74 rear gears

[Todd L.]

I sent my rear diff off to have it cleaned up and new parts installed.
I do not know for sure what gears I have in there now, but I am thinking 3:08's.
The company will let me know.
I have a 74, LS-4, M21 with Air. The motor is bored .030, as of me purchasing it. I do not know if it will need more or not.
I will have the car in Colorado and will have to deal with mountains. Should I go to 3:55's or even higher?
If I need to swap gears, what would be a good set to put in? The company recommended Strange; however, when I contacted Strange, the rep told me they get their gears from Motive.
Help?

Todd

[Joe R.]

Re: 74 rear gears

Hi Todd,

I'm glad you're getting back to working on your car. Please tell us a little more about how you plan to use the car. 4:11 gears will give the best acceleration, but gas mileage will suffer and it may be loud on the highway. 3:08's would be the opposite of that. Well, you get the idea.

Joe

[Michael W.]

Re: 74 rear gears

Standard ratio with LS4 and M21 is 3:36. Very nice combo, I'd leave it alone.

[James B.]

Re: 74 rear gears

Standard ratio with LS4 and M21 is 3:36. Very nice combo, I'd leave it alone.

With A/C equipped LS-4 Corvettes (such as Todd's vehicle), didn't they come standard with 3:08?

[Michael W.]

Re: 74 rear gears

With A/C equipped LS-4 Corvettes (such as Todd's vehicle), didn't they come standard with 3:08?

If I'm reading the book correctly, 3:08 with M20, 3:36 with M21.

[Joe L.]

Re: 74 rear gears

I sent my rear diff off to have it cleaned up and new parts installed.
I do not know for sure what gears I have in there now, but I am thinking 3:08's.
The company will let me know.
I have a 74, LS-4, M21 with Air. The motor is bored .030, as of me purchasing it. I do not know if it will need more or not.
I will have the car in Colorado and will have to deal with mountains. Should I go to 3:55's or even higher?
If I need to swap gears, what would be a good set to put in? The company recommended Strange; however, when I contacted Strange, the rep told me they get their gears from Motive.
Help?

Todd

Todd-----


Most likely, the car will have 3.36:1 but it's possible it could be 3.08:1. If 3.08:1, I might change to 3.36:1 but nothing numerically higher.

NOS GM gears are the best but ring and pinion gears manufactured by U.S. Gear and available from Tom's Differential are just about as good.

[Gary R.]

Re: 74 rear gears

I would not use Motive, which is owned by the same company that now owns Richmond. I only use Tom's R&P, the only ones still made in the USA, by US Gear. The rest are all imported. I just built a 355 & 373 set of diff's and the gears set up real nice. I hope you did your homework before you shipped out your diff, there are some weak points in the entire 63-79 iron diff run that should be addressed, especially if you have more power and a heavy foot.

[Duke W.]

Re: 74 rear gears

A big block has plenty of low and mid range torque/power to handle any mountain grade including the low air density up to the 12K foot top of Loveland pass on US 50. At 12K feet an engine can only make about half its sea level power, but a 454 has way more than than enough to keep up with traffic or pass when you need to.

Any car will feel somewhat soggy at high altitude, but a large displacement engine will feel less taxed than a small displacement engine, and you always have the option of downshifting if traffic slows you to less than 40 MPH or you need to make a pass. That's why it has a four-speed transmission.

Keep the OE ring and pinion.

A lot of aftermarket gears have a reputation for being noisy. If the OE ring and pinion is in good shape, and they get the pinion depth and backlash correct to maintain the OE tooth contact pattern, the OE gearset should be fine, but you should talk to them about this. Setting the pinion depth and backlash correctly is critical, and the final check should be a tooth contact pattern check and analysis. Setting up an axle is something that requires experience and patience, and sloppy work is absolutely unacceptable if you want a quiet and reliable axle.

Duke

[Todd L.]

Re: 74 rear gears

I started to work on it, and had the car completely torn down; then I received orders to Colorado. I am now trying to get the chassis to the point where I am able to drop the body back on it just to get it loaded in the trailer and tow it to CO. It is coming along and I would have liked to have it together by the end of this month, but I am waiting on parts.
I will be mostly driving around in the Denver area with an occasional run through the mountains to Glenwood Springs/Aspen area. I-25 speed limit is 75 and is always fun.

[Todd L.]

Re: 74 rear gears

Mike what book are you referencing? I was thinking of changing to 3:55, but 3:36 I would be content with. As others are noting, a LS-4 coupled to the M21 should cover me from launch to cruise. If the clutch is dumped on a start, what would the difference be between 3:08 gears, and say 3:73's? would the 3:73's cause the rear wheels to spin more than the 3:55's?

[Todd L.]

Re: 74 rear gears

Joe,
Why would you not go higher than 3:36? I'm not pushing back, just want to understand.

[Todd L.]

Re: 74 rear gears

fortunately or unfortunately the car's motor, besides the .030 bore is relatively stock. I do like to play but once the car is finished I would be afraid of really "playing". What are some weak points you refer to which should be addressed?
Thanks
Todd

[Todd L.]

Re: 74 rear gears

Duke,
are you saying even if the car has 3:08 gears the 454 has enough power driving in CO?

Todd

[Joe R.]

Re: 74 rear gears

Todd,

I would say amen to that.

Joe

[Joe L.]

Re: 74 rear gears

Joe,
Why would you not go higher than 3:36? I'm not pushing back, just want to understand.

Todd------

With a big block, there is no real need to go higher than 3.36:1 unless one is into drag racing. About 3.36:1 is the best ratio for the street out there. With a higher numerical ratio you end up losing comfortable cruising and fuel mileage. So, it's a situation where you gain nothing you need but you lose something of value.