3.08 gears with 2400 stall torque converter

[pacem]

I have a B-body (96 Buick Roadmaster) with:

LT1 5.7L, with LT-4 corvette valve covers with full comp cam, 1.6 roller rockers, 915 bee hive valve springs with new seals, etc.

The tranny is 4L60E automatic, 3 speed + overdrive.

The torque converter is 2400 stall, not sure if lock-up or not.

The rear gear is modded to 3.73 and that's what my question is about.
I get great performance at under 50mph but on the highway, the MPG is abysmal, about 18MPG and the RPMs are just too high for my taste, turning 2700 RPMs at 80MPH.

Question, could I get either 3.08 (The stock 96 Impala SS gears) or 3.23 without negatively affecting anything? I realize I will lose a bit of performance in the city and gain highway and gain MPG, at least 2-3MPG.

I was told my cam likes higher RPMs but what exactly does that mean? If I could get away with a numerically lower gear like 3.08 or 3.23 that would make me happy, as 3.73 is just not a great highway gear. Even 3.42 would be an improvement for me versus 3.73.

I had 3.23 gears once and liked them but that was with a stock L05 engine and stock stall (1400).

Basically the focus of my question is, I am trying to understand the relationship between cam, gears and stall speed. Can you use 2400 stall with 3.08 gears and a full comp cam? (It's not a stock cam in this LT1)

 

[eljefino]

18MPG is pretty good for a cammed V8 going 80. When you hop a motor up you're going to lose MPG due to valve overlap etc. Can you find a published torque curve for your cam/rocker combo? I bet the torque peak is higher and probably pretty close to 3k RPM and you're already in the sweet spot.

If you have tall gears and a high stall convertor you're going to make a lot of heat in the trans as it struggles. Lockup would be a must-have for the highway.

 

[JT1]

My bone stock caprice with L03/4l60/3.08 only gets 20.5mpg freeway at 70-75.

I think you should stop complaining, or go buy a honda.

 

[brsmith]

2400 stall with a 3.73 was what was used '84 Hurst Olds. The curb weight of that car was 3567.

If you are looking for quickness off the line then you are going to need that stall speed and gear to get that Buick moving. If you are more interested in higher cruising speeds, then switch back to the 3.23.

I think the best compromise would be around a 3.42.

You have a 2 ton car, there is no getting around that when you are running 350 cubes.

 

[punisher]

Agree with above, your mileage is pretty good for what you have. Before I burn money on gears and labor to swap them out, I would make sure I have a lockup converter. Youe should be able to feel lockup and see the RPM drop when it does. If it doesn't have a lockup, you are throwing money away going to taller gears.

I just don't see how you will amortize the cost of gears/labor unless fuel skyrockets in price, or you drive 2000 miles/month.

I ran a Camaro with 4.11s, 468ci, and a 3000RPM stall on the street for awhile. It would hit ~3600 RPm at 70mph. Driving two hours to the track on I35 it would average about 11-12 MPG. That's a lot of C12.

 

[pacem]

Originally Posted By: eljefino
18MPG is pretty good for a cammed V8 going 80. When you hop a motor up you're going to lose MPG due to valve overlap etc. Can you find a published torque curve for your cam/rocker combo? I bet the torque peak is higher and probably pretty close to 3k RPM and you're already in the sweet spot.

If you have tall gears and a high stall convertor you're going to make a lot of heat in the trans as it struggles. Lockup would be a must-have for the highway.


Good idea, I will find the torque curve. I am getting the most out of this engine and hitting the peaks at lower speeds due to the gears.

I am not real sure if it has lock-up but I will confirm. If not, I will swap it for a 2000 stall torque converter (stock is 1400). It should be slower in the city but also gain a bit of MPG.

 

[pacem]

Originally Posted By: JT1
My bone stock caprice with L03/4l60/3.08 only gets 20.5mpg freeway at 70-75.

I think you should stop complaining, or go buy a honda.


I know that L05/LT1 B-bodies with highway gears such as 2.53 or even 2.93 are capable of getting 23-25MPG on the highway, which is insanely good considering the sheer bulk of the car and the size of the engine.
In comparison, my Subaru only gets 29MPG.

I think you need a good tune up to up your mileage. CAFE standards were the reason for these highway gears such as 2.53 and L03 engine.

 

[Gator]

performance is a trade off,,, if you have a Hp cam then the low end torque is killed and needs to run in a higher RPM range to me a 3.73 with a small stall and Overdrive is the best for a street car,, you need a stall designed especially for overdrive cars that will lock up once in overdrive for gas mileage. a stall converter with a 3.08 is pure waste.

PS i noticed you saying the highway rpm was too high, check to see if you are getting into overdrive and is it locking up if so by my calculations you should be about 58 MPH at 2000 with converter locked up, so I would agree with you that this is not a interstate car but for short trips to work and around town it is perfect, besides who would mod or cam a car for long trips anyway.

 

[ARCOgraphite]

I'd leave it - you have a hot rod, any mods towards appreciable fuel mileage gain will kill the performance. Whats the cam spec and intended torque range on your setup? Like punisher says you would prob NEVER recoup the cost of parts and labour on a 2-3 MPG gain.

 

[ARCOgraphite]

A cheap way to "overdrive" is to fit the tallest tires on the back that will fit. It make a big difference like going one set taller gears. Do you have room for tires 1 inch taller?

 

[pacem]

Originally Posted By: punisher
Agree with above, your mileage is pretty good for what you have. Before I burn money on gears and labor to swap them out, I would make sure I have a lockup converter. Youe should be able to feel lockup and see the RPM drop when it does. If it doesn't have a lockup, you are throwing money away going to taller gears.

I just don't see how you will amortize the cost of gears/labor unless fuel skyrockets in price, or you drive 2000 miles/month.


I will check on the torque converter. I will pay attention to the tach.

I am past the point of doing something just for the frugality of it. I am doing this for the fun of it. My other set of wheels is a 10 year old BMW. Trust me, after BMW, there is nothing in the GM platform that I can consider expensive, in fact it strikes me how cheap the B-body platform is to mod.

 

[pacem]

Originally Posted By: ARCOgraphite
A cheap way to "overdrive" is to fit the tallest tires on the back that will fit. It make a big difference like going one set taller gears. Do you have room for tires 1 inch taller?


No, I don't have room. I have 255/50/17 in it now and I think even that is too big. The PO modded it with the F-body sway bar and lowered the spring and advised me to get 245/50/17 to get better clearance with the sway bar. Right now 255s don't clear with sharp U-turns.

I think I could go with 3.08 gears and 245/50/17 tires, smaller than stock to get some increase in RPMs.

 

[pacem]

Just talked to the builder, yes, it's a lock-up converter for sure.

He also OKed the swap to a numerically lower gear, suggesting 3.23.

 

[Steve S]

Kinda like worrying about the horse being gone after you left the barn door open. The mods done to the engine/driveline reduce the mpgs.

 

[SteveSRT8]

Yes, the 4L60E is a lock up converter. It also has a very high (numerically low) overdrive gear.

A boat that heavy needs all the gear it can get. 3.23 is the minimum I would consider.

I like 3.42 or 3.73 with a little more stall.

It is CRITICAL that the TC you use have enough torque capacity in the lockup for this heavy car. I recommend a Protorque with billet stator if you can afford it.

 

[pacem]

Originally Posted By: Steve S
Kinda like worrying about the horse being gone after you left the barn door open. The mods done to the engine/driveline reduce the mpgs.


Well, I didn't build it myself and had no input into what went into it, but got it as a package and a good deal. It has almost everything new and replaced.

The good thing is, with the B-body you can do anything you want, and experiment. A gear swap is not that expensive and is actually a lot cheaper than doing engine work. If you don't like one, you can put something else in.

 

[pacem]

Originally Posted By: SteveSRT8
Yes, the 4L60E is a lock up converter. It also has a very high (numerically low) overdrive gear.

A boat that heavy needs all the gear it can get. 3.23 is the minimum I would consider.

I like 3.42 or 3.73 with a little more stall.

It is CRITICAL that the TC you use have enough torque capacity in the lockup for this heavy car. I recommend a Protorque with billet stator if you can afford it.


You know, you have a point.

My torque converter is not stock, it's a 2400 stall lock-up converter, just installed. Nice around town.

 

[mechtech2]

First, is the speedo and odometer calibrated for the non stock gear? We don't know if it is accurate right now. Your speed and odo reading may be way off [therefore gas mileage computations].
If you change gears, don't count on 2-3 MPG. The load is going to remain the load - the power it takes to go at whatever speed is still the same.

Sure, you SHOULD get an improvement - this would be expected.
But we can't count on anything - odd stuff happens - all sorts of factors.

A lock up TC is a help - no waste or slipping. It also keeps heat down.

What we are talking about here is about a 20% reduction when going to 3.08 from 3.73. So at 80, you should be about 2250 instead of 2700. [remember to get calibrated correctly]

Your engine surely favors the high RPM range vs. a stock engine, But I see no problems with your present set up and 3.08 gears.

The car is set up for great fun around town and suburban driving. Compromises to make it a hypermiler on the highway are just that - compromises.

 

[pacem]

Originally Posted By: mechtech2
First, is the speedo and odometer calibrated for the non stock gear? We don't know if it is accurate right now. Your speed and odo reading may be way off [therefore gas mileage computations].
If you change gears, don't count on 2-3 MPG. The load is going to remain the load - the power it takes to go at whatever speed is still the same.

I know for a fact that a stock B-body will get 24-25MPG on highway only with a highway gear such as 2.56. Now that gear won't be useful for much else but it's possible to get good MPG.


A lock up TC is a help - no waste or slipping. It also keeps heat down.

That's what worries me about this 2400 Stall TC I have - if I install 3.23 gears, I will be hitting 2500 RPMS only at 90MPH. Does that mean it won't lock up before that? In practical terms, never locking up?
In other words, if I switch from 3.73 to 3.08 or 3.23, shouldn't I also upgrade the TC to something like 1800 or 2000 stall?


What we are talking about here is about a 20% reduction when going to 3.08 from 3.73. So at 80, you should be about 2250 instead of 2700. [remember to get calibrated correctly]

Yes, my PCM is calibered correctly, speedo, all else is right on the money. I checked with a GPS.

 

[ARCOgraphite]

The lockup will override the stall speed. You just need a hearty lockup. Maybe you can get the thing programmed so it wont auto-unlock everytime you take your foot off the throttle - this unlock always Teed me off when I was searching for lightest throttle cruise. You can always have it unlock with application of brakes or throttle angle or LOW threshold RPM in gear. IDK if the clutch is strong enough to handle a 3rd gear WOT lockup too? I use dto have a Buick Wildcat with a Nailhead and a super turbine 3 speed with switch pitch torque converter. Not as Beefy as a THM400 but much more efficient.