Differential Ratios!!!!!

[GHPT]

Hi Guys,

 

I need some help please.

 

I have an old racing Caterham which I am in the process of putting back on the road. It is has the escort 4 speed box and escort back end. I think it has a current ratio of 4.125 in the diff which is not ideal for the road! 60 mph at 6000 rpm!

 

I don't know whether to go for a 3.77 or 3.54 ratio. I hope someone has a bit of experience with these that they can share......?

 

 

 

I want to be able to drive to the South of France for example with ear drums left!

 

http://rs-garage.co.uk/speed.html here is some diff stats.

 

Cheers

 

 

George :)

[Mick Day]

Most K series cars with a 6 speed box run a 3.92 diff. Assuming your 4th is direct (as is 6th in a 6 speed) then I'd go for the 3.77.

[peterg]

Might be worth swapping the 4 speed to a T9 5 speed box to give you much wider ratios as well, they can be picked up for peanuts

[S47zz]

Your 4.1 ratio diff would be OK if you fit a 5 speed box with 0.82 5th gear ratio.[most standard type 9's]

I use a 3.9 with 5speed with 0.9 5th gear which is great for road use, when using 185/70x13" wheels, if you use 60 series 13" tyres then 3.7 or 3.5 diffs would be better IMO.

 

[OliverSedlacek]

What wheels and tyres are you using? A 3.9:1 diff on my old 4sp box gave 18 MPH/1000 RPM, not the 10 MPH/1000 RPM you quote.

 

I would go for something in the 3.6 to 3.7 range.

[Nigel Fox]

A good ratio to use is the 3.89:1 (35 teeth on crownwheel 9 on pinion). This is the Mk1 escort (on 12" wheels ratio). It has the advantage of being common and therefore cheap, you'll find one on ebay.

10mph per 1000revs is a ridiculously low ratio, I suspect that there is something wrong with your measurements.

 

 

[John Howe]

I'd go for the 3.7 and nod to Peter's suggestion of replacing the 4 speed with a 5 speed.

 

Fifth is normally by way of an overdrive ratio, which might suit your needs/ears even better.

 

JH

Deliveries by Saffron, the yellow 230bhp Sausage delivery machine

[peterg]

According to the link in the original post of this thread, a 4.125 diff on 185/60x13 tyres and a 4 speed Escort box with 1:1 top gear gives 90mph at 6000rpm, not 60mph so I think he's typed it wrong

 

here 😶‍🌫️

 

Just changing to a 5 speed box and keeping the diff you have would change this to 113mph at 6000rpm

 

Edited by - peterg on 14 May 2009 15:07:05

[GHPT]

Thank you for all your help. I did type that wrong. Those stats don't translate exactly as I have 8.0/22.0/13 on the back.

 

So do get the 5 speed box with 3.7 diff or rebuild my current 4.125 and get a 5 speed box with 0.82 fifth gear?

 

mmmm......

 

George :)

[w7rren]

I suppose if you have 100bhp then you want lowish gearing but if you have 200bhp you will have the torque to take higher gearing.

Don't lose the acceleration if you only have 100bhp.

My first 1600 xflow ( 110bhp ) was o.k with 3.9 ( 3.89 ? ) diff & 4 speed box.

 

Cheers,

Mark

 

 

Born again sevener

1st LAMBO back in 1971 my fun motors

[GHPT]

thanks. I got about 220BHP ford twin cam, brm head, dry sumped. I think what adds to the problem is that I have a racing exhaust on it which makes it sound a lot louder!

[peterg]

sounds like you need a new silencer from Ammo for the road then

[GHPT]

who is Ammo?

[peterg]

Amedeo Castellani, posts on here as AMMO, tunes Duratec engines to 300bhp+ and makes titanium repackable silencers that will get most engines quiet enough for any trackday: www.raceco.com

 

Raceco have designed and developed lightweight 6.25" (160mm)and 7.25" (185mm) silencers for the Caterham Seven. These titanium sleeved silencers weigh 4 and 5.6 kg respectively (for reference, the stock 5" factory supplied part weighs 6.5 kg).

 

The spun end caps are retained by stainless button head screws for easy dismanting and re-packing. Unlike some of our competitor’s products it is custom made with brackets and inlet pipe already fitted making it a direct replacement fit with no need for fabrication or welding. The internals are top quality with special attention paid to the open area and the design of the perforated core. This avoids packing material being burnt and blown out. The packing material is of the highest quality available. Because of the use of superior components the silencers rarely need repacking under normal use. Our road car has had a silencer fitted for over a year and 10,000 miles. In this time it has been repacked twice and on both occasions took a very small amount of material to bring it back to original spec. It has been noise tested at 98 dba @ 5,500 rpm on a 200 bhp + Zetec engined car. No expense has been spared in designing and manufacturing these silencers. They are the best available anywhere. Custom made with 2.25" or 2.5" inlet pipes. Different diameter inlet pipes available to order.

 

Titanium 6.25" £450.00 + VAT

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NEW Titanium "Long" Custom Silencer 2-3dB quieter £510.00 + VAT

 

and no, I'm not on commission

 

 

 

Edited by - peterg on 14 May 2009 21:44:41