How is it possible??.........Gas Mileage Q

[rsmithdrift]

Alright.......I have two cars, one is a 84 300zx 2+2 and the other is my new (to me) 240sx SE fastback.

The Z has a N/A 3.0l v-6 and the 240 has a N/A 2.4l i4

How is it possible that the Z gets better mileage than the 240?? It should be the other way around. Both cars make the same 160 flywheel hp and have stock motor with intake and exhaust (hi-flow cat, stock muffler, I like the sound )

I get 25-30mpg with my Z, and 22-25mpg with the 240. This should be theoretically impossible. The 240 should get much better mileage because it has 2 less fuel injectors and is 0.6l smaller displacement. ..............A N/A vehicle can only run right if it's A/F mixture is just right and both cars run perfect...............With a smaller engine you need less gas or the A/F will be ritch as hell, yet with the fuel mixture right in both car's the 240 uses more gas in a smaller engine???

WTF???

[crzycav86]

there's really a lot of factors, mainly the condition of both motors, but it also depends on what kind of driving you do with either car, and the gearing at highway speeds.

[KADreams]

The KA24DE runs way too rich on the OEM tune. Get an SAFC2, lean the fuel curve out, and run premium fuel.

[rsmithdrift]

I drive both cars exactly the same. Half city, half highway and I normally drive like a granny and never go above 2,500rpm. Occasionally I'll throw it up to 120+mph on the highway or do a 0- whatever I feel like on the terraces (3 lane one way, no traffic, no cops ever )

Both cars run perfect and both have 145k miles on the engine and both are geared EXACTLY the same. The only difference in the two cars is that the Z is an 84 and weighs 3200lbs and the 240's a 93 and is only 2600lbs. They almost look identical, just the 240 is much more aero dynamic looking.

Again, all of this points to the 240 should be getting MUCH better gas mileage than the Z.

[InsanityInc]

The Z likely has wider gearing, making for better gas milage.

[UK-SRi]

Also at part throttle engines are allowed to run at 16:1 or so as the heatflux at the throttle setting means the engine doesnt overheat, so ECU programming and cylinder head design, and VE ratio means you cant compare the engines properly on just displacement.

Mike

[rsmithdrift]

Did you hear me, both cars are geared EXACTLY the same. And I drive them exactly the same. The only difference I notice is that I run slightly lower rpm in the Z because it has so much more bottom end torque. I cruise at 1,500 rpm in the Z and 2,000 in the 240. I also need less throttle in the Z to keep my cruise speed compaired to the 240, but still, The 2 less cylinders and .6l smaller motor should more than compensate for this.

On the highway 75mph is 3,100rpm in both cars.

[UK-SRi]

Well as you dont need to rev the Z so far you will be in a taller gear more often, also as I said it is often not at stoich for cruising at light throttle. EGR is another gas saving variable. In the V6 there is an element of built in egr due to cam phasing, but I dont know which models have/use that.

Mike

[Ligouri Rd]

Quote »The only difference I notice is that I run slightly lower rpm in the Z because it has so much more bottom end torque. [/quote]-that is exactly why you get better gas mileage in the Z. Lower rpm means less gas burned, each cycle burns a specific amount of gas, less cycles = less gas. The larger displacement should require a slighly larger amount of gas but if the torque is so much greater that it can turn a much lower rpm you will get better gas milage. By the way, if they are turning a different rpm and running at the same speed they are either not geared the same or automatic. Tires are a part of the gearing too ya know.

Back in the day with the ole suburban the base model came with a 5L v8 and my silverado had a 5.7L. The larger motor got slighly better gas milage because it had the more torque to move the car at a lower rpm. Vette's (pre '06) got pretty good gas milage because at cruise the engine is pretty much sitting at idle.

[InsanityInc]

-that is exactly why you get better gas mileage in the Z. Lower rpm means less gas burned, each cycle burns a specific amount of gas, less cycles = less gas. The larger displacement should require a slighly larger amount of gas but if the torque is so much greater that it can turn a much lower rpm you will get better gas milage. By the way, if they are turning a different rpm and running at the same speed they are either not geared the same or automatic. Tires are a part of the gearing too ya know.

Back in the day with the ole suburban the base model came with a 5L v8 and my silverado had a 5.7L. The larger motor got slighly better gas milage because it had the more torque to move the car at a lower rpm. Vette's (pre '06) got pretty good gas milage because at cruise the engine is pretty much sitting at idle.

You're semi-right about how the operating range of an engine affects gas milage. Power is what determines the amount of gas you burn, not torque. Torque is essentially the amount of gas you burn per cycle of the engine, so obviously if you burn half the gas per cycle (half the torque) at double the RPM, your gas consumption will be the same (in a perfect world), and so is your horsepower.

However, that's in a perfect world. In a non-perfect world such as ours, your engine losses scale with engine speed, so the engine going twice as fast is less efficient because of friction, ring drag, reciprocating losses, etc.