3.4 Leakdown & Compression Check Q
07-15-2005, 12:34 PM
#1
3.4 Leakdown & Compression Check Q
I got a note on a car I am looking to purchase...it has 3.4 p/c's Leak down was 12% and the compression was 140 each. I don't know if this is hot or cold or what...
Any advice is appreciated.
Thanks
Any advice is appreciated.
Thanks
07-15-2005, 12:59 PM
#2
Nordschleife Master
12% cold isn't bad, but you really want to get a warm reading. The 140psi compression indicates camshafts without a lot of duration or higher than 7.5:1 compression. I would bet it has something like SC cams.
Last edited by Geoffrey; 07-15-2005 at 01:39 PM.
07-15-2005, 01:12 PM
#3
RL Technical Advisor
Just to offer another opinion,....................
I would be concerned if I saw a 12% leakdown figure; cold or hot. I would ask WHERE the leakage was observed coming from.
After all, thats really the true value of such tests.
I would be concerned if I saw a 12% leakdown figure; cold or hot. I would ask WHERE the leakage was observed coming from.
After all, thats really the true value of such tests.
07-15-2005, 05:39 PM
#4
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I'll throw my hat in too, Hi Geoff, 12% cold isn't insane , but there's a bBIG / to answer.. I run < and I think Geoffrey runs, HUGE ring gaps with ultra high power motors... even brand new they will pull 5-7%...
07-15-2005, 08:53 PM
#5
Its a "real" Ruf 3.4 twin plug. So who knows about the cams...I think the engine has done a lot of sitting...that's my big concern. Also w/the manual adjustable boost...who knows how much abuse the rings have taken.
07-15-2005, 09:37 PM
#6
RL Technical Advisor
One thing,.......
Ruf uses a VERY short-skirted piston in their 3.4 (Mahle) kits and those rock around in the bore more than the longer skirted ones to cause more ring wear.
Thats why I asked about WHERE the observed leakage came from,....
Manual boost control is an indicator of higher-than-average cylinder pressures so IMHO, this motor deserves closer scrutiny.
Ruf uses a VERY short-skirted piston in their 3.4 (Mahle) kits and those rock around in the bore more than the longer skirted ones to cause more ring wear.
Thats why I asked about WHERE the observed leakage came from,....
Manual boost control is an indicator of higher-than-average cylinder pressures so IMHO, this motor deserves closer scrutiny.
07-16-2005, 12:12 AM
#7
Nordschleife Master
I do what??? I'm not sure where you are getting your misinformation, I have factory ring gaps on my long skirt 3.4l Mahle pistons, and would only run factory specs.
As Steve mentions, I would want to find out more information.
As Steve mentions, I would want to find out more information.
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07-16-2005, 12:59 AM
#9
Rennlist Lifetime Member
Yes, 3.4 rings are very easy to get. I probably have a set on the shelf.
On the leak down numbers, I get worried with anything in the teens. On a new build after 300 miles or proper seating I get about 1% to 2% leak downs. The key with a leak down is consistency. If all the cylinders are 12 then you are probably ok. Could be anything. If you have a cylinder that is 12 and the others are 5 then you have a problem. Specifically if it is the 3/6 combo.
On the leak down numbers, I get worried with anything in the teens. On a new build after 300 miles or proper seating I get about 1% to 2% leak downs. The key with a leak down is consistency. If all the cylinders are 12 then you are probably ok. Could be anything. If you have a cylinder that is 12 and the others are 5 then you have a problem. Specifically if it is the 3/6 combo.
07-18-2005, 12:30 PM
#10
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Geoff,
Didn't meant to offend... dunno know about you guys(including Stephen)... But in 600-730 HP 3.3/3.4's if you're running 1.2-1.4 bar boost I wouldn't run 'porsche' factory ring gaps... I think it's .4mm off the top of my head... the risk of too tight a ring gap scares me.. i'd rather be safe than sorry... and there is always a leakdown consequence for the bigger gaps obviously..
With the water cooled engines I probably wouldn't care as much .. Stephen what do you run on Air cooled?
Didn't meant to offend... dunno know about you guys(including Stephen)... But in 600-730 HP 3.3/3.4's if you're running 1.2-1.4 bar boost I wouldn't run 'porsche' factory ring gaps... I think it's .4mm off the top of my head... the risk of too tight a ring gap scares me.. i'd rather be safe than sorry... and there is always a leakdown consequence for the bigger gaps obviously..
With the water cooled engines I probably wouldn't care as much .. Stephen what do you run on Air cooled?
07-18-2005, 12:48 PM
#12
Rennlist Lifetime Member
If you look at the latest rings for say a 98MM Mahle they have opened up the ring gap. It still shows the stock spec in the book, but the rings received are gapped bigger. I had 4 sets brought in before someone finally admitted there was an internal change.
