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Old 09-28-2017, 06:50 PM   #1
bcoke
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Advice Needed on Diff Re-Gear/Tranny Swap Dilemma

Hi! I'm new to the forum and have only had my blazer for 6 months, so you'll have to pardon the ignorance. I'm looking for some advice on a problem I'm having: how to get my 72 Blazer on the highway.

It seems like the last owner wanted it to be a rock crawler, but I'm looking for it to be more of a daily driver. Right now she screams at 55. My goal is to be able to eventually take her on a road trip and cruise the highway at 70.

Here are the current specs:
TPI 5.7L 350 engine
4 wheel drive
Turbo 350 automatic trans
NP205 Transfer case
4.83 gear ratios
1 ton axles
35in tires

I don't need the towing capacity of the 4.83s (at least not right now), but if possible I'd love to keep the torque while increasing the max speed. I know that that might be asking for too much, though, and just might not be practical.

I prefer a manual trans, so I'd like to eventually swap out the automatic for a manual, but I can hold off on that if there'd be a better way to get the truck up to highway cruising speeds right now.

It looks like re-gearing the diffs (have to do front and back since its 4WD) will be around a $2,000 job. That's creeping toward some tranny swap prices I've seen, and it seems like it'd be a shame to knock down a beast like this to something like a 3.55.

So my questions are...
1. Would a swap to a manual tranny like an NV3500 give me what I'm looking for in terms of top speed while keeping that low-end torque? Or would I have to regear the diffs even with the tranny swap?
2. Would adding an extra gear to the current trans setup give me the required speed at a price that's more reasonable than a tranny swap or a diff regear?

Any advice or ideas would be greatly appreciated. I'm open to everything. Thanks in advance for the help!
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Old 09-28-2017, 07:06 PM   #2
saxart
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Re: Advice Needed on Diff Re-Gear/Tranny Swap Dilemma

My vote says that you're (unfortunately) looking at a re-gear to something like 4.10 or 3.73's.
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Old 09-28-2017, 08:41 PM   #3
jocko
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Re: Advice Needed on Diff Re-Gear/Tranny Swap Dilemma

Yes, I'd say you have a dilemma. 4.83 sounds outrageous, but keep in mind that's with 35's, so it's not as drastic as it appears on the surface. Probably not a whole lot different than 4.10s with a stock tire dia. Also, keep in mind that with those tires and 4.83s, your speedo is totally out to lunch, so any speed vs rpm comparisons should be made with a GPS. Anyway, here's a chart to help you kinda see what benefits you'd get from a gear change alone, trans change alone, or both. 700R4 is shown, but NV3500 is .73 o/d rather than .70, so it gets you in the ball park at least. Note that TH350 with 3.73s (i.e. gear change only) nets you 1969 rpm at 55 mph whereas a 700R4 with 4.83s (i.e. trans change only) puts you at 1785 at 55. So, considering it's 4WD and the cost of both gear changes as you've highlighted - I'd say it's worth the relatively small "extra" cost to do the trans only - and you will maintain your rock crawler capability with the NV3500's low gear (might want to consider an NV4500 for strength) and still have the ability to cruise at decent highway speeds. Note that o/d + a swap to 4.10s (middle chart on right) puts you at too low an rpm at 55 mph to use 4th (in a 700R4) or 5th (in an NV3500) (so you'd cruise at 55 in 4th and 65 or above in 5th). I think this makes the case that you can achieve your goal with a trans swap only. Both might be overkill with negative results - I'd sink that potential extra gear cost into beefing up the new trans. Sorry, 700R4 numbers the only ones I had handy, would have to break out my excel geek bifocals to dig in and add a 5th gear to the chart...
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Old 09-28-2017, 09:17 PM   #4
57taskforce
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Re: Advice Needed on Diff Re-Gear/Tranny Swap Dilemma

I would not swap a 3500 into it. They are not very strong especially for something with big tires. A 4500 should be strong enough.
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Old 09-28-2017, 11:59 PM   #5
jocko
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Re: Advice Needed on Diff Re-Gear/Tranny Swap Dilemma

Ok, sorry, I couldn't help it.. Here's a better chart. Your current config outlined in red. Shows TH350, 2004R, 700R4, and NV3500/4500 at various diff ratios of interest. To me, your best choice is NV4500 with current diff gears - shown far right, bottom row. Good cruising gears while maintaining a DEEP granny gear, and decent 2nd gear for normal accels from a stop.
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Old 09-29-2017, 01:59 AM   #6
bcoke
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Re: Advice Needed on Diff Re-Gear/Tranny Swap Dilemma

@jocko you are THE MAN!! I was pumped about the first chart, but you've outdone yourself with the second one. I really appreciate you breaking that down for me - I'm a total n00b when it comes to gear ratios and corresponding RPM outputs etc. I would've never been able to do this on my own, so thank you so much.

And after looking at the numbers, I agree with you that I think I can cruise at the highway speeds I need (65mph at 2200RPM is perfect) by just changing the tranny. Unless it's a LOT cheaper to get the same results by gearing down to 3.54s alone (and let me know if you think it would be), I think I'll opt for the tranny because I could knock out the highway bird and the love of manual bird with one stone.

Which brings me to what I've been looking into for weeks: the 3500 vs the 4500. I actually hadn't even heard of the 2004r or the 7004r: those might be better. I'll need to research! But my knee jerk reaction says go with 5 gears over 4.

I originally decided on the 4500 because I heard the horror stories about people blowing 3500s up, but then I talked to a guy who rebuilds 3500s to be bulletproof and has put his through hell/swears by it, and said he would build one for me (obviously taken with a grain of salt because he has incentive to sell me on it).

In looking at all the forums and online, I've heard the 3500 shifts smoother and is a more normal drive (esp in terms of gear spacing). I've heard the 4500 shifts like a dump truck and can a bit annoying how it's spaced. I'm not going to be doing any rock crawling, maybe a little mudding and light towing, but mostly just daily driving. And yet I'd hate to go through all this to do a swap and wear out a 3500 after a few years.

Thoughts?
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Old 09-29-2017, 05:41 AM   #7
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Re: Advice Needed on Diff Re-Gear/Tranny Swap Dilemma

200-R4 cannot be used in a 4x4 configuration.
I doubt a NV3500 will tolerate the weight of a NP205 hanging off the rear of it's case.
A 700R4 could be made to work bt it's not a particularly heavy duty trans.
A NV4500 would be the best choice. It's a little more fabrication to install
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Old 09-29-2017, 08:12 AM   #8
Mike C
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Re: Advice Needed on Diff Re-Gear/Tranny Swap Dilemma

4.83 a weird gear #. 4..88? Either way as expensive as re-gearing both ends are, have you priced a built TH700R4 along with the adapter for the. 205? Building new crossmembers, front and rear driveshafts, etc.

I'd be looking for someone who wants to trade lowered geared axles for your 8 luggers.
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Old 09-29-2017, 01:21 PM   #9
bcoke
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Re: Advice Needed on Diff Re-Gear/Tranny Swap Dilemma

Ah I'm embarrassed...I just double checked and realized I misheard the mechanic when I was getting the diff serviced. It has 4.56s not 4.83s. Sorry! @jocko would there be any way you could change that bottom row of the chart from 4.83 to 4.56? if not, no worries.

Now I don't want to gear down even more than before...and if I'm going to go through the trouble to replace the tranny, I definitely want to throw in a manual, even though a 700r4 might be an easier swap.

So it's looking like a 4500 might be in my future. Now I'll have to begin the hunt for everything :/. I'm going to try to do it on a budget, so I guess the best place to head is the salvage yard. I know there's a bunch of threads on the 4500 but any off the top of the head thoughts?
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Old 09-29-2017, 01:52 PM   #10
jocko
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Re: Advice Needed on Diff Re-Gear/Tranny Swap Dilemma

Ha, no problem. Later tonight. I was kinda wondering about that too, thought it must be some rock crawler secret sauce or something I've never heard of....
Think trucks had 4.57, to be precise, 4.56 a car gear. Doesn't matter, RPMs will be the same. Keep in mind, if you can tell us exactly how many rpm you're seeing at exactly xx mph on a gps, we can confirm your gear ratio.
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Old 09-29-2017, 02:06 PM   #11
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Re: Advice Needed on Diff Re-Gear/Tranny Swap Dilemma

Since they are 4.56 and if you plan on keeping the 35s, I would be looking into an Overdrive. Don't bother with a 700. They just don't last in the long run. Step into a 4L80 or an NV4500. Any kind of a decent lift and an overdrive, it will be easier to move the engine forward to the next set of bolt holes first.
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Old 09-29-2017, 07:12 PM   #12
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Re: Advice Needed on Diff Re-Gear/Tranny Swap Dilemma

here u go
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