Base timing. Really ?
I am really confused..I do remember when i built this engine and using the stock distributor untouched, it would ping like crazy at high RPM and remember people over on moparts telling me that i would have to limit the slots to around 34 degrees because thats about all it could stand with that cam. So i JB welded the slots and installed the light spring and had to do a trial and error with a file to get the slots right. I ended up with 18 initial and 34 total and she didnt ping so i left it like that for years. This is with the factory mopar ECU. Then i did the GM module upgrade and at the same time, installed the FBO limiter plate and knocked out the JB weld from the slots. I never ran the vacuum advance. You guys are telling me to get rid of the limiter plate but when it goes beyond 34 degrees it pings like crazy. I am just confused.
Your engine requires exactly one timing, the one that causes the maximum amount of gas pressure to be delivered to the crank at the exact right time. IIRC this is ~28*ATDC. But I have read other numbers.
As rpm, loads ,temperatures, and elevations, change, the BEGINNING of the gas expansion strategy has to be adjusted to achieve that.
I talk
streeter only
so, there are at least four different timing strategies to achieve this;
1) power timing
2) idle timing
3) mid-rpm timing
4) cruise timing
5) auxiliary timing
6) timing from idle to stall
#1) power timing: Of the four, this is the most likely one to break parts so that is the one to nail down first. Thankfully, during the last 7 decades or so, there is a plethora of empiracle data, that says most HotRod engines want very similar power timings, in the range of 34 to 38 degrees, so mostly we shoot for 36* and if it don't detonate then it's done. see note #1
#2) idle-timing; this number falls out of the tune all by itself, during the setting of the transfer slot to mixture screw synchronization. Generally, your HotRod engine will like 14 to 28 degrees, but with an automatic, the only important number is the one at stall. see note 2
#3) mid rpm timing; oh yeah, this is where the hurting occurs; namely from stall-rpm to all-in rpm; somewhere around 2400(depending on your TC), and 3400 (the point you choose). see note 3
#4) the centrifugal advance system can only be right exactly twice, namely; at WOT after about 3600, and at idle. Every where else, at WOT, you can only be close, so don't bust your nuts looking for perfection ....... because, of the thousands of hours your engine will live, ask yourself how many of those might be at WOT from stall to 3600. You get the point? see note 4
#5) auxiliary timing; for guys with manual transmissions.
You guys know that driving below 5 or 6 mph is almost impossible, with a performance cam. see note 5
#6) idle to stall; with an automatic,who cares. see note 6
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note 1
On the street, your butt-dyno will not know the difference between perfect timing and 2 to 4 degrees not enough, so you ALWAYS aim low.Power timing does not begin until the engine can handle it without detonating. This depends on many factors, but usually falls in the range of 3200 to 3600. Again;
aim for the safe numbers closer to 3600.
Why?
Because; ask yourself how often are you gonna be at WOT at 3400rpm, that a couple to a few hp is gonna make a difference, with the tires spinning. I'll tell you; with 27" tires and 3.23s; at WOT, 3400 occurs at ~27mph in 2.45 low/~48 in 1.45 2nd/ and ~77 in 3rd; but, running to redline, only in first gear will it ever actually occur, and if your engine is not smoking street tires at WOT, in 2nd gear at 3400, then there is something wrong with it. Soooo, the true answer is your engine will never care about a power-timing deficiency at 3400! The bigger the engine the less it will care.
There is only one possible situation when it might make a few hp difference, and that is for passing. Here it is;
Again with 3.23s or less, and 27" tires;
Say you are cruising down the road at 50=2000(zero-slip), say 2100 on the tach@5% slip. And you catch up to a slow poke doing 48, but you gotta get around him in a hurry, so you floor it. The trans drops into 2nd and the Rs jump to 48=2800(zero-slip), say3200 on the tach@ 15% slip. Ask yourself how often this situation will ever occur. Then in the blink of an eye, your car will be pulling past his door doing 75mph at 5000. 7hp lost at 3400, will not make a difference.
So ideal power timing at 3400 is not a priority on my combo,
I say again; aim safe. You got thousands of miles ahead of you to experiment.
note 2
The only difference in that range (14 to 28) is how smooth the engine idles at; big deal. The more retarded you idle it, the lumpier it will sound. The more you advance it, the smaller the transfer slot will be, and the more likely it will be that you have tip-in problems. Get the tip-in right, by adjusting the transfer slot exposure right, and use timing to set the idlespeed; it's just about that simple. Above all; DO NOT set the timing to some pre-conceived number that you think she should have; instead, let your engine tell you what number is right, by the minimum idle speed required to escape tip-in problems, and that keeps the oil pressure reasonable..
note 3
if you have set the idle-timing too high,you MAY have problems with detonation at WOT, in this range, because it MAY be too high all the way to "all-in".
The only ways to fix this are by
a) reducing the idle-timing, or
b) SLOWING the flyweights down with springing, or
c) by reducing the weight of the flyweights.
Which method you chose, depends on if you have a spring selection, or a bench grinder, spare flyweights, or a limiter-plate, or just back up the idle timing, and mod the cam back to the required power-timing. Any method is acceptable, whatever gets the job done.
I like to get this timing worked out so I can run cheap fuel, cuz over 18 years, a guy can save a boatload of cash.
note 4
Ok so, what do you need to do, when not at WOT, and from idle to "all-in"?
This is called PT or Part Throttle. At PT, your engine, to achieve the optimum energy-transfer to the crank, will want between two and three times as much timing as the mechanical system can deliver. It is not uncommon to achieve maximum fuel economy with cruise-timing in the mid 50s to early 60s, this being almost wholly dependent on your specific build. This is how early the fire has to begin, to get the max-pressure to occur at the best point after TDC.
If you know your cruise rpm;
you can get a close idea of your best cruise timing, by holding the engine to that rpm, and with the vacuum advance hooked up, just pull in the timing until the engine rpm peaks; and then read the timing . That will be very close. Don't be surprised with what you see; the engine just told you what she wants. Many times, it will not be possible to achieve that, with factory parts. My engine needs a dash-mounted, dial-back, timing delay box, to make it happen.
note 5, part 1
Especially with a hi-compression engine. You have to either slip the clutch or ride the brakes. Have you figured out the why of it? IDK, but I'll tell you how to get as low as your gears will let you go.
timing is the answer.
Typically, with the Transfer slot synced up, your timing is gonna want to be about 12 to 16 degrees, but Ima thinking you guys are setting it much higher. So then, when you try to drive it idling at 700, the engine is so far down the transfer slot, it runs out of power and wants to stall. So you crank up the idle speed and it runs better, idling along in first gear, but the roadspeed with 3.55s is 6mph or more, so you almost never run down there. If you ride the brakes, the engine starts bucking the car, and you end up slipping the clutch.
The reason this is happening is because you have set the parameters to create the max pressure to the crank at or near the optimum point, which, for operation in this mode is just too daymn powerful.and so every power pulse, 4 of them per engine revolution, is hammering down your driveshaft and into the tires, multiplied by all the gearing in-between. Say your engine idles at 80 ftlbs, by the time that gets to the axles, that is multiplied by 80 x2.66x 3.55=755 ftlbs! Imagine some guy back there, with a long bar on your wheel nuts, and whacking it over as hard as he can 6 times per second. Yeah that will drive you nuts.
So what do you do? You already tried idling slower, but the engine just wants to stall. The trick is to cause peak cylinder pressure to occur with the piston further down the bore, which will simultaneously drastically reduce the pressure; problem solved right? And how you do it is to set your T-slot sync right, and then retard the timing until the engine hasn't got enough power to stay running anymore, and then give back just enough timing to prevent that.
By experimentation, my 230* cammed 367 combo that cranks over 180psi on the compression gauge, will idle just fine at 500/550 in gear and rolling, with just 5* advance. I do that with an aux-box. Mine has a 15* adjustability, and I set my idle-timing in Neutral, to 14*.
At 500rpm, you can get your roadspeed down to 4.26 mph with 2.66 x3.55 and 27"tires.
My trans has a 3.09 low in it, so 500=3.7mph, and smoooth sailing... it makes just enough torque to pull itself on flat,hard,level, ground at 500, but needs 550 to climb over a dime.lol.
Now driving this slow may not be important to most of you. But for me, I like Daisy Dukes. They remind me of those early years, in my marriage, 45 years ago, you know what I mean. . I like long reminders. So parading is high on my list of priorities.
Besides, that Hughes cam is finally sounding like something; music to my ears.
note 5, part 2
when your engine is cold, it wants more timing, not just at idle, but at PT, all the time. Mine likes at least 10 over the base idle-timing of 14*. At 24* I can drive away on the coldest September mornings. But three to four miles later, the speed-O has climbed 4 mph, and I know to take those 10* out before I floor it.
note 5, part 3
I use that dash-mounted,dial-back, timing delay box, for other things as well. especially for tuning. It helped me get my PT and cruise curves, to as close to perfection as possible.
And every once in a while I get some crummy gas, or it's just been in the tank too long. I just twist in/out some timing and away I go.
or, sometimes I just want a lil more timing at 35/40 mph cuz I'm gonna be there for a while, and want to eke out a lil more mpgs
or one time I went to Alberta and got into higher elevation, and my fueling was off, so I jut retarded the timing to be safe.
note 6
The only time you should care is if your combo has to cruise in there. With 3.55s your PT mph@stall rpms are;
20@2200, 22@2400, 24@2600,26@2800,28@3000 and so on. At PT your v-can will compensate for any mechanical shortcoming. As you move off PT to Power, stall-timing will takeover.
Only manual-trans cars need to work out the timing in this range.
more coming hit refresh,
ok I better quit for now