Ignition timing vs vehicle weight and tire size? (or overall Inertia)

Is there a general rule of thumb for pulling timing for some amount of weight increase (or even changes in tire size)? I'm tuning my 2010 f150 (4.6 2v, stock ecu) and my total timing is 11 degrees at 6k rpm. The majority of the mustang tunes I'm seeing are pulling between 26-30 degrees at 6k. (All assuming an air mass/load of ~.78 to .8 at WOT which is the highest I've seen it read)

Why would there need to be a difference in timing with an increased inertia? I would imagine there would be an increase in cylinder pressure because it takes more force and time to push the piston down. Does this cause increased cylinder temps to the point of detonation? I would think as long as there is no preignition there would still be an even burn and combustion process where the increased inertia won't make a noticeable difference to the actual combustion event outside the pressure increase.

Justin,

That's an awesome question, really insightful, so great job on that.

Tire size and weight impact on load is often forgotten and I've seen some interesting dyno testing on that.

I often use a Mustang dyno, so if knobby tires come off for dyno time I'll add load via vehicle weight or grade settings to account for it as I'm tuning. The same can be done to simulate pulling a load via a trailer or in the bed.

Anything resisting forward motion whether it be big knobby tires, weight of the vehicle or load it's carrying, body work or accessories that increase aero drag going down the road, going up a hill, all adds to the strain put on the engine.

Generally speaking for a given powerplant any additional vehicle load, aero drag, resistance to motion, will result in higher engine load to achieve a similar outcome, whether it be cruising at 60 mph down the highway, or accelerating.

That additional load and strain compounds over time in the form of heat, so that's one factor.

Another factor is perhaps gearing between the two vehicles. The shorter the gearing, the greater the mechanical advantage the engine has over driving the vehicle forward.

Another factor is completely unrelated to this discussion so far, and is the nature of the two vehicles. Ford expects a truck to be used like a truck and a mustang to be used like a sports car. They factory tune the truck leaving more margin of safety for lower quality fuel, hauling loads, being stressed for longer durations i.e. towing or snow plowing for example.

Also, I don't know if the cylinder heads are the same on the engine's you're comparing. Sometimes the cams are different for truck vs. sports car to tweak the power band better to the use case, and that can alter the ignition timing needed. Sometimes chamber size, vale size, or cam timing are tweaked between models as well, so that MAY be a factor in what you're seeing.

Back to what you can actually see in your logs, you'll likely see more cylinder air mass/load for a given behavior in the truck vs. car, and that additional cylinder pressure results in additional heat and stress, so that's what brings the engine closer to detonation. It does not give a 100% picture of load though in my experience. For example if you log the truck doing a dyno pull in 2nd then 3rd gear you might log the same air mass, so the PCM operates as if the load is the same, but the engine isn't really having the same experience due to the gearing, and then on the road the aero drag is not the same either. It's also a longer pull, and heat compounds over time, creating hot spots which can lead to detonation.

I always give people the comparison of thinking of waving your hand across the flame of a stove or campfire, how you barely feel it if you move your hand quickly, but knowing if you moved more slowly, or held your hand there, you'd get hurt.

I hope this helps!

Thanks for the response, Mike.

So basically, engine load will be largely different even with the same air loads, leading to a possibly increased cylinder temp which needs less timing to remain detonation/knock-free? In this case, does it mean the MBT timing will be different between the vehicles? (or does MBT stay the same and a higher octane is needed to stop detonation) Because on comparison of the MBT tables they are within +-2 degrees of each other. From my understanding, Ford finds the timing for MBT using 110 octane (no idea of true). In that case, could I copy-paste the MBT table and then remove timing for the change from 110 to 93 octane?

While I am Stroked and cammed I know I will have minor changes to the timing I need for peak torque compared to a stock engine, however I would think the MBT table should find me a starting point if I make changes based on a known octane change.

Justin,

I think higher average load over time is perhaps a good way to think of it when comparing the exact same engine being in a truck vs. a lighter car with reduced drag. Then the higher average load brings with it higher average temperature over time, and that heat has a compounding effect. If MBT values in the two stock tunes were almost the same, then perhaps they did start life the same or very similar.

That said, you've now mentioned a massive piece of info that changes everything. Having a stroker setup and aftermarket cam changes engine behavior in fundamental ways, and ignition timing needs to be optimized again. That means new MBT values, new actual values you'll want to operate at on 93 octane, and the gap between them likely will not be the same as before. The combo of air/fuel ratio and timing that works best may have also changed as well as other factors that impact optimal ignition timing on the pump gas you'll be running.

While triggering and displacement may not have changed, and the old tune can help you get the new setup running, I would think of it more like you swapped a different type of engine in the car, rather than you made a small tweak, and hopefully that mindset will help as you creat a new calibration for this new engine package.

Mike,

Thanks for all the amazing info. I believe I can get myself moving in the right direction.

On a side note do you have any suggestions on knock sensor devices (I'm road-tuning if that changes anything)? Or is there a way to change the "window offset" of the stock ford sensor settings to increase sensitivity? I, unfortunately, found out the hard way that the stock knock sensors on these motors are useless because upon a teardown for ring gapping I found abnormal skirt coating wear due to what I believe was spark knock. (I had been running 26 degrees of timing)

Justin,

I haven't seen a stock ECU solution for that engine which allows for that level of adjustment of how the knock system works. It's possible there's something out there that I'm not aware of though.

In terms of aftermarket ECUs, yes the good ones allow for adjustable windowing, adjustable frequency range, sensitivity/gain per cylinder and more.

If sticking with the stock ECU and it doesn't offer what you're looking for, an external knock monitoring device which has an output you can wire in and log alongside engine data is perhaps the best compromise.

Good question, and Mike covers almost everything that comes to mind.

Gearing is important, not only because it alters the acc'n rate on the dyno', but because in practice, at the same speed, rpm drops but torque load increases as the gearing is raised - eg, 3rd, 4th, 5th, will load the engine differently.

Gearing change will affect the time under load and heat soak! This is a HUGE problem when "tuners" use inertial dyno's for vehicles where they are under load for more than a few seconds - Mike mentioned heavy haulage and towing on ehills, where heat soak can require several degrees of timing pulled compared to a flash load. How often have you seen people doing a run then giving the engine a "cool down" period before the next? On the road the engine doesn't get that period, so realistically, other than a drag-only tune, should be done with everything hot. Worse, MANY "tuners" who REALLY should know better deliberately do their pulls at around 150f, well below the operating temperature, and engines they KNOW are 100% street vehicles!

As Mike also mentioned, you've made big changes to the engine, with the longer stroke not only potentially increasing static CR, and the compression rate from the rod to stroke ratio, but the camshaft will also affect the dynamic compression ratio and cylinder filling.

You may also have made other changes, such as exhaust/headers, intake, even the filtration can make a difference.

While you may be able to use the OEM settings as a base level, you can expect to have to change the WHOLE fuel and ignition curves throughout both the RPM and load ranges, and maybe even the coolant, air temp, etc, corrections, too.