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3.62 or 3.92 rear end with 5 speed?

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  • 3.62 or 3.92 rear end with 5 speed?

    What are you Zetec and Duratec guys running if you have a 5 speed, a 3.62 or 3.92 rear end?
    My Superlight has been ordered with the 3.62 but I read on Caterham UK website that 5 speeds are delivered with a 3.92. This is, of course, with the Rover K engine. Would the Zetec/Duratec work better with a higher rear end with their greater torque?

  • #2
    rear gearing

    Hi Steve,

    I have the 3.62 diff. It seems fine...perhaps it is better suited to the wide torque range of the Duratec. I did see some threads about this on blatchat a while back:

    Opinions are split...probably depends on what you are doing with it (street vs. track) and what gearbox/ratios you are running.


    Tom "ELV15" Jones


    • #3
      If you are planning to do much road driving get the 3.62. You will operate at lower RPMs @ hiway speeds and get better mileage. I have a 3.62 with a 6 speed (no overdrive) and wish that I had an overdrive for highway use. My car sees more track & autocross time than road use and the 3.62 is still just fine :!: If you will be only on the track or autocrossing then a 3.92 MIGHT make some sense. The more HP your car has the lower your need for a high # on the rear end. Specify a ZF for your car---it's considered the better choice for higher HP cars. 8)


      • #4
        Thanks both for your input. The choice seems like no easy matter. What exactly is a "ZF"?

        The 3.62 would be fine for a 6 speed as that's what Caterham recommends. But they go with the 3.92 for the 5 speed. My use will be mostly road but I just don't know how many miles will be on highspeed roads/freeways. Seems like there are lots of differing opinions on Blatchat regard this. I already have the tall first gear and the 3.62 will make it even taller which I am not sure is the best thing.

        Think I may have to do some gear ratio figuring and see what my RPM's will be in the different gears.



        • #5
          Steve, ZF is a brand, made in Germany. If you do not specify ZF you will likely get an AP or Quaiffe differential. The ZF contains a "clutch pak" and is thought to hook up better coming out of corners. Its' performance characteristics are also thought to better protect drive line components in higher HP cars from damage under adverse conditions. It probably does not cost any more $$$ to specify ZF.

          I think the Caterham factory website offers a calculator for rear end ratios and gearboxes which may be slightly useful.

          The best way to get opinions is just post your questions on blatchat and see what comes back. a phone call to SPC or Road & Race Transmissions might also be worth the cost. 8)


          • #6
            Freeway, what is that?

            Steve, I would not bet on driving much freeway. It may be at times necessary to get somewhere but I have yet to see a Sevens driver who likes to use freeways. Noisy, boring and more risk than worth it.

            Last year I had a rear-ender accident on the I10 in stop and go traffic but with more than sufficient damage. After that I bought a trailer to haul the Seven when it is more than an hour drive.

            BTW, I have an older model Birkin with a 4.11:1 diff on a regular 5-gear T9. With my 15" wheels that makes for reasonable cruising. If I remember correctly I am doing around 3500 rpm at 70mph and around 25mpg (even with Webers). I don't know if you are planning for 13" wheels but with 3.92:1 the RPM figure should be quite similar.



            • #7
              I tried to come up with more information regarding the rear end ratio over the weekend but still couldn't make up my mind as there were some missing links I still needed to know. Caterham made it up for me as I learned this morning the car is built and packed and it's too late to make a change without incurring a large charge. So I will have a 3.92,(that should be 3.62, not 3.92) 13" wheels, and a BGH 5 speed with a 2.80 first gear. Should net me good gas mileage and a good crusing rpm. Hope the track performance is still ok, too. Car should be here this month but won't get it registered until January. Lots of time to sit in it in my garage and dream.

              Update 6-3-04 Heard back from Caterham UK that they have 3.92's in stock and there will be no charge to go that route. My email was sent on to their engineering department where, hopefully, they will know what ratio will work best for me.


              • #8
                Got a 3.92

                It's all set that my car will come with a 3.92 rear end. Caterham's engineering department wouldn't be committed to saying that I SHOULD have the 3.92 but that it looked like the better ratio.

                A site I have found very interesting is here

                Click on DURAMODS under RECENT RANTS in the left column. I found this page after a lot of looking last week. Scroll down and Jackal has a lot of information and gear charts for his Duratec car based on the 3.92 rear. Now I know what I can expect and hope the 3.92 will prove best for a hot rod Caterham rather than a car used more for crusing around. Jackal has a slightly higher first and second gear in his gearbox but and a bit more HP than I will but, otherwise, things are very close. Now I am waiting to hear when my car will be shipped. Thank you all for your help on this gearing problem. Took a while to figure it out with the help of quite a few but I think the result will be worth it.



                • #9
                  I'm going with the 3.62 rear end, but part of the reason for the taller rear end, is I'm hoping for 275hp, and 200+ ft-lb torque from a Duratec 2.3 liter (through Autocourseinc). It's a works in progress and there's one more round (at least) to go with new rods, and a higher lift cam. When all is done, I'll post dyno charts.

                  If all this works out, then the car will be pulling somewhere between .8 and .9g's in first gear with the 3.62 rear end, and 16 inch wheels.


                  • #10
                    When do you plan to be on the road? Please join us at the Streets of Willow next month REGARDLESS of whether your car is running. We'll give you a preview of the fun you're gonna be having. Be sure to bring a helmet and some sunscreen. 8)