Vacuum Question
I have a early (66) 383 Mopar, pretty much stock, and just installed a Edelbrock 1406 carburetor. Timing set at 15 and 36 degrees. The motor had a flutter under slight acceleration between 1500 and 2200 RPM, with the vacuum hose connected to the "Timed Vacuum" port on the passenger side of the carb. -- I moved it to the "Manifold Vacuum" port on the driver side of the carb, and readjusted the idle, downward. I also moved the accelerator pump bracket from the middle hole to the hole nearest to the carb body (increased the squirt), and the flutter went away.. -- I moved the pump bracket 1st - slight improvement -- then switched the vacuum hose - presto, problem gone. -- Timing, vacuum, and carburetors are something that I am not very adept at, so please don't shoot Me. There are many different opinions regarding where to hook the vacuum to the carb. -- Have I done something wrong, and just got lucky -- or -- ???
Depends on your Vcan, and when the 36 was in by; but I'm pretty sure the engine didn't like the total timing it was seeing, and you may have interpreted detonation as flutter.
For instance if the centrifugal begins advancing at 1000 and is all in by 2800, then it is advancing 21* in 1800 rpm, or 1.67degrees per 100rpm. Therefore at 1500, she would be seeing 15* plus 5x1.67 plus the can which could be as little as 9 or as much as 22. So that totals
32* to 45*at light throttle. By 2200 it looks like 15* plus 12x1.67 plus 9 to 22=
44* to 57*, at light throttle.
These numbers are probably too high for a stock iron headed engine
Bottom line is this; if you can change the severity of the flutter by changing the amount of throttle, or if it disappears with a defeated Vcan....... then the flutter was probably detonation.
And you want to avoid detonation at any cost, cuz detonation can break your pistons pretty quick.
It seems there are some guys here on FABO, that are Gung-Ho to throw timing at every engine they come across.
EDIT;In your case, this might be a prime example of what not to do.
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The
factory timings on a 69 383-2bbl automatic,The only FSM I have, were pretty aggressive. I see it at; 7.5 initial,
and 2 to 12 at 1000,16 to 20 at 1500,34 to 38 at 4600,
and with 21 to 27 in the Vcan by 13.5 inches vacuum. The 4bbl specs are a bit tamer
IMO, IMO,IMO, it is better to be 3 to 5 degrees retarded at all points, even 3 short at WOT, than even just 1 too many. On the street, at WOT, I can almost guarantee you,that your butt-dyno will not be able to measure perhaps even 3 degrees short of optimum.
So, again, IMO, don't be in a hurry to throw timing at it, a mistake can be very expensive. Sneak up on it.
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EDIT; I only tune SBMs, and only for street.
I'm not against throwing timing at some combos, cuz it can and does work.
SBMs,with automatics and hi-stalls,almost anything goes.
But I have come across many cases of stock type engines when throwing timing at it doesn't work.
And street engines, with manual transmissions is a special case, requiring discretion.
But you need to keep in mind too, that a lot of initial timing,tames the lope in the cam, and everybody wants to hear the lope.
So if you had a hi-compression360 with a stick, and a 270 cam, and you throw 22* into it at idle.......your precious lope will be as good as gone. And the carb tune you put into it, with a poor-choice of gears, will tend to make it very jumpy at low rpms, and parading it will be impossible. Whereas with an automatic, most of that jumpiness will be absorbed by the fluid coupling.
But if you were to back the timing down to 5*, you would hear that lope pretty good. And the jumpiness would be pretty much gone. This is because your peak cylinder pressure will occur when the piston is much further down the bore, and the strength of that pulse will be much reduced.
Of course, 5* is pretty lazy, but that's sometimes what it takes, especially for a VERY-high compression stick car with hiway gears, and he wants to parade it (me).
The trick then, is to bring the advance curve in hard and fast to play catch up by mid 2000, and still hit the power-timing by the early to mid 3000s. And take care of the Part Throttle timing with the Vcan., to make up the difference.
Without computer controlled timing, it is impossible to hit all the marks, so compromise is always tempering the choices. In a stick-car;I tend to sacrifice all power-timing below about 2000rpm, to favor part-throttle response. I also tend to sacrifice initial timing for PT response. Cuz face it; probably 95% of the engines life is spent in PT operation. I don't care about idle-lope, and with a hi-C 360 my butt-dyno cannot tell if the power timing is 3* short of what the engine dyno says is perfect. My tunes rely heavily on the Vcan.
But I can sure tell if the engine is;lazy or jumpy, or Snapppppy.
