Regear after hemi swap? - JKowners.com : Jeep Wrangler JK Forum
 
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post #1 of 16 Old 04-01-2009, 09:48 AM Thread Starter
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Regear after hemi swap?

I'm wondering if it's necessary to regear after the hemi swap. I'm currently running the stock 4:10 ratio with 35's. I assume that the 5 speed tranny used in the swap has a different final drive ratio.

Would the 5.7 hemi, 4:10's and 35's work good with the swap?
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post #2 of 16 Old 04-01-2009, 10:07 AM
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You have plenty of power for 4.10s, but you may want to gear down to crawl better. If you dont do much rockcrawling I would leave it 4.10.

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post #3 of 16 Old 04-01-2009, 10:46 AM
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Really depends on where the OD puts the engine RPM at highway speeds.

Sure have the power to move the Jeep, but it may still want to gear hunt on the highway. The other thing to conside is with the 5.7L Hemi you retain the varible displacement feature. (shuts of cyl on the highway for fuel mileage). The wrong gears (too high) might keep the engine from using this feature and your mileage will suck.

I would ask the question on the AEV forum since they have more Hemi Jeeps than anyone.

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post #4 of 16 Old 04-01-2009, 10:59 AM Thread Starter
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Thanks for the replies so far. I posted on the AEV forum too.

I'm curious to see what the crusing rpms would be at around 65-70mph.
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post #5 of 16 Old 04-01-2009, 11:45 AM
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Quote:
Originally Posted by thebigtman View Post
I'm wondering if it's necessary to regear after the hemi swap. I'm currently running the stock 4:10 ratio with 35's. I assume that the 5 speed tranny used in the swap has a different final drive ratio.

Would the 5.7 Hemi, 4:10's and 35's work good with the swap?
Do you have an auto now? If you do and it's adequate for you the answer is no you do not have to re-gear.

The 545RFE's 1st gear is 3.0:1 compared to the 42RLE's 2.84:1 . If you have a manual and are converting to the automatic, you're probably going to really notice it in 1st. The Stick has a 4.46:1 1st. RFE's 5th is 0.67:1 compared to stock RFE's 0.69:1 and the sticks 0.84.

On the hi-way, it's a Hemi mated to the 545RFE. You could have 3.21's in there and still have a better experience.

I have 33's with 4.1 and it's more than adequate. 65 MPH is about 1800 RPM's in 5th. (Kind of hard to get it into 5th at 65) The MDS does not work in MDS engines.

The easiest way to know if you need to re-gear is to simply just drive it with th 4.1's
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post #6 of 16 Old 04-01-2009, 12:02 PM Thread Starter
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Thanks for the info.

I forgot to mention that I'm running an auto and plan to stick with an auto.

I was wondering about the gear swap b/c I didn't want to regear to 5:38's and then have to go back to something lesser once I do the swap. I figured that the 4:1's would be good but I wanted to get some confirmation first.
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post #7 of 16 Old 04-01-2009, 12:10 PM
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I'm @ 17-1800 RPMs @ 55 mph w/ 37 inch tires and 4.88 gears. Seems ok, but haven't really had it on the highway long enough to see what kind of rpms it has @ 65 or 70. I'd think it still would be fine with 4.56s & the 37s so you should be OK w/ 4.10s! FYI, I drive my jeep to and from the trails.

I wouldn't go with 5.38s though.


The 17-1800 rpms, is when the torque converter is locked up and in OD...

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Last edited by Prope; 04-01-2009 at 01:30 PM. Reason: edit
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post #8 of 16 Old 04-01-2009, 12:11 PM Thread Starter
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Thanks Prope!
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post #9 of 16 Old 04-01-2009, 12:15 PM
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Quote:
Originally Posted by edivebuddy View Post
RFE's 5th is 0.67:1 compared to stock RFE's 0.69:1 .

On the hi-way, it's a Hemi mated to the 545RFE. You could have 3.21's in there and still have a better experience.

Which means that the Hemi will be running at a stupid low rpm on the highway. Too low probably.

The engine may have more power through out the RPM range, but it still needs to opperate in the correct range.

My 3.8L/4:10/35 combo put the motor at about 1700 rpm at 65. Which was simply too low for it.

The real question is does the Hemi have enough power to run at 1700rpm on the highway AND use its variable displacement feature? It may also want to be at a higher RPM range. (2000-2250, I dunno?) Which would need a regear.


Last thing you want to to be "lugging" your Hemi down the highway or have it shifting too much because the RPM is too low. Much more to the equation than just HP/TQ

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post #10 of 16 Old 04-01-2009, 12:56 PM
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Quote:
Originally Posted by Prope View Post
I'm @ 17-1800 RPMs @ 55 mph w/ 37 inch tires and 4.88 gears. Seems ok, but haven't really had it on the highway long enough to see what kind of rpms it has @ 65 or 70. I'd think it still would be fine with 4.56s & the 37s so you should be OK w/ 4.10s!

I wouldn't go with 5.38s though.
One of the guys here has a Hemi in his 4door, with 4.88's. He regrets doing the 4.88's, and is changing to 5.38's. That decision was made when he had 37's, and now with 40's it's even more important to him.

THAT SAID, he doesn't drive on the highway a ton, mostly in-town driving. If he goes anywhere very far, it's on a trailer....so the deep gears may not suit YOUR style of driving as well as they do his.


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post #11 of 16 Old 04-01-2009, 01:22 PM Thread Starter
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Carl, thanks for your input. All of this is very helpful to my decision.
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post #12 of 16 Old 04-01-2009, 01:26 PM
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Quote:
Originally Posted by TeraFlex View Post
One of the guys here has a Hemi in his 4door, with 4.88's. He regrets doing the 4.88's, and is changing to 5.38's. That decision was made when he had 37's, and now with 40's it's even more important to him.

THAT SAID, he doesn't drive on the highway a ton, mostly in-town driving. If he goes anywhere very far, it's on a trailer....so the deep gears may not suit YOUR style of driving as well as they do his.
True, true, I'll add a disclaimer to my post... I drive my jeep to & from the trail & to work occasionally!

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post #13 of 16 Old 04-01-2009, 01:28 PM Thread Starter
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I do a lot of highway driving and tow a small enclosed utility trailer.

I travel through the Western states a lot in the winter.
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post #14 of 16 Old 04-04-2009, 10:46 AM
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I have 5.38's, 37's, 6-speed and it is a great combination. I use the 6 speed but not all that often. I have ordered my HEMI and is going to be installed within the next 3 weeks or so... I will be converting to an 545 auto and will let you know how this combination works when it is installed.

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post #15 of 16 Old 04-04-2009, 07:40 PM Thread Starter
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Sounds like a good combo Woody. Definitely let us know how it works out. I'm checking the prices and availability of some commander engines right now.
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post #16 of 16 Old 04-04-2009, 09:59 PM
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I think the bigger issue may not be gear ratio, but instead axle strength. Wtih the added HP I would think most people would be suffering various axle failure. Plenty of people are breaking axles with a 3.8.
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