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Old 06-24-2009, 12:29 PM
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Timing curve recommendation

I am trying to plan a timing curve prior to lite-off on my engine. This is a 383 with a 60,000V HEI (M.I.C.), no MSD, single pattern Hydraulic roller 224*@.050, 9.1:1 compression with older (just prior to the eliminator series) AFR heads. I am fortunate to have a distributor machine, so I can calibrate whatever seems right. Based on compression and cam timing (I suppose) BG Tech recommended 14* initial timing. My distributor curently produces 19-20* mechanical advance, all in at about 3100-3200 RPM. Vacuum too, of course, which I seem to recall is about 14* total. (Don't recall the signal levels for the vacuum can.)

As you'd probably guess, the primary use here is as a street engine, but I'll race it (at Infineon) at least once, someday. This all seems to be pretty close to where I should be, I think. Would any of you experts recommend a change, and if so, what change do you recommend and why?

Pat

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Old 06-24-2009, 03:12 PM
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You could safely go with what you've described:
  • 14 initial
  • 20 mechanical (you will want to bring this in earlier, most likely- but is fine for first start-up)
  • 14 degrees of vacuum advance (might be a bit much)
The vacuum advance might need tuning once you start driving it. Just keep tabs on any surge at cruise or ping at tip in from cruise.
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Old 06-24-2009, 08:04 PM
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Maybe I'm stupid , but , heres something that puzzles me.
You have vac advance adding timing at low RPM and losing timing at higher rpm.
Then you have centrifical that adds the timing as the rpm's increase.
The two kinda cancell each other out ,if not make your timing jump around.
Mild to wild i've been running 36 locked and a adjustable vacuum advance if the engine , convertor combo needs it.
Seems simple and has worked so far.
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Old 06-24-2009, 09:02 PM
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Quote:
Originally Posted by upstand2
You have vac advance adding timing at low RPM and losing timing at higher rpm.
This is wrong. Vacuum advance adds timing when vacuum in the intake is high. This can be at any engine RPM, as long as the throttle isn't WFO. If the engine is under a load, vacuum drops and the vacuum advance drops timing. And vice versa.

Without going into some long, drawn out dissertation, locking in 36 degrees of timing has its drawbacks.
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Old 06-24-2009, 10:21 PM
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I agree with Colbalt327. That is a good initial curve. When you are done I would start the mech about 250 rpm above idle and depending on rear gear finish the mech just below cruise rpm. 12 to 14 initial with 34 to 36 total works well when no vac advance is used. If you are going to use vac advance go for 6 in the vac advance and 16 mech. I would run the vacuum on a ported source.
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Old 06-25-2009, 10:05 AM
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Cobalt327 -- I agree that the Vacuum Advance (14*) might be a bit much, but it is adjustable. I cannot recall it's initial and Full retard vacuum points (in hg). I expect this engine to idle with about 15 in hg. Do you have a recommendation for intial full retard vacuum levels?

Blues 54, I was planning to run this with Manifold vacuum. The car has no smog requirements that I know of. Even here in CA. Except, maybe, a PCV Valve, which it does have. What leads you to recommend Ported Vacuum for this application?

The carb, BTW, is a very early (read, no idle-eze) 750 CFM, Vac Sec Street Demon.

Thanks both of you for your inputs.

Pat
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Old 06-25-2009, 10:20 AM
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Good Morning. I choose ported vacuum as it is drawn off of the venturi so there is no advance until the throttle blades are opened. When shutting down the engine it cuts some advance to make it easier to stop. When under part throttle acceleration the vacuum advance is in play to help with low end power. When using manifold vacuum, vacuum drops during initial acceleration which can cause a power drop off. Sometimes it is appropriate to use manifold vacuum as it will allow for timing retard during acceleration and will possibly stop detonation in a motor that is right on the edge of too much timing for that engine.
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Old 06-25-2009, 11:52 AM
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Quote:
Originally Posted by PatM
I am trying to plan a timing curve prior to lite-off on my engine. This is a 383 with a 60,000V HEI (M.I.C.), no MSD, single pattern Hydraulic roller 224*@.050, 9.1:1 compression with older (just prior to the eliminator series) AFR heads. I am fortunate to have a distributor machine, so I can calibrate whatever seems right. Based on compression and cam timing (I suppose) BG Tech recommended 14* initial timing. My distributor curently produces 19-20* mechanical advance, all in at about 3100-3200 RPM. Vacuum too, of course, which I seem to recall is about 14* total. (Don't recall the signal levels for the vacuum can.)

As you'd probably guess, the primary use here is as a street engine, but I'll race it (at Infineon) at least once, someday. This all seems to be pretty close to where I should be, I think. Would any of you experts recommend a change, and if so, what change do you recommend and why?

Pat
I think this is somewhat of a situation where "the cart is ahead of the horse".
I don't see that it's all that important to dial in a curve with a lot of rocket science at initial start up; as once you can put the engine on the road, assuming you don't have a dynamometer to go with the distributor machine, then you'll work to dial the advance in to what the engine likes given how hard it has to work for what it's installed in when its driven. This is a lot of work to do and can't be done till you're on the street.

The timing numbers you included seem a reasonable place to start up with, they're fairly conservative and for an initial firing that's a good place to get things going. Till you get some break-in time on it, get a feel for it including how it sounds under various loads, it's good to be conservative with the timing.

Later in the first week or so; you can begin to experiment with how much timing and where the engine likes it. Keep in mind that vacuum advance has to be coordinated with the centrifugal. The vacuum advance is high when the manifold vacuum is high and its low when manifold vacuum drops as the throttle is opened. Centrifugal is strictly dependent upon RPM. There is a point where both may be active and the engine can be over advanced in certain situations. Your job in putting the right curves into both the vacuum and centrifugal systems will be to identify those points and make adjustments to keep the cylinders slightly under the detonation limit.

Both vacuum and centrifugal advance tuning involves how many degrees and when (or rate of advance). Your starting base of 14 degrees and having 20 in the centrifugal gives a total of 34, is for a good burning chamber a pretty good place to start. If the vacuum can is delivering 14 degrees that means at idle, assuming the connection isn't to ported vacuum, the total advance will be 28 degrees with the cam and compression combination you have you'll probably need every bit of that as in my opinion your compression is low against the cams duration of 224 at .050 inch. The LSA will have something to say about that, a tight LSA would like more SCR while a wide LSA would be more effective with the SCR you have. The other big players in all this are the vehicle weight and the gearing as this establishes the load on the engine which in the end will define the detonation limit. Yes that limit changes with the gear the vehicle is running in, which is why modern computer selected advance is so much better than vacuum/mechanical systems as these vary the advance to the engine load rather than simply RPM or manifold vacuum.

With your combination of cam and compression, I'd highly recommend a multiple spark system, as this will greatly reduce low speed miss-fires which engines with long duration cams and low compression suffer from.

Bogie
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Old 06-25-2009, 01:52 PM
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I would use 16 initial mechanical and leave the distributor curve as is.

then plug the 14 degree vacuum advance into a manifold source. This will make 30 degrees at idle (16 + 14 = 30).

then at WOT, 16 initial plus 20 will make 36 degrees total which is a good number for AFR heads (fast burn) and a 4.030 bore.

then while driving down the highway, total timing will be: 16 initial + 14 vacuum + 20 mechanical = 50 degrees, which is perfect for good mpg.

Total timing will depend on what gears you have (what rpm the engine is running down the highway) and at what rpm the distributor makes full timing advance.

If it detonates (which I doubt with only 9:1 cr, 224 degree cam, and AFR's), then use stronger advance springs. You could run 10:1 with that setup with 91 octane. For now, I bet you can use 87.
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Old 06-25-2009, 03:31 PM
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Quote:
Originally Posted by PatM
Cobalt327 -- I agree that the Vacuum Advance (14*) might be a bit much, but it is adjustable. I cannot recall it's initial and Full retard vacuum points (in hg). I expect this engine to idle with about 15 in hg. Do you have a recommendation for intial full retard vacuum levels?
I really can't hazard a guess as to the exact points, there's just no way to estimate it w/all the different variables (gasoline quality, jetting/carb tune, vehicle weight, rear gear ratio/tranny splits and if OD, cam timing, DCR, etc.), and even w/all that info, you'd be surprised at how differently one engine can behave from the next- even w/very similar set-ups.

I suggest that you start w/the vacuum advance can where it's at (obviously ) and slowly bring it in sooner and drop off later until it's maxed out, then regress back to a safe/sane point where you have no ping.

You can dial both the vacuum advance and initial/mechanical curve in after you've broken the engine in and are ready to hit the road w/it, but the set-up you've outlined in your first post is just fine for that purpose.
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