Just Regeared to 4:88s, and have a couple questions
05-12-2011 | 10:16 AM
#11
JK Enthusiast
Joined: Nov 2010
Posts: 123
Likes: 0
From: Durham, North Carolina
So, ideally you will place that curve of the torque curve where it just begins that last bend to near level at your expected passing speed. That is, if you figure you'll hit 80 MPH when passing a vehicle, figure out your tach for a 70 MPH cruise speed, then back-calculate into the desired gear ratio using your tire size and tranny type. That will give you some kind of idea of the maximum you want that bend at so you don't run out of additional "oomph" when you really need it.
I've been kicking around various regearing ideas for my 6 speed lately, and what you're saying is in a lot of ways what I've been thinking too.
Our 3.8s don't hit max power until a little shy of 5k RPMs. I wouldn't be comfortable cruising around at THAT high of an RPM, but, realistically, with the 3.21s and stock 32" BFG MTs I've got to be 4th gear on the highway to even be up and around 3000 RPMs.
As ridiculous as it is to be doing 70 or 80 in 4th gear, the Jeep drives on the highway much better like that and can even pass folks going uphill. So in the stock form and without the tow package or Rubicon gearing, we're geared pretty darn far from where we should be. The poor auto transmission folks have it even worse.
05-12-2011 | 12:20 PM
#12
JK Enthusiast
Joined: Oct 2010
Posts: 139
Likes: 0
From: Hillsborough,NJ
You're just shifting too soon.
Start shifting at 3500 and see how you feel. Let it spin out to 4000 or even 4800 or so when you're trying to pass somebody or merge on the highway. It won't hurt your engine and you'll have much more power than you did on the 3.21s, when it would've taken much longer to rev up to those RPMs in low gears and they would've be unaccessible in high gears.
Start shifting at 3500 and see how you feel. Let it spin out to 4000 or even 4800 or so when you're trying to pass somebody or merge on the highway. It won't hurt your engine and you'll have much more power than you did on the 3.21s, when it would've taken much longer to rev up to those RPMs in low gears and they would've be unaccessible in high gears.
05-12-2011 | 01:15 PM
#13
JK Enthusiast
Joined: Nov 2010
Posts: 123
Likes: 0
From: Durham, North Carolina
And in fairness to you, the owners manual includes "recommended" shift points (based on speed, not RPM) that would have you shift well before 3500 RPMs if you had any of the stock gear ratios. And no, I've never seen anything from Chrysler saying it's safe to rev to 4k+.
But still, a few points . . . .
(1) I probably should have been clearer. In my view, if the situation allows you to shift at 3k or even 2500, then by all means that is what should be done. If nothing else, you'll get better mileage. With 3.21s on 32s, I usually shift around 2500 to 3000. Part of my ulterior motive was to get the OP to realize that he can shift at 3500+ without his Jeep bursting into flames. I think after he does it a few times and gets comfortable, he'll settle back down to numbers that are somewhere between his former shift patterns and the 4500+ numbers.
(2) The owners manual acknowledges that it's speed-based shfit points "may not apply" if you're pulling a trailer etc. Similarly, the redline is at 6k RPMs, and the owners manual says you shouldn't run it into the red. The manual makes no mention of 4k or 5k RPMs. If 4k was where damage really occured, wouldn't that be where the redline is? Wouldn't the owners manual say to keep it below 4k? Since it doesn't, does Chrysler have any basis to scold me (or deny warranty work) for revving up to 4k+? If revving to 4k+ (but below 6k) caused damage, wouldn't Chrysler say so in the manual and warranty materials so as to insulate itself from warranty claims?
(3) Dyno tests show that max hp/torque isn't reached until near 5k RPM. Redline, again, isn't until beyond that at 6k. Why would the engine design be such that you can't even safely run through just 80% of the power band or 70% of the way to the redline? Heck, Ripp is supercharging these things and they appear to be holding up great. Can that really be less stressful on the engine than revving it to a point thousands of RPMs below the redline?
(4) My in-laws' Town & Country has the same engine (
), though it's obviously an auto and I understand it's mated to a better transmission than is attached to JK autos. It performs great on the highway. If I give it gas on the highway and it downshifts, take a guess around where it will upshift again . . . . Yup, mid-3000s even up by 3700 if I really get on it. It's of course much better insulated than the JK, so you hardly notice. Aside from demonstrating that 3500+ is okay, is it realistic to think the T&C is tuned to operate at the very limits of the 3.8's RPM capacities?(5) Try it for a few days. When the situation permits, rev up to 3500+ for some shifts. It'll feel nervewracking at first just because you're not used to it and the JK's design makes the engine seem very loud. But once you get used to it . . . it actually feels natural. There's nothing (other than my dipping fuel economy) that indicates to me this is inappropriate. I'm not burning oil, my engine isn't running hot, it shfits very smoothly at those RPMs, etc.
(6) One thing you'll notice is that it will greatly cut on lugging and make for much smoother acceleration. When I shift at 3k, my RPMs generally drop to close to 2k before I can complete the upshift. Since I'm only putting ~40 hp on the ground at that RPM, it tends to lug until it gets to the mid-2ks. By contrast, if you shift at 3500, you'll still be above 2500 when you complete your upshift with a lot more hp at the wheels. It overall seems to be a less laborious process to get up to speed. You could do this with "rev matching" too, but if you always shift at 3k and you have already rev matched to 2500, you won't be staying in that gear very long.
(7) I will say that the power of the JKs is impressive at 3500+ RPMs. Like I said earlier in the thread, if I downshift such that I get to 3500 RPMs on the highway, I can accelerate and pass people going uphill. Imagine that! Back in my always-shift-at-or-below-2500 days, that was a fantasy. I can't wait until I regear so that that happens in 5th or 6th gear on the highway, rather than 4th. I truly believe that had Chrysler just spent the extra few bucks per Jeep to have every manual come with 4.56s and every auto come with 4.88s, all the "underpowered" crap that gets endlessly yammered about would have be greatly reduced if not totally avoided.
In light of all that, could I be wrong? Yes. But I feel pretty good about it.
Sorry for the book . . . .
Last edited by MTH; 05-12-2011 at 01:43 PM.
05-12-2011 | 01:50 PM
#14
JK Freak
Joined: Sep 2010
Posts: 565
Likes: 0
From: nyc
didnt read the whole thread but. i have a 6speed with 33's and 4.88's and i LOVE it.
6th gear on the highway is lovely, rev's a bit high but that's fine for me. when i went wheeling, my 4lo performed beautifully with the lower gearing.
no regrets on my 4.88's, and i have smaller tires than yours. and if i ever have to tow something i know it can handle it.
6th gear on the highway is lovely, rev's a bit high but that's fine for me. when i went wheeling, my 4lo performed beautifully with the lower gearing.
no regrets on my 4.88's, and i have smaller tires than yours. and if i ever have to tow something i know it can handle it.
05-12-2011 | 02:04 PM
#15
JK Enthusiast
Joined: Oct 2010
Posts: 139
Likes: 0
From: Hillsborough,NJ
Hah! I used to worry about the same thing.
And in fairness to you, the owners manual includes "recommended" shift points (based on speed, not RPM) that would have you shift well before 3500 RPMs if you had any of the stock gear ratios. And no, I've never seen anything from Chrysler saying it's safe to rev to 4k+.
But still, a few points . . . .
(1) I probably should have been clearer. In my view, if the situation allows you to shift at 3k or even 2500, then by all means that is what should be done. If nothing else, you'll get better mileage. With 3.21s on 32s, I usually shift around 2500 to 3000. Part of my ulterior motive was to get the OP to realize that he can shift at 3500+ without his Jeep bursting into flames. I think after he does it a few times and gets comfortable, he'll settle back down to numbers that are somewhere between his former shift patterns and the 4500+ numbers.
(2) The owners manual acknowledges that it's speed-based shfit points "may not apply" if you're pulling a trailer etc. Similarly, the redline is at 6k RPMs, and the owners manual says you shouldn't run it into the red. The manual makes no mention of 4k or 5k RPMs. If 4k was where damage really occured, wouldn't that be where the redline is? Wouldn't the owners manual say to keep it below 4k? Since it doesn't, does Chrysler have any basis to scold me (or deny warranty work) for revving up to 4k+? If revving to 4k+ (but below 6k) caused damage, wouldn't Chrysler say so in the manual and warranty materials so as to insulate itself from warranty claims?
(3) Dyno tests show that max hp/torque isn't reached until near 5k RPM. Redline, again, isn't until beyond that at 6k. Why would the engine design be such that you can't even safely run through just 80% of the power band or 70% of the way to the redline? Heck, Ripp is supercharging these things and they appear to be holding up great. Can that really be less stressful on the engine than revving it to a point thousands of RPMs below the redline?
(4) My in-laws' Town & Country has the same engine (), though it's obviously an auto and I understand it's mated to a better transmission than is attached to JK autos. It performs great on the highway. If I give it gas on the highway and it downshifts, take a guess around where it will upshift again . . . . Yup, mid-3000s even up by 3700 if I really get on it. It's of course much better insulated than the JK, so you hardly notice. Aside from demonstrating that 3500+ is okay, is it realistic to think the T&C is tuned to operate at the very limits of the 3.8's RPM capacities?
(5) Try it for a few days. When the situation permits, rev up to 3500+ for some shifts. It'll feel nervewracking at first just because you're not used to it and the JK's design makes the engine seem very loud. But once you get used to it . . . it actually feels natural. There's nothing (other than my dipping fuel economy) that indicates to me this is inappropriate. I'm not burning oil, my engine isn't running hot, it shfits very smoothly at those RPMs, etc.
(6) One thing you'll notice is that it will greatly cut on lugging and make for much smoother acceleration. When I shift at 3k, my RPMs generally drop to close to 2k before I can complete the upshift. Since I'm only putting ~40 hp on the ground at that RPM, it tends to lug until it gets to the mid-2ks. By contrast, if you shift at 3500, you'll still be above 2500 when you complete your upshift with a lot more hp at the wheels. It overall seems to be a less laborious process to get up to speed. You could do this with "rev matching" too, but if you always shift at 3k and you have already rev matched to 2500, you won't be staying in that gear very long.
(7) I will say that the power of the JKs is impressive at 3500+ RPMs. Like I said earlier in the thread, if I downshift such that I get to 3500 RPMs on the highway, I can accelerate and pass people going uphill. Imagine that! Back in my always-shift-at-or-below-2500 days, that was a fantasy. I can't wait until I regear so that that happens in 5th or 6th gear on the highway, rather than 4th. I truly believe that had Chrysler just spent the extra few bucks per Jeep to have every manual come with 4.56s and every auto come with 4.88s, all the "underpowered" crap that gets endlessly yammered about would have be greatly reduced if not totally avoided.
In light of all that, could I be wrong? Yes. But I feel pretty good about it.
Sorry for the book . . . .
And in fairness to you, the owners manual includes "recommended" shift points (based on speed, not RPM) that would have you shift well before 3500 RPMs if you had any of the stock gear ratios. And no, I've never seen anything from Chrysler saying it's safe to rev to 4k+.
But still, a few points . . . .
(1) I probably should have been clearer. In my view, if the situation allows you to shift at 3k or even 2500, then by all means that is what should be done. If nothing else, you'll get better mileage. With 3.21s on 32s, I usually shift around 2500 to 3000. Part of my ulterior motive was to get the OP to realize that he can shift at 3500+ without his Jeep bursting into flames. I think after he does it a few times and gets comfortable, he'll settle back down to numbers that are somewhere between his former shift patterns and the 4500+ numbers.
(2) The owners manual acknowledges that it's speed-based shfit points "may not apply" if you're pulling a trailer etc. Similarly, the redline is at 6k RPMs, and the owners manual says you shouldn't run it into the red. The manual makes no mention of 4k or 5k RPMs. If 4k was where damage really occured, wouldn't that be where the redline is? Wouldn't the owners manual say to keep it below 4k? Since it doesn't, does Chrysler have any basis to scold me (or deny warranty work) for revving up to 4k+? If revving to 4k+ (but below 6k) caused damage, wouldn't Chrysler say so in the manual and warranty materials so as to insulate itself from warranty claims?
(3) Dyno tests show that max hp/torque isn't reached until near 5k RPM. Redline, again, isn't until beyond that at 6k. Why would the engine design be such that you can't even safely run through just 80% of the power band or 70% of the way to the redline? Heck, Ripp is supercharging these things and they appear to be holding up great. Can that really be less stressful on the engine than revving it to a point thousands of RPMs below the redline?
(4) My in-laws' Town & Country has the same engine (
), though it's obviously an auto and I understand it's mated to a better transmission than is attached to JK autos. It performs great on the highway. If I give it gas on the highway and it downshifts, take a guess around where it will upshift again . . . . Yup, mid-3000s even up by 3700 if I really get on it. It's of course much better insulated than the JK, so you hardly notice. Aside from demonstrating that 3500+ is okay, is it realistic to think the T&C is tuned to operate at the very limits of the 3.8's RPM capacities?(5) Try it for a few days. When the situation permits, rev up to 3500+ for some shifts. It'll feel nervewracking at first just because you're not used to it and the JK's design makes the engine seem very loud. But once you get used to it . . . it actually feels natural. There's nothing (other than my dipping fuel economy) that indicates to me this is inappropriate. I'm not burning oil, my engine isn't running hot, it shfits very smoothly at those RPMs, etc.
(6) One thing you'll notice is that it will greatly cut on lugging and make for much smoother acceleration. When I shift at 3k, my RPMs generally drop to close to 2k before I can complete the upshift. Since I'm only putting ~40 hp on the ground at that RPM, it tends to lug until it gets to the mid-2ks. By contrast, if you shift at 3500, you'll still be above 2500 when you complete your upshift with a lot more hp at the wheels. It overall seems to be a less laborious process to get up to speed. You could do this with "rev matching" too, but if you always shift at 3k and you have already rev matched to 2500, you won't be staying in that gear very long.
(7) I will say that the power of the JKs is impressive at 3500+ RPMs. Like I said earlier in the thread, if I downshift such that I get to 3500 RPMs on the highway, I can accelerate and pass people going uphill. Imagine that! Back in my always-shift-at-or-below-2500 days, that was a fantasy. I can't wait until I regear so that that happens in 5th or 6th gear on the highway, rather than 4th. I truly believe that had Chrysler just spent the extra few bucks per Jeep to have every manual come with 4.56s and every auto come with 4.88s, all the "underpowered" crap that gets endlessly yammered about would have be greatly reduced if not totally avoided.
In light of all that, could I be wrong? Yes. But I feel pretty good about it.
Sorry for the book . . . .
Didn't want to quote the entire post but it deserved it! Great points!
You have me looking at things in a little bit of a different light now. Besides with my TopSpin on my snorkel revving to 3,500k RPMs sounds bad ass 
05-12-2011 | 02:24 PM
#16
JK Enthusiast
Joined: Nov 2010
Posts: 123
Likes: 0
From: Durham, North Carolina
Drive around for a week or so letting the RPMs go, and, if you're so inclined, post back here or PM me with an update. I think it'll feel like you've got a whole new Jeep.
12-15-2011 | 05:43 PM
#18
JK Newbie
Joined: Oct 2010
Posts: 38
Likes: 0
From: Madisonville, TN
I have the same setup as you and have run both 4:56 and 4:88 gears. I am gettting 16.5 MPG average with winch and heavy bumbers with tire carrier. My experience is that 4:56 would have gotten you 1 to 1.5 MPG more than 4:88s on average. I went to 4:88 for the crawl and power abilities of the setup. I commute 100 miles daily to work and average 3000 RPM @ 75 MPH for highway speeds. I usually run 2800 RPM @ 70 MPH when not passing or running in fast traffic. Here is another thread for reference...
https://www.jk-forum.com/forums/show...90#post2708690
https://www.jk-forum.com/forums/show...90#post2708690