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Old 03-13-2010, 01:01 PM
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Best ET - shift at max HP or max torque?

I have a 350 with 4-speed manual. When drag racing, where should you shift gears for best et time? Should it be at the engines max HP RPM range or max torque RPM range?
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Old 03-13-2010, 01:33 PM
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General rule of thumb for drag strip is shifting 400-500 rpm above your peak horsepower point so that the car "recovers" well from the rpm drop, but every car is different. Some with 3-speed autos like a slightly lower rpm shift to second, but a slightly higher rpm shift into high gear.

4-speed adds another couple of variables like different launch rpms and 1 more shift.

Run your car and test different points to see what works the best for your combo is the only true way to find the absolute best shift points,.... and it changes with the weather and track conditions, too.

Are we having fun yet??
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Old 03-13-2010, 03:48 PM
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Thanks for the info. From looking at the GM peformance crate motor website the lower end 350/290HP and 350/330HP have peak HP around 5,000 and 5,200 RPMs. Therefore, using your theory, I should shift around 5,400 RPM on the 350/290HP motor and at 5,700 RPMs on the 350/330 motor. However, I noticed on the specs on the website the max RPM rating on the 290HP was 5,100 RPMs and the 330HP was max RPMS rating of 5,500 RPMS. Its sounds like I may blow the motor in this range if that is what GM is calling the max RPM range. I'm not sure what the RPMs are for peak HP on my motor but I'm guessing it's in this ball park. I'll be having fun when I hit the dragstrip this Spring but guess I'll start off with keeping the RPMs under 5,100 RPMs when shifting between gears. Thanks again.
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Old 03-13-2010, 07:10 PM
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Quote:
Originally Posted by Hanks56Chevy
Thanks for the info. From looking at the GM peformance crate motor website the lower end 350/290HP and 350/330HP have peak HP around 5,000 and 5,200 RPMs. Therefore, using your theory, I should shift around 5,400 RPM on the 350/290HP motor and at 5,700 RPMs on the 350/330 motor. However, I noticed on the specs on the website the max RPM rating on the 290HP was 5,100 RPMs and the 330HP was max RPMS rating of 5,500 RPMS. Its sounds like I may blow the motor in this range if that is what GM is calling the max RPM range. I'm not sure what the RPMs are for peak HP on my motor but I'm guessing it's in this ball park. I'll be having fun when I hit the dragstrip this Spring but guess I'll start off with keeping the RPMs under 5,100 RPMs when shifting between gears. Thanks again.
In theory you get the Hp curve off a dyno, and work out where in each gear the horsepower is equal. In other words the Hp rises to the peak, then falls after the peak. That gives the MAXIMUM possible Hp during the whole time you are in that particular gear. Sometimes called horsepower under the curve.

The closer the gear ratios, the closer both your shift points will be either side of the Hp peak.

If the engine is not safe to run much beyond the Hp peak, then it is not much of an engine.
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Old 03-13-2010, 07:23 PM
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Quote:
Originally Posted by Silver Shadow
In theory you get the Hp curve off a dyno, and work out where in each gear the horsepower is equal. In other words the Hp rises to the peak, then falls after the peak. That gives the MAXIMUM possible Hp during the whole time you are in that particular gear. Sometimes called horsepower under the curve.

The closer the gear ratios, the closer both your shift points will be either side of the Hp peak.

If the engine is not safe to run much beyond the Hp peak, then it is not much of an engine.
Thanks for the explanation!
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Old 03-13-2010, 07:38 PM
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Quote:
Originally Posted by Hanks56Chevy
Thanks for the explanation!
Another thing to think about, is that the Hp curve for most high performance engines usually rises, then flattens out, then drops like a stone.

The equal horsepower points on the curve will not be equal distances either side of the horsepower peak. So the amount you need to rev it over the peak may only be a few hundred rpm.

If you have a dyno curve for your particular engine, and you know your gear ratios, here is some software to play with.

http://www.bgsoflex.com/shifter.html
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Old 03-14-2010, 01:09 PM
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I always shifter so the motor remained in the powerband at the end of the shift 3-500rpm over max Hp as stated , and went thru the traps at redline .
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Old 03-14-2010, 08:18 PM
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Quote:
Originally Posted by Silver Shadow
Another thing to think about, is that the Hp curve for most high performance engines usually rises, then flattens out, then drops like a stone.

The equal horsepower points on the curve will not be equal distances either side of the horsepower peak. So the amount you need to rev it over the peak may only be a few hundred rpm.

If you have a dyno curve for your particular engine, and you know your gear ratios, here is some software to play with.

http://www.bgsoflex.com/shifter.html
Thanks for the info and link. I know where to shift now, I just can't afford to miss a gear!
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Old 03-14-2010, 08:22 PM
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Quote:
Originally Posted by stimpy
I always shifter so the motor remained in the powerband at the end of the shift 3-500rpm over max Hp as stated , and went thru the traps at redline .
Thanks. you were dead-on with the 3-500rpm, according to the link " silver shadow" forwarded me.
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Old 03-14-2010, 08:22 PM
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Quote:
Originally Posted by Hanks56Chevy
I just can't afford to miss a gear!
An effective rev limiter may give you some peace of mind.
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Old 03-15-2010, 06:56 PM
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Quote:
Originally Posted by Silver Shadow
An effective rev limiter may give you some peace of mind.
Thanks, the limiter may prevent me from spending money on a new motor and help me be more relaxed at the track.
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