FlyLS6

Plenty of discussions about how much initial, how much vacuum, how much mechanical, how to balance between them all, etc.

My question is, presuming one get's past the detonation and general configuration issues, is what happens to top end horsepower and torque when total advance ends up greater than optimum? Is it as simple as horsepower doesn't build as quickly or reach highest levels? What would be a target maximum amount of advance for, say, a mild build, a hot street strip build, or a fairly radical build in just simple terms.

This may not be too clear a question, but if anyone wants to take a shot at commenting.......

Shark Racer

Proper advance curve has a lot to do with the combustion chamber design. A more efficient chamber will take less advance than an older one.

As you step up on your "builds", chamber efficiency will increase and you can likely get away with less advance.

A modern LS head takes ~24-26* spark advance at WOT, while a SBC prefers 34-36 with stock heads.

There's more to it than just that. A lean fuel mixture burns more slowly, thus requires more advance. (Why you have more advance at cruise vs WOT). Rich mixtures have higher burn rates thus less advance.

Hope that helps.

cardo0

So the crank turns twice for every turn of the cam and dist rotor - correct? Well if you have >45* of advance on the crank dampner then that make the dist rotor turn >22&1/2* under the cap - are you following this?? So where is the dist rotor pointing at >45* crank advance??? Yes, its pointing at the next plug terminal/wire/sparkplug! Electrons don't care what u want, they take the shortest path with least resistance - bang u fired the wrong plug. I call that too much advance.
Ok, Ok there is such a thing as dist rotor phasing but most posting here are unable to understand it - that and the fact vac advance drops out as WOT occurs and engine vac drops to something near zero.

Keep initial plus mechanical <35* and use vac adv to tune for cruise and economy.

Hopes this helps more than it hurts,
cardo0

MrForce

360 divided by 8 equals 45.

If optimum timing is exceeded, HP falls off. Shark Racers' points and numbers are in the ballpark. Cylinder pressure is another factor, the more you have the less timing you need/can get away with.

Rob

63mako

I must be a little thick. I am not following you at all. 45 divided by 2 is 22 1/2
That is totally wrong. The cylinder fires 50 times a second at 6000 rpm. The arc takes time to happen, then the flame travel front takes time to happen. This is why you need to fire the plug well before TDC on the compression stroke. More efficient chamber design makes the flame travel faster. A properly timed street SBC with optimal DCR will be running 52-54 degrees advance total advance with vacumn at cruise @ 3000 RPM. Without proper advance your power drops off drastically and you will burn an exhaust valve. Modern computer controlled timing gives the most advance possible prior to detonation. This gives you the best economy and efficiency not neccessarily the most horsepower. Best bet is time with base and mechanical @ 36 degrees @ 2500 to 3000 RPM depending on what your car will take. If you have fast burn chambers this can be dropped to 34-32 depending on the build. Total with vacumn can be 52-54 or more if the engine will take it for best economy without detonation. Some high compression builds with lower duration cams need less total or total with vacumn to avoid detonation. Drag cars will run locked out timing. They don't care about fuel mileage and basically set timing where the engine needs it at high RPM launch to redline for the most power. There are 3 timing systems, initial, mechanical and vacumn. Timing curve really need to be fine tuned to the engine and car build specs. Gearing, trans type. car weight. compression, cam, intended use and driving style will all affect how much timing you can run in all three systems and combined. With patiance and understanding of the systems you can get the most HP and efficiency out of your particular build.

billla

The topic has been covered MANY times and is in the timing sticky and Lars' papers - but that's the "rule of thumb: range to shoot for.

Best ~$300 you can spend is an hour on the dyno...

Shark Racer

Yuuuuuuuuuuuuuuup. The easiest way to know for sure whether you've got your advance right is on the dyno. Start low and keep going til you hit knock or the power drops off.

There are more expensive ways (very expensive diagnostic tools that determine cylinder pressure), but I wouldn't know where to begin.

cardo0

Yes your right 360/8=45. Would u care to elaborate? Is there something about 45* in post u don't agree with?

If some fails to believe that >45* of crank degrees is not >22.5* advance on the dist rotor then i cannot help them. If someone cannot understand that >45* of crank advance points (direction, not dist trigger) the rotor closer to the preceding cyl plug terminal (without dist phasing) i am just wasting my time. If someone can't understand electrons take the short path of least resistance to lower potential and can't visualize/see the spark jump rotor tip to the closest plug terminal they are using a different science for electricity. You cannot advance the spark timing more than 45* without misfire to the preceding plug in firing order (again without dist phasing).
And yes i have seen the result of >50* advance - a hole in the side of #8 cly on a brand new crate 383". This may answer the OP's question - but not others. If someone has addition questions they need to start thier own post.

cardo0

aussiejohn

[QUOTE=cardo0;1580798598]So the crank turns twice for every turn of the cam and dist rotor - correct? Well if you have >45* of advance on the crank dampner then that make the dist rotor turn >22&1/2* under the cap - are you following this?? So where is the dist rotor pointing at >45* crank advance??? Yes, its pointing at the next plug terminal/wire/sparkplug!

Cardo,

I know what you're trying to say but you've got it a bit wrong. As you say, if you have 45* at the crank, the rotor "advance" is only half that, i.e. 22.5*, exactly as you said. However, this puts the rotor HALF WAY between the 45* spaced distributor cap terminals, not directly underneath the next one in the sequence. I agree with you about the tendency of electrons, I just hope that the OP can understand all of this so let's all try to help him, eh?

Regards from Down Under.

aussiejohn

Shark Racer

Regarding the timing advance, there's a pretty big disconnect in the logic.

You can advance beyond 45* of timing advance. Why couldn't you? The distributor curve and travel of the cylinder are related mechanically, yes, but the distributor fires whatever cylinder post the rotor is under. If it fires with 60* of crank advance it certainly will. It only fires when the rotor tip is pretty much under the corresponding post (unless you have some ignition problems.)

What that means is the piston is only a third of the way up on the compression stroke when the ignition event happens for that cylinder.

It's not firing on the wrong cylinder, although it could be happening at the wrong time.

(it's probably not a good idea to have that much advance... but there are definitely cases where cars have advance in the 50* range at high engine speed, low load, etc.)

69 Chevy

I'd say 60 degrees.

billla

Yeek - another esoteric discussion for such a simple question...

'75

Seems like there are two different languages being spoken here. Timing referes to crankshaft degrees before tdc. So the generally accepted 52 deg including initial, centrifigul and vacuum advance is still only 26 degrees on the distributor.

pauldana

When i talked to Edelbrock (many times... ) They told me total timing should be about 36-38* and not to exceed 39*... I have 12* initial, and full in at 38+* @2500RPM.... at this i ping in the summer unless i use a little av-gas... if i went to 50 it would be KaPow...

am i not understanding something here???

please explain... p

Shark Racer

Read my post above for more clarity. WOT fueling will be richer than cruise fueling. Rich mixtures burn faster which means less timing at WOT. You can run more timing in lower-load situations and cruise situations where the mixture is leaner.

noonie

The distributor is still round 360°

FKING1

With my 383 modded anything over 34 and the engine bucks like mad under 1500 rpm.

Shark Racer

Starting again

Ignition timing: 45* BTDC
means that the ignition event happens (rotor tip reaches plug tower post) while the piston is halfway up on the compression stroke.

There's no need to worry about the distributor math. None. not at all. Helps when it comes to stabbing the dizzy or just understanding the principle behind how a cam driven distributor works, but it doesn't matter in this conversation.

Having your timing at 90* BTDC does not mean that the rotor will be pointed at the previous cylinder, it means that the ignition event for that cylinder occurs as the compression stroke starts.

FlyLS6

Guys...I'm really sorry...I thought it was a pretty innocuous question.

There's plenty of threads that involve timing, but they usually end referring to 36* or 38* total, or something along those lines without really going to much in what happens if it's "something else".

I really appreciate the responses, and the back and forth, and maybe the reason I asked the question might help. I didn't post this info because I thought the "general" repsonses wouldn't be so....controversial.

Not long ago I posted this, regarding recent dyno runs on my engine: http://forums.corvetteforum.com/c3-t...ate-motor.html

Actually, since '78 when it was originally built up, those are the only dyno runs (motor was installed at about 28K miles and car now has 40K miles on it).

I can't honestly remember the exact total timing in the distributor, because it was originally set up back in '78 and hasn't been messed with since. I'd always set the initial at 14*, "forever".

So, about three weeks before I took it in for the dyno runs, I had an issue with the Mallory breakerless point system in the distributor (installed in '78), and to make a long story short had to have it replaced. I told the 'vette shop I took it to be be sure the initial was set at 14*.

Got it back, ran fine, took it to Truestreet in McKinney, TX, and did the dyno runs and at no time was there ever any hint of pre-ignition. However, I had noticed that a couple of times when I would start it, I'd get just a hair of kick back, as if the timing was too advanced. But it was very minor and as I said, I'd had no other indications of any issues.

This past weekend was cloudy and cool around here, so I decided just for grins I'd recheck the initial timing. Well, it was off the scale, so to speak, and from what I could interpolate, set at between 18* to 20*.

As best I could recall, that could have put the total up into the 40* range, and wondered from a performance point of view if it might have hurt my dyno runs. Again, neither I nor the dyno guy at the time of the runs heard any indication of any pre-ignition at all. I've backed the initial off to 16*, to see how it'd work out, and so far so good. So my original question here was to try to get a feeling as to what might have been going on in the motor.

I didn't go into this originally, because I thought I could keep things "simple". I'm sorry if I should have to begin with.

63mako

BBC needs a little more sometimes. When Elle88's engine was dyno tuned we were at 19 or 20 initial and 38 or 39 on total for best power with modern aluminum heads. It has been a couple years and many runs so Im not quite sure but I remember thinking it was pretty high. I could see old iron heads wanting more if the fuel has enough octane for the additional advance.