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Join Date: Sep 2016
Posts: 47
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Camshaft overlap reading
I'm comparing some different camshaft specifications but I'm struggling to understand how much overlap this particular cam from Cat Cams has...
 Any kind and knowledgeable person out there who could help me out? Should I read it 15+8 degrees? Thanks!   camshaft details | CAT CAMS performance camshafts |
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05-02-2019, 04:33 AM
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Join Date: Oct 2006
Location: MYR S.C.
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if you are comparing you have to compare at the same amount of lift
some will give the specs at zero lash and/or at 1mm those are for 1mm. yes, 15+8 for overlap
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05-02-2019, 04:43 AM
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Thanks!
From what I've been able to read the general advice is to not have more than 10 degrees of overlap when you run CIS injection. So this particular cam has 23 degrees of overlap which is way over that. |
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05-02-2019, 05:07 AM
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It's a 914 ...
Join Date: Jan 2008
Location: Ossining, NY
Posts: 4,774
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I think the overlap is 15 degrees at 1 mm. I don’t see 23 degrees from this chart. Looking at the specs, they would be a sporty cam for a CIS motor but not necessarily unworkable. What is the application, I.e. daily driver. street sport, track, etc?
I’ll be curious what others think. Scott |
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05-02-2019, 07:03 AM
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This cam has been recommended by a local engine builder but I was worried it might be too aggressive. After some comparing it seems that perhaps it is not as far away from a 964 or 20/21 cam as I thought.
The engine will go in my Carrera 2.7 RS Lightweight replica based on a 78 SC. It will be a weekend driver, mostly street but occasionally track. The 3.0 CIS engine will be fully rebuilt with the following mods: 10.5:1 Wössner pistons SSI heat exchangers and freeflow muffler MSD ignition (still single spark though) |
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05-02-2019, 07:17 AM
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Join Date: May 2008
Location: Grapevine, TX
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Quote:
the 15 alone is the degrees BTDC of the intake valve open at 1mm you then have to add on the 8 degrees ATDC that occurs before the exhaust valve has closed down to 1mm, so the total duration of valve overlap with 1mm minimum opening is 23 degrees. |
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05-02-2019, 07:28 AM
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Join Date: Oct 2006
Location: MYR S.C.
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i think the overlap issues are at idle. you get pulsing back into the CIS.
some with more experience with a track car, CIS and a bigger cam will need to chime in,. I would talk to a cam grinder since you have 10.5 cr your intake closing gets more critical. have you considered EFI or carbs
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86 930 94kmiles [_ _] RUNNING:[__] NOT RUNNING: ____77 911S widebody: SOLD88 BMW 325is 200K+ SOLD 03 BMW 330CI 220K:: [_ _] RUNNING: [__] NOT RUNNING:01 suburban 330K:: [_ _] RUNNING: [__] NOT RUNNING:RACE CAR:: sold |
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05-03-2019, 05:28 AM
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