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GAFB
Join Date: Dec 1999
Location: Raleigh, NC, USA
Posts: 7,842
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Weird cam timing measurements on '76 2.7S
Checking a rebuilt longblock tonight, and got an unexpected timing measurement on the cams. They both measured out at 1.35mm lift at TDC overlap. My workshop manual says these should be anywhere from 0.40 to 0.54mm. The cam stampings (143 left and 144 right) are confirmed, these should be the correct cams. Measurements taken with PFTs installed; with idlers pinned/chains tightened, measurement drops about 0.1mm.
Is this some kind of secret cam advance trick the builder used to magically create HP, or should I go ahead and re-time these?
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Several BMWs |
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02-18-2007, 06:17 PM
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Registered
Join Date: Nov 2001
Location: Central Kentucky
Posts: 729
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Hey Dave, wonder if the builder had the original cams redone...seems pretty odd
btw..got the tranny parts..thanks!
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Richard Lane 1978 911SC 1973 "Clean" RS clone |
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02-18-2007, 06:37 PM
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Join Date: May 2003
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That setting would be correct for an SC cam. Maybe they were reground like Richard pointed out.
-Andy
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72 Carrera RS replica, Spec 911 racer |
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02-18-2007, 08:15 PM
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Join Date: Feb 2004
Location: California
Posts: 926
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Measure the full valve lift. 2.7 S cis cams should be .401" and .346" with lash. If the cams were ground to SC specs, you will see close to .450" intake and .390" exhaust.
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John Dougherty Dougherty Racing Cams |
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02-18-2007, 09:22 PM
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GAFB
Join Date: Dec 1999
Location: Raleigh, NC, USA
Posts: 7,842
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John,
Actually I already measured the full lift, I should have posted that too. 8.47mm which equals the .346" you mentioned. Looks decisively like I have stock 2.7 CIS cams and wrong timing spec. No worries, I can retime them no problem. Any recommendations on where in the range spec to set them? 0.40 to 0.54 is quite a bit of room to work.
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Several BMWs |
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02-18-2007, 09:37 PM
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Location: California
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.40- .54 is less than 1/2 of a degree spread. I would shoot for .54
btw, 1.35 is around 2.5 degrees advanced. 8.47mm was on the exhaust side right?
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John Dougherty Dougherty Racing Cams |
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02-18-2007, 09:50 PM
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GAFB
Join Date: Dec 1999
Location: Raleigh, NC, USA
Posts: 7,842
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OK John, are you ready to laugh at the rookie? It turns out when you don't set your dial gauge with enough preload, and use a conversion rate of 24.5 instead of 25.4, you can magically come out to .346" - the right measurement for the wrong side. Duh.
I adjusted the preload, and got .402" on the #1 intake valve. Still a 2.7 cam, still gotta re-time. Darn.
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Several BMWs |
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02-18-2007, 10:44 PM
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Location: California
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Have fun!
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John Dougherty Dougherty Racing Cams |
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02-18-2007, 11:03 PM
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