I have a 91 notchback with gt40p heads, trick flow track heat intake, e303, stock bottom end, stock t5, and other mild mods. I was wondering if I should go with 3.73s or 4.10s, this will be a street strip car. I've read that there is very little difference between the rpms at 70 mph or so. I will probably spray it occasionally but I would like to hear some opinions on which I should use.
3.73s Or 4.10s
- Thread starter Roller1991
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With a t5 and 4:10 if it hooks at the track your more then likely going to fry the stock clutch. The pressure plate was never designed to hold the brute torque that 4:10s launch at. And you'll find yourself grabbing a handful of second gear quicker then you'll anticipate. Redline sneaks up fast and becomes annoying on the street.
liljoe07
5 Year Member
I agree with the second part of you post since I just switched to 3.73's from 4.10's. It is very annoying.
First part I dont agree with. 4.10s should put less load on the engine, transmission and clutch due to the higher torque multiplication at the rear wheels.
liljoe07
5 Year Member
I posted over at the corral on a topic just like this.(Might even be the same OP?) But, since I run a 26" tire, 4.10s and 3.35 first gear is very, very short. Its actually useless, and I never used first when I was driving around on the street. I would be a 4500-5000 rpms before I could even get across an intersection. LoL! If I had a 28'' tire, I would definitely go with 4.10s. For street driving, I think a T5Z gearing and 4.10s would be a blast to drive with my tire size.
Running 3.73's in my car now....actually thinking about switching back down to a 3.31. Much nicer cruising around not feeling like you should be looking for another gear all the time.
I speak like an engineer or physicist sometimes so here's the bottom line up front: 3.73s may indeed be the way to go for you, but the information about 4.10s being harder on a clutch than 3.73s is nonsense. Actually, the opposite is true. Yet I still recommend that you don't base gear selection on this fact alone.
Discussion:
4.10s do multiply the torque at the rear wheels, but that torque multiplication only happens after the pinion gear touches the ring gear in the rear end. So for example, 300 ft-lbs (peak torque for a stock 5.0) is applied through the clutch through the input shaft and to the gear cluster in the transmission where it is multiplied by 1st gear by 3.35. That makes the torque at the output shaft 1005 ft-lbs minus any mechanical losses. At that point the output shaft, yoke, driveshaft, yoke, and pinion gear all feel the twist of 1005 ft-lbs. The ring gear multiplies it by 3.73 or 4.10. The sum of the torque to the axles, wheels, and tires will be 1005*4.1= 4030 ft-lbs minus drive train losses.
There are 4 common things in racing that are particularly hard on clutches:
1. The torque your engine makes may simply over power the friction and clamping force that clutch provides. This results in clutch slip.
2. Shock (brief but massive torque) to the drivetrain, which takes into account vehicle weight, engine torque, transfer of engine and drive train rotational inertia, tire sidewall flex, and traction all at once when you drop the clutch to launch hard, or when you power shift.
3. The driver purposefully slips the clutch. This puts less shock through the drive train, but it builds more heat in the disk and pressure plate because of the kinetic friction between the slipping clutch disk and the pressure plate. "Slipping the clutch" severely decreases clutch, pressure plate, and flywheel life. The heat and friction can, even in one event, kill a perfectly new clutch by damaging the friction surfaces badly enough.
4. The RPM of the input shaft spins too high, which results in mechanical damage or destruction of the clutch disk. This will only happen if you're in the wrong gear or if there is a defect in the clutch.
In fact, short (numerically high) gears allow you to launch just as hard with less engine torque. This means you can launch from lower RPM. Therefore, you can slip the clutch less, or dump the clutch without as much inertia in the rotating assembly. That actually turns out to be easier on your clutch.
Now, if you want to pick your gears based on what will get you the best 1/4 mile ET, then choose one that will put you through the top of your power band in 4th gear through the traps. What do you think you'll trap? I'd guess something like 100mph give or take. You'll also want to cross the traps at around 5500 RPM. You didn't mention your tire size, but I'll assume its in the realm of stock, say 225/55/16. With this 25.7" diameter tire and 4.10 gears, you'd go through the traps at 102 mph at 5500 RPM. You also have plenty of room to grow. 6000 RPM will take you through at 112 mph. The stock rev limiter at 6250 RPM will put you through at over 116mph.
At 70 mph, the difference is 2530 RPM with the 4.10s and only 2300 with the 3.73s.
Good luck with your decision.
Discussion:
4.10s do multiply the torque at the rear wheels, but that torque multiplication only happens after the pinion gear touches the ring gear in the rear end. So for example, 300 ft-lbs (peak torque for a stock 5.0) is applied through the clutch through the input shaft and to the gear cluster in the transmission where it is multiplied by 1st gear by 3.35. That makes the torque at the output shaft 1005 ft-lbs minus any mechanical losses. At that point the output shaft, yoke, driveshaft, yoke, and pinion gear all feel the twist of 1005 ft-lbs. The ring gear multiplies it by 3.73 or 4.10. The sum of the torque to the axles, wheels, and tires will be 1005*4.1= 4030 ft-lbs minus drive train losses.
There are 4 common things in racing that are particularly hard on clutches:
1. The torque your engine makes may simply over power the friction and clamping force that clutch provides. This results in clutch slip.
2. Shock (brief but massive torque) to the drivetrain, which takes into account vehicle weight, engine torque, transfer of engine and drive train rotational inertia, tire sidewall flex, and traction all at once when you drop the clutch to launch hard, or when you power shift.
3. The driver purposefully slips the clutch. This puts less shock through the drive train, but it builds more heat in the disk and pressure plate because of the kinetic friction between the slipping clutch disk and the pressure plate. "Slipping the clutch" severely decreases clutch, pressure plate, and flywheel life. The heat and friction can, even in one event, kill a perfectly new clutch by damaging the friction surfaces badly enough.
4. The RPM of the input shaft spins too high, which results in mechanical damage or destruction of the clutch disk. This will only happen if you're in the wrong gear or if there is a defect in the clutch.
In fact, short (numerically high) gears allow you to launch just as hard with less engine torque. This means you can launch from lower RPM. Therefore, you can slip the clutch less, or dump the clutch without as much inertia in the rotating assembly. That actually turns out to be easier on your clutch.
Now, if you want to pick your gears based on what will get you the best 1/4 mile ET, then choose one that will put you through the top of your power band in 4th gear through the traps. What do you think you'll trap? I'd guess something like 100mph give or take. You'll also want to cross the traps at around 5500 RPM. You didn't mention your tire size, but I'll assume its in the realm of stock, say 225/55/16. With this 25.7" diameter tire and 4.10 gears, you'd go through the traps at 102 mph at 5500 RPM. You also have plenty of room to grow. 6000 RPM will take you through at 112 mph. The stock rev limiter at 6250 RPM will put you through at over 116mph.
At 70 mph, the difference is 2530 RPM with the 4.10s and only 2300 with the 3.73s.
Good luck with your decision.
I'm kinda leaning towards 4.10s but in the near future I am going to do the 5 lug swap and go with a set of drag radials, either 275/50/15 or 275/60/15. I'm also planning on putting in the ford racing limited slip dif when I switch the gears. What rpm will I cruise at 70 with the 3.73s and the 4.10s
Years ago, I built an excel 1/4 mile estimator that has a gear/tire/transmission calculator integrated and I've fired it back up for you! Hope it helps. It was actually nice to use it again and refine it with some newer skills.
275/50/15 with a T5 and 3.73s
275/60/15 with a T5 and 3.73s
275/50/15 with a T5 and 4.10s
275/60/15 with a T5 and 4.10s
275/50/15 with a T5 and 3.73s
275/60/15 with a T5 and 3.73s
275/50/15 with a T5 and 4.10s
275/60/15 with a T5 and 4.10s
FWIW, a set of 275/50/15's is waaaay more tire than you'll ever need at your power levels. I'd go with a slightly narrower 235/60/15 if it were me. It's quite a bit lighter (wheel and tire combo) than the 275/50/15, but has the same over all diameter. Cars have gone well into the 9's on a 26x9.5" tire, so don't worry about traction....and with it's lighter rotational mass, you'll definitely go faster on it, than the 275.
Great info posted above. I would like to mention there is a big difference between a 302 type build(302-347) compared to a stroked 351 build for your gear selection. A 393-408 is going to make a lot more torque and lesson your need for torque multiplication.
All the math or engineering in the world doesn't mean a thing if you use a short gear ratio on the street and all you do is spend your time spinning tires and instantly shifting out of first gear.
IMO, use the 3.73, it's a nice happy medium between 4.10's and 3.55's. It will never really be too tall or too short.
When you actually build the stroker and break the t5, you can change the ratio better to suit a tko600 and your engine. Run what's best for you now, you never know when an engine is going to be finished.
IMO, use the 3.73, it's a nice happy medium between 4.10's and 3.55's. It will never really be too tall or too short.
When you actually build the stroker and break the t5, you can change the ratio better to suit a tko600 and your engine. Run what's best for you now, you never know when an engine is going to be finished.
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All the math and engineering in the world!? Yeah, why use math or engineering when you take jimbo's opinion? BECAUSE Math and engineering can help you make the best decision! It can help you weigh the trade-offs! There are pros and cons to every decision. Try to at least figure them out before making the decision.
For example, what's more likely to hook, 4.10s with a 28" tire or 3.73s with a 26? Look at the table above and compare the maximum speed in each gear. Since, the gear change and tire swap practically cancel each other out, going with the bigger tires provides more sidewall to store energy and a better footprint for launches. Even though the overall effect is essentially the same gearing, the 4.10s will provide the better launch. That's not something that's obvious until you look at the math. So, based on what you said, why wouldn't that be your recommendation?
Don't be confused about my stance. I didn't make any recommendation. I just provided the information to help him make a more informed decision, and to directly answer his questions.
I'm just curious if your hostility was aimed at me, or if you really prefer to avoid information when making decisions.
For example, what's more likely to hook, 4.10s with a 28" tire or 3.73s with a 26? Look at the table above and compare the maximum speed in each gear. Since, the gear change and tire swap practically cancel each other out, going with the bigger tires provides more sidewall to store energy and a better footprint for launches. Even though the overall effect is essentially the same gearing, the 4.10s will provide the better launch. That's not something that's obvious until you look at the math. So, based on what you said, why wouldn't that be your recommendation?
Don't be confused about my stance. I didn't make any recommendation. I just provided the information to help him make a more informed decision, and to directly answer his questions.
I'm just curious if your hostility was aimed at me, or if you really prefer to avoid information when making decisions.
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