Anyone else striving for better MPGs by adding HPs?

[Lonesome Trail]

Just curious, feel like a black sheep with my lists of car modifications, lol.

I've always been a fan of fuel economy comes with a better controlled right foot (or left for the Euro folks) and though it's labeled me a granny driver, it also makes for more money I can buy Dr Pepper with when I fill up the tank.

That said, the engineer in me (software engineer for Intel for almost a decade before striking out on my own) is all about numbers, and the mechanic is all about power and efficiency, so for me achieving better volumetric efficiency (HP per L or CC) naturally goes hand in foot with better economy. Yes you can argue that more power requires more fuel, however that's only when the engine is producing that peak power output. The rest of the time a more efficient engine is just droning along, AND because it is more efficient in terms of power production you are producing more energy to the wheels and less to the exhaust and atmosphere at the same 'load' you had before (Newton's Laws).

Ergo, a more efficient engine of same displacement makes more power AND 'can' achieve better economy, assuming the same driving environment. Having your cake and eating it too.

Anyone else thinking along the same lines?

[oil pan 4]

Only if it's a turbodiesel.

[Xist]

Well, your explanation makes sense to me, but probably the only consensus here is that I am not an expert. However, I wonder if increasing the horsepower and maintaining speed with lower RPMs would necessarily equate to the engine being more fuel efficient.

I was a substitute teacher out in Queen Creek and San Tan Valley before joining the Army in 2008. My sister lived out there and we called the whole area Queen Creek before it officially became San Tan.

I just remember trying to find new schools for the first time and intersections were missing street signs. That is my main association with Queen Creek, although I wonder if that was the unincorporated portion.

I once stopped at the Filiberto's and had never seen orange salsa. I loved it!

Welcome to the site!

[V6MustangFTW]

I would think more power would let you shift earlier and have lower rpms which would improve mpg but I'm not an engineer so I don't really know.

[Lonesome Trail]

LOL, yeah I'm not a non-believer, more of a sideways thinker...

Queen Creek is huge now, have our own Harkins, WalMart and everything! Even pass through 2 counties, Maricopa and Pinal (I'm on the Maricopa side near Sossaman and San Tan Blvd, still rural though)

I just let the results speak for themselves. It's pretty amazing the looks I get when folks see my little notebooks of fuel numbers (I keep a log for tax purposes, small business owners FTW!) It freaks people out that my lopey cam Fox Body pulls pretty consistent 21-23 depending on if I get stuck in traffic, but I always point this out to folks: When I'm on the freeway and am going up the hills (we have overpasses every mile, it's fabulous for fuel economy) I just sort of imagine my throttle increasing a hair and the car maintains speed just fine. Every 'econo' car I've owned with a stock 4 banger required rolling into the throttle to maintain speed. Same scenario, similar vehicles in terms of weight, aerodynamics, speeds, but the big 'thirsty' engines don't have to work as hard to accomplish the same tasks as the little thrifty engines.

There was an episode of Top Gear where they raced a Prius and a BMW around their track for a bit, the BMW was chasing the Prius which the Stig was driving as hard as he could. They compared fuel economy numbers at the end of the race, and the BMW M-Series got twice the fuel economy as the Prius...

Blasphemy? Maybe. Different driving situations? Absolutely. Just like if you put the same cars in rush hour stop and go for an hour with the a/c running on a 100 degree day, the Prius obviously would get waayyy better fuel economy because it gets to sit there in electric a LOT of the time while the BMW idles away. All depends on how you drive, where you drive, etc.

[Lonesome Trail]

Originally Posted by Old Tele man

If the vehicle requires only 15 road HP to do 50 mph, increasing the POSSIBLE output is not going to produce a noticeable difference. However, if that increased HP occurs at LOWER, and not HIGHER, engine RPMs, then, yes, it will improve fuel economy; for example, the great leap downward in displacements coupled with concurrent turbo-charging, where TORQUE becomes "flat" from slightly above idle until just before maximum HP occurs.

Very true mostly, lol. If the potential power under the curve is increased via increased engine efficiency, all else being equal, then engine 'load' is decreased vs the less efficient engine and it doesn't have to burn the added fuel (ie convert one form of energy into another) to accomplish the same results, in your scenario achieving 15HP to maintain speed, with no change in RPM.

Absolutely the new tiny turbo motors are amazing, but so are the variable valve timed NA motors, such as my '12 GT. It makes more power than my 5.0L pushrod Fox Body, weighs more, and gets better fuel economy despite the fact that the engines have the 'same' displacement, 5.0L and 8 cylinders. The new Coyote engine is freakishly efficient with it's DOHC and variable cam timing vs the dinosaur pushrod engine, and as such in the same scenarios it's able to accomplish the same tasks while using less fuel (same 91 octane).

Pretty amazing stuff, physics

[jamesqf]

Originally Posted by Lonesome Trail

There was an episode of Top Gear where they raced a Prius and a BMW around their track for a bit, the BMW was chasing the Prius which the Stig was driving as hard as he could. They compared fuel economy numbers at the end of the race, and the BMW M-Series got twice the fuel economy as the Prius...

Easily explained: they lied. It's TV, so if they don't get the results they want, they fake it, as with the exploding gas tanks: NBC Admits It Rigged Crash, Settles GM Suit - latimes And since "Top Gear" is all about performance cars, they're pretty well known for lying about anything that has to do with fuel economy or non-IC engined cars.

[Vman455]

^^^That, and...

Originally Posted by Lonesome Trail

There was an episode of Top Gear where they raced a Prius and a BMW around their track for a bit, the BMW was chasing the Prius which the Stig was driving as hard as he could. They compared fuel economy numbers at the end of the race, and the BMW M-Series got twice the fuel economy as the Prius...

...you misremember--they claimed 14mpg for the Prius and 16mpg for the BMW. And the BMW was drafting the Prius for the duration of the "test."

[Vman455]

Originally Posted by Lonesome Trail

I just let the results speak for themselves. It's pretty amazing the looks I get when folks see my little notebooks of fuel numbers (I keep a log for tax purposes, small business owners FTW!) It freaks people out that my lopey cam Fox Body pulls pretty consistent 21-23 depending on if I get stuck in traffic, but I always point this out to folks: When I'm on the freeway and am going up the hills (we have overpasses every mile, it's fabulous for fuel economy) I just sort of imagine my throttle increasing a hair and the car maintains speed just fine. Every 'econo' car I've owned with a stock 4 banger required rolling into the throttle to maintain speed. Same scenario, similar vehicles in terms of weight, aerodynamics, speeds, but the big 'thirsty' engines don't have to work as hard to accomplish the same tasks as the little thrifty engines.

Also, a flaw in your reasoning here; gas engines are less efficient at smaller throttle openings. That 4 banger that has to "roll into the throttle" to go up an incline does so at a lower BSFC (i.e. less fuel consumed per power unit produced), and, assuming the same weight, aerodynamic load, etc., and consequently same power produced, the car with the larger engine that seems to do less work to produce the same power to go up the same hill, actually consumes more fuel to do so because the BSFC is higher. This Autospeed article explains the concept; the crux of it being:

"For example, anything that allows you to keep the throttle open wider and the revs lower (like changing up to a tall gear and then holding it) will reduce fuel consumption because BSFC will be improved."

Modding the engine to produce more power at a smaller throttle opening without changing the gearing does the opposite.

[Lonesome Trail]

Originally Posted by Vman455

Modding the engine to produce more power at a smaller throttle opening without changing the gearing does the opposite.

Hi Vman, do you have some numbers to back up this statement? It's pretty much opposite what my tuner and Torque Pro tools have told me, plus the old pencil and paper method.

Let's run your example another way...

You're climbing a hill, it requires 'x' amount of energy to maintain speed.

Car A is a 4 banger with typical 4 banger torque band with a curve and peak around 4k RPM, Car B is an efficient V8 with flat torque curve and peak around say 2500RPM as is very common in today's engines.

For sake of argument both cars are geared the same, same size tires, etc.

Now both cars hit the hill at the same time, and to maintain speed Car A must really open the throttle to achieve enough power production to climb the hill, Car B cracks its throttle.

By your logic Car A is using less fuel because it's a small engine making more power per displacement, hence is more 'efficient' because of that ratio, correct?

However! To achieve this type of efficiency the engine is 'working' harder. When the throttle blade opens to allow in more air, you have to have more fuel added at the same time to make things work, air alone can't produce 'energy' from an engine. To see this in real life hook up your tuner or scanner device and set to watch your injector pulse width, short term fuel plan, etc.

Or if you're analog another fun tool is a vacuum gauge. Your 4 banger with the throttle open is drawing much less vacuum than the V8 with cracked throttle. You can argue that means the 4 banger is more efficient based on the above statement, OR you can also realize that on all vehicles with vacuum regulated fuel systems your fuel pressure regulator is now opened wide allowing 40psi to the injectors, which are also pumping at say 85% duty cycle in order to meet the power requirements of this hill, vs Car B which isn't working hard, still drawing good vacuum, fuel pressure is lower, injector duration is lower, etc.

Making sense so far? The problem with the efficiency models the way you describe is they look good on paper, but tend to sink in real life. When you look at the numbers in terms of how the engines are functioning, all the non-black and white perfect physics variables that are going into making Car A much more complicated than 'HP per L' efficiency...

To use another example that's really going to mess with your head, aerodynamics. Why does reducing drag improve fuel economy rather than improve it? According to your statement above (new Car A and Car B are now the same cars except Car B has some aero mods) Car A should get better fuel economy because the engine is working harder to maintain the same speed due to increased drag on the vehicle, requiring more power to stay at speed. Higher power requirements means the engine's throttle is open more, means that engine is more 'efficient' as it is producing more power than Car B's engine that is working less hard to maintain speed as the vehicle is more slippery and doesn't require the same energy to overcome drag.

Ruh roh...

Yep that's right, in real life the more slippery version, Car B, gets better fuel economy than Car A even though its engine is 'less efficient' than Car A's as it is producing less power to keep the car moving at the same speed.

Same thing with the Top Gear episode everyone poo-poo'd because it's obviously against popular edict here, let's change the scenario one more time but keeping with their test ideas:

You have 2 trucks pulling say 8,000lbs of trailer. Truck A has the smaller gas engine option, truck B has the bigger gas engine (no diesel here to complicate things). Which truck will get better fuel economy? All else is equal, rigs are essentially the same weight, same aerodynamics, same gearing, etc.

Truck A has to work really hard to get its load moving vs Truck B because it has less power under the curve, which according to your logic means Truck A is getting better fuel economy because it's throttle is wide open and is making more power per L vs the bigger engine Truck B which is just loafing along. Truck A has to work really hard to KEEP its load moving at highway speed vs Truck B, same reasons as above.

Which truck is going to have the better fuel economy? The more 'efficient' Truck A, efficient because it's making more power per L to move its load, or Truck B which I argue is the one that's really more 'efficient' because the engine isn't working nearly as hard, ergo the engine is in fact burning less fuel per L displacement because it has much more power under the curve and you don't need to open the throttle as wide to achieve the same power output as Truck A required to achieve the same end results.

Head hurt yet? I need some caffeine, and my chickens are getting hungry...