Best option for increasing HP?

[Blmpkn]

Problem with that graph, It must be using a transmission final drive of 1:1 not over drive and a cruising speed of 65 mph.

My truck in overdrive at 65 mph and 3:73 gears with 30 tires = is about 2100 RPM.

That chart is saying I should be 2715 RPM

Good catch. I didn't notice that.

 

[don4331]

@Roert42 slapped a blower on his for moar power.

The red on that graph is telling you the rpms are a bit higher than ideal at 60mph I'm relatively sure.

Roert42 has the SOHC motor for which there is a blower installation kit from aftermarket. There is not equivalent for '96 4.0 OHV engine. (There are adapters, but they assume you are mechanically competent and can sort out all the rest of the pieces to get it working).

 

[don4331]

Fantastic reeeeally appreciate all the knowledge and advice! Sounds like regearing the diff is the way to go.
How do I interpret the color code on the graph? I see my current 31's are in the red with 4.56, red usually means bad? I have thought about putting 33's on, not entirely sure what that would mean mpg, handling, performance wise. But would definitely be beneficial for how and where I like to take it.
Don't want to kill MPG or regularly operate at borderline RPM. Overall, I want it to be reasonable on pavement, because that's where most of its time is spent. But only because I have to drive pavement to reach dirt. I'm more concerned with off-road performance. But not looking to go super rock crawler. As is, I've went down the flint trail (?) In canyonlands maze district once. Never, ever again. Pucker factor was high. But it performed. But that is also beyond what I would ever be willing to do again.

Note: If you change rear axle ratio, you have to change the front's as well as you have 4x4. And finding 4.10s in pre-'98 front axle is limited = $$$. So, cost might not be that different from re-gearing to 4.56s.

As pointed out, as soon as you start changing things, your reliability changes.

 

[franklin2]

Right now your final gear ratio is .79 (0verdrive 5th) times 3.73 which is 2.94 to 1.

If you change to 4.10, that final would be .79 times 4.10 which is 3.23

I was wondering if you could do a test and run down the highway in 4th. 4th in your transmission is 1 to 1, so the final drive ratio in 4th is 1 times 3.73 or 3.73. But that is a little higher than the final drive ratio we got before with 4.10 at 3.23. But if you can tolerate running down the highway with a final ratio of 3.73 (4th gear) you could certainly tolerate a final ratio of 3.23.

4.56 would give you .79 times 4.56 which would be 3.60 final ratio. That is closer to 3.73 so 4.56's would be more like running down the highway with what you have now in 4th gear.

You could drive it for awhile only using 4th gear as the top gear, and see what it did to your fuel mileage also.

 

[idontknowwhatimdoing]

Don't understand all that, but feel I'm getting the gist of it.
In 5/OD I am at 3k RPM at 70, approximately. I'll verify next time I drive.
In 4, I can operate at hiway speed np. Done it more than a few times unintentionally. Not sure what the RPM is. Gotta be over 3k I'd think. Check that next time I drive as well. But it doesn't roar or feel different, I only notice because I see my stick there, or I go to shift again, or see the RPM's.

So if I go those 4.56 (or 4.10, but feeling the 4.56 is being favored?),1st and 2nd might come quicker, closer together? Hiway RPM might increase for OD at 70? Front diff or 4wd transfer might need regearing? Transmission can remain unaltered? MPG will suffer some? Should lift more for 33's (oh,no! Not THAT! )? But my off-road performance will improve, and pavement will remain reasonable. And all told, even though a change has been made, so long as it is properly done I could the expect uninterrupted reliability and performance I have experienced?
You all are great! Appreciate the responses.

 

[2011Supercab]

In 5/OD I am at 3k RPM at 70, approximately. I'll verify next time I drive.

Those RPM's seem awful high for 31" tires and 3:73 gears

That would be closer to having 4:56 gears already

 

[franklin2]

Translation of my post. If you drive down the highway in 4th gear, that will be the same as having 4.56 gears and being in 5th. All your other gears with 4.56 will have more pep.

 

[stmitch]

Yes, you'll need to match the front and rear ring/pinion gearing. It's going to be expensive if you're paying somebody to do it.
Yes, your cruising RPMs will increase which is likely to hurt your fuel economy. The faster you drive, the more it will hurt. (This will also increase NVH levels.)

Before you mess with the cost and hassle of regearing, I'd verify that the engine is in good condition. Run some seafoam through it, then replace the spark plugs.
Do the 'throttle cable mod' to make sure that your throttle is actually opening in proportion to your pedal position.
Consider an e-fan conversion to free up some parasitic losses and improve your fuel economy a bit.
Start with simple, cost effective things to make sure that you've got a decent foundation, and build from there until you're happy.

 

[don4331]

Don't understand all that, but feel I'm getting the gist of it.
In 5/OD I am at 3k RPM at 70, approximately. I'll verify next time I drive.
In 4, I can operate at hiway speed np. Done it more than a few times unintentionally. Not sure what the RPM is. Gotta be over 3k I'd think. Check that next time I drive as well. But it doesn't roar or feel different, I only notice because I see my stick there, or I go to shift again, or see the RPM's.

So if I go those 4.56 (or 4.10, but feeling the 4.56 is being favored?),1st and 2nd might come quicker, closer together? Hiway RPM might increase for OD at 70? Front diff or 4wd transfer might need regearing? Transmission can remain unaltered? MPG will suffer some? Should lift more for 33's (oh,no! Not THAT! )? But my off-road performance will improve, and pavement will remain reasonable. And all told, even though a change has been made, so long as it is properly done I could the expect uninterrupted reliability and performance I have experienced?
You all are great! Appreciate the responses.

If you go with higher numerical gears, specifically, 4.56s, each gear needs ~20% more rpm for same speed or you're ~20% slower at same rpm depending which way you want to look at it. Same for all gears. In your case 4th just happens to be ~20% different than 5th, hence @franklin2 's test suggestion. And you should be seeing more "pep" when driving in 4th.

Transmission and transfer cases don't get re-geared, well not by individuals with only mortal amounts of money.

With street-able 4x4, you need front and rear axles to be within 4%, ideally identical, when using same size tires front and back.

There is a "U" curve for failures - they happen early in component's life and when worn out. All your vehicle's parts have been happily working together for 25+ years. If they aren't worn out, they will continue to do so. A new set of gears needs to properly break-in before they will run reliably. When you pull things apart and put them back together, they need to get "reacquainted" before they run reliably again or they fail. New parts usually fail less often than "experienced" parts. That's why changing u-joints & bearings are recommended if you have front axle apart to re-gear. So, for the initial period after change, there is higher risk of failure - the better the workmanship, the lower the risk, but there is always some. E.g. Why you re-torque your wheels after ~30 miles after changing them.

 

[idontknowwhatimdoing]

This has given me lots to think about for the future. I plan on taking it to the aamco here, that did the clutch years back, to do a motor/transmission inspection. See what exactly I have. I'll have to phase in mods over time, once that time comes.
In the meantime I'll just keep on truckin'! Need to get a shell before March so I can go back to the Colorado Plateau for 2 or 3 months. Then it's mountain time again!
You're all fantastic, greatly appreciate your time and knowledge.

 

[rubydist]

In 5/OD I am at 3k RPM at 70, approximately. I'll verify next time I drive.

If you have 31" tires and 3.73 axle, then you are not running 3000 rpm at 70. The math doesn't work out.

When you are doing these calculations, it is best to use the rpms at 60 mph. That is 1 mile per minute, which makes all the math easier. 31" tires are about 97" or 8.1' in circumference. That means about 650 revs per mile. At 3.73 * .79 drive ratio, that means the engine is turning about 1900 revs per mile. At 60 mph, that is 1900 rpm. (around 2200 at 70). So, if your tach is reading somewhere in the range of 3000 rpm at 70, that means 1. your speedometer is geared for the smaller tires originally on your truck, or 2. your speedometer is not working properly, or 3. your tach is not working properly.

 

[idontknowwhatimdoing]

My bad. Not sure why I had 3k stuck in my head.
At 60 I'm at 2.1k, 70 is 2.4k. 3.73 gears.
Y'all are great!

 

[Eddo Rogue]

If you have find yourself in situations where the rpms are too low to push, but will be too high if you downshift, then your final drive gearing is too tall (numerically too low). On the contrary, if you can shift up or down without much difference, then your gearing is too low (numerically high).

The low output of the 4.0 OHV comes from lack of flow. It is a very choked down motor, and responds greatly to helping it breathe by getting more air/fuel in, through, and out of it.

Porting/polish and cams/rockers can help. Besides that there is one guy that makes cool stuff for the 4.0

There are also many thing you can do to increase rear wheel horsepower with same engine HP.

Switching to an electric fan frees up a lot of power.

As mentioned, re gearing can help too if rpms allow.

Tire size and weight also matter, rotating mass is a big deal.

 

[Blmpkn]

If you have find yourself in situations where the rpms are too low to push, but will be too high if you downshift, then your truck obviously doesn't have the engineering masterpiece that is the 10r80 as its transmission.

.

 

[Eddo Rogue]

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Long forgotten truth.