Author Topic: Theoretical gearbox modification  (Read 2220 times)

billyausten

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Theoretical gearbox modification
« on: April 12, 2021, 09:58:43 PM »
Having thoroughly enjoyed the first couple of months of mk1 jazz ownership, and having put some decent miles on it already, I'm getting a decent feel for its strengths and weaknesses. The i-DSI engine is a peach, and while I understand it's design intent, I can't help feel that 5th gear is a touch on the low side.

Looking at the gearbox specs from the 1.2 and 1.4, they have subtly different ratios. 1st and 2nd are identical, however the 1.2 has a longer 3rd, 4th and 5th coupled with a shorter final drive to achieve essentially the same overall 5th ratio.

I wonder if it would be possible to swap jusy the 5th ratio from a 1.2 into a 1.4 box? This would raise 5th from standard, and would in theory lower the revs for a given speed. I can't help think his would be nice on dual carriageways etc. It wouldn't be a radical reduction, but even a 250rpm reduction at 70 would be pleasant.

My gut feel is that the 8v i-DSI easily has the guts to pull a slightly longer gear. The gap from 4th to 5th would be perceptible, but it would simply be a feature. I don't think the Speedo would be affected as the output is taken off the final drive. I can't imagine transmission casings and bearings are any different.

Interestingly most breakers don't seem to differentiate between the 1.2 and 1.4, they offer gearboxes as fully interchangeable (they probably are) , they are just slightly different internally.
« Last Edit: April 12, 2021, 10:04:01 PM by billyausten »
Current: '08 GE3 Sport, '97 CE1 Aerodeck 2.2iES
Previous Hondas: CA5 CC1 EE8 EE9 CC9 CH1 ED7 ZE1 CN1 CE1. - Non Honda: E-RNN14 GTiR E-BNR32 GTR

billyausten

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Re: Theoretical gearbox modification
« Reply #1 on: April 12, 2021, 10:03:21 PM »
Gear ratio-
L12A1
1st 3.142 2nd 1.750 3rd 1.166 4th 0.857 5th 0.710 Final 4.294

L13A1
1st 3.142 2nd 1.750 3rd 1.241 4th 0.969 5th 0.805 Final 4.111.
With standard ratios at 3000rpm:
Tyre Circumference = 1836.26 mm
1st Gear = 25.59 km/h - 15.87 mph
2nd Gear = 45.94 km/h - 28.48 mph
3rd Gear = 64.79 km/h - 40.17 mph
4th Gear = 82.97 km/h - 51.44 mph
5th Gear = 99.88 km/h - 61.92 mph

Modified ratios would be:
1st 3.142 2nd 1.750 3rd 1.241 4th 0.969 5th 0.710
Final 4.111
Therefore at 3000rpm:
Tyre Circumference = 1836.26 mm
1st Gear = 25.59 km/h - 15.87 mph
2nd Gear = 45.94 km/h - 28.48 mph
3rd Gear = 64.79 km/h - 40.17 mph
4th Gear = 82.97 km/h - 51.44 mph
5th Gear = 113.24 km/h - 70.21 mph

This is actually a 402rpm drop at 70mph! Food for thought.
« Last Edit: April 12, 2021, 10:20:24 PM by billyausten »
Current: '08 GE3 Sport, '97 CE1 Aerodeck 2.2iES
Previous Hondas: CA5 CC1 EE8 EE9 CC9 CH1 ED7 ZE1 CN1 CE1. - Non Honda: E-RNN14 GTiR E-BNR32 GTR

Jocko

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Re: Theoretical gearbox modification
« Reply #2 on: April 12, 2021, 10:18:20 PM »
Fitting the modified ratios actually lead to higher revs for a given top speed in 5th.

https://spicerparts.com/calculators/transmission-ratio-rpm-calculator

Jocko

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Re: Theoretical gearbox modification
« Reply #3 on: April 12, 2021, 10:34:08 PM »
Fitting the modified ratios actually lead to higher revs for a given top speed in 5th.

https://spicerparts.com/calculators/transmission-ratio-rpm-calculator
Forget that. I had the numbers round the wrong way.

UKjim

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Re: Theoretical gearbox modification
« Reply #4 on: April 17, 2021, 09:06:30 AM »
Very interesting, seems like a major job to a dunce like me. When we had our 2 manuals some years ago, I always thought it could do with an ‘overdrive’ 5th gear as it would easily cope with it.

bus_ter

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Re: Theoretical gearbox modification
« Reply #5 on: April 18, 2021, 07:44:38 PM »
I have noticed the MPG drop from 70mph Vs 60mph is quite significant. Cruising at 70mph does seem to be revving quite high.

Jocko

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Re: Theoretical gearbox modification
« Reply #6 on: April 18, 2021, 08:02:36 PM »
Drag is a function of the square of the speed and that will have a considerable greater effect on mpg than the slight increase in revs.

embee

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Re: Theoretical gearbox modification
« Reply #7 on: April 19, 2021, 08:36:25 PM »
As Jocko says, drag force is proportional to speed squared, drag power is proportional to speed cubed. Going faster needs more fuel as a rule.
 The significant factor in which gear you use (and hence engine speed) is that the brake specific fuel consumption (BSFC), the fuel used to produce one kW.hr of energy at the wheels, generally improves with more load and less speed (up to a point).
The BSFC map below is an example of a car petrol engine.
The "road load" in 4th gear is shown dotted to give an example of where the engine runs as vehicle speed increases. Higher gears will show as curves above this example, lower gears will be below it.



The top line in bold is essentially the maximum torque curve at wide open throttle measured in brake mean effective pressure BMEP (Bar), this is in essence the torque per litre with a constant factor applied (Nm = 7.958 x BMEP x swept volume). BMEP alllows you to compare different engines directly, taking swept volume out of it.
The lines curving from top left down towards bottom right are constant power lines (see right hand axis). If you run a car at a given speed you need a fixed power, lets say 30kW. Follow the 30kW line up towards the left and you'll see that at for example 4000rpm (low gear) the BSFC is about 400 g/kW.hr.
If you use a higher gear and run the engine at say 3000rpm, the BSFC for 30kW is around 300 g/kW.hr, and at 2500rpm it is about 270 g/kW.hr
Thus you can see that for cruising at steady speed using a higher gear generally speaking puts the engine nearer the "eye" of the map and thus gives a lower amount of fuel needed to produce the required power (and thus energy).
If you go too far it can make it worse, but that tends to be at extreme low rpm and extreme load (torque). For a typical petrol engine it is usually not beneficial to run at high load much below around 1500-1750rpm (heat losses in the slow cycles means lower thermal efficiency), and at near full load (open throttle) the engine usually runs slightly rich so fuel efficiency reduces.
« Last Edit: April 19, 2021, 08:40:13 PM by embee »

Jocko

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Re: Theoretical gearbox modification
« Reply #8 on: April 19, 2021, 09:01:31 PM »
I try to drive with the revs at or about the maximum torque (orange curve).



I change up around 2,500 - 3,000 rpm and try to cruise around a similar range.

billyausten

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Re: Theoretical gearbox modification
« Reply #9 on: April 19, 2021, 11:31:51 PM »
As Jocko says, drag force is proportional to speed squared, drag power is proportional to speed cubed. Going faster needs more fuel as a rule.
 The significant factor in which gear you use (and hence engine speed) is that the brake specific fuel consumption (BSFC), the fuel used to produce one kW.hr of energy at the wheels, generally improves with more load and less speed (up to a point).
The BSFC map below is an example of a car petrol engine.
The "road load" in 4th gear is shown dotted to give an example of where the engine runs as vehicle speed increases. Higher gears will show as curves above this example, lower gears will be below it.



The top line in bold is essentially the maximum torque curve at wide open throttle measured in brake mean effective pressure BMEP (Bar), this is in essence the torque per litre with a constant factor applied (Nm = 7.958 x BMEP x swept volume). BMEP alllows you to compare different engines directly, taking swept volume out of it.
The lines curving from top left down towards bottom right are constant power lines (see right hand axis). If you run a car at a given speed you need a fixed power, lets say 30kW. Follow the 30kW line up towards the left and you'll see that at for example 4000rpm (low gear) the BSFC is about 400 g/kW.hr.
If you use a higher gear and run the engine at say 3000rpm, the BSFC for 30kW is around 300 g/kW.hr, and at 2500rpm it is about 270 g/kW.hr
Thus you can see that for cruising at steady speed using a higher gear generally speaking puts the engine nearer the "eye" of the map and thus gives a lower amount of fuel needed to produce the required power (and thus energy).
If you go too far it can make it worse, but that tends to be at extreme low rpm and extreme load (torque). For a typical petrol engine it is usually not beneficial to run at high load much below around 1500-1750rpm (heat losses in the slow cycles means lower thermal efficiency), and at near full load (open throttle) the engine usually runs slightly rich so fuel efficiency reduces.

Great post, thank you.

In my mind, this lends support to my proposal that a higher 5th ratio will reduce fuel consumption with all other things equal. Is this correct?

The increase in ratio would in my mind not labour the engine excessively, it would conversely keep it closer to (but still slightly over) its 'sweet spot'. The car seems to rev out beyond this sweet spot in 5th over about 50mph, and tbh 50 is not practical or sustainable for decent length journeys with young irritable children.

Ive been looking at various gearboxes from cars being broken. and 95% of breakers offer jazz boxes as being suitable for both 1.2 and 1.4. Can anyone with a 1.2 please confirm what they have in the way of markings on their own gearboxes so that I may try to validate my options?
Current: '08 GE3 Sport, '97 CE1 Aerodeck 2.2iES
Previous Hondas: CA5 CC1 EE8 EE9 CC9 CH1 ED7 ZE1 CN1 CE1. - Non Honda: E-RNN14 GTiR E-BNR32 GTR

embee

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Re: Theoretical gearbox modification
« Reply #10 on: April 20, 2021, 10:02:31 AM »
..... a higher 5th ratio will reduce fuel consumption with all other things equal. Is this correct?

The increase in ratio would in my mind not labour the engine excessively, it would conversely keep it closer to (but still slightly over) its 'sweet spot'. ........

Very broadly speaking, yes, the higher the gear the better the economy, ideally you want to run the engine as near the eye of the onion plot as you can. The compromise is usually keeping enough "reserve" power (difference between road load and max available at that engine speed) available to cope with changes in gradient, acceleration etc., which is often what is behind the actual ratios selected for a road car. More reserve means less need to change down a gear.
This is also what is behind the adoption of more and more ratios in auto gearboxes, up to 10 these days, but of course an auto can easily switch to lower ratios when needed, in a manual it becomes much more tedious to have to keep changing gear so a slightly lower than ideal top gear might be used to keep it flexible. The old toggle switched overdrive was actually a nice solution, albeit heavy and expensive, essentially an auto gear added to a manual box.
« Last Edit: April 20, 2021, 10:04:32 AM by embee »

Jocko

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Re: Theoretical gearbox modification
« Reply #11 on: April 20, 2021, 10:02:58 AM »
70 mph in 5th is well at the top area of the torque curve and on a flat road my ScanGauge shows 60+ mpg. As for gearbox markings, sorry but the only part of my grearbox I have ever seen is the gear lever and even that is probably remote.

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