3.73 vs 4.10 - getting off the line

gandolphxx · June 16, 2002

The late model Vortecs seem to produce a lot of horsepower in the upper RPM's - so here is the question:

Will you get improved "off the line" [0-60 times] with a 4.10 vs a 3.73? I know that this is a better setup for heavy towing, just trying to understand [perhaps the obvious] why it will improve the "off the line" performance.

Thanks.

Avalancher · June 17, 2002

Cmon gandolfxx,

Even I know that one, it's called mechanical advantage.

gandolphxx · June 17, 2002

My question has to do with the torque curve for the 8.1L BigBlock in particular - and any side effects such as high speed effects - yse I understand the basic, just trying to find the caveats. ;)

Avalancher · June 18, 2002

My understanding is that the mechanical advantage applies off the line as well. Exactly like when towing, instead of every 3.73 driveshaft revs turning your tires 1 rev, your driveshaft gets to turn 4.10 revs per tire rev. This equates to more "net torque" from engine to pavement. This will also allow your engine to get into the power band at a lower speed. Remeber, you are "towing" a three ton vehicle even when nothing is attached to your hitch, lower gears allow you to apply that mechanical advantage to get it moving!

I have seen some of these guys post actual expected improvement in 1/4 times. Hopefully someone will chime in.

MarkD · June 18, 2002

I may be stating the obvious, but I think of it this way...

(these are just arbitrary numbers to make the math easy)

Assume your engine puts out the following torque numbers...

1st gear speed w/ 3.73's 4.10

1000 rpm --------- 450 ft/lbs 10 9

2000 rpm --------- 500 ft/lbs 20 18

3000 rpm --------- 600 ft/lbs 30 27

4000 rpm --------- 650 ft/lbs 40 36

4445 rpm --------- 675 ft/bls 45 40

5000 rpm --------- 700 ft/lbs 50 45

(yeah,yeah, I know, read disclaimer above)

If it takes the following power levels to hold a steady speed

10mph -------------200 ft/lbs

20mph -------------220 ft/bls

30mph -------------240 ft/bls

40mph -------------260 ft/bls

50mph -------------280 ft/bls

(remember, these are the official numbers of thin air, because that's where I pulled them from)

Ok, let's look at 40 mph

260 ft/lbs required to maintain 40mph

3.73's put rpm at 40mph at 4000 rpm, and 650 ft/lbs

4.10's put rpm at 40mph at 4445 rpm, and 675 fl/lbs

That leaves 390 ft/lbs at 40 mph to accelerate with 3.73's

Where there is 415 fl/lbs available with 4.10's

So in this case , you would have 25 fl/lbs more available at 40mph. Of course this benefit would apply throughout the entire rev range.

CMNTMXR81 · June 24, 2002

Well, I know it's not the same and I can't compare a 3.73 truck that I converted to a 4.10 as I already have 4.10's.

Anyway, stock in the Z, I had 3.42's. I went to 3.73's and dropped .3 of a second off my E.T.'s right there. More importantly, I cut my 60ft times from 2.0 seconds, to 1.9's and once a 1.8.

Simple mechanical leverage is the case here and the quicker you can get a mass up and moving, the quicker it will be down a measured distance (such as 1/4 mile). If you want top speed, the mechanical leverage with a smaller gear, say a 2.73:1 would net your best.

What you need to consider though is whether or not, going to such a steep gear will hurt you on the big end. You need to find a gear ratio that works best for both launch and and the big end as well as your tire size.

For instance, I find that 3.70 - 3.73's work best in my application as I get decent 60ft times, E.T.s and MPH without running out of gear at the big end and having to shift into 5th (overdrive). 4.10's would simply overwhelm my tires and I'd run to close to the rev limiter in 4th and potentially need to shift to 5th.

gandolphxx · June 27, 2002

Folks, I want to thank you all, great insight, my truck is getting changed to 4.10's today - will post results as soon as I figure out how to use the damn HPP3.

CMNTMXR81 · June 28, 2002

You're getting your front one changed to 4.10 too right? Otherwise if you only change the rear, the minute you engage 4WD you're gonna bind the driveline and break something...most likely a driveshaft as one will want to spin faster than the other.

GreenRado · July 8, 2002

Ouch - now that would be NASTY!!!

gandolphxx · July 8, 2002

You're getting your front one changed to 4.10 too right? Otherwise if you only change the rear, the minute you engage 4WD you're gonna bind the driveline and break something...most likely a driveshaft as one will want to spin faster than the other.

UPDATE: 230 miles on the new gears and the PCM is finally reset to know what is there. Fabulous - my engine practically hugged me, these 4.10's are a natural with the 8.1L, plenty of power, no shift hunting on the highway - keeps that monster right in the sweet zone, and great startup - now I need ET Streets.

And, yes, I did the front also.

CityTruck · July 8, 2002

Let us know when you get a new 1/4 time with the 4.10's. I'm curious what the gears did for you. I would have guessed the other way on the gears. I always thought a higher gear would more benefit a higher revving engine with lower torque. The 8.1 is not a high revving engine and has tons of torque so the challenge would be to "stick" the launch now. With 4WD that may be easy, but I'd still like to know what it did for you. Thanks,

gandolphxx · July 8, 2002

Let us know when you get a new 1/4 time with the 4.10's. I'm curious what the gears did for you. I would have guessed the other way on the gears. I always thought a higher gear would more benefit a higher revving engine with lower torque. The 8.1 is not a high revving engine and has tons of torque so the challenge would be to "stick" the launch now. With 4WD that may be easy, but I'd still like to know what it did for you. Thanks,

The 4.10 gears move the engine up into the power curve better - can feel it on the road real quick, lees hunting in OD. As to the track, well, hard to tell in 90 degree weather, but I do have to run in Auto all the time now to hook up.

CMNTMXR81 · July 8, 2002

I have the 4.10's from the factory in mine and you can definetely feel the "Snap" to it when you hit the gas.

I would have guessed the other way on the gears. I always thought a higher gear would more benefit a higher revving engine with lower torque. The 8.1 is not a high revving engine and has tons of torque so the challenge would be to "stick" the launch now.

Think in terms of a mountain bike or other multi-speed bicycle. A 4.10 gear would be comprable to starting off in 1st gear from a dead stop vs. a 3.73 which would be like starting off in 2nd from a dead stop. A 4.10 allows a higher amount of leverage force for the same amount of power and weight moved.

Conversely at high speed, such a steep gear now hurts as the engine (or your legs) run faster no matter what. Now you want leverage to work the other way.

CityTruck · July 8, 2002

I understand gearing (somewhat ;) ). I upgraded gears in my last vehicle (3.27's to 3.73's). The reason I thought that it might not make sense for the 8.1 is because if you do a chasis dyno on an 8.1 you'll see a relatively flat curve (oximoron sp?). The engine hits peak torque and hp very quickly and holds it to shift point. If you put in a taller gear, you're essentially keeping the engine at higher revs as you shift from gear to gear. Since the 8.1 doesn't suffer from low hp/tq at the shiftpoints with the 3.73's, you're just limiting your runout aren't you?

Let me try to explain another way: If you've got a high revving engine that produces it's max hp/tq at say 5K RPM and it shifts from 1st gear at 6K to 2nd gear at 4K... you've got to build up from 4Kto 5K in 2nd before you're putting max hp/tq to the ground. If you put in a taller gear, you shift from 1st gear at 6K to 2nd gear at 4.8K and you're practically in the max hp/tq curve at the shift.

This example is Fictitious of course, but you see my point right. If you put a taller gear in an engine that developes max hp/tq at 2K and runs out of breath at 5K and already shifted at 3K with the shorter gear, it seems like you're just shifting more. :idontgetit: