Why an NV4500?
By David Gray for Off-Road.com's Reviewer's Notebook
In just a few words: Bullet-proof Rockcrawling. YJ-1 is being purpose built as tough, but streetable, rockcrawling machine.
When your out on the rocks slower is better. Slow gives you control over the rocks and lets you idle over things easily. To go slow you need more than just low engine RPM, you need the torque (power) to get you over the obstacles as well. Torque multiplication in the gear box is what lets you go slow enough to maintain control while still having the power to get where you need to go, like the old-timers say "It's all in the gears".
The overall measure of how slow you can go is "crawl ratio". To determine your crawl ratio you multiply your transmissions first gear ratio x the transfer case low range x R&P ratio. For project YJ1 that turned out to be:
3.73:1 x 2.72:1 x 4.10:1 for a total crawl ratio of 41.6:1.
The "holy grail" of crawl ratios is around 100:1 and we were a long way away from that & the 35" tires that YJ-1 wears makes the actual trail crawl ratio even worse than the 41.6:1 would indicate. Now we could change the R&P out (again) at both ends (for between $1,500 & $2,500). If I substituted 4.88:1 then the total crawl ration would be 49:1, but highway cruising would suffer tremendously. So for YJ-1 I decided on 2 major modifications: New transmission with a very low first gear and overdrive, and a change in the transfer case (see Atlas review).
So at this point we had decided we needed a bulletproof 5 speed transmission with a low 1st gear and an overdrive 5th gear. The NV4500 is just such as beast, it has quickly become one of the most popular manual transmission swaps for off-roaders and may be one of the best overall off-road transmissions. It has a 5.61:1 first gear (a 6.31 is available but rare) and a 0.73:1 overdrive 5th. In contrast the AX15 has a 3.73:1 first gear and a 0.79:1 overdrive 5th.
| The NV4500, also known as the GM MT-8, has been available in heavy duty GM & Dodge Pickups since 1992. Early GM units had a 6.34:1 unsynchronized first & reverse gear, all Dodge units and later GM units have a 5.61:1 synchronized first & reverse gear and 0.73:1 OD fifth gear. |
With the NV4500 installed our crawl ration would improve from 41.6:1 to 62.6:1 and that would actually result in much better performance on the trail! In addition the higher OD ratio would allow lower engine speeds on the highway and improved gas mileage (or the use of lower R&P gears without ruining highway performance).
The NV4500 is found in both late model GM & Dodge Heavy Duty trucks (GM versions of the transmission require a new input shaft). It is not strictly a bolt in installation, but a few vendors have the parts to make it much simpler. Advance Adapters supplied the transmission and all of the parts to install it in our Jeep.
If you buy the transmission & adapters from Advance Adapters you are getting a brand new factory transmission and they do virtually all of the tough work for you: all input shaft modifications are done and the necessary adapters are assembled into place. This lowers the difficulty rating for the driveway mechanic a notch or two.
INSTALLATION
REMOVING OLD COMPONENTS
Special tools needed / difficulty rating
This is not rocket science but you do need to be comfortable doing major work on your vehicle to attempt this. You will also probably need a few tools that you may not have laying around your garage.
- Transmission jack*
- Clutch bearing puller*
- Clutch alignment tool
- 4-6 Tall jack stands
- Torx sockets
- Friend
Fortunately these tools are commonly available at most tool rental places. These tools include a transmission jack and a clutch throwout bearing puller. The transmission jack is made to both hold the transmission securely and position the transmission so the input shaft lines up well. Since transmissions are heavy and hard to handle I also strongly suggest that you have a friend to help you with this project. You will need at least 4 jack stands (the more the better). A set of female torx sockets will be needed to remove one of the bellhousing bolts and lastly air tools are recommended but not necessary.
You should also replace the clutch and the clutch slave cylinder while the transmission is out.
Vehicle Preparation
It is suggested that you use a good degreaser and clean the area around the bellhousing, transmission & transfer case before you begin.
Inside the Jeep remove the shift knobs, the shift boots, the cover assembly and finally the shifter levers.
Important: Chock the rear wheels securely, disconnect the battery.
Jack the vehicle up as high as possible (quite a bit of clearance is necessary to move the NV4500 under the vehicle when it is mounted on the transmission jack) and support it with jack stands on the frame just ahead of the skid plate. Remove the front tires (for maneuvering room).
Alternatively, If you do not have tall jack stands it may be possible to drive the front up onto a set of ramps.
Unbolt the exhaust system from the exhaust manifold in preparation for it's removal. We did not do this and it was in the way the whole time.
Removing Skid Plate
Support the rear of the engine with a jack stand underneath the oil pan. Place a large block of wood between the two to keep the pan from denting. Place a jack stand or jack centered underneath the skid plate (it is quite heavy and you do not want it landing on you - if it has mud encrusted on it it will be even heavier). Unbolt the bolts for the transmission mount (located in a cutout near the center of the skid plate) and then undo the six bolts holding the skid plate to the frame. We used a floor jack to support the skid plate and then to lower it carefully while we checked and made sure there were no hang-ups.
After the skid plate is off go ahead and complete the removal of the exhaust system.
Removing Driveshafts
Unbolt the driveshafts from the transfer case outputs. Be careful to save any or all of the various nuts, bolts and straps in a zip lock bag. Use tape or a rubber band to hold the end caps on the U-joints and then tie the shafts up and out of the way. We bungy corded them to the frame.
Removing Transfer Case
Well, our shop manual recommended removing the transmission and transfer case as one unit. But since we were doing this in our driveway we decided to remove them separately so it would be easier to manuever and handle the heavy components.
Remove the speedometer cable, the vacuum lines, disengage the shifter linkage and anything else that may be attached to your transfer case.
Support the transfer case with the transmission jack and unbolt the six nuts attaching the transfer case to the transmission.
Carefully maneuver the transfer case back until it disengages from the transmission output shaft. Lower and remove the transfer case. Do not tilt the transfer case or fluid may leak out the input. Cover the input to prevent contamination and set the transfer case aside.
Removing Existing AX-15 Transmission & Bellhousing
 At this point it is advantageous to tilt the engine and transmission assembly so the transmission points down a bit by repositioning the support at the engine oil pan. This gives us more room to reach the bellhousing bolts.
Remove the hydraulic lines going to the clutch cylinder. The fluid will drain out of these lines so be prepared by having something to catch the fluid in (it is brake fluid - so don't get it on painted surfaces). Cover the tube ends so they will not be contaminated and tie them up out of the way. Remove the reverse gear switch connection. Unbolt and remove the starter (if you did not remember to disconnect the battery this could cause a fireworks show and severe burns).
Support the transmission with the transmission jack - make sure to use the safety chain.
Unbolt all of the bellhousing bolts. On this model two of the top bolts required torx sockets.
Double check that all of the connections between the Jeep and the transmission have been removed - then slide the transmission back and down slightly until the input shaft disengages from the clutch assembly. Carefully lower the transmission and slide it out from under the vehicle on the jack.
|
|
Side by side it is easy to see how much beefier the NV4500 is in comparison to the AX15 it is replacing. This is a heavy duty 1 ton transmission and in a lightweight jeep it is practically bulletproof. |
INSTALLING NEW COMPONENTS
- Clutch Preparation
Remove old clutch components: Note: Most clutches contain asbestos and extreme caution should be used not to breath the dust that may be present. Place the clutch alignment tool through the splined hole in the clutch - this will prevent the clutch plate from falling out as you remove the cover. Support the clutch cover/pressure plate and remove the bolts securing the cover. Remove the cover and then remove the clutch plate. Now, using the clutch pilot bearing puller (one type shown) remove the pilot bearing. This was the difficult part as the bearing came apart as we attempted to remove it. The puller did it's job though and soon all the parts were out. Inspect the flywheel surface, if it is damaged, grooved or excessively worn the flywheel should be either resurfaced or replaced as necessary. Again, if there is any question about it, do it now while it is easy and save yourself a big job in the future. Using an aerosol brake cleaner clean the surface of the flywheel thoroughly and make sure to keep it free from any contamination.
Install new Advance Adapter pilot bearing:Advance Adapters provides a new solid brass pilot bearing for use with the NV4500 transmission and thus a Jeep pilot bearing is not re-installed. The bearing supplied to us was slightly over sized and we could not fit it in as supplied. Advance Adapters assures us that this was an anomaly. The solution was fairly simple, we fitted the bearing over a rubber bit that is normally used to hold sanding sleeves. We installed this in our drill press and using some fine sandpaper we turned a small amount of material from the outer bearing surface. The bearing is then pressed into position, this is a fairly tight press fit and the bearing was encouraged into place with a bit of 2in left over roll bar tubing and a heavy mallet.
Install new clutch components: Rinse the flywheel surface off again with the brake cleaner. Using the alignment tool position the clutch plate carefully, observing the markings indicating which side goes next to the flywheel. Carefully position the pressure plate and cover and start the bolts securing it. Tighten the bolts to the specified torque while making sure that the clutch plate stays aligned, if any misalignment occurs it will prevent you from being able to install the transmission and it is likely the entire cover assembly will have to be removed again.
 Attaching Bellhousing to NV4500
The stock bellhousing is reused from from the AX15 transmission which was removed from the vehicle. Advance Adapters also has a GM based bellhousing available that utilizes an external slave cylinder. When used this saves about 1" off the total driveline length. We decided that the extra expense for this modification was better invested in the Atlas Transfer Case which shortens the driveline more than 2". If you are sticking with the NP231 transfer case (with the tailshaft eliminator kit) or have a low pinion rear end (such as a Ford 9") then it might be wise to consider the shorter bellhousing as rear driveshaft length will be at a premium.
 Due to the size and shape of the adapter plate for the NV4500 the hole that the hydraulic clutch lines pass through needs to be enlarged. We used a air powered die grinder with a 1/4" carbide cutter head which made very quick work of the aluminum with minimal clogging. It could also be done with a round file or even a metal cutting jig saw.
After this is done the Advance Adapter adapter plate is bolted to the bellhousing. This assembly is then bolted to the NV4500 transmission.
 The new slave cylinder is carefull slid over the transmission input shaft and the locating pin (supplied with the adapter) is slid into place. Be careful when handling the slave cylinder, if the restraining straps pop thee cylinder will expand, never to go back together again, and you will be making another trip to the Jeep parts store for another new slave cylinder before you can finish the job. These straps are designed to pop off automatically after the clutch is installed, the first time you press on the pedal. The input lines are slid through the newly enlarged hole in the bell housing. It is a good idea to put a thick rubber hose or other material between the metal hydraulc lines and the rough edges of the hole to prevent vibration & rubbing from destroying the lines.
 Installing NV4500 in Vehicle
This is the tricky part. Make sure you have a friend to help you and the transmission is firmly secured to the transmission jack.
Roll the transmission & jack under the vehicle. Since the NV4500 is significantly taller than the old AX15 you need to make sure the vehicle has been raised high enough for the transmission to clear (if you jack up the vehicle at this point make sure the engine is properly supported).
-
Raise the transmission carefully on the jack making sure that everything clears. Make sure the clutch inspection shield is in place and align the transmission input shaft carefully with the splines in the clutch and slowly move the transmission forward. If you are extremely lucky the splines will be aligned and the shaft will slide easily into the pilot bearing - ya right... If you are like the rest of us a considerable amount of jockeying into position will have to be done before the transmission can slide into place. Even though the transmission jack has adjustments in 3 axis for this, it can sometimes be a very frustrating experience. In our case after fighting it for a couple of hours we gave up for the night. Strangely enough, the next morning it slid on in less than 10 minutes - go figure. With the transmission in place bolt the bellhousing to the engine. Remember those 2 #%@)&@ torx bolts up top? They have to go back in too. They are always the toughest. After the bellhousing is snugged up against the engine replace the starter motor.
Attach the clutch slave cylinder hydraulic lines, fill the system with hydraulic fluid and bleed the system. If you have problems with the new clutch slave cylinder it is better to know about them now and not discover them after everything else is bolted into place.
The reverse indicator switch requires a new GM connector to match the switch on the transmission. The old connector is removed and the new connector installed.
You may reattach the exhaust system at this time. Make sure that any emission hoses that have been disconnected are reconnected - especially if you have the carburated motor with the Pulse Air emission control system.
Fill the transmission with it's special gear lube (expensive stuff, ours was supplied by Advance Adapters). Attach the shift lever and make sure that the transmission shifts easily - again, it is better to discover any problems now instead of later.
  Installing Transfer Case & Linkage
Once the transmission is in place and bolted in correctly the toughest part of the job is over.
If you are staying with the stock transfer case the adapter for the NP231 shifter must be bolted onto the transfer case adapter on the NV4500 assembly. In our case the floor pan had to be trimmed somewhat before the transfer case shift lever would fit.
 The AA adapter is supposed to use the stock shift linkage, but in our case the linkage was not long enough to permit the shifting of the transfer case through all positions. The solution was very simple, we took a piece of 1/4" steel rod, threaded the ends, screwed on 2 spherical rod ends and shaped a linkage that would fit.
- Skid Plate Modifications
 If our skid plate had been dropped, for example when a lift kit was installed then it is likely no modifications would be necessary. In our application we wanted to keep the skid plate in it original position so using a sawzall and a cutting wheel in our die grinder we had to cut a channel into the front and back lip and cut a section out to relieve the crossmember. Since this weakens the cross member we uses some 1/4"x2" steel stock and welded some additional support to the skid plate. Since this skid plate is battle scarred from its encounters with Sierra granite, at some point we will probably reinforce the skid plate further.  We then test fitted the skid plate into position and marked where the holes would need to be drilled for the new support provided by Advance Adapters. The bolts heads that protruded through the holes would likely soon be savaged by contact with rocks so two small squares of 1/4" plate were welded just in front of the holes to help protect them. The edges of these plates were beveled and they were welded on 3 sides to prevent them from getting hung up easily.
 Driveline Modifications
The NV4500 assembly is longer that the stock AX15 and this necessitates driveline modifications. If using the stock NP231 transfer case then a tailshaft eliminator is mandatory or the rear drive shaft will be too short to be practical. Our project Jeep had previously had the MIT tailshaft eliminator kit and a CV driveshaft installed so the only driveline modification necessary was to have the front and rear driveshafts lengthened and shortened respectively.  With the reverse cut Dana 60 in our Jeep and the 4" lift we had reached the shortest possible length for our rear driveshaft. A few weeks later as we were installing the AA Atlas transfer case the shafts were restored to almost their original lengths. The Atlas is a better overall solution to driveshaft length problems than the tailcone eliminator in this application.
 Installation of skid plate & drivelines.
TEST-DRIVE
Well we may have cheated a little bit by running the transmission before the transfer case was installed, but this was the first time we would drive with it in. We double checked to make sure that we had not forgotten anything and with the transmission in 5th gear and the emergency brake on (so if the clutch did not release the engine would die and we would not run over anything). We depressed the clutch pedal and started the engine. We worked the clutch a few times and once we were satisfied that everything was working right we put in reverse and backed on out of the driveway. The first impression is that this transmission is much louder than the AX15 it replaced - the sound has been characterized as being like a "box of rocks". It shifts smoothly through every gear and our trip around the block is a success.
ROAD TEST
On this project it was important that while we improved trail performance that we maintain it's highway manners. Many granny 4 speeds are available, some with gearing as low as 7+:1, but without an overdrive gear highway performance can be a distant memory unless compromises are made in axle gearing. With this version of the NV4500 and the 4.10:1 axle gearing all 5 gears are usable on the street, we get the great low first gear for the trail and the .73:1 OD fifth gear allows us to cruise 70mph at around 1900rpm. In fact the best gas mileage project YJ has ever seen (even when stock) was on several hundred miles of flat 55mph road after th NV4500 was installed - 22mpg!
TRAIL TEST
The street performance was nice, but we installed this beast for thee trail. Our trail test for this installation was done at the 30th anniversary Jeepers Jamboree held on the Rubicon Trail. This is where low gears shine. Rock Crawling is transformed from a series of barely controlled jumps and crashes, burning out your clutch to make it over boulders into an art of finely controlled driving. The synchronized first gear and reverse also make shifting easy, even when your posterior has clenched into the absolute tightest vacuum hold on your seat ever. The NV4500 is also practically bulletproof in this application. It is very unlikely we will ever be limping home from the trail having busted any part of this transmission!
SUMMARY
The NV4500 was installed in project YJ to provide vastly improved rock crawling ability, increased strength and reliability and still maintain highway performance. With the NV4500 and all necessary hardware supplied by Advance Adapters the installation procedure was transformed into a relatively painless affair that anybody comfortable with doing major work on their jeep should be able to accomplish in the driveway with a few special tools and a friend. The NV4500 met all of our expectations of it, and it's test on the Rubicon proved why it is probably the best choice in modern, off-road manual transmissions. Some may think of this as an expensive modification, but it is not so much more than many other modifications that even mildly serious off-roaders make on their vehicles. It is a particularly wise thing to consider if you are contemplatng either major repairs on or replacement of your existing transmission.
| Article Statistics: |
| Viewed:7316 |
| Current Reviews: 0 |
|
)
)
)
)
)
)
)
)
)
)
)
)
)
)
)
)
