Optimum gear change ?

[mark123]

OK so going on this link http://www.datsuns.com/Tech/whentoshift.htm. Based on my 172 cup when it had just a K-Tec stealth exhaust and no other mods.

Even shifting at 7250 RPM the torque at the wheels is higher in the lower gear than when you shift up.

So the optimum shift point is 7250 rpm just before you hit the limiter.

For those interested in the maths:

Here are my gear ratios calculated by noting speed and rpm in each gear. They are relative, I have made 1st gear 1.

1st 1.00
2nd 0.62
3rd 0.44
4th 0.34
5th 0.26

So to work out torque at the wheels at shift point in both gears you need the rpm in both gears at shift point. Then take the torque at each of those points and multiply by the gear ratio for the appropriate gear.

So for example:

1st to 2nd gear shifting at 6500 rpm.

1st gear rpm 6500 torque is 140 lbft
2nd gear rpm 4044 torque is 141 lbft

Multiply the torque by the appropriate gear ratio:
1st 140 x 1 = 140
2nd 141 x 0.62 = 87

At 6950 when the shift light comes on the torques are 141 and 90 for 1st and 2nd respectively.

So torque is much higher in first at the shift point but you're on the limiter so you don't have a choice.

Even best/worse case scenario 4th to 5th at 6500 rpm:

4th gear rpm 6500 torque is 141 lbft
5th gear rpm 5021 torque is 145 lbft

Multiply the torque by the appropriate gear ratio:
4th 141 x 0.34 = 48
5th 145 x 0.26 = 38

So in summary the torque does not drop off fast enough as you approach the limiter to make it worth changing up early. Modern multi valve engines normally maintain good torque all the way to the limiter.

 

[Jonny1*2]

I'll have to take your word for that one mark, way over my head. sounds good though

 

[mark123]

Also I should add that torque at wheel values are all relative and are not "true" torque values since they are multiplied by my relative gear ratios. They are only useful for comparing which is best.

 

[MarkCup]

There's no optimum that applies to all gears...at high speeds as drag comes into play so short shifting can be more effective plus the different spacing/drop in revs between each gear makes the change point for each shift vary.

I've found that in the lower gears (2nd + 3rd) it's often worth hanging on to a gear past peak power, as the ratio still gives as good or better acceleration as you'd get in the meat of the torque curve in the next gear up.

In 4th though forget it - after 6,500rpm it has no torque to push through the wind/drag and hits a brick wall. Much better shove to be had in 5th at those speeds despite the really loooooooooooong gearing.

So back to the OP - 0-100mph?

Go for the light in 1st (with no wheelspin unlike my poor effort below), again for the light in second, then just before the light in 3rd and you're done shortly after in 4th.

It's been up before and I know many have seen this...but for those that haven't here's a video of my 172 Cup going from tick-over to limiter in every gear. Watch the revs almost stop after 6,500 in 4th...it's gutless at those speeds

[YOUTUBE]odTwrUXNlsM[/YOUTUBE]

LOL at the gear-change indicator/light/whatever being the 'optimum'.

 

[mark123]

There's no optimum that applies to all gears...at high speeds as drag comes into play so short shifting can be more effective.

I've found that in the lower gears (2nd + 3rd) it's often worth hanging on to a gear past peak power, as the ratio still gives as good or better acceleration as you'd get in the meat of the torque curve in the next gear up.

In 4th though forget it - after 6,250-6,500 it has no torque to push through the wind/drag and hits a brick wall. Much better shove to be had in 5th at those speeds despite the really loooooooooooong gearing.

So back to the OP - 0-100mph?

Go for the light in 1st (with no wheelspin unlike my poor effort below), again for the light in second, then just before the light in 3rd and you're done shortly after in 4th.

It's been up before and I know many have seen this...but for those that haven't here's a video of my 172 Cup going from tick-over to limiter in every gear. Watch the revs almost stop after 6,500 in 4th...it's gutless at those speeds

[youtube]odTwrUXNlsM[/youtube]

LOL at the gear-change indicator/light/whatever being the 'optimum'.

That can be true but not in the case of a 172 as my calculations show.

 

[172Ben]

The revs are crazy as you say at 6500 in 4th.

Awesome tune

 

[MarkCup]

That can be true but not in the case of a 172 as my calculations show.

Calculate away all you want fella, but that video proves visually that past 6,500 rpm in 4th, the speedometer accelerates no faster than it does in 5th at the same speed.

 

[mark123]

Calculate away all you want fella, but that video proves visually that past 6,500 rpm in 4th, the speedometer accelerates no faster than it does in 5th at the same speed.

I just timed your vid:

4th 100 - 120 6 seconds
5th 100 - 120 8 seconds

So it is faster in 4th.

 

[172Ben]

Very scientific

 

[Jonny1*2]

where on earth did you do that mate? be careful putting up vids of illegal speeds.

 

[simon135]

Basic physics - maxium torque at the wheels creates greatest acceleration. This is what mark123 is calculating. The speed your doing at the time and wind resistance has sod all to do with this. The optimum point is all about the engines torque curve and the difference between one gear ratio and the next. The only place to short shift (ie short of the technical optimum point) is in lower gears, to avoid wheel spin - depending on what you're driving and levels of grip.

 

[Lee-182]

Why was you MPG showing you what you was doing instead of average.

 

[Jonny1*2]

if you recently reset the clock it does that Lee. Give it a try

 

[Tony Hunter]

Lol at you lot,who really gives a flying f..k.

 

[Henry]

:quiet:

 

[mark123]

This is mega important stuff!

The "change before the gear change indicator light" myth has been dispelled forever.

We can now happily rev the nuts off our 172/182s safe in the knowledge that it is the fastest way to drive it and changing gear at the optimum moment.

:rasp:

 

[Henry]

except in second:dapprove:

 

[Tony Hunter]

 

[Matt@Stealthsport]

Jesus you lot need to get out more who gives a f**k and also unless you've had it on a rolling road how do you know where peak power is?

 

[Mark One]

wait till it goes "raaan, daaan, daaan, daaan daaan" and then change.

Jesus you lot need to get out more who gives a f**k and also unless you've had it on a rolling road how do you know where peak power is?

full cycle.

 

[MonkeyKnifeFight]

i was waiting for you to hit the limiter in fifth

 

[D1CKY]

i was waiting for you to hit the limiter in fifth

Same here :S

 

[rich-6]

1st and 2nd & 3rd 7k after that your going fast enough

When theyre at 7k they rattle like fook though so it tends not to go up there very often lol