is in degrees absolute,

Substitute aand transpose to get P

Any way, those seem to be good compressions, worth doing them cold and hot,

A standard gauge will not do very well for a leakdown test.

Now check your oil pressure....

Cordialement,

Is there a method for obtaining my oil pressure quantitatively? If so, what kit would I need? Cheers.

what2d wrote:Is there a method for obtaining my oil pressure quantitatively? If so, what kit would I need? Cheers.

An oil pressure gauge


(sorry, I couldn't resist, I'm off to bed now)

Oil pressure is usually measured by using a seperate oil pressure gauge tapped into the oil gallery (normally by taking out the oil pressure switch and replacing it with an accurate gauge).
There are certain criteria...ie: oil needs to be hot, from memory I think the reading is taken at 2000rpm .

Yes...2 bar at 2,000rpm (about 29 PSI) minimum
Having said this....something in the back of my mind says idle oil pressure needs to be over 7 PSI ....not sure where I remember that from.

Martin