guzziriders.org - moto guzzi forum Homepage
Forum Home Forum Home > Technical > Big Block Tonti
  New Posts New Posts RSS Feed - cylinder compression test v1000
  FAQ FAQ  Forum Search   Events   Register Register  Login Login

cylinder compression test v1000

 Post Reply Post Reply Page  <12
Author
Message
v7john View Drop Down
Senior Member
Senior Member
Avatar

Joined: 14 May 2014
Location: West Wales, UK.
Status: Offline
Points: 588
Post Options Post Options   Thanks (1) Thanks(1)   Quote v7john Quote  Post ReplyReply Direct Link To This Post Posted: 07 Nov 2017 at 10:41
Years ago I was told a way to get a rough idea of what the reading should be when you do a compression test. This is what I put in my blog when checking my V7 700 loop.

"As I’ve got a compression tester I thought I might as well use it! It’s a cheap one I bought many years ago when I was into classic cars. It takes both hands to hold it against the spark plug hole while the Mrs holds the throttle wide open and pushes the starter. I got a reading of 132psi on the left and 140psi on the right. These are close enough to each other to be acceptable and are high enough. Both figures are reached in 4 revs. Apparently you can work out roughly what the reading should be in psi from the engines’ compression ratio. As the V7 compression ratio is 1:9 then the reading should be 9 x 14.7, which is the atmospheric pressure at sea level, plus an amount for an increase caused by the heat generated by the compression process. That comes out at 132 plus a bit. Pretty good then I reckon".

If you think the compressions are down I would put a bit of oil in the spark plug hole and repeat the process. If that gives a significant improvement the problem could be the rings. If not then I'd check the valves. One other thing. Make sure the valve clearances are correct before doing the test.
1972 V7Sport "The Racing Rhino".
1972 V7 700cc "The Fire Bike".

Read my blog https://racingrhino.wordpress.com/
Back to Top
v1000-tog View Drop Down
Senior Member
Senior Member
Avatar

Joined: 08 Apr 2016
Location: East Yorkshire
Status: Offline
Points: 126
Post Options Post Options   Thanks (0) Thanks(0)   Quote v1000-tog Quote  Post ReplyReply Direct Link To This Post Posted: 07 Nov 2017 at 22:38
the G5 has a compression ratio of 9.1:1 so 134 PSI using your calculation. 
so at 125 psi it has lost 9 psi in nearly 40 years.  I will put a bit of oil in and test again posting the result.
see if it gets the lost 7% back. - thanks
V1000-G5
past: Cali-vintage (2yrs) cali-special sport(8yrs)
Back to Top
red leader one View Drop Down
Senior Member
Senior Member
Avatar

Joined: 07 Oct 2014
Location: Cullercoats
Status: Offline
Points: 2822
Post Options Post Options   Thanks (0) Thanks(0)   Quote red leader one Quote  Post ReplyReply Direct Link To This Post Posted: 08 Nov 2017 at 10:30
That's the Angels share.
Back to Top
PHIL H View Drop Down
Falcone
Falcone
Avatar

Joined: 18 Jan 2016
Location: SW Yorkshire
Status: Offline
Points: 52
Post Options Post Options   Thanks (0) Thanks(0)   Quote PHIL H Quote  Post ReplyReply Direct Link To This Post Posted: 08 Nov 2017 at 18:17
I got similar on my G5 about 125 to 130
On my SP I got 85 on left and 125 on right...time to open tool box I think!
1978 SP 1000
1979 G5 1000
YAM XJ600 Street Tracker
Back to Top
Mike H View Drop Down
Senior Member
Senior Member
Avatar

Joined: 21 May 2014
Location: East Anglia
Status: Offline
Points: 5059
Post Options Post Options   Thanks (0) Thanks(0)   Quote Mike H Quote  Post ReplyReply Direct Link To This Post Posted: 09 Nov 2017 at 17:35
Originally posted by v7john v7john wrote:

Apparently you can work out roughly what the reading should be in psi from the engines’ compression ratio. As the V7 compression ratio is 1:9 then the reading should be 9 x 14.7, which is the atmospheric pressure at sea level, plus an amount for an increase caused by the heat generated by the compression process. That comes out at 132 plus a bit. Pretty good then I reckon".






"Chicken nuggets don't dance on a Tuesday."
Back to Top
Phil View Drop Down
Falcone
Falcone


Joined: 20 Jan 2016
Location: Yorkshire
Status: Offline
Points: 30
Post Options Post Options   Thanks (0) Thanks(0)   Quote Phil Quote  Post ReplyReply Direct Link To This Post Posted: 09 Nov 2017 at 20:39
Erhmm not quite you are mixing pressure units atmospheric pressure (a) and gauge pressure (g). atmospheric pressure of 1 bar a is 0 bar g. 9 times atmospheric pressure 9 bar a is 8 bar g ( 116 psi g )

A compression test is more complicated than boyles law accounts for as it takes no account of cam timing, heat caused by compression and any fuel in the gas charge.

Edited by Phil - 09 Nov 2017 at 21:58
Back to Top
Jools View Drop Down
Falcone
Falcone
Avatar

Joined: 19 Aug 2016
Location: Milton Keynes
Status: Offline
Points: 73
Post Options Post Options   Thanks (0) Thanks(0)   Quote Jools Quote  Post ReplyReply Direct Link To This Post Posted: 09 Nov 2017 at 23:15
And again, ah, er ..... no.

'Gauge pressure' is the only referenced measurement here. 

The type of pressure gauge used by 99.9% of us are calibrated to read '0.0' under STP, i.e. standard temperature and pressure of 20 degrees C and 1 Atmosphere/14.696psi/101.325kPa (the last being the SI unit of pressure). The instrument will then read in terms of 'Gauge Pressure'. In addition, the type of gas used in the calibration process will be the same as that which the gauge is expected to measure in use (air/nitrogen/acetylene/etc).

Only in those instances where the pressure gauge has been calibrated to read '0.0' against a total vacuum, will the instrument read in terms of 'Absolute Pressure', e.g. like a meteorlogical Barometer. 

Thus, as we are dealing solely with the former type of calibration, there is no need to apply any correction. 

Strictly speaking, any deviation from STP will make a difference. Cam timing will also make a difference if either valve is open when the piston begins to rise from BDC on the compression stroke, and any fuel mixture drawn in will similarly affect the gauge reading as its compressibility will be different from the gauges' calibration gas (air in the majority of cases). 

All in all, the 'rough guide' figures are quite adequate for our purposes.
Jools

V1000G5
1972 T150V
1961 Royal Enfield Constellation
1971 Suzuki ACC50
erm... 1963 Philips Panda
(and other assorted detritus)
Back to Top
Phil View Drop Down
Falcone
Falcone


Joined: 20 Jan 2016
Location: Yorkshire
Status: Offline
Points: 30
Post Options Post Options   Thanks (0) Thanks(0)   Quote Phil Quote  Post ReplyReply Direct Link To This Post Posted: 10 Nov 2017 at 07:40
No...   you are missing the point ... you cannot say 9 x atmospheric pressure should give you 9 bar gauge.
Back to Top
v7john View Drop Down
Senior Member
Senior Member
Avatar

Joined: 14 May 2014
Location: West Wales, UK.
Status: Offline
Points: 588
Post Options Post Options   Thanks (0) Thanks(0)   Quote v7john Quote  Post ReplyReply Direct Link To This Post Posted: 10 Nov 2017 at 09:03
You’re probably right Phil. As you say, the gauge reads zero when atmospheric pressure is already at about 1 bar. 1 bar = 14.5 psi, 1 atmosphere = 14.7psi. They aren't quite the same.

Remember, I did say it was a "way to get a rough idea of what the reading should be" in the first sentence of my original post on this. It's only an estimate. Making a more accurate calculation gets much more involved and is well beyond me. In my notes I have a more complete method to find pressure at TDC but it’s still only a more accurate way of making an estimate;

p(TDC) = p(BDC) x CR raised to the power of 1.3 for the heat ratio of an air/petrol mixture X 60% for the volumetric efficiency of a petrol engine, minus the 1 bar not registered by the gauge.

So in my case that would give;

P(TDC) = 1 x 9 ˄ 1.3 x 0.6 - 1 (in bar).
P(TDC) = 1 x 17.40 x 0.6 - 1 = 10.44 -1 = 9.44 bar or 136.92 psi (using 9 x 14.7 gives 132.3 psi)

For V1000-tog it would be;

P(TDC) = 1 x 9.1 ˄ 1.3 x 0.6 -1 (in bar).
P(TDC) = 1 x 17.65 x 0.6 -1 = 10.59 -1 = 9.59 bar or 139.1 psi (using 9.1 x 14.7 gives 133.77 psi)

As you can see, the very basic method isn’t too bad if you’re just looking for a rough idea and I did say plus a bit “for an increase caused by the heat generated by the compression process”.

You’re right about cam profiles being able to make a difference. The valves might not be closed for the whole of the piston stroke so reducing the pressure. There are a number of other factors as well such as the temperature of the cylinder etc. Quoted compression ratios are about a change in swept volume while the cylinder pressure increase relies on much more.

1972 V7Sport "The Racing Rhino".
1972 V7 700cc "The Fire Bike".

Read my blog https://racingrhino.wordpress.com/
Back to Top
 Post Reply Post Reply Page  <12
  Share Topic   

Forum Jump Forum Permissions View Drop Down

Forum Software by Web Wiz Forums® version 11.10
Copyright ©2001-2017 Web Wiz Ltd.

This page was generated in 0.094 seconds.