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Rear Ratios

B

BBB454

Guest
This is probably a stupid question, but I'm trying to learn more about the Blue Beast and what to expect from her...

Since I just had the rear-end work done, I asked him what ratio it was and he said 3.08. So I looked in my 71 Corvette Specifications guide and it looks like that was the standard ratio for the 454 4-speed with air. My question is, why are there so many different ratios to choose from? I understand that it's how many times the tires spin per rotation of the axle, but can somebody elaborate on why some people would want a 3.08 and others want a 4.11?

Thanks!
Barb :w
 

Ken

Gone but not forgotten
Joined
Jan 30, 2001
Messages
8,237
Location
Hermosa Beach, CA
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1987 Z51 Silver Coupe
Barb, I won't even start with this one (except that I already did!); you're gonna get a lot of feedback here. :D

Ken

FYI, I went from a 3.07 (.08?) to 3.73's in mine. ;)
 
Joined
Jun 5, 2001
Messages
2,231
Location
Northern Virginia
Corvette
71 Conv. (Sold) / 98 Pewter Coupe (Sold)
My 2 Cents

Barb,
Ken's right, lots of feedback and Ill get it rolling (no pun intended) and you're right in what the ratio really amounts to. The short version of this is that the 3.08 (there was a 2.73 in 70) gives you less gitty up when you're sitting at a light and romp on it since the tires are spinning less, however, it also gives you a higher top end speed. The 4.11 and 4.56 offered in 71 are drag kings because they transfer a lot more torque to the wheels and do so a lot faster so they are the tire burners but to get that starting line excelleration you sacrifice a little top end speed. The higher ratios tend to be more popular because most people like the seat of the pants performance they give. The physics of this all is set in stone but a lot of the theory is just that because a higher torque/hp engine (LT-1, LS-6) can often blow the doors off of their lower powered siblings (L-48, LS-5) even w/ lower ratio rears. My 71 has the base L-48 and the 3.08 rear so it's not the quickest out of the gate but when I punch it at cruising speed there's still no mistaking it's a vette.

The higher torque engine/rear end combos also require a much more substantial type of U-joint because of the torque. These are the cars that more often end up w/ half-shafts ripping through the underside when the pop off a U-juoint. Don't think I'm against higher ratios, the next shark I get (many years from now) will have one.

Oh yeah, if you think you get bad gas mileage, you should compare yours to someone with a higher ratio rear.

That's my 2 cents and there will be a few more dollars worth following close behind. :s
 
Joined
Jun 5, 2001
Messages
2,231
Location
Northern Virginia
Corvette
71 Conv. (Sold) / 98 Pewter Coupe (Sold)
Okay, I just read my own post and I think I got part of that backwards but oh well!
 
P

pro2

Guest
Before choosing a rear ratio you need to think about a few things.
1. Where is the peak torque and HP of your engine in RPM's?
2. What type of driving do you do?
3. What trans do you have?
4. tire size?
For me the easiest way to decide on the ratio was to figure what RPM I wanted while on the highway at 65mph in 4th gear.
You can figure out the exact RPM/MPH, by knowing what trans gear ration you have and the tire size. I ended up with 3.55 rear ratio with my M21 trans. I intend to lower that ratio as I plan on doing more highway driving. If you install 4.11's and have a 1:1 gear ratio in 4th(manual trans) you will not enjoy driving over 50mph unless you have earplugs.
Thanks,
Phil
 
B

BBB454

Guest
A few weeks ago the same mechanic worked on the tranny and I asked if it was an M21, he said yes. Then when I was looking in the spec book it said the only the wide ratio (M20) was available with the 454 with air, not the M21. So I'm not sure what tranny I have, I'll check the numbers next time it's in the air. Either way, it's a 1:1 ratio in 4th gear, but first gear for the M20 is 2.52 and for the M21 is 2.20. So which one would be faster off the line with the same engine?

As far as the rest of it goes, I do mostly rural roads or state highways with stop lights, etc... rarely go on the highway (though we did go to Bloomington Gold this year and it wasn't comfortable to cruise over about 72mph).
Tires are Radial T/A P225/70R15s all around.

Thanks for your help!
Barb :w
 
C

C5Phil

Guest
Barb,

All things being equal (and assuming the tires hook up same in both cases), the wide ratio box will get you off the line faster. The acceleration rate of your car will exactly follow the curve of the torque applied to the rear wheels.
However, the close/wide story is more complex than that. If your primary purpose were to drag race in the ¼ mile, you would want the close ratio box and lower rear end gears. Sounds like a contradiction at first, I know.
To get the multiplier for the torque applied to the rear wheels one multiples the selected transmission gear ratio times the rear end gear ratio. For instance, 2.54 x 3.08 = 7.8 and 2.20 * 3.55 = 7.8 (approximately). Both will produce the same acceleration rate in 1st gear then. However, the close ratio car with the 3.55s will outrun the wide ratio car in the ¼ since it will be shifted at a more efficient point in the torque curve of the engine. Additionally, as you probably see, once out of 1st the close ratio box produces a higher numeric ratios over all and will put more torque to the rear tires. This produces higher acceleration rates. Since 4th gear is 1 to 1 in both cases the 3.55 car will get less gas mileage, produce more noise, and more wear on the engine than the 3.08 car. Since in 4th the engine will be turning faster for a given speed.
The wide ratio box was designed for the street in that it will give you comparable “off the line” or “street light to street light” performance with higher rear end gearing than the close ratio box, plus better fule economy, less noise, less wear, etc.
For your kind driving the wide ratio box is probably just what you want.
If this does not make sense (I think I confused myself), let me know.

Regards,
C5Phil
 
C

C5Phil

Guest
Barb,

All things being equal (and assuming the tires hook up same in both cases), the wide ratio box will get you off the line faster. The acceleration rate of your car will exactly follow the curve of the torque applied to the rear wheels.
However, the close/wide story is more complex than that. If your primary purpose were to drag race in the ¼ mile, you would want the close ratio box and lower rear end gears. Sounds like a contradiction at first, I know.
To get the multiplier for the torque applied to the rear wheels one multiples the selected transmission gear ratio times the rear end gear ratio. For instance, 2.54 x 3.08 = 7.8 and 2.20 * 3.55 = 7.8 (approximately). Both will produce the same acceleration rate in 1st gear then. However, the close ratio car with the 3.55s will outrun the wide ratio car in the ¼ since it will be shifted at a more efficient point in the torque curve of the engine. Additionally, as you probably see, once out of 1st the close ratio box produces a higher numeric ratios over all and will put more torque to the rear tires. This produces higher acceleration rates. Since 4th gear is 1 to 1 in both cases the 3.55 car will get less gas mileage, produce more noise, and more wear on the engine than the 3.08 car. Since in 4th the engine will be turning faster for a given speed.
The wide ratio box was designed for the street in that it will give you comparable “off the line” or “street light to street light” performance with higher rear end gearing than the close ratio box, plus better fule economy, less noise, less wear, etc.
For your kind driving the wide ratio box is probably just what you want.
If this does not make sense (I think I confused myself), let me know.

Regards,
C5Phil
 
S

sscam69

Guest
good explanation

that could not be better put
 
B

BBB454

Guest
Wow!

Thanks for all of that insight! I'm going to have to print a copy of it and study it for awhile, then I might "get it"... :L

Barb :w
 

Yoda

Well-known member
Administrator
Joined
Oct 12, 2000
Messages
4,885
Location
Amarillo, TX
Corvette
1981 UL5
WoW..

C5Phil,

That was a great explaination. I printed it and put it in my hard copy archives.. Thanks

BudD
:w
 

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