diff ratios

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diff ratios

Post by numbnuts on Sun Mar 15, 2015 7:15 pm

I currently run a Ford type 9 gearbox with a v long 1st and close ratios with a short 5th which is great for getting off the line in a race but due to the final drive 3.77 means that it can still bog down in 2nd in a hairpin and on the road I have to slip the clutch for a slow getaway. I have never used 5th on a circuit near limit in 4th on some circuits so I have been thinking about changing the diff ratio so that all of the above are improved. To give you an idea I will only change gear10 Times on a lap of Cadwell. I have in the garage what I believe is a 3.86 diff and I have just got a 4.11 that I won on ebay for under £20!

So my questions are

1 how many people use 5th on circuit? (moped drivers need not supply)

2 how much jump will there be between the ratios?

3 will the 4.11 be to short and all the extra gear changes actually slow me down?

NB I have done the online gear calc thing and it only seems to make marginal differences on paper but I cannot translate these to how the car will feel?

Any help will be appreciated as I really don't know what to do and I really would kick myself if I spend money getting it fitted only to make me slower!!
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Re: diff ratios

Post by Sean Graham 72 on Sun Mar 15, 2015 9:37 pm

If it's any help, the diff of choice for Locost formula is the 4.1  They normally go for about £150, so you have done well there.  The Locosters will normally run it with a 4 speed box up to 7500rpm at Snetterton with their Crossflows.  

When I ran a 3.9 at Pembrey for example, I found I was still having to round the hairpin in 3rd to stop the 80BHP spinning the rears up on exit.  Once off the line, it was 3rd and 4th only at most UK circuits, using 5th to relax the engine at places like Donington.

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Re: diff ratios

Post by billy f on Mon Mar 16, 2015 8:24 am

Hi Ed

Ideally you would need to know the following to make an educated guess at your proposal:

Max speed of you car on track (actual, not theoretical), I guess this would be about 125mph to 127 mph?

RPM for max power?

RPM for max torque?

Gearbox ratios?

Turns per mile of your tyres (if not available make and size of your tyres?

How ponderous is the change across the gate from 4th to 5th?

If you can send me the info, I would be happy to have a look at it Smile

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Re: diff ratios

Post by IAN51 on Mon Mar 16, 2015 8:43 am

What ratio is your 5th Ed ? They are normally 1:1 on some others they are 0.87 but im sure the type 9 is 1:1

As for diff ratio I used to run 4:11 in the Spitfire with a 7500rpm ref limit and found the Diff quite short with the 4 speed box we ran.

The mk2 Hornet from memory runs a 3:63 but again needs to because of its 4 speed box. Like you say at most circuits I was only using 3rd and 4th with only circuits like the hairpin at Mallory requiring second gear.

My Cossie fiesta again runs a 3:62 which suits the car well

to answe your question you need to ascertain your top speed and the likleyhood that your going to hit it. With yours being a 7 with barn door aerodynamics I would suggest that its somewhere around the 130mph mark ! In which case I reckon a 4:11 would suit it well assuming that you have a 1:1 fifth gear.


Last edited by IAN51 on Mon Mar 16, 2015 8:49 am; edited 1 time in total
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Re: diff ratios

Post by IAN51 on Mon Mar 16, 2015 8:48 am

Sorry never seen Billy post until id finished mine but we are in agreement with the actual top speed. Billy is very good at this in the info pack I had with the mk2 he had calculated all kind of graphs and plots for the gearing depending on which tyres it was using impressive stuff when you read it sadly wasted on me lol
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Re: diff ratios

Post by billy f on Mon Mar 16, 2015 10:08 am

As we've discussed previously, one other possibility is to swop over to 13" wheels, which will also reduce your gearing. This would also:

Significantly reduce the combined weight of your wheel/tyre.
Worthwhile reduction in cost of replacement tyres.
Reduce your CofG height by about 1/2" to 3/4" which will improve cornering speed and braking. It will also reduce roll, which you are suffering with at the moment because you cant add any more negative camber to the front tyres.
The reduction in rotating mass and the smaller diameter will also improve braking and acceleration.
The reduction in gearing may be enough to make 5th a usefull gear on the longer/faster straights and also be enough to stop bogging down at hairpins.

Your 15" wheels and tyres can still be used for on the road, to keeps revs reasonable and help you over sleeping policemen!

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Re: diff ratios

Post by numbnuts on Mon Mar 16, 2015 2:13 pm

the gear ratios that I have are as follows

1 2.482
2 1.610
3 1.225
4 1.000
5 0.870

peak power 6500
rev limit 7200
205/50 15 tyres R888's

the change from 4th to 5th feels slow and unnatural to me as I have never really used it apart from on road as an overdrive. But I assume I could get used to it??????

The cost to change the diff and LSD over will be aprox £500 (axle off job on the six)
to change the wheels and tyres will cost £1000 as I dont want to go back a step and would want pro race ones. I will also have to pay someone to weld up the trailer as I will have to alter it so that the car will fit on as I have ZERO ground clearance at the mo

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Re: diff ratios

Post by billy f on Mon Mar 16, 2015 3:11 pm

DOH! I'd forgotten about the trailer problem Neutral

Thanks for the info Ed, do you know what revs your peak toque is?

I will look at the figures over the next couple of days and try and give some further insight.

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Re: diff ratios

Post by numbnuts on Mon Mar 16, 2015 3:39 pm

Peak power 6500
Peak Torque 5250
rev limit 7200
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Re: diff ratios

Post by billy f on Tue Mar 17, 2015 7:53 am

Hi Ed

as discussed last night I have looked at the gearing and will scan and send you an email with all the info.

For the benefit of the others the current set-up has redundant 5th gear in that when you are just reaching your max speed (estimated at 124 mph from previous experience ) in 4th gear the power curve is dropping off, and if you change to 5th it puts you to before the peak power, and in effect you get no further acceleration.

Additionally, the drop from 1st to 2nd gear drops you to well below peak torque and first will also take you to 37 mph.

The best option from the diffs you mention is the 4.11. the 1st, 2nd gear ratios are still high but not as bad with the 4.11. The 5th gear is now a very effective gear and the top speed now more or less co-incides with the peak power of your engine. 4th gear will also take you to 114 mph, so you should only need to use 5th on the longer straights.

To speed up the change from 4th to 5th see if it already has a quick-shift lever and change to a thinner oil but high quality oil.

The exhaust could also be tuned to bring the max power rpm up a bit (reduce primaries length). As it is suggest running to red line in 1st and 2nd, change at about 7,000 in third and about 6,800 in fourth.

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Re: diff ratios

Post by numbnuts on Wed Mar 18, 2015 7:11 pm

Good news I have had a look at my ebay diff and it is deffo a 4.11 Very Happy so once I can confirm the part no. is for my axle I will be good to go
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Re: diff ratios

Post by numbnuts on Thu Apr 02, 2015 12:03 pm

Might have to delay the diff and quick shift as I have had the car on ramps and found a few other problems!

Chassis might need welding!
Brake caliper might need replacing
Rear break lines replacing
master cylinder/ pedal stuff as brakes are currently binding.

So depending on how bad the chassis is will affect my go faster budget Sad
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Re: diff ratios

Post by numbnuts on Thu Apr 02, 2015 6:03 pm

Good news is the chassis is not too bad Very Happy bad news is that the gearbox mount and prop shaft need replacing. Cheaper than fixing both UJs and shortening it Sad Sad so the will be fully fit in a couple of weeks.
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Re: diff ratios

Post by billy f on Tue Apr 07, 2015 7:27 am

Best of luck with the repairs Very Happy

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