Author Topic: Engine Compression  (Read 2178 times)

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Halfpint

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Engine Compression
« on: October 21, 2011, 09:43:22 PM »
Hi, Oh great Guru's,
Its been 4yrs since I put this motor in! But being my 4th A series motor, and never ever checked compression, I wonder what it should be.
My tester has a PSI value,(never really used it till now) but all my books say 8 to 9.7:1 ?
I have a bit of understanding, I think, And a search for info gives me this link
https://www.mokeforum.com.au/index.php/topic,5365.msg57040.html#msg57040
Your right Terry, needs lots of info and input from us all.
Still, what is a good number for an A series?
HP
« Last Edit: October 21, 2011, 09:54:30 PM by Halfpint »
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Re: Engine Compression
« Reply #1 on: October 21, 2011, 09:54:04 PM »
HP..
I think that "?" is the modern "BAR"


mav.

ps...   http://www.centauro-owners.com/articles/psibar.html
« Last Edit: October 21, 2011, 09:58:02 PM by mavro »

moemoke

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Re: Engine Compression
« Reply #2 on: October 21, 2011, 10:10:53 PM »
HP are you wanting to check the compression ratio ot the compression pressure, they are 2 completely different things.
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Tim

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Re: Engine Compression
« Reply #3 on: October 21, 2011, 10:14:12 PM »
Compression ratio (8:1 or 9.75:1) is how much the engine compresses the air by. So 8:1 means the engine takes a full cylinder of air and squashes it down to 1/8th the volume. If life was easy we'd take the pressure of air at one atmosphere (about 15 PSI) and multiply it by eight to get 120 PSI. Its not that simple because the cylinder doesn't necessarily completely fill and there are losses as you try to compress it.

What you want is compression somewhere around 120-150 psi across all 4 cylinders. The absolute pressure is less important than them all being reasonably equal, especially if you only have a cheap gauge.

Tim
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Terry

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Re: Engine Compression
« Reply #4 on: October 21, 2011, 10:17:31 PM »
Hi,

For the few 1275 engines I have done a compression test that i can remember  they were around the 150psi mark and all were pretty tired or of average condition.

The compression ratio isn't going to come out of a gauge, unfortunately, as it comes down to how much space there is at the bottom of the stroke vs how much space/volume at the top of the stroke.

...and seeing as Tim has posted while I was typing, his point about them all being close or the same, the rule of thumb I was bought up on is there should be no more than 10% difference across all cylinders.

Terry
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Halfpint

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Re: Engine Compression
« Reply #5 on: October 21, 2011, 10:27:52 PM »
       
Hi Mav, I love numbers, but a conversion still gives me just numbers.
Im after what it should be.... what ever the number is  :o


1 pound per square inch = 0.0689475729 bar
So 120 PSI =8.273708724 BAR which is just a big push from the plug hole untill there is a value.
Even PSI or BAR doesnt show up in books I have.
Moe, just caught your post. Am after the pressure, I can measure it easy at the moment. Just curious. 

Ah ha, thanks guys you type so fast. Appreciate it. Just playing today with some kit that come from my dads shed and always wondered about but never bothered to check cause it ran ok.
So at 120 psi across all 4 gives me that warm and fuzzie A serries feeling 8) 8)
HP

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Maddog

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Re: Engine Compression
« Reply #6 on: October 22, 2011, 01:48:28 AM »
I was pretty happy with this....... :)


There are a few tricks to get a good reading, but no easy cure for a bad one. ;)


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spider

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Re: Engine Compression
« Reply #7 on: October 22, 2011, 06:52:58 AM »
What you want is compression somewhere around 120-150 psi across all 4 cylinders. The absolute pressure is less important than them all being reasonably equal, especially if you only have a cheap gauge.

Tim

That is a healthy range for sure. If a bit of a cam has been fitted or if the timing chain has stretched, you can expect these numbers to be lower than this. But, as Tim has pointed out, rather than looking for a specific pressure, a consistancy of pressure across all cylinders and the number of times you need to spin it over to get there is more meaningful. These gauges I've never found to be teribly accurate, but a good one will give repeatable results.

For best results, it should be done with the engine at running temp.

MD, that's almost a worry! Sure it's not a Diesel?
« Last Edit: October 22, 2011, 07:09:00 AM by spider »
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Re: Engine Compression
« Reply #8 on: October 22, 2011, 09:44:54 AM »

MD, that's almost a worry! Sure it's not a Diesel?

maybe the head had been shaved off abit in the past, coz if its too hight it will blow a headgaskit wouldnt it ?


mavro

Tim

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Re: Engine Compression
« Reply #9 on: October 22, 2011, 10:05:53 PM »
If its too high you get pinking or knocking caused by pre-ignition (i.e. it starts to work like a diesel). Its especially bad with modern fuels that aren't high enough in octane rating.

In the old days you could legally run road cars on avgas which is high octane. A lot of Minis around here used it. It was a bit of a pain though because the airport in Hobart is a reasonable distance out of town and its a fair way to go just to fill up. Once it became illegal they had all sorts of hassles getting their compression ratios back down again.

Tim
1951 Morris Commercial J Type Van
1955 BSA C11G
1961 Morris Mini Traveller
1969 Triumph TR6R
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spider

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Re: Engine Compression
« Reply #10 on: October 23, 2011, 07:21:30 AM »
Yeah, I'd be worried about pinking and running on. The head gasket will be OK.

Of interest, but ambitious, from the factory Morris 1100 workshop manual, they quote cylinder compression tests numbers of 155 to 180 psi for the LC (8.8:1) Engine and 185 to 210 psi for the HC (9.75:1) Engine. Single atmospheric pressure is a nominal 14.7 psi, these numbers are simply not possible.
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