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2.7 compression
Does 120-125 across all cylinders seem low for a 2.7 from a 77 911s?
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09-02-2015, 04:18 PM
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Registered
Join Date: Sep 2012
Location: Newport Beach CA
Posts: 1,873
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If they are consistent, it's a runner. But 140 is what I would consider as "good".
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I will use a FAF when I am dying of thirst 
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09-02-2015, 04:23 PM
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76 911S Targa
Join Date: Oct 2010
Location: Idaho
Posts: 1,150
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Wayne Dempsey's book on rebuilding Porsche 911 engines is a good source on this topic. Look on pages 14-15 where he discusses this very topic. If the 2.7 is fitted with hotter cams, these readings may mean a good engine. If fitted with standard 2.7 cams, 120 is low. Also depends on how the test was done. The numbers are consistent which is good. Did you do the test yourself? Was the throttle wide open during the test? Did you add a tablespoon of motor oil to each cylinder and test again? Is this engine in Ontario or in Denver? In Denver, 125 psi is not bad. In Ontario, not so good.
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76 911S, 2.7, Bursch Thermal Reactor Replacements, Smog Pump Removed, Magnecors, Silicone Valve Cover Gaskets, 11 Blade Fan, Carrera Oil Cooler, Turbo Tie Rods. |
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09-02-2015, 04:58 PM
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Yes in Ontario. Described as stock. Compression results are from a bench test by an experienced mechanic completed about 6 years ago before the motor was placed in storage.
Leak down numbers were 8-12% with one cylinder at 20%. |
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09-02-2015, 05:04 PM
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Max Sluiter
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If you assume 14.5psi atmospheric pressure then you should get 123psi gage with a 9.5:1 compression ratio and 100% VE. Your numbers sound good to me.
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1971 911S, 2.7RS spec MFI engine, suspension mods, lightened Suspension by Rebel Racing, Serviced by TLG Auto, Brakes by PMB Performance |
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09-02-2015, 06:12 PM
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abit off center
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You can change those numbers a lot by timing your cams different in a 2.7 too
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______________________ Craig G2Performance Twinplug, head work, case savers, rockers arms, etc. |
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09-02-2015, 06:51 PM
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gearhead
Join Date: Nov 2007
Location: Loverland, CO
Posts: 23,599
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Am I the only one concerned by the leakdown?
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1974 914 Bumble Bee 2009 Outback XT 2008 Cayman S shop test Mule 1996 WRX V-limited 450/1000 |
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09-02-2015, 06:52 PM
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Wer bremst verliert
Join Date: Jan 2005
Location: Toronto, Ontario
Posts: 4,767
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I wouldnt place much faith in those numbers. Thats a few big gaps, a lot of time to pass and variables. Pretty leaky too.
What are you looking at and what are you trying to achieve? Also here in Ontario....
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2007 911 Turbo - Not a toy 1985 911 Cab - Wife's toy 1982 911 3.2 Indiash Rot Track Supercharged track toy 1978 911 3.0 Lichtbau toy "Gretchen" 1971 911 Targa S backroad toy |
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09-02-2015, 07:40 PM
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Functionista
Join Date: Jul 2011
Location: CO
Posts: 7,717
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My first 2.7 made 160-170 at 8500 ft. CIS with S cams. No idea on internals but it was a strong runner.
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09-02-2015, 07:53 PM
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It's a basically complete motor but with carbs. I'm looking for something to drop in and run.
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09-02-2015, 08:29 PM
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Project Addicted
Join Date: Jul 2012
Location: Eastern Shore. MD
Posts: 919
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Here is a data point for you:
 This was done on a '77 - 2.7 with 100K miles in storage for about the same amount of time, maybe a bit longer. How was the test done?? All plugs out? Throttle wide open? strong battery? cranked until the needle would no longer rise? On a cold engine that has been sitting it's a bit if a crap shoot with these tests.
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Jon 1966 912 1976 911 3.4 Backdate Project 1986 944 |
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09-03-2015, 04:56 AM
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Hi Jon, thanks for your post, if this motor showed those numbers it would be in the back of my truck already. It's because the comp seems low and the leakdown seems high that I'm concerned.
The guy who did the testing is a legendary porsche guy (heck he could find 6 year old detailed records in ten minutes) so my guess is his numbers are accurate. |
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09-03-2015, 05:57 AM
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Registered
Join Date: Apr 2005
Location: outta here
Posts: 54,260
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Quote:
A '77 didn't have 9.5:1, though. It had 8.5:1. His numbers are low, even for that compression ratio. The leakdown numbers are worse. If the test was done properly, that motor could use a rebuild. JR |
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09-03-2015, 06:59 AM
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I think I know the answer to this but figured I'd post anyway... Looking at a '74 US Carrera with 132k miles. owner performed a comp test and got these figures:
Cylinder 1: 125 Cylinder 2: 135 Cylinder 3: 135 Cylinder 4: 125 Cylinder 5: 150 Cylinder 6: 150 assuming these aren't great since not consistent and that cylinders 1 & 4 are 25 psi off.
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TBD Prior: '84 911 3.2 Carrera, '83 911SC Targa IG: @heuersandcarreras "I want to use it. That's what 911s are for." - Richard Hammond |
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11-13-2025, 10:51 AM
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PCA Member since 1988
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You know the answer. Plus, with 132Kmi, it's probably time for a head refresh, and once you go that far, check the pistons, rings, and cylinders, and then the slippery slope to opening up the bottom end.
On the plus side, it's a 1974, which did not have the thermal reactor, which increased operating temps, which increased magnesium case distortion. You're not in CA, but 1974 is easier to register in CA, since it's exempt from smog testing. For the right price and condition, I would buy it. Just figure a future rebuild or engine swap into your offer. Then drive the piss out of it until you have to rebuild it. Which you may never do if you don't drive it pretty frequently.
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1973.5 911T with RoW 1980 SC CIS stroked to 3.2, 10:1 Mahle Sport p/c's, TBC exhaust ports, M1 cams, SSI's. RSR bushings & adj spring plates, Koni Sports, 21/26mm T-bars, stock swaybars, 16x7 Fuchs w Michelin Pilot Sport A/S 3+, 205/55-16 at all 4 corners. Cars are for driving. If you want art, get something you can hang on the wall! |
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11-13-2025, 04:24 PM
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Registered
Join Date: Jan 2003
Location: Langley,B.C.
Posts: 12,043
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Quote:
Was the test done with fuel pump relay removed, battery charger connected and WOT? Cheers
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Turn3 Autosport- Full Service and Race Prep www.turn3autosport.com 997 S 4.0, Cayman S 3.8, Cayenne Turbo, Macan Turbo, 69 911, Mini R53 JCW , RADICAL SR3 |
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11-16-2025, 07:47 PM
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1973 Porsche 911T
1971 Porsche 911S
1969 and 1986 Porsche Targa and Coupe 1720cc and 3
'83 Targa
