Question for you engineering-types...
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From: Pittsburgh PA
Question for you engineering-types...
Given the hp and torque curves for the stock LT-1, my question is simple. Is there an optimal gear-set that maximizes RWHP, for the street. (Assuming engine stays in stock form, and normal cruise speed is... ~65 MPH).
I'm wondering about what the net effects are, taking into consideration parasitic losses, tire inertia (assuming stock 275/40 R17), etc.
While on the surface, it seems that such a change would have no net effect... But I have never seen any data to confirm or dispute such an assumption.
Asking for an engineering guy, because I don't have access to the information necessary to determine the answer, and while I've been tossing this around in my head for several years, I've never been able to find a way to answer the question, one way or t'other.
I'm wondering about what the net effects are, taking into consideration parasitic losses, tire inertia (assuming stock 275/40 R17), etc.
While on the surface, it seems that such a change would have no net effect... But I have never seen any data to confirm or dispute such an assumption.
Asking for an engineering guy, because I don't have access to the information necessary to determine the answer, and while I've been tossing this around in my head for several years, I've never been able to find a way to answer the question, one way or t'other.
Advanced
Joined: Mar 2008
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From: Klamath Falls Oregon
Optimum performance
In order to determine the proper gear set one has to look at the horsepower and torque curve for the engine that is installed, plus the transmission and tire size. It also depends upon whether you want performance off the line, or gas mileage. Gear ratio should be selected depending upon what you want the car to do. Also if your transmission has lots of gears then you can get both. Then it is just a matter of shifting. I have a C-5 with an automatic and that transmission gets me both performance and gas mileage. I have not found some of the gears yet, but it does shift down at 85 MPH and I do not know where it upshifts, because I had run out of road. I also had an 89 C-4 and it was pretty doggy. It needed more horsepower to pull the car in overdrive. I also had a 95 and it had enough horsepower to pull it off the line and in overdrive. Seems like the earlier C-4 models needed a horsepower injection rather than a gear ratio change
Last edited by fbelz; May 7, 2009 at 05:02 PM.
Pro
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From: Austin Texas
Given the hp and torque curves for the stock LT-1, my question is simple. Is there an optimal gear-set that maximizes RWHP, for the street. (Assuming engine stays in stock form, and normal cruise speed is... ~65 MPH).
I'm wondering about what the net effects are, taking into consideration parasitic losses, tire inertia (assuming stock 275/40 R17), etc.
While on the surface, it seems that such a change would have no net effect... But I have never seen any data to confirm or dispute such an assumption.
Asking for an engineering guy, because I don't have access to the information necessary to determine the answer, and while I've been tossing this around in my head for several years, I've never been able to find a way to answer the question, one way or t'other.
I'm wondering about what the net effects are, taking into consideration parasitic losses, tire inertia (assuming stock 275/40 R17), etc.
While on the surface, it seems that such a change would have no net effect... But I have never seen any data to confirm or dispute such an assumption.
Asking for an engineering guy, because I don't have access to the information necessary to determine the answer, and while I've been tossing this around in my head for several years, I've never been able to find a way to answer the question, one way or t'other.
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From: Pittsburgh PA
In order to determine the proper gear set one has to look at the horsepower and torque curve for the engine that is installed, plus the transmission and tire size. It also depends upon whether you want performance off the line, or gas mileage. Gear ratio should be selected depending upon what you want the car to do. Also if your transmission has lots of gears then you can get both. Then it is just a matter of shifting.
Each gear-set, I know, has its own parasitic losses, and from what I've read, the taller the gears, the less this is. But, given that lower gears increase torque to the rear wheels, wherein lies the trade-off between increased torque vs. decreased RWHP? And is there an optimum gear-set, to maximize both?
Trial and error, IMO, is not an efficient way to go. My checkbook isn't water-cooled, so I have to choose carefully (and get it past "momma"), and then go for it...
As I stated originally, it seems to be a simple question, but I know there's a lot of different factors coming into play, that I can't blindly predict...
Team Owner
Joined: Mar 2001
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From: Cincinnati, Oh USA
A number of facts have to be juggled to get maximum acceleration from stop. Higher differential gear ratio allows more torque to the wheels, but there is a limit to tire adhesion and there is also a limit to engine rpm. You have to chose the gear ratio for the maximum torque the wheels can handle without spinning and you also have to chose the gear ratio that allows engine operation within the largest area under the hp/rpm curve. Usually a compromise is chosen.
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From: Hinckley Ohio
Safety Car
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From: Marianna FL
In theory, you shouldn't lose or gain any HP with a gear change. This isn't exactly true because different gearsets have different friction characteristics. What you are changing with a different set of gears is the torque at the wheels.
You can put a 6.xx set of gears in your car and have a helluva lot of torque, but its going to be dog slow after you get moving. Likewise you can keep the 2.59's and your available torque at the wheels suffers, but you have low cruising RPM.
You can put a 6.xx set of gears in your car and have a helluva lot of torque, but its going to be dog slow after you get moving. Likewise you can keep the 2.59's and your available torque at the wheels suffers, but you have low cruising RPM.
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Team Owner
Joined: Mar 2001
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From: Cincinnati, Oh USA
Ideally you would want to run the engine at the rpm it generates its highest horsepower and then adjust the gear ratio for maximum torque at each vehicle speed. This will allow the engine to add kinetic energy (vehicle speed) to the vehicle at its highest rate. Unfortunately transmissions that can handle hundreds of HP are not made continuosly variable, so you have to adjust the differential gear ratio so that each transmission gear works over a range of rpm that is safe for the engine AND provides the maximum area under the engine HP vs RPM curve. This provides the highest average horspower in each gear.
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For crying out loud, don't take things so literally. He didn't say torque = HP. Since HP is a function of torque, increasing torque will improve HP -- at the rpm where torque is improved. Even my dumb self knew that....
To the OP: It's probably a better question to query those who've installed steeper gears on their (stock) A4. Have people reply about traction, mpg, rpm at 70mph, satisfaction, etc....
Unfortunately, I pay more attention to M6 cars where 3.73-3.93 seems to be the favorite (sweet spot for racing). And, rarely to people change gears with 3.54's out back. I've seen several posts where people have been unhappy with 4.xx gears because they didn't get any faster.
You could try calculating the max rpms you feel you want to be running at 70mph, then see what ratio comes nearest that (using tranny gear, tire diameter, and axle gear). And, go from there....
To the OP: It's probably a better question to query those who've installed steeper gears on their (stock) A4. Have people reply about traction, mpg, rpm at 70mph, satisfaction, etc....
Unfortunately, I pay more attention to M6 cars where 3.73-3.93 seems to be the favorite (sweet spot for racing). And, rarely to people change gears with 3.54's out back. I've seen several posts where people have been unhappy with 4.xx gears because they didn't get any faster.
You could try calculating the max rpms you feel you want to be running at 70mph, then see what ratio comes nearest that (using tranny gear, tire diameter, and axle gear). And, go from there....
Safety Car
Joined: Sep 2000
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From: Hinckley Ohio
[QUOTE=GREGGPENN;1570003501]For crying out loud, don't take things so literally. He didn't say torque = HP. Since HP is a function of torque, increasing torque will improve HP -- at the rpm where torque is improved. Even my dumb self knew that....
GM choose 2.59 gears for economy and performance, but offered 3.07 gears as a "performance" axle...
So GM's answer to the OP would be 3.07 gears...still great economy
Another approach for great street launches would be a slightly higher stall torque converter...not sure how that would work with the 2.59's
GM choose 2.59 gears for economy and performance, but offered 3.07 gears as a "performance" axle...
So GM's answer to the OP would be 3.07 gears...still great economy
Another approach for great street launches would be a slightly higher stall torque converter...not sure how that would work with the 2.59's
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Which goes back to the core question: what gears, for a stock LT1/A4 combination, maximizes RWHP?
Ok. As someone else said, choose the highest RPM I'm willing to tolerate at 70MPH, and go from there... which, in my case, would be 2400-2500 RPM, at 70. Going backwards from there, this yields a 3.73 gearset. 
However, I'm still curious... as to whether there is an optimum gearset that maximizes RWHP, specifically... In other words, IF I have a 3.55 gearset in my car, and yield... (for the sake of argument) 235 RWHP, will the installation of a 3.73 gearset increase or decrease that peak number?
I know, I know.... it's kind of an esoteric question, but I would have guessed that this would be a very attractive puzzle for the mechanical engineers on the forum...
Ideally you would want to run the engine at the rpm it generates its highest horsepower and then adjust the gear ratio for maximum torque at each vehicle speed. This will allow the engine to add kinetic energy (vehicle speed) to the vehicle at its highest rate. Unfortunately transmissions that can handle hundreds of HP are not made continuosly variable, so you have to adjust the differential gear ratio so that each transmission gear works over a range of rpm that is safe for the engine AND provides the maximum area under the engine HP vs RPM curve. This provides the highest average horspower in each gear.
However, I'm still curious... as to whether there is an optimum gearset that maximizes RWHP, specifically... In other words, IF I have a 3.55 gearset in my car, and yield... (for the sake of argument) 235 RWHP, will the installation of a 3.73 gearset increase or decrease that peak number?
I know, I know.... it's kind of an esoteric question, but I would have guessed that this would be a very attractive puzzle for the mechanical engineers on the forum...
Race Director
Joined: Mar 2006
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From: Sherwood, Arkansas
Hasn't this thread been done many times and the same arguments surface. The torque vs horsepower always developes into a pissing match. I understand what you are trying to do OP but don't be surprised if this one eventually winds up locked.
Le Mans Master
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From: The Old Dominion
However, I'm still curious... as to whether there is an optimum gearset that maximizes RWHP, specifically... In other words, IF I have a 3.55 gearset in my car, and yield... (for the sake of argument) 235 RWHP, will the installation of a 3.73 gearset increase or decrease that peak number?
If you changed the rear axle ratio, then strapped it on a dyno, the horsepower the car makes would stay the same (within some tolerance). Changing the gearset will not produce or remove horsepower. It does significantly change rear wheel torque though.
Safety Car
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From: Hinckley Ohio
[QUOTE=Aurora40;1570018113]
If you changed the rear axle ratio, then strapped it on a dyno, the horsepower the car makes would stay the same (within some tolerance). Changing the gearset will not produce or remove horsepower.QUOTE]
Actually there would be power lost in the rear axle due to various frictions, windage, lubricant viscousity, etc.......
So the real question is what rear axle design, lubricant, ratio etc results in the most efficient design...
My guess would be the lower ratio numerically would tend to be the most efficient...that is a 2.59 would tend to be more efficient than a 3.73 from the standpoint of internal losses...
If you changed the rear axle ratio, then strapped it on a dyno, the horsepower the car makes would stay the same (within some tolerance). Changing the gearset will not produce or remove horsepower.QUOTE]
Actually there would be power lost in the rear axle due to various frictions, windage, lubricant viscousity, etc.......
So the real question is what rear axle design, lubricant, ratio etc results in the most efficient design...
My guess would be the lower ratio numerically would tend to be the most efficient...that is a 2.59 would tend to be more efficient than a 3.73 from the standpoint of internal losses...
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Maximize RWHP ???? Do you mean RW torque. The only way to increase the RWHP would be lesson the loss between the motor and rear wheels. As the gears get taller the RW torque is increased. On to your engine, hypothetically, the torque curve your engine produces max torque at 4000 rpm. The taller gears or changes to a torque convertor would be the better bang for the bucks. There are so many variables including tire size. The best way to understand it would be a 4x4. When you use low range you get that head jerking action off the line. Tall gears in the transfer case tall gears in the rear end with no top end. Adjusting that line is all in the application.
Horsepower = Torque X RPM/5252
Horsepower = Torque X RPM/5252
Team Owner
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From: Cincinnati, Oh USA
Partial Quote from deerkill10:
Maximize RWHP ???? Do you mean RW torque. The only way to increase the RWHP would be lesson the loss between the motor and rear wheels.
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No the OP means RWHP. HP is the time rate of work and it is HP that causes acceleration. Almost all engines have their torque peak lower in rpm than the HP peak. If you could have a transmission whose input/output ratio was continuosly variable, for maximum acceleration you would choose the xmsn ratio curve that would allow the engine to operate at its peak HP and NOT the peak torque. At peak HP, you would provide the highest torque at the rear wheels.
For the highest acceleration in real cars that have fixed stepped ratio's in their transmissions, you choose a differential ratio that allows the engine to operate with the highest average HP (area under the HP/RPM curve) AND with the consideration that you limit the top speed of the vehicle if the differential ratio is too high.
Maximize RWHP ???? Do you mean RW torque. The only way to increase the RWHP would be lesson the loss between the motor and rear wheels.
++++++++++++++++++++++++++++++++++++++++ +++++++++++++++++
No the OP means RWHP. HP is the time rate of work and it is HP that causes acceleration. Almost all engines have their torque peak lower in rpm than the HP peak. If you could have a transmission whose input/output ratio was continuosly variable, for maximum acceleration you would choose the xmsn ratio curve that would allow the engine to operate at its peak HP and NOT the peak torque. At peak HP, you would provide the highest torque at the rear wheels.
For the highest acceleration in real cars that have fixed stepped ratio's in their transmissions, you choose a differential ratio that allows the engine to operate with the highest average HP (area under the HP/RPM curve) AND with the consideration that you limit the top speed of the vehicle if the differential ratio is too high.
Safety Car
Joined: Sep 2000
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From: Hinckley Ohio
What can really maximize the horsepower in each gear is to adjust the transmission gear ratios...that is the whole idea of a close ratio gear box..that is to keep the engine near the horsepower peak..
As you noted the ultimate close ratio gear box would be a CVT..