yakmastermax

Is it worth $450 to upgrade from 3.45 rear gears to 3.73?

I am having a D44 rebuilt and refreshed with all new bearings, seals, and posi trac-lok clutches.

It has 3.45 gears currently.

Going in a ZF6 time attack and aggressive street car with a 383 and a pretty high RPM camshaft meant for a Miniram or converted LT1 intake.

Currently I am slated to spend $500 or so on the rebuild, but to regear from 3.45 to 3.73 adds $450... is that going to be worth it?

Or should I try 3.90? I can do 3.90 for the same cost, basically $450 extra.

tequilaboy

Pick your poison. More gear is easier on the clutch but is harder on the D44, stub-axles, u-joints, 1/2 shafts, spindles and tires. Which of these items would you prefer to replace more frequently?

If you're already traction limited in first gear for a given tire, more gear won't really help unless you run a taller and/or stickier tire which will further exacerbate the issues with the D44, stub-axles, u-joints, 1/2 shafts, spindles.

For a drag car, you can optimize gearing for the launch, the 1/8 mile or the 1/4 mile (and sometimes all 3). For a road race car you can optimize the gearing for the longest straight on a specific track. Its silly to try and optimize a street car's gearing for any specific condition or interval unless its going to be tested in a magazine where things like 0-60 mph, 1/4 mile, and top speed are over-emphasized, unless bragging rights are important. This is C4 General after all.

For reference: my own car is optimized for just about every possible condition with 4+3 ratios (MH5), 3.73 gear and 767 rev/mile tire and can be further fine-tuned via tire selection.

https://www.andysautosport.com/learn...ors/#GearRatio

Here's a log (4th gear pull) showing pretty good correlation with respect to the chart: https://datazap.me/u/tequilaboy/dyno...93-407-421-435

ULTM8Z

I have a 3.42 axle in my '71 Camaro with a Miniramed 383 (albeit not a high winding cam).

With my gear vendors behind my TH7004R, I can switch back and forth between 3rd gear at 1:1 and 2nd gear with the GV activated (which is close to an equivalent final drive with 4.10 axle).

Its a bit faster, but not as much as I would expect.

So going from 3.45 to 3.73... I'm not sure you'd see a huge difference. I'd probably opt for the 3.90 since the six speed has a double OD to calm it down on the highway.

yakmastermax

Yeah I am leaning now towards doing 3.90 if I do anything at all... but I don't know, maybe someone out there did 3.73s from 3.45 and can comment if it was worth it?

I plan on running some decent tires back there. Right now running 200 treadwear Kumho V730 275mm wide, but I also have a set of 315mm wide Nitto NT05 which are 200TW as well.

Hoping for right around 460-480 crank hp out of the motor...

I wish I could find a cheap 3.90 D44HD gearset... like $200 cheap. That would sway me.

Phobos84

I have 3.73's in mine and love it. But keep in mind if you pay the extra $450 to have 3.73's put in you could sell your other gear set and make a lot of that money back.

yakmastermax

Yeah so I probably couldn't sell the 3.45s since I have a 3.07 D44 that would almost certainly need those.

Gah hard decision...

I am leaning towards 3.90s

I have 3.54s in my ZF6 L98 TPI 1989 and I like em, but even in that car...

What about 60mph-120mph pulls? or like 75-120mph pulls... you know, highway gapping... if I was trying to optimize that for my build...

I should bust out a calculator probably?

I think my cam and intake combo is supposed to produce a pretty flat torque curve but rev to 6500 or so...

C4 Steck

My experience is from driving my dad’s LT4 six speed car. I’m sure cam technology has come a long way even since 96 and you’re 383 should offer more on the bottom and mid range. However, the one thing I feel driving my dads car is that the 3.45’s don’t get you where you want to be in the power band fast enough. I think it will be even more pronounced with a big cam. So I agree with you on the gear swap and I think the 3.90 is where I would go. I think that’s where your mind is as well. Sixth gear is very tall in the ZF6 so I don’t think you will notice a huge hit in the MPG department for cruising. I’ve also heard others who have done this say that it makes 6th more “useable”. Winding county roads at 55 MPH can be 6th gear friendly instead of it being just a highway gear.

yakmastermax

Yeah I am leaning towards 3.90 now
I found a source for a 3.90 ring and pinion gear set for $350 shipped or so. Maybe I can convince the guy rebuilding my diff to swap the gears as part of the rest of the labor...

The $350 3.90 ring and pinion set is listed as being for Dana D44HD, so correct for the viper vette application. Ring gear dimension is listed as 8.5" so presumably doesn't need to have the housing clearanced.

I understand there are two ring gear bolt sizes?

Will either work with my carrier or do I need to get a specific ring gear bolt size early? 3/8 vs 7/16?

Considering the cam, the intake, and the intended use of the car, I think 3.90s might be the right calls.

Could anyone help with a calculator? I am curious about what gears and what RPMs a "highway" 60-120 pull might end up with, or a 75-100 pull etc

1Hotrodz

Engine RPM will be ~13% higher using the 3.90 set as opposed to the 3.45 set. Don't know your trans gear ratios or tire circumference . That's all I can tell you.

C4 Steck

im working on the calculator for you

84 4+3

Since I did this math once before... assuming a 1:1 gear ratio, 6000 RPM and an early C4 tire size...
3.45 = 135 mph
3.73 = 125 mph
3.90 = 119 mph

Those are all rounded whole at 6000 engine RPM, but figure 4th gear on the 6 speed is 1:1 iirc. Do with that what you will.

yakmastermax

Awesome thanks!
For a given trans ratio and rear end ratio, road speed is linear with engine RPM right?

So if 4th gear ZF6 1:1 and 3.90 rears yields 119mph at 6000 RPM, then 60mph is right around 3000 RPM for the same gears? So a 60-120mph would be all in 4th gear from 3000-6000 RPM? That seems good right?

3rd with the ZF6 is 1.29, so if 4th gear 1:1 and 3.90s at 3000 RPM is 60mph, then 1.29*3000 RPM is 3900 RPM so 60mph in 3rd ZF6 gear with 3.90s is 3900 RPM.

That gives 92mph at 6000RPM in 3rd gear, or 98mph at a 6400 redline...

So an on ramp from 45mph-92mph in 3rd would be 2900RPM-6000RPM

Hmmm

That all seems decent to me? What do yall think?

The main straight at my local time attack track people rarely get above 100mph, so that could fit well.

And just to touch base and reality check here, assuming all these RPM to mph relationships are acceptable, the benefit of using a shorter gear is that at any given moment during a pull the instantaneous torque and thus acceleration applied will be greater than if a smaller numerical gear ratio were utilized?

tequilaboy

I posted a handy gear calculator above. Input the tire size in Revolutions Per Mile for realistic results. It should answer almost all of your questions.

yakmastermax

Oh cool I missed that, thanks.

I ran the calculator and things look decent I guess?

Highway 75mph-85 cruising is around 2100-2200 RPM in 6th which seems solid..

But all these numbers, is it really going to drive better than 3.45?

WVZR-1

I've used similar and the more popular one that I used/liked also calculated RPM shift loss gear to gear. Your offering certainly supplies enough information.

You need to enter your ratios etc but this actually should be quite useful. There's another with better graphics but this Wallace works quite well.

http://www.wallaceracing.com/calc-rpm-drop-shift.php

**** 6th gear cruising shouldn't even be considered. By some the use of 6th gear consistently led to their failures.