Help me understand power vs economy as it relates to tuning [Archive]

Power Hungry

Tue, June 1st, 2021, 02:42 PM

Hypothetically: Let's say that I drive around, I'm light on the throttle, and I never exceed 2200 RPM.

Outside of shift points and torque converter lockup, how can a tune impact my fuel economy?

I'm asking this question, because every "custom" tuner seems to offer a wide variety of "economy" vs tow vs various power level tunes and I just don't really understand what I'm really picking between (behind the scenes).

If higher EGTs come from more fuel with less air, then lower EGTs should mean more efficiency, right? And, towing tunes are focused on EGT management? So is an "economy" tune basically just a towing tune with earlier shifting?

Also, when using very light throttle input, what's the functional difference between a super aggressive performance tune and a towing tune (for example)?

I'm just looking for a deeper understanding of what's actually going on with the myriad of tunes that seem to be available from every tuner, and what I'm actually choosing between (functionally speaking).

Hey there....

So, I understand the reasoning for the questions, but I'm afraid you might not like the answers. There are as many different schools of thought on tuning as there are tuners, so there is very little consistency for any given type of calibration, whether it be performance, towing, economy, or otherwise.

With that said, I will do the best that I can to explain our process and logic behind the way we tune. This may compare to some degree with other tuners, or it may be completely different. Hard to say. :shrug:

Moving forward, your observation about economy, efficiency, and EGTs are pretty spot on. When the engine is running at its most efficient, the heat generated by the combustion process is turned in to both downward pressure on the piston as well energy to drive the turbo. This results in minimal waste heat coming out of the exhaust and is reflected as reduced EGTs. Of course, high fueling conditions under heavy loads are always going to cause a rise in EGTs, so when we say "low EGTs", when mean that in a relative sense. For the same fueling, boost, and load, lower EGTs are going to that the engine is generally running more efficiently.

When you are looking at "economy" tunes (at least from us), you are going to see certain characteristics in those tunes. Among these are:

Earlier shifting is a common attribute as this helps to keep RPMs in a lower, more efficient range. You'll find the shifts and converter lock points to be close to stock, if not lower that stock in some cases.
Conservative Low Boost fuel controls that are pretty near stock are common as well. This helps to reduce the amount of smoke generated until the turbo can spool, which ultimately reduces the amount of fuel that isn't being utilized to make any power.
Increased Injection Pressure has several benefits to both performance and economy. By increasing discharge pressure at the nozzle, you are subsequently increasing fuel atomization which helps to more completely consume the fuel during the combustion process. This also increases the fuel quantity injected without extending the injection event length, which helps reduces the amount of unburnt fuel reaching the exhaust and reduces EGTs.
Advanced Start of Injection (timing) also helps to keep more of the heat energy in the cylinder, and ultimately makes more power from a specific volume of fuel while also reducing the EGTs.

By contrast, performance tunes will almost always have later shifts, more aggressive low boost fueling, and extended pulsewidths to help get as much raw power as possible. This can often equate into higher EGTs, although good tuning will still be designed to help to improve the combustion efficiency, so you'll still see things like advanced Start of Injection and increased Injection Pressure as they still make power by increasing efficiency.

There's also some mentality that needs to be considered when designing different tunes, based on the specific expectations in regards to drivability. Performance tuning often contain shift patterns that are both late and aggressive because this is the old school, gasser mentality created by decades of automatics with high stall converters, shift kits, modified valve bodies, and adjustable vacuum modulators. The idea is that if you're not close to redline before the shift, something isn't right. It's hard to get around this concept, so we just deal with it as best as we can.

Tow tunes, in effect, are a blend of both performance and economy. Being that we are anticipating a load, we construct the shift patterns, converter lockup, low boost fueling, and other operational characteristics to achieve good pulling performance, reduced shift hunting, and keep the engine speed in a higher RPM range than would otherwise be considered the most efficient in an effort to keep EGTs down.

It's understood that diesels are generally most efficient under 2000 RPM, and this has to do with characteristics of diesel engines that are far beyond the scope of this conversation. However, under heavy load, high fuel conditions, the airflow through the engine under 2000 RPM simply isn't adequate enough to keep EGTs in a reasonable range. Downshifting to increase the RPM helps to improve airflow through the engine as well is getting the turbo moving, both of which help reduce EGTs while still proving solid pulling capability.

Of course, we could go an about this all day but I think what I've outlined here should at least give you some idea of what's going on.

I hope this is helpful.