shizzl Posted January 26, 2009 Share Posted January 26, 2009 how do you work out what diff gear you need to run? the issue is,i really wanna use a IRS diff set up in my new stockcar, and will be changing the gearbox set up also. so may change to either a 3 or 4 speed HQ type box.. however i dont know what those gearbox ratios are.. all i know is that my current set up is a zf box with 1.68 3rd gear,and mazda diff, most likely a 4.1,at the end of the straights the engine is pulling 6500rpm... running a 215R15 on the right rear and a 215/70 on the left.. ive been told that the 4 speed holden box is all wrong for the normal diffs they run in them.. so if i can use a 4 speed box i will cos theyr cheap and easy to get.. but what gear ratio should i need to run??? will try to either use a nissan(skyline or sim) or rx7 diff.. if anyone can shed some light on it that would be mint,if not,tough shit' id also like to be running a lower profile tyre like a 60,,ive got some mint michelin 225/60s for it id like to run.. Quote Link to comment Share on other sites More sharing options...
Unclejake Posted January 27, 2009 Share Posted January 27, 2009 Do an internet search on "gear ratio calculator". There are heaps of online ones so take your pick. Quote Link to comment Share on other sites More sharing options...
slipsittin Posted January 28, 2009 Share Posted January 28, 2009 how do you work out what diff gear you need to run?the issue is,i really wanna use a IRS diff set up in my new stockcar, and will be changing the gearbox set up also. so may change to either a 3 or 4 speed HQ type box.. however i dont know what those gearbox ratios are.. all i know is that my current set up is a zf box with 1.68 3rd gear,and mazda diff, most likely a 4.1,at the end of the straights the engine is pulling 6500rpm... running a 215R15 on the right rear and a 215/70 on the left.. ive been told that the 4 speed holden box is all wrong for the normal diffs they run in them.. so if i can use a 4 speed box i will cos theyr cheap and easy to get.. but what gear ratio should i need to run??? will try to either use a nissan(skyline or sim) or rx7 diff.. if anyone can shed some light on it that would be mint,if not,tough shit' id also like to be running a lower profile tyre like a 60,,ive got some mint michelin 225/60s for it id like to run.. Quote Link to comment Share on other sites More sharing options...
forced Posted January 28, 2009 Share Posted January 28, 2009 You need to know what your max speed on the straights is likely to be first, then you can calculate backwards to work out how many revs you want to be doing at that speed. So at max speed you'd probably want to be towards the top end of the torque curve, so you don't drop out of it in the slower parts. Steve Quote Link to comment Share on other sites More sharing options...
slipsittin Posted January 28, 2009 Share Posted January 28, 2009 You need to know what your max speed on the straights is likely to be first, then you can calculate backwards to work out how many revs you want to be doing at that speed. So at max speed you'd probably want to be towards the top end of the torque curve, so you don't drop out of it in the slower parts. Steve Well said Quote Link to comment Share on other sites More sharing options...
shizzl Posted January 29, 2009 Author Share Posted January 29, 2009 exactly whats i was after..but we dont run speedos so at the moment the car is pulling 6500rpm,in 3rd with is 1:68, through a mazda diff that is most likely a 4:1. tyres are 205/70/15 and a 215R 15... can speed be worked out by that? Quote Link to comment Share on other sites More sharing options...
forced Posted January 29, 2009 Share Posted January 29, 2009 Maybe about 139Km/h. Of course the other way to look at it would be to look at your overall gearing now at about 5.88 and look whether you want to increase or decrease the revs and by what % then change diff / box gearing accordingly. Whenever I visit Pick a part the diffs from the Hi ace vans always seem to be missing, I've got an idea where they miight be going. Steve Quote Link to comment Share on other sites More sharing options...
slipsittin Posted January 29, 2009 Share Posted January 29, 2009 exactly whats i was after..but we dont run speedos so at the moment the car is pulling 6500rpm,in 3rd with is 1:68, through a mazda diff that is most likely a 4:1. tyres are 205/70/15 and a 215R 15... can speed be worked out by that? I think the mazda diff will be lower than you think, if not there are heaps of ratio's for them. Quote Link to comment Share on other sites More sharing options...
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