when to shift
Thread Starter
2nd Gear Member
Joined: Oct 2006
Posts: 311
From:
when to shift
now that my T5 is installed I am planning on taking the car to the strip once I break in the clutch. How high can I bring the RPMs while still being safe; I do not have forged internals[:@]. also with my mods where is the car gonna make optimum power.. off the top of my head I think the cam is for between like 1500-6000
5th Gear Member
Joined: Apr 2007
Posts: 2,100
From: Texas Hill Country
RE: when to shift
Your are safe upto about 3500fpm piston speed with stock stuff (4000fpm with good aftermarket stuff and 5000fpm with race quality motor). The max rpm while staying below 3500fpm piston speed is relative to your stroke, so that puts you somewhere around 7000rpm max with a 3.00" stroke. This is assuming you do not have valve trane problems. Valve flow is always in issue to contend with at high rpms.
Now when to shift will depend on traction, gearing, horsepower, etc. A general rule is 500-700rpm past peak horsepower. That insures that you stay in the power band in the upper gears (i.e. you don't shift until your power in that gear is less than the power at the speed in the higher gear). Confusing I know, so the best way to figure it out is to chart your horsepower by rpm or use a sim program. Drag race guys spend a lot of test and tune time figuring out when to shift in what gear for peak performance. A question you did not ask is what rpm to launch in. That will depend on your traction, but basically as high as possible without boiling the tires when you side step the clutch.
I have a drag sim that will give you some general ideas about shift points and launch point, but I would need your hp curve, tranny gearing, rearend gearing, and tire traction info to get good data.
Now when to shift will depend on traction, gearing, horsepower, etc. A general rule is 500-700rpm past peak horsepower. That insures that you stay in the power band in the upper gears (i.e. you don't shift until your power in that gear is less than the power at the speed in the higher gear). Confusing I know, so the best way to figure it out is to chart your horsepower by rpm or use a sim program. Drag race guys spend a lot of test and tune time figuring out when to shift in what gear for peak performance. A question you did not ask is what rpm to launch in. That will depend on your traction, but basically as high as possible without boiling the tires when you side step the clutch.
I have a drag sim that will give you some general ideas about shift points and launch point, but I would need your hp curve, tranny gearing, rearend gearing, and tire traction info to get good data.
5th Gear Member
Joined: Sep 2006
Posts: 2,266
From: Australia
RE: when to shift
urb where does peak torque come into the equation?
I've never considered the shift points in this way, so sorry for the stupid questions...but wouldn't 500 over peak hp be way past peak torque? I always thought you wanted to change so that you dropped back onto or slightly below peak torque?
if that were my engine with stock internals i'd be reluctant to go over about 6000. With your heads it will probably go pastthere (pending on cam)but that's putting exponential strain on the bottom end.
if you wanna know why, go down to an engine builder and pick up a stock piston compared to a lightened one then imagine that x8fying round at 6000+
I've never considered the shift points in this way, so sorry for the stupid questions...but wouldn't 500 over peak hp be way past peak torque? I always thought you wanted to change so that you dropped back onto or slightly below peak torque?
if that were my engine with stock internals i'd be reluctant to go over about 6000. With your heads it will probably go pastthere (pending on cam)but that's putting exponential strain on the bottom end.
if you wanna know why, go down to an engine builder and pick up a stock piston compared to a lightened one then imagine that x8fying round at 6000+
5th Gear Member
Joined: Apr 2007
Posts: 2,100
From: Texas Hill Country
RE: when to shift
FPM is feet per minute.
A car accelerates the hardest (i.e. acceleration is the highest value) with gearing selected to stay as close as possible to peak power (i.e. horsepower). In other words you want to shift so that you stay as close to peak horsepower as possible through all the gears. You can determine the optimum shift points by graphing horsepower vs. velocity or transmissiontorque vs. rpm. Engine torque alone will not determine shift points.
Another view. If you shift so that your are centering around peak torque, yes the engine is making more torque but the transmission will have a less advantaguous gear ratio so you actually end up with a net loss of power planted to the ground.
Force(lb)=Power(hp)*374/velocity(mph)
This means the maximum force pushing the car is when the Power is the most it can be at a given velocity. Lets put some play numbers up (I got these from an exercise explaining the physics of shiftpoints).
RPM HP
1000 10
1500 19
2000 30
2500 44
3000 59
3500 76
4000 91
4500 107
5000 124
5500 139 (peak torque)
6000 149
6500 151 (peak power)
7000 147 (red line)
Let's say shifting between gears moves you 2500 rpm (not completely acurate but good for explaination).
So if you shift based on___________:
500 past peak Torque 500 past peak Power
149hp->76hp147->107
The net loss of shifting based on peak torque is 73hp. The net loss of shifting based on peak horsepower is 40hp. Which seems better to you? I want to lose the least amount of power I can by shifting to a higher gear because F=374*P/V. Yes torque is involved but only as a function of horsepower. You ask where does peak torque come into the equation? It does not except that peak torque is going to affect peak horsepower becaue P=T*RPM/5252.
If torque is what you really want to look at, take a look at this table from the same data as above.
Engine Transmission output torque (ft-lb):
Torque 1st 2nd 3rd 4th 5th
RPM (ft-lb) 3.54 2.13 1.36 1.03 0.72 <- gear ratio
---- ------- ---- ---- ---- ---- ----
1000 50 177 107 68 52 36
1500 65 230 138 88 67 47
2000 80 283 170 109 82 58
2500 92 326 196 125 95 66
3000 104 368 222 141 107 75
3500 114 404 243 155 117 82
4000 120 425 256 163 124 86
4500 125 443 266 170 129 90
5000 130 460 277 177 134 94
5500* 133 471 283 181 137 96
6000 130 460 277 177 134 94
6500* 122 432 260 166 126 88
7000 110 389 234 150 113 79
So if you shift from 1 to 2 based on___________:
500 past peak Torque 500 past peak Power
460tranny torque->243tranny torque389tranny torque->266tranny torque
That is a net loss of 217ft-lbs vs. 123ft-lbs. Again, I want to shift to minumize my torque lose at the tranny so I pick the second option.
So what does all this mean? You should shift so that you stay as close to peak horsepower as possilble. For my engine this endsup being redline or within a few hundred rpms of it. In some cases, you are better off by the numbers to shift higher than your engine will redline because of where peak power is and how much rpm you loss in shifting. In those cases, you just shift at redline so you don't blow anything up. For some engines, peak power make be low enough in the rpm band to shift before redline.
A car accelerates the hardest (i.e. acceleration is the highest value) with gearing selected to stay as close as possible to peak power (i.e. horsepower). In other words you want to shift so that you stay as close to peak horsepower as possible through all the gears. You can determine the optimum shift points by graphing horsepower vs. velocity or transmissiontorque vs. rpm. Engine torque alone will not determine shift points.
Another view. If you shift so that your are centering around peak torque, yes the engine is making more torque but the transmission will have a less advantaguous gear ratio so you actually end up with a net loss of power planted to the ground.
Force(lb)=Power(hp)*374/velocity(mph)
This means the maximum force pushing the car is when the Power is the most it can be at a given velocity. Lets put some play numbers up (I got these from an exercise explaining the physics of shiftpoints).
RPM HP
1000 10
1500 19
2000 30
2500 44
3000 59
3500 76
4000 91
4500 107
5000 124
5500 139 (peak torque)
6000 149
6500 151 (peak power)
7000 147 (red line)
Let's say shifting between gears moves you 2500 rpm (not completely acurate but good for explaination).
So if you shift based on___________:
500 past peak Torque 500 past peak Power
149hp->76hp147->107
The net loss of shifting based on peak torque is 73hp. The net loss of shifting based on peak horsepower is 40hp. Which seems better to you? I want to lose the least amount of power I can by shifting to a higher gear because F=374*P/V. Yes torque is involved but only as a function of horsepower. You ask where does peak torque come into the equation? It does not except that peak torque is going to affect peak horsepower becaue P=T*RPM/5252.
If torque is what you really want to look at, take a look at this table from the same data as above.
Engine Transmission output torque (ft-lb):
Torque 1st 2nd 3rd 4th 5th
RPM (ft-lb) 3.54 2.13 1.36 1.03 0.72 <- gear ratio
---- ------- ---- ---- ---- ---- ----
1000 50 177 107 68 52 36
1500 65 230 138 88 67 47
2000 80 283 170 109 82 58
2500 92 326 196 125 95 66
3000 104 368 222 141 107 75
3500 114 404 243 155 117 82
4000 120 425 256 163 124 86
4500 125 443 266 170 129 90
5000 130 460 277 177 134 94
5500* 133 471 283 181 137 96
6000 130 460 277 177 134 94
6500* 122 432 260 166 126 88
7000 110 389 234 150 113 79
So if you shift from 1 to 2 based on___________:
500 past peak Torque 500 past peak Power
460tranny torque->243tranny torque389tranny torque->266tranny torque
That is a net loss of 217ft-lbs vs. 123ft-lbs. Again, I want to shift to minumize my torque lose at the tranny so I pick the second option.
So what does all this mean? You should shift so that you stay as close to peak horsepower as possilble. For my engine this endsup being redline or within a few hundred rpms of it. In some cases, you are better off by the numbers to shift higher than your engine will redline because of where peak power is and how much rpm you loss in shifting. In those cases, you just shift at redline so you don't blow anything up. For some engines, peak power make be low enough in the rpm band to shift before redline.
5th Gear Member
Joined: Apr 2007
Posts: 2,100
From: Texas Hill Country
RE: when to shift
Sorry for the criptic answer. This is not an easy thing to explain. If youwant, I can post my power and torque numbers from the engine I am building or even a stock type 302 and show you shift points on that data.
6th Gear Member
Joined: Apr 2007
Posts: 10,468
From: California
RE: when to shift
The OP's setup would prolly have the best shift point at like 6,200-6,500 range.
And yeah, FPM = feet per minute = the way inertia is calculated. RPM effects piston speed, higher speed means higher inertial loading on the piston/pin/rod/rod bolts/cap when the piston changes direction. RPM limits are different for every engine, based on stroke length and materials used(strength, weight etc).
And as far as shifting, remember again that horsepower is a way of measuring torque, peak hp occurs when there is less torque than peak(usually) but at a higher rpm. The higher rpm allows you to either run more gearing(multiply the torque back to the wheels) or to stay in any transmission gear longer. The net result becomes that a lower transmission gear at higher rpm with less torque(peak hp) is producing MORE torque at the WHEELS that a higher transmission gear at lower rpm with more torque. Gear multiplication is a HUGE factor in performance. If the gears are wrong you either bog or can't use the powerband effectively, or you end up taching out and shifting too quickly or having traction issues or taching out before you get to the finish line. Remember that peak torque in 2nd gear is putting less torque to the wheels than peak hp(less ENGINE torque) is in first gear.
Back to the OP's engine though, what rods/pistons/rod bolts/valve springs do you have? That's going to determine what rpm the engine will safely take.
And yeah, FPM = feet per minute = the way inertia is calculated. RPM effects piston speed, higher speed means higher inertial loading on the piston/pin/rod/rod bolts/cap when the piston changes direction. RPM limits are different for every engine, based on stroke length and materials used(strength, weight etc).
And as far as shifting, remember again that horsepower is a way of measuring torque, peak hp occurs when there is less torque than peak(usually) but at a higher rpm. The higher rpm allows you to either run more gearing(multiply the torque back to the wheels) or to stay in any transmission gear longer. The net result becomes that a lower transmission gear at higher rpm with less torque(peak hp) is producing MORE torque at the WHEELS that a higher transmission gear at lower rpm with more torque. Gear multiplication is a HUGE factor in performance. If the gears are wrong you either bog or can't use the powerband effectively, or you end up taching out and shifting too quickly or having traction issues or taching out before you get to the finish line. Remember that peak torque in 2nd gear is putting less torque to the wheels than peak hp(less ENGINE torque) is in first gear.
Back to the OP's engine though, what rods/pistons/rod bolts/valve springs do you have? That's going to determine what rpm the engine will safely take.
5th Gear Member
Joined: Apr 2007
Posts: 2,100
From: Texas Hill Country
RE: when to shift
Just ran the numbers on my car and got a little conflicting data but it is very close:
Numbers based on engine sim
Peak Torque is 467ft-lb@4500
Peak Horsepower is 463@6000
Shiftto minimize horsepower loss I should shift at 6500.
Shiftto minimize transmission torque loss: 1-2 shift at 7000, 2-3 shift at 7000, 3-4 shift at 7000.
Ironicaly, the redline of the motor I am building is between 6800-7200, and I should shift bewteen 6500 and 7000to maintain peak accerlation. Sounds about right to me.
Numbers based on engine sim
Peak Torque is 467ft-lb@4500
Peak Horsepower is 463@6000
Shiftto minimize horsepower loss I should shift at 6500.
Shiftto minimize transmission torque loss: 1-2 shift at 7000, 2-3 shift at 7000, 3-4 shift at 7000.
Ironicaly, the redline of the motor I am building is between 6800-7200, and I should shift bewteen 6500 and 7000to maintain peak accerlation. Sounds about right to me.
Thread
Thread Starter
Forum
Replies
Last Post