Fuel Economy and RPMs and VEs

[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.