I read a lot about timing curves and vacuum advance. I am helping my brother tune his 67 Chevelle. It is an 8.5 to 1 468 with a voodoo hyd ft cam and msd vacuum distributor.Idle vacuum at 14hg. I set the init at 18 at 900 rpm and black bushing gives another 16. When I plug in the vacuum can it jumps to 48. It does not ping but at light acceleration the vacuum doesn't drop off much more than 4 or 5 hg. I am concerned about all that advance as he rows through the gears stop light to stop light. Seems to run fine and no pinging at WOT or cruise surging. I set initial down to 10 and it had a flat spot off idle. Am I doing this right?
street performance timing curve
blown496
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I read a lot about timing curves and vacuum advance. I am helping my brother tune his 67 Chevelle. It is an 8.5 to 1 468 with a voodoo hyd ft cam and msd vacuum distributor.Idle vacuum at 14hg. I set the init at 18 at 900 rpm and black bushing gives another 16. When I plug in the vacuum can it jumps to 48. It does not ping but at light acceleration the vacuum doesn't drop off much more than 4 or 5 hg. I am concerned about all that advance as he rows through the gears stop light to stop light. Seems to run fine and no pinging at WOT or cruise surging. I set initial down to 10 and it had a flat spot off idle. Am I doing this right?
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I like the timing curve you had at first 18* initial. Pluggin in the vaccum advance always jumps the timing way up, that's what it is designed to do. Is it an adjustable vacuum can? Are you looking at a vacuum gauge while driving? Won't see full vacuum any other way.
If you are running the black bushing and you set your initial at 18* you should have 36* total timing with the vacuum advance disconnected. Most MSD vacuum advance cans add 10 distributor degrees of timing with equates to 20 crankshaft degrees. With the vacuum advance employed you COULD have a total of 56 degrees during a high vacuum cruise situation.
Did you plug the vacuum advance into ported or manifold vacuum? What springs are you using on the centrifugal advance weights?
Did you plug the vacuum advance into ported or manifold vacuum? What springs are you using on the centrifugal advance weights?
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If it doesn't ping, it's not too much timing.
The engine does not whup put a timing light and decide on that basis, whether it's "good" or not. It KNOWS and will tell you in no uncertain terms if it doesn't like it.
Don't outsmart yourself over lights, marks, and "specs". Keep it simple.
In the words of one of The Great Ones in an entirely different field, paraphrased for use with cars:
"If it RUNS good, it IS good".
The engine does not whup put a timing light and decide on that basis, whether it's "good" or not. It KNOWS and will tell you in no uncertain terms if it doesn't like it.
Don't outsmart yourself over lights, marks, and "specs". Keep it simple.
In the words of one of The Great Ones in an entirely different field, paraphrased for use with cars:
"If it RUNS good, it IS good".
Stick with 18* base --48* @ cruze with the vacuum can is just fine if no ping.
Heck I would even add another 2-4 * out of the can if no ping.
My engine will slight ping in 4th, light load if I dont limit the V- can to 8- 10*.
A lot of our muscle cars factory stock distributors had around 52* total timing with vac advance @ cruz.
Heck I would even add another 2-4 * out of the can if no ping.
My engine will slight ping in 4th, light load if I dont limit the V- can to 8- 10*.
A lot of our muscle cars factory stock distributors had around 52* total timing with vac advance @ cruz.
I have a hard time biting this one off and chewing it. This advice reminds me of the, " keep advancing the timing until it kicks back when starting then back it off a little and run it!"
Use a good timing light and know whats exactly going on with the timing.
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I have an even HARDER time biting this one off and chewing it.
First: a timing light is only as accurate as the "marks". I wonder if the OP (or 99.99% of all the other people that post stuff like this, including yourself) has verified those?
Second: EVEN IF the "marks" are "accurate", a light only tells you where the timing IS. It does not tell you where it SHOULD BE. It'd be a little bit like having a GPS with no terrain or road database, along with even a compass, in an unfamiliar place a blind fog; sure, you know EXACTLY where you are, and even which way you are facing, but you still have not the vaguest clue which way to go.
Third: the engine knows AHELLUVALOT BETTER than you ever will, what makes itself run right. If you are A TOTAL FORKING GENIUS, and you get all your "calculations" and whatever all else EXACTLY RIGHT, the best you can do, is to agree with WHAT THE ENGINE WANTS. Which, if you just pay attention to the engine, it will TELL YOU, without having to rely on geniosity. See the last sentence in my post.
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Absolutely, ALWAYS.
So, how about the OP's brother's motor?
Does your mark tell you where the timing SHOULD BE? If it does, you need to go into business selling those, because nobody else's does that, but it SURE WOULD be handy. Most of us, the best we can do, once we adopt a dose of humility and get past the idea that were such brainiacs that we somehow "know" before we start tuning where the "best" timing "should be", is to tweeeek on it until the engine runs THE BEST IT CAN.
I used a degree wheel and positive stop TDC setting up the cam. I always ran 18 and 18 with light springs even on my blown 496 but have not used VA for years on anything. I was just surprised that it ran so well with it hooked up and no ping. I will build a travel stop and trim it to 10 degrees on VA and see how it goes. Don't want to have an oops with his motor. I appreciate the input. By the way.....my bro set the timing by ear when he fired it up the first time with my old 85551 in it. His initial was 40. I spent too many years drag racing to guess at any of this. That is why I ask so many questions. These forums give some pretty good insight sometimes.
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Here is a tip if you ever have a chance to put it on a Dyno.
Find at what wide open throttle timing the engine makes it's best power. Say it's 36 degrees.
Unplug the vacuum advance and time the engine where the mechanical advance is all the way in at 36 degrees. Then re-plug the vacuum advance and forget about it.
In all honesty, with just 8.5 to 1, I doubt detonation will be much of a problem, unless you let it run hot.
We have all been where you are. There is a distinct line drawn in the sand about vacuum advance. Some very reputable engine men will tell you to do away with it, for the very reasons that concern you. Other just as reputable engine men will say it is a necessary incredient for good street performance. From my personal experience with my 540 incher, (9.5 to 1), vacuum advance that sets the timing at about 46-48 degrees at cruise speed is just right.
Find at what wide open throttle timing the engine makes it's best power. Say it's 36 degrees.
Unplug the vacuum advance and time the engine where the mechanical advance is all the way in at 36 degrees. Then re-plug the vacuum advance and forget about it.
In all honesty, with just 8.5 to 1, I doubt detonation will be much of a problem, unless you let it run hot.
We have all been where you are. There is a distinct line drawn in the sand about vacuum advance. Some very reputable engine men will tell you to do away with it, for the very reasons that concern you. Other just as reputable engine men will say it is a necessary incredient for good street performance. From my personal experience with my 540 incher, (9.5 to 1), vacuum advance that sets the timing at about 46-48 degrees at cruise speed is just right.
So my original issue was set initial at 18 based on total mechanical timing all in at 2700 for a total of 36. Then when I plug in the vacuum advance to manifold idle timing goes to 28. Now we add in mechanical and at 2700 it is at 46. At wot it would be right on at 36. When I set it up with 8 init and add VA it is 18 at idle but total mechanical is only 24 and cruise is 34. So why couldn't I use the silver bushing and bring in 25 mechanical? Then set init at 11 and idle at 21 with VA. Then wot would be 36 and cruise 46. Seems like there are several ways to go about this with all the MSD kit parts.
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