A/F ratio, RPM, and gas mileage

[AGlobalThreat]

This is all about low throttle, closed loop cruising, not going over 3000 RPMs. I'm curious if the RPM affects gas mileage, or if the A/F ratio can simply be read as how much fuel is being used. I imagine the RPM has an effect on the exhaust velocity, but am unsure if the change is big enough to make a noticable difference in the A/F reading and how much fuel is actually being used.

For example, I normally shift into 5th at 45mph (4.3 LSD). If I'm cruising in 4th at about 45mph, my engine is running much leaner than it is if I shift into 5th and run at lower rpms.

I'm basically wondering if it's better to stay in 4th and if it's saving fuel since the A/F reading is much leaner, or if I'm actually using less fuel by shifting into 5th since the RPMs are lower, even though the A/F reading is much richer.

 

[Poodles]

I'm in 5th at 35MPH and I have a 3.73 rear...

I knock down 20MPG easily that way, the more RPM, the more fuel being used, but in your case I would figure out why the AFR's are fluctuating...

 

[CRE]

Keep in mind that in some instances driving in the highest gear possible may require more fuel to keep things moving smoothly than driving in a lower gear at a somewhat higher RPM but a much lower load.

For example, I found that cruising in one gear puts my manifold pressure at 73kPa on a mild incline on what's generally thought of as a flat road, that's a real big load when the ambient pressure is around 80kPa (damned near WOT). One gear lower and I'm at around 40kPa and my injector duty cycle is a ton lower... more than enough that it makes up for the difference in RPM.

Yeah, you want to avoid sitting at 5000RPM all the time, but cruising at 1800RPM as opposed to 2400RPM may actually cost you. It's not very common that people try to go that far with it though.


AGlobalThreat, how are you reading your AFRs?

 

[AGlobalThreat]

I know why it's fluctuating (TPS signal isn't coming in correctly) but can't fix it for a couple weeks. Tried a new TPS today and got the same results so the problem resides in either the ECU or the wiring harness.

Reading my AFR from an AEM UEGO Wideband.

Like I said in the first post this is talking about all below 3000rpms. I understand how its important to find the best RPM to shift and in every car it will be different. I would only be taking 4th gear 500rpms higher than I usually do.

 

[Poodles]

I shift at 2K RPM usually, 3K if I'm surrounded by idiots in traffic...

 

[AGlobalThreat]

In CA I'm ALWAYS surrounded by idiots in traffic. People ride my ass enough when I shift at 2500 RPMs, makes me feel like I'm driving like a grandma. They punch it from like to light thinking they're saving time but in a CA city with stop lights everywhere all they're doing is wasting gas.

 

[Poodles]

Also, CRE, you can hear when the engine is loading (hell, watch the boost gauge), so I'll downshift.

After all the mods I've done my car will pull from idle pretty easily, it's why I always say people of full of shit when they say they lost low end power putting exhaust on their car...

 

[CRE]

Also, CRE, you can hear when the engine is loading (hell, watch the boost gauge), so I'll downshift.

I'm sure you know as much... but you should also know not to presume that anyone else here does.



The amount of lost power on the low end from the engine having to push the gas out itself is hardly measurable... but, it's still there. It's just a fact that gets passed up too often and when you let too many of those slip by that's when you start getting HUGE groups talking about crap like the benefits of back pressure.

I'm not nagging, just informing. Chill. :icon_bigg

 

[Poodles]

Oh no, not arguing

People used to say it was valve float if you bogged the engine like that, not sure on how true that is, but it's definately audible...

 

[gaboonviper85]

Oh no, not arguing

People used to say it was valve float if you bogged the engine like that, not sure on how true that is, but it's definately audible...

if I'm not mistaken...valve float is actually caused by very high rpm....in which the the valves don't get to close before the cam comes back around...so the valve is in a limbo cause the springs aren't strong enough to move the valve quick enough...more common in motorcycles that push 9-10,000 rpm....someone smart please correct me if I'm wrong...

 

[Poodles]

Yep, correct, that why I kinda doubt it's true, but it does make a god aweful racket...

 

[gaboonviper85]

sounds like an old diesel motor to me.

 

[Nick M]

This is all about low throttle, closed loop cruising, not going over 3000 RPMs. I'm curious if the RPM affects gas mileage, or if the A/F ratio can simply be read as how much fuel is being used. I imagine the RPM has an effect on the exhaust velocity, but am unsure if the change is big enough to make a noticable difference in the A/F reading and how much fuel is actually being used.

For example, I normally shift into 5th at 45mph (4.3 LSD). If I'm cruising in 4th at about 45mph, my engine is running much leaner than it is if I shift into 5th and run at lower rpms.

I'm basically wondering if it's better to stay in 4th and if it's saving fuel since the A/F reading is much leaner, or if I'm actually using less fuel by shifting into 5th since the RPMs are lower, even though the A/F reading is much richer.

The amout of fuel the ECU delivers is first dependent on the engine speed and postion from the CPS and air flow meter. They are primary inputs. More engine speed, more fuel. Higher frequency from the airflow meter, more fuel. Lower TPS value, correction lower.

A program to give you lean cruise like Ford did back in the 80's with SEFI and GM does now on the Corvette is how we could improve highway milage. That falls under 3p's thread. Closed loop technically isn't a part of that.

 

[figgie]

Keep in mind that in some instances driving in the highest gear possible may require more fuel to keep things moving smoothly than driving in a lower gear at a somewhat higher RPM but a much lower load.

that is never the case unless the load changes. Higher RPM = More work per unit time. Load being the same, a lower RPM will give better mile/gallon.

For example, I found that cruising in one gear puts my manifold pressure at 73kPa on a mild incline on what's generally thought of as a flat road, that's a real big load when the ambient pressure is around 80kPa (damned near WOT). One gear lower and I'm at around 40kPa and my injector duty cycle is a ton lower... more than enough that it makes up for the difference in RPM.

Ambient pressure hovers around 100kpa at sea level (101kpa to be a bit more precise but still not accurate). Sucks to live in Denver

Yeah, you want to avoid sitting at 5000RPM all the time, but cruising at 1800RPM as opposed to 2400RPM may actually cost you. It's not very common that people try to go that far with it though.


AGlobalThreat, how are you reading your AFRs?

It is dependant on the motor and where the power is produced. Vettes and most of the LSx 6-speed cars roll around 1800 rpm at 80 mph but that is because they produce a significant amount of power at that level.

On our cars, that is dependant on numerous things but also dependant on how the EMS in question measures load (ie just MAP, Just TPS a combination of MAP + TPS etc).

 

[AGlobalThreat]

Im at 20 vac cruising in 4th and when i shift into 5th its running at 10 (in g)

 

[Doward]

It all boils down to throttle needed to maintain a certain speed.

 

[AGlobalThreat]

I'm pretty sure I'm using more throttle in 5th, especially if I have to accelerate up to 50 mph. I think my vac readings prove that its better for me to stay in 4th until at least 50 mph.

 

[figgie]

TIME OUT!

Vac readings are not a good indication of load. Why?

Quite simple. Unless you remeoved the EGR system, the EGR system will be injecting inert gases into the system. Even though the pressure increases, the load will still be the same That is why toyota measures the air directly and not assumes via MAP.

 

[AGlobalThreat]

1JZ doesn't have EGR, but thanks for correcting me.

So I need to finish tuning the fuel map, and then to correctly determine the best RPM to shift at I need to be reading my load meter, throttle %, and RPM, correct?

 

[IJ.]

It's a big fat car with a little engine....

You're going to use more fuel if you lug it in a lower gear as you'll need the same if not more throttle % to accelerate the mass.

Light throttle and moderate rpm through the gears to cruise will give the best results.