2.56 vs. 2.73 vs. 3.08 - Chevelle Tech
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post #1 of 29 (permalink) Old Sep 11th, 19, 9:25 PM Thread Starter
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2.56 vs. 2.73 vs. 3.08

Interesting debate in my mind...

Stick with the 2.56 12 bolt in my MC or change to 2.73 (I have a second 12 bolt with 2.73, and it is free) or look for a 3-Series carrier and pick up a 3.08?

The more I think about it, the more I'm inclined to stay with the 2.56.

So hear me out...

Car is very nice, my goal is at most an 11.50-12.00s cruiser with as conservative package as possible. If it ends up running 13.50 at first, so be it, but is it possible to have your cake and eat it too? Near stock stall and highway gears, at 4300 lbs?

Anyhow, the debate between 2.56 vs 2.73 vs. 3.08

Based on:
2.56 (rear gear) working with 1.52 (2nd gear)
117 mph @ 5700 rpm,
124 mph @ 6000 rpm
1911 rpm at 60 mph - cruising speed
2549 rpm at 80 mph - highway speed
5569 rpm @ 115 mph - hopeful 1/4 mile trap speed, finishing in 2nd gear

2.73 (rear gear) working with 1.52 (2nd gear)
110 mph @ 5700 rpm
116 mph @ 6000 rpm
2038 rpm at 60 mph - cruising speed
2718 rpm at 80 mph - highway speed
5939 rpm @ 115 mph - hopeful 1/4 mile trap speed, 2nd gear

3.08 (rear gear) working with 1.52 (2nd gear)
103 mph @ 5700 rpm
98 mph @ 6000 rpm
2300 rpm at 60 mph - cruising speed
3066 rpm at 80 mph - highway speed
6700 rpm @ 115 mph - hopeful 1/4 mile trap speed, 2nd gear
4408 rpm @ 115 mph - hopeful 1/4 mile trap speed, drive gear

I'm inclined to go no deeper than 2.73, because of the highway cruising rpm. As little difference as switching to 2.73 would appear to make, it seems sticking with 2.56 would make sense.

I don't plan on an engine capable of 6700 rpm in this car, and shifting to Drive with the 3.08 will kill power at the end of the track.

I'm thinking a big stroker, conservative cam, good working heads, and my goal is achieved with 6000 rpm shift points. This makes sense if you can get off the line with prodigious amounts of torque.

Kudos to anyone willing to read this babble and comment.

1981 Malibu wagon
406/350/4.10
Best ET NA 12.20 @ 110
Best ET w/N2O 11.47 @ 117
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post #2 of 29 (permalink) Old Sep 11th, 19, 9:33 PM
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Re: 2.56 vs. 2.73 vs. 3.08

You are going to find with the torque of the BBC that changing gear will not help that much, I switched my 2.56's out for 3.42's back in the day the car did not feel that much faster or quicker being mine was a big torque engine only pulling 5K . A new converter will make a bigger difference. While I think 12.0 is possible with what you have, 11.50's will take a lot more effort with 4300 lbs. Think of 2.56's like a power glide only with a more aggressive 1st gear. You will most likely go through just before the end of the 1/4 shifting into 3rd.
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post #3 of 29 (permalink) Old Sep 11th, 19, 9:35 PM
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Re: 2.56 vs. 2.73 vs. 3.08

I don't see much difference in all three to be honest...

There was a guy locally that ran a 327 with about 375 horse with a 3 speed manual and 3:08 rear gears....this is back in the early 90's.....car was lowered and it would shoot sparks off the asphalt at night between shifts.....was very quick.....not body every out ran this guy on the street to my knowledge.
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67 chevelle malibu / SS/ custom

Currently undergoing a metal work to remove all rust, if I ever get there.

Cant decide on prostreet or cruiser with a 4 speed.
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post #4 of 29 (permalink) Old Sep 11th, 19, 10:04 PM
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Re: 2.56 vs. 2.73 vs. 3.08

I went from 3:31 to 3:90 in my Chevelle with the 355 in it. It ran 12.90s @104-05 before the switch. Just shifting into drive at the 1/4 mile stripe...after the switch it ran 12.90[email protected] I was crossing the stripe at 5900/6000 in drive.

So no difference in et or mph, but being able to use all three gears.

The 454 wagon I have has a 700r4 /stock stall and 3:73 gears. It cruises at 2000rpm @62 mph. It has alittle giddy-up but no idea what it would run.
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'71 Malibu
3580 #, Carbed 6.0l LS swap in process
T-350, PTC 9" converter, 12 bolt, spool, 3.90 gears, MT ET R 275/60/15

355 [email protected]
125 [email protected]
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'72 Greenbrier Wagon: 454, 700R4, 12 bolt with 3.73
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post #5 of 29 (permalink) Old Sep 11th, 19, 10:21 PM Thread Starter
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Re: 2.56 vs. 2.73 vs. 3.08

Quote:
Originally Posted by Geo71 View Post
I went from 3:31 to 3:90 in my Chevelle with the 355 in it. It ran 12.90s @104-05 before the switch. Just shifting into drive at the 1/4 mile stripe...after the switch it ran 12.90[email protected] I was crossing the stripe at 5900/6000 in drive.

So no difference in et or mph, but being able to use all three gears.
Not sure what that means...other than that was a healthy 355. What were the details? Stall?

1981 Malibu wagon
406/350/4.10
Best ET NA 12.20 @ 110
Best ET w/N2O 11.47 @ 117
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post #6 of 29 (permalink) Old Sep 11th, 19, 10:43 PM
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Re: 2.56 vs. 2.73 vs. 3.08

Here is one I had in the race car. I have 4.10 gears in it and replaced them with 4.88's, guess what the car ran?, the same ET 11.42 at the time. Did not pick up anything, waste of money on that one.
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post #7 of 29 (permalink) Old Sep 11th, 19, 10:52 PM Thread Starter
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Re: 2.56 vs. 2.73 vs. 3.08

It's really hard to go wrong with a 4.10 for the quarter mile...unless the engine makes too much torque, or you like to drive on the highway.

1981 Malibu wagon
406/350/4.10
Best ET NA 12.20 @ 110
Best ET w/N2O 11.47 @ 117
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post #8 of 29 (permalink) Old Sep 11th, 19, 10:53 PM
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Re: 2.56 vs. 2.73 vs. 3.08

Quote:
Originally Posted by dragginwagon406 View Post
Not sure what that means...other than that was a healthy 355. What were the details? Stall?
It was nothing special:

9.7:1 Edelbrock Performer RPM heads and intake, Clay Smith Hyd roller 221/235 .516 lift. 1-5/8 headers into 2-1/2 exhaust.

The converter stall did change from 4000 with 3:31 to 3800 with 3:90s.

We thought it might pick up something, but no real difference...still running the 3:90s now.

'71 Malibu
3580 #, Carbed 6.0l LS swap in process
T-350, PTC 9" converter, 12 bolt, spool, 3.90 gears, MT ET R 275/60/15

355 [email protected]
125 [email protected]
383 Mud Motor- [email protected]
100 shot [email protected]
'72 Greenbrier Wagon: 454, 700R4, 12 bolt with 3.73
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post #9 of 29 (permalink) Old Sep 11th, 19, 11:09 PM Thread Starter
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Re: 2.56 vs. 2.73 vs. 3.08

Some inspiration:

https://www.chevelles.com/forums/155...ack-wow-2.html


12.985 @108.72 with 2.56 and stock stall.

The motor is a 454 .030 over, 781s with 2.19/1.88 valves, .133 dome KB pistons, 286H Isky hydraulic cam (.510/.510 224/234), edelbrock RPM Air Gap, and a Quick Fuel 750 carb.

9.3 or somewhere around there when we were planning out the motor. The package dynoed 435 hp @5500 and 506 lb/ft @ 4300 with the carb straight out of the box.

1981 Malibu wagon
406/350/4.10
Best ET NA 12.20 @ 110
Best ET w/N2O 11.47 @ 117
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post #10 of 29 (permalink) Old Sep 12th, 19, 8:12 AM
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Re: 2.56 vs. 2.73 vs. 3.08

I have a 69 el Camino with a 454/Turbo 400 and non posi, ten bolt 2:73. I like everything about it. I wonder if having this ratio lets the stator multiply torque higher. I'd like to find a non posi 12 bolt 2:73...

If you want to sell the 2.73, let me know, I'm in Pittsburgh.. Thanks..
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post #11 of 29 (permalink) Old Sep 12th, 19, 8:25 AM
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Re: 2.56 vs. 2.73 vs. 3.08

I find that 3:55 or 3:73's are ideal for most street use.

Claude.
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1971 Chevelle SS
1987 Buick Grand National 9.48 @ 142 mph
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post #12 of 29 (permalink) Old Sep 12th, 19, 8:31 AM
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Re: 2.56 vs. 2.73 vs. 3.08

Quote:
Originally Posted by hydro462 View Post
I have a 69 el Camino with a 454/Turbo 400 and non posi, ten bolt 2:73. I like everything about it. I wonder if having this ratio lets the stator multiply torque higher. I'd like to find a non posi 12 bolt 2:73...

If you want to sell the 2.73, let me know, I'm in Pittsburgh.. Thanks..
If you don't find anything let me know in a few weeks, I'll be home then.
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Jim

77 Monte
421 Profilers ported by Eric Weingartner, Straub HR 3200 conv. from Jake 200-4r w/ 3.73's
Machine work by Torvinen's machine
11.93 @ 112.7 & tuning
67 Imp.SS for the wife 275 hp 327-TH350
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post #13 of 29 (permalink) Old Sep 12th, 19, 8:36 AM
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Re: 2.56 vs. 2.73 vs. 3.08

Quote:
Originally Posted by 77 cruiser View Post
If you don't find anything let me know in a few weeks, I'll be home then.
Thank you.. I'm in no hurry... I'd love to get a 12 bolt under there though..
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post #14 of 29 (permalink) Old Sep 12th, 19, 8:54 AM
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Re: 2.56 vs. 2.73 vs. 3.08

I know theres a lot of opinion and anecdotal evidence posted here and I mean no disrespect with my response. I dont see how a 20% increase in torque could not improve ET significantly.

I had a 70 SS 396 with a fresh stock engine, t400 and 3.31 gears. I wanted to drive it more easily at 70 highway speeds so changed the rear to a 2.73. It was definitely less snappy.

I also had a 71 El Camino with a hopped up LS3 (MKIV) and changed it from a 3.08 to a 2.55 (10 bolt) and it was a resultant dog to drive.

Without an overdrive, I think a 3.08 is the perfect rear gear ratio for highway driving. I have one in a box waiting for my next project.

Mencken: The whole aim of practical politics is to keep the populace alarmed by menacing it with an endless series of hobgoblins, all of them imaginary.
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post #15 of 29 (permalink) Old Sep 12th, 19, 6:38 PM
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Quote:
Originally Posted by Alan F View Post
I know there’s a lot of opinion and anecdotal evidence posted here and I mean no disrespect with my response. I don’t see how a 20% increase in torque could not improve ET significantly.

I had a 70 SS 396 with a fresh stock engine, t400 and 3.31 gears. I wanted to drive it more easily at 70 highway speeds so changed the rear to a 2.73. It was definitely less snappy.

I also had a ‘71 El Camino with a hopped up LS3 (MKIV) and changed it from a 3.08 to a 2.55 (10 bolt) and it was a resultant dog to drive.

Without an overdrive, I think a 3.08 is the perfect rear gear ratio for highway driving. I have one in a box waiting for my next project.
I’m a little confused how a well matched gear produced similar times as well but can reason that lack of improvement with, what appears to be better gearing, comes down to one thing - average power applied over the 1/4 mile. This could be lack or traction at the beginning or lack of rpm at the end.

Now my babbling in my initial post becomes clear...I believe with enough torque (or converter), the 2.56, or possibly 2.73, is better than 3.08 since you can finish the 1/4 without shifting to drive, and dropping the engine out out it’s peak horsepower range. This fact will increase the average lower applied over the course of the 1/4 mile.

The bonus of the 2.56 gears is better highway manners.

Proving every day hindsight is 20/20.
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