Please help me design my next gearbox!

[Mcalvert]

Dear All,

 

I would appreciate your help in choosing the best options for gearbox ratios when I have the gearbox hopefully rebuilt by BGH at the end of the year. This is a long message but please read on as I would really appreciate some input to help me make the right decisions.

 

The car is a 1700 Crossflow on a 244 cam with approx 155 BHP – It is rev-limited at 7300 and gives peak power around 7000 (I believe). It only really comes on cam at 4000 revs. I have the standard 3.92 diff and run 185/60R13 tyres.

 

 

I have the standard 5 speed box, which I believe has the following ratios;

 

1st 3.360

2nd 1.810

3rd 1.260

4th 1:1

5th 0.825

 

Playing with Gearcalc gives the following results for the set-up as it is today:

 

Gear Mph per 1000 RPM Mph @7000 RPM Mph @7300 RPM

--------------------------------------------------------

1 4.91 34 36

2 9.12 64 67

3 13.09 92 96

4 16.50 115 120

5 20.00 140 146

 

Mph RPM (in Gears)

-------------------------------------------------

1 2 3 4 5

-------------------------------------------------

5 1018 549 382 303 250

10 2036 1097 764 606 500

15 3055 1646 1146 909 750

20 4073 2194 1527 1212 1000

25 5091 2743 1909 1515 1250

30 6109 3291 2291 1818 1500

35 7128 3840 2673 2121 1750

40 4388 3055 2424 2000

45 4937 3437 2727 2250

50 5485 3818 3030 2500

55 6034 4200 3334 2750

60 6582 4582 3637 3000

65 7131 4964 3940 3250

70 5346 4243 3500

75 5728 4546 3750

80 6109 4849 4000

85 6491 5152 4250

90 6873 5455 4500

95 7255 5758 4750

100 6061 5000

105 6364 5250

110 6667 5500

115 6970 5750

120 7273 6000

125 6250

130 6500

135 6750

140 7000

145 7250

 

Gear Change RPM drop (change @7000) RPM drop (change @7300)

-------------------------------------------------------------

1 -> 2 -3229 (to 3771) -3368 (to 3932)

2 -> 3 -2127 (to 4873) -2218 (to 5082)

3 -> 4 -1444 (to 5556) -1506 (to 5794)

4 -> 5 -1225 (to 5775) -1277 (to 6023)

 

 

What I am looking for is a better spread of gears low down (car gets used for sprinting) but still to maintain reasonable road manners for touring as we do 8-10K miles a year.

 

Is the RMP drop the best figure to monitor when choosing the ratios? I guess that I am looking for each drop to put me back in the power band? Assuming that this is the correct approach, what is the logic behind the number of revs dropped decreasing at each step as you move through the gears? I am sure that this is for a good reason, but I am having trouble getting my head around it!

 

The best option seems to be what BGH calls the E7 box which gives me the following choices:

 

1st 2.66 or 2.75

2nd 1.750

3rd 1.260

4th 1:1

5th 0.82, 0.84, 0.86. 0.89

 

My best guess for a choice would be as below which would give the following figures.

 

1st 2.75

2nd 1.750

3rd 1.260

4th 1:1

5th 0.86

 

However, this is built on gut feel rather than real logic so any comments and things to consider in making this choice would be greatly appreciated!

 

Thanks –

Michael.

 

Gear Mph per 1000 RPM Mph @7000 RPM Mph @7300 RPM

--------------------------------------------------------

1 6.00 42 44

2 9.43 66 69

3 13.09 92 96

4 16.50 115 120

5 19.18 134 140

 

Mph RPM (in Gears)

-------------------------------------------------

1 2 3 4 5

-------------------------------------------------

5 833 530 382 303 261

10 1667 1061 764 606 521

15 2500 1591 1146 909 782

20 3334 2121 1527 1212 1042

25 4167 2652 1909 1515 1303

30 5000 3182 2291 1818 1564

35 5834 3712 2673 2121 1824

40 6667 4243 3055 2424 2085

45 4773 3437 2727 2346

50 5303 3818 3030 2606

55 5834 4200 3334 2867

60 6364 4582 3637 3127

65 6894 4964 3940 3388

70 5346 4243 3649

75 5728 4546 3909

80 6109 4849 4170

85 6491 5152 4431

90 6873 5455 4691

95 7255 5758 4952

100 6061 5212

105 6364 5473

110 6667 5734

115 6970 5994

120 7273 6255

125 6516

130 6776

135 7037

140 7297

 

Gear Change RPM drop (change @7000) RPM drop (change @7300)

-------------------------------------------------------------

1 -> 2 -2545 (to 4455) -2655 (to 4645)

2 -> 3 -1960 (to 5040) -2044 (to 5256)

3 -> 4 -1444 (to 5556) -1506 (to 5794)

4 -> 5 -980 (to 6020) -1022 (to 6278)

 

 

 

[Joe 90]

Mike,

my car has a similar engine spec (but I haven't sprinted it yet). I went for the BGH 2L long first box (E2 on my old version of gearcalc) with a 0.87:1 fifth and I'm very pleased with the ratios. Do you want to meet up one evening and compare notes.

 

SEP field working, not spotted in 102,200 miles. Some photos on webshots, updated 10 June

[Luke Beaumont]

I've just had an E7 built. I went for the 2.75 1st, but chose the tallest top gear* as I wanted 5th to be an ultra low revs cruising gear.

 

* that happened to be 0.84, as it was the tallest they had in stock. I initially chose the 0.82 but it would have meant a long leadtime to order the gear in, apparently.

[christian.v]

I'm in the same boat.

Have you any approx costs on changing the first 3 ratios at BGH?

[Bare]

OR... you could obtain a Mazda Miata (mx5?) transbox from a wreckers and have almost exactly what you need.. Your 90 hp to the rear wheels will be v well taken care of.. Plus the thing shifts pretty darn well as a bonus.

You may also discover that Mazda and Ford Bits are surprisingly 'compatible' as one would expect from the same Parentage.

[BrianHorn]

Has anyone done the Mazda 'box conversion? Does it fit staight onto the x/flow?

 

 

Brian

 

'The Wizard' is a 1989 de Dion SuperSprint in Red with Silver stripe..

[Luke Beaumont]

I'm thoroughly prepared to be wrong here, but IWHT that at the inception of the Miata 5 speed, Ford & Mazda had no connection whatsoever.

 

Wooly: the E7, which changes 1st, 2nd and 5th over the 2.8 Sierra box is costing me 900+vat because I'm bringing him a 2.0 gearbox to convert. If you're starting with a std caterham 5 speed it's a good deal cheaper.

[Colin Mill]

Seems to me you need rather a lot of input data before you can really optimise the ratios. I think a torque curve would be important as well as an idea what the traction limited acceleration is going to be on the rubber and surface of your choice. There are a few other things like how hard a time you are prepared to give the clutch in the initial launch. A tall first gear is good so long as you are prepared to hold the revs in the launch up at a level that allows you to be traction limited. The taller the first gear the higher the revs will need to be to have enough torque to (just) unstick the rear tyres and the longer you will have to slip it before you can let the clutch all the way in. With your engine you have a power loading of about 210Watts/kg. How much of that you can expect at the rear wheels I don't know. However allowing 5% for transmission losses and if you reckon on getting 0.8g acceleration (7.85m/s) then you can be traction limited up to about 25m/s (56mph). If the traction limited acceleration is 0.95g then the traction limit is 21.5m/s (48 mph). So the more traction you have the lower first gear

you can make use of. I would reckon that a first gear that takes you to only 34 mph at max power will allow you to light the rears up with only 60% to 70% of max torque. With a torque curve you could see the minimum revs you would need to hold during the launch.

 

I think the reason why you have bigger gaps in the lower ratios is down to the length of time one spends accelerating through the gear. In second you can afford to be a bit lower down the power curve just after the change than you can in third because you will spend less time in the less favourable part

of the engine's power curve in the early part of second than you will in the early part of third if you had the same rpm drop in both gears.

 

OK I think I have wittered on enough!

 

Colin

 

[christian.v]

Whereabouts are BGH?

Could you post up a phone number please

[Joe 90]

BGH Geartech

Red Roofs

New Road

Cranbrook

Kent

TN17 3LE

01580-714114

 

There's no point in going mad on the theory when you're limited by what's available.

 

SEP field working, not spotted in 102,200 miles. Some photos on webshots, updated 10 June

[Colin Mill]

Why do you think you want the 2.75:1 first rather than the 2.66:1 first? Caterham's 6 speeder has a 2.69:1 first. Assuming they got their sums right the 2.66:1 is closer to the 6 speeders first than 2.75:1 is. Also you would expect that the optimum first gear for a 5 speeder would be longer than the optimum first for a 6 speeder. All of which points to the 2.66:1 option.

 

Edited by - Colin Mill on 7 Aug 2004 14:47:54

[Nifty]

I have a 2.66:1 first, as supplied by BGH.

My only comment would be that in standing/slow moving traffic, as I encountered on the M25 the other week, I'd rather a slightly lower gear just to make trundling slightly easier. With my relatively low torque 1.4 I need to give it more revs to start it moving and then I find it is slightly higher geared than the other cars and so I can't let it trundle at idle because I'm running into the back of the car in front.

On the open road, when pulling away from junctions etc on wideopen throttle, then the higher gearing is great.

 

I would listen carefully to Brian Hill with regards to the gear ratios...he designed the Caterham 6 spd so he must be aware of the differing gear ratios.

 

Luke,

Ford and Mazda have had a link for many years..though I would agree that there is unlikely to be any similarity between the Type 9 box and that of the MX5.

The old box shaped Mazda 323 ('80's design) used to be sold in Aussie as a Ford (Laser, I think).

 

 

Keep off the straight and narrow 😬