Original cam specs
Thread Starter
Racer
Joined: Aug 2011
Posts: 429
Likes: 24
From: Charlotte NC
No I have the specs for that, I would like to get an idea what originally came with the base engine. I have 218 duration at .050 lift with this but doesn't idle below 1100 rpm very well. I'd kind of like to know what came with the Mark IV 270hp BB. This info may not be at your finger-tips any more. Thanks.
Dr. Detroit
Joined: Mar 2012
Posts: 10,095
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From: New Braunfels Texas
NHRA list cam specs under the rules section.
http://www.nhraracer.com/content/gen...635&zoneid=132
Jebby
http://www.nhraracer.com/content/gen...635&zoneid=132
Jebby
Le Mans Master
Joined: Mar 2004
Posts: 7,465
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From: Little Rock AR
Specs are available but not very useful. The designs are not comparable to aftermarket cams due to the long slow OEM ramp rates. Best to compare aftermarket cams to aftermarket cams.
Dr. Detroit
Joined: Mar 2012
Posts: 10,095
Likes: 4,026
From: New Braunfels Texas
No I have the specs for that, I would like to get an idea what originally came with the base engine. I have 218 duration at .050 lift with this but doesn't idle below 1100 rpm very well. I'd kind of like to know what came with the Mark IV 270hp BB. This info may not be at your finger-tips any more. Thanks.
Jebby
Thread Starter
Racer
Joined: Aug 2011
Posts: 429
Likes: 24
From: Charlotte NC
218 @ .050 is very mild for a Big Block........it should idle very well. I believe you have other issues. Go through and check timing....set at 36 degrees @ 3000 rpm and let the initial fall wherever it falls. (Do Not go by the GM initial spec for timing). Also check vacuum advance for proper operation and that the mechanical advance mechanism is free and moving as it should.
Jebby
Jebby
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Le Mans Master
Joined: Apr 2001
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From: Was New Orleans but swam to Baton Rouge LA
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Last edited by Sigforty; Dec 30, 2019 at 11:02 AM.
Thread Starter
Racer
Joined: Aug 2011
Posts: 429
Likes: 24
From: Charlotte NC
After you get the timing straight, look for vacuum leaks and carb idle. I have a 229/229 camshaft on a 109 LSA and it idles under 1000 rpms.
https://m.youtube.com/watch?v=MZijJ18MXBA
https://m.youtube.com/watch?v=MZijJ18MXBA
The spark plugs appear to be firing exactly as in the previous engine with the same plugs getting gas fouled at lower rpm. The exact same cylinders so I tend to think it's the cam. Compression is very good all cylinders.
The cam was very popular back in the day they tell me and has .518 lift. But it is old school, and was thinking maybe switching to a modern cam may clean up the low rpm fouling problem?
Le Mans Master
Joined: Apr 2001
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From: Was New Orleans but swam to Baton Rouge LA
Cruise-In IX Veteran
Thanks for the video, that does idle smooth it seems. Only I may not have any glaring issues with timing or vacuum as I just had the engine tuned at a very expensive Classic Speed and Custom shop less than 500 miles ago. They said they would set the timing to the GM setting called for. The engine was just rebuilt at another shop. I did not change the cam because I liked what I had previously. I did drop the compression to 9.5 going with different pistons. I changed the intake from a performer to a RPM which gave it a little more top end but took away bottom end.
The spark plugs appear to be firing exactly as in the previous engine with the same plugs getting gas fouled at lower rpm. The exact same cylinders so I tend to think it's the cam. Compression is very good all cylinders.
The cam was very popular back in the day they tell me and has .518 lift. But it is old school, and was thinking maybe switching to a modern cam may clean up the low rpm fouling problem?
The spark plugs appear to be firing exactly as in the previous engine with the same plugs getting gas fouled at lower rpm. The exact same cylinders so I tend to think it's the cam. Compression is very good all cylinders.
The cam was very popular back in the day they tell me and has .518 lift. But it is old school, and was thinking maybe switching to a modern cam may clean up the low rpm fouling problem?
Thread Starter
Racer
Joined: Aug 2011
Posts: 429
Likes: 24
From: Charlotte NC
Why would they set the timing to stock GM if the cam has been changed? GM timing specs are there more for emissions. A lot of time the timing can be advanced more and it will run better. Having changed the intake it is possible a gasket did not seal properly and there is vacuum leak somewhere.
Le Mans Master
Joined: Sep 2001
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From: Unreconstructed, South Carolina
https://www.gmheritagecenter.com/doc...t-Corvette.pdf
about page 67
Thread Starter
Racer
Joined: Aug 2011
Posts: 429
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From: Charlotte NC
They must know that about factory timing specs and relation to HP, what would the advantages be? A cooler running engine, a richer mixture and better for break-in on a new engine? I best get a timing light and see where it is at, my old light is toast. I'll try bumping up a couple degrees but I doubt that will completely correct the plug fouling on a few cylinders that are a problem.
Thread Starter
Racer
Joined: Aug 2011
Posts: 429
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From: Charlotte NC
AC Delco R45TS for 5 cylinders that are doing well. NGK UR56P for the others that are having some issues. The iridium plugs do help it seems but I have not lowered the idle yet, it is still about 1050 rpm after warming up. Takes about 5 mins this time of year to get warm so I throttle it to keep revs up. It has an auto choke that is disconnected and I might connect for winter driving.
I checked the timing and it is at 16 BTDC which is factory. I'll check with the shop I'm working with about advancing it and see what they say about taking it to 36 BTDC which I see has been recommended on this forum in other threads.
I'm still thinking a cam change would eliminate possibly these issues.
I checked the timing and it is at 16 BTDC which is factory. I'll check with the shop I'm working with about advancing it and see what they say about taking it to 36 BTDC which I see has been recommended on this forum in other threads.
I'm still thinking a cam change would eliminate possibly these issues.
Team Owner
Joined: Oct 2004
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From: altered state
St. Jude Donor '05
run it to 3k and see where your total is bet it runs a lot better. now its retarded low on power and I bet running warn. Thats a lot of your idle problem.
260ish@050 and can idle under 1100 easy. Its timing.
260ish@050 and can idle under 1100 easy. Its timing.
Le Mans Master
Joined: Sep 2001
Posts: 7,739
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From: Unreconstructed, South Carolina
Perhaps there's been a mix-up/misuse between stated duration of the two Common but Different standards: Advertised versus @0.050" lobe lift ?
e.g. three hydraulic Flat tappets: a relatively mild Comp 260H High Energy duration is BOTH Advertised 260* while 212* @ 0.050" lobe lift ... in a 350, it'll idle good
https://www.summitracing.com/parts/c...make/chevrolet
and several steps hotter Comp 280H Magnum cam duration is BOTH Advertised 280* while 230* @ 0.050" lobe lift ... in a 350, it'll idle fair to rough
https://www.summitracing.com/parts/cca-12-212-2/overview/make/chevrolet
but Many steps hotter is Comp 312H special cam duration is BOTH Advertised 312* while 260* @ 0.050" lobe lift ... in a 350, it's really a race cam: crappy-no idle YMMV
https://www.summitracing.com/parts/c...make/chevrolet
AFAIK, 'Advertised' is usually interpreted as AT either 0.004" OR 0.006" lobe lift. YMMV
Happy, Healthy New Year for all
e.g. three hydraulic Flat tappets: a relatively mild Comp 260H High Energy duration is BOTH Advertised 260* while 212* @ 0.050" lobe lift ... in a 350, it'll idle good
https://www.summitracing.com/parts/c...make/chevrolet
and several steps hotter Comp 280H Magnum cam duration is BOTH Advertised 280* while 230* @ 0.050" lobe lift ... in a 350, it'll idle fair to rough
https://www.summitracing.com/parts/cca-12-212-2/overview/make/chevrolet
but Many steps hotter is Comp 312H special cam duration is BOTH Advertised 312* while 260* @ 0.050" lobe lift ... in a 350, it's really a race cam: crappy-no idle YMMV
https://www.summitracing.com/parts/c...make/chevrolet
AFAIK, 'Advertised' is usually interpreted as AT either 0.004" OR 0.006" lobe lift. YMMV
Happy, Healthy New Year for all
Last edited by jackson; Jan 1, 2020 at 04:50 PM.
Thread Starter
Racer
Joined: Aug 2011
Posts: 429
Likes: 24
From: Charlotte NC
no actually it runs cooler than the previous engine that was running advanced timing. But that is because I switched to the RPM version of intake and had the headers hot coated. That might be why anyway.