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Old 08-08-2008, 03:44 PM
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Locking full centrifugal advance and vacuum question

First time poster, long time reader

Couldnt locate any pics online so I have to ask.

I have my 383 stroker locked at 38' total advance. I use an msd 8981 box to retard timing at start and give me about 18' idle timing. Works great as far as I can tell.

Question: Does it matter how I achieve total timing? I can think of many ways to lock the total timing, each of which will put the distributor components in different postions

1) lock the vacuum advance in place, remove the weights but spin out the centrifugal advance and lock that in place. Whatever TOTAL timing is missing can be gained by a slight rotation on the distributor.

2) OR, lock the centrifugal advance and NO advance at all (weights out). Vacuum advance is locked. Any TOTAL TIMING needed can be gained by turning the distributor even more that method 1

3) Lock the centrifugal (either all out or all in), dont turn the distributor at all, instead use an adjustable rod of some sort to gain advance where the vacuum advance is normally connected until total is reached.

Am I wrong?

Anyone got pics of a locked out distributor. The only pics I found online show only the vacuum advance. I have yet to see any with weights removed and acccurately showing weather or not its locked at full centrifugal advance.

I use method 1 and havent noticed any faults for years but perhaps a different method is better for some reason or another??

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Old 08-08-2008, 10:06 PM
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Welcome Bubba.
Is this a race engine?
I can't imagine a street vehicle running right with locked timing. Aircraft engines run with a locked timing system such as you mention, but their requirements are very different.

If this vehicle is street driven, I'd recommend running normal graduated timing.....
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Old 08-08-2008, 11:40 PM
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If your engine has a camshaft with a lot.lot of duration and overlap, and low manifold vacuum at idle, you need a lot of timing at idle. Right up to the point of locking out the advance.
On a GM HEI distributor I simply remove the weights and springs and tie wrap the advance mechanism locked fully advanced using electrical cable ties.
Orient the knots down under so you can get the rotor back on.
Fire it up and reset the now locked timing at max advance (34-36deg).
The timing does not move with rpm.
You can (and should) still employ functional vacuum advance for part throttle cruiseing althou the amount limit and rate of vacuum advance will need to be dialed in.
Crane replacement adjustable vacuum advance. Usually 10 to 15deg of vacuum advance at high speed part throttle cruise is good.
Stock OEM GM HEI distributors typcially have too much (20-25deg) vacuum advance. This needs to be limited down a bit and the rate dialed in to best suite your car and engine.

I find that hyd cams with more than 240@.050 and solid cam with more than 247@.050 duration (expecially with tight LSA {more overlap}) want so much timing at idle for a clean idle you might as well just lock out the mechanical advance.

The increased exhaust reversion (EGR effect) at idle and low rpm of a cam with lots of duration and overlap slows the fuel burn speed down, requiring this extra timing at idle to burn the fuel efficiently.
When you cam up a motor it is no longer a "stock motor" and does not use a stock motor's timing curve.
On radical cammed motors with automatic transmissions this gives a much cleaner, more stable idle in gear than depending on full source vacuum advance to get a good idle.
If the motor is hard to start when hot with locked timing, install a start retard box "starter saver" or install a simple spark power interupt switch to kill the spark while first cranking over the hot motor. Get 'er cranking and throw the switch and it will fire easily without starter strain.

If your motor's camshaft falls somewhere in between a stock camshaft and a racey radical cam, it can benefit from increased static timing at idle (16 to 24deg) {for your typical cam with 221 to 235@.050 duration}
This requires that the mechanical advance travel be limited to avoid over advanced mechanical timing at high rpm/max advance.
This requires modification of the mechanical advance stop limit.
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Old 08-09-2008, 09:19 AM
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I ran a locked distributor on my 454 with 8.7:1 and a 218/224 cam. Worked fine, but thats partly because of the inefficient 049 heads I used.

I locked mine by removing the weights and then drilling through the top plate and advance plate in two places, then using a #10 machine screw to lock them together. You'll have to reset the endplay on the drive end, but that's a good way to lock it out if you want to change it.

I've also seen guys braze them or tack weld them.
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Old 08-09-2008, 02:35 PM
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Great responses.

The cam is pretty radical if you ask me and its a street driven weekend car. 236/246 cam if I remember. Any idle under 20' is choppy.

I did mention that I use an MSD box that has a built in 20' retard at startup so I've never had issues starting. Idle timing is also adjusted via the MSD box so I can idle at lower than TOTAL. The box does require LOCKED advance.

Im wondering how itd work with the vac though? I know that the box deducts from my TOTAL timing. Like, if I have 36 total, and dial the box to give me 20' idle its deducted 16'. But now if I were to turn the distributor and give MORE advance the box will also increase my idle. So, that should imply itd work with vac advance attached.

And, is it correct that at full highway speed the timing is TOTAL plus the vac? I thought vac was only "in" at idle, off at highway so how can it add additional timing during highway?

EDIT: The msd 8981 box I have can only lower the TOTAL timing upto 20' for an idle setting (36' total means I can get idle timing at 16' and higher only). So if I had a vac advance on there, at idle, wouldnt the vac increase the total way way over 36? Meaning, the msd would be forced to raise my idle time the preset amount PLUS the vac advance. Anyone follow me?

Last edited by bubbahotep; 08-09-2008 at 04:14 PM. Reason: learned something
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Old 08-09-2008, 04:15 PM
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Vacuum advance works at part throttle cruise also. As the throttle is opened more the vacuum drops as does the vacuum to the vacuum advance canister.
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Old 08-09-2008, 04:51 PM
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Quote:
Originally Posted by SSedan64
Vacuum advance works at part throttle cruise also. As the throttle is opened more the vacuum drops as does the vacuum to the vacuum advance canister.
Vac drops at part throttle so doesnt the vac advance NOT advance at partial throttle? It advances at idle (its full amount) and then at WOT theres no vac at all. Correct? I was always taught that VAC advance never gets added in to total timing so the 52* advance doesnt exist?
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Old 08-09-2008, 05:07 PM
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Quote:
Originally Posted by bubbahotep
Vac drops at part throttle so doesnt the vac advance NOT advance at partial throttle? It advances at idle (its full amount) and then at WOT theres no vac at all. Correct? I was always taught that VAC advance never gets added in to total timing so the 52* advance doesnt exist?
Vacuum advance is highest when cruising with the throttle open just enough to keep your speed up. Or under hard decelleration. If this is wrong someone will correct me.
Total advance = Initial + Mechanical + Vacuum, unless you're not using vacuum advance then just I + M.
Vacuum doesn't get included with Initial timing and neither does Mechanical.
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Old 08-09-2008, 05:46 PM
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Most street cars have mechanical AND vac advance. (best drivability and mileage)

Most drag cars have mechanical and NO vac advance. (not enough advance at WOT)

Most dirt track and ALL nascar motors have NO mechanical and NO vac advance. (high RPM ALL the time, most WELD everything up so theres no chance of it moving)

As far as the vac advance......with it hooked to the manifold you get high advance at cruse and part throttle, but lesser advance under hard throttle.

With it hooked to the port on the carb you get no advance at cruse and more and more advance as the throttle plates are opened, however under hard throttle there is little to no vac advance.

I have found better mileage with manifold and better power with ported, but my 350 camaro does not like ported and runs better on the manifold
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Old 08-09-2008, 06:05 PM
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I hate to get off my original post...but, show me where Im going wrong here:

If any car is in the driveway at idle. Youd set your idle time, then check your mechanical advance works at the higher rpm (rev the engine, watch for timing advance). When done, you reconnect the vac advance and your IDLE timing will go up a bit because you have vacuum and the vac "advance" puts in a little more advance like when you're stuck in traffic at a light. Something about saving gas.

If my car is still in the driveway WITH the vac advance connected, I rev my engine rpm up. As I do so, vacuum begins to drop so I lose "vac advance" but my weights take over. If Im in the driveway and have the car revving at 2500rpm, Id see maybe 36 timing or something close. No where near 52*. Isnt that the SAME as driving on the highway?? Where is the additional vac advance being added in?

The reason for my confusion is straight from conversations with MSD numerous times. Im no master of timing, just repeating what I was told and that is "total timing never goes above 36*, ever, regardless of vac canister or not".

Can I include a link to an article (one of many) regarding vac advance NOT being involved in total timing? http://rockridgefarm.com/vettdoc/Timing_101.pdf
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Old 08-09-2008, 06:13 PM
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TIMING AND VACUUM ADVANCE 101

The most important concept to understand is that lean mixtures, such as at idle and steady highway cruise, take longer to burn than rich mixtures; idle in particular, as idle mixture is affected by exhaust gas dilution. This requires that lean mixtures have "the fire lit" earlier in the compression cycle (spark timing advanced), allowing more burn time so that peak cylinder pressure is reached just after TDC for peak efficiency and reduced exhaust gas temperature (wasted combustion energy). Rich mixtures, on the other hand, burn faster than lean mixtures, so they need to have "the fire lit" later in the compression cycle (spark timing retarded slightly) so maximum cylinder pressure is still achieved at the same point after TDC as with the lean mixture, for maximum efficiency.

The centrifugal advance system in a distributor advances spark timing purely as a function of engine rpm (irrespective of engine load or operating conditions), with the amount of advance and the rate at which it comes in determined by the weights and springs on top of the autocam mechanism. The amount of advance added by the distributor, combined with initial static timing, is "total timing" (i.e., the 34-36 degrees at high rpm that most SBC's like). Vacuum advance has absolutely nothing to do with total timing or performance, as when the throttle is opened, manifold vacuum drops essentially to zero, and the vacuum advance drops out entirely; it has no part in the "total timing" equation.

At idle, the engine needs additional spark advance in order to fire that lean, diluted mixture earlier in order to develop maximum cylinder pressure at the proper point, so the vacuum advance can (connected to manifold vacuum, not "ported" vacuum - more on that aberration later) is activated by the high manifold vacuum, and adds about 15 degrees of spark advance, on top of the initial static timing setting (i.e., if your static timing is at 10 degrees, at idle it's actually around 25 degrees with the vacuum advance connected). The same thing occurs at steady-state highway cruise; the mixture is lean, takes longer to burn, the load on the engine is low, the manifold vacuum is high, so the vacuum advance is again deployed, and if you had a timing light set up so you could see the balancer as you were going down the highway, you'd see about 50 degrees advance (10 degrees initial, 20-25 degrees from the centrifugal advance, and 15 degrees from the vacuum advance) at steady-state cruise (it only takes about 40 horsepower to cruise at 50mph).

When you accelerate, the mixture is instantly enriched (by the accelerator pump, power valve, etc.), burns faster, doesn't need the additional spark advance, and when the throttle plates open, manifold vacuum drops, and the vacuum advance can returns to zero, retarding the spark timing back to what is provided by the initial static timing plus the centrifugal advance provided by the distributor at that engine rpm; the vacuum advance doesn't come back into play until you back off the gas and manifold vacuum increases again as you return to steady-state cruise, when the mixture again becomes lean.

The key difference is that centrifugal advance (in the distributor autocam via weights and springs) is purely rpm-sensitive; nothing changes it except changes in rpm. Vacuum advance, on the other hand, responds to engine load and rapidly-changing operating conditions, providing the correct degree of spark advance at any point in time based on engine load, to deal with both lean and rich mixture conditions. By today's terms, this was a relatively crude mechanical system, but it did a good job of optimizing engine efficiency, throttle response, fuel economy, and idle cooling, with absolutely ZERO effect on wide-open throttle performance, as vacuum advance is inoperative under wide-open throttle conditions. In modern cars with computerized engine controllers, all those sensors and the controller change both mixture and spark timing 50 to 100 times per second, and we don't even HAVE a distributor any more - it's all electronic.

Now, to the widely-misunderstood manifold-vs.-ported vacuum aberration. After 30-40 years of controlling vacuum advance with full manifold vacuum, along came emissions requirements, years before catalytic converter technology had been developed, and all manner of crude band-aid systems were developed to try and reduce hydrocarbons and oxides of nitrogen in the exhaust stream. One of these band-aids was "ported spark", which moved the vacuum pickup orifice in the carburetor venturi from below the throttle plate (where it was exposed to full manifold vacuum at idle) to above the throttle plate, where it saw no manifold vacuum at all at idle. This meant the vacuum advance was inoperative at idle (retarding spark timing from its optimum value), and these applications also had VERY low initial static timing (usually 4 degrees or less, and some actually were set at 2 degrees AFTER TDC). This was done in order to increase exhaust gas temperature (due to "lighting the fire late") to improve the effectiveness of the "afterburning" of hydrocarbons by the air injected into the exhaust manifolds by the A.I.R. system; as a result, these engines ran like crap, and an enormous amount of wasted heat energy was transferred through the exhaust port walls into the coolant, causing them to run hot at idle - cylinder pressure fell off, engine temperatures went up, combustion efficiency went down the drain, and fuel economy went down with it.

If you look at the centrifugal advance calibrations for these "ported spark, late-timed" engines, you'll see that instead of having 20 degrees of advance, they had up to 34 degrees of advance in the distributor, in order to get back to the 34-36 degrees "total timing" at high rpm wide-open throttle to get some of the performance back. The vacuum advance still worked at steady-state highway cruise (lean mixture = low emissions), but it was inoperative at idle, which caused all manner of problems - "ported vacuum" was strictly an early, pre-converter crude emissions strategy, and nothing more.

What about the Harry high-school non-vacuum advance polished billet "whizbang" distributors you see in the Summit and Jeg's catalogs? They're JUNK on a street-driven car, but some people keep buying them because they're "race car" parts, so they must be "good for my car" - they're NOT. "Race cars" run at wide-open throttle, rich mixture, full load, and high rpm all the time, so they don't need a system (vacuum advance) to deal with the full range of driving conditions encountered in street operation. Anyone driving a street-driven car without manifold-connected vacuum advance is sacrificing idle cooling, throttle response, engine efficiency, and fuel economy, probably because they don't understand what vacuum advance is, how it works, and what it's for - there are lots of long-time experienced "mechanics" who don't understand the principles and operation of vacuum advance either, so they're not alone.

Vacuum advance calibrations are different between stock engines and modified engines, especially if you have a lot of cam and have relatively low manifold vacuum at idle. Most stock vacuum advance cans aren’t fully-deployed until they see about 15” Hg. Manifold vacuum, so those cans don’t work very well on a modified engine; with less than 15” Hg. at a rough idle, the stock can will “dither” in and out in response to the rapidly-changing manifold vacuum, constantly varying the amount of vacuum advance, which creates an unstable idle. Modified engines with more cam that generate less than 15” Hg. of vacuum at idle need a vacuum advance can that’s fully-deployed at least 1”, preferably 2” of vacuum less than idle vacuum level so idle advance is solid and stable; the Echlin #VC-1810 advance can (about $10 at NAPA) provides the same amount of advance as the stock can (15 degrees), but is fully-deployed at only 8” of vacuum, so there is no variation in idle timing even with a stout cam.

For peak engine performance, driveability, idle cooling and efficiency in a street-driven car, you need vacuum advance, connected to full manifold vacuum. Absolutely. Positively. Don't ask Summit or Jeg's about it – they don’t understand it, they're on commission, and they want to sell "race car" parts.
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Old 08-09-2008, 06:20 PM
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inital + mechanical= total mechanical...... add vacuum advance at part throttle hiway cruise speed (manifold vacuum is very high), say 15deg, now you have 51 deg of combined spark timing.

From this point if you floor the gas, the manifold vacuum drops to 0 and the vacuum advance drops to 0. WOT. timing would be 36deg. initial+mechanical with no additional vacuum advance.
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Old 08-09-2008, 06:35 PM
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Holder350 - Ive read that post on this website MANY times over the years Thus my confusion since it goes against most of what Ive been told or read.

F-Bird: I understand what you're saying but please show me how Im wrong in this statement: What is the difference between "cruise speed" on the road and the car sitting in the driveway at the same rpm as "cruise"? The engine doesnt know its going 70mph with no load. At 70mph on the open road its the same rpm as sitting in the driveway. No? So timing in the driveway (which never goes over 36*) should be the same at highway too.

I do agree 100% that if you can get the vac advance canister to "advance" at higher rpm you will increase your total timing beyond 36*. But, as rpm goes up your vac goes down so you lose the advance. No?
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Old 08-09-2008, 07:48 PM
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Install a vacuum gauge to the full manifold vacuum port on your car and go for a ride. Watch the gauge at different speeds and engine loads (throttle opening).

The manifold vacuum is not the same while cruising down the road at 70mph as it is while just reving the motor to 2500rpm in neutral (close, but not the same).
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Old 08-09-2008, 07:53 PM
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Quote:
Originally Posted by bubbahotep
F-Bird: I understand what you're saying but please show me how Im wrong in this statement: What is the difference between "cruise speed" on the road and the car sitting in the driveway at the same rpm as "cruise"? The engine doesnt know its going 70mph with no load. At 70mph on the open road its the same rpm as sitting in the driveway. No? So timing in the driveway (which never goes over 36*) should be the same at highway too.
There really is not much diffrence.

your TOTAL timing is base+mechanical without the vac can

so if your base is 8* add in the mechanical advance (roughly another 23*-25*) and you've got your 32* TOTAL.

Now, sitting in the driveway before you start up you've got 8* witch makes starting a breeze, because the plugs are firing long after the pistons reach TDC. Then when the motor fires up and starts making vacuum, the timing jumps up to 23*, thus firing the plugs as close as possible to TDC, making the most of the motors CR and making more power, and idling nicely.

So running down the road at 55 barely into the throttle and the motor running at 1,500 RPM you still only have 23* of timing, BUT as the motor spins faster and faster you need to be in the throttle more and more, thus REDUCING the vacuum in the manifold and reducing the vac advance, but at the same time the mechanical advance is taking over and should be all in by 3,500 making your timing 8*+24*=32*

Clear as mud right?
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