NZR: 3.27 -> 2.73 too steep?

[rudypoochris]

I have a truck with specs:

 

155hp @ 4200rpm

220ftlbs @ 2400rpm

4300lb loaded

OD: .79

Tire: 235/75/15 (28.88" OD)

 

Basically I am trying to pull better fuel economy out of this truck. I have 3.27 rear gears right now and cruise 1960rpm at 65mph. I am wondering if switching to 2.73's will gain me fuel economy? It will shift me down to 1630rpm at 65mph which will essentially make the car a dog. Will this load the engine too much and someone cause me reliability issues? Increased fuel consumption? I don't see it causing either, but people have told me that it will be an issue. I would prefer to get some real 'experts' advice here, at least I consider you all 'experts' compared to the other 95% of forums I find. Thanks guys!

[johnc]

That's a 16.5% drop in ratio which is a big drop given the engine torque numbers you list. If you can split that with a shorter tire you will probably see a small increase in fuel mileage, but since your torque peak is at 2,400 rpm you don't want to get too far away from that number.

[rudypoochris]

That's a 16.5% drop in ratio which is a big drop given the engine torque numbers you list. If you can split that with a shorter tire you will probably see a small increase in fuel mileage, but since your torque peak is at 2,400 rpm you don't want to get too far away from that number.

 

Is it that the engine will be too far out of its power band? I know it isn't exactly the same thing, but don't the LSx's run something like 1600rpm at freeway speed. Surely their torque peaks are quite high up compared to such a low number?

 

Thanks! I am really thinking the better option is to get a new OD gear cut from a local gear cutter.

[Pete84]

Does he NEED peak torque? He is only overcoming wind and rolling resistance after all.

[rudypoochris]

I am figuring it this way.

 

The engine is probably most efficient at peak torque or peak horsepower or somewhere about there. That is that the ratio of power received for fuel used is the highest. That being said, I do not need that much power. So you can gear out of it. The low point should be determined by when the engine is so geared out that it is so far out of its efficiency that despite lower revs and less pumping loss, the amount of fuel going in respect to the power back is not a favorable ratio. If only I could get a chart that showed this... is that basically what a VE chart is? If so, does anyone have a VE chart for anything like a stock 5.0L or 350 or 4L, etc. Big cubes, low power.

[johnc]

The following is pure conjecture, but that's what this thread is asking for anyway...

 

Without seeing the engine torque curve from a dyno chart we are all assuming a bunch in this discussion. Generally the rise to torque peak is steeper then the fall off after torque peak: 500 rpm below the torque peak has less torque then 500 rpm past the torque peak.

 

Let's assume the engine makes no usable power (at least for this discussion) at 1,000 rpm. So there's a 1,400 rpm range where 220 ft. lbs. is built up. Take away the top 800 rpm of that range with the changes proposed and what kind of torque numbers do you think the engine is producing?

 

At 65 mph the aerodynamic drag is probably (wild ass guess) 75 ft. lbs. of torque. How much more torque is needed for friction losses, tire drag, MOI, etc.? I don't know.

[Gollum]

Displaimer: This post is only based off of what I've seen, not experienced. I've been working on building a database of dyno charts and I've gone through literally hundreds of dyno charts and I've become, to some degree, versed in knowing what kind of power curves to expect from motors based solely on displacement, block, and head design. I'm in no ways saying I know everything or trying to prove anyone wrong. If someone wants to correct me, feel free.

 

I really wish I could find a dyno for the 4.0L cologne motor.

 

You're right JohnC about the rise and fall of torque, but as I'm sure you're aware, peak torque isn't always placed right in the middle of the torque curve.

 

An engine's torque band might start much lower than it actually peaks, due to having such a soft peak, which is quite typical of 2 valve pushrod engines.

 

My point is that there's no way to tell where the torque curve STARTS just by knowing the RPM of the peak. Maybe if you know the cam specs, valve size, bore, stroke, header flow, and intake flow you could calculate the curve.

 

But just by guessing... that's a super low PEAK number, which would make me guess that the curve probably starts to ramp up at around 1,000 rpm or so, and you probably have 80% of your torque by at least 1,400.

 

Most high torque, low power engines like these have at least a 80% of it's torque for a good solid 3k RPM.

 

I think weather or not you see a significant difference in mileage will depend on how well the ECU keeps the mixtures lean at highway speeds still. But in that regard I think your engine will manage just fine.

[rudypoochris]

You bring up a good point about torque rising abruptly and then falling slowly. Hmmm. Food for thought.

 

Additional information... Aerodynamic drag in HP, then in lbs, then in % change from previous. This information isn't exactly 'useful' with out the dyno sheet still, but it gives a good idea of what is demanded.

 

10mph - 0.1hp - 3.1lbs - N/A

20mph - 0.7hp - 12.3lbs - 297%

30mph - 2.2hp - 27.8lbs - 126%

40mph - 5.3hp - 49.4lbs - 78%

50mph - 10.3hp - 77.2lbs - 56%

55mph - 13.7hp - 93.4lbs - 21%

60mph - 17.8hp - 111.1lbs - 19%

65mph - 22.7hp - 130.4lbs - 17%

70mph - 28.3hp - 151.2lbs - 16%

75mph - 34.8hp - 173.6lbs - 15%

80mph - 42.3hp - 197.5lbs - 14%

90mph - 60.2hp - 250.0lbs - 27%

100mph - 82.5hp - 308.6lbs - 23%

105mph - 95.5hp - 340.3lbs - 10%

110mph - 109.9hp - 373.4lbs - 10%

115mph - 125.5hp - 408.2lbs - 9%

 

Wonder where I can get a fuel flow meter?

[GabeRoc]

If the goal is improved milage then there are several other questions that need to be asked.

 

Presently how often does the truck downshift durring normal driving? if the truck is downshifting on hills with any frequency then the steeper rear end will make this worse.

 

how much of the pedal are you currently using? do you run close to down shifting right now at highway speeds? there are situation where going to higher numerical ratio rear end will improve milage.

 

how much highway driving do you do? this rear end will most likely mean that for city driving OD will be useless and you will need to use D for speeds below about 50 mph.

[rudypoochris]

If the goal is improved milage then there are several other questions that need to be asked.

 

Presently how often does the truck downshift durring normal driving? if the truck is downshifting on hills with any frequency then the steeper rear end will make this worse.

 

Whenever I move the shifter down from 5th to 4th.

 

how much of the pedal are you currently using? do you run close to down shifting right now at highway speeds? there are situation where going to higher numerical ratio rear end will improve milage.

 

I am using very little, I would say <10% at 65mph.

 

how much highway driving do you do? this rear end will most likely mean that for city driving OD will be useless and you will need to use D for speeds below about 50 mph.

 

80% of driving is at highway speed if not more.

[EMWHYR0HEN]

Just cuious, what kind of milage is the truck getting?

[rudypoochris]

Just cuious, what kind of milage is the truck getting?

 

I usually see around 18-19mpg combined per tank. I drive it like a 20 year old around town, so figure that into the equation. Last all highway trip, I averaged 74.5mph at 20.6mpg with 4 wheels and tires in the trunk to sell to a fellow HBZ'er. That is with passing spurts to the mid 80's and slows to the high 50's for truck blockages on I-5. I think for that average speed, that is very reasonable. Tires were at 36psi all around. Best tank I ever remember getting was 23mpg and that was up to the mountains and back down. I think the altitude which kills your power also helps your economy. I am sure I could get MUCH better than I do, if I just slowed down. Realistically all the highways around my area though are traveling ~70-80mph and I am just impatient maybe. The truck doesn't make 300hp though, so it isn't quick, which I assume is why it gets decent fuel economy. It is ~155hp and 220ftlbs.

[Gollum]

When I was riding with chris I noticed that even when he's "driving like a 20 year old around town" he's still dipping bellow 1k when shifting...

 

The engine has plenty of torque to be able to cruise at 1500 on the freeway no problem.

 

And you'd talked about RPM at 65mph, but as you've stated most of your highway travel is a bit faster. I think 2.73 might even be leaving some RPM on the table honestly. I think you'll be fine under load conditions like high wind or going up a hill.

[rudypoochris]

Depends on wind speed. I don't want higher than 2.73 as it really will be a dog. I am shaving off RPMs and a bit of torque since I am going before the torque curve. That kills the horsepower really.

 

In any case, I wasn't driving like a 20 year old. I was driving like I would consider I normally do about 1/2 the time.

[Gollum]

At any rate right now the car pulls just fine bellow 1k.

 

And I don't think you end up loosing HP in the long run, just torque.

 

HP is just an equasion of torque and RPM. If you gear to have more torque you loose RPM. If you gear for more RPM you loose torque, but the overall work done is the same.

 

That said, it WILL feel less powerful no doubt.