Ian Mackenzie Posted January 19, 2015 Share Posted January 19, 2015 Hi ThereI'm just deciding on the best Diff as I have two!The Duratec that I'm fitting should have Circa 200hp and rev limit of 7,500 with a 5 speed gearbox with the following ratios:1st 3.36:1, 2nd 1.81:1, 3rd 1.26:1, 4th 1:1 and 5th 0.82:1 (I think)With the 3.6 Diff I get the following 'calculated' top speeds in each gear: 1st = 39mph, 2nd = 73mph, 3rd = 104, 4th = 131, 5th = 160mphWith the 3.9 Diff I get the following 'calcuated' top speeds in each gear:1st = 36mph, 2nd = 67mph, 3rd = 90mph, 4th = 113, 5th = 148mphGut feel looks like the 3.9 Diff, but would welcome opinions....CheersIan Link to comment Share on other sites More sharing options...
DJ. Posted January 19, 2015 Share Posted January 19, 2015 I would go with the 3.9. First is a little longer with the 3.6, but with the 3.9 you'll get better acceleration in 3rd and 4th, and also 5th will be a more useable gear rather than overdrive.Of course if you're mainly touring and want better range/economy then go for the 3.6 Duncan Link to comment Share on other sites More sharing options...
7 wonders of the world Posted January 19, 2015 Share Posted January 19, 2015 3.9 mated to a decent set of 3J ratios Ian Link to comment Share on other sites More sharing options...
Grubbster Posted January 19, 2015 Share Posted January 19, 2015 With 200 bhp I'd go with the 3.9 to give you a more useful 1st and 2nd, you'll have more than enough oomph for 3rd and 4th - unless you are only really doing trackdays then the 3.6 would be marginally better. I run a SPC long ratio 5 speed (long 1st, 2nd and 3rd) with a 3.9 (230 bhp) and it's excellent. Link to comment Share on other sites More sharing options...
Mankee Posted January 20, 2015 Share Posted January 20, 2015 I would go for a 3.92 as well, but like 7WOTW says, a decent set of ratios is recommended, as 1st is just silly short and 5th is slightly too long. Even with 200bhp, I don't think you'll get much more than 140mph, due to the crap aerodynamics of our cars. I couldn't get above above 7200rpm in 5th out of my 200bhp K-series, which, with the tyres I was using at the time, equated to just under 139mph.Correction: I actually got to only 6800rpm in 5th and 137.45mph. Limp. Link to comment Share on other sites More sharing options...
Member Jonathan Kay Posted January 20, 2015 Member Share Posted January 20, 2015 What Duncan says: how much do you value the lower engine speed when cruising at 70 mph: 3,280 v 3,550 rpm?Jonathan Link to comment Share on other sites More sharing options...
Area Representative Richard Price Posted January 20, 2015 Area Representative Share Posted January 20, 2015 Ian,You don't say what your planning on using the car for.For track use, the stock 5 speed 'box is hopeless with either diff. 1st is way too low.If you're predominantly using the car on the road and touring, then the overdrive top AND a tall (3.6) diff would make for more relaxed cruising and better economy.Additionally, you don't say what wheels and tyres you're using, which, again, affect overall gearing.7WofTW is right, a completely different set of ratios in the gearbox would make the car much nicer to drive. Link to comment Share on other sites More sharing options...
Slipper man Posted January 20, 2015 Share Posted January 20, 2015 3.6 for the road, 3.9 for the track. Depends where you will use it most. Link to comment Share on other sites More sharing options...
Grubbster Posted January 20, 2015 Share Posted January 20, 2015 That's a good top end Mankee, is that with an aero screen? I still use a full windscreen and top end in fourth is a vbox 130 mph (indicated 146) and even with a slightly shortened 5th gear it won't pull any more. I wonder what difference an aero would make? Link to comment Share on other sites More sharing options...
Mankee Posted January 20, 2015 Share Posted January 20, 2015 Yeah, that was with aeroscreen, 205/55R13 CR500s on the back and standard Caterham/Ford V6 Type 9 with the 0.825 5th and a 3.92 diff. The speedo was reading just over 160mph, so it was pretty far out! I think you need to invest in an aeroscreen for this year. I've got a box with a slightly shorter 5th now. Link to comment Share on other sites More sharing options...
Grubbster Posted January 20, 2015 Share Posted January 20, 2015 I've got an aero, never tried it though, this will be the year that I do! Link to comment Share on other sites More sharing options...
Ian Mackenzie Posted January 20, 2015 Author Share Posted January 20, 2015 Hi thereThanks for all the comments......Yes ideally a 5 speed box with better ratios is the way to go, but all the pocket money has been used puting the Duratec in at present (But I do have 2 diffs in garage to choose from without spending anymore cash).The speeds I've used have been based on the rolling circumference of some Yokohama 048s (185x60x13), though I'll probably have 205s on the back when the car is finnished so the speeds will change slightly again.Regarding the use of the car, I'm looking at building close to a sprint spec though not full sprint spec (so performace rather than touring) as I'll still be on a medium compound tyres....CheersIan Link to comment Share on other sites More sharing options...
7 wonders of the world Posted January 20, 2015 Share Posted January 20, 2015 Ian, I will BM you a spreadsheet for you to look at................ you know you want those ratios really Link to comment Share on other sites More sharing options...
F355GTS Posted January 20, 2015 Share Posted January 20, 2015 Ianfwiw I'm going 6 speed and 3.62 diff Link to comment Share on other sites More sharing options...
Support Team Shaun_E Posted January 20, 2015 Support Team Share Posted January 20, 2015 How about 6 speed and 3.92 diff? Not ideal for touring but great on the hillclimbs!! Our car will do 130mph @ 8500rpm in 6th - the only problem occurs at Goodwood where we are on the limiter for some time on both straights.I'm not helping am I? Link to comment Share on other sites More sharing options...
paul jacobs Posted January 21, 2015 Share Posted January 21, 2015 Quite frankly, I think that the 3.6 will be more than long enough for mainly road with a bit of track work thrown in from time to time. Even with longer 1st and 2nd ratios it will be fine. Whilst my 2.3 Duratec is a little more torquey, that is what I have in my CSR and I could really use a 3.3 diff, and thats with a BGH geartec modified box with longer 1s & 2nd ratios too.The only use for the 3.9 is on a revy low[ish] torque motor like a 1400 & 1600 K series, I know everyone will have their own view on this, but I've had numerous combinations over the years and would always advise a 3.6 with any gearbox behind a Duratec.However, I'm sure what you put in will work, just be sure to resurect this thread and let us know, honestly, what you think of your chosen setup. Link to comment Share on other sites More sharing options...
Recommended Posts
Create an account or sign in to comment
You need to be a member in order to leave a comment
Create an account
Sign up for a new account in our community. It's easy!
Register a new accountSign in
Already have an account? Sign in here.
Sign In Now