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  #16 (permalink)  
Old 03-29-2012, 09:23 AM
v8hed's Avatar
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Custom10... this motor makes 8-9" idle vacuum.

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  #17 (permalink)  
Old 04-04-2012, 11:41 AM
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Quote:
Originally Posted by v8hed
383 with GM HEI distributor (modded with aftermarket module, curve kit, coil and shimmed for end play). Before going to the chassis dyno, I'd set my timing up (with a good timing light) for 34* total. Checked it up to about 4,000rpm and I could see the advance curve adding more timing until around 3,000rpm then it remained static. As I said, I only went up to about 3,500-4,000rpm with the timing light... just enough to see that the advance curve had finished adding timing. On the chassis dyno, we were messing around with the timing (just adding/subtracting a couple of degrees to do some fine tuning). I asked the dyno operator to take it up to 6,000rpm and keep the timing light on the balancer. He said total timing went up to something like 50* by 5,500rpm! We checked with another light and same thing. Couldn't hear any pinging (although the car is v.loud). What I don't understand is how could this possibly be happening? I still don't believe everything is at it seems, since taking timing out resulted in less power. If is was really running up around 50* total timing, there's no way the motor would be making power. I've previously tried experimenting locking the mech advance out and didn't see over 36* (with about 14* initial). Since the holes in the base plate physically limit max advance, what's going on with this wacky timing reading? It had occurred to me that perhaps the ProStreet balancer I'm using could be slipping, but it's pretty new and looks perfectly fine to the naked eye.

Anyone got any other ideas or explanations? If not, my next step will be to try a brand new MSD HEI just to eliminate the distributor from the equation. What about spark scatter or some other kind of effect?

I have been reading up on ignition timing and found there is much confusion on vac advance.I havnt read all you discussion here so you may have covered it already. timing may go to 50 deg in certain instances as I understand it. Not being an xpert on the subject here are some discussions here on hotrodders about timing.

http://rockridgefarm.com/vettdoc/Timing_101.pdf
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Old 04-04-2012, 07:57 PM
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F-BIRD'88 is right on the money...

The HEI distributor does NOT limit maximum centrifugal advance like its non-HEI predecessor. Previously, the max advance was limited by a pin in a slot. The maximum could be limited by adding a bushing to the pin to shorten the travel, but this is for the pre-HEI distributors ('73 ('74?) and earlier). The HEI limits its advance by when the short arm on the flyweight reaches nearly to the center of the oval cam under the springs. HOWEVER, some aftermarket flyweights, have a much shorter arm and will have greater leverage to twist the distributor to a still greater advance after the flyweight appears to have reached maximum advance.

The stock flyweights have worked better for me with stiffer springs to tailor the curve. The stock weights have a longer cam arm and appear to limit more robustly than aftermarket "performance" curve kits.

In any event, F-BIRD'88 has the best solution to add a positive stop to the centrifugal advance with the HEI distributor.
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Old 04-05-2012, 05:38 PM
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Timing your engine

Many years ago I had the opportunity to get hooked up with an old NASCAR mechanic. He never used a timing light! He used a vacuum gauge to set timing. I know it sounds odd, but with his help I learned how to do this. Hook up a good vacuum gauge to the engine, a port near the carburator works best. Start the engine, bring the RPM's up to between 1700 and 2000. The vacuum gauge should be rock steady. If it wavers you need to adjust timing until it becomes steady. Of course if you want to run advanced or retarded timing use a very good timing light and set it to the degree's you want. But the vacuum gauge works and I have been using one ever since.
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  #20 (permalink)  
Old 04-05-2012, 06:00 PM
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Quote:
Originally Posted by dbrow6272
Many years ago I had the opportunity to get hooked up with an old NASCAR mechanic. He never used a timing light! He used a vacuum gauge to set timing. I know it sounds odd, but with his help I learned how to do this. Hook up a good vacuum gauge to the engine, a port near the carburator works best. Start the engine, bring the RPM's up to between 1700 and 2000. The vacuum gauge should be rock steady. If it wavers you need to adjust timing until it becomes steady. Of course if you want to run advanced or retarded timing use a very good timing light and set it to the degree's you want. But the vacuum gauge works and I have been using one ever since.
The problem w/this as I see it is there's no way to know what the total timing is, or at what rpm the mechanical or vacuum advance starts and is all in by, nor the amount of mechanical and vacuum advance.

But other than that...
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Old 04-05-2012, 06:35 PM
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NASCAR engines of the 1980's and 1990's ran fixed, locked timing.

I dont understand setting timing for a racing engine at no load.
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Old 04-05-2012, 08:19 PM
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tIMING

So no load means they set the timing running around the trackk? No seriously, this can be done on a dyno and I'm thinking it probably is although I haven't been near a dyno in just about as many years. I stick by the vacuum method and have used it on many hot rodded engines. Then I have used a light with an advance dial on it to "fine tune" timing. I do not trust all balancers, some yes, but not all. Many times I have found the timing ring on a balancer slipped or plain just improperly installed. Use the vacuum gauge first, check the timing mark to see where the balancer is and go from there.
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Old 04-05-2012, 09:09 PM
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not the answer but feedback

i usually advance the cam then fine it with the distributor ,minor.it would make you lower the timing to hit 34 at 6000 rpm.that would hurt you low.but maybe youll have to retrace youre steps to the point of failure and adjust from there.i run a HEI with sbc vortec 350.dot to dot on the cam on comp cam mother thumper is fine 2-4 degree advance there or go alittle more.so i dont really want to have to do much more than an 1" to 1.5"advance manually on the dis.like the part bout mech advance only affecting low drive to an idle imma keep that in mind for when i next tune.love carbed out monsters and its true fine tuning and constant adjust comes with the territory.good luck with it brother.out
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  #24 (permalink)  
Old 04-05-2012, 09:49 PM
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Huh????? I read that twice and still have no idea on 90% of it.
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  #25 (permalink)  
Old 04-06-2012, 12:14 AM
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Another explanation is called spark scatter, high cylinder pressure causes a higher load on the ignition system which can cause the coil to delay or jump across the plug gap only after the mixture self detonates.

High compression ratios combined with weak stock style ignition systems are a deadly combination, my bet is this phenomena you are seeing only occurs under load above a certain rpm...if so you are getting a clue of your ignition system weakness.

A good test is to physically lock the timing at a high initial like 18 degrees and repeat the test on the dyno, if your still seeing it assume your ignition system isn't up to the task and needs upgrading.
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  #26 (permalink)  
Old 04-07-2012, 02:18 AM
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ignition

no its just if the cam was dialed you have advanced the engine.so the ignition is just tuning the spark timing.i was allways taught the distributor is the moved bout an inch to a inch and a half prior to the spark of the .so if the #1 spark connection on the distributor is moved to spark bout an inch or so of travel before it lines with its own cable headed out to the #1 spark plug.so the spark is fired then by the time it shoots out to the spark plug ,it has occured ,if it went off at the same time of the distributor #1 being lined up to spark plug #1 it would be to late and the engine wouldnt run.i didnt understand the timing in that engine changing as the rpm raised.i allways buy the best i can afford in equipment and follow the instructions and research on you tube and internet and never once ran into that problem.i helped build a 65 mustang we built and now did a chevy 350 vortec 4 bolt main ,1 piece rear seal.just a 350 sbc but never bore to 60 for heat problems,i try to stay at .030 maybe .040 but even that i dont know cause you alter one part to much and then it starts throwing you Off in other areas.so i just port and polish the heads maybe 1mm off the exhaust chambers to wake it up,dont want my headers overlapping a 1.5 or 1 5/8 primariey.i really get technical bout the bowls ill just buy a pair of junk valves and port like any of us would go ahead and do.want he bowls like mirrors,mirrors i tell ya.lol .hope that cleared it up a little.sorry for confusing the masses.lol.if it makes sense and you wanns motor chat with ,hit me back anytime.bye dude
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  #27 (permalink)  
Old 04-07-2012, 08:43 AM
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Can somebody please translate the above gibberish?
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Old 04-07-2012, 12:43 PM
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spark

Hi guy`s

Is it possible that voltage rise time is the real issue.What I mean is hei pick up voltage will rise with r.p.m. and alter timing.


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  #29 (permalink)  
Old 04-07-2012, 01:02 PM
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If you want to prove if it is mechanical or electronic it is simple and has been stated in this thread several time.
Disconnect the vacuum advance
Lock the timing at something reasonable and see if it still jumps.
If you lock it and it still moves then it is not the advance weights or related parts. I would then look at the module or pickup. Either could cause the issue. A bad pickup may not be up to the rpm task and could be collapsing the field a little to soon at higher rpm or not building the field long enough. I would also check the air gap on the pickup it becomes more critical at higher rpm. The module could also be an issue but the pickup would be my first guess if it is electronic.
I think if the timing chain was stretching you would be loosing timing not adding and it would be inconsistant.
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  #30 (permalink)  
Old 04-08-2012, 11:25 AM
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Quote:
Originally Posted by cool rockin daddy
Can somebody please translate the above gibberish?
I see the word bowls used allot, I am thinking the bowls are ablaze
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