metheglin

When you set the timing "all in" by 2800 RPM at about 36º, are you getting all you can out of your engine?

When an engine turns faster, the ignition system needs to light the spark sooner, in order to build up peak pressure by the optimal cycle point (about 16º ATDC). So at 5600 RPM (double of 2800 RPM), why should you still be sitting at a now measly 36º of advance? Don't faster rotations call for more advance?

The "36º total timing, all in by 2800 RPM" mantra generates a rather steep timing curve, compared to the factory spec curve. I don't argue the slope of the curve. Steep is better.

But why stop abrubtly at 2800 RPM? This I do question.

No extra advance will be forthcoming from the vacuum system. At WOT, you've got no vacuum, and thus no vacuum advance. You really do need to rely on a centrifugal advance mechanism to keep bringing up the advance.

I realize that the mixture will be richer at higher RPMs with WOT, and richer mixtures burn faster, and so less advance is desired. However, by how much? Stoping hard at 36º seems, well, unnatural. Why should there be such a hard bend in the curve, without a matching hard-change in the engine demands.

I realize that the distributor is a limited mechanism - you can't just freehand an optimal timing curve and make it happen, you only have a few mechanisms to work with, and they are mostly linear.

Still, I'm thinking that you could do better by:
(1) Using soft centrifugal advance springs, as otherwise suggested, to generate a steep curve,
(2) Time the engine so that at 2800 RPM, there is 36º of advance,
(3) Extend the centrifugal advance by removing the bushing or even grinding out the slot, so that after 2800 RPM, advance continues advancing.
(4) If the advance curve needs to "flatten out" as the RPMs get up there (for a reason I have yet to learn, so please educate me), a soft compressible rubber bushing might do the trick.

I've run at 44º total advance at higher RPMs (measured without vacuum at around 4000 RPM, but driven with manifold vacuum) and I've never heard any pinging, knocking, or tapping... no detonation problems (using 95 octane gas).

It seems, in theory, to be a fine idea.

In practice, in my car, it works. I still get the steep early timing curve, where it most counts, since lower RPMs are where most of my driving occurs. Yet at higher RPMs, I also get just the extra bit more performance.

So go ahead... shoot holes in it.

WRC7732

Hmmm....

Glassbowtie77

greyghost79

well, i'm no engineer, but my understanding is that increasing advance also effectively leans the A/F ratio. if true, then there has to be a limit to how much advance you can dial in before you reach a dangerous (for your engine's life) lean burn situation. also, and i don't know at what point this would be, at some point of advance wouldn't you be firing spark in the cylinder before the intake valve even starts to open, thereby firing with a dry cylinder??

gofastvette

If your fuel delivery system is adequate 40+ is not unheard of. Fuel engines {Nitro} run as much as 60 degrees or more.

Z-man

Think about it - if the spark is too advanced, the fuel starts burning way before compression of the mixture is complete. The force of the burning gas is expanding against the piston as it is rising, causing it to slow down. Even if you can avoid detonation, the power produced is not optimal and the power output starts to decrease.

After billions of engines have been produced, most find that 34 - 38 degrees are best, but like folks say - different engine configurations and fuel will allow a wide variation in timing.

Solid LT1

If your motor has bad cylinder heads, low compression and isn't very efficient, 40 degrees will work. Late model "smog" type open chamber motors will go to 40-42 degrees with good gas. At very high RPM most engines like a little retard put back into the timing curve 2-6 degrees has worked well for people I know that are WAY smarter than I am about motors. If your motor has good iron heads like Dart, Pro-Action (now owned by Comp Cams) or Vortec heads 40 degrees is too much advance. With better cylinder heads and small chambers, most SB Chevies like 32-34 degrees max on good fuel.

onaqwst

.....

lars will be on site in a minute

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tune in lars....

metheglin

Good point. I might be getting less performance. I haven't dynoed it. I guess I'm just suprised by the utter lack of detonation that everyone seems to speak of in fear and trembling.

But to be honest, I haven't really run it hot on a hot day, so maybe it just never got into pinging temp range. I did let it lug in 4th gear and step on it, with no problems.

Still, this is more a theoretical question, so regardless of what my car wants to do, I still don't see the technical reasons why timing should stop advancing at 2800 RPM, 36º.

This makes sense, under the assumption that the spark is actually too advanced. But that is the condition I am challenging. What causes such a condition? If the combustion chamber pressure builds up too much before the crank gets over far enough past TDC, then we have a problem.

The time it takes from spark ignition to a significant combustion chamber pressure is fairly fixed, at the same fuel ratio. Reverse-engineering the figures, I calculate it at about .003 seconds.
At low RPM and low amounts of advance... take idle for instance at 750 RPM and 8º advance, that .003 seconds is spread out between 8º BTDC to 16º ATDC. Most of that expansion period falls ATDC, which is good since that shoves the piston downwards.

At higher RPMs and high amounts of advance, take 2800 and 36º, that .003 seconds is spread out from 36º BTDC to 16º ATDC. A much greater proportion of that fuel-burning-pressure-building time is happening BTDC when we really don't need or want any pressure yet. However, due to a significant delay from the point of the spark to the point when the flame front truly takes off, it is concievable that all the pressure is happening right near the end of those .003 seconds, and we're still getting pressure in the right place.

Following this reasoning, there should be a theoretical limit to the amount of advance you can have, at any RPM... and if so, I have no idea what it is, but 36 or 38 degrees, as emperically tested tried-and-true, could quite well be that limit.

Until I hear otherwise, this is the theory that I am going to go with. But I'd prefer if someone who actually knows the real answer could chime in.

Because I have another theory. My car runs beyond 36º, and I'd say 44º is well beyond 36º. So maybe the pinging problems that people typically get are due more to heat and turbulence issues than to "too much advance", and if so, limiting the advance is a band-aid solution. Detonation issues might be solved by other means such as keeping a clean and smooth combustion chamber, not running lean, eliminating vacuum leaks, keeping the cooling system in top shape, running cold plugs, running manifold vacuum, etc.

I have a stock 1969 corvette RPO L46 HX (A/C) 350ci small block, 350HP, cast iron everything, original 11:1 spec compression (but probably closer to 10.5:1). It was rebuilt by a previous onwer at 70,000. It's now 20 years later, at 85,000 miles. I haven't tested the compression, but I've been meaning to, just to baseline it. I'm running 95 octane petrol (NZ).

tpiselli

So having just spent last week at an EFI101 class I'll throw in my two cents.
The best way to tune for timing is to get the car/engine on a dyno which will hold the engine at a specific rpm while changing the load. Think of it as a fixed rpm vs. throttle position. As you change the throttle position change the timing to achieve a max torque value. Once found any more advancement is not really helping and may start to actually lower your torque. Do this across the rpm band and build a map of your torque values.

stingr69

I ain't an expert but I do think this question boils down to one thing - "Why does a mechanical theory dissagree with empirical evidence"?

Answer: Because we have incomplete data. There is more to it than just some fuel, air, and a spark. The Otto Cycle is easy in theory and complex in practice. Fuel is the limiting factor. The physical form of the combustion chamber has an effect as well. LOTS of incomplete data.

In practice, 36° does work very well for a wedge combustion chamber in a typical SBC running on gasoline. If you go over that you can damage parts and not improve efficency either.

-Mark.

Tonys96

I can't give you the technical answer, but when we did the dyno tune on my motor, after getting the jetting correct, keep adding jet size until HP decreases then go back to new peak. We did the same with timing, added 2 degrees until peak HP dropped then backed off 1 degree to new peak HP

Z-man

This may not be as fixed as you think. The flame propagation will depend on compression and octane of the fuel.

I found this paper with some interesting points about this. It looks like no matter when the spark occurs, you want the pressure of the explosion to happen about 15 degrees ATDC.
Link

gkull

Real race motors pull timing out as RPM climb. So up to 3000 some rpm you need to add timing. up to 5000 rpm your still Okay at max. Then from then on up you would want to retard the timing.

The reason is fuel atomization....Blah blah blah at higher rpm. Flame travel speed goes up with rpm. Race motors exceed 100% cylinder filing during certian rpm. The heat, with inlet speeds near the speed of sound, and rapid piston compression create super fast flame travel.

These systems that pull timing out are just cost prohibitive. Modern fast burn technology type heads use 32 or less degrees of max advance. Mine are set to 32 max with 17 initial and all in at 3200 rpm

evil herbavore

The timing isn't completely to make the mixture burn in time for the quicker revs......



Part of the timing game is to increase cylinder pressure when the cam makes it a bit low.....a lumpy cam doesn't make much power down low, but does after 3K (very generally speaking).


It's more about how dense the mixture is in the cylinder which is a direct function of volumetric efficiency. When VE is high, such as when RPMs build in a cammed up engine, you don't have to start igniting the mix so soon.

0Corvette Engineering

Everyone I know starts at 36 degrees and then takes it to the track and adjusts the timing et al for the best times.

but I know from personal experience that too much timing will slow you down

Keith

MotorHead

Simple dyno tuning has proven beyond any doubt a small block chevy produces the most power ~34-36deg advanced, sure mine runs fine too at or above 40deg advance but why run it there when it is making less power.

You don't need a degree and alot of theory and equations. It is very easy to see for yourself, spend 50 bucks for 3 chassis dyno pulls, one at 32 deg, one at 36 and one at 40degs, I know which one I would bet gets you the most power.

lars

That's it. I've done a lot of dyno tuning, too. Talking about it and arguing theory is a waste of time. The dyno does not lie: Engines with conventional heads (non fast-fast burn) will produce best power in the 34 to 38 degree range from 2500 rpm and up. That's a fact. You lose power on almost all GM engines over 38 degrees at any rpm at WOT (the only time I have seen any imrpovement in 1/4-mile mph above 38 degrees has been on low-compression engines run at high altitude, and these can make power up to 40 degrees in some cases, but this is an exception). The fast-burn show best power on the dyno at about 32.