D110/LS/6L80 Owners: Ratio Question!

w2hx

Member
Callsign: W2HX
Hi friends. I recently swapped a 2.5L NA power plant with a 5.3L Vortec/6L80 in my 1986 D110 Tithonus. We replaced the tcase gearing from 1.667 to 1.222. At the time, we thought the diffs were 3.54. Now, at 70 MPH, the truck runs at 2500 RPM (31.7" tires). I am now thinking I may actually have 4.11 diffs (maybe military used them?)

Can other owners of an LS/6L80 let me know what kind of RPMs they get at 70 MPH? Just for comparison? the 2500 seems a lot.

To reduce the rpms, I could swap the diffs to 3.54 and get RPMs to 2152. Or, I could swap the tcase gears from 1.222 to 1.003 and get 2051. Or heck, i could do both and run 70 MPH at 1767. Not sure if the 315 horses would like that or not but this set up has a lot of power to spare.

But my question is mostly about what other LS swapped people are getting for RPMs at 70 MPH.

Thank you!
Eugene
 

Z.G

Well-known member
1. Do you know your tach is correct?
2. I've been selling a bunch of 1.0 and 1.1 LT230s to LS swaps, the engines have plenty of power for it
 

w2hx

Member
Callsign: W2HX
1. Do you know your tach is correct?
2. I've been selling a bunch of 1.0 and 1.1 LT230s to LS swaps, the engines have plenty of power for it
Z.G. Thanks for the info. Do you know what diffs the buyers of your 1.0s are using? Very curious about that. Also which LS's? I know some people are getting 400+HP from some LSs. I've probably got 300 or so.

Also, do you still sell 1.0s? And are you selling just the gears or complete TC?
As for my tach ,I think it is. It is an electronic one coming from some pulse somewhere. Also, the revs do sound high so while it may not be totally accurate it is directionally accurate if you get my drift.
 
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w2hx

Member
Callsign: W2HX
Hey Pmatusov, can you share a link for that? Would be good to educate myself.
Also, pmatusov, with 6 gears I'm really hoping for the feel of an overdrive gear. I dont know if I need optimal torque while driving for a few hundred miles on flat highways. If i need to pass, I'm happy to kick down to 5th gear and pass. but it sure would be nice to lower the RPMs while cruising. thanks
 

Z.G

Well-known member
Not sure as far as engines go, I know they've used the LS3/LT1 etc. Diffs have mostly been geared stock at 3.54, but I also have a shop doing it with 4.12s. Not sure what size tires they're running though
 

abraded

Member
It's not complicated to spin an axle and count revolutions to determine what diff gears you have.

Otherwise, there are any number of online calculators to figure out rpm for speed--like this:

Just look up the ratios for the 6l80e and plug them in. If the rpms you are seeing at a given speed don't match up there are only a couple of explanations (tach not programmed/measuring correctly, trans tuning wrong[not shifting into 6th, not locking up, etc].

The reason you would change t-case ratios is if you didn't like the performance for a particular use case--lower rpm for highway cruising, staying in first longer pulling away from a stop, matching powerband to particular long uphill usually taken at x speed, etc.
 

pmatusov

Technical Excellence Contributor
Hey Pmatusov, can you share a link for that? Would be good to educate myself.
Google for 5.3 Vortec torque curve. This is one of the plots I saw - others are similar.
Most of them seem to have the peak torque around 4000-4100 rpm, which means if you run at 2500 on the highway, you have plenty of torque and power to pass lesser vehicles.

1626890257821.png


 

Napalm00

Technical Excellence Contributor
as stated above you're going to want to verify that your tach actually is accurate. The tach no matter what the signal is needs to be adjusted for the engine.

A cheap laser tachometer from Amazon with a reflector on the crankshaft will tell you your exact RPM. You can then adjust the tachometer with the dip switches and tiny little potentiometer in the side of the unit to make it accurate.

If you're off by let's say one or two hundred RPM at the bottom of the range you could be off by five or 600 at the top of the range.

You're going to need to verify what you have before worrying about if your correct or not.

Verify what diffs you have, takes about 10 min
Verify that your tachometer is correct, takes about 30 minutes.

With these two pieces of information then you'll be able to decide whether or not you need to move down in ratio on your transfer case.

Locally we have two guys with LS is here in the Bay area in the club. Both are running 1.001 transfer cases. One of them has a 5.3 like you The other one has a 6.2 l. Both are running taller tires than you at 33in with stock 3.54 differentials.

It would be really really weird to have 4.1 diffs in a truck that's running 31-in tires. Especially in an ex-military truck that had a 1.6 transfer case in it originally.
 

w2hx

Member
Callsign: W2HX
It's not complicated to spin an axle and count revolutions to determine what diff gears you have.

Otherwise, there are any number of online calculators to figure out rpm for speed--like this:

Just look up the ratios for the 6l80e and plug them in. If the rpms you are seeing at a given speed don't match up there are only a couple of explanations (tach not programmed/measuring correctly, trans tuning wrong[not shifting into 6th, not locking up, etc].

The reason you would change t-case ratios is if you didn't like the performance for a particular use case--lower rpm for highway cruising, staying in first longer pulling away from a stop, matching powerband to particular long uphill usually taken at x speed, etc.
abraded, this is precisely what I did. this is how I arrived at the suspicion that my diffs were not what I expected. 31.7" tires, 0.67 6th gear for the 6L80, 1.222 tcase should not produce 2500 rpm at 70 mph for 3.54 diffs, according to ashcroft's calculator. I know I have 31.7" tires, I know I have a 6L80, I know I have a 1.222 (because we bought it from ashcroft and installed it).

Yes, I will certainly do the wheel rotation/shaft check to be sure. But I am mostly interested in what other LS engine folks have done. If others tell me 2500 rpm @70 mph is what they think is best, then great. If others tell me they are getting 2,100 rpm or less, with adequate power, then I'd like to know that too.

thanks for all the input and feedback.
 

w2hx

Member
Callsign: W2HX
as stated above you're going to want to verify that your tach actually is accurate. The tach no matter what the signal is needs to be adjusted for the engine.

A cheap laser tachometer from Amazon with a reflector on the crankshaft will tell you your exact RPM. You can then adjust the tachometer with the dip switches and tiny little potentiometer in the side of the unit to make it accurate.

If you're off by let's say one or two hundred RPM at the bottom of the range you could be off by five or 600 at the top of the range.

You're going to need to verify what you have before worrying about if your correct or not.

Verify what diffs you have, takes about 10 min
Verify that your tachometer is correct, takes about 30 minutes.

With these two pieces of information then you'll be able to decide whether or not you need to move down in ratio on your transfer case.

Locally we have two guys with LS is here in the Bay area in the club. Both are running 1.001 transfer cases. One of them has a 5.3 like you The other one has a 6.2 l. Both are running taller tires than you at 33in with stock 3.54 differentials.

It would be really really weird to have 4.1 diffs in a truck that's running 31-in tires. Especially in an ex-military truck that had a 1.6 transfer case in it originally.
ok you're right. I'll check the accuracy of the tach. Not a big deal as you point out. Same with confirming the diffs. All good, just requires some time. But, I will say, the truck definitely SOUNDS like its revving higher than it should in 6th gear. AND it appears to have plenty plenty plenty of power. I can easily accelerate up hill in 6th gear. I wouldnt mind having to kick down to 5 to pass up hill to save some rpms (and fatigue) in 6th on a straight-away. But you are right, I need to do more homework.

In the meantime, if other LS owners have some data for comparison, I'd be grateful
thank you everyone!
 

abraded

Member
FWIW a 2015 Chevy Suburban (5.3l, 6l80) has either 3.08 or 3.42 (trailer pckg) axle gears and a 1.0 t-case and 31.6" tires. So cruising at 70mph rpm would be 1500-1700ish depending which axle gears. That's gonna be GM engineers trying to thread the needle of tolerable available power, fuel economy/emissions and NVH for a vehicle that presumably has lower Cd but higher curb weight than a defender.

Do you have a display telling you what gear your transmission is in? Are you sure you're getting into 6th? Is the torque converter locking up?
 

w2hx

Member
Callsign: W2HX
FWIW a 2015 Chevy Suburban (5.3l, 6l80) has either 3.08 or 3.42 (trailer pckg) axle gears and a 1.0 t-case and 31.6" tires. So cruising at 70mph rpm would be 1500-1700ish depending which axle gears. That's gonna be GM engineers trying to thread the needle of tolerable available power, fuel economy/emissions and NVH for a vehicle that presumably has lower Cd but higher curb weight than a defender.

Do you have a display telling you what gear your transmission is in? Are you sure you're getting into 6th? Is the torque converter locking up?
Hi Abraded. Yes, i have a can bus display for the gear. As for luck up, I am not sure, but it is very consistent. Although I dont really know the symptoms of a CV not locking up, I would imagine it might feel like a clutch slipping? Would I get an OBD error light? I dont have those.
Your thought about the suburban is very à propos. Lots of similarities. Thanks for the data point on the suburban. Would seem getting sub-2000 RPMs at 70 MPH might be possible. Thank you.
 

w2hx

Member
Callsign: W2HX
Hi Abraded. Yes, i have a can bus display for the gear. As for luck up, I am not sure, but it is very consistent. Although I dont really know the symptoms of a CV not locking up, I would imagine it might feel like a clutch slipping? Would I get an OBD error light? I dont have those.
Your thought about the suburban is very à propos. Lots of similarities. Thanks for the data point on the suburban. Would seem getting sub-2000 RPMs at 70 MPH might be possible. Thank you.
hmm seems I cant edit my response. I meant "I dont really know the symptoms of a torque converter not locking up..."
 

madscientist

Well-known member
I have done dozens of LS/LT and Coyote V8 conversions in Defenders. Gear the LT230 to 1:0 to 1. No need for gear reduction in the LT230 with the 6l80 and a GM V8.
 
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