When to shift for Drag Racing?
Team Owner
Joined: May 2002
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From: San Diego , CA Double Yellow DirtBags 1985..Z51..6-speed
Originally Posted by JCD
I found that the LT1 is a pain becase the torque (to the ground) at the rev limiter in any gear is greater than the torque at the same speed in the next higher gear, so shifting before the rev limiter is losing time in the 1/4.
Your results may vary since you have the L98.
Your results may vary since you have the L98.
That's exactly right on shifting. I'm sure there's other factors to consider, but shift where the torque is the same coming out of 1st as it is going into 2nd, and etc. You can determine shift points with a dyno curve and all your gear ratios. 
Melting Slicks
Joined: Apr 2003
Posts: 2,150
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From: NS
Originally Posted by Nathan Plemons
Slamming into the rev limiter is also usually a pretty good indicator too 
. For instance my limiter is set to 6500 but it'll hit the rev limiter when it reads around 6000. In the other gears it will blow well past 6000 before it ever hits the limiter.
. For instance my limiter is set to 6500 but it'll hit the rev limiter when it reads around 6000. In the other gears it will blow well past 6000 before it ever hits the limiter. Yep, that's exactly the way my '95 is. From watching Mr. Mojo's video it looks like this is a common thing for our tachs.
Instructor
Joined: Apr 2005
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peak HP
you want to shift at the peak horsepower so when it goes into the next gear you are near peak torque. Torgue is good to get going horsepower is better at speed.
If you pull on the shifter it will pop out of gear at the max RPM. Just remember to let off the gas and put the clutch in so you don't blow the motor. Now you shift 200 RPMs earlier.
If your good you can pull the next gear with no clutch by pedaling the gas pedal. That way the clutch doesn't disengage and slip.
You can also interrupt the spark with an ignition switch. This is a cheap way to shift like a pro stock. A washing machine switch works good. They only kill the power for a milisecond. When the ignition stops for that split second the next gear meshes perfectly. So you are already in the next gear when normally you would still have the clutch in. It works just like the air shifters in pro stocks. You would need a short throw shifter.
If you pull on the shifter it will pop out of gear at the max RPM. Just remember to let off the gas and put the clutch in so you don't blow the motor. Now you shift 200 RPMs earlier.
If your good you can pull the next gear with no clutch by pedaling the gas pedal. That way the clutch doesn't disengage and slip.
You can also interrupt the spark with an ignition switch. This is a cheap way to shift like a pro stock. A washing machine switch works good. They only kill the power for a milisecond. When the ignition stops for that split second the next gear meshes perfectly. So you are already in the next gear when normally you would still have the clutch in. It works just like the air shifters in pro stocks. You would need a short throw shifter.
Last edited by JETSET; Jun 9, 2005 at 10:47 PM.
Elite Torch Red Member
Joined: Sep 1999
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From: Exit 89 GSP,Lakewood,NJ The Land Of Mojo
Cental/South NJ Events Coordinator
CI 1-2-3-4-5-6-7-8-9-10-11-12 Vet
CI-II Burnout & Drag Champ
St. Jude Donor '03-'04-'05-'06-'07
Originally Posted by JETSET
If your good you can pull the next gear with no clutch by pedaling the gas pedal. That way the clutch doesn't disengage and slip.
Drifting
Joined: Feb 2004
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From: Champaign, IL
St. Jude Donor '05
Originally Posted by Mr Mojo
I'm good, but I can't shift without the clutch in my vette, any other car I can, but not the vette, and yes I have tried and I don't recommend it to anyone.
Team Owner
Joined: May 2002
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From: San Diego , CA Double Yellow DirtBags 1985..Z51..6-speed
Originally Posted by ToyC4
I can do it but not at the strip or under full acceleration
I just bought some. Little brass pieces of crap. Sacrificial parts.
I was shifting without clutch, had no choice. Hard to get going from a stop without a clutch though.
Team Owner
Joined: Dec 2000
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From: SE NY
Cruise-In II Veteran
If you can get consistent passes at the track you can use the empiracle method of varying each shift point for best trap speed & ET.
The idea is to maximize the area under the power curve between each shift point. This means that you will shift somewhere past peak HP so that the RPM after the shift will give the most power, without going so far before the shift as to lose time. Remember that indeed a stock TPI falls off pretty sharply after peak power, so the shift point won't be far above the peak power.
Then of course there's the issue of accurately measuring RPM....
The idea is to maximize the area under the power curve between each shift point. This means that you will shift somewhere past peak HP so that the RPM after the shift will give the most power, without going so far before the shift as to lose time. Remember that indeed a stock TPI falls off pretty sharply after peak power, so the shift point won't be far above the peak power.
Then of course there's the issue of accurately measuring RPM....
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Safety Car
Joined: Sep 2000
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From: Hinckley Ohio
Originally Posted by GreyHairedTopGun
I have an 89 with a 6 speed. What is the optimum RPM to shift at for drag racing? Do I look at the torque curve and shift at the rpm for maximum torque or isn't it all that simple?
Think about it this way.........pick a speed........say 60 mph......then figure out what your rpm would be for 1st, 2nd , 3rd, & 4th gear.......in this example you would find you can't be in first gear because the rpm would be way above redline.....so now you have 2nd, 3rd, & 4th to choose from......figure the rpm for each gear & look at the horsepower for each of these rpms.......to get maximum acceleration you want to be in the gear that gives the highest horsepower......pretty simple...
Yes it is actually rear wheel torque that accelerates the car, but at any given speed you get the max rear wheel torque when the motor is at maximum horsepower...
Some fundamentals, motor horsepower equals wheel horsepower, but rear wheel torque and motor torque are related by the overall gear ratio.....example with 2:1 gearing the rear wheels go at half the speed of the motor but have 2X as much torque....so you cut the speed in half, double the torque, which means the horsepower is the same...
Anyhow the rule for shifting is be in the gear that gives you the maximum horsepower at every speed plus stay under redline....
The ideal transmission for drag racing would be a CVT ( Continously variable ratio), like in the Saturn, that puts the engine at its horspower peak and holds it there....
Pro
Joined: Mar 2005
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From: Walton KY
Originally Posted by JETSET
If your good you can pull the next gear with no clutch by pedaling the gas pedal. That way the clutch doesn't disengage and slip.
It can be done smoothly. When my slave cylinder blew I drove about 15 miles back to my shop with no clutch....little hard on the starter at red lights!!
I have done it a couple of times under acceleration. YOU MUST BE CAREFULE!!!!!!!I have a CBR929 I take to a road course pretty often. The cassette style transmission makes full throttle, clutchless shifts pretty easy. Just have to blip the throttle or hit the rev limiter.
Team Owner
Joined: Mar 2000
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From: Boston, MA
Originally Posted by JETSET
If your good you can pull the next gear with no clutch by pedaling the gas pedal. That way the clutch doesn't disengage and slip.
Racer
Joined: Sep 2003
Posts: 261
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From: Edwardsville, Illinois 88 stroked s10, 92 stock 6spd
i may be wrong- but i think i know what ski is trying to say. he waits for 6200 to show- fact in the lag time it takes his brain to get the signal to his hand/feet to shift- by the time he acually does shift- the car is at 6800 rpms- which is what he found works best for his car- to be shifted at 6800. what he is saying is- shift before you reach your desired shift point. kinda like leaving on the last yellow at the strip- don't wait for the green.
Team Owner
Joined: May 2002
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From: San Diego , CA Double Yellow DirtBags 1985..Z51..6-speed
You got that all right except this part... hp is a factor of engine speed, so it'll always be higher than torque above 5252, and lower than torque below 5252 rpm.
Originally Posted by LT4BUD
Yes it is actually rear wheel torque that accelerates the car, but at any given speed you get the max rear wheel torque when the motor is at maximum horsepower...
Team Owner
Joined: Dec 2000
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From: SE NY
Cruise-In II Veteran
Very good analogy; you have about 1/4-1/2 sec or so of reaction time from the shift light flash till the shifting action is completed.
When you combine human reaction time with the time it takes your car to roll out of the staging beams you have the reason that we can leave on the "3rd yellow" and not (usually) go red.
The shift light would need to be set up flash progressively closer to the target engine RPM with each gear change, as the engine is reving more slowly in each higher gear. So the 1-2 shift light flash would have the biggest offset from target RPM with the 3-4 shift having the shortest.
When you combine human reaction time with the time it takes your car to roll out of the staging beams you have the reason that we can leave on the "3rd yellow" and not (usually) go red.
The shift light would need to be set up flash progressively closer to the target engine RPM with each gear change, as the engine is reving more slowly in each higher gear. So the 1-2 shift light flash would have the biggest offset from target RPM with the 3-4 shift having the shortest.
Former Vendor
Joined: Jun 2001
Posts: 7,204
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From: St Marys PA
That is exactly what I was trying to convey....and your exactly right that each gear would need its own "setting" - which is why at the strip you see some guys with up to 3 different shift lights. 
THey know that 1st gear RPMs increase much quicker, hence you need more of a head start to hit the same end RPM.
For the average Joe, one light is enough and for the street - you wouldn't know the difference anyways if you were off a little. But when trying to be within a few thousands of a second on your dialin - you need all the help you can get.
With the DFI it has an extra output signal for a shift light, and I was going to employ it for exactly what we are now talking about.
THey know that 1st gear RPMs increase much quicker, hence you need more of a head start to hit the same end RPM.
For the average Joe, one light is enough and for the street - you wouldn't know the difference anyways if you were off a little. But when trying to be within a few thousands of a second on your dialin - you need all the help you can get.
With the DFI it has an extra output signal for a shift light, and I was going to employ it for exactly what we are now talking about.
Race Director
Joined: Dec 2002
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From: SCMR Rat Pack'r Charter Member..Great Bend KS
Originally Posted by Spankyellow
I cant speak for Ski but when I datalog my car, the lag in the tach is obvious. For instance, if I shift at 5200 on the dash, Datamaster will show the shiftpoint at 55 or 5600 RPM.
Thanks.
Larry
code5coupe
