Advance the timing before TDC. Why?

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69cudaownr

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Hello guys. My question is to learn the purpose of what I did.
This is the engine; 318ci, bored .020 over, stock pistons, 302 heads decked to 58cc (ported), Edelbrock Performer intake (ported), Edelbrock 1406 600cfm Carb, Comp cam XE262H Duration 262/270, Lift .462/.470, stock distributor

It was recommended I advance the timing on the engine when I tune it. Stock tune is 0 degrees, or TDC. I've tuned it to about 9 degrees before TDC.

Why do I want it tuned that way?
Still related to ignition. What would you suggest for a distributor to replace stock?
 
With it cammed up even 9BTC is likely not enough for "best."

TDC is a smog setting. If you go download a few service manuals from MyMopar, and get back into the electrical section "specs" You'll find that this zero crap came about sometime around after '70...when engine emission controls came about in '68 and got tighter year by year

Some smog stickers called for timing to be AFTER TDC

Retarded ignition specs are about SMOG CONTROL and not mileage or performance.

There is a "famous" thread on here about timing you should read. See if someone can find it............see post no4

Ignition timing chart
 
One awful dictatorial policy "back when I" was stationed at NAS Miramar, in San Diego, CA ARB mandated that OLD vehicles be retrofitted with engine emission controls. None of these cars did well

The most basic of these "kits" were a cap to plug off the vacuum advance forever, and caps and green silicone to "lock" the carb idle screws. Installing the "kit" went something like this:

1....Remove the vacuum advance hose and put a special kit supplied cap on each fitting
2....Retime (retard) the engine timing to TDC and see if you can get it to run. Ignore the boiling sounds and the antifreeze gushing from the overflow
3....Adjust the idle screws for a reading of ***.X on the air fuel analyzer AKA unbelievably lean.
4....Shut the hood

After about a month the owner would call the tow truck because the engine had run so hot, for so long, that now it just won't.........run.
 
With it cammed up even 9BTC is likely not enough for "best."

TDC is a smog setting. If you go download a few service manuals from MyMopar, and get back into the electrical section "specs" You'll find that this zero crap came about sometime around after '70...when engine emission controls came about in '68 and got tighter year by year

Some smog stickers called for timing to be AFTER TDC

Retarded ignition specs are about SMOG CONTROL and not mileage or performance.

There is a "famous" thread on here about timing you should read. See if someone can find it............see post no4

Ignition timing chart
 
Time. It’s all about time. You advance the firing of the plug so that peak cylinder pressure occurs at the point where the most leverage is placed on the crankshaft at the point in its rotation where it does the most work.
 
i run about 8 to 10 initial timing, But i have 11.25 to 1 cr 205 cranking psi. every engine is different
 
And speaking of "time" bear in mind that a spark/ combustion/ power is not instantaneous. It takes TIME (and crank rotation) for the combustion explosion to happen and develop pressure and power, and meanwhile, the crank is still moving. You must "get ahead" of that game
 
67Dart is correct about smog timing- I was there. When I "new car prepped" these cars I always advanced the timing to 5*BTDC, they ran so much better. Ignore factory specs and tune it for performance. Fellows here can help.
 
you light the mixture ahead of top dead centre so the biggest part of the "Boom" occurs just as the piston is heading down.
.. that's the simple explanation
 
Hello guys. My question is to learn the purpose of what I did.
This is the engine; 318ci, bored .020 over, stock pistons, 302 heads decked to 58cc (ported), Edelbrock Performer intake (ported), Edelbrock 1406 600cfm Carb, Comp cam XE262H Duration 262/270, Lift .462/.470, stock distributor

It was recommended I advance the timing on the engine when I tune it. Stock tune is 0 degrees, or TDC. I've tuned it to about 9 degrees before TDC.

Why do I want it tuned that way?
Still related to ignition. What would you suggest for a distributor to replace stock?
You need to get the fire lit at the right time... not too early, not too late. Sparking at TDC can get the fire started to late (or explosion should we say :D) maybe this may help....


 
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Why do I want it tuned that way?

as to WOT Timing
Because there is a time (in degrees ATDC) in the power cycle when the crank is in the best position to receive the energy of the hot expanding gases atop the piston. If yur too early or too late, the engine cannot make power to it's potential. I have read variously that this point in time is from 25 to 28 degrees ATDC. So this is, or should be, what your timing is designed to do.
As to Part Throttle Timing,
It is critical to get your sub-3600rpm timing to that ideal point, for the engine to make the most amount of power it can, with the least amount of throttle. This can take over 50 degrees of lead to achieve.
As to Idle-timing
At idle your engine might like 20 to 25degrees of timing. I mean it is easy to figure out, just keep adding timing while simultaneously maintaining your idle-rpm by reducing your throttle opening, until the rpm no longer rises with more timing.
But just because the engine likes it, doesn't mean you should give it.
The reason being is that as the throttle is reduced, less and less fuel will be delivered from the Transfer slots, so more and more will have to come from the mixture screws. Ok fine, until the mixture screws get maxed out. And then, it won't take throttle because the transfers are either dried up, or too slow to get moving again, and this results in a tip-in sag, a hesitation, or a bog. Sometimes you can cover that with pump shot, but there is no good reason for that, except to satisfy the tuner's personal desire to throw mega idle-timing at it. Tuned this way, she will be a pig on gas, and possibly prone to detonating when the Vcan comes on line..
Your engine does not care about low-speed timing, until it hits it's stall rpm, and again at power-timing, usually after 3200 to 3600 rpm. Below stall, with an automatic, it doesn't much care about the timing, as long as it is close.
However, the mechanical timing is designed for WOT operation and can only be made ideal at three points; the rest of the time, at anything but WOT, the mechanical timing will be 100% wrong! The three points are as mentioned, stall rpm, all-in-by, rpm (usually 3200 to 3600), and whatever it takes at idle to not have a tip-in sag, or too high an idle or if it bangs or stalls, going into gear.
At all other times when NOT at WOT, your Vcan has to make up the difference from what it's getting, to what it wants.
 
FYI, 67 dart, 273 2bbl, with CAP, factory setting is 5 ATDC. I have it set to 5 BTDC and it is much happier. Total advance might be a bit too much though.
 
Nobody ever really talks about the real reason why. It's actually very simple from a physics standpoint. Everything with the engine running happens very quickly. Even at idle, when you talk about a valve opening and closing, for example, by the time you've talked about it, it's opened and closed probably a hundred times......or more.

With that mindset, you can see that if you set timing to say 0 degrees, or top dead center (TDC), that's very late in regards to engine real time running. And all of this timing talk is relative to when the spark is lit. Light the spark at 0 degrees, for example and you're "missing" a lot of time that could have been used for making power. At 0 degrees, you're lighting the mixture with the piston at TDC. That makes the power stroke very short and decreases engine performance.

However, lighting the mixture several crankshaft degrees BEFORE TDC (BTDC) lengthens the power stroke. It allows combustion to be created much earlier in the power stroke, so it lasts a longer time, creating a harder "push" against the piston going down. Once the piston is on the way down, it's too late. That's why it's important to create the combustion before top dead center (BTDC).

This also has some negative effects (according to some). It burns more hydrocarbons, since the power stroke becomes longer, burning more fuel. This is why through the beginning of the emissions era, you saw timing specifications down around and even some AFTER TDC. I've always said the EPA is stupid there. I say that, because more advanced timing burns more fuel, but it creates FAR more power than the extra fuel burned, so its efficiency goes way up. In other words, more power and more work being done per gallon of gas. So in the long run around the barn, advanced timing is far more efficient. This is why if you still have inspections in your area, you need to retard the timing before the inspection, so the vehicle will pass. Less timing advance = fewer hydrocarbons out the tailpipe.

I've always chosen to give one as much timing as it can stand without dragging the starter, or having spark knock.....whichever happens first. I run just under 15* initial on my stock 170 slant six and it runs fantastic. I tried right at 15* and it ran a little rough so I backed it down. How much timing one can stand is directly related to how much cylinder it produces. The more cylinder pressure, the less timing it can stand......unless of course, you run race gas.....but we're not talking about "that".
 
.... or, another way of saying it is you have to start early to be on time. Take a foot brake drag car racer. He may leave on the last yellow to cut a perfect light. If you wait till the light is green to go, you'll be late on the light. Why? It's the order of events that take place before the car leaves the beams. Same with timing....
 
if you really look at the process it's hard to believe it evens runs. It happens so
many times so fast it's hard to wrap your head around it being that exact?
 
if you really look at the process it's hard to believe it evens runs. It happens so
many times so fast it's hard to wrap your head around it being that exact?

My high school friend and I who've been mechanics all our lives are constantly saying the same thing. Engines are mechanical miracles, no doubt about it.
 
With late timing, an engine becomes partially external combustion. Horribly inefficient. It heats heads, and exhaust system. Something invented by politicians, not engineers.
 
I've tuned it to about 9 degrees before TDC.

Why do I want it tuned that way?
Much of this has been more or less answered in the thread.
Because people here have some sense of what a 318 with that cam will need, the range of initial timings suggested here is going to be about right.
Your hot rod cam has more valve overlap at idle than stock. This means more exhaust dilution and slower burns. That's why it needs more time at idle. You'll have to experiment a bit because the more compression, the more efficient the burn.
A non-smogged 4 bbl 273 had initial timing spec'd around 10* BTC at 650 rpm. Tech's had discretion to bump that or reduce that depending on fuel quality and elevation. (That's the last non-CAP high performance factory smallblock, and your cam is more radical)

Still related to ignition. What would you suggest for a distributor to replace stock?
Stock is fine.
If it has an early CAP or CAS advance curve (initial timing of TDC or ATC is a big clue), then the advance off idle is going to be too long and too fast.
If so, then clip the start of the curve by welding up the inside of the slots. Or, send it to @halifaxhops and he'll set it up for ya. (But yes you have to pay him. )

I'd start by measuring the timing from idle, as slow as you can go, to at least 2800 rpm.
Then plot that.
From there you can make decisions
 
The goal at idle is maximum power efficiency. With an automatic transmission, the engine should hardly lose any rpm or vacuum when placed in gear. Timing and Fuel Work Together

Here's an example of a 318 clean air system timing (from a shop manual) that had an initial of TDC at 725 rpm..
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Notice how fast it advances from 750 to 950 rpm. This was to make up for the fact it was purposely retarded at idle to make the cylinders hotter and more completely consume the hydrocarbons. But no way to know what the curve looks like unless you measure it. Well if the tag is still on the distributer, then the shop manual will have the specs. So that is a way to get an idea of the advance without measuring.
 
Couple of comments.
- Mother Mopar, like other brands, did not know what the car would be used for once it left the showroom. Would grandma just use it for church on Sundays? Or would someone use it for towing a loaded trailer up a mountain pass on hot summer days?
They didn't know, so ign timing was set conservatively.
- I would NEVER buy a DUI dist. Why? Because in the catalog that I have they talk about the mixture 'explosion'. If DUI knows that LITTLE about ign, then I would be worried about the product. The mixture does not explode [ unless it detonates, then you have other problems...], it burns & expands at a controlled rate, the expansion forcing the piston down.
- It is generally accepted that the most advantageous position for the burning mixture to reach max expansion force is 14-20 crank degrees AFTER TDC. This takes into account both the chemistry of the A/F & the mechanical advantage of the crank.
 
It is generally accepted that the most advantageous position for the burning mixture to reach max expansion force is 14-20 crank degrees AFTER TDC.

Yes and isn't it totally ironic that in order to accomplish that, the mixture needs to be lit almost that exact same amount BEFORE TDC? Interestin' stuff.
 
Burn rate varies due to many things: pressure, mixture, quantity, temperature, chamber volume and shape, stratification....
 
Burn rate varies due to many things: pressure, mixture, quantity, temperature, chamber volume and shape, stratification....
I was astounded to learn that total advance on G3 hemis is about half what a typical wedge head wants.
 
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