Gas mileage theory question

[HeatRaveR]

Stock-vs-stock, I know that, all other factors being a constant (ie car body weight, tire combo, etc.) a 4 cyl engine should theoretically consume less gas than an equivalent 6 or 8 cyl of the same aspiration.

 

Now I know souping up a 4 cyl to be as powerful as a larger engine is ultimately going to require more fuel. So my question is, if you soup up a 4 cyl engine, and a 6 cyl engine to the same horsepower level, will the 4 cyl still consume less gas? I realize the lighter weight would contribute to fuel efficiency, but I'm wondering if the engine itself is still consuming less fuel. I would imagine that for normal driving, the 4 cyl of the same horsepower would consume less since it wouldn't be at peak power production all the time.

 

Now let's say we have two engines with the same # of cylinders, but different displacement, both tuned to the same power level. I would ask the same question if the smaller displacement engine would still consume less gas.

 

Any thoughts, confirmations, or discussions would be appreciated.

[randy 77zt]

fuel milage is more about the weight of a vehicle(in town driving) or aerodynamics.my 77z with 81zxt engine has got 24 mpg on trips running fast with ac.newer (96 up) cars with obd2 run engines very lean.many times a vehicle with a bigger motor will get the same or better milage than a vehicle with a small engine.best example is a 4 cylinder jeep vs a 6 cylinder.the 6 gets better.i dont know about a v8 z though.maybe if you keep the revs down with a 6 speed trans.newer corvettes get better milage than old z cars on the freeway.engine in the vette is turning under 2000 rpms in 6th at 70mph.

[TheNeedForZ]

If we only evaluate friction and weight, I would say the 4 banger save more gas. But I could be wrong, I am no engine theorist.

 

The lighter rotating mass require lesser fuel to maintain idle speed when stopped at traffic lights. smaller engine, lesser bearing surface, lesser friction in engine, lesser friction loss. The lighter mass require lesser energy to get up to a certain vehicle speed as well.

 

But all is not relevant because power and fuel economy cannot be had in the same setup---If a small engine is to have big power, then it probably uses a rich mixture.

[Pop N Wood]

There is less internal friction with a smaller number of cylinders. More valves also equals more power to drive them. Finally tuning has a huge impact on overall efficiency. That is why fuel injected engines generally get better mileage than carbed. A high compression ratio will take more power on the compression stroke, but should yield more power on the burn stroke.

 

Trying to build a 4 cylinder to have the "same power output" as a V8 generally means engine components will be geared toward high RPM operation. Add a big enough cam and the engine will scream at high RPM but run like crap at low RPM's. That "running like crap" usually mean it will waste a lot of fuel just trying to turn over.

[mtcookson]

an engine's maximum efficiency is at its peak torque. if you have a low peak torque (i.e. a couple thousand rpm) that should be where its peak power is.

 

most 4 cylinders have a high peak torque (4,000 rpm or higher). for instance my VG30E's peak torque is produced at about 2800 rpm. i can cruise on the highway at that rpm and get over 30 mpg. my brother's 240sx at the same rpm gets way less fuel economy most likely due to its higher torque peak.

 

now, you're probably thinking well for instance the new Corvette gets peak torque at about 4,000 rpm and gets great gas mileage on the highway. being a V8 it would have a lot of low end torque compared to a 4 cylinder, which means its at a higher efficiency at a lower rpm than the 4 cylinder.

 

that's pretty much my take on it. a lot of it has to do with the afr as well as other things but that's one of the bigger things that i've read about.

[johnc]

all other factors being a constant

 

But they never, ever, not in a million years, even in a lab, are constant. The precursor statement to a lazy man's arguement is: "All else being equal."

 

Let's say the 6 cylinder engine is designed to run Miller Cycle. Even with more cylinders and a larger displacement it will most likely get better fuel mileage then an "all other factors being constant" 4 cylinder. BMW's eta 6 cylinder engines were designed to run with large throttle openings (to reduce pumping losses) at low rpms. BMW couldn't get a 4 cylinder to work with this type of tuning because of the lack of low end torque.

[savageskaterkid]

alot of it does hafta do with the wieght of the car......the low torque peak will help out in a heavy car, so that it will get it goin alot easier a high torque and high hp 4 banger with a lower peak.......around 2-3000 Rpm will prolly get alot better gas mileage then a V8 with the same torque curve.

[Ferd/289]

the simple answer. The higher the revs the more effective burning of fuel. 4 cylinders rev higher over their power band, and thus is why they use less fuel per torque pound of resistance. Disclaimer: this may be bulls*it.

Ferd

[HeatRaveR]

This' some good discussion guys. I heard there was a conversion rate for like, every X amount of pounds removed from the car = Y secs shaved off the quarter mile time. Is there a similar conversion like, for every X amount of pounds removed from car = Y gas mileage improvement?

[Guest tony78_280z]

I'm piping up here to subscribe to this thread for it is rather interesting. I'll shut up now.

[Brad-ManQ45]

Lack of low end torque?

 

For a given displacement and horsepower @ the same rpm, the smaller # of cylinders will have more low end torque.

 

Whay do you think that Mistubishi (and Porsche - who licensed the balance shaft technology from Mistubishi) utilized 2.6 and 3 litre engines? Do you think Porsche would do this if a 6 cylinder was better? (remember that no other cars utilized the engine in the 944/968's)

 

I had a Conquest w/HKS intercooler and later a Starion ESIR and they were all ABOUT low end - even before the turbo kicked in.

 

They both would stomp my '83 ZXT (intercooled). Same amount of boost. Off Idle to just shy of boost - no contest - they'd just SQUIRT through traffic.

 

Remember too - the Mitsu's had THROTTLE BODY INJECTION - not the individual port injection of the ZXT. Of course the lousy MCA Ject heads are prone to cracking...

[icewtr]

There is no set answer to this almost all comes down to weight and tuning . lean any motor out will get better fuel mileage. toon it bad use alot of fuel .. i tune with a wide band 02.. car cruises about 14.7 to 15. 0 around town light throttle and on highway .. seems to get about 25 to 27 mpg thats a 77 350 chevy in a 240z.... making 300 or so hp .. have a small turbo motor making 400 hp gets almot 35 mpg.. car is a geo metro and only about 1700 lbs .. weight is the biggest difference .. but ya also have to remember alot of hp tuned right can go the same speed as a less hp car at light throttle .. i have an 800 hp turbo 408 that gets about 17mpg the stock motor in that car never got but 14 to 17 and had 225 hp .. its all just numbers ..lol