Diff ratios

[AMMO]

I quite like high gearing. Seeing that I'm not out to win any traffic light Grand Prix I'm thinking of fitting a 3.62 crown and pinion.

 

I have a 5 speed box. I think the spacing between gears for every day road use is about right. First gear is almost useless at the moment as you have to change up after just a few yards. When I'm cruising in fifth I'm always looking for another higher gear.

 

If I do the swap fifth would end up with about 24 mph per 1000 rpm (if i've done my sums right) which I'm pretty sure it would pull. Fifth would be a proper overdrive. Great for long distance work. For track days I'd probably never get into fifth even with the current set up.

 

A better solution than having a closer ratio gearbox with higher first because it still would not fulfill my top gear cruising needs.

 

What do you guys out there think? Anyone done this?

 

 

 

AMMO

[Blatman]

I have a "normal" type 9 and 3.62 Sierra diff in my "road" car. It's definitely better than a 3.9, which is what I had in my hillclimb car, and for road use, is just about right. The hillclimb car now has a 2.39 first gear, and is OK on the hills, but would be tiresome on the road, as it requires at least 2,500 rpm to pull away smoothly and not bog down, even with a torquey atmo cossie engine.

For a gearing calculator, have a look here

 

Edited by - Blatman on 8 Jul 2001 12:59:33

[AMMO]

Thanks for that Blatman. Downloaded your gearing calculator. Turns out 22.05 mph per 1000 rpm with the 3.62 against 20.36 mph with the 3.92. 8.5% difference approx.

 

Next dilemma is do I get a crown wheel and pinion or avoid all the hassle and get a complete diff? Do some (Sierra 4x4?) 3.62 diffs come with lsd as standard? Are these any good?

 

AMMO

[Blatman]

I believe the XR 4x4 rear diff is the diff of choice, as it is a viscous LSD as "standard". I have an XR4x4 as my tow/road car, and with 130,000 miles plus, the diff is dry, (as in no leaks, not devoid of fluid....) no noise, and functions as it should. If you are going to buy a diff/CWP, and have the funds, I have a deep liking for the Quaiffe ATB, which is what my Escort diffed hillclimb/sprint car uses, and I love it.

[Blatman]

If you want to try the set up, and are anywhere near the West London area, you are more than welcome to a trip in the plastic fantastic. I will provide a disguise of course...........biggrin.gif

[CHRIS CLARK]

Big wheels and tyres!!!!

 

 

[AMMO]

Thanks again for the info Blatman. If I plan any trips to West London I'll email you, I'd like to have a go. I used to live in Chiswick and Shepherds Bush, now reside in Suffolk. Where are you?

 

Chris, regarding big wheels and tyres, I recently did a track day and was let loose in a couple of Caterhams with 15 and 16" wheels. I definately prefer my 13" Yoko A021 shod wheels. Good point though. Easy way to raise gearing (and ground clearance).

 

AMMO

[Petrolhead]

I had a thread on the ChitChat section called 'Unusual 7' There Fast Arnie suggested a 3.38 diff. Don't know where to get one but that would give you very long gearing.

 

I know what you mean about the 1st gear though, not much use.

 

I have a 1.4 SS and have been thinking about the SPC CR Box with the 2.39 1st. Blatman has concerned me though with his comments. Would I have problems with this with my current setup - type 9 5 speed with the 3.92 diff which I think is the diff currrently in my car?

 

Better to go in a blaze of glory than to fade away teeth.gif

[Blatman]

Petrolhead

You wouldn't have "problems", and my feeling may be different to yours. However, when poodling around the paddock, I seem to spend a lot of time riding the clutch. When the clutch is fully out, I seem to be travelling at considerably more than walking pace, making town traffic, or traffic jams potentially tiresome for the old left leg, and possibly causing extra wear on the clutch. As for Ammo, if you wanna come have a try, you would be more than welcome.

Ammo

Can't say where I am, too many folk wanna know my real name as it is (not here, somewhere else!!!!). I also keep a house in North London, so there would be good too. If I happen to venture north of the M25 Essex/Suffolk way, I'll be in touch.

[Mike C]

Ammo- I'm thinking of doing the same, it's a real pain driving to St Moritz at 5000rpm!!!!

 

Let me know what you decide to do, I'm also thinking of larger wheels but the diff is probably the way to go.

 

Cheers

 

Mike C

K9 JPE

[AMMO]

The 3.38 diff will give 23.62 miles x 1000 rpm with Yoko 285/60-13 (22" diameter). Almost 14% taller than the stock 3.92. This would be great for a powerful 200bhp + engine in a road car in my opinion.

 

As I'm planning on about 180 bhp out of an 1800 Zetec think I'll try a 3.62 first and see how I get on. That will give me 90 mph at around 4000 rpm in fifth gear.

 

If it will rev out to 7000 rpm in fourth should just manage to squeeze 120 / 125 mph. It would geared for 150 mph in top, but I doubt it will pull that. Maybe 135 mph at 5,900?

 

Fast enough for me.

 

AMMO

[oilyhands]

Interesting that you chaps are considering raising your gearing, I have a 2wd Cos diff with 3.62 CWP and a CR 5 speed box, 2.39 first and .88 5th gear and 15/195/50 tyres giving 21.1MPH /100RPM in 5th gear. I find this much overgeared for most situations and am looking to go for a 3.92 or 4.1 CWP, the higher diffs (3.3 and 3.1CWP) can sometimes be found on automatic Sierras.

 

Although the engine has more than enough torque to pull in 5th gear (185ft/lb) it's still achingly over the top with 100MPH being just under 5000RPM and 70MPH being 3300RPM and well out of the serious power band.

 

Oily

[Blatman]

Both my 16v engines (atmo cossie, and Vx xe) produce around 180-200 bhp, and I don't find a problem in not having enough torque, from as low as 2000 rpm. Ammo did state that he has his cruising requirements at the forefront of his thinking.

One man's meat and all that...smile.gif

[John]

Dear Mr Blatman,

 

Why all the secrecy? In the words of Phil Mitchell..."I know who you are!!!". Serves you right for getting your car on the front cover of certain magazines...

 

John

[Blatman]

I really must make time to come and say hello at the next sprint, John. Will you be at Boscombe, or Mallory soonquestion.gif

As for the magazine, I didn't think that any-one would remember that far backwink.gif

[AMMO]

Oilyhands

 

You have to decide how you want to drive and then choose the gearing. I know that what I have is too low for me even with a shagged Crossflow. If you are out of the serious power band maybe you want to go down one or two gears and rev the tits off the thing. I like high gearing for cruising but I do like to give it some stick as well. On the basis that I'm not interested in going much over 120 mph and I can do that in fourth, I can just keep fifth as an overdrive for those long trips.

 

A proper race car would probably have more gears and a 1:1 top, engine wear, noise and fuel consumption be damned. I drive my car every day and have reached the age when a bit of comfort does not go amiss.

 

Each to his own though. Fancy swapping diffs?

 

 

 

AMMO

[Graham Perry]

Ammo/Blatman , The viscous LSD of a 4x4 Sierra won't fit. It has a different design of driveshafts that fit via a rubber cushdrive into the diff via two perminant flanges. Even you if withdraw the flanges from the diff you still have problems because the splines on the new driveshafts that go into the diffs have to be of differing lengths to accomodate the Viscous unit. From what I recall one is about five inches and the other three. In theory one of these can be made to work but you will need to find someone who can make some bespoke shafts. I looked into this in a big way and discovered that it was possible to get some shafts modified but the cost would be similar to a Quaife ATB or AP Suretrack. This was a couple of years ago mind , so maybe someone now makes them.

 

Edited by - Graham Perry on 11 Jul 2001 06:58:25

[oilyhands]

dont have a problem with revving the tits off it, the problem is that with a 3.62 diff first gear is a tad too high for comfort and it would be nice to have 5th as more of a driving gear. I would certainly swap CWP, but the diff is pumped... so I wouldn't want to swap the whole thing.

 

Graham, BGH Geartech/shaftech will sort your driveshafts out for you at a reasonable cost... pretty sure they graft the Lowbro joints in place of the existing tripods allowing them to mate with the diff flanges.

 

Oily

[Peter Carmichael]

While being firmly one of the *rev the tits off it* brigade, I still think there is a flaw in your plan for a long striding diff.

 

The yawning gulf between 1st and 2nd that you get with the standard box will still be there. Imagine the situation in traffic where you are in the speed range of 10-20 mph. Where previously you might have left it in 2nd, 2nd will now be too high. You will have to cross the gulf down into 1st and then when you get moving again you will have to cross the gulf again to get back into 2nd.

 

This is a simple observation based on the fact that a longer diff ratio is scaling the gear selection points further up the road speed range, moving the low changes into a range of speeds more often encountered.

 

I think you may find this very unsatisfactory, with first still being too low and most of the comments you have attracted on this thread have been along the lines that the most glaring fault in the car's gearing is related to the gap between 1st and 2nd. Even if you had a 3.1 diff, 1st would still be lower than the Caterham 6 speed matched with a 3.62 diff (standard Superlight configuration), which is quite easily manageable even with a light flywheel and no torque (K-series).

 

The span of gearing you have with the Ford box is greater than you need; the scaling is wrong and the distribution is wrong. Something like a BGH long 1st conversion would fix the distribution issue and bring the span down to be much more manageable. The diff change would fix the scaling. I think most other commentators suggest changing the gearbox first, but I can see how your priorities might lead you to change the differential first.

 

 

[John]

Blatman,

 

Next time out is Llandow and Llys y Fran.

 

John

[AMMO]

Oilyhands

 

Spoke to Brian Hill (a very nice man) today at BGH about fitting a 3.62 with lsd. Making shafts with different joints and all that. He couldn't help but put me in contact with Phil Stewart (another very nice man) who also thought the 3.62 was a bit high. He recommended staying with the 3.92 and fitting 205/60-13 tyres at the back. This would raise the gearing a tad and according to him improve various other aspects of the car. This appeals to me as there is less lying around under vehicles swopping things over and the rear tyres are rapidly wearing out anyway.

 

Peter

 

I realise that the gearbox is basically wrong. The higher diff may help to make it less wrong for my application (at a price I can afford). I understand that first gear will still be too low but at least fifth will be about right for me. In an ideal world I'd just get a five or six speed box with closer ratios and have a couple of different diffs to play with. Regarding the traffic / gearing problems, I live in Woodbridge, Suffolk. The rush hour starts at 5.30 pm and finishes at 5.35 pm and I'm usually still at work. I agree if you lived where they have proper traffic it might be a problem.

 

AMMO

[Blatman]

John

 

Curborough in Augustquestion.gif

[Pierson Broome]

Ammo,

 

Having been brought up in Suffolk, knowing Woodbridge well and now owning a cottage not far away, I'd venture to suggest that any 'rush hour' that lasts a mere 5 minutes is, indeed, my definition of 'proper traffic' - especially if enjoying a Seven....

 

smile.gif

 

Pierson