Choosing Gear ratios or gearboxes - Using dyno data?

We all know that the right gearbox can transform a car. But i've been thinking how you can translate a dyno torque curve to help you choose ratios. I know its to do with axle torque etc etc but what methods are used to calculate this using commonly available boxes and ratios.

I was recently reading another forum where they go on about a CTN(Mk3 TDI) gearbox on a G60. And how it transforms accelaration and cruising. For education purposes id like to see how data can be used. Which would also help decide what i'm doing regarding my gearbox.

In other words there must be a mathematical way of working all this out without actually swapping and changing things?

Example below.

CTN Ratios

1st	3.778
2nd	2.118
3rd	1.36
4th	1.029
5th	0.755
FD	3.158

Dyno plot of said G60.(Hope its ok to post)


Graph of CTN Gearbox including a shorter 5th from a CDA/DPA



More Data

Gearbox:- CTN 1.9L D
Tyre:- Michelin all patterns 205/50 x 15
-------------------------------------------------------------------------------------------
Top Gear gives 28.274 MPH/1000 RPM and a top speed of 175.298 MPH at 6200 RPM

Engine speeds in top gear:-
30 MPH = 1061 RPM 40 MPH = 1415 RPM 50 MPH = 1768 RPM 60 MPH = 2122 RPM
70 MPH = 2476 RPM 80 MPH = 2829 RPM 90 MPH = 3183 RPM 100 MPH = 3537 RPM

Top Speed in 1 gear = 35.061 MPH
And changes into 2 gear at 3475 RPM dropping 2725 RPM
Top Speed in 2 gear = 62.550 MPH
And changes into 3 gear at 3982 RPM dropping 2218 RPM
Top Speed in 3 gear = 97.393 MPH
And changes into 4 gear at 4428 RPM dropping 1772 RPM
Top Speed in 4 gear = 136.355 MPH
And changes into 5 gear at 4823 RPM dropping 1377 RPM
Top Speed in 5 gear = 175.298 MPH
************************************************** ****************************************
Gearbox:- CTN 1.9L D CDA 16v 5th
Tyre:- Michelin all patterns 205/50 x 15
-------------------------------------------------------------------------------------------
Top Gear gives 25.518 MPH/1000 RPM and a top speed of 158.212 MPH at 6200 RPM

Engine speeds in top gear:-
30 MPH = 1176 RPM 40 MPH = 1568 RPM 50 MPH = 1959 RPM 60 MPH = 2351 RPM
70 MPH = 2743 RPM 80 MPH = 3135 RPM 90 MPH = 3527 RPM 100 MPH = 3919 RPM

Top Speed in 1 gear = 35.061 MPH
And changes into 2 gear at 3475 RPM dropping 2725 RPM
Top Speed in 2 gear = 62.550 MPH
And changes into 3 gear at 3982 RPM dropping 2218 RPM
Top Speed in 3 gear = 97.393 MPH
And changes into 4 gear at 4428 RPM dropping 1772 RPM
Top Speed in 4 gear = 136.355 MPH
And changes into 5 gear at 5343 RPM dropping 857 RPM
Top Speed in 5 gear = 158.212 MPH

 

Nice thread idea, may help answer a lot of questions in future. Chance of fixing the images?

There must be a mathematical way of working it out, but I have only ever looked at it from the point of view of width of torque & power band + rev drops between gears, and the key distinction between n/a widths and forced induction widths.

Obviously the choice of ratios and indeed final drive are often constrained by what's available.

 

But there's question of what torque value (lb/ft) and rpm drop is acceptable for a given engine. Supercharged have a simlar shape to NASP cars (wider) just more of it. A turbo car tends to have more torque generally but more of a peak (narrower).

Some gears are probably easier than other like 5th as you can judge by your desired cruise rpm. Plus its a failry simple swop. Ie in mine it seems as if i could drop the rpms quite low at motorway speeds and still have overtaking abilty, but in real life it may not work too well.

Calculators are all very well but the dyno printout is mentioned, so how is it used?

 

I think the first step here is to decide what part of the power band matters to the end user.

Let's (for once) try and shove race mentality aside, aside from briefly noting that race gearing tends to major on top end revs because circuits allow revs to be maintained (therefore power /torque all at the top end) whilst (say) rallying recogises more flexibilty is required at lower revs.

I assume for enjoyable fast road, we are looking at upper mid range delivery? And also looking at top gear for cruising rpms.

Does that move this forward?

 

Its not a thread on one particular engine. Maybe the supercharged one is not a good one due to the broad spread of torque. Chris i think your on the right lines.
But even though, i'd like to see if or how that gearbox would work well on it.

 

I have a recollection that the gear needs to drive the engine from the peak torque rpm to the peak power rpm. And repeat in next gear. Could be n/a applications however?

 

Sounds fair. But what if you don't shift at peak power all the time? Suppose thats why its difficult to judge?

Anther thing i notice is for nasp cars, standard or modified, it seems that lowering the FD is the best way forward, but going the otherway ie when there is alot more power/torque from forced induction it isn't so clear cut.

 

CTN Gearbox Torque Table

Driveshaft Output Torque (ft - lbs)​
		Gears and Ratio​
	Engine Torque	1st​	2nd​	3rd​	4th​	5th​
RPM	(ft-lb)	3.778	2.118	1.360	1.029	0.755
1500​	115​	1372​	769​	494​	374​	274​
2000​	150​	1790​	1003​	644​	488​	358​
2500​	175​	2088​	1171​	752​	569​	417​
3000​	180​	2147​	1204​	773​	585​	429​
3500​	190​	2267​	1271​	816​	617​	452​
4000​	190​	2267​	1271​	816​	617​	452​
4500*​	195​	2326​	1304​	838​	634​	464​
5000​	180​	2147​	1204​	773​	585​	429​
5500​	175​	2088​	1171​	752​	569​	417​
5800**​	170​	2028​	1137​	730​	553​	405​

* = Peak torque (Figures are a rough estimate from above trace)
** = Peak power (No point going to 6200 as power drops sharply)


Gear graph for camparison



Took Me while But did a table that would show axle torque for each gear using above multiplication that gave effective gearing.

Cant see this on the table? But looking at the graph i can see the ratios will drop Rpm into the high torque areas.

 

Now work out tyre Force as the torque in the gears acts on the tyres radius.
With this data you can work out the acceleration via the vehicles weight.
The results per rpm will resemble the "Forward Gs" shown in the dyno plots thread.
Time for you to start breaking out the spreadsheet methinks.

 

How would that translate to knowing you have chosen the right gearbox/FD etc? As in too much acceleration would cause wheelspin and less forward G's. Also too low a ratio would do the same?

Trying not to get too technical here.

 

Found this statement on the www. Sort of explains my thinking.

 

Did that because power drops off sharply on that engine. Boost bypass or something IIRC. 5.8k was peak power. 6k is more like 150ftlbs . Can't see the point of shifting much past there, torque drops rapidy.

It's not so much about building from scratch. More like finding something that suits from whats available or just a FD change. ie a reason why it is suggested that the gearbox shown (CTN) is suited to this engine.

 

Yea i have. Ok so you can see it works. Don't quite get the bit about 3rd and 4th not too far from 2nd. ( do you mean drop in revs?) and the numerical 5th shift point bit?

The point now is comparing that to my own torque curve. Seeing as that is supercharged and has a lower but widespread torque. And mine is turbocharged has more torque but more peaky. Obviously if its on the road you wont be shifting at 6k all the time but its a science i'd like to understand a bit more.
I'll put my plot up.

An older plot. May be better now up top



In theory it looks ok although i seem to have ~10 ftlb less at 2k but more else where.

Torque Table for this engine using CTN gearbox

Driveshaft Output Torque (ft - lbs)​
		Gears and Ratio​
	Engine Torque	1st	2nd	3rd	4th	5th
RPM​	(ft-lbs	3.778	2.118	1.360	1.029	0.755
2000​	145​	1729​	970​	622​	471​	346​
3000​	205​	2446​	1371​	880​	666​	489​
3500​	250​	2983​	1672​	1074​	813​	596​
4000​	260​	3102​	1739​	1117​	845​	619​
4500*​	255​	3042​	1706​	1095​	829​	608​
5500​	225​	2684​	1505​	966​	731​	536​
6000**​	210​	2506​	1405​	902​	683​	501​
6500​	190​	2266​	1271​	816​	618​	452​

* = Peak torque (Figures are a rough estimate from above trace)
** = Peak power (The last dyno trace confirmed peak @6k)

 

I'll give it a go.

 

Table now up. Want to do another table for what i proposed to do. ABF CDA/DPA box with VR FD

 

Proposed solution for me. CDA/DPA box with VR(3.389)final drive.

CDA/DPA + VR FD

1st	3.3
2nd	1.944
3rd	1.308
4th	1.029
5th	0.837
FD	3.389

Effective Ratios

1st	11.183
2nd	6.588
3rd	4.433
4th	3.487
5th	2.837

Driveshaft Output Torque (ft - lbs)​
		Gears and Ratio​
	Engine Torque​	1st​	2nd​	3rd​	4th​	5th​
RPM​	(ft - lb)​	3.3​	1.944​	1.308​	1.029​	0.837​
2000​	145​	1622​	955​	643​	506​	411​
2500​	170​	1901​	1120​	754​	593​	482​
3000​	205​	2293​	1351​	909​	715​	582​
3500​	250​	2796​	1647​	1108​	872​	709​
4000​	260​	2908​	1713​	1153​	907​	737​
4500* ​	255​	2852​	1680​	1130​	889​	723​
5000​	235​	2628​	1548​	1042​	818​	667​
5500​	225​	2516​	1482​	997​	785​	638​
6000** ​	210​	2348​	1383​	931​	732​	595​
6500​	190​	2125​	1252​	842​	663​	539​

* = Peak torque (Figures are a rough estimate from above trace)
** = Peak power (The last dyno trace confirmed peak @6k)

Comparing two gearboxes

 

Expected a bit more out of this! Seems like gearboxes are a black art.

 

you need to break out a spreadsheet and make some thrust curves......just googled for this, had a glance and looks quite interesting

 

Russ.
You have to study your engine's torque curve from idle to rev limit and work with the gear ratio's you posted here to determine which shift points would be best to keep the engine around the maximum torque or say 85% of this peak. If the engine can spin up the tires in low ratios, then a tall diff will help to reduce axle torque with no loss of acceleration ( as you will now be just under the slip angle). Boost controlled turbo engines with a wide and flat band of torque ( 2.5 to 5K say) can have wider spacing than peaky engines with a torque spread of just around 1000rpm. However with OE VW boxes you will be limited on gear set spacing.

Most later 02A and 02J units have the same 1-5 gear ratios with different FD combinations. So the spacing or drop in rpm will be the consistent for gear sets but speed in gears will vary based on FD.

Use the tables to determine what axle torque causes slip and what will happen if you make the overall gearing higher with a VR6 3.389 or 1.9 TDI 3.125 FD from a std 3.68 job.
Note though, if your engine does produce enough force at high rpm you will multiply the drag exponentially and slow the car down even further, reducing its top speed.

On the other hand if you have a peaky little banshee of an engine that is gutless sub 4K and good at making 70% of its maximum torque at 8500rpm then short ratios with good spacing is your friend.
The overall gearing will boost axle torque at low revs when the engine torque is 65% or less than the peak.
Of course if your engine does not have a high rpm peak but maybe geared high for fuel economy, then you can fit a very short diff and significantly increase the axle torque for quick acceleration. The speed in gears will not be high and you will soon hit a much lower V-MAX.

 

Ah. So ideally, I would do a table for the existing box and it will give me a guide as I know when slip occurs ie in the 1st 2 or 3 gears. Larger spacings on OE boxes usually come in the diesel boxes, but I guess that could mean using the box more due to not being in the torque zone especially during low speed driving.