NEO RB25DET Gas Mileage???

[1slowsilvia]

So I have searched google and 2 things came up jack and shit... I'm considering doing a RB25DET NEO 6 swap in my 1998 S14. just curious how the MPG is on them? I'm not expecting SR20 MPG which in my SR's ive seen over 33 mpg on hwy and 27 in the city :)

Anyone with knowledge if it gets better MPG than a KA ill be happy less and I might start looking into some LS1 swaps.

[davirene]

Hmmmmm so you got 33mpg out of a stock sr???

[1slowsilvia]

Hmmmmm so you got 33mpg out of a stock sr???

Yes sir driving from FL to Indiana. staying out of boost cruising all on interstate. car had Greddy FMIC, walbro fuel pump, trust down pipe, apexi n1 cat back.... and no 02 sensor lmao.... I assume the car was tuned to run without it as the bung for the o2 sensor was welded shut.

I know cause I set my trip and everytime id pull over my dad would be like what?!?!?! you're still on a half a tank! was pretty funny. was a VERY healthy SR too 162 psi across all 4 cyl.

[Sileighty_85]

My S1 RB25 gets 16 city ( I have a heavy foot) 25-26 Highway

[1slowsilvia]

My S1 RB25 gets 16 city ( I have a heavy foot) 25-26 Highway

ok so im assuming since i'm buying a Neo which is from a R34 skyline. Then the fuel economy should be a little bit higher... (usually how that works since prob has a different tune id assume than a series 1)

[singlecamslam]

My S1 RB25 gets 16 city ( I have a heavy foot) 25-26 Highway

yes city mileage on these things suck. Highway is ok. Gas mileage should be your last worry when picking a swap. Just saying.

[Kingtal0n]

I love fuel economy threads.

Heres a little insight. 1 Joule = 1 Newton*meter = [(1kg*m^2)/s^2]
also, 1 calorie = 4.184 Joules

When calculating fuel economy, you are actually comparing Joules of work to dollars, essentially how much you pay per gallon of gas vs how far you actually get moved.

If we just compare gallons of gas to how many miles we go, that would be miles per gallon or gallons per mile. lets use 1 gallon of gas for example purposes.

If I have 1 gallon of gas in my tank, and I run my engine (any engine) and do not drive anywhere at all... I could use up all 1 gallon and have nothing and havn't moved an inch. 0 Miles per gallon. 0 Joules of work? NO! Work was done on the piston, the crankshaft, the rods, the flywheel, etc... Parts are being moved around so Joules of work are being done on components. But we are simply not moving. So there is more to these equations than just JOULES of work done; we could know the maximum potential joules of work that could be done, and compare it with how much work was actually done, if we wanted, but it wouldnt matter because work done by our engine while it does not move our vehicle does not make us happy; we want miles per gallon, which is examination of the physical distance our vehicle moved with all of the sloppy energy production of our barbaric combustion engine factored in.


Let's ask ourselves- One rolling 240sx that might weigh 2800lbs, is easy enough to push? I push my 240sx around all the time; even when I dont need to. If I push my 240sx 1 mile, did I use any gasoline? Obviously not; but does that make it free? again, no, because I had to eat something to provide the energy to move the vehicle by hand. How many calories? Recall that 1 Joule = 4~calories. Not Kcal! Human food calories are measured in Kcal, so 1 Joule is actually .004184 food calories (Kcal). If I decide to push my 240sx one mile, I still use energy and much of it is wasted as heat and other forms of energy as the process of metabolism via enzymatic pathways provides the force necessary to push the vehicle. Also take into account the earth is not a perfect circle, gravity in different places on our planet's surface differs. As does temperature- which affects to some insignificant degree the size of objects, including the vehicle itself and it's internal components, and even the height of the water in our oceans, and I guarantee there are different number of atoms per piston in your engines; no two pistons are the same. So no two pistons are the same shape, or same weight, ever, unless somebody actually counts every single atom and removes the excess. Which is technically possible- but not heard of. And even then the bonds between the atoms could be different shapes, still giving a different shape piston despite it having the exact same number of atoms as the next one.

So here is our imperfect vehicle. Imperfect engine. Imperfect gasoline. How much of your 93 octane is actually true 2,2,4 trimethylpentane? How much energy is available for extraction from that gasoline? Sometimes we can get more joules from 1 gallon of gasoline than other types... so mileage with that respect will vary as well.

For just one minute, forget about engines, and vehicles. Just think about weight, and distance. The more weight, the more distance, the more fuel/food you will need. If your 240sx gets 30mpg today, and you add a couple hundred pounds to it, it is very likely that it will no longer achieve the same fuel economy given the additional weight. That said, it doesnt matter what kind of engine you install! A RB25 NEO engine with low mileage should get similar fuel economy as an SR20DET engine from any year, due to it's efficiency and design, and low vehicle weight for both species. the ONLY difference between the two engines, assuming identical drivetrains, will be the energy lost due to friction of turning more components. But on the other hand, the RB25 may have better efficiency in it's design, meaning perhaps less energy is lost while turning, for instance maybe the stroke/bore is more friction friendly, and maybe the rings are lower tension, and maybe the additional weight of the flywheel or drivetrain parts gives a better control for the driver to accelerate lightly; those insignificant differences can add up and give you energy back in your pocket in the form of dollars.

And as examples from vehicles I measured:
1995 240sx OEM redtop OEM tires/parts everywhere: 30+Mpg highway steady cruise 65MPH
1997 240sx OEM RB25 OEM tires/parts: 30+MPG highway steady cruise 70MPH
1997 240sx 2jz-gte @ 450RWHP OEM internals, heavy showy wheels: 26MPG highway steady cruise 65MPH

now, I take the sr20det and install a wideband, and configure a narrowband output for 15.2:1 air fuel ratio for cruising. Install the lightest wheels/tires. Take everything out of the vehicle possible.
Now you are getting 32+MPG highway. I have not been able to get 35mpg, I am thinking about where else to remove weight from, what else can be lighter? Lightweight driveshaft is on my mind. already using OEM valve springs. Maybe a gearing change? install a 6-speed and the S15 differential will lower final cruise RPM, thus giving less friction for internal components among other benefits. that might work. The point is: lightweight, easy to turn. Installing heavy valve springs for instance makes the engine harder to turn.

[korniyan]

get the weight out from your body

[tinys s14]

I dont know exactly how mpg I get with my neo but I drive it 5 days a week about 35 mile to and from work and fill up 30 bux once a week.

[zerooath]

I dont know exactly how mpg I get with my neo but I drive it 5 days a week about 35 mile to and from work and fill up 30 bux once a week.

Wow that's not bad at all. 70 miles total everyday. 350 miles total for the week.

Now when you say "fill up" does that mean it takes $30 to fill up your tank from Gas Light on to Full?

I'm assuming this is all highway driving?

[2.5T_/<ouki]

I've got an S1 RB25 and I only get 200 miles to a full tank :'(.

[Syncade]

My Neo is great on gas. I was getting a rough calculated 27.

Im selling my full S14 Neo swap you're interested. Fully stock motor never beat on (I dont drift/track), and many brand new parts not yet installed.

[smoked240]

King pretty much hit the nail on this one. If your still not sure read his post once more.
I can pull about 300-350 a tank highway cruisin. That's with a s2 rb25.

[steve shadows]

Love you Kingtal0n - No Homo. Someone else with real engineering knowledge on the forum Gotta love it. Listen to him folks.