difference in differentials?

MetalHead · 02-20-2002, 03:34 AM

Could someone please either explain to me the specs of differentials? I understand open and limited slip, but after that I get confused reading about 3.5 to 4 gears, and 8 inches to 9 inches. What do these measurements mean? How do they affect performance? Is there any good websites that I could visit that could thoroughly explain this to me? thanks for any imput you guys give.

Jason

PKRWUD · 02-20-2002, 04:40 AM

The smaller numbers you are referring to is the gear ratio. It is the number of times the driveshaft rotates for one complete rotation of the driven axle. For example, a 4.10:1 gear set means that for every complete revolution the driven axle does, the driveshaft will rotate 4.1 times. The ratio is always given as a number (xx) to one (1). there are 2.73:1, 3.08:1, 3.23 :1, 3.56:1, 3.73:1, 4.10:1, etc. Each of those number 1's equals the axle rotating one time, while the number in front of it is how many times the driveshaft turned. Higher numbers are actually lower gears. The lower the gear (higher numerically), the faster the car will accelerate, to a point. You see, the more times the driveshaft must turn, then the more times the engine must turn, and a quickly turning engine is a faster engine. The problem comes when your engine reaches it's limits. If your engine can only handle a certain rpm, you need to understand that by spinning the engine faster, you will reach that limit sooner. In otherwords, a set of 4.10:1 gears will make a car very, very quick for a short distance, while a higher gear (lower numerically), like 2.73:1 gears, will be slow at first, but will not reach their rpm limit for quite a while, and by the time they do, their top speed will be almost twice that of the 4.10's (everything else being equal). Last analogy:

Did you ever own and ride a multi-speed bicycle, like a 10 speed? Same thing. In first gear, you had to spin the crank and pedals really fast to get the back tire to move, but because you were spinning so fast, you initially moved quickly. before you knew it, you had to shift because you had reached your rpm limit. If you had started out in 10th gear, it would have taken you forever to get going, but once you did, you would fly.

Really simple: Number of driveshaft revolutions (4.10) to (:) the number of driven axle revolutions (1)

The larger numbers you asked about refer to the size of the ring gear. The rear end has two gears: a pinion gear, that is directly attached to the driveshaft, and a ring gear, which is attached to the axle(s). They are in constant contact with each other. In a 4.10:1 ring and pinion set, the pinion gear has 10 teeth, while the ring gear has 41 teeth. Divide the pinion by the ring, and you come up with 4.1. The larger the ring gear, the stronger it is, and the more force it can handle. Ford's 7.5" rear end is called that because the ring gear measures 7.5" across. They suck. The Ford 8.8 rear end has a ring gear that measures, you guessed it, 8.8" across. It's a decent rear end. The Ford 9" rear end is not only stronger (with it's 9" ring gear), but is convenient too, because of it's design. That's a whole nother story, though, and my fingers are getting tired.

I hope that helped!

Take care,
-Chris

MetalHead · 02-20-2002, 02:22 PM

So basically if you are willing to sacrifice some top speed for acceleration, you would want to go with a lower gear (higher numerically), and if you still want that top speed maybe a 3.56 or a 3.73 gear would be better.

One more question: what is the correlation between the gear ratio and the size of the ring gear in inches? Is 4.1 gear the only 9'' rear end? Does that mean the 3.73 gear is an 8.8'' rear end? Or are the two measurements interchangeable?

What rear end would you go with?

Thank you very much for your information, it really has helped a lot in me understanding how this works!

Jason

fordkid68 · 02-20-2002, 05:26 PM

I am not sure but I think that A 4.10:1 gaer could go with diferent rear ends as well as any other gears.

and when choosing A rear end it is really A question of how much HP you want any thing over 300HP you will need the 9". if you have under 300HP then you could go with A 8" rear end with no problem.

ford kid

red · 02-20-2002, 07:20 PM

As Chris said, the size of the ring gear determines what "kind" of ford rear end (7.5", 8", 8.8", 9"). The nine inch rear has many (the most?) gear ratios available due to its popularity in racing (strong but heavy). The 8" rears have many ratios but the most popluar in street/strip classic mustangs are 3.55 (usually used with a C-4, 3 speed auto trans), 3.78 (OK for the street, manual trans), 4.11 (you better have an overdrive trans or don't plan on going on the highway).

take a look at the currie web site for more details

-red

PKRWUD · 02-20-2002, 07:49 PM

Metaldude-

You picked up on that well. Good. I can never tell if I'm explaining something well enough when I can't see facial reactions! Any gears lower than 4:1 (ie: 4.10:1, 4.56:1. 5.56:1, etc.) are considered by most to be racing gears. Any gear higher than 3:1 (ie: 2.83:1, 2.53:1, etc.) are considered by most to be "highway gears". Most ring and pinion gear manufacturers offer a wide variety of gear ratio combinations in most sizes. The only thing that is significant about the ring gear size is it's strength. It's important to note that other components are not interchangeable, either. For example, a 7.5" ring gear will not fit in an 8.8" rear end, and will not work with a 9" pinion. The rings and pinions are always a matched set, and should remain that way, and a specific size (7.5", 8.8", etc.) can only be used in their respective housings. otherwise, like I said, the only real significane is the strength. Bigger is better, from a strength standpoint. The 7.5" sucks eggs, and I wouldn't recommend one for use with a driveline exceeding 150hp. The 8.8 is much stronger, and can comfortably handle up to 400hp, and a Ford 9" can take on levels above that. Installing a ring and pinion set can be cumbersome, and takes a while. The Ford 9" rear end is a different design, and has a third member design that allows the entire gear set to be removed and exchanged with others in a matter of minutes. Very handy for circle track racing, where changing track conditions often require changing the rear gears.

lastly, the only thing that determines the gear ratio is the number of teeth on the ring gear divided by the number of teeth on the pinion gear. If you want a different gear ratio, you substitute a ring and pinion with a different number of teeth on them. For example, the ring and pinions that Richmond makes for a Ford 8.8" rear end have the following tooth counts:

3.55:1 39-11 (that is 39 teeth on the ring gear, and 11 on the pinion)
3.73:1 41-11
4.10:1 41-10
4.56:1 41-9

Their Ford 9" selection is alot bigger, and ranges from 2.80:1 all the way to 6.50:1. Their tooth count, for example, of those two sets are:

2.80:1 42-15
6.50:1 39-6

If you ever run across a ring and pinion set, again, all you have to do to determine the ratio is to divide the ring teeth by the pinion teeth.

As for my personal recommendation for you, that's hard because I don't know you, or your car. It depends on alot of factors, like your current and planned mods, and what kind of driving you will do. if you spend alot of time on the freeway, I would not recommend anything smaller than 3.73:1. 3.55:1 might be even better.

The general rule of thumb, though, is in order to acheive similar results, a vehicle with an automatic transmission requires a lower ratio than a car with a manual transmission. As such, I wouldn't exceed 3.73:1 with a 5spd, or 4.10:1 with an automatic.

Take care,
-Chris

thunderbolt · 02-21-2002, 01:07 PM

Hey PK
I thought you did a great job with the gear explainations. If only all of our explainations were that good.

PKRWUD · 02-21-2002, 01:59 PM

Thanks! I was in a good mood that day.

Take care,
-Chris

MetalHead · 02-21-2002, 03:35 PM

PKRWUD,

Thanks a lot for all your explaining, you have really explained things thoroughly. I just have one more technical question, and then an advice question.

Technical: If the 8.8'' and 9'' figures are the measurement of the diameter of the ring gear, and as you say they make a variety of different gear ratios for a single diameter (9'' = from 2.8 to 6.5), then how can they make a ring gear with different numbers in teeth, and yet still have the same diameter? Is it because they only change the number of teeth by 1 or 2? Or something else?

The advisory question is what you think I should go with. I have a 67 coupe with a 289, 600 cfm edelbrock performer rpm carb, edelbrock airgap intake, MSD ignition system, true dual exhaust with flowmasters, hooker competition headers. I have an edelbrock cam in my backseat, but i'm currently saving to put a new tranny in my car first, i think i'm going with a toploader or a T5, not sure yet. After that I want to go with a new rear end, which is the reason for all the questions, and after that some new heads. Any advice would be greatly appreciated, and thanks again PKRWUD, you've been a big help!

Jason

Frankenstang65 · 02-22-2002, 12:35 AM

Metalhead.....You mentioned that you are planning on doing a tranny swap in the near future and are debating between a top loader 4spd and a T5. The decision you make in this department will have a huge affect on what gears you throw in out back. Since you're going to change the tranny anyways, I'd pop a T5 in there and take full advantage of the overdrive. Having overdrive will allow you a large drop in rpms at higher speeds so that you can put lower gears in the rear. That way you get a more powerfull launch and can still take her on the freeway for a nice easy cruise. The way I see it....it's the best of both worlds.

Hope that helps...sounds like a sweet project...good luck

PKRWUD · 02-22-2002, 09:25 AM

Brass Noggin-
A Ford 9" ring gear can have as few as 31 teeth or as many as 47 teeth. They are different sizes (the teeth, not the ring). Kinda like comparing two bolts that are the same diameter, and the same length, except one is fine thread and one is coarse thread. The pinion gears are cut to fit that ring, which is why I said they are a matched set.

Recomendation: You haven't said what types of driving you plan on doing, so I'll give you a rough, safe answer. 3.73's w/ a 5 speed, and 3.55's w/ a 4 speed.

Take care,
-Chris

02-20-2002, 03:34 AM	#1
MetalHead Registered Member Join Date: Feb 2002 Location: Monterey, CA Posts: 76	difference in differentials? Could someone please either explain to me the specs of differentials? I understand open and limited slip, but after that I get confused reading about 3.5 to 4 gears, and 8 inches to 9 inches. What do these measurements mean? How do they affect performance? Is there any good websites that I could visit that could thoroughly explain this to me? thanks for any imput you guys give. Jason

02-20-2002, 04:40 AM	#2
PKRWUD Junior Member Join Date: Mar 1999 Location: Ventura, California Posts: 8,981	The smaller numbers you are referring to is the gear ratio. It is the number of times the driveshaft rotates for one complete rotation of the driven axle. For example, a 4.10:1 gear set means that for every complete revolution the driven axle does, the driveshaft will rotate 4.1 times. The ratio is always given as a number (xx) to one (1). there are 2.73:1, 3.08:1, 3.23 :1, 3.56:1, 3.73:1, 4.10:1, etc. Each of those number 1's equals the axle rotating one time, while the number in front of it is how many times the driveshaft turned. Higher numbers are actually lower gears. The lower the gear (higher numerically), the faster the car will accelerate, to a point. You see, the more times the driveshaft must turn, then the more times the engine must turn, and a quickly turning engine is a faster engine. The problem comes when your engine reaches it's limits. If your engine can only handle a certain rpm, you need to understand that by spinning the engine faster, you will reach that limit sooner. In otherwords, a set of 4.10:1 gears will make a car very, very quick for a short distance, while a higher gear (lower numerically), like 2.73:1 gears, will be slow at first, but will not reach their rpm limit for quite a while, and by the time they do, their top speed will be almost twice that of the 4.10's (everything else being equal). Last analogy: Did you ever own and ride a multi-speed bicycle, like a 10 speed? Same thing. In first gear, you had to spin the crank and pedals really fast to get the back tire to move, but because you were spinning so fast, you initially moved quickly. before you knew it, you had to shift because you had reached your rpm limit. If you had started out in 10th gear, it would have taken you forever to get going, but once you did, you would fly. Really simple: Number of driveshaft revolutions (4.10) to (:) the number of driven axle revolutions (1) The larger numbers you asked about refer to the size of the ring gear. The rear end has two gears: a pinion gear, that is directly attached to the driveshaft, and a ring gear, which is attached to the axle(s). They are in constant contact with each other. In a 4.10:1 ring and pinion set, the pinion gear has 10 teeth, while the ring gear has 41 teeth. Divide the pinion by the ring, and you come up with 4.1. The larger the ring gear, the stronger it is, and the more force it can handle. Ford's 7.5" rear end is called that because the ring gear measures 7.5" across. They suck. The Ford 8.8 rear end has a ring gear that measures, you guessed it, 8.8" across. It's a decent rear end. The Ford 9" rear end is not only stronger (with it's 9" ring gear), but is convenient too, because of it's design. That's a whole nother story, though, and my fingers are getting tired. I hope that helped! Take care, -Chris __________________ Webmaster: Rice Haters Club Jim Porter Racing Peckerwoods Pit Stop Support Your Local RED & WHITE!

02-20-2002, 07:49 PM	#6
PKRWUD Junior Member Join Date: Mar 1999 Location: Ventura, California Posts: 8,981	Metaldude- You picked up on that well. Good. I can never tell if I'm explaining something well enough when I can't see facial reactions! Any gears lower than 4:1 (ie: 4.10:1, 4.56:1. 5.56:1, etc.) are considered by most to be racing gears. Any gear higher than 3:1 (ie: 2.83:1, 2.53:1, etc.) are considered by most to be "highway gears". Most ring and pinion gear manufacturers offer a wide variety of gear ratio combinations in most sizes. The only thing that is significant about the ring gear size is it's strength. It's important to note that other components are not interchangeable, either. For example, a 7.5" ring gear will not fit in an 8.8" rear end, and will not work with a 9" pinion. The rings and pinions are always a matched set, and should remain that way, and a specific size (7.5", 8.8", etc.) can only be used in their respective housings. otherwise, like I said, the only real significane is the strength. Bigger is better, from a strength standpoint. The 7.5" sucks eggs, and I wouldn't recommend one for use with a driveline exceeding 150hp. The 8.8 is much stronger, and can comfortably handle up to 400hp, and a Ford 9" can take on levels above that. Installing a ring and pinion set can be cumbersome, and takes a while. The Ford 9" rear end is a different design, and has a third member design that allows the entire gear set to be removed and exchanged with others in a matter of minutes. Very handy for circle track racing, where changing track conditions often require changing the rear gears. lastly, the only thing that determines the gear ratio is the number of teeth on the ring gear divided by the number of teeth on the pinion gear. If you want a different gear ratio, you substitute a ring and pinion with a different number of teeth on them. For example, the ring and pinions that Richmond makes for a Ford 8.8" rear end have the following tooth counts: 3.55:1 39-11 (that is 39 teeth on the ring gear, and 11 on the pinion) 3.73:1 41-11 4.10:1 41-10 4.56:1 41-9 Their Ford 9" selection is alot bigger, and ranges from 2.80:1 all the way to 6.50:1. Their tooth count, for example, of those two sets are: 2.80:1 42-15 6.50:1 39-6 If you ever run across a ring and pinion set, again, all you have to do to determine the ratio is to divide the ring teeth by the pinion teeth. As for my personal recommendation for you, that's hard because I don't know you, or your car. It depends on alot of factors, like your current and planned mods, and what kind of driving you will do. if you spend alot of time on the freeway, I would not recommend anything smaller than 3.73:1. 3.55:1 might be even better. The general rule of thumb, though, is in order to acheive similar results, a vehicle with an automatic transmission requires a lower ratio than a car with a manual transmission. As such, I wouldn't exceed 3.73:1 with a 5spd, or 4.10:1 with an automatic. Take care, -Chris __________________ Webmaster: Rice Haters Club Jim Porter Racing Peckerwoods Pit Stop Support Your Local RED & WHITE!

02-21-2002, 01:59 PM	#8
PKRWUD Junior Member Join Date: Mar 1999 Location: Ventura, California Posts: 8,981	Thanks! I was in a good mood that day. Take care, -Chris __________________ Webmaster: Rice Haters Club Jim Porter Racing Peckerwoods Pit Stop Support Your Local RED & WHITE!

02-22-2002, 12:35 AM	#10
Frankenstang65 Registered Member Join Date: Sep 2001 Location: Seattle, WA Posts: 79	Metalhead.....You mentioned that you are planning on doing a tranny swap in the near future and are debating between a top loader 4spd and a T5. The decision you make in this department will have a huge affect on what gears you throw in out back. Since you're going to change the tranny anyways, I'd pop a T5 in there and take full advantage of the overdrive. Having overdrive will allow you a large drop in rpms at higher speeds so that you can put lower gears in the rear. That way you get a more powerfull launch and can still take her on the freeway for a nice easy cruise. The way I see it....it's the best of both worlds. Hope that helps...sounds like a sweet project...good luck __________________ 1965 Coupe: 351W, comp cam, Aluminum Systemax II heads (2.02/1.60), 1.7 Cobra full roller rockers, Edelbrock intake, Holley 650 double pumper, Mallory Unilite, MSD-6AL, MSD Blaster 2 coil, MSD heli-core plug wires, Hedman headers, 2 1/2" exhaust (GT style) w/ 40 series delta flows, 69 C4 w/ shift kit, shelby traction bars, 3.55:1 rear Daily Driver: 1989 GT- 5spd, K&N (no silencer), Shorty Headers, King Cobra Clutch, Flowmaster Force II Cat-back

02-20-2002, 02:22 PM	#3
MetalHead Registered Member Join Date: Feb 2002 Location: Monterey, CA Posts: 76	So basically if you are willing to sacrifice some top speed for acceleration, you would want to go with a lower gear (higher numerically), and if you still want that top speed maybe a 3.56 or a 3.73 gear would be better. One more question: what is the correlation between the gear ratio and the size of the ring gear in inches? Is 4.1 gear the only 9'' rear end? Does that mean the 3.73 gear is an 8.8'' rear end? Or are the two measurements interchangeable? What rear end would you go with? Thank you very much for your information, it really has helped a lot in me understanding how this works! Jason

02-20-2002, 05:26 PM	#4
fordkid68 Registered Member Join Date: Jan 2002 Location: fremont Posts: 306	I am not sure but I think that A 4.10:1 gaer could go with diferent rear ends as well as any other gears. and when choosing A rear end it is really A question of how much HP you want any thing over 300HP you will need the 9". if you have under 300HP then you could go with A 8" rear end with no problem. ford kid

02-20-2002, 07:20 PM	#5
red Registered Member Join Date: Sep 1998 Location: Millburn, NJ Posts: 310	As Chris said, the size of the ring gear determines what "kind" of ford rear end (7.5", 8", 8.8", 9"). The nine inch rear has many (the most?) gear ratios available due to its popularity in racing (strong but heavy). The 8" rears have many ratios but the most popluar in street/strip classic mustangs are 3.55 (usually used with a C-4, 3 speed auto trans), 3.78 (OK for the street, manual trans), 4.11 (you better have an overdrive trans or don't plan on going on the highway). take a look at the currie web site for more details -red

02-21-2002, 01:07 PM	#7
thunderbolt Registered Member Join Date: Oct 1998 Location: Houston, TX, USA Posts: 380	Hey PK I thought you did a great job with the gear explainations. If only all of our explainations were that good.

02-21-2002, 03:35 PM	#9
MetalHead Registered Member Join Date: Feb 2002 Location: Monterey, CA Posts: 76	PKRWUD, Thanks a lot for all your explaining, you have really explained things thoroughly. I just have one more technical question, and then an advice question. Technical: If the 8.8'' and 9'' figures are the measurement of the diameter of the ring gear, and as you say they make a variety of different gear ratios for a single diameter (9'' = from 2.8 to 6.5), then how can they make a ring gear with different numbers in teeth, and yet still have the same diameter? Is it because they only change the number of teeth by 1 or 2? Or something else? The advisory question is what you think I should go with. I have a 67 coupe with a 289, 600 cfm edelbrock performer rpm carb, edelbrock airgap intake, MSD ignition system, true dual exhaust with flowmasters, hooker competition headers. I have an edelbrock cam in my backseat, but i'm currently saving to put a new tranny in my car first, i think i'm going with a toploader or a T5, not sure yet. After that I want to go with a new rear end, which is the reason for all the questions, and after that some new heads. Any advice would be greatly appreciated, and thanks again PKRWUD, you've been a big help! Jason

02-22-2002, 09:25 AM	#11
PKRWUD Junior Member Join Date: Mar 1999 Location: Ventura, California Posts: 8,981	Brass Noggin- A Ford 9" ring gear can have as few as 31 teeth or as many as 47 teeth. They are different sizes (the teeth, not the ring). Kinda like comparing two bolts that are the same diameter, and the same length, except one is fine thread and one is coarse thread. The pinion gears are cut to fit that ring, which is why I said they are a matched set. Recomendation: You haven't said what types of driving you plan on doing, so I'll give you a rough, safe answer. 3.73's w/ a 5 speed, and 3.55's w/ a 4 speed. Take care, -Chris __________________ Webmaster: Rice Haters Club Jim Porter Racing Peckerwoods Pit Stop Support Your Local RED & WHITE!

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