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 (http://www.chevelles.com/forum/ultimatebb.php/topic/4/22173.html) 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. :hmm:

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.

[High Plains Mopars]

True, to truly validate this as a test my combo should be run with a lower ratio gearset or an OD transmission through the same loop. However, despite the higher rpm range of my combo, I'm putting down mileage figures that are comparable to the "published" figures of any modern full size truck, and I'm doing it with a carburator and conventional ignition system. Modern trucks are not doing much, if any, better using EFI, coil on plug ignitions, and turning the engine at lower revolutions per minute. I also think we could all agree that the real world figures for any newer vehicle is going to be less than any published figures. So despite turning the engines slower, comparable displacements and comparable vehicles specs are not necessarily seeing improved mileage figures over runnig an engine at its VE point.

As I said before, this is using my experience with a fullsize truck with a pretty large displacement engine that is purpose built. Some of the smaller displacement engines that would need to cruise at 4000+ rpm to be at their VE may indeed have a lower fuel cost to spin slower compared to buzzing them that high. I don't know, I don't play with those powerplants and with the multi displacement, variable valve timing systems out there, it isn't a simple apples to apples comparison anymore. Heck even the port configuration in the new LS motors have an impact on how efficiently the whole packages operates over a broader rpm range, which should allow it to achieve greater efficiency at lower rpm levels compared to previous versions of similar engines.

Modern maufacturers are saddled with mass production, epa regulation, and the need to fulfillthe needs of every driver out there purchasing a new vehicle , be it Grandpa Cruiser to Boy Racer. Just because they have greater resources doesn't mean they are producing the ultimate answer. They are simply producing the best answer for the biggest number of people for the most reasonable cost while meeting ever increasing warrenty requirements. As hot rodders, we have the luxury of creating the best answer for our own individual purposes for whatever cost we're willing to pay.

[Jim Nilsen]

It would be a different world if we were like the autobahns and we could drive at the efficiency level instead of the speed limit posted for the vehicle we drive. Let's think about what it would be like if the tachometer had a set point for cruising that gave the best mileage by the EPA. Post tachometer limits instead of speed limits on the roadways. Some would be driving 75 to 85 and maybe even a few would be doing 90 to 95 and then we would have trucks like my brothers that would be doing 50 with the low gears he has. It would be a different world to drive in but I think I could handle it with the rest of you guys on the road but with my sister and young kids along with the other incompetent drives I know it would make it a disaster.

I still want it to be harder to get a drivers license in my state and it may happen someday the way kids are getting killed these days. It is just too easy to get a license.

I see the future in the hydraulic hybrid vehicles that can make a Ford Explore get 35mph in the city and 21mph hiway. It seems odd but it is the future and it is a good start and the offline acceleration of the hydrualic hybrids is amazing with all wheel drive. You can get more info on hydraulic hybrids by going to the SEMA site and other places online.

There will never be a happy medium with mileage and torque curves with todays speed limits controlling the issue so it doesn't even pay to debate it. Just build what you can live with and afford and go from there.

I believe one day we will find an even faster way to go and gasoline will be something we use for nastalgia racing and fun to remember the good old days of controversy.

Jim Nilsen

[pdq67]

Onna the reasons rich people bought the big Cad's was b/c GM did their homework a bit better on them than the cheaper Chevy cars and such.

A big Cad., even weighing like 5500 pounds or so would knock down darn good road mileage b/c their motors were always so big and so had gob's of low rpm torque!!

Especially the 500" jobber!!

Next point is that for out and out gas mileage, look at weight!!

My '87 Chevy Sprint ER/Suzuki Swift 3-banger, 5-speed hatch weighed right at 1500 pounds and if i woulda driven it like the EPA driving loop said to drive it for max. mpg, it woulda gotten like 72 mpg or so!!

Now I drove it just like a SK-8 Board, above 70 most of the time's, and the little corn-popper would knock down 50+ mpg!!

Fast forward to my 2000 3-banger, 5-speed hatch Metro and due to it weighing 1950 pounds, I could only get 50+ mpg out of it running say 58/62 mph!!

Most WON'T lower themselves to drive a "corn-popper" commuter car when they really could use one b/c of driving costs b/c of so-called SAFETY reasons and pride!!

But I will and am b/c since I flat can't buy a cheap Metro, high gas mileage, corn-popper anymore, I bought a 2003 Accent 5-speed, hatch that gets about 40 running 60 mph down the road!!

You don't know how much I wished Honda would put their 1.3 l, i-vtec engine (or whatever it is) in a 1500 pound, four pass. corn-popper car with only a 5-speed, radio and A/C as an option and then sell it for say $11,500!!

I bet they would sell every f-- one they made!!

pdq67

[Rabidhamster]

... I think they called it the FIT .. that might be a 1.5 though

I dunno if anyone mentioned it.. but another issue then gearing for tiresize and your cruising mph... is where your engine is tuned to run yto begin with... if your tuned to run between 3200 and 7000... lugging your poor engine down to 1500 rpm right off your cam isn't going to hurt it... infact you'll probably foul your plugs and get WORSE mileage

one way around this to a point is EFI and elec timing control.. but you can only tune around where the engine was built to be driven too a point.

personally with all my cars I've found the best mileage at an rpm/gearing low enough that the engine isn't lugging, and if I roll on the throttle the car will still accelerate. usually right around the low side of when the cam is starting to come "on"

basically mileage your playing a balance act between power, rpm, and aero. but maybe that's a bit to generic for the advanced section ;)

[bnickel]

i have found that there a rea bunch of things you can do improve mileage when you're building an engine. run as much compression as you can possibly get away with. if you have aluminum heads and fuel injection then 10.5:1 or 11:1 is not out of the picture, yes you'll have to run premium fuel but the mileage difference can be more than worth it. run a lower rear gear ratio and use an overdrive trans, the lower gears get the car moving more efficiently at low speeds and the OD keeps them reasonable at highway speeds. turning an engine at too low and RPM on the highway will hurt gas mileage because it lugs the engine down. use as low a cam duration as possible to achieve your desired performance with the given compression ratio. certain engine coatings go along way, mainly piston and head combustion faces and header coatings but ant-friction coatings on the pistons help. try to run as lean a fuel mixture as possible at cruise RPM. tire pressures help a lot as well, i've run as high as 38-40 PSI in my tires on long trips and mileage can improve as much as 3-5 mpg just by doing that.

the last engine i had in the mustang had a mild 270 sealed power cam, 10.5:1 compression, carter 600 carb, performer intake, stock exahust manifolds with 2 1/4" exhaust FMX 3 spd auto and 3.25 gears. it got an average of 22-24 mpg on the highway. probably would have done even better if i'd had a lower rear gear and OD but i thought that was pretty respectable. obviously not a 450hp engine but it was no slouch either.

[LS6 Tommy]

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.

Detroit's not always right, ya know. ;)

Seriously, my 79 Cutlass with a 305 Quadrajet and a TH 200, 2.73 rear ratio got better mileage than my 87 Monte LS did with the same basic engine and rear ratio, but a TH 200R4. BTW, the Cutlass had 220k miles on it and the Monte had less than 70k.
My carb guy's 73 AMX Javelin got close to 20 MPG with a T10 4 speed and 3.90's.

O/D ratios get to a point of diminishing return, just like others here have said. Once you start to run the engine at an rpm below where it's relatively efficient, mileage will go down even at a lower rpm then with a less overdriven combo at a higher rpm.

Tommy

[redhead]

just to throw in another overlooked factor into the rpm vs effeciency equation, friction. many for the recent improvement in
mileage can be attributed to leaps in lubrication technology of both
the overall system as well as the lubricants themselves. you can
move that redline as high as the motor will rev but you are giving
up a lot of energy in term of heat (wear).
also consider the benefits of the cvt which keeps the engine at
the same rpm regardless of vehicle speed thus always operationing
within the range of highest VE. on paper it makes sense but to me
it seems incredibly boring.
da.

[pdq67]

Anybody have a car with a shift light on it??

I think the '87 Chevy Sprint ER had one, but I forget.

It's a light that's hooked up to a vacuum gauge so that when you reach the point of highest vacuum, it come's on to tell you to shift.

Work's like a charm if you are driving for gas mileage..

pdq67

[BluEyes]

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.

That BSFC map is the key.

Definately hard to get ones hands on as well. If you've got Engine Analyzer or something like that you can build one up with a bit of work though. Find the RPM's that your engine will be turning with the gearing options available and then drop the carb/TB CFM rating untill you get down to cruise HP at the right RPM's. The gear ratio that gives the lowest BSFC at cruise RPM and cruise HP will the the best for economy. I guess I repeated what you said but maybe a bit easier to understand. Let me tell you, it's amazing what little CFM numbers you've gotta punch in - around 20 or so. And the BSFC in a throttled situation is very different from WOT BSFC (much higher). For some real fun you could change the ambient conditions in Engine Analyzer and see the effect of weather/elevation...

I've used the BSFC output from Engine Analyzer to calculate the theoretical MPG that some of my cars should be getting and it actually agrees pretty well with real world numbers from road trips and such so I'll say the technique is definately worth a shot.

[greygoose006]

basically, in generic terms, we have established that it is better to have a high displacement engine, tuned for low end torque, running at its highest BSFC, at the desired cruising speed.

in my car, an 84 caprice classic, that speed seems to be around 55 mph.
it takes very little throttle to keep it going 55 on the highway.
any lower, and it takes less throttle, but feels like it is working harder.
same with higher speeds.

my car is running a 229 V6 with a TH-350 trans ,2.73 gears, and 225/70/15 tires.
i usually get around 20 on the highway if i can force myself not to go over 63.
if i drive at 75, i will get 15 or less mpg.

[BluEyes]

If you want economy AND high performance, then go with the displacement. The more economy you desire, then go with the smallest displacement possible that can maintain your desired cruising speed (not as much fun though) The lowest BSFC is at peak torque RPM at *WOT*. Less than WOT and the BSFC again rises.

So, a 350 cruising at 75mph and a given RPM is using a greater % of its available torque than a 454 under the same conditions so will have a larger throttle opening, a lower BSFC and better economy, everything else being equal.

[BillyShope]

Back in the fifties, I'd sit for 8 hours each day at a Friden mechanical calculator at Chrysler, calculating fuel efficiencies. This is, I'm certain, a task which is done far more efficiently now on the computer.

Anyway, the minimum brake specific fuel consumption usually occurs at about 90% throttle and at an engine speed roughly corresponding to that at the WOT torque peak. So, it's pretty obvious that simply lowering the engine speed is not always equivalent to improved fuel economy under normal driving conditions.

[Fuelie Fan]

What billy said about the location of PEAK BSFC is correct. However this does NOT mean that best economy will occur while running the engine at, or even close to, that rpm. Look again at those BSFC maps. BSFC is relatively insensitive to RPM, but very sensitive to load. You lose maybe 5-10% by slowing a typical engine from 3000 to 2000 rpm, but you will gain well beyond that through the increased load on the engine.

If you want performance AND economy, turbo/supercharged small displacement engines are the answer, NOT large displacement engines. Pressurizing the intake is the most effective "variable displacement" technology in my opinion. It does require some discipline on the behalf of the driver, or an adjustable boost controller.

[BillyShope]

Afraid it doesn't work that way. You WILL get maximum fuel economy at that "sweet spot." The problem is that, when you start plotting road load torque curves across a torque curve plot with lines of constant BSFC included, you'll quickly see that reasonable engine sizes, axle ratios, and force required data simply do not combine to allow for operation there for any appreciable amount of time.

And, I'm sure it was carelessness, but it's MINIMUM BSFC that we're after and not a peak.

[BluEyes]

er, yeah. Operating a typical hopped-up 350cid at 90% throttle at peak torque RPM would not result in a legal speed so the sweet spot is still pretty much theoretical unless you want to run a 'peanut' cam.

Personally, I would love to have a CVT so I could set the throttle to 90% and then regulate speed via gearing. Basically, gear down the engine through an insane overdrive until the torque produced at 90% throttle equals the torque needed to move the car down the road at, say, 75mph. If you look at the current 6+ speed transmissions out there that is pretty much what is being done. There are a number of cars out there which will only be doing 2000rpm (or less) at 80mph. The result is going to be a larger throttle opening and lower BSFC.

[PTAddict]

It's been 25 years since I took Internal Combustion Engine theory in grad school, but there are some fairly simple principles to understand when it comes to maximizing fuel efficiency.

First off, there are two major sources of parasitic losses in a gasoline engine - frictional losses, and throttling losses. Frictional losses are self explanatory; throttling losses are the energy the engine wastes "sucking" against the air throttle, and are proportional to the intake vacuum, and inversely proportional to engine load. This is why BSFC is maximized near WOT (there are some effects due to the richening of fuel/air ratio near WOT as well, of course, which depend on the carburetor/EFI tuning). Note: diesels have no air throttles, which is a major factor in diesel fuel efficiency.

Now, taller gearing helps reduce both these losses. Frictional losses are lower due to the lower RPM, and throttling losses are lower because engine load is higher and therefore manifold vacuum lower. Hence, the very tall top gearing we see in OEM applications.

In performance applications, there are some other considerations. Camshafts with large overlap, and that hold the exhaust valve open well into the intake stroke, cause considerable mixing of exhaust gas into the intake charge at low RPMs. This causes the famous idle "lope", but also reduces lower RPM combustion temperatures and thermal efficiency. So there is a point at which taller gears can begin to work against you.

[BillyShope]

Again, BSFC is MINIMIZED. (Commonly used units are pounds of fuel per horsepower-hour. In other words, thermal efficiency is the inverse of BSFC.)

[PTAddict]

Again, BSFC is MINIMIZED. (Commonly used units are pounds of fuel per horsepower-hour. In other words, thermal efficiency is the inverse of BSFC.)

Yes. Sloppy wording on my part.

[derekf]

Thanks, y'all, exactly the type of discussion that I was looking for.

Given that I'm a hobbyist I'm gathering that without the type of test equipment that I'm not likely to ever have access to, I won't be able to really nail down gear ratios that I should look at to maximize efficiency; Billy, should I take

Anyway, the minimum brake specific fuel consumption usually occurs at about 90% throttle and at an engine speed roughly corresponding to that at the WOT torque peak. to indicate that my best bet for economy is to dyno the engine and then gear the car such that the torque peak is 500-1000rpm below what I'd be running at highway speed -- or is there a better formula to use without spiffy diagnostic equipment?

[BillyShope]

Again, there is no quick response for a question like this. The procedure, at Chrysler, was to calculate the mpg for various points on the road load curve and then plug the numbers into an equation which was supposed to represent the average "tank" economy. The results would sometime surprise us. We'd look at road load curves for a couple of different axle ratios, for instance, and come to a knee-jerk conclusion as to which would be better, but find that...according to the magic equation...we'd picked the wrong one.

In short, it all comes down to personal driving habits. What works for you might not work for another.

[PTAddict]

Thanks, y'all, exactly the type of discussion that I was looking for.

Given that I'm a hobbyist I'm gathering that without the type of test equipment that I'm not likely to ever have access to, I won't be able to really nail down gear ratios that I should look at to maximize efficiency; Billy, should I take to indicate that my best bet for economy is to dyno the engine and then gear the car such that the torque peak is 500-1000rpm below what I'd be running at highway speed -- or is there a better formula to use without spiffy diagnostic equipment?

As BillyShope points out, it's not as simple in practice as it is in theory. But even the theory is very complicated. Essentially, you're dealing with optimization in more than 2 dimensions. You have BSFC vs. engine load as a function of RPM, then you have RPM vs. engine load as a function of gearing (a mostly linear graph, but not quite). On top of that, you apply different driving styles in the form of f(RPM ,ENGINELOAD) over time. And it's impossible to generalize some of these curves across engine designs and drivers.

But if you want to limit the solution space to highway driving at a constant speed, you might be able to come up with some rules of thumb that are close. Here's a rule of thumb I propose for performance cammed engines, to be debated by others: for max highway mileage, gear your car to run at the minimum operating RPM, at highway speed, for the cam you've chosen. If you choose a Comp Cams XE274 for SB Chevy, RPM range 1800-6000, gear it to run at 1800 RPM on the highway for max economy.

The rationale for this rule: the bottom of the cam's RPM range is when the manufacturer deems the overall combination of volumetric and combustion efficiency to be above the "bad" range. Gearing the car to run at this RPM keeps RPM as low as possible to minimize frictional loss, and engine load as high as possible to minimize throttling loss.

Arguments against this rule:

- Operating range is a somewhat arbritary definition dependent on cam manufacturer.
- Operating range in practice depends on engine displacement and a host of other factors.
- The real BSFC vs. Load vs. RPM matrix is only loosely related to cam operating range.

Anyway, given that in general none of us has access to the necessary BSFC/Load/RPM data to do our own optimization, is this or any rule of thumb useful?

[BillyShope]

It's not all that complicated. And, it doesn't involve any theories. We can calculate, with sufficient accuracy, the mpg for a particular operating point. The problems arise when you include the driver in the mix. That tank economy equation, from my last post, was the result of someone's educated guess as to average driving habits. I should also include the terrain parameter. The same driver/car combination won't get the same mileage in Oklahoma City and Denver.

But, as you pointed out, the average person doesn't have access to the data I used in my calculations. And, the rule of thumb is still valid in most instances: The lower the axle ratio (numerically), the better the tank economy.

[Fuelie Fan]

Afraid it doesn't work that way. You WILL get maximum fuel economy at that "sweet spot." The problem is that, when you start plotting road load torque curves across a torque curve plot with lines of constant BSFC included, you'll quickly see that reasonable engine sizes, axle ratios, and force required data simply do not combine to allow for operation there for any appreciable amount of time.

And, I'm sure it was carelessness, but it's MINIMUM BSFC that we're after and not a peak.

should've said "best" instead of peak, sorry about that.

I think We're actually saying the same thing, billy. I just was beginning my conversation from the consideration of road load, whereas you're starting more from principle and then bringing it back a step. Yes, if you put a 1L, 40hp engine in a fullsize car, you'll probably get best economy at the torque peak on the highway. Nobody here has that combination. With overpowered vehicles, almost without fail reducing engine speed below torque peak will still increase economy. Corvettes didn't double their economy between 1977 and 2007 through improvements in combustion efficiency, and they haven't improved their aero that much either. That double overdrive is the key. Anyone that disagrees, drive your new vette/camaro M6 in no higher than 4th gear for a week and report back about your mileage.

[MonzaRacer]

Ok first of all a well tuned engine is key and if its slobbery around tome its gonna be slobbery on highway regard less.
Dial upa smooth running torque engine that is a tight rocket from idle to redline and a team it up with reasonable gears(enough to pull the car) figure a Caprice with a set 3.23 or 3.42s is going to accelerate well and get good economy wth a decent overdrive trans.
Take a car like mine a 77 Monza, it comes in at only 2900 to 3000 lbs so a well built torque engine that uses the heads,intake/induction ,exhaust to its best ability. Add in an overdrive and some decent gears to getit moving without over rpming and its going to getgreat mileage.
The trick is, in my engine building to get a smooth ,flat ,broad torque curve from 2000 rom or so up.
If its real peaky its gonna be hard to tune.
In my yellow 77 I had a milder csplit duration/split lift cam and some smaller port heads that I had opened up to a smooth transition and good flowing bowl work. also the AFR 180/AFR 195s are going to get better mileage on a 355 over a set of AFR 215/AFR225 heads.
The trcik I was taught was to get the intake tract running with enough velocity to get within the Hemholtz resonance peak but not at full peak ,just under.
What this means is that the air/fuel charge is going fast enough to have gravity, atmospheric pressure and inertia actually push the charge in at a rate greater than expected.
That 355 had a set of 991 truck heads and I had set them up with bowl work, smoothed with a custom contoured grinding stone , raised the guides 5/32s , an old Edelbrock Scorpion intake and 600 Carter AFB and the engien would knock down an honet 18 to 20 mpg with 3.42 gears, th350 and 275/60/15 rear tires. it was torque monster lethal on launch if not violent.
port velocity is key too much and the swirl will be too fast and fuel can spin out and wash down the cylinder walls too little and its a bottom end dog.
This is why we always say build a lower to middle rpm range peak torque engine and you will out accelerate and out mileage the guy that has to rev the wee out of his set up.
Just remember flat torque readings and start loww and keep them in a gracefull rise and fall and your car iwll pull and get better mileage.

[pdq67]

Try this on for size!

Take a stock 9.5 to 1 CR'd 195hp/283 and install an old E-brock SP-2P intake on it whether you want a 2GC, 2- or 390 cfm, 4-barrel, then install a set of 1.375" or 1.5" LOOOOOOONG 4-tube headers and a little bitty solid lifter cam like this!!

250/210, 104/108, .440" lift net lift.

Now you may have to watch out b/c the old SP-2P just may suck so WELL that your DCR goes too high so watch that!

Run the numbers through D2k b/c I figure this little combination will have onna the flattest torque curves that you will ever see!

Then install it in a little-bitty light car and gear and tire it accordingly and go get you gas mileage!!

And fwiw, one year a while back when the wife and I were fighting, I moved back home w/ Mom and was driving 180 miles round trip a day for like 4.5 months in my new 2000 Metro 3-banger 5-speed hatch and got right at 50 mpg running 58 to 62 mph w/o the A/C on!!

I have NO delusions of wanting a big car b/c it's safer! I just drive SAFE!! For wanted gas mileage...

pdq67