AJ/FormS
68 Formua-S fastback clone 367/A833/GVod/3.55s
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Here's a clue to answer that.
Loosen the distributor-clamp, then without using the timing light, just turn the distributor CCW to advance the timing; keep advancing it until the rpm stops rising. Now read the timing/rpm.
Don't be surprised to see a number close to or over, 30 degrees.
You see; at idle, your engine timing is NEVER correct.
But you cannot drive it like that cuz sooner or later the piston skirts are likely to end up in the pan from detonation.
If you had a computer to adjust your timing, then you could give it the idle-timing she wants, but as soon as you put a load on the crank, the timing would have to be retarded to prevent detonation.
And So, because of detonation, combined with the limitations of the mechanical distributors, we have to retard the low-rpm timing from idle to about stall-rpm. Once the engine has wound up, One timing will usually carry it from there until she floats the valves.
Since we are limited by a mechanical-advance system the timing has to hit the magic number at or near 3500, and the number has to be no more than the engine can take, at stall rpm. So then, if your stall is 1700, and at WOT detonation begins there at 20 degrees, then your timing curve can be from 20 to say 35*, going from 1700 to 3500. Doing the math, that is 15 degrees over 1800 rpm equals 0.83 degree per 100 rpm. How the engine gets to 20*@1700, she doesn't care!, so you can idle it with whatever timing it gets.
So then, if you extrapolate that .83 degrees back to say 900, from 1700stall, you get 6.64 degrees, so you set the Idle-timing to 20 less 6.64= 13.36*.
Now when she revs up, she will go from 13.36 to 35 degrees, over the rpm range of, 900 to 3500; providing that the Distributor can be calibrated that way. Then from 3500 and on, the mechanical timing would be, 35 degrees.
Now suppose you install a 3500 convertor, that stalls at, um, 3500. Now the engine as I have talked about above, may not see any detonation, even idle-timed at 35 degrees.
And so, you can see that, the Stall-speed has a great deal to do with how you determine your Power-Timing curve.
Most factory convertors are gonna stall in the window between 1700 and 2400. And most iron-headed street SBMs are gonna be able to take full-timing by, in the window of 3400 to 3800. So then, you can curve your distributor, according to what your combo wants, and idle-timing be damned.
Except for one thing.
adding timing at idle, causes the rpm to rise. So then, in response to that, you have to close the throttles to reduce the idle-speed. This action simultaneously reduces the fuel coming out of the transfer slots, which usually causes the engine to stall. In response to that, you increase the fuel coming from the idle-mixture screws. and so, the engine idles.
But then, when you roll into the throttle, nothing happens until the transfers start up again, and that manifests as; a tip-in sag, a hesitation, or even a bog, or a stall, until the transfers wake up. In response to that, you mess with the accelerator pump for hours, but can never get it right, principally because, the fuel coming from the nozzles was never designed for that, and so, the fuel particles are to big, come too late, or the linkage is just not reliable enough to respond consistently. In the end, you adapt your driving style, to sortof put up with all that crap.
Furthermore, with the mixture screws now set rich enough to overcome the lack of fuel from the transfers at idle, the entire low-speed circuit, off idle will be rich. So then some 80% or more of the time, the engine is running fat. Guess what happens to your oil, your cylinder-walls, your sparkplugs, and even to the valves..... but you know, 20* of idle timing sure makes for a snappy throttle response!
Whereas, all you would have to do, is open the throttle a bit to keep the transfers on line, and retard the doggone idle-timing to get the idle speed back down, cuz like I said, the engine doesn't care about more degrees of idle-retard cuz it is always retarded some 20 to 25* or more, anyway! Then reset your mixture screws.
The cure for these blues is to install a dual-rate timing curve, that ramps up rapidly from idle to stall, then slows down so as not to overshoot the target of say 35degrees at 3500. Now maybe, you can idle it down to 550 in gear at 5*, and hear the lovely rumpidy-rump of the performance cam, and yet, with a 2800 stall, the engine takes WOT right away..
Or you can just set the timing to 20* and muck the rest up.
As to the controversy of running ported or full-time advance, in an automatic trans car;
At Idle;
as your cam gets bigger and bigger, the engine loses idle-power. As you increase the throttle opening, to find enough power to idle on, the transfers start flooding the engine. So you gotta get that power from somewhere else. As has been found, increasing the Idle timing, very quickly gets the power, with little to no additional bypass air.
However, it comes with some penaltys;
The higher power, slams the trans on the shift from N/P into gear. and
The engine may have a tendency to keep running after you shut it off, and
it becomes a bit of an art to find the right transfer fuel to idle fuel ratio, to sustain the engine until the mains come on line.
So, the easiest way to overcome this, is by plumbing the Vcan to full-time vacuum, and up-stalling the trans, and/or reprogramming it.
And finally, for guys with manual transmissions, the higher your engine idles and/or the more Idle-power it has, the harder it becomes to drive slowly; and parading with typical street gears of 3.55s or less, is just not possible. So for US gear-jammers, plumbing to full-time vacuum-advance is not an option. In fact, my engine likes just 5* to parade with, approaching walking speed at 550@4mph, down to [email protected]. this can only be achieved by retarding the Idle-timing.
And BTW, this is just enough power to pull itself on a flat level even hard surface, with no help from slipping the clutch. And this, with a 276/286/110;230@050 cam, a 750 DP on an Air-Gap, and with big-port alloy heads; So don't anybody tell me I need no stinking 20*@800 ...... By it's 93 mph/Eighth-mile trapspeed, this 367 is putting out over 400hp by the on-line calculators. If it can idle for an hour or so, parading around at 5* at 550/500 rpm, tell me why a lesser engine needs more idle-timing.
The answer is, it doesn't.
The fact is, that automatics just get away with it, because there is almost no load on the crank below the stall-speed, which eliminates detonation. This doesn't mean it's right for a stock-ish engine, nor is it the thing to do, but like said, they get away with it.
and, you know, lotsa idle-timing smooths a big cam right out.
Parting shot;
smarter guys than me have figured out that, for most gasoline ICE engines the ideal point of max combustion pressure should occur in the window of 25 to 28 degrees AFTER TDC, almost irrespective of load or rpm. Therefore all our timing systems should target that crank position.
At WOT, full load, and after about 3500rpm, it takes about 32>36 degrees of ignition advance, to get the peak pressure to occur in that window.
From stall to 3500, it's more or less a straight shot.
But from idle to stall, it's kindof a free-for-all cuz of all the variables, and the constraints of the mechanical systems. The biggest deal here is to keep the transfers on line ready to take throttle. If your engine suffers from a tip-in sag, a hesitation, a bog, or an outright stall when you roll into the throttle atta stop, I'll take a guess that 99 times out of a 100, you can blame it on the transfers. Get the transfers on-line and most of your problems will disappear, regardless of what the idle-timing is.
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