Timothy Stewart

At a recent AutoX I made the mistake of
upshifting rather than just staying in
2nd.

I was just looking in my User Manual
(Hi Ray!) and trying to get the basis
for a rule of thumb along the lines of
"if I can't be accelerating in 3rd for
at least 2 seconds (say), then its not
worth it to upshift".

The acceleration curve (time vs speed) and
the transmission diagram (speed vs rpm)in the
manual were counter to my experience. These
diagrams show upshifts much earlier than
I would have expected - about 4K rpm - based
on my AutoX times and butt-dyno.

Optimizing on performance, at what rpm
do you upshift?

tim

Robin 993DX

I would think based on optimal performance the shift point would be at maximum horse power output of the engine RPM.

993RS

Really depends on whether you want to accelerate fast or just normal driving. On just normal driving, around 3500-4000 rpm. When accelerating fast, above 6000 rpm. (my max hp is around 6250 rpm)

Tom W

Tim,

Are you refering to the Candlestick Autocross? Once I started to shift to 3rd on that straight (at 5500 rpm) and had the guts not to lift for the kink, my times dropped from 43 to 41 seconds pretty quickly. (Somehow I didn't get credit for my fastest time of 40.7 ).

No dyno results (other than butt dyno) but my car seems quickest if I shift at 5000-5500. I'm betting the LWF and altered gear ratios play a role here.

Speedraser

I know there is more info elsewhere, but the optimum shift points for acceleration depend both on the power/torque figures while you're in a given gear and on the power/torque at the revs the engine will see when you shift into the next gear. Although acceleration should fall off after peak power (6100 rpm for my '95, I think), it is often faster to delay the upshift so that the engine is turning faster, and therefore produces more power/torque, after the upshift.

Flying Finn

Speedracer is totally right,

You should shift right before red line, (even though your over the maximum hp) because after the shift, on higher gear, your rpm's are higher (and closer to the maximum hp).

Look at any race car driver, they don't shift before red line either.

SundayDriver

[quote]Originally posted by Robin 993 DX In Atlanta:
<strong>I would think based on optimal performance the shift point would be at maximum horse power output of the engine RPM.</strong><hr></blockquote>

Actually you want to bracket the max HP, but you have the right idea. If you envision the HP curve, is is an inverted J. Your shift should be such that you move straight across the curve and you 'jump' to the lower RPM with the higher gear.

If the vehicle is engineered right, then the power curve is matched with transission ratio gaps and you usually want to shift at redline for max performance. If you want to get real nit-picky, you would need a power curve and tranmission chart to find the right shift point for each gear (they are not all going to be the same).

tom_993

This has been discussed before, here’s a thread:

<a href="http://forums.rennlist.com/forums/ultimatebb.php?ubb=get_topic&f=3&t=002802" target="_blank">http://forums.rennlist.com/forums/ultimatebb.php?ubb=get_topic&f=3&t=002802</a>

When to upshift for maximum acceleration is one thing, but I think Tim was also kind of asking if he should upshift if he’s only going to be in the higher gear for a short time: "if I can't be accelerating in 3rd for at least 2 seconds (say), then its not worth it to upshift." I seem to remember a thread on this recently, but I searched and searched and couldn’t find it. If I recall, the consensus was that holding the engine at redline is tough on the engine, but so is a missed downshift. Perhaps someone can come up with the correct search term and find the old thread?

STLPCA

[quote]Originally posted by tom_993:
<strong>I seem to remember a thread on this recently, but I searched and searched and couldn’t find it. If I recall, the consensus was that holding the engine at redline is tough on the engine, but so is a missed downshift. Perhaps someone can come up with the correct search term and find the old thread?</strong><hr></blockquote>

Tom
Is this it?

<a href="http://forums.rennlist.com/forums/ultimatebb.php?ubb=get_topic&f=3&t=003106" target="_blank">Redlining/revs </a>

tom_993

[quote]Originally posted by Dan 96C2 St.Louis:
<strong>
Tom
Is this it?
</strong><hr></blockquote>

Thanks, no, that doesn't look quite right. Perhaps it was on another board.

SundayDriver

[quote]Originally posted by tom_993:
<strong>This has been discussed before, here’s a thread:

<a href="http://forums.rennlist.com/forums/ultimatebb.php?ubb=get_topic&f=3&t=002802" target="_blank">http://forums.rennlist.com/forums/ultimatebb.php?ubb=get_topic&f=3&t=002802</a>

When to upshift for maximum acceleration is one thing, but I think Tim was also kind of asking if he should upshift if he’s only going to be in the higher gear for a short time: "if I can't be accelerating in 3rd for at least 2 seconds (say), then its not worth it to upshift." I seem to remember a thread on this recently, but I searched and searched and couldn’t find it. If I recall, the consensus was that holding the engine at redline is tough on the engine, but so is a missed downshift. Perhaps someone can come up with the correct search term and find the old thread?</strong><hr></blockquote>

The general racing view is that you lose 1 second for a shift up and down, so if you can't gain more than 1 second with the higher gear, don't upshift.

Timothy Stewart

So, it seems we all agree.

"The general racing view is that you lose 1 second for a shift up and down, so if you can't gain
more than 1 second with the higher gear, don't upshift. "

The above is the kind of info I was looking for.
Thanks!

Curious why the Owners Manual shows upshifts
far below concensus.

Tom W - Yep it was Candlestick. Other factors
involved, but staying in 2nd got me down from
41 to 39.

tim

Flying Finn

[quote]Originally posted by Mark D - TT Alumni:
<strong>...so if you can't gain more than 1 second with the higher gear, don't upshift...</strong><hr></blockquote>

But then you're hitting the redline for two seconds?!
Or was the original question so that he wanted to upshift, even though he wasn't at the redline yet?

SundayDriver

[quote]Originally posted by Flying Finn:
<strong>

But then you're hitting the redline for two seconds?!
Or was the original question so that he wanted to upshift, even though he wasn't at the redline yet?</strong><hr></blockquote>

Yes, exactly. If you can't gain more than a second with the upshift, then hang at the redline (below the rev limiter) and hold the lower gear.

ScottMellor

Hi Mark,

Actually you want to bracket the max HP, but you have the right idea. If you envision the HP curve, is is an inverted J. Your shift should be such that you move straight across the curve and you 'jump' to the lower RPM with the higher gear.

So if you have your handy dandy gear ratio chart, and can figure out the rev drop between gears, all you really need to know is where the top of the inverted J is.
Where do you think that would be on a modified TT?