I cant decide! you choose (diff ratio)
Discussion
I cant decide! you choose (diff ratio)
Below are the figures for the 3 diffs available for my atlas. (without paying a fortune)
Even though the car has lots of power, it is turbo charged and has a little lag, I currently have the 3.09 in it at the moment and its ok to poodle about and nice and low reving but on track it is sluggish out of the tighter bends and putting back in first is just pointless. My only concern with going for something too high is wheel spin, which it does a lot of already. Its mainly a track day toy!
What do you guys think?
3.75
70 mph in 5th is doing 2750 rpm
Gear Mph per 1000 RPM Mph @6500 RPM Mph @7000 RPM
1 5.25 34 37
2 9.08 59 64
3 13.16 86 92
4 18.30 119 128
5 25.46 165 178
3.44
70 mph in 5th is doing 2550 rpm
Gear Mph per 1000 RPM Mph @6500 RPM Mph @7000 RPM
1 5.73 37 40
2 9.90 64 69
3 14.34 93 100
4 19.95 130 140
5 27.75 180 194
3.09
70 mph in 5th is doing 2250 rpm
Gear Mph per 1000 RPM Mph @6500 RPM Mph @7000 RPM
1 6.38 41 45
2 11.02 72 77
3 15.97 104 112
4 22.21 144 155
5 30.89 201 216
Below are the figures for the 3 diffs available for my atlas. (without paying a fortune)
Even though the car has lots of power, it is turbo charged and has a little lag, I currently have the 3.09 in it at the moment and its ok to poodle about and nice and low reving but on track it is sluggish out of the tighter bends and putting back in first is just pointless. My only concern with going for something too high is wheel spin, which it does a lot of already. Its mainly a track day toy!
What do you guys think?
3.75
70 mph in 5th is doing 2750 rpm
Gear Mph per 1000 RPM Mph @6500 RPM Mph @7000 RPM
1 5.25 34 37
2 9.08 59 64
3 13.16 86 92
4 18.30 119 128
5 25.46 165 178
3.44
70 mph in 5th is doing 2550 rpm
Gear Mph per 1000 RPM Mph @6500 RPM Mph @7000 RPM
1 5.73 37 40
2 9.90 64 69
3 14.34 93 100
4 19.95 130 140
5 27.75 180 194
3.09
70 mph in 5th is doing 2250 rpm
Gear Mph per 1000 RPM Mph @6500 RPM Mph @7000 RPM
1 6.38 41 45
2 11.02 72 77
3 15.97 104 112
4 22.21 144 155
5 30.89 201 216
It really depends on what you want from the finished car, the lower the diff ration the better it will be on the longer runs touring etc, if however you just want to blat about the higher ratio will be better for your needs.
Overall performance will also depend on your gearbox and how that works with the diff, then theres the camshaft and how the power is delivered at what revs and ultimately what gear etc, you really do need to consider more than just the diff ratio when making your decision.
Overall performance will also depend on your gearbox and how that works with the diff, then theres the camshaft and how the power is delivered at what revs and ultimately what gear etc, you really do need to consider more than just the diff ratio when making your decision.
Go for the shortest gearing you can go for without limiting top speed by gearing. There is no point gearing a track car for a speed it cannot get close to, and I wouldn't recommend gearing it so it would sit at max rpm on a straight in top gear either.
You probably want to gear the car so that the aerodynamic limit of the car, and the top speed gearing are about the same.
If you are worried about wheelspin, gearing the car down is as good as not having the power in the first place. You might at least give yourself the option of using it. If you gear longer the car to gain traction, you would be better off shortening the gearing again and turning down the power which will achieve the same thing but use less fuel to do it.
You probably want to gear the car so that the aerodynamic limit of the car, and the top speed gearing are about the same.
If you are worried about wheelspin, gearing the car down is as good as not having the power in the first place. You might at least give yourself the option of using it. If you gear longer the car to gain traction, you would be better off shortening the gearing again and turning down the power which will achieve the same thing but use less fuel to do it.
Camshaft, whats that ??? 
(its rotary powered)
The standard car that my engine and gearbox came from has a 4.1 but with 16inch wheel with a 205 /50 tyre, mine has 15 inch wheels with a 205/50 ratio but weighs half as much.
So it will probably still be lowered geared that the original car. I am going for a 3.75 and see how it goes. It should transform the acceleration which is already blistering to say the least
(its rotary powered)The standard car that my engine and gearbox came from has a 4.1 but with 16inch wheel with a 205 /50 tyre, mine has 15 inch wheels with a 205/50 ratio but weighs half as much.
So it will probably still be lowered geared that the original car. I am going for a 3.75 and see how it goes. It should transform the acceleration which is already blistering to say the least
LaurenceFrost said:
You probably want to gear the car so that the aerodynamic limit of the car, and the top speed gearing are about the same.
I'm inclined to agree, for a track only car. If it's going to do any motorway work, it might be better to gear it so that that effect is achieved in 4th, then have 5th as an overdrive.Yes indeed... I replied Saturday morning. I'll check my sent items folder later this evening (still in the office at the moment, so can't access my personal e-mail), to make sure it went to the right address, but you might want to check your spam folder, too.
ETA: yep, definitely replied Saturday.
ETA: yep, definitely replied Saturday.
Edited by Sam_68 on Tuesday 23 March 20:23
what is your max rev limit (I would have thought quite a bit higher than 7k for a rotary)
then using that look at the max speed
I would have thought you would need something around 140 but you need to look at the tracks you are going to use. You may need to look at ones higher than the ones you suggest
If you are already suffering wheelspin then getting a shorter diff may mean having to get stickier rubber (and better throttle control - it is not a switch
then using that look at the max speed
I would have thought you would need something around 140 but you need to look at the tracks you are going to use. You may need to look at ones higher than the ones you suggest
If you are already suffering wheelspin then getting a shorter diff may mean having to get stickier rubber (and better throttle control - it is not a switch
i put 7k into the gear calculator but i could go to 7500 or even 8 but my power goes flat from 7 to 7500 and then tails off, its turbo charged hence having a slightly lower rev range, its not that he engine wont do it, but there is no benefit, my shift light is set to 7 although everything seems to happen so quickly, its usually about 7500 before i manage to change gear.
Throttle control is harder with a turbo, its often easier to 'switch' it on and hold it a little bit sidways than try and feather it i'm out to have fun not be competitive
Throttle control is harder with a turbo, its often easier to 'switch' it on and hold it a little bit sidways than try and feather it
i'm out to have fun not be competitive Well the diff is in having had a refresh and new CWP, gone from a 3.09 to a 3.75
I also had a new prop shaft made, (well i sent the old home made one off to be balanced and he said he cant balance it cos its not straight , )
I also fitted new half shafts as one of mine had a slightly bent hub.
Apart from the fact that the whole car is obviously a lot smoother with no vibration the car is certainly a lot more lively to say the least, i have only run it on low boost but by god its quick. It will out accelerate my 1000cc sports bike!
Its still very civilised and cruises at 70 at 2700 rpm and does 70 in second which is nice. Oh, and if i ever hit 7000 rpm in fifth, it would be doing 180 mph so top speed isnt restricted too much
I was worried that with a higher ratio diff i would get more wheel spin but it doesnt seem to.
I am at woodbridge on saturday doing a Javeline Trackday so the test will be then to see if it gets out of the corners a little quicker.
Thanks for your advice
I also had a new prop shaft made, (well i sent the old home made one off to be balanced and he said he cant balance it cos its not straight , )
I also fitted new half shafts as one of mine had a slightly bent hub.
Apart from the fact that the whole car is obviously a lot smoother with no vibration the car is certainly a lot more lively to say the least, i have only run it on low boost but by god its quick. It will out accelerate my 1000cc sports bike!
Its still very civilised and cruises at 70 at 2700 rpm and does 70 in second which is nice. Oh, and if i ever hit 7000 rpm in fifth, it would be doing 180 mph so top speed isnt restricted too much
I was worried that with a higher ratio diff i would get more wheel spin but it doesnt seem to.
I am at woodbridge on saturday doing a Javeline Trackday so the test will be then to see if it gets out of the corners a little quicker.
Thanks for your advice
i'm not entirely sure but i would guess between 650 and 700 kg.
My bike is a thunderace with 135hp at the back wheel when dynoed. (just dont tell fat arnie )
I havent tested this theory but it certainly seems to get to speed as quick if not quicker. Also going by some 1/4 mile times that have seen people get on a thunderace and what the stylus has done previously with the taller gear ratio, 70 hp less and a slipping clutch, i would guess im quicker over the 1/4 mile now although im sure the bike would soon catch up in treble figures. Dont think i will ever know for sure.
My bike is a thunderace with 135hp at the back wheel when dynoed. (just dont tell fat arnie
)I havent tested this theory but it certainly seems to get to speed as quick if not quicker. Also going by some 1/4 mile times that have seen people get on a thunderace and what the stylus has done previously with the taller gear ratio, 70 hp less and a slipping clutch, i would guess im quicker over the 1/4 mile now although im sure the bike would soon catch up in treble figures. Dont think i will ever know for sure.
In a Stylus the shortest gearing gives you a theoretical max speed of 178mph?
No Stylus will remain on the road without becoming airborne at that speed, so in in interests of not numbing the acceleration at all lower speeds go for the shortest ratio. If you can get something which tops you out at about 150mph the car will be even more entertaining, shorter gears still if you can get the RPM up.
FWIW, years ago I had an RX-7 twin turbo - I'm sure that revved to more than 7k. (i think the imports were 8k) No torque though - again an enemy of tall gearing.
Revs and short gears make the best track cars unless you are going on tracks like the Ring or Spa.
No Stylus will remain on the road without becoming airborne at that speed, so in in interests of not numbing the acceleration at all lower speeds go for the shortest ratio. If you can get something which tops you out at about 150mph the car will be even more entertaining, shorter gears still if you can get the RPM up.
FWIW, years ago I had an RX-7 twin turbo - I'm sure that revved to more than 7k. (i think the imports were 8k) No torque though - again an enemy of tall gearing.
Revs and short gears make the best track cars unless you are going on tracks like the Ring or Spa.
You are right, i wouldnt want to be in the stylus at more than 150 mph.
The engine will rev to 8k or more but with the medium sized turbo there isnt much of a benefit of going above 7500 as power goes flat. Also, with the mazda box i'm using the first 4 gears are quite short so in 4th at 7500 rpm it is only doing 135 mph but fifth is very tall and takes it the rest of the way. 135 is probably about right for most tracks i will ever do. I use my M3 for the ring. The Twin turbo rx7's are good cars in the right hands, i have had several and currently have a late model twin turbo which is stock. They do rev to 8 k but the power is tailing off.
The other reason for going for a 3.75 ratio is that it is the highest ratio that was a stock item from ford, anything higher is after market and fetches big money and are harder to find.
Arnie, are you going to be rebuilding K2RUM? i trust you wernt hurt in the mishap.
The engine will rev to 8k or more but with the medium sized turbo there isnt much of a benefit of going above 7500 as power goes flat. Also, with the mazda box i'm using the first 4 gears are quite short so in 4th at 7500 rpm it is only doing 135 mph but fifth is very tall and takes it the rest of the way. 135 is probably about right for most tracks i will ever do. I use my M3 for the ring. The Twin turbo rx7's are good cars in the right hands, i have had several and currently have a late model twin turbo which is stock. They do rev to 8 k but the power is tailing off.
The other reason for going for a 3.75 ratio is that it is the highest ratio that was a stock item from ford, anything higher is after market and fetches big money and are harder to find.
Arnie, are you going to be rebuilding K2RUM? i trust you wernt hurt in the mishap.
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