3.23 to 3.73 w/ 4L80e Auto

[kmkalkhoff]

I am contemplating swapping to a 3.73 rear end in my 2013 ZL1, which has been converted to a 4 speed 4L80e auto.

I've attached the comparison of the factory 3.23 to the 3.73 with respect to the gear ratios in the 4L80e.

Other than increased cruising rpm in OD and maybe a decrease in mpg, am I missing any other drawbacks from this swap? Looking at the chart, it seems like the 3.73 would be a great fit with the 4L80e auto.

Just looking for opinions on whether it's worth the swap or not.

FYI - the car recently made 652 rwhp on a dynojet, with 93 pump gas, and the car is strictly a weekend warrior that cruises around to car shows and occasional trips to the 1/4 mile drag strip.

[G Money]

Your car is going to launch much beter if you race 1/4 mile tracks.....I have a M6 and swap to a auto rearend with the 3.23 gearing for all my halfmile events I run. Word of sugestion is, its much easier and faster to change out the whole rear diff assembly than changing the ring and pinion......Under 3 hrs compaired to 6+ hrs for ring and pinion.......

[kmkalkhoff]

Since traction is already extremely limited in first gear, I fear that swapping to 3.73 make it worse on the street - although I am hoping that it would pull harder through the other gears.

[Chrispy]

Couple of questions
- what stall converter
- what tires?

With your horsepower I'd stay 3.23s, run a good DR (MT ET street radial) and a fairly tight 2800 stall converter. That should run 10s easily with that combo at 650rwhp.

I'd personally leave the diff alone with 3.23s. It'd be a perfect street cruiser / strip car.

[kmkalkhoff]

The tires are 305/35/20 approx 28" which is actually listed and used to calculate the speeds in the chart above.

Converter stall is 2800

Since I'm never going to see 190+mph, does it not make more sense to go with a 3.73 to help get into the higher rpm faster where the car makes the most power? I'm pretty new to this so I could be missing something, which is why I am reaching out to others that might have experience.

If I were to stay with the same tire, I'm only giving up a couple hundred rpm at highway cruising speed going with the 3.73.

I haven't ran the calcs on 3.73 and a 30" tire with this setup.

Thoughts?

[kmkalkhoff]

Forgot to mention that the tires are MT ET streets

[Chrispy]

Personally I don't think it's worth the cost for "maybe" a tenth in ET compared to greater challenges hooking up, higher cruising rpm etc. Positive displacement blowers generally don't need deeper gearing as the torque curve is basically square. The hit is so immediate. I've run pretty much every gear ratio from 2.73 to 4.30 in different F-bodies. (NA and supercharged, m6 and automatic). For a PD blower car with 2800 stall and 650rwhp a 3.23 / 28 inch combo would be perfect for street/strip use.

[kmkalkhoff]

Originally Posted by Chrispy

Personally I don't think it's worth the cost for "maybe" a tenth in ET compared to greater challenges hooking up, higher cruising rpm etc. Positive displacement blowers generally don't need deeper gearing as the torque curve is basically square. The hit is so immediate. I've run pretty much every gear ratio from 2.73 to 4.30 in different F-bodies. (NA and supercharged, m6 and automatic). For a PD blower car with 2800 stall and 650rwhp a 3.23 / 28 inch combo would be perfect for street/strip use.

You make a good point, and the feedback is much appreciated.

I'm happy with the way the car performs with the 3.23, I've just been kicking around the idea of the swap after looking at the rpm chart and trans ratios.

I've got it in my head that being a 4 speed (2.48 first gear ratio), versus the factory 6 speed auto (4.03 first gear ratio), it might be better suited with the 3.73 given the trans ratios in the 4l80e, but like you said, it's a moot point if the car won't hook up.

The effective gear ratio with my 4 speed versus the factory 6 speed would be 8.01 to 13.02 respectively. Which seems like a pretty big difference, and it make sense why the autos come with a 3.23.

So the question is, can I get it to hook well with 3.73? I would think so, especially comparing it to a car with equal power with the factory 3.23 and A6 trans. Even if I swapped to 3.73, the effective gear ratio at launch/1st gear would only be 9.25 versus the factory 13.02.

Anyone out there with 650+ rwhp with factory 3.23 and A6 want to chime in on how well your car hooks, and if traction is a serious struggle?

Or do you agree with Chrispy that the power comes on so quick with the PD blower that I wouldn't benefit much from the swap?

[bct10985]

I have been wondering the same thing with my setup. I have a centrifugal supercharger(power will come on higher in rpm range), 4l80 with a 3600 stall and a 3.73 zl1 rear. I do have a 15" conversion with 325 mt et street R's so traction should be there but dont know if it will be enough

[kmkalkhoff]

TTT

[hairtrigger]

It's all about available traction with the tires and surface you are racing on. Look back to old school TH400 which has the same ratios. 3.73 to 4.10 rule this trans on the strip with 600hp to the tune of high tens and 1.5x sixties. With what you do with the the car, no not worth it,,,but if you want to have fun playing and modding go for it.

[Chrispy]

Yup traction (street and strip) is the limiter. Also when you do your calcs for rpm in your 1:1 gear factor in slippage unless you have the converter locking in third gear. My preference with PD cars (and turbo) is to have a slightly taller rear gear (lower numerically) as you can surf the torque curve more. It just feels awesome.

Like I said though try it with a he 3.23s and see how you like it and what sort of ET/mph you run.

Have fun with whatever route you go

[kmkalkhoff]

Well I've decided to give the 3.73's a shot. Does anyone know if the factory manual trans rear assembly, GM part # 20931133, is a direct swap with the auto rear?

I can buy a complete brand GM new assembly online for right around $2,000 without a core, so I can keep the factory 3.23 rear that's currently on the car.

I also saw where I could buy just the factory 3.73 ring and pinion for about $500. Would this be a direct swap with the 3.23 that are currently in my auto rear assembly?