A question of torq and rear end ratio.

Mo65 · 12-21-2000, 05:50 AM

Can any body tell me how the concept of rear ends work. In my 65 I hace a 2:80 rear-end ratio. I am still working on my car and have not driven it yet. I have a 302 and a c-4 transmission from a Ford Futura.I do not know what I will expect as far as rpms, torq, etc. I am not building this car for racing purposes. I want low rpms in freeway speeds. However, I preffer low-end torq. 0-60 performance. In the car I have petronix ignition, and petronix 40,000 volt coil. The engine is stock and has a new holley 2 barrel carburator with a K&N fitler and a 289 high perfomance type air filter.I have a pair of tri-Y headers and panning to install a 2 1/4 inch double exhaust with a balance tube and Dyno Max super turbo mufflers. I did these mild modifications to make it run more effecient and aquire a little bit more low end torq and not for top end speed racing.
Will the 2:80 rear end give me low Rpm at freeway and better low end torq? I have not purchased tires yet, and some people say that I need taller tires to aquire low end torq and drop the rpms. Other say that I need tires that are about 24-25 inches tall to get a better low end torq.
Can anyboby that has the knowlege, explain to me what the 2:80 will give me, and with the combination I have, what height tires do i need to aquire low-eng torque and keep the freeway rpms to a minimum.
One last thing, is the dyno max super turbo the best bet for a quit performance muffler. I was planing to instal a Flowmaster 3 chamber, but everyones said it is noiser and it is an overkill because the engine is under 300 hps, and it would not make a diffrence.
I appreciate anyones input and advise, thanks.

12-21-2000, 09:37 AM

The 2:80 (this means the driveshaft turns 2.8 times to make the rear wheels turn once) gears will give you a high top speed and good fuel mileage, But a 3:55 would be a good choice for better acceleration. The rear end gears work similar to bicycle gears. On a 10 speed or mountian bike, the lowest gear will be easy to pedal and you can accelerate fast, but you will have a low top speed beacause your speed will be limited to how fast you can move your legs. This would be similar to a 4:10 gearset where acceleration would be fast, but top speed would depend on how high you want to spin your motor. In the highest gear on a bike, you have trouble starting because it is so hard to pedal, but once you get moving it gets a little easier. You can go much faster moving your legs at the same speed as before. Your acceleration would be slow, but you would have a high top speed. This would be similar to a 2:80 gear. I would recommend 3:55 for your application for the best combo of acceleration and reasonable highway rpms.

------------------
67 Fastback - Arctic White Pearl paint
351W 415 hp, Trick Flow Alum. Heads, C-4 Trans, 3.55 gears, Front Disc Brakes, 1-1/8" Fr. 3/4" rear sway bars.

John Z · 12-21-2000, 10:29 AM

TOM351's answer is very good and the 3.55:1 rear end is generally a good overall compromise recommendation. However, just as you use low gears for accelerating on a bicycle and high gears for cruising, the automatic transmission in the car slects a low gear for acceleration and a high gear for cruising. The basic equation for estimating speed from engine speed and drive train data is:
speed = (tr/168)x(rpm/(tg x rg)
where
vehicle speed in mph
tr = tire radius in inches
rpm = engine revolutions per minute
tg = transmission gear ratio
rg = rear gear ratio
168 = units conversion factor
(This equation assumes no slip in the transmission.)
example
tr = 14
rpm = 2400
tg = 1
rg = 2.8
Speed = 71.4
If you change the tire radius to 17 in the speed would increase to 86.7 for the same rpm. Or for the same speed, the rpm would be 1975.
As a rule of thumb, when you lower rpm at cruise speed, you increase economy but hurt acceleration. You cannot select a tire size that gives you both good acceleration and cruizing economy. That can be acheived with selection of the right transmission and rear end. An over drive transmission combined with a low (high number) rearend is the ticket to get both acceleration and reasonable economy.

Mo65 · 12-23-2000, 02:25 AM

Thanks guys for answering my question. Your examples make it clear to me and make sense.

12-21-2000, 05:50 AM	#1
Mo65 Registered Member Join Date: Feb 1999 Location: Ontario, California Posts: 43	A question of torq and rear end ratio. Can any body tell me how the concept of rear ends work. In my 65 I hace a 2:80 rear-end ratio. I am still working on my car and have not driven it yet. I have a 302 and a c-4 transmission from a Ford Futura.I do not know what I will expect as far as rpms, torq, etc. I am not building this car for racing purposes. I want low rpms in freeway speeds. However, I preffer low-end torq. 0-60 performance. In the car I have petronix ignition, and petronix 40,000 volt coil. The engine is stock and has a new holley 2 barrel carburator with a K&N fitler and a 289 high perfomance type air filter.I have a pair of tri-Y headers and panning to install a 2 1/4 inch double exhaust with a balance tube and Dyno Max super turbo mufflers. I did these mild modifications to make it run more effecient and aquire a little bit more low end torq and not for top end speed racing. Will the 2:80 rear end give me low Rpm at freeway and better low end torq? I have not purchased tires yet, and some people say that I need taller tires to aquire low end torq and drop the rpms. Other say that I need tires that are about 24-25 inches tall to get a better low end torq. Can anyboby that has the knowlege, explain to me what the 2:80 will give me, and with the combination I have, what height tires do i need to aquire low-eng torque and keep the freeway rpms to a minimum. One last thing, is the dyno max super turbo the best bet for a quit performance muffler. I was planing to instal a Flowmaster 3 chamber, but everyones said it is noiser and it is an overkill because the engine is under 300 hps, and it would not make a diffrence. I appreciate anyones input and advise, thanks.

12-21-2000, 09:37 AM	#2
tom351 Guest Posts: n/a	The 2:80 (this means the driveshaft turns 2.8 times to make the rear wheels turn once) gears will give you a high top speed and good fuel mileage, But a 3:55 would be a good choice for better acceleration. The rear end gears work similar to bicycle gears. On a 10 speed or mountian bike, the lowest gear will be easy to pedal and you can accelerate fast, but you will have a low top speed beacause your speed will be limited to how fast you can move your legs. This would be similar to a 4:10 gearset where acceleration would be fast, but top speed would depend on how high you want to spin your motor. In the highest gear on a bike, you have trouble starting because it is so hard to pedal, but once you get moving it gets a little easier. You can go much faster moving your legs at the same speed as before. Your acceleration would be slow, but you would have a high top speed. This would be similar to a 2:80 gear. I would recommend 3:55 for your application for the best combo of acceleration and reasonable highway rpms. ------------------ 67 Fastback - Arctic White Pearl paint 351W 415 hp, Trick Flow Alum. Heads, C-4 Trans, 3.55 gears, Front Disc Brakes, 1-1/8" Fr. 3/4" rear sway bars.

12-21-2000, 10:29 AM	#3
John Z Registered Member Join Date: Feb 1999 Posts: 102	TOM351's answer is very good and the 3.55:1 rear end is generally a good overall compromise recommendation. However, just as you use low gears for accelerating on a bicycle and high gears for cruising, the automatic transmission in the car slects a low gear for acceleration and a high gear for cruising. The basic equation for estimating speed from engine speed and drive train data is: speed = (tr/168)x(rpm/(tg x rg) where vehicle speed in mph tr = tire radius in inches rpm = engine revolutions per minute tg = transmission gear ratio rg = rear gear ratio 168 = units conversion factor (This equation assumes no slip in the transmission.) example tr = 14 rpm = 2400 tg = 1 rg = 2.8 Speed = 71.4 If you change the tire radius to 17 in the speed would increase to 86.7 for the same rpm. Or for the same speed, the rpm would be 1975. As a rule of thumb, when you lower rpm at cruise speed, you increase economy but hurt acceleration. You cannot select a tire size that gives you both good acceleration and cruizing economy. That can be acheived with selection of the right transmission and rear end. An over drive transmission combined with a low (high number) rearend is the ticket to get both acceleration and reasonable economy.

12-23-2000, 02:25 AM	#4
Mo65 Registered Member Join Date: Feb 1999 Location: Ontario, California Posts: 43	Thanks guys for answering my question. Your examples make it clear to me and make sense.