Question About 3.73 Gears And Torque Converter

[dogma]

I have a B-body (Buick Roadmaster) with LT1 and the following cam + TC installed.

4L60E tranny, the gears are 3.73, the tires are 255/50/17.

 

I don't race or tow anything, just want a good MPG car and decent power. I don't like 3.73 gears, the RPMs are too high on the highway for my taste (between 2500-2600 at 70-80) and the corresponding MPG is terrible, I am not even making 18mpg.

 

Question, possible to swap in either 3.23 or 3.42 gears for better MPG and to lower the RPMs a bit? (At 70, the RPMs should go from 2300 to 2000) On paper, it seems my cam is pretty mild going from 1000-5000 and the TC is also pretty mild and 3.23 should work nicely.

 

I heard one opinion that it's better to keep the 3.73 gears but swap the TC for a much more efficient 9" TC to transfer the torque more efficiently, something much higher like 3000 stall. My concern here is that it might not be streetable and the RPMs will remain high.

 

Yet another problem is that my PCM could be tuned for power and not efficiency, I also don't know what octane gas it was tuned for.

 

I heard that it's possible to get mid 20's MPG in a B-body (1990's Chevy Caprice, Impala, Roadmaster) with 3.73 gears and a high-efficiency TC, is that true? I have a hard time believing that. Seems to me the only way to improve MPG is to go a numerically lower gear, correct me if I am wrong?

 

 

 

Transmission Specialties BigShot 12LS

 

12'' STREET CONVERTERS Our 12" Street converters are the standard of the Industry by which all others are measured. Maybe that's why we sell more than all others combined.

Designed for vehicles with mild intake or camshaft changes, our street converters give you the increased stall speed to complement the HP and torque increase of your combination, plus a smooth ride for the daily driver.

Available for GM and some Ford applications.

 

GM Big Shot 12LS

Primarily used in cars that are slightly modified, and mostly street driven. An excellent converter for the GM Target Master crate motor. Rear ratios should be in the range of 3.23 to 4.10. Hydraulic and flat tappet cams with good low-end and mid-range torque work well with this converter.

Stall speed range: 2000-2400 rpms.

• New pump hub
  
• New bearing between stator and pump
  
• Computer balanced and pressure leak tested
  
• Rebuilt sprag assembly
  
• Pump and turbine fins tig spot welded for extra strength
  
• Competitively priced
  

 

The 12" Street Converter is available in Chevy, 700R4, 2004R, 4L60E and AOD in both high stall and low stall.

 

Here is the cam:

 

Competition Cams Comp cam LT-1 LT-4 Chevy 93-96

 

· Application 1993-1996 Chevy 350 LT-1 and LT-4 engines

· Power Range 1000-5000 RPM

· Idle Quality/special requirements-Best for original LT1 with stock computers

· Valve Lift 500/510

· Adv Duration 266/276

· Duration @.050 210/220

· Lobe Centerline 114

[nickvrebel]

torque converter is not you answer.

 

That cam isn't too big to affect too much of the whole equation.

 

a modern torque converter does not affect fuel economy at steady highway cruising speed when the lockup clutch is active, a tighter torque converter may help with in town economy, but don't expect miracles and expect some low end power issues.

 

Being tuned for power won't drastically hurt efficiency, in fact in a lot of cases it helps fuel economy (I know of a low 11 sec. Corvette with a nasty cam that gets over 30 mpg on the highway, much more than stock).

 

Gears are your answer.

[UndeadGMC]

If you want fuel efficancy deffinatly don't get a higher stall torque converter. Gears would be the way to go for highway driving and gas mileage. Torque converter for more torque off the line, which basically boils down to you have to rev the engine higher before the converter locks in and you start rolling. Also the higher stall your converter the louder the transmission is, especailly at high speeds like highway driving.

 

I had my 4L60E transmission rebuilt for my 93 4.3L V6 about 2 years ago now. First it was rebuilt with all stock parts and a new stock converter. It wouldn't even peel the wheels froma dead stop so I took it right back and said this wont do because it didn't work like when I brought it in. Second built was shift kit, larger check balls and springs, huge servos (bigger than corvette ones), some better solenoids and a couple other odds and ends. This acted better but it still wasn't back to how it was when I brought it in so back it went. The third time in the mechanic suggested to simply put in a higher stall torque converter from a corvette as it bolts right up and requires no modification so it would only take him 30 minutes. This changed the truck entirely, so much so that for the 1st month it was scary to press on the gas (partly because this project took place during a 1 month long ice storm). It has no trouble peeling teh tires from a stop now thats for sure. Barely touch the the gas pedal and it will burnout untill you let off a little bit. The few times I've actually stomped the gas since then its as if I was competing at a burnout competition. My truck has stock tires of 235, 75 R 15 so the tires will burn untill I let off the gas because there just isn't enough rubber contact for that much power. My gas mileage went from pretty sad when I brought the truck in to now I keep a $20 bill hidden in the truck just incase. Its also so loud on the highway that it feels like the transmission is going to explode so I don't even use the truck on roads that require me to go above 45 now. Seriously I have been on the highway twice since the rebuild, once when i brought it home the day it was done and 2nd the day after I went to work. That was over 2 years ago.

 

One of these days I am going to get a new stock converter or at least something less than the vette ones thats in there now so I can finally get back on the highway.

[dogma]

My main concern would be that the 3.23 gears are low enough to make the TC lockup in city driving. It's rated from 2000-2400 but I am not really sure at what speed it will lockup. Looking at the TC specs, they say any gear from 3.23 to 3.73 should work with it. Is the lockup speed controlled by the PCM or is more complicated by that and is determined by weight and engine torque? e.g. different weight cars with different engines but the same TC will lockup at different speeds?

 

What is the difference between lockup and stall speed?

 

I realize it will be a little slower at 0-50 speeds but I should gain some top end speed. Now my cruising RPMs are 2300 at 70 and should come down to 2000.

 

In my experience with 3.73 gears, the MPG drops like a rock between 70 and 80 as it's turning between 2300-2650 RPMs.

 

The other reason why I want to keep the RPMs a little lower is because I think it will enable me to accelerate easier, e.g. it's easier to go from 2000 to 2400 RPMs instead of 2300 to 2700 RPMs. Or so it seems anyway.

 

I had a pickup once with 4.10 gears and 3 speed auto, it was very fast 0-50 but on the highway, if you wanted to pass someone, good luck. The RPMs were already so high at 3,000 - 3300 that it was tapped out. Of course this is not a really good comparison as in my case I have 3.73 with overdrive transmission, but still. Seems that gears like 3.08 and 3.23 have more acceleration or passing capability at nice highway speeds than 3.73 or 4.10 (which have more punch at slower speeds in the 0-50 range).

 

I suppose a different cam would make 3.73 behave differently towards this end, but to be perfectly honest, I am more interested in drivebility and good MPG than performance. The 3.23 gears combination I am contemplating is pretty snappy. If I had to drive for hours and hours on the highway at 80, which is not that uncommon in some states, 3.73 would drive me nuts, even with overdrive.

 

Once upon a time I had a Caprice wagon with L05 5.7L and 3.23 gears and it had respectable power for a daily driver, I never felt it lacked power daily driver even though it wasn't LT1.

 

I hear that those who swap gears from 3.08 to 3.73 usually lose about 3MPG, which is huge for these cars that don't get all that great MPG to begin with. So my thinking is by going from 3.73 to 3.23 I should gain almost 3MPG on the highway only, now with the city driving, it's more complicated.

 

FWIW, Stock Impala SS came with 3.08 gears... not sure about the cam specs.