Fuel Economy and RPMs and VEs

[derekf]

Is it true that for a given car and gearing, you'll always get better gas mileage by lowering the RPMs?

I've got a suspicion that this is not true, but I can't pin it down.

Is fuel economy directly related to Volumetric Efficiency? In other words, should I be tuning my car/choosing components so that cruise speed on the highway is at peak torque/peak VE/something like that? Or is it indeed just a matter of spinning the motor a little less?

[derekf]

1 week, 87 views and no responses?

Surely someone has an opinion, some knowledge, or at least a pointer at the right folks to ask? Or did I not really phrase the question well?

[KendallF]

Is it true that for a given car and gearing, you'll always get better gas mileage by lowering the RPMs?

I've got a suspicion that this is not true, but I can't pin it down.

Is fuel economy directly related to Volumetric Efficiency? In other words, should I be tuning my car/choosing components so that cruise speed on the highway is at peak torque/peak VE/something like that? Or is it indeed just a matter of spinning the motor a little less?

Peak WOT VE usually occurs around the torque peak. But..to run at a constant cruise speed at higher RPM requires less throttle opening. I suspect that, for at least midrange off-idle to torque peak RPM, VE gets worse as the throttle is closed and the motor must pull turbulent air around the throttle blade. Also pumping losses and rotational losses are proportionate to RPM (or RPM^2).

So..I'd conclude from these factors that the lower the RPM and the wider open the throttle the better for cruise efficiency, until you get it so low that the VE falls off due to the intake, cam, and head design. Does that make sense to anyone else?

[Q ship]

1 week, 87 views and no responses?

Surely someone has an opinion, some knowledge, or at least a pointer at the right folks to ask? Or did I not really phrase the question well?

Derek, it's a good question and one that is actually vitally important to me. Once my Caprice is running again, it will become my daily commuter for a 200 mile/day trek. (And no, I will not consider anything smaller-I'm doing that now, and it sucks!).

First, I'd say you have to look at what GM is doing to get high 20's out of the C5 Vettes. Low RPM's at cruising speed, VERY advanced computer mapping, and an aerodynamic shape go a long way.

I have wondered a lot about what causes the most internal losses in an I.C. engine. Would a short stroke 327 be more efficient than a 383? A hydraulic roller cam would seem to be a given, as would a well dialed in EFI setup.

My ballpark plan? TKO tranny(500 or 600), 350ish displacement, and 3.08's. The boxieness of the car can't be helped, and I have been led to believe the drag figures aren't as horrible as they would seem.

I would be very interested to see this thread advance-everyone wants to talk about HP and timeslips, and the room gets very quiet when you start talking MPG.

[Q ship]

Some more random thoughts.

My dd right now is a Ford ZX2, 2.0 dohc/5 speed. At highway speeds it runs right at 31-33 MPG, and RPM's are 3500-3700(!). 80 is the normal speed, starting elevation is 5000 ft going down to 1250 ft.

Before the trans let go, I got 21 consistently from the Caprice with a 305/700r4/2.73 combo. With a computer controlled Q-jet.

Kendall, every once in a while I get a wild hair and think about a V-6/hairdryer combo-didn't the stock turbo engines do pretty good for mileage?

[CHILI]

I've always heard that your greatest efficiency (best MPG) is at the highest vacuum reading. Mount a cheap vacuum guage in the car and play around. It would probably help if you have cruise.

I don't think it's just a factor of engine speeds. With my car I turn about 2100 RPMs at 80, but I'm only pulling around 10" of vacuum (big cam). My mileage sucks too for an EFI car with so much overdrive (TKO and a 3.08 rear).

[boodlefoof]

and if you're trying to get the highest vacuum reading, that means not accellerating from too high a gear... meaning, don't shift sooner just to keep rpms down, because you'll need more throttle to get moving.

I'm not sure if the vacuum - mpg correlation is correct... but that is what I thought as well.

[derekf]

Definitely the types of answers I was hoping for, thanks all.

It does seem like a couple of these answers are somewhat contradictory though - wouldn't vacuum go down as the throttle was opened (to meet max throttle opening at min rpms)?

[Q ship]

Here's a very timely link. Funny how stuff pops up at the right time.

By the by, I'm thinking the throttle plate position concern is not a factor in this. The RPM's can't be too low for the engine combo(cam, specifically), but need to be as low as possible. Air/fuel ratio needs to be closely monitored and as lean as possible at cruise. The latest issue of GRM has an article on wide band O2 monitors, def. worth looking into.

I know the common answer to this is buy a 4 cylinder, but if I'm going to do all this driving I want it to be in something I enjoy driving. My little ZX2 has been a good appliance, that's all. Not really fun. I'm too old to just tolerate what I can influence.

[MuscleRodz]

As an small aircraft mechanic and pilot, I will try to explain it a little differently. Manifold (which is also vaccum pressure) directly relates to fuel consumption. the more vaccum you pull the more fuel you burn. I basically have two throttles in a high performace airplane. One controls maifold pressure, the throttle (your foot in the car) and the other controls rpm, propeller (the transmision, rear gearing, and tire size).

By properly adjusting the propeller for the correct operating rpm, I can reduce mainfold or vaccum press to a certain point at which I start to lose airspeed or mph, that will be the most efficient point. Rpm does not effect fuel burn accordingly. You can rev it to the moon and if there is no load, then there will be no vaccum press, and no fuel consumption.Now that is a figurative claim so don't think I think it will run on no fuel.

In cars, this becomes a science becuase of all the variable and choices in part combonations. In airplanes we are stuck with what we got for the most part. Start with more motor than you need, for a caprice i would say at least 300 hp. If its underpowered like many OE cars are, then you will waste fuel efficiency just trying to keep the vehicle moving. By properly selecting rear gearing, tire size, and the right trans gearing, you will be able to come up with the best efficency for that power to weight combo.

Now I have not even mentioned aerodynamics which also play huge on fuel efficiency. The same combo in a sleaker same weight car will get better mpg just due to aerodynamics.

By using the vaccum gage, it will help you find best effiency point for mpg. If you can tap into the intake somehwhere will give you the best readings, at WOT, you should be able to achieve close to 29.92 which is barometric press. Only a turbo motor will make more than barometric press. Hope this helps. If I confused you, ask cause I gave myself a headache trying to think and type at the same time.

Mike

[Fuelie Fan]

How you would figure it out if you had all the information (equations simplified):

Your first “known” is your desired speed, V_car
From this, you can compute losses due to aero and rolling losses in the tire. You can also make a first estimate at drivetrain loss (this is an iterative problem)

Road Load Power = P_tire+P_drivetrain+P_aero

At constant speed, the engine produces just enough power to match all the losses. If it were producing more, you would accelerate. If it were producing less, you would decelerate.

P_engine = RLP

So, you know how much power you need to produce. Now what you need (and probably don’t have) is a complete BSFC map for your engine. This gives you the engine’s efficiency in fuel consumtion per horsepower (lbs_fuel/[hp*hr]) versus torque and engine speed. Since horsepower = T*N_engine/5252, you can draw a constant power equal to RLP, and use it to find the highest efficiency point ALONG that line. Follow the intersection point down to the RPM axis to give you the engine rpm.

KEY POINT
You will NOT be at the engine’s peak VE/BSFC. If this were the best for economy, GM would gear its V8s to cruise 70 mph at 3000 rpm.

Now that you have the desired engine speed and desired vehicle speed, gearing can be calculated:

N_engine = V_car*gearing factor (NOT just rear end gearing, also has tire size)

Using the engine speed and gearing, you can now check you estimated drivetrain losses using engine, transmission, and rear end data, and re-calculate using more precise info for road load power.

General guidelines: every single component of the road load power is speed-sensitive. In other words, the best way to save fuel is to slow down! Assuming you don’t want to slow down, you can minimize the drivetrain losses by at least slowing IT down with tall gearing.

You engine’s BSFC map will have it’s highest values at high torques, in other words closer to WOT. To have a high torque without having high power, you need low RPMs, so here is another reason that tall gears help increase efficiency. More throttle means less pumping losses, greater efficiency.

As pointed out, an engine’s efficiency will fall off if the rpms get TOO low, but to say precisely where would require the BSFC map

[harshman]

k... I'm going to go back to the stupid threads now and take some aspirin.

[ProdigyCustoms]

I have never been a gas mileage guy, always a ultimate performance guy. I do have instant fuel economy reading in my new DuraMax, and I can assure you, the best mileage, Does not happen at the lowest RPMs. I get less mileage at a 45MPH cruise, then a 65MPH cruise. And thats about as much as I know or care about mileage.

[Fuelie Fan]

I've never seen what an efficiency map looks like for a diesel, but i'd be curious. With Diesels, you're probably not trying to achieve near-WOT torque levels, becuase you have no throttling losses (for the obvious reason: no THROTTLE). So, it would probably make sense that you'd be closer to peak torque RPM for best cruise economy with a diesel instead of below it.

[Ralph LoGrasso]

I have never been a gas mileage guy, always a ultimate performance guy. I do have instant fuel economy reading in my new DuraMax, and I can assure you, the best mileage, Does not happen at the lowest RPMs. I get less mileage at a 45MPH cruise, then a 65MPH cruise. And thats about as much as I know or care about mileage.

Just out of curiosity, what are you averaging with the truck? I've heard guys say they're getting as much as 18mpg while towing (!) with the DMax. Pretty impressive, if it's true.

[Jim Nilsen]

I can only say from my experience that the most torque you can get at the lowest rpm and the tallest gear you can live with for the performance level you desire will get you the best mileage.
This is a problem when you have speed limits and you have too tall of a gear. Best mileage can come at any speed and rpm combo as long as you are utilizing torque at its peak.With todays cams you can have a really flat torque curve.

In my old 67 with a torque built engine I would get 19 mpg at 80mph at around 3000rpm and get 15-16 at 55 and 2000 rpm with 2:73 gears. Less gear would have helped bring it up at 55 and more gear would have probably brought it up a bit at 80 so it is a tough choice for those who like to go fast and get there quick.

For the most part the best way to get mileage is to go for the most torque at a lower rpm and and have a taller gear to pull and live with the rest of the reality that comes with it.

I can tell you that the 67 would pull an honest 150mph top end and was stated to be geared forever by a drag racer that tuned it. 0-30 was like a stock 2bbl car and after 2500rpm it ran like a big block for excelleration. It was fun to drive and economical for as fast as it was.

I always told people that it wasn't quick but it was fast, so what ever you think you can live with is what makes you happy.

Jim Nilsen

[High Plains Mopars]

I'll side with the arguement that running at peak volumetric efficiency is what will generate the highest mileage figures, within reason. Certainly in a competition engine with a peak VE of 4500, running down the highway at that rpm will be detrimental to the longevity of your engine.

I built the engine for my truck using computer modeling. Going into it I knew that my trans/gear/tire combo ran at 3200 rpm while cruising 80 mph down the highway. So I built the engine to reach peak VE at this rpm.

Once it was all together and on the street, mileage confirmed that running at the VE was better than simply spinning it slower. I tested this theory on a trip from Denver to Phoenix and back again. Running in 3 hour blocks, I recorded the highest mileage figures at 80 mph, turning 3300-3500 rpm. The lowest mileage was at 60 mph. I split the test up by 5 mph increments. At 85 it started to drop off again, but by then I was past the VE point of the powerband.

FWIW, I was knocking down about 18 mpg at the peak, 12 at the lowest. This is a 74 Dodge 3/4 crew cab running a bored 440, 3 spd trans, 4:10 rear and 33" tall tire. I was pulling a 20 ft open trailer with 3500# worth of car on it.

[derekf]

Outstanding. That's the sort of detailed response I was hoping to get. Do you have the detailed breakdown and maybe some sort of dyno sheet that shows VE vs RPM?

[High Plains Mopars]

I've got all the engine info saved in a Dyno2000 file, so whenever I can figure out how to save it to a word document, I'll post the whole chart.
When profiling cam patterns, I went for the one that provided the smoothest curve, not necessarily the one with the highest peak numbers.

I'll try a rough mock up here;

rpm VE% tq hp

2000 84 474 181
2500 86 486 231
3000 87 486 278
3500 89 480 320
4000 87 456 348
4500 85 409 351

[Fuelie Fan]

I don't think the test is complete until the combo is tested again with taller gears so that you're cruising at 80 mph at lower engine rpms. Your experience goes 100% AGAINST what all of detroit is doing to improve mileage in their cars, which is tall cruising gears for low engine speed.
I don't think recording mileage in the midst of a trip in which you change about 4000ft in elevation is valid either, by the way.
I challenge anyone here to find a car produced today that is geared so that cruising speed is at peak torque.