When to change gear.....

[James FR]

On petrols its easy - you just red line 'em

but..

Whats the best revs to change gear on a standard 1.9 FR 2005 to get the best acceleration?

 

[Nath.]

At an educated guess I would say 4k revs.

However I disagree with the red line thing on petrols, the power on a petrol drops off before the red line.

 

[ATP]

i normally change about 3200 unless im really going for it... then 4500

 

[muddyboots]

On a standard PD130 I'd say not much over 4k.
Remapped engines tend to hold on to the power for a bit longer up the rev range, I usually take mine up to 4400/4500-odd if I want to get the most out of it. Power may tail off a little from peak, but you've got the advantage of holding a lower gear (and therefore less gearing loss) for longer.

 

[Fl@pper]

However I disagree with the red line thing on petrols, the power on a petrol drops off before the red line.

not on all of em

 

[techathy]

On petrols its easy - you just red line 'em

Red herring, some cars it's well past the red line others it's well before, depends on the engine.

Whats the best revs to change gear on a standard 1.9 FR 2005 to get the best acceleration?

Get it on a rolling road & thrust graphs done in each gear, where they cross you change up It'll be different for each gear unless you're hitting the limiter before the trust drops off.

 

[uzi_cupra]

i normally change about 3200 unless im really going for it... then 4500

normally in these engines after 4000 rpm you are infact in negative power, i.e you have passed your optimum bhp and now you are loosing power so i'd definately not shift at 4500 rpm on the cupra tdi.

on a torque/bhp graph for my pd160 cupra max bhp is reached at 3950 rpm, so id say its roughly the same for ur 1.9 fr, but get it on a RR to find your optimum for each gear (as techathy says). change just before 4k (short shifting first gear to eliminate wheelspin). however on a more economical drive change at 2k rpm, just before boost and gt 50+MPG...ahhh the pleasures of having a deisel

 

[techathy]

normally in these engines after 4000 rpm you are infact in negative power, i.e you have passed your optimum bhp and now you are loosing power so i'd definately not shift at 4500 rpm on the cupra tdi.

However changing up at peek power isn't the fastest thing to do, it'll be somewhere passed peek power. If your turbo is hitting a flow brick wall at say 180bhp, until the turbo starts getting inefficient it's probably worth holding onto the gear well past peek power. The Panda for instance is worth taking through to the limiter in the lower 4 gears & in 5th it's still fairly close to the limiter, the reason is it's only dropped 9bhp over the 1200 rpm after peek power.

 

[uzi_cupra]

However changing up at peek power isn't the fastest thing to do, it'll be somewhere passed peek power. If your turbo is hitting a flow brick wall at say 180bhp, until the turbo starts getting inefficient it's probably worth holding onto the gear well past peek power. The Panda for instance is worth taking through to the limiter in the lower 4 gears & in 5th it's still fairly close to the limiter, the reason is it's only dropped 9bhp over the 1200 rpm after peek power.

fair point, and this is true for higher gears, i.e 4th onwards. however doing this in earlier gears is doing no good to the engine. worth taking into consideration on when flow would be most restricted, if you change before peak power you're not causing any damage and the next gear can flow from where youve left with higher torque. however when you pass peak power your restricting continuous flow...hence why it would choke, what your doing is making the turbo work up all over again, if that makes sense...

 

[techathy]

Where & why is the damage happening? No one has ever mentioned this issue to me, the engine is good for those rpm, the turbo isn't running to fast it's just not trimmed to be efficient at those speeds, nothings getting out of control thermally. This is the way most engines are set up from the factory, if there was a problem pushing the engine to 7000rpm, Fiat state in my hand book "happy at 7000 rpm" (my engine builders set the engine up for a 7700 rpm limiter) & if taking the engine beyond 4000 rpm was an issue Seat would have set the red line at 4000 not 4700.

 

[steerinfromrear]

id have to agree and say change of gear just before 4k on the revs, hits a dead spot afterwards and no real gain of speed is noticed (in my car anyway)

 

[muddyboots]

However changing up at peak power isn't the fastest thing to do, it'll be somewhere passed peak power.

I agree, as you've also got to take gearing into account, as well as just engine figures.
Each successive higher gear knocks another percentage off what's transmitted from engine to wheels.

Once past the peak, power may start to drop, but you're still in a lower gear which will help - so there's a tradeoff to work out. You might change up so the engine is developing more power again, but the power at wheels could be less due to the higher gearing effect.
What you need are graphs of power at the wheels for each gear (ie taking into account the different gear ratios), so you can work out the optimum shift points for each gearchange

 

[Adam R]

Get it on a rolling road & thrust graphs done in each gear, where they cross you change up It'll be different for each gear unless you're hitting the limiter before the trust drops off.

im sorry but thats rubbish!

you want to change gear after where the curves meet. this way when you change in to the next gear your higher up the power band climbing to the 'meet' again. this way you hold the best boost / power and torque.

 

[st10587]

normal driving around 2000-2500rpm for economy, spirited driving 4000rpm, track 4500 when pulling hard out of the corners.

 

[muddyboots]

im sorry but thats rubbish!

you want to change gear after where the curves meet. this way when you change in to the next gear your higher up the power band climbing to the 'meet' again. this way you hold the best boost / power and torque.

I'm inclined to agree with Techathy. I think... unless I'm misunderstanding what you're saying. (Not the easiest thing to explain in text).

Incidentally you won't "hold the best ... power and torque". One is mathematically related to the other, and peak torque on a TDI will probably be nowhere near the peak power region...most likely somewhere between 1800-2500 rpm (ish) depending on engine/map.

 

[techathy]

Adam R, basic physics in a very simple form acceleration = thrust / mass
You don't get better acceleration by not using the highest value of thrust end of story. This is why you have thrust plotted per gear to get it bang on the nose back working from a whp run is better & bhp is the worse short of blind gut feeling.

 

[Adam R]

right here is a piccy taken from a JKM RR graph on a dyno dynamics.

as you can see the 2 curves cross @ roughly 3160rpm.... this is NOT the place to change gear as your bhp will drop off once the engine has started spinning in the next gear. (say a 700 rpm drop.

if that was my car and i wanted to drive it as quickly as possible i would be taaking the car up to 3400 - 3500 rpm then changing gear, this means that on the rpm drop the engine will be producing near peak torque WITH a steady rise of BHP (roughly 2800 rpm)


what i was trying to explain is, you want to curve 'cross' to be in the middle of the power band through each gear change.

any better ?

 

[muddyboots]

I think you're misunderstanding what Techathy is trying to explain

He's not talking about where power/torque curves cross.

He's saying that what you would need to look at, is a plot of power at the wheels, in each gear. The change point would be where the power at the wheels in a particular gear starts to drop below what would be generated at the wheels in the next gear.

I think...

 

[mardon]

Bit off topic but wonder if all this is worked out on the new DSG auto box's that are coming out.

 

[techathy]

Yeah, you're looking at the wrong graph. This is a thrust (lbs) v's speed (mph) graph based on data from when the Panda was missing in the lower/mid range at WOT, anyway I've used the stock gearing show where the change points are.

This clearly shows that theoretically you should go to the limiter in 1st through 3rd, 4th you should change at about 98mph & 5th at 114mph.

I may have got a conversion factor wrong here but it's a systematic error & the reason for the differing curve shape is because I had to hand sample the power from a printed graph as OOo doesn't do XY graphs properly.