m998 and the 700R4

[Donnie]

Hi

I`m looking to get a little extra oomph out of my stock 85 m998.

The 700R4 swap seems to do that but I just wanted some opinions.

My H1 mechanic will be doing the work on the m998 - so I have to source the parts and want to see if you guys have thoughts on what parts I should be collecting together as well as if there is anything else that should get worked on at the same time?

Thanks

 

[Iceman3005]

You will have to run a 12 volt wire for lockup, not sure if the transfer case will bolt up. Also will have to fabricate or find a 700r4 tv cable bracket. Other than that should bolt right up, all the linkages should be the same.

 

[NDT]

How about the 4L80E option and keep it all stock?

 

[Donnie]

How about the 4L80E option and keep it all stock?

Would this still give the performance boost? I`m just looking to make highway travel more manageable.

Thanks

 

[Donnie]

You will have to run a 12 volt wire for lockup, not sure if the transfer case will bolt up. Also will have to fabricate or find a 700r4 tv cable bracket. Other than that should bolt right up, all the linkages should be the same.

Thanks Iceman. I`ll look into those.

You did this correct? Do you get the extra top speed I have heard about? Would anything else play a part in doing that? Thanks again,

 

[Donnie]

Tv bracket...



This would do the trick?

Thanks

 

[NDT]

Would this still give the performance boost? I`m just looking to make highway travel more manageable.

Thanks

Reduced engine RPMs at highway speed. Still no hot rod.

 

[Iceman3005]

installing a 4l80e is an expensive way to go, you will have $1000 dollars just in the stand alone computer, not to mention the cost of the trans, and other modifications. The 700r4 is the cheaper way to go, plus no electronic computer to mess with. With a few simple internal upgrades the 700r4 will be just as stout as the 4l80e. I have done quite a few of the 700r4 swaps, really simple and the results are great. The 700r4 has a very low fist gear, making an underpowered vehicle seem like it has more power from a stop, plus the overdrive will drop rpm's about 35%. So if your at 2800 rpm's at 55mph with the turbo 400 then rpm's will be 1850 with the 700r4 in overdrive! You should also see a mpg increase, mine went from 15 mpg to 18-20mpg.

 

[Donnie]

Thanks. So what would the new comfortable top speed be?

 

[Iceman3005]

Tv bracket...



This would do the trick?

Thanks

No, this is the bracket you need.

 

[Donnie]

You wouldn't happen to have a part number for that?

Iceman the man.

 

[Iceman3005]

the 6.2 diesel tv cable bracket part number is 14038643, the rod is 14038642. It can be found here, http://parts.nalleygmc.com/showAsse...l=14483&ukey_driveline=0&ukey_trimlevel=15548, it is not available anymore so most likely will be a NOS part.

 

[Donnie]

Most helpful!

 

[Blackmagic94]

The 4L80E is based on the TH400 trans while the 700R4 is based on the TH350 and are a weaker deesign.


Another option may be to add a Gear Vendor overdrive unit to the tailshaft of the TH400, it gives overdrive for each gear so a 3 speed Th400 becomes a 6 speed.



What kind of MPG do people get with the 3 speed vs the 4 speed OD on M998s?

 

[Retiredwarhorses]

Please keep in mind these trucks were made for convoy speeds and off-road capabilities, not to go fast.
You are, after doing any trans upgrade limited in speed by the 2:56 diffs and the geared hubs that are geared down 2:1.
I won't do any other upgrade other then the 4L80E...the rest is Jury rig and not correct for the truck...my opinion.
But yes, expensive, the single largest cost is a new body harness...yes, you can hack and wack, but I don't do that.
TCM's are not 1k, they can be had for 250.00 for a take out too 500.00 for NOS.
there are far to many things to list for this conversion to make it go right...Crank pos sensor, throttle pos sensor etc....
PM me if you are serious...thanks

 

[Warthog]

This HMMWV thread was hijacked by a user wanting to install a 700R4 in a CUCV. Moved unrelated posts to the CUCV forum.

For the HMMWV I also would only install the 4L80E

 

[TedG]

I have had a 700R4 behind a 6.2 in a hmmwv slantback. It would go fast but it cannot handle the weight of the truck especially off roading. The 700R4 will fail sooner or later when used in this application. I done 4l80e conversions and Steve is correct, lots of work for just some speed gain.

 

[Blackmagic94]

They can be built up to handle more power with typical hot rod techinques. **** even the TH400 has a weak link of the stock Sprag, the upgraded Sprag makes the power cabablity nearly double with proper clutches.