How Low is too Low? Cruising at Low RPM

bob64

bob64

G-Body Guru
Mar 30, 2017
716
93
Niagara Falls, Canada
If you go HV oil pump make sure you get the quart bigger oil pan with the baffle in it. Baffle will help at high rpm against sucking the pan dry.

Going turbo/adding HP you will want to swap that rearend to a heavier rearend so maybe some lower gears.

I guess l should write down all the specs on my cars so l could remember them, 😁 but l'll tell you what l can remember to give you idea.

Camaro, 496BBC, 4L80E, 3.42 gears, HV with 6 quart, at about 55mph it will buck until 60mph with lock-up on, so l just keep my foot in it until 65mph then flip toggle.
Cutlass, 468BBC HV with 6 quart, 4L60E, was 2.14 gears(factory 7.5) now has 8.5 4.10 so much nicer to drive, no bucking. The 2.14 was stupid on the highway, speed limit wasn't proper, 😁 never even got lock-up ingaged in keeping the speed limit.

Both engines are LS3 70/71 so a little older than yours and have been worked, heads, cam, rotating, blah blah. As l don't do much highway(maybe once a year) it was pointless to spend money on lock-up set-up so l just toggle it. Ya ya l'm bad!!

So yes as mentioned you'll want to change the gearing and rearend if your going turbo for sure.
 
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Turbolq4

Turbolq4

Royal Smart Person
Sep 25, 2017
1,316
113
Nampa Idaho
It's a stock big block from a truck. 1500 rpm is inside the meat of the torque it makes. It will work really well for crushing miles of highway. It doesn't need steep gears or fancy oil pumps. What it really needs is a bigger gas tank.
 
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olds307 and 403

olds307 and 403

Comic Book Super Hero
Oct 14, 2008
4,788
113
Melville,Saskatchewan
It will probably be slightly below 1500 rpm if his current car is running at 2100 rpm at 70 mph without a lock up converter. Yah, give it a try, even used 2.56 through 3.08 gears can be swapped right in if it lugs too much. Thick 3.23 gears are available new that fit his current carrier and will give the same rpm he currently runs, locked up in OD.
 
64nailhead

64nailhead

G-Body Guru
Dec 1, 2014
916
93
Upstate NY
First of all I would put pen to paper and see if it's really worth money and effort and how much you need to pick up to make it worth while. Personally I feel your going loose mpg - the trans is heavier and going to take more power and take you further out of the torque curve. I know quite a few people that have stepped up their gear and lost no mpg and a few actually gained because it took less effort to get their junk moving. The guys have brought up a lot of concerns and I'm sure the engineers didn't design it to be efficient that low. I don't know if you have gotten into tuning but there are gains to be had there. (wide band)
^^^This is what's going to happen. The 4L80 is darn near twice the weight of a TH350 with converter and fluid included. It takes a bunch more power just to turn it. So your fuel economy is going to take a turn for the worse with the current setup. 70mph at 1500rpm - no thanks. This is th edown side.

The upside is your future plan will require a PG, TH400 or 4L80, so you might as well get it done. The rear gears will be nothing more than bad no matter what is done in front of them with a performance motor. But you will need a different rear no matter what, so that will be the time to upgrade.

A couple of things to keep in mind when power adders are being put onto a BBC. The stock rods, pistons and crank aren't up to the same task that the block is. And for that matter, they aren't up to the task of a mild boost, sub 10 psi, build. A BBC turbo build has crazy potential, but it needs a crazy pocketbook to match if you expect the motor to stick it out. It truly makes turbo LS builds looks like peanuts price wise. A stock 4.8, 5.3, or 6.0 will take more power with Gen 3 rods than a stock BBC. Throw in a set of Gen 4 rods, then it's not even close financially until you want a reliable 800+whp.
 
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DRIVEN

DRIVEN

Geezer
Apr 25, 2009
5,877
113
Owyhee County
For best efficiency aim for peak torque. Modern cars (where mpg is a real concern) have very flat torque curves for this very reason. I think a 1500rpm cruise will be fine with a mild, stock BBC. You can always experiment when it's done by running a tank at 1500rpm, then a tank at 1700rpm, etc.
My dad played this game with his El Camino and found that his supercharged LT1 liked 3.08s better than 2.56s.
 
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O

oldsmobile joe

Royal Smart Person
Nov 12, 2015
1,010
113
mpls
can you actually lug an auto trans? a properly set up auto trans should not lug, it might hunt between lock and unlock or between gears but not lug.

i say go for it. sounds like its step one on a multi step upgrade. you may not improve your mpg, but you never know. that abc will have a ton of torque down low as well as the mass of the reciprocating assembly going in your favor.

fwiw, back in '88 i had a 79 calais with a 305. i rebuilt a sbc 400 with 8:1 cr, a torquer cam, edelbrock intake, quadrajet and 1.5" primary headers. stock 305 carb, dist and egr, combined with a 700r4 and 2.29 rear gears. i would pull 21mpg on the hwy. performance was ok because of all that torque, but it never lugged or pinged.
when i did my final step and put in 3.23 gears, mpg dropped to mid teens. but performance was very good, eventually ran 13.2 on street tires.
 
Firetruck1

Firetruck1

Apprentice
Nov 1, 2017
92
33
I find that more issues associated with manual transmission cars then automatics. If you engage the lock up you will experience the same issues as they are both 1:1. If it were my ride, I would swap the overdrive in and leave the current gears you have but not use the lock up. Yes i understand you are giving up some of the efficiency you are looking for but you are gaining drive ability which is priceless compared to the price at the pump.

Another thing in my mind is I would not invest a penny outside of maintenance in any 7.5 rear end! Especially one behind a big block that will have more power!
 
O

oldsmobile joe

Royal Smart Person
Nov 12, 2015
1,010
113
mpls
I would swap the overdrive in and leave the current gears you have but not use the lock up
i'm not sure if that would work. maybe things have changed since i was trained at gm back in the 80's, but with those freeway gears and running an OD auto trans with stock converter, you need to lock up the torque converter or risk burning the trans up. the way it was explained to me is, the torque converter generates heats when not locked up. from there, the fluid is sent to the trans cooler and back to the trans to lubricate its internals. if the lock up is not used, the fluid is not sufficiently cooled and, over time, will destroy the trans.
if I'm wrong, please correct me. its been awhile since i have been educated in this area.
 
L

LS6 Tommy

n00b
May 6, 2018
3
3
Your gearing is just about spot on for a stock 454 now... IMO, it will lug if you go to a 4L80.

Tommy
 
TURNA

TURNA

Geezer
Jul 24, 2009
8,257
113
Socialist NY
Hey Tommy welcome!!!!!!!!!!
 

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