I'm in the process of developing a timing curve (a baseline prior to track time) and have invested in some new software to get a leg up on it.
Calculated results aren't what I expected.
Conventional thinking had generally been "all in" by 3000 RPM or so with the requisite total being dictated by engine architecture and external factors such as inlet air temperature, etc.

What I'd like to see, should anyone have anything representative, is what the baseline tables look like as supplied by a manufacturer like Holley or similar.
Holley LS based software (so it's not apples to apples here as this is an iron headed Gen 1) is indicating a dip in the timing amount at and around peak TQ. Then climbing to redline. Very much outside the convention of the "stone age" where I got my chops.

Performance Trends Engine Analyzer (basic) is showing something similar although with a 100 octane fuel spec'd, timing rises steadily with about 1° for every 500 RPM starting at 2500 up to 6500.



Does anyone have a tuner based timing table handy for a SBC?

 

I don't have a table on hand, butt the engine does not care whether the timing is controlled by a computer, or by a mechanical distributor. All it cares is that the spark occurs at the optimum time. Therefore in the absence of any other data, start out with the same sort of curve you aways would have used with a mech controlled distributor.

Good place to start would be, set the base to about 12° BTDC; and then the "advance" should of course be 0°, up to around 1200 RPM, and increase smoothly to about 22 - 24° from there. Then add about 15° more when the vacuum is above 12" or so and the RPM is above 1500. You'll want that to start backing off when the vacuum drops to 12" or so, and be fully out by the time it drops to maybe 6". You can fine-tune it later for things like IAT and coolant temp as well as knock sensor inputs.

 

It's not that the engine cares where the curve is developed from, it's just there are thousands of hours of development in those EFI tables and what's presented looks to be out of the ordinary. The ordinary being just what you've described and has been my "modus operandi" for longer than I care to recount.
But they're laid out that way for a reason. At least it would appear so to me.
Now, that said, have you seen a curve that continues to go to redline?
Bill Jenkins (no, I don't have a Pro Stock engine from 1970) suggested around 36 to 38 at 3000 and no limit on the upper end, shooting for 1 degree per 1000 rpm. with no end (as related to me by a student of Jenkins work). That compares to my software developed curve although the total amount is different (but not by much) and it's given me food for thought. Start at 30 at 3000 and get to 35-36 by the 6500 redline. Seems odd. What happens before 3000 is another matter but that'll not been seen on track days other than getting there.

The vacuum advance side is something I have to revisit altogether. This new spec engine is quite dissimilar to the old version and timing changes are certainly needed. It's less a highway cruiser than it was as I'm more orientated to drag racing this time around, but proper timing at cruise has yielded some impressive MPG (impressive being relative) and I'll have to dial that part in again. That and the centrifugal side of things.
Ultimately, I'll make the move to MSD's digital box and can finally get away from the limitations of springs, advance stops, et al. But in the meantime, there's a new distributor on the way (long story as to why) and I'll have to get back to it old school style. Makes my back hurt just thinking about.

 

You can't go by aftermarket efi timing tables, they seem to have no idea what they are doing in general.
Programmers for those systems seem to be rarely if ever, also educated as engine mechanical engineering experts.

And that goes double for many OEM table designs.

Part reason OEM tables get ridiculous numbers is because of the way they test those engines for emissions and efficiency. Pushing too much timing to knock an engine and letting knock sensors deal with it for example, should NEVER be done in aftermarket and at hobby level installations and yet many OEM use that 'idea' as an awful way to control and 'learn' a peak timing for the ECU depending on the fuel choice of the owner, since there is no way to sense the difference between 87 and 93 or bad gas @ 84 octane when its all the same E content.
Another factor is the load-rate principle, where the engine is set to develop maximum cyl pressure with max timing in OEM apps , for max BSFC (economy) and torque. They can get away with that kind of timing map with OEM engines that tend to develop poor cylinder VE at higher rpms, so its like running negative boost pressure past a certain point, timing can be added that a typical performance engine would never take.

you have to look at engine design, cylinder fill, fuel quality, temperature indices, injection timing, plug placement, and so on, to determine timing features.

bottom line You can't go strictly by their or any numbers really. You have to make your own from your own knowledge, and testing over time for any particular combo.

There are also different timing strategies depending on the application. Most people have reliability apps and don't even realize it. If you have an engine built for 2k hp but use it for 'pleasure' e.g. no sponsorship racing, unlimited class, no money to win or trophies to win for personal gain, its just for 'fun' then that is a reliability app and not a performance app, because when the engine breaks you have to pay for it with time/labor and there is no sponsor or kick back for ruining an engine. Therefore the logical setting is to minimize engine abuse / stress, because what is the point of squeezing out the last ~5% power at the cost of having to replace everything? Why not just build the combo to make that extra little % included at part of the baseline setting rather than the last 5% of headroom it has to offer?

In a reliability apps always use the least amount of timing possible that still gives a reasonable BSFC and EGT/CHT for the engine, at the most possible load in the longest possible gear (e.g. 5th, 6th, 7th, etc... slowest rate of accel). This is done to minimize the peak cylinder pressure which will maximize the lifespan and reliability in those conditions. It does not generate the most torque/power, so many people are confused by the notion of 'why would i want less torque if i could have more torque by changing a number in the timing map' the reason is because as you add timing, you add peak cyl pressure, and that is what starts to eat rod bearings and chew up the cylinder when the load-rate changes, i.e. when the vehicle is tuned in 'fast' (numerically lower) gears, or when fuel quality changes, or vehicle weight changes, etc... in other words timing values can only be dialed into one specific load-rate and acceleration-rate for an engine, they technically require a different timing map for each gear and each vehicle weight. But since there is (usually) only 1 map that 1 map has to work for all those variable conditions, it needs to be tuned for the slowest rate and most load possible, otherwise the timing will be too advanced later when the driver decides to use that gear or adds weight to the vehicle or drives somewhere that the load changes like up a long steep hill or in the case of heavy slow trucks, towing extra weight with extra friction like raining.

The spark is only the beginning or start of the reaction, it does not control the speed of the reaction. Chemical reaction speed is dependent on how fast / violently molecules slam into each other, so chamber design is key point(reaction vessel properties, surfaces, heat transfer, etc... Year and technology matters), and it is based on partly compression-rate (how fast and how much the mixture is being compressed), cylinder volume change rate, and temperature (more internal energy = more molecules overcome reaction barrier energy per unit time). Another dominating factor is engine rate of change of rpm. If engine load changes for example remove 1000lbs from the vehicle, the rate of change of the engine goes up, which means the piston position after a set amount of time becomes lower and lower after TDC in the bore after a set spark advance in any given gear ratio, which requires more and more advance to place the optimal pressure integral over a useful set of crankshaft angles. the same thing happens in numerically lower gear ratios, it is like removing load from the engine. Compression-rate also went up, which is helping to maintain the difference (it is why a single number can often suit a large number of conditions). If we then increase temperature the fuel quality comes into question, if the fuel can handle the temp increase the reaction rate speeds up a little bit, which takes less advance, and maybe you throw in some extra fuel to cool it down, but if the fuel cannot handle the new high temp (gasoline at say, high density + 150*F iat mixture) it will suddenly explode violently and create a large pressure spike when the spark occurs, it doesn't matter how low the timing is adjusted the reaction rate went up too quickly. The same can happen with poor fuel quality at high compression, too much input energy develops too many chemical reaction components per unit time for the engine to rotate out of the way. Another example of what not to do is dyno the vehicle in 1:1 gear for max effort and then use it on the highway in overdrive at max effort. The overdrive gear ratio lowers the engine rate of change, which puts the piston higher up in the bore after the same time period as it was tuned in the 1:1 gear, so now you've got more cylinder pressure peak and a smaller volume of cylinder to work with in the same amount of time as it was tuned previously, so now the engine is at risk of eating rod bearings and busting a gasket/ring land. If you had tuned the engine in 1:1 for minimum timing instead of max effort, then the timing on the table makes up the difference for when the rate of change of acceleration got reduced, keeping cyl pressure down to a safe range

 

None of that is new to me Mr K.
I've built dozens of this style of engine and have gone deeper into each one.
What I'm looking for are trends and ideas rather than the old tried and true. Which may or may not be suitable.
I haven't used an OEM example in any analysis. Especially late model stuff as they're too complex and as such the timing isn't representative.
But, what the aftermarket has to offer is another matter.
Some are far removed from the old school approach. Not that there's necessarily anything wrong with the old school but there evidence and examples to suggest that there's more to it
This reduced timing at peak TQ comes to mind. And continuing with the advance past that peak.
THAT'S the part I find intriguing and want to explore it further.
One fellow I chat with has put his SBC on a dyno with the Progression Ignition programmable distributer. He can make changes in real time and observe cause and effect. One of his tables exhibits that continued advance past peak TQ and is about 1° per 1000 RPM or so.
That's where I'm at presently.

 

Good tuning in reliability apps will place a soft spot at peak torque to keep max peak cyl pressure down for safety

then ramp back in the timing after peak torque because cylinder VE is falling, its like negative boost pressure so = more timing

If the engine has completely flat torque curve like a particular S2000 I know of, there is no need to add timing back for more rpm unless chamber design is poor , like an SBC probably would take more vs a late model chamber, sbc style chamber has inefficiency issues and typically needs anywhere from 5 to 10 more degrees of advance at high load.

if its an sbc tune it like a distributor is the best advice given so far.

 

Exactly as I understand it and yet the traditional approach, which has been in play for the many decades I've been involved (with a family raising hiatus in the middle) ignores that peak cylinder pressure factor. Or appears to.
We still discuss "all in by 3000" (or thereabouts) with maximum timing achieved there. No further advance.
Some newer timing models are showing less than maximum advance at peak TQ and then continues to redline. I understand that to be a factor or reduced VE and subsequently cylinder pressure is also reduced .The charge needs more lead time to create maximum pressure at the appropriate crank angle.
The old boys don't seem to pay attention to that although Bill Jenkins back in 70's references in his book the need to continue to advance the timing to the end. That's Pro Stock though and like the OEMs, hardly comparable.
So, I'm in the research stage and not having a dyno to facilitate my tuning, I want to start with a reasonable baseline. It would seem that the baseline of old isn't what it was cracked up to be.

 

Dyno playing is not engine tuning or viable strategy. Its just numbers game with dynometer data.
They can extract the max possible output from the engine on a dyno - in that controlled environment. And possibly even to the brink of engine damage without realizing it.
But that is not how you want an expensive hobby level engine to reside in YOUR vehicle. That is a disaster setting for any engine, to be on the brink of absolute most possible advance thrown in it.

This is the lesson #74:
Don't look for power with timing, instead: Increase air density and reduce timing.
The most possible reduced timing with the best density is a reliability high output setting.
The most possible added timing with any density is the failure setting.


Enthusiast goal is to make the chamber as efficient as possible first (LS swap) , and then, use the least amount of timing possible by taking advantage of the modern chamber efficiency.

 

Yes. It IS dyno tuning but it helps with observing trends.
That there's a curve becoming defined is the interesting part. How it translates to the drag strip is another matter. But it could be a reasonable starting point.
In my case, as I've done many times, I'll tune at the track for ET and MPH. Thing is , you can't hear detonation at the track. That's the disconcerting part. So, what do you do?
Rattle past peak TQ and then things settle down? Or settle for a detonation free pass through that peak but give up power at the top?
At least dyno tuning would reveal any tendencies the engine might have.
I'm in full agreement with the increased charge density. Something I'll also be working through with making my cowl induction functional.

 

This seems like a real issue here.
You, are a reliability app. Not a racing app. There is no need to use max timing or best timing for best ET.
The answer to your question is: use the minimum timing, giving up ET and MPH by a percent or two.
Make the car 1 or 2% slower, if that?
And now it will stay together.

If you want more power use forced induction.
It will make the car 200% as fast. 300% as fast. Forget about this 1.3582% nonsense in the timing.
You should be making 600rwhp E10 5.3L Daily driver minimum using a free engine with 11* of timing.
That is the least expensive way to make 600 dynojet right now

 

Naturally, reliability is a factor. It is for everybody when put in context.
Again, I'm in agreement in that the minimum timing is best. Also a safety margin.
The point of this thread, if I may bring it back around, is what is the shape of the curve I'll start with? All in by 3000 at what ever amount keeps it from knocking it's way past peak TQ? Or get through that critical part and then keep advancing to redline?
There's no conclusive way to say other testing.
But I'm asking just the same so as to solicit from others, with engine and vehicle similarities, to post up what is working for them.

As for forced induction, I'm very much in the twilight phase of this hobby. Other than dialing in the heap that I have, I wouldn't take on the investment let alone the learning curve that would accompany a switch from NA.

 

For daily drivers and reliability ,
The shape is found by comparing timing profiles near max output conditions (weight, load, gear, temp, etc...) as you reduce timing to find the minimum. It can be done with or without a dyno. With experience a dynojet makes it easier.

for example at reasonable gasoline compression ratio, at 3000rpm on a dynojet, smoothing=0:
11* 187lbf-ft 1350*F + misfires
13* 217lbf-ft 1268*F
15* 225lbf-ft 1224*F
17* 232lbf-ft 1217*F
19* 234lbf-ft 1212*F
21* 245lbf-ft 1210*F -spikey wavy curve, no longer trust torque graph
23* 248lbf-ft 1212*F -big spikes yield erroneous peak torque values

If the vehicle was 4400lbs on this dynojet result, we know the actual load and rate of change of engine will be less than on the dynojet. So whatever we decide as 'optimal' on this dynojet result will be too far advanced for the actual vehicle.
however in the range of 2800-3200lbs the dynojet does a fair job simulation.
Also if the dynojet was done in 1:1 but the owner plans to run 0.70:1 gear at wide open, our dynojet result will also be too advanced.
Also if the temp of the IAT/CTS/OIL will be higher in practice, again the timing we decide from dynojet will be too advanced

Do you see how the dyno is a tool, only meant to get us close to what we want.
What we want in my example is max reliability. Lets say vehicle is 3200lbs. The dyno was done in 1:1 and the car is only ever raced in 1:1 no highway overdrive pulls.
We look for percentage change that timing makes, and compare to EGT to see whether it is reasonable, at the most vs least timing that can produce a smooth trustworthy graph. In this case, it is
13* vs 19*
What is the difference?
234 vs 217lbf-ft of torque as an example,
or about 7% difference
We look at EGT, the number is not important but the difference is,
1212 vs 1268 about the same, 5 to 6% difference, this information is important to correlate to torque, more energy going to exhaust with timing reduction should happen.
The misfires at 11* give us a rock bottom for the lowest possible load (dynojet) to stay up from, the onset of spikey/wavy curves at high timing values give us the ceiling to avoid at low-mid load (standard dynojet load is low when the vehicle is heavy and mid when the vehicle is 2700-3300lbs)

For a daily driver app using 93 octane fuel and taking all of those other variables into account which I commented on before this,
the ideal timing for this vehicle at 3000rpm will be approx 14 to 15.5*, and the IAT tables can throw another degree at it when the temp drops 20*F or so, and another half a degree for another 20*F, diminishing returns on that.
If this was a 87 octane vehicle then we would remove a bit more timing from the highest end.
If this was a racing vehicle with many air oil coolant cooling abilities, high quality oil cooling, excellent radiator systems, good safety ECU controls, and so forth, the timing can be moved up a bit more, because the engine can maintain its environment better. Perhaps 15.5 to 16.5* is acceptable in that case, for a couple more percent of safe power, assuming it can keep those oil temps and coolant / air temps in check as promised. In an OEM daily driver app sometimes that traffic situation brings oil temps up, the air temps go high, its more abusive, it requires less timing, better fuel. And when on 87 octane especially that temp can quickly damage a performance compression ratio engine.

 

I suggest a review of fluid mechanics
Natural aspiration is forced inducted by the atmosphere, for example sea level is 14.5psi of boost which is the absolute pressure.
If you drive up mountain, boost drops with elevation.
'boost' is being produced by the column of air molecules extending into the atmosphere, it is their combined mass or weight pressing down on us that forces air into the engine and our lungs.

If you drive below sea level , pressure also increases above 14.5psi, boost increases at natural aspiration.

Thinking about NA vs FI is one dimensional: It is the same thing to the MAP Sensor which simply reports absolute pressure. The MAP can tell the difference between elevation pressure and below sea level to some point. Other map sensors go further under sea level (2-bar, 3.5bar, etc...)
The main issue with novice hobby level FI setups is negligence with respect to the energy input on behalf of the compressor.
In an NA setup, leaking air intake tubes do not have any cost because the atmosphere provides unlimited boost energy input to it's atmospheric pressure.
You could run an engine without any intake manifold for example, with direct injection, its just at WOT all the time, no throttle body no intake system is fine.
that is a bare bones concept of 'boost' from the atmosphere into an engine with no intake or throttle body.
In FI, the 'boost' coming from some compressor pump. Pumps add heat to their fluids so there is temp rise and density loss. E10 does not respond well to high temps so intercooling is needed, even though intercoolers reduce pump energy and reduce the pump capability and so all intercooling will reduce engine output and create more resistance to flow for the pump (little dot moves up and to the left on the compressor map when adding intercooler or any other plumbing).
The concept 'conservation of energy' needs to be applied to the pumping of the compressor to avoid mistakes by novice and hobby level installation.
Compressor impart energy to fluid molecules (air is a fluid). If the fluids are able to leak out (boost leaking) the energy is lost. In turbo apps this can cause engine damage because the wastegate can sense when boost is dropping off, and it will demand more from the turbine which rises EGT and EGP. With enough boost leak, the turbocharger can over-speed and creates high IAT and high EGT EGP which ruins the engine. Therefore all turbo systems are particularly sensitive to boost leaking and precautions must be taken to avoid them, such as the PRESSURE test. I always pressure test every turbo system before tuning it or I the tuner am at fault when it blow up because of a boost leak.

 

Should have vacuum advance built into the curve at idle. Idle timing should be 26-34° on most SBC engines. If it has idle stabilization timing control, generally that advances/retards the timing up to ~10° to help control the idle speed. Low-mid 20s is where the base idle timing should be leaving the PCM the ability to advance the timing to raise the idle rpm. My 383 is set at 25° BTDC base idle timing with +/- 7° of idle control timing allowed.

 

What kind of babbling BS are you spewing today after being banned or run out of other groups? RPM is RPM. The engine does not care if it is in a reduction gear, 1:1 or overdrive gear. If the engine is at 2,000 rpm or 3,000 rpm or whatever the piston dwell near TDC is the exact same amount of time regardless of what gear it is in. You can overtime the engine in lower gears with an inertia dyno but not on a load bearing dyno or holding the engine at a steady state. I tuned my engines on a load bearing chassis dyno in 2nd gear to keep driveshaft and tire speeds sane and safe. The Eddy brake on that dyno can slow down the pull or even stall the pull at a steady state on just about any powertrain combination to create acceleration rates that are slower than anything a real vehicle would see moving down the road, the exception being a truck pulling a heavy trailer. Automatics are going to downshift at heavy throttle or WOT, especially a truck pulling a load. Your IAT comment is also not something I have seen even with 200F IATs.

 

Here's where I'm at:
First, the subject in question is my 3500 lb race ready, auto equipped, 3800 stall, 3.73 geared Camaro. Lets say it's not traction limited (at least that's my hope).
Working RPM is essentially 4500-6500. There'll be a brief run from ~3000 RPM foot brake but 4500 happens in a hurry.
As it stands, going back over what I have presently and that's:
MSD magnetic pickup distributor with one heavy and one light silver spring.
Mechanical advance is limited to 18°.
Initial timing is 16°.
Forget vacuum advance for a minute (although I use one and it's very dialed in).
IIRC, advance starts at 1250 (+/-) and is all in by 3000. That's about as old school as it gets on the mechanical side.





What I see here, while it hadn't hurt the engine (as it was taken apart for a rebuild and there was zero evidence of detonation. Pistons, rings, ring lands, bearings, plugs, were all tip top.) is that at peak torque, I was arguably passing through with maximum mechanical advance. That being 34°.
If you go back to the sim generated timing table (post #1) you'll see that 33-34° was calculated as a target value. So, in that regard, I'm on the right track albeit that was, for analysis purposes, done with 100 octane fuel. That is very much a possibility when racing as well as it's cheap insurance, relatively speaking.
What wasn't happening was this continued increase in advance past peak TQ and onto the redline.
This is where this whole conversation started.
Several curves are suggested and naturally it's very engine / vehicle/ running conditions dependant. 1° per 1000 RPM was calculated via simulation.

Would this suggest that up to an including 4500 RPM I was on track, but by stopping the advance at that point, there's potential power output being unrealized?
It would appear so.
There's also evidence from other's testing that say there's a need for high RPM timing retard as at high RPM and high load, as in the top end of the drag strip in top gear, the engine can't take as much advance.
The jury is still out on that although I will say that a programmable ignition and a chassis dyno would be really handy right about now.

 

Im not reading all the posts

yes some factory efi tables show dips in peak torque. Tpi cars especially and ive found this to be the case that peak torque tends to want a few deg less especially when pushing higher compression ratios. Thats peak VE and cylinder pressure.

ive also found timing to increase with rpm and that can help extend the powerband. When the power curve fell off rapidly after peak it sometimes is not enough timing at those spots. So the all in by 3600 and lock out at 34-36 deg isnt always right for the combo. Thats always been the way because you didnt have accurate enough programmable timing control methods like we have today. A loaded dyno like a hub dyno can easily dial this in for those spots and overlay curves to verify. You can hold an rpm load and press the up and down arrows and watch hp change on the fly to dial it in if you want. Great tool if you have access to one. Dynojets dont load as good but can be fine for most lower power na deals. When max power is found i usually back off 1 deg or 2 for track when leaving a dynojet. Hub dyno if loaded right and simulating an actual power pull on a track you should be ok but can always play it safe and yank 1 deg too

 

Thanks for those insights Orr.
I figured that somewhere along the way this peak TQ timing dip would be verified through somebody's EFI tables. This is why I asked. Similarly, the advance past peak as well.
While a dyno might be a ways off (maybe not) I think I've got the means to build a curve with not so much a dip at peak TQ but passing through it with safe timing and rather than as before and calling it a day there, continue with another couple of degrees to the RPM limit.
Certainly something to work with.
The programmable CD box will definitely make that work easier, and even allow for a dip at peak, but in the meantime it's old school tuning here.

 

Please allow me to introduce myself, my name is Dr. Kal, I teach mathematics and engineering at University level with a PhD in mechanical engineering since 2023.

I know many are not all mathematicians, I will mention needed info and many important parts, feel free to work through these with math tutor some time.

Needed Definitions
Omega = angular velocity = dtheta/dt = dθ/dt
Alpha = dOmega/dt = d²theta/dt² = d²θ/dt² = angular acceleration
dOmega/dt in rad/s²
Change in heat with respect to crankshaft angle
dQ/dθ = dQ/dt * dt/dθ = (1/omega) * dQ/dt
We know from newtons 2nd law for rotation,
J * Alpha = Torquecombustion(theta) - Torque(resistance or LOAD) = Describes how torque results with net accel or decel of crankshaft
J = moment of inertia = kg*m²
The J represents how difficult it is to speed up or slow down the rotating parts being forced on by external forces, in this example it is constant, just be aware when J is larger because of heavy engine/transmission/tires etc... rotating parts, the engine has a smaller Alpha and Omega given the same input Torque(combustion).
I will leave Omega as 'Omega' and only swap theta for θ some places to tighten things up below, so theta = θ , be aware of that I use them interchangebly

Now we know what these terms stand for,
Whenevr Load increases Torque(resistance), Alpha becomes smaller, and can even be negative (engine decelerating)
Since Alpha = angular accel = dOmega/dt (change in angular velocity per unit time),

This is pressure vs crank angle equation for combustion thermodynamics:
dPressure/dθ = gamma*(Pressure/Volume)*(dV/dθ) + ((gamma - 1)/Volume)*(dQ/dθ)
Here is an example of spreadsheeting cylinder pressure over crankshaft angle to generate a pressure integral of torque over time example for a 2jz-gte engine:

We relate heat per crankshaft angle to Pressure:
dQ/dtheta = dQ/dt * dt/dTheta

We can do this because we go from time to crankshaft angle,
Omega = dtheta/dt then dt/dtheta = 1/omega, a reciprocol
In other words, If crank angle (theta) increases with time at rate (omega), then the amount of time it takes to sweep one unit of crank angle is a reciprocal: dt/dtheta = 1/Omega
Now we have change in heat with respect to crankshaft angle
dQ/dθ = dQ/dt * dt/dθ = (1/omega) * dQ/dt
When load increases(resistance torque), engine is decelerating, Alpha and Omega is smaller at any point in time t,
Volume(theta(t)) The volume of the cylinder is smaller at time t,

When load increases (resistance torque rises), engine decelerates, alpha becomes smaller, and angular velocity will be smaller over time
Its another way of saying that, the higher the load, the more slowly the engine accelerates, because its producing the same torque with more resistance.
Since dQ/dθ = 1/omega * dQ/dt, Alpha affecting omega at time t,

When Omega is getting smaller over time due to Alpha decreasing, dQ/dθ is getting larger per degree of crankshaft angle
Which contributes to larger pressure rise per degree of crankshaft angle


Take it up with combustion thermodynamics equations. I am not suggesting anything; I am teaching. You may require more education. Feel free to point out any math errors I have made...

 

Apparently, according to Bill Jenkins, there is a difference with respect to what gear the car is in.
Relating to drag racing, the top end charge, when hanging it out in top gear for an extended period of time (compare 1/4 mile to 1/8 mile racing) then he found a need to pull the timing back at the top.
From his book:



Naturally, his reference is to his Pro Stock engine from the 60's/70's but the concept might apply in other applications too.
In my case, which isn't a Pro Stock engine, this reduction at peak TQ is an interesting thing to work with.
As for the top end, I can see how it's kind of split between continuing to advance past peak, holding steady (as is the old school method of "all in by") or high speed retard.
Dyno time or a lot of fiddling at the track required to determine what's best in this case.

 

I think you did not understand what I was saying. I said it is possible to overtime an engine with an inertia dyno, but also stated that is not what I used to tune. I said when you test on a loaded dyno that can hold the powertrain to a static state or even decelerate the rollers at WOT there is no need to remove further timing even if you are in 2nd gear during that loaded pull. If the acceleration rate is stalled to less than anything you would see in the vehicle the RPM IS the factor for timing advance. I have not seen any street engine really benefit from retarding timing at higher rpm in any gear or should I say instead over advanced timing in lower gears. The basic engine in question by the OP does not have the crazy acceleration rate of a ProStock and thus most of what KingKnowItAll and for that matter Grumpy states in that paragraph does not overly apply to it.. Most want a timing bump at lower rpm to help flash the converter and traditional kind of timing curves. Using a LS computer the timing map is based off cylinder air mass. For the record I really do not care what he writes in any of these long drawn out paragraphs as I trust my over 20 years of actual tuning experience and hundreds of tuned vehicles.

 

I have real world education as in 20 years of actually tuning stuff to run. I understand what is going on inside the engine and I do not need to do math like that to understand where a SBC needs to be timed to make power. As I stated, how I calibrate MBT for a combination, engine acceleration is NOT a factor because there is none to extremely little. I have made pulls with the acceleration rate at 100 rpm/sec. When I am doing timing adjustments it is often steady state load as well, the dyno is set to hold a specific RPM. You load the engine to a specific throttle, increase the timing, torque output increases or decreases and the RPM is held steady. With low acceleration rates, a full pull from 1,000 rpm to 6,500 rpm in 2nd gear takes 55 seconds. I ran the Texas mile in a previous vehicle I owned and I ran nearly 160 MPH at ~4,700 lbs race weight and that run was a much shorter than 55 seconds.

I have also done a lot of high load, steady state tuning in marine engines and even some fan boats powered with modified SBCs and LS engines as well.

Like I stated as well I have run full load and full throttle with IATs at ~200F with a stock GM engineered airbox and air filter setup and never ran into runaway detonation that cannot be stopped. More fuel to pull out the charge heat, slow and cool the burn is the answer not trying to control it via spark retard.

 

I would agree and that's where the difference would be then.
Loaded on a dyno wouldn't make a difference regarding what gear you might be in.
That however, that can't necessarily be translated to what happens on the track. This is where Jenkins was describing the extended time spent in high gear as opposed to the amount of time it spends in the lower gears. Engine load is increased, acceleration is reduced and therein, in Jenkin's case, is the need for reduced timing in 4th gear, WOT.
But, as it's been stated, every engine or even the running conditions will have an impact on the timing needed.

I envy you your dyno time. I wish I could say the same. Very few and far between but it's in the grand plan to get back to one.

 

it may not have a dip, it depends. If its on a race gas and not going to detonate, it may not have a dip but a gradual rise to max rpm. Dip is more for timing sensitive engines riding edge of detonation based on pump gas or something lol. Or weak head sealing engines, like stock bottom end turbo ls stuff. You dont want high cylinder pressure so you dont run timing down low and dont put boost down low rpm. You run them high rpm for less average cyl pressure.

ideally we would have in cylinder pressure sensors to monitor the combustion events and find what gives max pressure where we want it and be safe. Dyno you can basically find what makes max torque at the rpm point in question and when you see little to no increase in torque there for each deg or half deg, its done. And potentially overtimed by 1 deg or two if you added some and no change.

Regarding time in throttle and timing, yeah you tune differently as heat builds up with time. So stuff that goes wot for minutes or hrs at a time are done way different than a short seconds long drag pull

 

What Grumpy was doing was over advancing the timing in lower rpms. I bet he pushed his cars to the line as well and had the combustion chambers and cylinder heads as cool as possible at the start of the run too. Likely what he was seeing that was affecting the advance required in the biggest manner was actually the rising combustion chamber temperatures more than the load of being in high gear. GM ECMs and PCMs have had had timing modifier tables in relation to coolant temperature for decades. Cool engines want more timing and vice versa. If the engine is overtimed as the coolant temps increase, chamber temps increase and the engine can start to knock especially during a steady state dyno test. You can actually watch the torque fall off as the combustion chamber heats up.

 

Being a former student of mechanical engineering I can appreciate all of the math and science that is used to calculate, predict and model something like a timing curve.
Thank you for that.
While the theoretical side can be great fun, especially if one enjoys mathematics as I do, for a hobbyist such as myself, the methodology can be somewhat simpler.
I gather anecdotal, real world, theory (as in the math posted above) and simulation based results, which helps form my timing table "hypothesis".
Then, at the very least, I'm not blowing through hours of testing and analysis at the track to get to where many have gotten to before. The phrase "standing on the shoulders of giants" comes to mind.
What this thread has done, and you're very detailed equations and computations notwithstanding, has given me what I had surmised from the beginning.
Does an SBC need timing extended past peak TQ? Should it be trimmed at that point? Is "all in" at X RPM a viable approach?
The answer to all of those questions is, it depends.

The other takeaway is that there's really no substitute for dyno time. Although it has its limitations, presented by the likes of Fast 355 and Orr, it's still a useful tool for discovering trends. If some or most of that can be eliminated prior to track days, then there's more that can be done outside of that particular bit of tuning. Or allowing more time to get into the details of that rather than fumbling with wholesale changes.

It's going to be a timing light, tachometer and vacuum pump to build the base curve(s). The it's track time. I'll be able to step it up a notch with a programmable ignition and explore it all further but first things first.
One step at a time. Yet again.



I miss my old friend.

 

I will say my bbc turbo car was able to greatly extend the power curve with increased timing past where i thought peak hp would show. I had a drop off in power that I thought was valvetrain related, perhaps too small a cam. I had basically had timing almost locked above 5500 rpm. It might have only increased 1 deg to 7500. I started going up past 6500 to 7500, with nearly 2 degs total, just a straight line increase. (Holley will fill in rows for you, place a number in the high end of the row and the low end, then just highlight the cells and fill rows, it will do a linear fit.). So instead of holding basically 30deg across 6000-7500, i basically ramped from 29-29.5 to 32-33 across that span. Hp gained past previous peak and basically moved from 6400-6500 ish to 7100-7200. So it was definitely under timed up high and really needed more. Since peak torque came in with peak boost in that 5500-6000 range, i didnt want full timing there, like locking it out at 32-33 deg. Stuff like this the dyno is a valuable tool to see the affects. Using timing as a power curve manipulation and just finding what the engine wants rather than what ppl say to do

 

I think that last half of that last sentence is the big takeaway from this.
It'll be a case of test and tune as it's always been.
More than anything, I'm exploring possible outcomes, ie, locked out, extended advance, etc as I don't want to waste time persuing something that's been proven to be less than effective.
New distributor lands at the end of the week so I'll be plenty busy getting a debrief on what I've got presently, what the new distributor is out of the box and how I'll go about building the curve that, from what I've gathered through these and other conversations, might be best suited.
Mechanically speaking, I like to start at 20°, ~1500 RPM and be basically linear to 34° at 6500. More than likely though, I'll try what's worked (but untested really) and that's the "all in" approach at 3000-3500 RPM, which IIRC, is what the old MSD is curved for.
I do run a vacuum advance and previously I had great results with full manifold vacuum. I'll have to experiment with that and ported vacuum this time as this new engine has different idle characteristics than the old engine.
Was 14° static, 16° idle vacuum (for 30° total idle timing) and another 18° mechanical. That meant, more less, 45° of advance at cruise RPM. The engine seemed very happy and just on the edge of light rattling. A functioning cold air intake would probably have solved that. Another project for another day.

 

Your question may be loaded. If timing peaks are low (in terms of total advance), peak torque would theoretically be limited to a lower RPM. Since insufficient time for the burn of a fuel charge with the piston near the top of stroke results in lower effective pressure (torque), the point at which that pressure begins to decrease, and the rate of that decrease compared to RPM, will define the answer. There are probably practically innumerable ways to build an SBC, given the variety of potential components. Add the variables of fuel, environmental conditions, and total load, and the potential variations are multiplied further. If total advance is limited to 30°, for example, and that provides a torque peak at 2,700 RPM, NOT increasing that advance can prevent any chance of maintaining that output power (torque) level much above that 2,700 RPM. If, however, total advance is allowed to reach 40° that may support power at a higher RPM since the fuel charge burn would still achieve sufficient pressure while the piston is still near the top pf stroke. Conversely, allowing more advance may decrease power at various RPMs, but math alone is an impractical method to determine that, even though it can be a somewhat reliable predictor.

The results of dyno testing usually reveal those points of peak torque, and indicate how much of that torque is maintained to a given RPM, and THAT'S the experimentally derived optimum. Your statement "one step at a time" is certainly applicable.

 

Interesting insight into the advance vs TQ output vs RPM.
Something I was aware of but hadn't worked that into possible outcomes.
I've a simulation curve generated by EA and stated maximum advance at peak torque is about 31-32°.
It would be interesting to see if can manipulate that sim to have reduced timing at peak TQ and see how it moves the torque curve.
As it is, as always, I'll sneak up on it. But the dragstrip can be a difficult place to tune. Especially on a long summer nights up here. The DA swing from late afternoon (start of test and tune) to closing at 10 pm is huge. And traction tends to go away as the moisture moves in and the track cools. Cause and effect can be hard to determine.
If I'm struggling, I may have to move up the dyno date before I go programmable next year. Or invest a little more and a little sooner and get digital box before the season is over.

 

I have not seen an engine not respond to increased advance past peak torque. Cylinder filling is greatest near peak torque and falls off as the engine revs higher.

 

Did a little more digging into how I could implement a curve as posted above with a weights and springs distributor.
Turns out Grumpy Jenkins had this same issue and revealed his solution in his book. (Which I don't have but will be looking for a copy). He was looking to build about 1 1/2° per 1000 RPM with no upper RPM hard stop.








While the numbers he achieved aren't relevant here, it's how it was done that has given me some direction. I hadn't given much though to lightening the weights although I was aware of this modification. Should be easy enough to experiment with. The springs might take some doing though. I don't have any that heavy in my collection of springs. I'll need to get creative.
Anyway, as I was saying, this may or may not be anything that'll be a benefit in terms of performance but I'm keen to give it a test toast when I'm back trackside.
Could be this engine likes the all in approach. Then again, as has been stated here, engines do respond to this shape of timing curve.

 

Personally if I were still running a carb, I would just get a Progression Ignition HEI and be done with it. No mechanical advance or vacuum advance and the timing curves can be remapped with your smart phone. It also has a kill switch function that will prevent the ignition system from creating spark until you disable the anti-theft with your smart phone. Another nice feature is that it has a built-in function that helps control idle speed. You program what idle speed you want and the ignition system advances and retards the idle timing to help maintain the correct idle speed.

This was a test of a Progression vs Davis Unified with a mechanical curve.

https://youtu.be/_MMCnP8570U?si=M1OtGhn3B5jDwbHh

 

Progression Ignition has been on my radar for a couple of years.
I thought I had a workaround in that once I dialed in my distributor (because it's what I had) I'd get an MSD Digital box to take the place of my old 6AL. The Ultra 6AL Plus gives the same programmable options as Progression. What I liked about it was the big company name that supports it rather than some smaller shop proprietary bit of software that I'd have to rely on for possibly several years. That said, by the accounts I've read or heard about, Progression Ignition has a solid track record. Truth be told, if I had seen that they offered a slip collar version, I may have gone that route rather than a like for like replacement for my well worn MSD distributor.
As it stands though, the new Pertronix may get returned. It's behaving in the oddest way, in that it retards on RPM increase. I thought it was a rotor phasing issue but I' m reasonably certain I've addressed that. If the old MSD works as it did before, I'll send the Pertronix back and get the programmable distributor.