Z06Vette.com - Corvette Z06 Forum

JACK FRISSINA · 01-28-2003, 12:52 PM

For getting maxium power and lowest et. i shift at

**FRC Tom** · 01-28-2003, 02:35 PM

Moving over to racing Jack.
I hope to see you Thursday nite.
Bradenton has recently cracked down on the helmets and I am going to try and borrow one from Jake or Brian.

To answer your poll, none of the above.
I speed shift 1-2 and power shift 2-3 and 3-4 but at 6000 rpms as that is were my hp peak is. My problem is the launch, crappy 60' times.

JACK FRISSINA · 01-28-2003, 03:02 PM

Tom Your time slip looks like mine , last thurs the track was going away in 3rd gear ,5 runs later i packed it in ,30 mi a hr winds blowing sand , they also warned me about a helmet in the last run . only car there with a better time was a pontiac turbo pace car ,he was mid 12 s also but he had slicks .im going to wait till sat ,the track is to cold at nite right now ,plus all the rice burners drag all the water up , good luck thurs if you go ,you might consider sat . see ya

Hye06 · 01-28-2003, 05:03 PM

Whats the Highest RPM according to the Stock Tach that someone Speed shifts/power shifts on the
1-2 shift?
2-3shift?
3-4shift?

I Grannyshift through all gears.
1-2--->6200
2-3--> 6300
3-4--->6500

These Numbers are "according to the tach" Shift the 1-2 earlier since the tach lags.

McRat · 01-28-2003, 06:10 PM

Shift right before the limiter. There is more HP at 6500 than there is at 4500, which is where you drop to on all M12 redline shifts.

McRat · 01-28-2003, 06:24 PM

OPPSS!! I don't powershift. .1 second isn't worth it.

Your car does not accelerate with the clutch in. Having the clutch in is what slows you down. Powershifting allows the engine to pick up RPM (inertia) while the clutch is in and transfers it when you release the clutch.

If you could shift infinitely fast, powershifting would have zero effect. The slower you shift, the more gain you get from powershifting. Get your shifting speed lightning fast first, then powershift to get the 0.1 second from the captured inertia. Or not.

Speed shifting takes alot of practice though. You must time the clutch and the gas, and the shift perfectly or you miss the gear.

With powershifting, you just pull/push hard on the lever, and stab the clutch to the floor. It is easier to do, but harder on the forks and the synchro's.

pat · 01-28-2003, 06:29 PM

FRC Tom,

For someone new to Z06's (but not new to speed) and soon to be racing the 1/4 mile, can you explain what you mean by "power Shifting" and "speed shifting" ???

Thanks for the help,

Pat

Ranger · 01-28-2003, 07:14 PM

Last time I posted this reply to this question, I was attacked by a member for being untruthful. But here goes again with what I do. Not saying it will work for everyone....

I shift at the highest possible rpm without hitting the limiter. For the first two shifts, I do this by feel rather than watching the tac. I estimate that I execute most of those shifts at 6450-6550. For the shift to 4th I usually glance at the tac and try to start the shift at an indicated 6500.

I began doing this when my 02 was running in the 11.9s and find that higher shift points are rewarded by lower times. When I shift at lower rpms, it generally costs a couple tenths. When I get the shifts just right, the car makes its best times. A shift light set at 6400 would probably improve my average time each day at the track. I purchased a very small one late last year but haven't installed it.

I have found that nudging the limiter costs me well less that a tenth and banging the limiter hard costs "only" 1 or 2 tenths if the recovery is fast.

Making this approach work well requires very fast leg speed and well coordinated movements of the right hand and clutch foot. I practice this alot, including in the staging lanes between runs.

I normally power-shift the 3-4, but only occasionally the 1-2 or 2-3. Experience with BFGs DRs has taught me that powershifts can induce wheel spin on all but very well-prepped tracks, and spinning the tires on the shifts usually wastes time. A speed-shift with slight easing of the throttle can be faster if done quickly and without inducing much wheel-spin.

Ranger

McRat · 01-28-2003, 07:26 PM

Quote:

Originally posted by pat
FRC Tom,

For someone new to Z06's (but not new to speed) and soon to be racing the 1/4 mile, can you explain what you mean by "power Shifting" and "speed shifting" ???

Thanks for the help,

Pat

Speed shifting involves lifting the throttle pedal during a shift, but still stabbing the clutch to the floor as quick as is humanly possible while changing gears.

Powershifting? Known also as "flat-footing", you bury the throttle pedal at the launch, and NEVER lift it. You shift by pulling on the lever prior to shifting, then stabbing the clutch to the floor.

With powershifting, the engine is under load (reving) when you shift, so it spins the syncro's faster (like slipping a clutch) than if you unload the engine. Also, because you need to pull harder than a speed shift, the shifter forks get more side wear, and can actually bend.

You still have to "preload" the shifter to do a speedshift, but since there is less load, you don't have to pull as hard.

Believe it or not, you CANNOT pull a car outta gear at WOT with the clutch engaged. You'd break the shift fork or linkage before you could apply enough force on a non-modified trans.

**FRC Tom** · 01-28-2003, 08:27 PM

Quote:

Originally posted by pat
FRC Tom,

For someone new to Z06's (but not new to speed) and soon to be racing the 1/4 mile, can you explain what you mean by "power Shifting" and "speed shifting" ???

Thanks for the help,

Pat

Glad to see Ranger and McRat jump in here, you can't ask for any better.
Thanks guys.

RocketSled · 01-28-2003, 09:16 PM

Velocity varies as a function of acceleration and time. Distance as acceleration and the *square* of time.

The car is not accelerating when it is not in gear. It is not in gear when you are shifting. More time shifting equals less time accelerating. Less time accelerating means lower velocity and distance.

The less time spent shifting, the more time spent accelerating, and the faster and further you go.

Minimizing shift time is critical to achieving fast ETs.

Granny Shifting (GS) typically takes the most time, and is therefore the least efficient method available. And because you lifted throttle on that GS, once you *are* back in gear, you need to spool-up the intake and exaust gas flow rates again before the motor starts making decent power and the car accelerates efficiently. Your slow shifts cost you in the ET, they maximize neither the time spent accelerating, or the acceleration you get immediately after each Shift is completed.

The big advantage of a GS is that it puts the least stress on the driveline.

Speed Shifting (SS) is the next fastest way to shift. As pointed out above by McRat, SS is just a fast GS. You lift throttle to keep the motor from hitting the rev-limiter, but only for as long as it takes you to clutch and change gears. Done right, this the time interval can be fairly short (nearly as short as for Power Shifting). Airflow *is* disturbed by the closed throttle, but not as badly as for the slower GS. That faster shift has gone at least part of the way to maximizing the time under acceleration and the acceleration you get after the Shift is completed, and you *still* aren't abusing the driveline (at least, not too badly).

Power Shifting (PS) is the fastest way to shift. As defined by McRat, again, you keep your foot buried, preload the shifter, and stab the clutch. Clutch on-shift-off, bang, you're done. You might hit the rev limiter (depending on the RPM you shifted at), but the throttle stays open, so it's only the fuel and spark that gets cut. The shift time is as short as can be, and the motor stays "up on step" because the throttle never closes. You have maximized both the time spent accelerating, *and* the acceleration you get immediately after each shift is completed.

Unfortunately, you've also maximized the stresses applied to your driveline (and especially the gearbox). Accordingly, Power Shifting isn't the kind of thing you should expect to be able to do a lot of.

As to the question of *when* to shift, here's my theory...

The real answer is, it depends on the shape of the torque curve and the ratio between gears. So long as the engine is making more HP (which is a function of Torque and RPM) at the current RPM than it would make if shifted to the next gear (which causes RPM to drop to a different area of the torque curve), it's better to stay in the lower gear, since the acceleration is greater.

The LS6 torque curve is fairly flat pretty much right up to the rev-limiter. That means that it's probably better to stay in gear to the highest RPM you can before hitting the rev-limiter (and when you do shift, do it *fast*).

pat · 01-28-2003, 09:36 PM

FRC Tom, Rocketsled,

Thanks a ton for the great information on shifting. I'll try it when the snow disappears here in Michigan. To help me learn I just purchased a Gtech/Pro, I hope it will give me some good feedback on my shifts. Since I have never run the quarter on a track I want to get some practice in before hand.

On another note, I do own a 125 shifter racing go cart. With the right gearing it will do 0-60 in about 2.5 seconds. Maybe I'll post a thread about it some one of these days.

Thanks again,

Pat

WA 2 FST · 01-30-2003, 02:23 PM

Quote:

Originally posted by RocketSled

As to the question of *when* to shift, here's my theory...

The real answer is, it depends on the shape of the torque curve and the ratio between gears. So long as the engine is making more HP (which is a function of Torque and RPM) at the current RPM than it would make if shifted to the next gear (which causes RPM to drop to a different area of the torque curve), it's better to stay in the lower gear, since the acceleration is greater.

The LS6 torque curve is fairly flat pretty much right up to the rev-limiter. That means that it's probably better to stay in gear to the highest RPM you can before hitting the rev-limiter (and when you do shift, do it *fast*).

This is right on the money. Shifting AT peak HP is wrong. You will go slower this way and have a lower trap speed b/c you are not maximizing the HP under the curve.

Take a look at some of the times that some of these guys with centrifugal blowers are running. They do not line up with the trap speeds they pull. Why? Because they are making peak HP at redline (where they have to shift). This is not the most efficient power curve.

You're always going for HP under the curve (not TQ) when you're trying to accelerate fast.

That being said, as Powershifter and others have correctly pointed out... on a stock, near stock Z06, you should be shifting right up near the rev-limiter.

vortech · 01-30-2003, 02:46 PM

I shift at 6200 using a autometer min-light with a 6200 rpm pill. I never powershift or even try, I've tried different rpm pills but 6200 seems to be the best and most repeatable.

zul8r · 02-02-2003, 08:41 PM

What are you guys starting out with at the light on the stock tires? I get so much wheel hop and spin at 3000rpm's, much less 6000.
sg