Fuel Mileage Discussion

[mitch_04]

One of the reasons I love this forum is because the average user seems to be reasonably intelligent It's a place where theory can be discussed without a "do it this way" attitude. That being said, I've got a discussion I'd like to see get some really steam behind it.

Fuel mileage.

Y'up, something normally reserved for the daily driver, right? As far as the engine goes, there isn't much more to increasing the fuel mileage of a set engine size other than increasing the volumetric efficiency with intake/exhaust flow efficiency. Intake, heads, cams, so on. However, assuming we are all speaking about a pre-existing engine already in a car, what can be done to increase fuel efficiency.

Off the top of my head I would think aerodynamics, rotating weight of drivetrain, weight of vehicle, and gearing for a specific speed so your engine is running an ideal rpm for your mileage. However, I would love to discuss not only other area's I'm forgetting or don't know about, but also the ranking of importance and ways of dealing with them. If the engine and vehicle are not variables, what could we do to increase this imaginary car's fuel mileage? Feel free to use certain cars for example, for instance if I was to attempt to make my C10 more aerodynamic I would assume lowering it and removing bulky mirrors would help. However, I'm sure someone on here understands aerodynamics better than me and could also include things I wouldn't consider, like an underbelly pan or maybe a grill modification?



Anyways, I just thought this could be an interesting discussion. Things to do, or that you have done, to see improvements in a vehicle without altering the engine and what you found was the most effective.

Lastly, let's assume the driver has a light foot and is driving with the purpose of fuel mileage, since that is the most important aspect!

[Rod]

The newer LS engines are more efficient by design, my Camaro had a 383 with a 4speed and a edelbrock 650 thunder series carb and I was around 13mpg, when I did the LS swap still using the same 4speed and 650 edelbrock now the Camaro gets 18+mpg

[Ron Sutton]

Rod,

What was the power output of your 383 versus your new LS?

[CarlC]

The easiest and most cost efficient means of increasing fuel mileage is to focus ones efforts on the delicate use of the throttle pedal.

We knocked down over 24mpg in the Camaro on OLoA on the highway with two people and 450lbs of gear.

[mitch_04]

That's awesome Carl! How much power is that putting down as well?

I totally understand the LS being more efficient, that's one of the reasons I'm going that route myself. A car isn't as useful if you have to stop for gas every 100 miles, or can't afford to drive it more than an hour away!

I wonder if anyone has a weight/fuel mileage relationship... maybe a graph that shows how directly the weight of the vehicle affects fuel mileage. If you had been by yourself and no gear, what would have been possible?

Obviously a car like a Corvette is able to get near 30 mpg with a LS because of its weight and aerodynamics. It makes me think if a Camaro was more aerodynamic, you would have been able to obtain nearly the same results.

Also, if you decreased rolling resistance, how much do you think it would affect mileage? Obviously not something you want to do, but you could look at the flip side and say "How many MPGs do you lose going to larger tires?". Corvettes have managed great mileage with pretty beefy tires, so I'm thinking it isn't too much.

This also makes me wonder what kind of mileage you could get out of a car meant for MPG's if you stuck an LS in it. I remember seeing a build where a guy dropped a Civic body onto a Corvette chassis. Didn't beef up the motor much because it wasn't really necessary! I would imagine he got great fuel mileage, but that's a discussion that never comes up.

Maybe an interesting thread/chart would be Car-Engine-Trans-Weight-Mileage..... could be surprising!

[MonzaRacer]

You kn ow I keep hearing about how bad of gas mileage everyone got, YET my 402bbc could knock down 14 mpg beatin g on it and on long trips I could squeak 18+, with a 800 dbl pumper and th400/3.55 gears in a 70 Monte Carlo.
My LT4 HOT roller 350 in my 71 could get better with TCI th350, same rearend and was fairly efficient, back when the car was running its Quadrajet I could push 20ish, swapped in the same 800 DP and retuned and properly jetted and could pull 22+.
When I built my 77 Monza, 355 4bbl, the TCI trans, 3.42 gears it would push 24+ on long cruises.
Its odd I learned long ago to tune and so far the my tuning has gotten me great mileage.
If you ever watched older GM timing on the FI cars, those cars would run as much as 60+ DEGREES. My 402 had over 50deg timing at cruise. So many people forget to give engines timing they seem to limit the timing because they THINK its too much.
But good luck to all who are looking for gas mileage.

[Rod]

Rod,

What was the power output of your 383 versus your new LS?

the LS is a little better

[CarlC]

490hp/490tq to the rear wheels but there's a big lung on top of the engine.

[Matt@BOS]

490hp/490tq to the rear wheels but there's a big lung on top of the engine.

I keep forgetting your car has "only" 490/490 at the wheels. The butt dyno always gets confused when I go for a ride on the autocross with you. That engine of yours just flat out pulls, especially in the low to mid range. Now I find you're getting mid 20 MPG going down the highway? That LS + Magnacharger is a hard combo to beat, especially considering I seem to recall you had the cooler figured out as well.

As for my environment killing machine, it is an 454ci LS engine with 600rwhp pushing around an old pile that weighs 3700lbs with me in it. It averages about 13-16 mpg at slightly above a leisurely granny pace, which is where I like to stay on the road. Additional tuning might net improvements though...

Matt

[mitch_04]

I am going with a power adder as well, a GT45 (Chinese knockoff) that a large amount of people have had great success with over on the turbo forums. It's crazy to hear roughly 500HP and 24 mpg. How much are you getting "around town" and when you beat on it a little more? Not all out racing, but maybe a fast pass or two down the highway, little burn out here and there.

I'm hoping we have an engineer who specializes in aerodynamics or a related field kick in. Or someone who has spent a lot of time chasing mileage and learned a thing or two.

[High Plains Mopars]

This also makes me wonder what kind of mileage you could get out of a car meant for MPG's if you stuck an LS in it. I remember seeing a build where a guy dropped a Civic body onto a Corvette chassis. Didn't beef up the motor much because it wasn't really necessary! I would imagine he got great fuel mileage, but that's a discussion that never comes up.

Well, my old commuter CRX Si with a injected 4 banger had the same power to weight ratio as a Vette, and knocked down 50mpg when new so I'm not sure an LS conversion in it would net any benefit other than giving it lots more power and a ridiculously nose heavy weight distribution.

An experience in my truck demonstrated that there was a lot of mileage to be had in managing the torque production in the engine. The more torque across the useable rpm range meant I needed less throttle angle to get things moving and keep things moving which tended to increase the mileage produced. Torque was the result of increased dynamic compression ratios. It also peaked at 3500 rpm, which coincided with the highway cruise rpm of the rest of the combo.

With a 11.5 static ratio engine, combined with a small overlap cam and 650 carb, my 4500# truck with 3 spd auto, 4:11 gears, 33" tires, no aero aids and pulling a trailer with 3500# of car average 18 mpg over a 1000 mile trip. I also discovered the limits/benefits of rarified atmosphere as this combo had detonatation problems at lower altitudes but ran like a scalded dog at anything over 4500'.

A subsequent buildup in the same truck for a 9:1 engine saw a huge drop in the seat of the pants power meter and the resulting torque that produced the mileage. Average mileage figures for this combo also dropped and I had to struggle to get 12 mpg on the highway thereafter, but it never detonated at low altitude again.

So therein lies the key to more modern powerplants ability to produce high power with high mileage...their ability to maximize the squeeze while controlling the potential damage of detonation.

[Scott Parkhurst]

My wagon routinely knocks down 20 MPG on the freeway. Gearing is a big part of that.

I run a 3.89:1 gear in the Currie 9-inch, and in 6th gear in the RSGear T56, that put the 383 around 1,400-1,600 rpm at freeway speeds. The cam is big, but it just starts to smooth out at those rpm. It doesn't buck or complain, and with gentle persuasion, can accelerate nicely on the freeway in 6th between 65-85 mph easily.

This is a carbureted Gen1 383 SBC with an HEI. Nothing tricky- just a good combo. Stock fuel system and Doug's Tri-Y headers. Basic stuff...kept in good tune.

The engine made 545hp and 500 ft-lbs on the dyno. With 150hp of spray it made 579hp to the wheels. 10:1 C/R.

[mitch_04]

I always love hearing about carbed engines that get great mileage, seems like much more of a challenge! The reference to a Civic with an LS was more to see what difference weight would make with fuel mileage, not saying it was a great idea!

Although it is great hearing about guys getting good mileage with higher power engines, I would like to try to steer the conversation back towards what we could do to help mileage assuming we can't touch the engine or gearing. Reducing resistance to air/road, things like that.

[High Plains Mopars]

Unfortuantly, IMO, once the engine build is done, improvements after the fact are like trying to dam a river with pebbbles. Can you make some minor improvements, probably, but you will be battling a case of diminishing returns.

Some quick thoughts on things to do;

reduce tire width
increase tire pressures
replace dino oils with synthetics
lighten up overall weight
functional ram air
install a vacuum gauge and use it to change driving habits
install a wideband O2 sensor and tune using it

[rockytopper]

23+ here on mail order tune and bad driving habbits.

[Z06vet]

I get around 12-14 in town but get on it quite a bit here & there. On the highway I get a solid 25mpg at 75-80mph even with the AC on. I have put over 10K mi on the car in just over 2 yrs. Its an LS1 with heads/cam, T56, and 4.10's. At 80 mph its turning about 2100 rpm. It put down about 410 to the wheels.

[mitch_04]

I understand the engine and driving habits are probably the 2 most important components, but for the sake of discussion I would just like to address other contributors.

Would ram air help the fuel mileage? In my mind, it would be resistance to air flow, although it would also create a small boost effect at high enough speeds.

When you do things to add downforce, it should hurt mileage because you are increasing resistance to air in an effort to press down the car. So removing spoilers and wings would, in theory, add mileage.

Do you(anyone) think if you fabricated a lightweight underbelly pan it would increase mileage?

[Scott Parkhurst]

The aero stuff you're talking about will work, sure. But it will make such a trivial difference at freeway speeds (65-70 mph) it's hardly worth the effort.

With really high end runs over 100 mph, aero starts to have a dramatic effect- and becomes a genuine concern for any car hanging out over 120 for any period of time. Silver State proves that.

But for typical freeway driving, the big stuff being mentioned here is what makes the numbers work. RPM at freeway speed is a really big one, and nothing will make a bigger boost in your fuel economy than reducing the rpm your engine must maintain to keep you at 65-70 mph. If you're smart and you do the math and nail the lowest rpm point the engine can comfortably maintain those speeds at, you'll see the best-possible performance. Since rear gears are expensive, do the math carefully the first time...! LOL!

You can tweak it with tire height, but tires are such a big part of the Pro Touring game that they need to be optimized for performance and fit. Fuel economy is a secondary consideration.

[19,69camaro]

Ram air doesn't do anything at all. There's a good article somewhere busting that whole myth. Plus the last thing you want is more air being pushed into the engine compartment in a musclecar.

[mitch_04]

I do understand that the engine and driving habits are important, just thought it may be a fun discussion. Other things have to be somewhat important, otherwise a '99 Corvette and Camaro with similar LS1's should get the same mileage, but the 'Vette does tend to win. It is lighter, but also has more rolling resistance from the large tires. I'm assuming the wheel and tire combo's are heavier than the Camaro, but I don't know that. I would believe that the aerodynamic differences have to be a large amount.