How Low is too Low? Cruising at Low RPM

Dayzedandkonfuzed

Dayzedandkonfuzed

Master Mechanic
Feb 9, 2010
274
70
28
Salmon Arm, BC
#1
So I'm tossing around the idea of throwing a 4L80 in the Malibu to try and squeeze a bit more MPGs out of my 454 (I know, I know, I'm the only guy trying to get efficiency out of a big block,) and am wondering if there is anything I'm overlooking here. With my 2.28's, I cruise at about 2100 rpm at 70mph. With the 4L80, that should come down to about 1600rpm, before the converter locks up. Not sure how much more it will drop with the converter locked. Should I be worried about anything here? Only thing that crosses my mind would be proper oiling on my hour long commute to work. Would a high volume oil pump be the answer for that? Do I really need to worry about sucking the stock pan dry with a high volume pump? Any other issues with cruising at such a low RPM? Anyone know how many more RPMs I'll drop with the lockup?
 
motorheadmike

motorheadmike

Comic Book Super Hero
Nov 18, 2009
3,186
5,738
113
Ottawa, Ontario Canada
#2
100-200 RPM drop depending upon the torque converter and torque production at that RPM.
 
bob64

bob64

G-Body Guru
Mar 30, 2017
660
570
63
Niagara Falls, Canada
#3
They say that 1500 rpm is the sweet spot for the best fuel mileage. BBC is just playing with the weight of your car so l wouldn't worry to much about an oil issue.
 
marcar1993

marcar1993

G-Body Guru
Aug 31, 2007
669
154
43
New Jersey
#4
The biggest thing you will have to contend with is cam profile and how reactive your lock/unlock and kickdown are.

The cam in my car is fairly aggressive (232/240 .568/.568 in a 408ci engine) and with the stick it will buck and be unhappy at lower rpms in .5 overdrive (even with 4.10s) depending on throttle position and engine speed.

The other issue is dependent on cylinder pressure, chamber design, temperature of the cylinder: as rpm drops, piston speed decreases (obviously) which gives a greater potential for preignition issues. At higher speeds, there is less time for the fuel to preignite.

I don't know your build, but some background could be helpful.
 
Clone TIE Pilot

Clone TIE Pilot

Royal Smart Person
Aug 14, 2011
1,871
501
63
Galaxy far far away
#5
If you run the engine too low, you can end up lugging it which is bad for the crank bearings. HV oil pumps have quite a few downsides, they use up more HP which uses more gas. They are also more prone to cavitation which again is bad for your bearings as well as causing ignition timing scatter.
 
Dayzedandkonfuzed

Dayzedandkonfuzed

Master Mechanic
Feb 9, 2010
274
70
28
Salmon Arm, BC
#6
The biggest thing you will have to contend with is cam profile and how reactive your lock/unlock and kickdown are.

The cam in my car is fairly aggressive (232/240 .568/.568 in a 408ci engine) and with the stick it will buck and be unhappy at lower rpms in .5 overdrive (even with 4.10s) depending on throttle position and engine speed.

The other issue is dependent on cylinder pressure, chamber design, temperature of the cylinder: as rpm drops, piston speed decreases (obviously) which gives a greater potential for preignition issues. At higher speeds, there is less time for the fuel to preignite.

I don't know your build, but some background could be helpful.
Motor is bone stock right now, out of a 79 pickup. Will be getting a turbo and a custom grind cam in the future. I'll be running a TCI EZ TCU to control the trans, so kickdown and TCC should be taken care of. I never thought about the potential for preigniton, so that's something to keep an ear on.

If you run the engine too low, you can end up lugging it which is bad for the crank bearings. HV oil pumps have quite a few downsides, they use up more HP which uses more gas. They are also more prone to cavitation which again is bad for your bearings as well as causing ignition timing scatter.
Lugging is one of my concerns for sure.

Sounds like I'll just have to try it out, worst case I find some slightly lower gears to compensate.
 
TURNA

TURNA

Geezer
Jul 24, 2009
6,328
8,012
113
Socialist NY
#7
MPG, efficiency, big block ..........no

A HV oil pump in a BBC is a no no IF you are running a stock capacity pan.

A turbo and a cam is a complete waste of time unless you plan on putting a set of heads on it as well
 
lilbowtie

lilbowtie

Royal Smart Person
Jan 7, 2006
2,378
2,133
113
Canton Mi
#8
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)
 
8

81cutlass

Royal Smart Person
Feb 16, 2009
1,455
1,799
113
Western MN
#9
I have a ls2 with a blower and mild cam with a t56 (.5 OD) and 3.73. I have a hard time not lugging and going into boost at low RPM at 65 or less going up hills. I have to drop it to 5th. If I am going 70+ on a more open road like an interstate I can keep it in 6th going up hills.

A 4l80e isnt a bad idea but go ahead and put 3.42 or so in the rear and enjoy the car more. Even 3.08 would be good, but 2.28 would be tragic.
 
olds307 and 403

olds307 and 403

Comic Book Super Hero
Oct 14, 2008
3,777
1,648
113
Melville,Saskatchewan
#10
The lock up torque converter drops 250 to 300 rpm in my experiences. My 2017 Challenger runs around 1600 rpm at 70 but has variable valve timing, knock sensors and a sophisticated ZF 8 speed auto. I had a 8 to 1 Olds 403 with a 204/214 cam with the 2004R and 2.56 gears. It lugged at bit running 1600 rpm at 60 mph. I found out the lock up torque converter wasn't locking. This was a 2400 stall lock up converter, 1300 rpm would have be insane. I swapped in 3.42 gears, made the car massively better to drive. With my current 215/75R14 it will run at 1750 rpm at 60 mph locked up. I am aiming for 30 mpg imperial with the Olds 260 in my 88 Cutlass with combo. With a 454 consider 3.08 or 3.23 gears with the 4L80E. You should really upgrade to the Yukon spider gears anyways if you are doing any hard launches. The stock ones are ticking time bombs. I find this calculator accurate it has the lock up included in the final number. http://tech.oldsgmail.com/axle_RPM.php
 

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