Fuel economy discussion goes here

[g6civcx]

Due to protest in the other thread, all posts were copied here for discussion if you so choose.

I wonder what kind of gas mileage this will get

This [v8] project is making me question the rb25 240 I want to build

last one got around 15 mpg , but that was romping on it 24/7 pretty much, im sure it could do in the 20's if you tried. My sr with 350 whp got 25mpg

Duh, because it's fun to jump to conclusions and flame people on forums ;)

I think it's a great question to ask from the curious side of things, as I too wondered how it would do. Seems to me no better/worse than an LSx based V8 (when not using 6th) and or the typical SR20. Good stuff to know!


Looks like this is a hard question for Zilvians to simply anwser. It didn't come off as : ''yeah your MPG is shit now'' .

To give you an idea MPG is largely affected by weight .

90-93 Q45 Curb weight : 4039 14 city / 18 highway *active suspension w/o 15 city / 20 highway

This engine is very well made and will be very efficient it's design trickles down to the SR20 with it's bottom and and upper closed deck design, and it's 4500cc's (4.5L) displacement with 1.76:1 Rod to Stroke ratio make it a very good motor on fuel today's EMS can make it better.

However this engine in a GSRacers S14 with is what 1000lbs lighter +? will have higher MPG think 25 and better.

Damn, that's a very good point. The weight factor totally slipped my mind as far as gas mileage is concerned.

Mine gets like 17 or so i think. idk my speed sensor hasnt been working properly.

i really doubt your gonna get 25. I dont think i am. but my timing i think is still off.... so who knows.

Im interested in that powersteering bracket you made. as i didn't have time to make one before i went out to school. so i'm driving it without PS.

You will get better mileage as long as you do not have a heavy foot. Since the s-chassis is lighter than a q45, the motor does not need to work as hard to move that weight.

If you did some good tuning had it set to lean burn on low loads and at idle, you could definitely get good gas mileage with a large motor.

If you're worried about fuel economy, don't forget final drive ratio and cruising engine speed.

I dont... if i did i wouldn't have done it and spent 2 fucking years figuring out how the hell to put it into an s13... durrrrrr


but now that its all said and done it would be nice to not go through a tank in less that a week.


Please be nice.



I talked to a Nissan factory technician about fuel economy. He informs me that the engine is the most efficient at torque peak, and therefore would yield the best fuel economy if cruised at torque peak RPM.

My V8 has a flat torque curve. I cruise at 1,500 RPM. Small displacement engines doesn't have enough torque to cruise at 1,500 RPM so they need a higher cruising RPM.

The problem is that V8 tends to come in heavier car. So V8 has a bad rep for getting bad fuel economy.

If you took a light car, and compare a V8 to say a KA-T, the V8 would get mid to high 20s, whereas the KA would struggle to get high 20s, and any acceleration at all would knock it back down to low 20s.

Keep in mind this is not theoretical. I compare my V8 to his KA-T. Granted I have carbs so I'm only getting low to mid 20s, but think of LS-series that get mid to high 20s on the highway. Now drop the weight by 500 lbs and I think you can break into 30s.


We both make about the same amount of torque ~400 ft-lbs, but I make that from 2-5000 RPM. Same car. Both of us have S13 fastback. He doesn't make close to that until 4,500RPM and revs to 8K.

On paper he makes more power, but we're about the same on torque.

The advantage is that I don't have to rev so much for regular driving to be in my torque peak = higher fuel economy for V8.



[who cares about fuel economy?]

people who are intelligent? who wants a car that gets poor fuel economy specially with gas prices rising?

yeah, because all V8's get poor fuel economy :rolleyes: LS1 gets 28MPG

try this new thing it's called thinking before you say/write all the 'cool' people are doing it.

[g6civcx]

Originally Posted by CrazyTrance
Mine gets like 17 or so i think. idk my speed sensor hasnt been working properly.

David Steele
Doesn't mean you were driving for best economy.

all I'm saying is fuel economy is an important factor and people shouldn't be abused because they asked the question.

/done

BSFC(brake specific fuel consumption) rarely happens around peak torque because it has a variety of factors that affect it(friction, pumping losses, manifold/port harmonics etc). It's typically at a lower RPM than peak torque on modern engines. Sometimes much much lower.

How is BSFC related to throttle position and minor fluctuations in speed?

There's a BSFC curve for every throttle position. RPM is related to speed. Unless that's a rhetorical question, in which case I've got nothing.

What I mean is that you generally don't hold a steady cruising speed.

You generally slow down a little bit and speed up a little bit even when cruising.

So it's not practical to calculate the max point of the BSFC curve for a single speed.


So would a single max point on the BSFC or more like an RPM range, say 500RPM or 1000RPM range, that would make more sense to analyze for fuel consumption?

I never said anything about a max POINT of BSFC. It's a curve, there will be a minimum which will move around wrt RPM depending on other variables.

Changing speed slightly or not, the BSFC curves for say every 10% throttle opening will show you where an engine is most efficient.

Enough of the tangent, BSFC and how it relates to fuel economy isn't that complicated. I'm sure there's a wikipedia article on it or something.

Anyone else who wants to contribute please feel free to continue. Otherwise please hold your comments.

[Bushido]

I read today that the new zr1 corvette gets 30mpg on the highway. impressive.

[g6civcx]

I read today that the new zr1 corvette gets 30mpg on the highway. impressive.

Ultra low cruising RPM = fuel economy?

The older fbody and c5s did pretty well too so it's no surprise.

[GSXRJJordan]

I thought the BSFC convo was interesting - naturally, when an engine is under conditions that produce peak torque (which for almost all internal combustion piston engines is up in the revs quite a bit), you'll need less fuel to produce the same amount of energy - the problem is that keeping the engine at that 'condition' means losing a bunch of that energy to moving all the reciprocating mass, the friction involved, and to a lesser extent the rotating mass of the driveline.

That's why you get better mileage below your peak torque... mileage is something that is complicated enough that it needs to be tested in the application - go out and drive as "grandma" as possible for a set number of miles, then normal, then "track day" status, and report back.

[g6civcx]

peak torque (which for almost all internal combustion piston engines is up in the revs quite a bit)

I respectfully disagree.

Exhibit A: http://img239.imageshack.us/img239/6151/dynovn9.jpg


My peak torque is somewhere around 1200-1500 RPM, but the dyno doesn't read that low :p

[SexPanda]

so basically... what your saying is...

Grow a mullet, date your sister, and drive an f-body to get good gas mileage, right?

lol I kid. There are plenty of ways to increase your mpg. Better intake, better spark plugs, new o2 sensors, tune it to run a little lean... a little. And I'd do that on an NA car before I'd to it on a turbo car.

[g6civcx]

Grow a mullet, date your sister, and drive an f-body to get good gas mileage, right?

Actually, yes. If nothing else, f-body motor - f-body + lightweigth Nissan = fuel economy?

I am growing a mullet though...

tune it to run a little lean... a little

I'd rather get .5 fewer mpg than running too lean.

[bkfill]

longer gears would be great

i read someone did a 3.2 rear end? is that true, they took the ringland out of a pathfinder front diff or something or a xterra

i get 15-17mpg street/highway

i got 22mpg once on highway, some street

SR20DET S14 NVCS// GT2871R .64
550cc, 255walboro, 50fuel pressure nismo fpr
stock s14 VLSD 4.08
stock sr20 s14 tranny

[g6civcx]

i read someone did a 3.2 rear end? is that true, they took the ringland out of a pathfinder front diff or something or a xterra

That's total BS. That person doesn't know what he's talking about. If he were here I'd beat him senseless.

No Nissan R200 ever came with a 3.2.

[mrmephistopheles]

longer gears would be great

i read someone did a 3.2 rear end? is that true, they took the ringland out of a pathfinder front diff or something or a xterra


A ring land is on a piston. A ring gear attaches to a differential planetary gear.

3.2 doesn't exist (for Nissan R200, anyway).

I'll be using a 3.692 in my S13 as soon as I can figure out how to change a bearing over without a press. :keke:

[Dutchmalmiss]

I'll be using a 3.692 in my S13 as soon as I can figure out how to change a bearing over without a press. :keke:

Wow that's still long haha. From what is that from?

[g6civcx]

3.2 doesn't exist (for Nissan R200, anyway).

It does exist, but not as an OEM option. Offroad Nissan guys get custom ratios made for their R200 diffs, but I don't hang out with that crowd enough to know.

I'll be using a 3.692 in my S13 as soon as I can figure out how to change a bearing over without a press. :keke:

It may be easier to order a new one from the manufacturer. Not Nissan. The manufacturer is printed on the bearing somewhere.

If not let me know and I'll set up the info.

$40+shipping. It was cheaper than trying to find a press.

[g6civcx]

Wow that's still long haha. From what is that from?

http://zilvia.net/f/tech-talk/213491-argh-final-drive-swappin.html

[bbejj123]

a little off topic but wouldnt a lower ratio'd rear end theoretically help in emissions?

[SexPanda]

a little off topic but wouldnt a lower ratio'd rear end theoretically help in emissions?

Lower ratio-lower engine RPM at cruising speed.

Lower RPM-less fuel burned. So yeah I suppose if you look at it that way... sure it'd help. In a superficial kind of way.

[g6civcx]

a little off topic but wouldnt a lower ratio'd rear end theoretically help in emissions?

It depends on that standards used to judged emissions.


If you're talking about low speed dyno tailpipe testing, then it depends if your engine makes more emissions at lower RPM or not. If yes, then lower ratio will not help.

Some state requires certain gears to be used for testing, and if you go too low in gearing your engine will bog.


If you're talking about cruising, then maybe yes maybe no. Lower cruising speeds may produce more emissions in ppm, but you're spewing out less nominal count so it may be better.


It really depends. Nobody knows.

[JRas]

I should be getting great MPG with a 4.63 ring and pinion than :sarcasm:

with the stock ring and pinion ( 3.08? ) I already am at like 3k going 60-65 ( in 5th )

I need a 6th gear to save gas

[mrmephistopheles]

I should be getting great MPG with a 4.63 ring and pinion than :sarcasm:

with the stock ring and pinion ( 3.08? ) I already am at like 3k going 60-65 ( in 5th )

I need a 6th gear to save gas

You have it backwards (on purpose?).

higher number = more engine revs to turn the wheel one time.

Stock final drive ratio is 4.08:1

number if gears isn't relevant to engine revs - it's the drive ratios of those gears.

4th gear is almost always 1:1.

[WangonwWarrior]

My LT1 f body just got 15 MPG today and i paid $5.29 a gallon for that tank of gas:faint: After i bought that tank it runs like shit even after a new fuel filter and boost of octane i dunno. I got a bad distributor i need to fix so i can go back to 26mpg.

And i dont date my sister but a mullet would be nice

[blownmotor]

Tons of torque and long gears plus lower ring and pinion ratio will get you good gas mileage. I usually shift around peak torque on my KA and it gets me pretty good mileage. On the SR I seem to waste more due to the lack of torque and it's boost dependent.

[mrmephistopheles]

I thought 4.63 gears would be shorter, therefore the engine revs would climb more quickly?

I'm a bit confused please explain how this stuff works, Please

Thanks

Think of it like this - Let's say we're working with a button and a light, and 3:1 and 4:1 ratios.

3:1
Press the button 3 times, and the button lights up for one second.

Press the button 180 times and you've got light for a minute.

4:1
Press the button 4 times, and the button lights up for one second.

Press the button 240 times and you've got light for a minute.


You can see there's a difference of sixty button pushes between the ratios. Consider that as a difference in how much an engine has to work.

Let's change the button presses to revs/min (rpm) and increase by a factor of 10.

1800rpm for a minute will get you a mile, or 2400 rpm for a minute will get you a mile.

Seeing the difference here?

4.36 or 4.6 gearing makes the engine work harder throughout the range of speeds.

Off-the-cuff example (not accurate by any means - just an example).

60mph

4.08:1 (stock) = 3500rpm
4.36:1 (some Silvias) = 3850rpm
4.6x:1 (something) = 4200 rpm
3.69:1 (S15 SpecR) = 3200rpm

That's messing only with final drive ratio, not tire/wheel sizing or transmission gearing.

As you can see, the engine has to spin more, work harder and burn more gas to do the same amount of work.