Modified engine crewcab gas mileage?

[Nick88]

I definitely get the time crunch aspect of it. Not trying to rain on your parade or anything, just discussion. Maybe you will get lucky and see closer to the 12-14 end of things keeping speeds down. Sounds like it's gonna be a badass truck either way

Yeah man I gotta have the whole truck completely driveable in 2 months and with the rain in ny and the block sitting in the machine shop I haven't been able to make much progress last two weeks. I don't speed and I don't punch it out of stops ever so hopefully I'll see a bit better mileage but who knows. Yeah man I'm gonna black it out fully and stuff, gonna be a really sweet rig when everything is done.

 

[Matt69olds]

What's wrong with gear vendors?

No lockup converter, and more expensive than a junkyard 4L80 build.

 

[Matt69olds]

4l80 will just bolt up to the T-case if the T-case is already behind an '85-later TH400. They use the same round bolt pattern after 85. You will want to get a 4wd 4L80 and get the adapter that was on the 4l80 at the same time. Honestly, most people just get a decent junkyard 4L80 that doesn't have burnt fluid for a few hundred bucks and send it. There are guys running straight from the junkyard 4L80's behind turbo LS's for years with no issues. I decided to rebuild my donor because it had 288,000 miles on it behind a Gen VI 454.

It cost me ~$750 for ALL new frictions, plates, bushings, bearings, pistons, sonnax no walk bushing, sonnax lube regulated pressure valve, Transgo TCC and AFL valve kit, internally dual fed the direct drum, and reman torque converter. $10 for a new 2wd output shaft because my NP205 is a figure 8, and $20 for a rear drum reluctor gear to drive the OSS sensor. I also needed a $250 3/4" adapter plate from Northwest fab because I needed to use my original figure 8 adapter. You shouldn't need to do any of that adapting because yours is newer.

The biggest issue is going to be controlling it. You can do a full manual conversion for next to nothing electronically. You can actually buy a harness/box for under $50 to do that, or just build one yourself. There are various instructions on how to do that online. There are also various electronic controller out there that can do the controlling for ~$500-1000 dollars and are pretty good. Best solution would be a GM controller and wiring harness from an LS engine with a few required inputs. Namely TPS and RPM. It will behave even better if you can give it a 4wd lo signal and MAF signal, but that is harder with a carb setup.

Unless you’re making 800hp, you don’t NEED a built 400 or 4L80. Either one in good condition will handle easily handle 500plus hp with nothing more than a fairly cheap valve body kit. And unless the 400 you’re using is a 71 or older model, it’s going to have the weaker roller clutch direct drum. Every 4L80 comes factory with the hood drum, most already have a 34 element sprag, and the come factory with a better clutch hub.

 

[Nick88]

No lockup converter, and more expensive than a junkyard 4L80 build.

Either way I'm installing a lockup converter on the th400

 

[Nick88]

Unless you’re making 800hp, you don’t NEED a built 400 or 4L80. Either one in good condition will handle easily handle 500plus hp with nothing more than a fairly cheap valve body kit. And unless the 400 you’re using is a 71 or older model, it’s going to have the weaker roller clutch direct drum. Every 4L80 comes factory with the hood drum, most already have a 34 element sprag, and the come factory with a better clutch hub.

I know it doesn't need to be built, I'm just building mine because I was planning on overhauling it anyway (no idea the mileage but leaked fluid bad so it probably is a bit burnt) but a kit to build it is so cheap it was only like 270 bucks, which is the same price for when I rebuilt my 700r4 to stock specs besides a couple small goodies.

 

[Matt69olds]

Either way I'm installing a lockup converter on the th400

How do you install a lockup converter on a 400???

I have seen the kits to modify a 400 for a lockup converter. It’s intended for big money class type racing, with huge by large, money is no object race cars.

The lockup converter kits cost enough to buy 2 gear venders, with money left over. It consists of a pump, input shaft, the hydraulics to make it work, and an extremely expensive custom made converter.

 

[Nick88]

How do you install a lockup converter on a 400???

I have seen the kits to modify a 400 for a lockup converter. It’s intended for big money class type racing, with huge by large, money is no object race cars.

The lockup converter kits cost enough to buy 2 gear venders, with money left over. It consists of a pump, input shaft, the hydraulics to make it work, and an extremely expensive custom made converter.

Well let me rephrase, I shouldn't have called it a lockup because it technically isn't, what it is is a fuel miser torque converter by Hughes, it acts like a lockup converter by greatly reducing slippage and drops rpms a couple hundred rpm when cruising. Essentially a lockup converter but not technically a lockup.

 

[Matt69olds]

Well let me rephrase, I shouldn't have called it a lockup because it technically isn't, what it is is a fuel miser torque converter by Hughes, it acts like a lockup converter by greatly reducing slippage and drops rpms a couple hundred rpm when cruising. Essentially a lockup converter but not technically a lockup.

Ok, that makes sense.

FYI, you could buy 50 of those fuel miser converters for the cost of ONE lockup th400 trans!

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[Nick88]

Ok, that makes sense.

FYI, you could buy 50 of those fuel miser converters for the cost of ONE lockup th400 trans!

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Yeah I should have clarified haha. I just kept calling it lockup because it acts similarly

 

[Ricko1966]

If you go full manual control on a 4l80e ,which is a basically free modification. Do a 48mod which replaces the pwm modulator with a vacuum modulator.

 

[Grit dog]

What's wrong with gear vendors?

Basically what huck said. I was being harsh. Apologies.
From a cost/benefit standpoint I believe swapping in an OD trans is a better option, that’s all.
I applaud yours and anyone’s love and effort for these trucks. It’s awesome. And we all use vehicles differently. For me, it doesn’t make sense from most all angles to try to use an old squarebody as a high miles daily driver.
Would be a totally different scenario I imagine if my daily drive was 20-30miles rural and relaxed vs 60-100 miles of the 3rd worst traffic in the country and rain or early morning trips up and over the mountains and back the same day. 2-300miles.
Your truck sounds awesome and solid. And fun!

 

[Nick88]

Basically what huck said. I was being harsh. Apologies.
From a cost/benefit standpoint I believe swapping in an OD trans is a better option, that’s all.
I applaud yours and anyone’s love and effort for these trucks. It’s awesome. And we all use vehicles differently. For me, it doesn’t make sense from most all angles to try to use an old squarebody as a high miles daily driver.
Would be a totally different scenario I imagine if my daily drive was 20-30miles rural and relaxed vs 60-100 miles of the 3rd worst traffic in the country and rain or early morning trips up and over the mountains and back the same day. 2-300miles.
Your truck sounds awesome and solid. And fun!

I should've also clarified also, this truck will be my "daily driver" but I hardly drive the blazer anymore nor will I drive this much, I take the train to work now, when I mentioned a lot of highway miles, that's when I'm gonna be driving maybe twice a year a 700+ mile trip each way. That's also why I'm not swapping in an OD trans or gear vendors or anything yet, I don't need it at the moment. I appreciate yours and everyone else's help, once I'm done with the truck I'll post some pictures on the forum.