I'm building a tranny right now for my race car and I'm tempted to make it a nice close ratio but I've read that it might not be the best idea for a high-powered turbo car like what I'm building... Especially for drag racing... The thing is that I can make the close ratio tranny for practically free (accounting both time and money)... so why not? What's better shifting 3 times in a quarter mile and loading the engine more or shifting 4 times and loading the engine less on each gear?
The engine should peak at about 9 or 10 grand, not sure yet, won't know until the top end is bought and installed and put on the dyno, approximately 600whp, 450 ft-lbs. it will have 26" tires probably (235/60/25) Mickey Thompson ET streets:
here's the current stock gear ratios:
1st 3.250
2nd 1.909
3rd 1.250
4th 0.909
5th 0.702
FINAL DRIVE 4.250
here's what I am planning for it:
1st: 3.250
2nd: 1.909
3rd: 1.650
4th: 1.250
5th: 0.702
FINAL DRIVE 4.250
thoughts?
selecting tranny ratios
Moderator: Team
You need a dyno sheet before we can say conclusively.
http://www.hotroddersauctions.com/tech/ ... ricks.html
scroll down to the it says: "Set Your Gears Up " near the bottom if
http://www.hotroddersauctions.com/tech/ ... ricks.html
scroll down to the it says: "Set Your Gears Up " near the bottom if
I was no longer driving the car consciously. I was driving it by a kind of instinct, only I was in a different dimension.
West Siloam Dispensary
West Siloam Dispensary
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Justin Jones
-
OldSStroker
- Guru
- Posts: 1261
- Joined: Wed Feb 16, 2005 6:14 pm
- Location: Upstate New York
Re: selecting tranny ratios
Look at the rpm drops if you shift at 10K.Justin Jones wrote:I'm building a tranny right now for my race car and I'm tempted to make it a nice close ratio but I've read that it might not be the best idea for a high-powered turbo car like what I'm building... Especially for drag racing... The thing is that I can make the close ratio tranny for practically free (accounting both time and money)... so why not? What's better shifting 3 times in a quarter mile and loading the engine more or shifting 4 times and loading the engine less on each gear?
The engine should peak at about 9 or 10 grand, not sure yet, won't know until the top end is bought and installed and put on the dyno, approximately 600whp, 450 ft-lbs. it will have 26" tires probably (235/60/25) Mickey Thompson ET streets:
here's the current stock gear ratios:
1st 3.250
2nd 1.909
3rd 1.250
4th 0.909
5th 0.702
FINAL DRIVE 4.250
here's what I am planning for it:
1st: 3.250
2nd: 1.909
3rd: 1.650
4th: 1.250
5th: 0.702
FINAL DRIVE 4.250
thoughts?
First option:
1>2 10K >> 5875
2>3 10K >> 6550
3>4 10K >> 7275
4>5 10K >> 7725
2nd option:
1>2 10K >> 5875
2>3 10K >> 8650
3>4 10K >> 7575
4>5 10K >> 5625
With the second option you won't be in 3rd very long, maybe 1-2 seconds, depending on your weight and torque curve.
You will be in 5th a long time if you are planning 9-10K in the lights. 4th will carry you farther (1st option), quicker than shifting into 5th (2nd option).
Once you have a dyno torque curve, put the engine and vehicle data into a good drag racing simulator and play with all the ratios you have available.
Good luck.
[i]"There are some people who, if they don't already know, you can't tell 'em."[b]....Yogi Berra[/b][/i]
[i]"Being able to "think outside the box" presupposes you were able to think in it." [b]--Bob Lutz[/b][/i]
[i]"Being able to "think outside the box" presupposes you were able to think in it." [b]--Bob Lutz[/b][/i]
OldSStroker definitely has could the issue. The 1-2 drop is over 40% of the rev range. In the second option the 2-3 drop is less than 15%.
In both cases are you skipping 5th? Are you even going to get into 4th? My guess is this is a comparison of a three speed vs. a 4 speed, where the extra ratio has small advantage that the shift time may make it a detriment.
Answer...neither choice is particularly good.
In both cases are you skipping 5th? Are you even going to get into 4th? My guess is this is a comparison of a three speed vs. a 4 speed, where the extra ratio has small advantage that the shift time may make it a detriment.
Answer...neither choice is particularly good.
As I see it there are basically 3 hard data points you have to address and a few discretionary decisions, in picking gear ratios.
You must chose a starting ratio (diff + low gear + tire diameter) that gives you the max gear you can effectively put to the ground on launch.
Your top gear needs to be tied to your trap speed and the dyno plot so you clear the traps on the down hill side of the power curve. This will get you as much acceleration as early as possible in the run.
You want gear ratios the get progressively smaller in percentage change as you go through the gears because (especially in a fast car). In the last 330 ft or so your not only acclerating the weight of the car but pushing a lot of air resistance.
Next you need to decide how many gear changes you want to make in the quarter which is tied to 2 variables, how broad is your engine torque curve, and what is your rpm range (back to the dyno plot), and how fast do you shift gears.
Most drivers take from 0.5 - 0.3 seconds to complete a shift (ignoring the folks named Ronny Sox for a moment). If you have the torque curve/power to pull 3 gears and you know you shift a bit on the slow side, favor running 3 gears. If you are like most of us plan on running 4 gears, and trapping near red line in 4th at your expected ET/mph.
After you have an answer to all those questions, then the simulation programs will get you real close.
You might also want to ask around to see what similar cars are running for differential gearing, tire size and low ratio to help pin down how much gear you can get to hook on your tires, chassis, and engine combo.
Most high performance street cars (with tires near 24" in dia), if you take the engines max torque and multiply it by the starting ratio ( diff x low ratio) and divide by the cars weight you will come out with a number between 1.10 and 1.25. With typical street tires the rolling radius is so close to one foot it is not worth figuring out the lever arm of the tire/wheel.
If you get much higher than 1.25 you'll spend all your time trying to keep from blowing the tires off it.
I've not found reliable numbers for cars with slicks so can only guess that the numbers are up in the 2.0 - 3.0 range or there abouts.
(anyone want to donate some representative numbers for torque/weight and gearing for a few cars running slicks?)
I have some of the info for the Ford Thunderbolt but need rear end ratios and tranny low gear ratios.
1964 Ford Fairlane Thunderbolt = = = 480 lb-ft 3225# =
Here are some representative cars I've found data for to work out this concept. As you can see only a couple of true "super cars" ever get up in the 2+ range on very sticky racing tires.
Larry
You must chose a starting ratio (diff + low gear + tire diameter) that gives you the max gear you can effectively put to the ground on launch.
Your top gear needs to be tied to your trap speed and the dyno plot so you clear the traps on the down hill side of the power curve. This will get you as much acceleration as early as possible in the run.
You want gear ratios the get progressively smaller in percentage change as you go through the gears because (especially in a fast car). In the last 330 ft or so your not only acclerating the weight of the car but pushing a lot of air resistance.
Next you need to decide how many gear changes you want to make in the quarter which is tied to 2 variables, how broad is your engine torque curve, and what is your rpm range (back to the dyno plot), and how fast do you shift gears.
Most drivers take from 0.5 - 0.3 seconds to complete a shift (ignoring the folks named Ronny Sox for a moment). If you have the torque curve/power to pull 3 gears and you know you shift a bit on the slow side, favor running 3 gears. If you are like most of us plan on running 4 gears, and trapping near red line in 4th at your expected ET/mph.
After you have an answer to all those questions, then the simulation programs will get you real close.
You might also want to ask around to see what similar cars are running for differential gearing, tire size and low ratio to help pin down how much gear you can get to hook on your tires, chassis, and engine combo.
Most high performance street cars (with tires near 24" in dia), if you take the engines max torque and multiply it by the starting ratio ( diff x low ratio) and divide by the cars weight you will come out with a number between 1.10 and 1.25. With typical street tires the rolling radius is so close to one foot it is not worth figuring out the lever arm of the tire/wheel.
If you get much higher than 1.25 you'll spend all your time trying to keep from blowing the tires off it.
I've not found reliable numbers for cars with slicks so can only guess that the numbers are up in the 2.0 - 3.0 range or there abouts.
(anyone want to donate some representative numbers for torque/weight and gearing for a few cars running slicks?)
I have some of the info for the Ford Thunderbolt but need rear end ratios and tranny low gear ratios.
1964 Ford Fairlane Thunderbolt = = = 480 lb-ft 3225# =
Here are some representative cars I've found data for to work out this concept. As you can see only a couple of true "super cars" ever get up in the 2+ range on very sticky racing tires.
Code: Select all
Car diff low ratio starting ratio max torque weight Max torque/weight
2006 Saleen S7 3.70 2.86 10.582 700 2950 2.510983
1996 Mclaren F1 LM 3.27 3.23 10.5621 520 2336 2.3511
2002 Corvette Z06 3.42 2.97 10.1574 400 3130 1.298070
AC Cobra 427/428 3.31 2.32 7.6792 480 2350 1.27509
2003 Dodge Viper Comp cp 3.07 2.66 8.1662 540 3995 1.11608Larry
Thought I would post up a link to a software program for gearing in case you are interested. I picked up a copy for my project.
http://www.enginemanagementsystems.com/gearsplit.html
http://www.enginemanagementsystems.com/gearsplit.html
ECTA
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Justin Jones
wow thanks for all the good replies! there's some good stuff in this thread.
the 1-2 shift sucks and there's nothing i can do about it unless i win the lottery and get a custom gear set machined (that would be pretty nice) that is my achilles heal i think. well, one of them.
I think the best I can do is keep the gearsets stock... For about 500 bucks I could get a shorter final drive, (4.5,4.7 and 4.9 are options) I'll consider it.
BTW the car is 2200 pounds, front wheel drive...
the 1-2 shift sucks and there's nothing i can do about it unless i win the lottery and get a custom gear set machined (that would be pretty nice) that is my achilles heal i think. well, one of them.
I think the best I can do is keep the gearsets stock... For about 500 bucks I could get a shorter final drive, (4.5,4.7 and 4.9 are options) I'll consider it.
BTW the car is 2200 pounds, front wheel drive...
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Grocerius Maximus
- Pro
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- Joined: Fri Jun 18, 2004 11:08 pm
- Location:
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Cobra
You are trying to maintain equal horsepower before and after shift. Using your "inner gear ratio" for 1-2 shift, 3.250/1.909=1.7025 inner ratio and a 10000 RPM shift point, your RPM after shift would be 10000/1.7025=5874 RPM. If the horsepower you were trying to maintain was found only at 10000 RPM your engine will be in pieces before you reach an RPM to achieve this goal. This is where those dyno sheets come in. Unless all your inner gear ratios are the same you will have individual shift points for each gear change. Wide ratio boxes force higher RPM shifts. Close ratio boxes, (lower inner ratios), allow you to run closer to chosen power level. Class restrictions, car weight, tire size, etc., etc.... all enter into the equation. Please base your choices on the math, dyno, science, and the race track. This is way too much to cover with a single post!