Wales
by Joske Vermeule, September 1
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Jun 14th, 2026
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Joined: Dec 2017
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Talk Morgan Expert
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Talk Morgan Expert
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Could anyone advise me on the ideal psi range for compression testing on a 1994 3.9 Rover engine?
Bruce
1964 4/4 Series V Comp (Megan)
1994 +8 (Maurice) tis no more
2013 M3W (Olga)
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Joined: May 2014
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Talk Morgan Expert
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Talk Morgan Expert
Joined: May 2014
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Someone will come along to advise but memory tells me it's around 170-180?
Worth a google if not to confirm.
Steve
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Tricky Dicky Member of the Inner Circle
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Tricky Dicky Member of the Inner Circle
Joined: Dec 2009
Posts: 37,434 Likes: 978 |
170 psi Minimum in my manual.
2009 4/4 Sport Henrietta
1999 Indigo Blue +8
2009 4/4 Sport Green prev
1993 Con Green +8 prev
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Joined: Dec 2017
Posts: 2,223 Likes: 183
Talk Morgan Expert
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Talk Morgan Expert
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Steve: I think I have answered my own question and you are correct. With a compression ratio of 9.35:1 the range is 170 - 180.
Thanks for the quick response.
Bruce
1964 4/4 Series V Comp (Megan)
1994 +8 (Maurice) tis no more
2013 M3W (Olga)
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Joined: Jul 2007
Posts: 27,816 Likes: 749
Member of the Inner Circle
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Member of the Inner Circle
Joined: Jul 2007
Posts: 27,816 Likes: 749 |
The S1 Roadster is 10:1. I compression tested the cylinders last week at 220 psi which I was told was fine.
JohnV6
2022 CX Plus Four
2025 Renault Scenic Alpine etech "Yvette"
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Joined: May 2014
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Talk Morgan Expert
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Talk Morgan Expert
Joined: May 2014
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It's not just the individual readings that are important, the variation between cylinders too. Ideally you don't want more than 10% variation. Of course lots of factors determine the ideal.
I remember way back when I checked a straight 6 vauxhall running on 5. Zero compression on one cylinder and running as rough as anything. Head off suspecting a burnt valve and found no piston or rod fitted and a jubilee clip covering the crank journal oilway. No wonder it was running rough!
Steve
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Joined: Nov 2015
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Just barreling along Needs to Get Out More!
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Just barreling along Needs to Get Out More!
Joined: Nov 2015
Posts: 8,684 Likes: 493 |
It's not just the individual readings that are important, the variation between cylinders too. Ideally you don't want more than 10% variation. Of course lots of factors determine the ideal.
I remember way back when I checked a straight 6 vauxhall running on 5. Zero compression on one cylinder and running as rough as anything. Head off suspecting a burnt valve and found no piston or rod fitted and a jubilee clip covering the crank journal oilway. No wonder it was running rough! Did the owner lust after a 5 Cyl Audi perhaps ? 
Jon M
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Joined: Dec 2017
Posts: 2,223 Likes: 183
Talk Morgan Expert
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Talk Morgan Expert
Joined: Dec 2017
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Richard: Thanks, I'm running one cylinder at 178, one at 178.5 and the others at 179, so I think it's OK.
Bruce
1964 4/4 Series V Comp (Megan)
1994 +8 (Maurice) tis no more
2013 M3W (Olga)
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Joined: Dec 2009
Posts: 37,434 Likes: 978
Tricky Dicky Member of the Inner Circle
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Tricky Dicky Member of the Inner Circle
Joined: Dec 2009
Posts: 37,434 Likes: 978 |
Richard: Thanks, I'm running one cylinder at 178, one at 178.5 and the others at 179, so I think it's OK. Bruce, that's about as good as it gets I would say a nice healthy engine I am glad it's good news. I've got the Haynes Land Rover Discovery manual that covers our 3.9 engines so if there is anything else you need just shout and I'll help if I can.
2009 4/4 Sport Henrietta
1999 Indigo Blue +8
2009 4/4 Sport Green prev
1993 Con Green +8 prev
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Joined: Nov 2016
Posts: 122
L - Learner Plates On
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L - Learner Plates On
Joined: Nov 2016
Posts: 122 |
Hi. Probably sucking eggs, but don't forget to have the throttle fully open during the test.
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