Diff ratio for Brands GP circuit

[Copperbottom]

Anyone know which ratio I'll need for my mk1 escort? I run a 3.77 for the indy circuit and only use 4th gear along the start/finish straight where the engine revs to 7200rpm.max revs are 8k,I usually reckon on 7.5k as being the max continuous limit. I'm thinking 3.54:1 which is the highest available but don't want to overgear her.

[Al Weyman]

There is a good long straight but it is approached up hill so if your car is wanting in torque you may not get up as much top speed as you may think, you also will be entering the start/finish straight a lot faster so you will get up more speed there. I would suggest 3.54 may be a tad high but should be OK all depends on the rolling diameter of your wheels and tires of course. I have a 3.77 on 18inch rims and that would be fine but I did race it with a 3.24 on very low profile 16 inch and that was well over geared. If it were me I may contemplate leaving it as is and gain accelleration even if I lost a tad at top end.

[phoenix]

Originally Posted by Copperbottom

Anyone know which ratio I'll need for my mk1 escort? I run a 3.77 for the indy circuit and only use 4th gear along the start/finish straight where the engine revs to 7200rpm.max revs are 8k,I usually reckon on 7.5k as being the max continuous limit. I'm thinking 3.54:1 which is the highest available but don't want to overgear her.

What is the maximum speed you achieve at Snetterton and at what engine speed? If you don't run out of revs there you probably have the right diff for the GP circuit.

[GORDON STREETER]

If I was you I would leave the 3.77 in it and give it a try as I doubt you would get max revs in top along the back straight as IMHO you are overgeared anyway !
I know I use a lot more revs but the highest diff I use including Spa and Thruxton is a 4.4. on 13 inch tyres.

[zefarelly]

I'd say a 3.9, like Gordon I run a 4.4 or a 4.7 on 13" rims and the equivalent of 60 or 70 profile tyres

my revs are about the same as yours.

[Copperbottom]

Thanks,I'm on 20" tyres this year which are more like 50 profile,I used a 4.1 and 21" tyres last year at BH and seemed undergeared.I'd use a 3.5 at Snetterton. I think that you and Zefarelly would rev your engines by about 1000rpm more than I use (no steel parts) Gordon,I run a race ratio rocket box btw.

[GORDON STREETER]

The point I am making is that if you are not getting maximum revs along the top straight you wont get them along the back as you can't get round the last part of the aproaching corner that fast.
Zef has already said that he uses the same revs as you , but my engine is a different kettle of fish and is over 9000 most of the time. As for the gearbox that's nothing to do with it, as top is still a direct drive on the boxes that we three use.

[Copperbottom]

I've always thought that the ratios in the 'box are relevant as being in the right gear/revs for each corner is probably more important than for the straight?
We haven't got a base line for the GP circuit as we haven't driven on it,Snetterton isn't really any better as I had a cr*p day there last year and didn't get a chance to see if the gearing that I was using was right or not,the only circuit that I really know is Goodwood,how does the gp compare?

[GORDON STREETER]

Agreed that it is important to be in the right gear for different parts of a circuit but the higher the final drive ratio the more you have to change gear. This is obviously more important in a smaller cc car without "Yank Tank" torque and HP. If you run a lower diff the gears wont loose as many revs between changes.
Brands GP loop isn't that amazing after you have been round it a few times and since altering the corner before Dingle Dell (and Dingle Dell ) it is not as flowing as it used to be, so you can't hold as much speed as you could in the "good old days".
As for saying that top speed isn't as important, what can be better than overtaking on a straight !!!
Believe it or not but if you gear it down you will more than likely find that you will have a better top speed.
But then what would I know

[Copperbottom]

Thanks for your input,it looks like I'll run with the 3.7 but take the 3.5 to the circuit with me incase I want/have to change. I've just worked out how many gear changes I do per flying lap......................6,I'm now wondering whether to try a 3.89 for the indy as I change from 2nd to 3rd after druids then back to 2nd for the left hander,staying in 3rd until 1/4 -1/2 of the way along the pit wall where I change up to 4th at 8k then down to 3rd at paddock until druids where I go back down to 2nd for the hair pin.(hope this makes sense as I've added to it) I've found that 3rd is too high but 2nd too low around clearways.

[GORDON STREETER]

Well you've just proved to me that you are well over geared for the Indy circuit.
Take my word for it !

[Copperbottom]

Please explain but please remember that I'm running a rubber cranked crossflow that won't last long at 8k all day ;-).

[Eric Falce]

Dont take any notice of these old f###s,Zaf is in a Cortina,a lot different to a Mk1 and the other bloke doesnt know what he's talking about anyway,he revs to 10 plus so that goes to show how much he knows,just try and catch him if he ever comes out again,(should read when).
I ran a 3.7 in my Mk1 Lotus and ran out of revs on the straight onto the GP circuit,that was reving to 8 so leave the gearing as it is and see,depends on what power your getting but I think you have about the same as the twin cam.

[Tim Falce]

Why not change the wheel/tyre size?

[Copperbottom]

Originally Posted by falcemob

Why not change the wheel/tyre size?

I just have and it's transformed the car,to go taller would be a backward step in my eyes.Diff's are easy to change and I have one of each ratio.

Eric the magic number is 160

[GORDON STREETER]

Originally Posted by Copperbottom

Please explain but please remember that I'm running a rubber cranked crossflow that won't last long at 8k all day ;-).

The very fact that you said you only hit 7200 in top along the finish straight ! Even if you pull more revs along the back straight (that I doubt) it won't be "all day"

You can't compare a "twink" with a crossflow because it revs more freely

[Copperbottom]

IF I went down to a 3.9 I'd have to change up to 4th going up the hill before druids and probably along Cooper straight,then back down for clearways so I'd change gear 4 more times..........(I think)

Out of interest: maximum power is made at 7200

[GORDON STREETER]

OK don't take any notice of anything I have said

[Eric Falce]

Originally Posted by Copperbottom

I just have and it's transformed the car,to go taller would be a backward step in my eyes.Diff's are easy to change and I have one of each ratio.

Eric the magic number is 160

That was the same BHP as my T/C,but do take on board what Gordon is saying,he's been doing it for a long time and nobody can catch him so he must be doing something right,have a word with your engine builder,see what it will stand for short periods,and try the 3.9, see if it's better.

[Copperbottom]

Originally Posted by Eric Falce

That was the same BHP as my T/C,but do take on board what Gordon is saying,he's been doing it for a long time and nobody can catch him so he must be doing something right,have a word with your engine builder,see what it will stand for short periods,and try the 3.9, see if it's better.

I AM the engine( and gearbox,suspension infact the whole car) builder,and have already said that I'm going to try a 3.9 on the Indy. I believe what Gordon's saying,I just can't afford to build another engine

[morninggents]

I agree with Gordon.
I run 5-25 M X 13 Dunlop woodies (23.2" diameter) and run a 4.7 diff on both the short and GP circuits and rev to around 8,200 ( though perhaps only 8,000 on the longest straight). If your rear tyre o/d is around 21 inches I would think you would need a 4.1 diff or 3.9 at worst. 3.77 is definitely overgeared.

[Copperbottom]

Originally Posted by morninggents

I agree with Gordon.
I run 5-25 M X 13 Dunlop woodies (23.2" diameter) and run a 4.7 diff on both the short and GP circuits and rev to around 8,200 ( though perhaps only 8,000 on the longest straight). If your rear tyre o/d is around 21 inches I would think you would need a 4.1 diff or 3.9 at worst. 3.77 is definitely overgeared.

Thanks for that, the rear tyres are 20.6" tall.

[morninggents]

Originally Posted by Copperbottom

Thanks for that, the rear tyres are 20.6" tall.

I reckon a 4.125 diff should be ideal. Don't forget that although you may change through the gears at 7,500 to run to 8,000 in top should be OK as the rev build up is slower.

By the way, just my humble opinion, but we are talking about the finest circuit in the UK, just shading Oulton Park full circuit for that honour.

[R59]

Originally Posted by morninggents

By the way, just my humble opinion, but we are talking about the finest circuit in the UK, just shading Oulton Park full circuit for that honour.

I thought we were talking about Brands, not Cadwell Park.

I recall running my Nova GTE around Brands with a 4.53:1 diff on boots of similar rolling radius. It was hitting 8800 in 5th before the start/finish. I found the best diff on the Indy circuit with that engine was a 3.94 as a 4.18 would run out to well over 8500 down towards Westfield.

If you don't want to see 8000 too often, then yes, the 3.9 in your Anglebox would be idea.

Can you not fit a steel crank and let it rip?

[Copperbottom]

Right! I've built up a 3.89:1 to try next time I'm on the Indy circuit,lets hope it gains me a least 2 seconds a lap

Now back to the original question: Which ratio for the GP circuit