initial vs. total timing
Thread Starter
Drifting
Joined: Jun 2006
Posts: 1,912
Likes: 12
From: Schaumburg (Chicago) Illinois
initial vs. total timing
I read and followed the sticky on timing. So far I have the following results and concerns:
Set dwell to 30 but it fluctuates up to 35 at higher RPM's. (I already have another thread on this)
Set base timing to 4* as per specs for a 68 327 350 hp.
Set total timing to 36 and fortunately it is all in at 3000 rpm.
But now initial timing is 17. That makes my centrifugal advance now 19.
Lars seems to say forget about base as long as you have the total. He does mention at the end of his article that 18 is ideal. That would make my numbers pretty good!
But If I am reading things right I think barry and Hinckley would say that my initial going to 17 is a problem.
I am not sure what to think.
I didn't actually check the timing but I then attache the vacuum line to the can and there was no increase in the idle!!!! When I put my finger on the end of the vacuum line it doesn't seem to suck much.
Any comments are appreciated.
Set dwell to 30 but it fluctuates up to 35 at higher RPM's. (I already have another thread on this)
Set base timing to 4* as per specs for a 68 327 350 hp.
Set total timing to 36 and fortunately it is all in at 3000 rpm.
But now initial timing is 17. That makes my centrifugal advance now 19.
Lars seems to say forget about base as long as you have the total. He does mention at the end of his article that 18 is ideal. That would make my numbers pretty good!
But If I am reading things right I think barry and Hinckley would say that my initial going to 17 is a problem.
I am not sure what to think.
I didn't actually check the timing but I then attache the vacuum line to the can and there was no increase in the idle!!!! When I put my finger on the end of the vacuum line it doesn't seem to suck much.
Any comments are appreciated.
Race Director
Joined: Sep 2004
Posts: 11,874
Likes: 263
From: Arlington Va Current ride 04 vert, previous vettes: 69 vert, 77 resto mod
you have just timed the engine for performance....now just enjoy it. Lars is a timing guru for those of us who were lucky enough to have met him and have him tune our cars, i can tell you if he is leading you down the path follow him......
did you change your advance springs ?
there is both ported and direct vacuum. do you have a big cam in the engine or is it stock?
did you change your advance springs ?
there is both ported and direct vacuum. do you have a big cam in the engine or is it stock?
Last edited by bobs77vet; Sep 13, 2009 at 09:07 PM.
Thread Starter
Drifting
Joined: Jun 2006
Posts: 1,912
Likes: 12
From: Schaumburg (Chicago) Illinois
It is not stock but not too big either. I made sure of that. I am pulling the vacuum off the carb from what I believe is the stock port. My engine is a 383 stroker.
Race Director
Joined: Sep 2004
Posts: 11,874
Likes: 263
From: Arlington Va Current ride 04 vert, previous vettes: 69 vert, 77 resto mod
you need to read lars article on vacuum cannisters......on my 350 with a com cams xe268h i had to go to a vacuum cannister that pulls full vacuum at 6 " of mercury
Thread Starter
Drifting
Joined: Jun 2006
Posts: 1,912
Likes: 12
From: Schaumburg (Chicago) Illinois
I'll read it but Lars rebuilt the distributor. BUT, I don't think he knew it was being upgraded to a 383 stroker, if that makes a difference.
Race Director
Joined: Sep 2004
Posts: 11,874
Likes: 263
From: Arlington Va Current ride 04 vert, previous vettes: 69 vert, 77 resto mod
i dont know what the difference the stroke makes in relationship to timing but i am sure Lars does send him an email and ask him.
Thread Starter
Drifting
Joined: Jun 2006
Posts: 1,912
Likes: 12
From: Schaumburg (Chicago) Illinois
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Le Mans Master
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From: Seattle WA
St. Jude Donor '14
The advance being discussed is mechanical only - so has nothing to do with how the vacuum can is hooked up.
17 degrees initial is on the high side but as noted it's in an acceptable range.
17 degrees initial is on the high side but as noted it's in an acceptable range.
Drifting
Joined: Feb 2005
Posts: 1,879
Likes: 38
From: Los Gatos CA
In 68, Chevy had a lot of mechanical advance built into their distributers. My 68 L36 has 16* in the distributer, which is 32 at the crank. This assumes a stock distributer that is working correctly... If you installed a lighter advance spring the distributer may be advancing at idle speed. Just a possibility... By the way, a lot of lead (like 30*) at idle is generally considered a good thing resulting in good off-idle transition. Sounds like you have a good curve.
Thread Starter
Drifting
Joined: Jun 2006
Posts: 1,912
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From: Schaumburg (Chicago) Illinois
What do you think about the big changes in the dwell? 5+ degrees between idle and about 3000 rpm.
Thread Starter
Drifting
Joined: Jun 2006
Posts: 1,912
Likes: 12
From: Schaumburg (Chicago) Illinois
In 68, Chevy had a lot of mechanical advance built into their distributers. My 68 L36 has 16* in the distributer, which is 32 at the crank. This assumes a stock distributer that is working correctly... If you installed a lighter advance spring the distributer may be advancing at idle speed. Just a possibility... By the way, a lot of lead (like 30*) at idle is generally considered a good thing resulting in good off-idle transition. Sounds like you have a good curve.
Race Director
Joined: Sep 2004
Posts: 11,874
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From: Arlington Va Current ride 04 vert, previous vettes: 69 vert, 77 resto mod
changes in dwell.....man this is going back a long time....
i think the dwell reading is the relationship of the points being open to the lobe that they ride on.....as these points start opening fast i would think that there would be some float to them. i suppose the spring tension on the points could become less over time so that the force to close them would be come less so they stay open longer. are they new points?
i just don't recall measureing the dwell at anyting other then at a set rpm .....
i think the dwell reading is the relationship of the points being open to the lobe that they ride on.....as these points start opening fast i would think that there would be some float to them. i suppose the spring tension on the points could become less over time so that the force to close them would be come less so they stay open longer. are they new points?
i just don't recall measureing the dwell at anyting other then at a set rpm .....
Thread Starter
Drifting
Joined: Jun 2006
Posts: 1,912
Likes: 12
From: Schaumburg (Chicago) Illinois
changes in dwell.....man this is going back a long time....
i think the dwell reading is the relationship of the points being open to the lobe that they ride on.....as these points start opening fast i would think that there would be some float to them. i suppose the spring tension on the points could become less over time so that the force to close them would be come less so they stay open longer. are they new points?
i just don't recall measuring the dwell at anything other then at a set rpm .....
i think the dwell reading is the relationship of the points being open to the lobe that they ride on.....as these points start opening fast i would think that there would be some float to them. i suppose the spring tension on the points could become less over time so that the force to close them would be come less so they stay open longer. are they new points?
i just don't recall measuring the dwell at anything other then at a set rpm .....
Thread Starter
Drifting
Joined: Jun 2006
Posts: 1,912
Likes: 12
From: Schaumburg (Chicago) Illinois
Here are my numbers now. I am sure I have a distributor problem and have posted that in another thread so for now I am going to leave things as they are until I get the distributor fixed. But I believe this is good anyway:
Dwell 30 deg.
Base 16 deg.
Total 36 deg. all in at 3000 rpm
advance 33 so vacuum is giving me 17
total 53. I know I need to stay under 52 but until I get the distributor worked out I don't really know exactly where I'm at. And I am not driving the car yet anyway.
Dwell 30 deg.
Base 16 deg.
Total 36 deg. all in at 3000 rpm
advance 33 so vacuum is giving me 17
total 53. I know I need to stay under 52 but until I get the distributor worked out I don't really know exactly where I'm at. And I am not driving the car yet anyway.
Melting Slicks
Joined: Jul 2006
Posts: 2,266
Likes: 478
From: Salmon Arm, BC
This suggestion might not be what your looking for......but as your not rebuilding your car "NCRS" why don't you try an stock style GM HEI. They are easy to work with, give you a hotter spark, easy to upgrade and you don't have to worry about point bounce.......only a suggestion
Thread Starter
Drifting
Joined: Jun 2006
Posts: 1,912
Likes: 12
From: Schaumburg (Chicago) Illinois
This suggestion might not be what your looking for......but as your not rebuilding your car "NCRS" why don't you try an stock style GM HEI. They are easy to work with, give you a hotter spark, easy to upgrade and you don't have to worry about point bounce.......only a suggestion