Engine sweet spot and highway RPMs - Defender Source
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  #1  
Old September 5th, 2007, 09:51 PM
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Charles Galpin
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Engine sweet spot and highway RPMs

Ok, I'm caught in the tangle of upgrades I got a set of ARB lockers at a decent price and although I wasn't ready for them, they are sitting in my garage taunting me. So I went to order bearings and decided to replace not just the carrier bearings but also the pinion bearings and seal - I mean why go half a$$? So you know what's coming next. Heck if you have it all apart, now is the time to re-gear!

My truck is a '94 with an automatic transmission from a discovery, stock 3.54 diffs, 285/75/R16 (~ 33") tires and the low end gearing is terrible (although I honestly don't know what it is).

That said, I have always liked my low RPMs on the highway and have been telling myself that I wouldn't re-gear in the axles, but keep them stock, and one day rebuild my transfer case and at that time give it better low end gearing on the low range thus keeping my low RPMs at speed. I do slow down on hills right now, sometimes much more than I'd like, but considered this "normal" for a brick on wheels. But when this happens it does feel like I just have no oomf at the low RPMs and that higher RPMs would handle it better.

But while innocently reading up on gearing, I noticed people saying their highway RPMs were much much higher than mine, and also saw people saying that not only can the V8 run at relatively high RPMs comfortably, but that the sweet spot is above 2500 RPM. I'm starting to think I might be able to benefit from going to 4.1 gears at both ends as it might bring my highway RPMs up closer to the sweet spot and still be relatively low.

So i'd like your comments as to what the engine sweet spot is and what are good RPMS for highway driving.

In my case I don't think I have ever pushed the RPMs over 3000 before today. In testing, I floored it from a standstill and the highest I got the RPMs to was 3200 before it downshifted (probably 2-3 or 3-4). Usually I never break 2500 and mostly putz around under 2000.

At 60 mph I run about 1800 - 1900 rpm
At 70 mph I run about 2100 - 2200 rpm

What do you guys run, and what would you guestimate my RPMs would rise to with 4.1 gears?

tia
charles
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  #2  
Old September 5th, 2007, 10:00 PM
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I have 4.10 with ARB's and run ~3000 RPM at 75 mph. But, that is with a supercharger so it uses a little more RPM.
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  #3  
Old September 6th, 2007, 08:33 AM
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Charles Galpin
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Thanks Paul. Do you recall what your RPMs were before the 4.1 gearing? And how does 3000 RPM feel? Seems high.
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  #4  
Old September 6th, 2007, 08:35 AM
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S. Smith
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There is a very helpful calculator which can help you figure out what effect gearing changes have on RPMs etc.

http://www.4lo.com/4LoCalc.htm

Cheers
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  #5  
Old September 6th, 2007, 08:39 AM
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My only mods are in the Disco ... same math though, right?

235/85's and 4.10s in the diffs = ~2900rpm @ 75mph ... it gets a little busy about halfway across Nebraska

The 285's should reduce that by what, ~7% ... not a big number ... Weren't there some 3.90's available somewhere??? or were those just the old P5 gears?
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  #6  
Old September 6th, 2007, 10:57 AM
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Charles Galpin
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Keith, what did you use for a transmission ratio? If I use the specs listed int he owners manual for a '94 in 5th high, I get way higher rpms than I am experiencing. I have to use a 0.42:1 transmission ration to get the RPMs I am seeing. Assuming this is acurate, if I change to 4.1 gears in the diff, my RPMs rise to just under 2500.

Our transfer cases are 2:1 right?
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Old September 6th, 2007, 11:34 AM
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The ZF autobox, what is it, an HP22 or something.

The o.d. fourth is, what, 0.8:1; Hi in the LT230 is 1.2:1; diff gears are 4.10:1 ... overall top gear is 3.94:1

Stock ... 0.8:1 * 1.2:1 * 3.54:1 overall top gear would be 3.40:1

Looks like the difference - w/o regard to tire size - is ~ 16% increase in rpm at a given road speed.

Hmmm ... 2,500rpm * 1.16 = 2,900rpm ... whadya know

...edit ... also see http://www.expeditionlandrover.info/gear_ratio.htm for a nice collection of gear ration figures.

KAA
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  #8  
Old September 6th, 2007, 11:59 AM
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Charles Galpin
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Mmh, my transfer case ratio was way off. Using 0.8 for the gearbox/transmission still gives me much higher rpms than I am seeing. It matches my rpms at about 0.7.

I'm not sure what's different about my rig, but it does give me a way to see what the change would be for re-gearing, thanks.

But that still begs the questions:

1. What is the engine's sweet spot
2. What are good RPMs for highway use (running for hours)

It looks like I'd still be around 2500 or less with 4.1 gears which I think is acceptable, but would still like opinions.

thanks
charles
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  #9  
Old September 6th, 2007, 12:01 PM
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I don't know about all these numbers and ratios but I replaced my stock gears with 4:11's and I had the same problems on the hills with the stock gears. I noticed that the truck was a little more snappy across the RPM ranges. I also noticed that the hills were slightly easier with the 4:11's. But I wouldn't say it was dramatic at all. My RPM"s are typically higher than you are describing, however. I will pay attention today to the RPM's and get back to you on this. I was runnings 33's all this time but I have just put 35's on so things have changed somewhat.
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Old September 6th, 2007, 12:41 PM
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Ok, before anyone has the urge to jump up and down here ...

The R380 5th gear is reported to be 0.78:1, not 0.8 in my example. Additionally, the LT230 high range is 1.22:1, not the 1.20 that I used previously.

So:

a) .73 * 1.22 * 3.54 = 3.15
b) .73 * 1.22 * 4.10 = 3.65

A difference of 0.50, or still roughly 16% of the original 3.15 overall ratio.

All those figures aside, the Disco was a pig with 235/85's and 3.54's. It's more driveable with the 4.10's and they give a nice extra gear multiplication in low range too.

Back to your regularly scheduled programming ...

KAA
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  #11  
Old September 6th, 2007, 12:47 PM
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Charles Galpin
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so where does the 0.73 come from
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  #12  
Old September 6th, 2007, 01:05 PM
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http://www.expeditionlandrover.info/gear_ratio.htm#r380
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  #13  
Old September 6th, 2007, 01:16 PM
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Charles Galpin
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Aah ok, so the 0.78 was a typo. Cool. These numbers are close enough to what I see with my ZF.

My manual had 0.795:1 for 5th gear so go figure.
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Old September 6th, 2007, 02:28 PM
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It is a lot easier to plug in the numbers for different scenarios than to bolt it in and see what happens!
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Old September 6th, 2007, 02:29 PM
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Sheesh, plut ito both sets of numbers and see of you get a (significanlty) different % change in the overall gearing when the only change is from 3.54 to 4.10 .

You're going to experience a 15 to 16% change in overall gearing ... and with a constant tire size, the same change in rpm at a given road speed.

So, yeah, it'll spin faster going down the road, and yeah, it'll go uphill better ... having said that, you do still have 4th gear, right?

Drive around in 4th and see how you like it. The change from 3.54 to 4.10 will be about halfway between 4th and 5th.

Your ball...

KAA
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Old September 6th, 2007, 03:07 PM
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I am going to a taller tire on my 110 because I do not want to change the diff gears and on the hills 5th is too high and 4th is to low. With another inch of tire, I will still have to shift to 4th but I won't go 50 because of the revs, I should be able to keep 55mph.
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  #17  
Old September 6th, 2007, 03:10 PM
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Scott, it sure is easier (and cheaper)!

Keith, I'm not sure what gear I end up in with the auto box if I am one notch back, but I'll give it a try. My gut feeling is I'll be ok with the RPMs and 4.11 gears.

Perhaps my kickdown cable needs adjusting or I need to floor it, but when it does slow down on hills, it doesn't kick down a gear easily.
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Old September 6th, 2007, 04:36 PM
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I just put a new HP 22 and lt230 in my RRC, the adjustment of the kickdown cable made a HUGE difference in drivability. It drives better than it did before and drops down at about 3/4 throttle which is how I like it. My vehicle doesn't go any faster after 3/4 throttle anyway, it just burns more gas.
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Old September 6th, 2007, 04:40 PM
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What method do you use? I followed directions i found on the 'net which basically had me loosen it until you can head the "click" when the cable retracts, and tighten just to the point where you no longer hear a click.

thanks
charles
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Old September 6th, 2007, 04:49 PM
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Here is what the manual says.

Release and remove kick-down cable clevis pin.
2. Loosen locknut and release kickdown cable from
abutment bracket.
3. Release outer cable sleeve from clip on rear of
LH cylinder head.
4. Holding cable as straight as possible, extend
inner cable to first detent.
5. With inner cable extended, check dimension A
between end of threads and crimped sleeve.
Dimension A = 42 mm 3 mm, 1.65 0.10 in.
6. Connect cable to abutment bracket and fit
locknut.
7. Secure outer cable sleeve to clip on rear of LH
cylinder head.
8. Connect kick-down cable to throttle lever and
secure with clevis pin.
9. Adjust outer cable until crimped sleeve is
dimension B from end of outer cable.
Dimension B = 1 mm, 0.04 in.
10. Tighten locknut.
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