Reading KM to MPH and when to shift

[Ben C34]

What if you also put your foot on the brake to slow down if you are  accelerating throttle and going too fast?

[niZmO_Man]

What if you turn a corner?

[joshuaho96]

13 hours ago, Murray_Calavera said:

Can you explain how you came to this conclusion? 

I am interested in the math and any other data you are relying on to make this statement. 

 

Depending on the engine the map might look different but you can see that full throttle 1500 RPM to have the same power as you would at 2500 RPM you need to basically be right around full load. Your BSFC is going to be in the high 280 g/kWh range if you keep the engine at 1500 RPM. If you instead let the car sit at the 2500 RPM or whatever RPM it wants to really be at you can instead be in the 240 g/kWh sweet spot for thermal efficiency. So the conclusion for turbo engines like this one is basically try and stay right at the edge of boost if you're accelerating. If you're cruising you want to maximize load by using your gearing, but you don't want to overdo it either.

The US EPA has a lot of very nice public data on some engines where they took the time to run through the dyno with a fuel flow meter to derive BSFC and thermal efficiency: https://www.epa.gov/vehicle-and-fuel-emissions-testing/combining-data-complete-engine-alpha-maps

Maybe you guys have less traffic but around here you're almost entirely limited by traffic and lights unless you get out of the city and into the back roads. Trying to drive efficiently helps to make things a little less boring.

[Murray_Calavera]

I don't know how relevant this is to anyone with an aftermarket ECU and bigger turbo. The manufacturers have a very different set of goals when tuning, emissions control being top priority for example.

There are things we can do, for example targeting a AFR of 15 during cruise and low load, which the manufacturers can't due to the massive spike in NOX by doing this. 

Then factoring in AFR targets for when boost builds, I would imagine that most would richen their mixtures very quickly when boost is coming on. 

Then we have the matter of larger turbos not used by the manufactures. Safe to say that at 1500 RPM, anyone with a big boy turbo will not be making boost. However 2500 RPM, boost will be building and depending on the tune you will be targeting richer mixtures. 

I've attached my AFR target map for a visual aid. Feel free to laugh at my tune, I do not care about fuel economy. 

[Murray_Calavera]

If it wasn't clear what I was trying to say - 

What uses more fuel, a non-turbo 2.5L motor or a turbo 2.5L motor? It's pretty obvious. 

And if you care about fuel economy, you can make your 2.5L naturally aspirated motor target a leaner mixture. Leaner then what the manufactures can.   

[joshuaho96]

7 hours ago, Murray_Calavera said:

I don't know how relevant this is to anyone with an aftermarket ECU and bigger turbo. The manufacturers have a very different set of goals when tuning, emissions control being top priority for example.

There are things we can do, for example targeting a AFR of 15 during cruise and low load, which the manufacturers can't due to the massive spike in NOX by doing this. 

Then factoring in AFR targets for when boost builds, I would imagine that most would richen their mixtures very quickly when boost is coming on. 

Then we have the matter of larger turbos not used by the manufactures. Safe to say that at 1500 RPM, anyone with a big boy turbo will not be making boost. However 2500 RPM, boost will be building and depending on the tune you will be targeting richer mixtures. 

I've attached my AFR target map for a visual aid. Feel free to laugh at my tune, I do not care about fuel economy. 

Your 14.7 AFR gas target is stoichiometric. That's pretty normal and where the TWC operates best. You can run leaner like 16 or 17:1 AFR gas for cruise but as a general rule the efficiency benefit is not going to magically turn a throttled PFI gas engine into a diesel engine in that regard. I don't know RBs well enough to say whether they can efficiently cruise lean, some engines really need to stay near stoichiometric, some particularly bad engines like the L26 4.9L actually had to run rich to avoid stalling and did an "interesting" trick where it would operate at stoich and run like complete crap until you turned on the AC or heater at which point it would enrich the mixture.

Even a turbo RB26 can wait until ~3-5 psi to start enrichening, it's a 8.5:1 CR naturally aspirated engine until the turbos really wake up and the ITBs really screw with intake runner pressures so it's really easy to hit 0 psi. Helps a bit with the spool as well to keep EGTs as high as you dare. I don't blame you for staying conservative beyond that though, RBs are ridiculously expensive to build and kind of fragile. 

Generally speaking the problem with staying at too low an RPM is that the engine is just not tuned to breathe well at those RPMs, either because of valve timing or intake runner tuning or exhaust runner tuning or some other fluid dynamics problem I don't understand. If injection timing is not adjusted to avoid the low lift portion of the intake cam opening you can also experience higher covariance of IMEP from stuff like pre-ignition. So the best strategy is to increase the RPM to the sweet spot, whatever it is, and stay right around 100 kPa MAP to maximize efficiency. NA engines tend to have less dynamic range and you tend to want to be closer to ~70-80 kPa MAP instead.

It's not a perfect comparison but we can compare the 2.0L Skyactiv-G to the 2.5L Skyactiv turbo engine:

 

If you squint and compare between these two engine maps actually at 180 Nm of torque at 2500 RPM the Skyactiv turbo engine is more efficient, not less. If you drop down to 140 Nm of torque the 2L NA has a ~4% advantage in BSFC. As you go down you can tell the turbo engine is less efficient due to the extra displacement, but that just means you need to apply some more load, either in the form of accelerating harder, gearing, or increased speed. Or your car is just too light. So as a general rule I think if you drive a turbo engine correctly and use the lower compression ratio to run right around atmospheric while accelerating and keep the load high the efficiency hit is not terrible, and when you compare to a bigger displacement NA engine the efficiency will be better at low loads, but if you have a lot of boost enrichment like these early turbo engines then pretty much any kind of scenario where the turbo engine has to run boost + boost enrichment and the NA engine can stay out of high load enrichment is an easy win for the NA engine. If you toss tricks like E85 or water injection in though you could keep the same amount of torque and reduce enrichment for better efficiency. Same goes for GDI which allows for quite lean AFRs even at WOT.

[Ben C34]

What the f**k,?

Just drive it. You don't need a map of the engine

[joshuaho96]

If you want the answer to the question of when to change gear the map helps though.

[GTofuS-T]

If you're concerned with fuel consumption maybe don't buy a skyline?

[joshuaho96]

I too disable DFCO and run a lambda 0.7 target globally for all my tunes because fuel consumption doesn't matter.

[Murray_Calavera]

Ah mate, don't be like that. You're on Skylines Australia, not Corollas Australia after all. 

[joshuaho96]

So clearly then fuel consumption does matter? Or it doesn't? I'm unclear here. Just because a high performance vehicle has a high energy per mile requirement doesn't mean that it's pointless to try and optimize the efficiency with which fuel is converted to KE.

[r32-25t]

No one except you and old mate that started this thread gives a shit about fuel economy in a skyline

[Ben C34]

Dfco.  I have it off, man I am bad. Oh the theoritacal savings out the window..

[joshuaho96]

30 minutes ago, r32-25t said:

No one except you and old mate that started this thread gives a shit about fuel economy in a skyline

I have no idea who started this thread, it's bizarre that you would suggest this is some conspiracy to troll people. OP asked a question, I gave the actual answer to the question. Do you guys just sit around ridiculing everyone that has a potentially dumb question like that's a productive use of time? You must be a very cool person to spend your life like that.

[r32-25t]

I’m sorry that you see the need to get defensive about the fact no one cares about fuel economy in these cars 

[joshuaho96]

4 minutes ago, r32-25t said:

I’m sorry that you see the need to get defensive about the fact no one cares about fuel economy in these cars 

Excellent, why does it matter in the context of answering OP's question?

How badly other people screw up their cars with bad engineering and tuning is really not something I'm concerned with, it's not like I'm the one that has to live with the thing. It just seems ridiculous to answer a question about shift points with non-answers and gatekeeping.

[GTofuS-T]

5 hours ago, joshuaho96 said:

Excellent, why does it matter in the context of answering OP's question?

OP said nothing about shifting for fuel efficiency... so you're answer os way out of context on a Skyline forum... tbf... OP was very vague how he came up with his own shifting guide, but don't claim your answering his question directly.

[joshuaho96]

7 minutes ago, GTofuS-T said:

OP said nothing about shifting for fuel efficiency... so you're answer os way out of context on a Skyline forum... tbf... OP was very vague how he came up with his own shifting guide, but don't claim your answering his question directly.

If fuel efficiency is immaterial then there's no reason to have any shifting algorithm at all. It literally doesn't matter as long as you don't stall the car or lug it to death. Or you could just stay in the lowest gear possible at all time which I suppose is an algorithm, just a terrible one that I've never seen anyone do. Anything other than those two choices implies that fuel efficiency matters at least a little bit, if only because running an engine at WOT all the time will kill it sooner than later.

You can data log pretty much every automatic transmission within the last 10-20 years and see that it's going to try very, very hard to put you right at the max efficiency point for any possible power request give or take some hysteresis and other second order considerations. My original recommendation is to emulate that behavior, with the added benefit that you can be more intelligent and predictive than a TCU that will never be able to know whether you want to maintain momentum when coasting and select the highest possible gear without stalling to reduce engine braking or if a long red light is ahead and to downshift aggressively to maximize engine braking. There is nothing in that guidance that says you shouldn't rev the car past 3500 RPM under any circumstances. It depends on the power you're requesting from the engine. If you want to extract maximum power then you have to go WOT and keep RPMs as close to the peak power RPM as possible on average. If you want something in between max power and max efficiency then it's further down the RPM range.

If anything if you think carefully about what I'm saying there is a minimum amount of power you should be using from the engine because below that point you're going to be running it inefficiently. If you're going to accelerate you should accelerate enough to apply a good amount of load on the engine. Gasoline engines are also terrible at doing something like cruising at 30 kph unless you're driving a kei car so driving below the speed limit for the sake of "fuel efficiency" is stupid. If you can do 120 kph it makes sense to do 120 kph because it's likely that your mpg will be constant, unless you're limited by gearing and lack a ratio tall enough to keep RPMs low enough.

[GTSBoy]

2 minutes ago, joshuaho96 said:

Or you could just stay in the lowest gear possible at all time which I suppose is an algorithm, just a terrible one that I've never seen anyone do

You haven't met my sister-in-law, clearly.