Help me understand gear ratios

Daff_Duck · 4 Dec 2017 at 11:51

I drive a 200bhp German diesel car
Having driven a few different German brands the one thing that irritates me is the close gear ratio. What I mean is you hardly move before you are to put in 2nd and again in 3rd before you have even hit 30 you are promoted to be In 4th

How the heck are you to accelerate to what is ( yes I know open to interpretation) 0-60 if sub 8 s? Is it unique to German diesels? I have seen transits shift far quicker than my car ever will in fact a 320D blew me out I’d the water only yesterday

So is it that I drive like a granny, or this is similar scenario for many diesel drivers, on which case what is the point in having 200bhp when 140 will do

Howard · 4 Dec 2017 at 11:54

Driving for economy (what the car promotes) and for maximum acceleration require completely different driving styles. One requires you to get into as high a gear as possible without labouring the engine, and the other requires you to hoon it like a loon.

sovietspybob · 4 Dec 2017 at 12:00

Diesels tend to have very narrow power bands so you do have to 'row through the gears' in them compared to a petrol car, i find it very frustrating driving our works van (VW Caddy) as i feel like i'm forever changing gear due to the tiny power band.

[TW]Fox · 4 Dec 2017 at 12:09

Double post

Liquidfox · 4 Dec 2017 at 12:55

arfoll said:
There's some good videos on engineering explained YT's channel that explain optimal shift points but on most cars that's very close to the redline.

On most naturally aspirated petrol engines it likely is, but on a turbocharged engine it's most likely not, even moreso on a diesel engine.

Alibaba99 · 4 Dec 2017 at 12:59

Are you just following the gear change indicator on the dash and not driving to the conditions. That is primarily there for the emissions testing, as without it, the gear changes are carried out at prescribed points taken from the test manual which may not be optimum for that car to achieve best possible figures.

Joe T · 4 Dec 2017 at 13:00

Indeed, do not redline a diesel for optimum acceleration!

Alibaba99 · 4 Dec 2017 at 13:01

Liquidfox said:
On most naturally aspirated petrol engines it likely is, but on a turbocharged engine it's most likely not, even moreso on a diesel engine.

If you are talking max performance then the best change point is just before the limiter. Very, very few cars have more tractive effort in a higher gear than can be achieved at peak rpm in a lower gear.

Joe T · 4 Dec 2017 at 13:07

Alibaba99 said:
If you are talking max performance then the best change point is just before the limiter. Very, very few cars have more tractive effort in a higher gear than can be achieved at peak rpm in a lower gear.

If you're quoting EE then his calcs were not based on a turbo-charged diesel. They have very different torque characteristics.

Joe T · 4 Dec 2017 at 13:17

arfoll, are you saying the 275nm is the torque at max bhp? If so, that's not at redline is it?

Nasher · 4 Dec 2017 at 14:08

Short gear ratios are normally for better performance on track. They are harder to get a smooth shift from though as you need to be more accurate with the changes. Try a BRZ, they have very short gears for a road car and 2nd barely reaches 60mph, you need 3rd to hit 62.

In most cars you won't want to go right to the red line as they drop off before that, a diesel will drop off well before that.

theone8181 · 4 Dec 2017 at 14:28

Am I right in thinking that optimum is when the torque drops off?

Howard · 4 Dec 2017 at 14:32

Once torque and power start dropping off, you may as well change gear.

I know mine redlines at about 7k but you can feel the power dropping off much past about 5750.

Edit: Here's a power/torque graph from Superchips.

http://www.mybluefin.co.uk/curves/FordEcoboost2.0203ps.pdf

My RR graph looks basically the same. No point revving past 6k. I've tried both scenarios - in auto mode, my gearbox will rev right to the redline before changing gear, but if I use it in manual mode and shift a bit before, there's less of a momentary reduction in acceleration. If I power up to about 5.5k in 2nd and change to 3rd whilst keeping my foot down, it just keeps going.

Alibaba99 · 4 Dec 2017 at 15:12

Howard said:
Once torque and power start dropping off, you may as well change gear.

I know mine redlines at about 7k but you can feel the power dropping off much past about 5750.

Edit: Here's a power/torque graph from Superchips.

http://www.mybluefin.co.uk/curves/FordEcoboost2.0203ps.pdf

My RR graph looks basically the same. No point revving past 6k. I've tried both scenarios - in auto mode, my gearbox will rev right to the redline before changing gear, but if I use it in manual mode and shift a bit before, there's less of a momentary reduction in acceleration. If I power up to about 5.5k in 2nd and change to 3rd whilst keeping my foot down, it just keeps going.

Howard, you need the graphs of tractive effort in each gear to make the gearchange decision. If the tractive effort at the redline in 2nd is greater than the tractive effort at the rpm you drop down to when you change gear you will be faster in the lower gear. For most cars the minimum tractive effort in a lower gear is greater than the max tractive effort in the next ratio up, therefore you will be quicker by maximising the use of the lower gear.

Howard · 4 Dec 2017 at 15:21

Alibaba99 said:
Howard, you need the graphs of tractive effort in each gear to make the gearchange decision. If the tractive effort at the redline in 2nd is greater than the tractive effort at the rpm you drop down to when you change gear you will be faster in the lower gear. For most cars the minimum tractive effort in a lower gear is greater than the max tractive effort in the next ratio up, therefore you will be quicker by maximising the use of the lower gear.

I see what you're saying, but the way I see it is my engine is producing max torque over quite a lot of the power band, and the turbo will be producing maximum boost as I change, so is wringing it out for the extra 500-1000rpm whilst actually being able to feel the decline in torque output through the "butt dyno" really going to help? Can it be worked out if I knew the gear ratios? I'm sure I've seen them somewhere before.

Interesting discussion though, and I'm happy to be educated.

sovietspybob · 4 Dec 2017 at 16:19

My understanding is even though you're past peak torque of for example 400nm at 5000rpm, it'd be worth still going 'till red line at say 300nm at 6500rpm as you'll still be making more torque at the wheels due to being in a lower gear. It does all depend on the actual torque curve and gearing of a specific car.