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  • #31
    I'm not sure if you missed it above. Here is what I wrote previously about the 4.10 ratio.

    Originally posted by Jake View Post
    The 4.10 would have put my RPM at the braking zone at about 6550rpm, which is about right on my peak torque, but still 1000rpm below peak HP.
    Do you see the disadvantage? Only peaking at 6550 at the end of the straight in 4th is not what I'd call ideal. You're pretty convinced it would have been better though. Wish I had tried a 4.10. But I just don't see how anybody could make sense of that considering all the factors.

    Originally posted by robertm View Post
    Avoiding 3-4th shifts is not a good strategy on track. A higher diff gear ratio acts like a power multiplier (torque) plain and simple. Why do you think they call it a poor mans supercharger?
    I'm not sure if you understood me correctly on "avoiding 3rd/4th". Just to be clear I wasn't skipping over it, or holding off, or short shifting, or anything like that. I just didn't want to select a set up that put the majority of my shifts at 3 to 4.

    No matter how you slice it, 4th gear is quirky in the CR box. The reason as I understand it was so that 1 - 3 was "close ratio" for acceleration, and then 4th & 5th were for autobahn cruising. So there's a weird jump when going into 4th. The RPM drop from 2 - 3 is 2284. Now when it should be less in the next gear, it's not. Its 2507 when going from 3 - 4! 4 -5 has a 1795 RPM drop.

    And I agree that in pure acceleration through the gears a shorter diff ratio will be better. Like you said, it is a poor mans supercharger. But this test eliminates certain elements present on a curvy track. Mainly, how are you set up into and out of the turns? Are your max speeds being met? How close to red line are you at the end of the straight? All that stuff matters. What you appear to be saying is that is doesn't. Not only am I saying it does, but I've given an example as to how it could (and did) matter.

    Maybe you think I'm trying to say that a taller diff is better? I hope not. I'm only saying a taller diff was better in my situation at that track and with that car. If I could have made a shorter diff work I would have gone that route instead.

    Originally posted by robertm View Post
    Your situation may have warranted doing something out of the ordinary due to an uncommon set of criteria.
    I don't think it's so uncommon, but to each their own. If you had a track with 10 straights and 6 of them have you stuck between gears with other than ideal RPM ranges, and a shorter diff ratio that gets you out of that doesn't give you the top speed you need, then the next choice is to try the other direction.

    Originally posted by robertm View Post
    In your case (based on the tire circ listed in your spreadsheet) I would go with a 4.10:1 rear end and shift at 8000 rpms in 5th gear. that will get you to 141 mph. This would be ideal for your setup and needing to reach 140mph.
    I'm not sure what's listed on the downloadable spread sheet. I made the tire dimensions a variable so you can put in whatever you wanted. The numbers you saw on there were whatever I had in there when I saved it last and uploaded it. I was actually using 205/40R17. Put that in the CR chart with a 4.10 diff and see what you get. I get 136.2mph. Unless I missed something, that wouldn't work under my conditions.

    Originally posted by robertm View Post
    On a dog leg tranny 3-4-5 are extremely usable gears. 2nd gear too occasionally. With enough gearing advantage through a higher geared diff the perceived problem of 3-4th shift will not be an issue. If it is then shift at a little higher rpms to avoid dropping out of the power band.
    I do wish I had tried this to see how effective it was at mitigating the 3 - 4 spread. Regardless, anything but a 3.46 wasn't right for the situation. With the 4.10 diff, I would have had to shift at 8250 just to touch 140mph. That's about 750 rpm past peak HP not to mention my valve train was not up to the task; may have been interesting though.

    FWIW, here is a vid of a hot lap with the final set up. You can gauge RPM and what gear was selected to get an idea. Keep in mind it's only a mildly tuned 2L S14 which dyno'd at 222hp @ 7580 & 158ftlbs @ 6750. All things considered, it was a fairly quick car.

    Click for Youtube.

    Jake <-- bed time


    • #32
      A few things I'm not understanding. There is simply no way a 'mildly' tuned 2.0L s14 is making 222 hp at the wheels. It must be more than just mild. Regardless going to 8200 rpms shouldn't be much of an issue at all, especially with the 2L motor. Throw in some motorsport valve springs and you're done. If you are really putting down 222hp then you obviously already have some pretty aggressive cams. If you have aggressive cams then you almost certainly have improved valve springs.

      A big part of your issue is running those skinny tires. The overall diameter is ~23.5" Most guys running 17" wheels are running 225/45/17, or 245/40/17 tires. These will give you a dia of ~24.6". With your tires, running 4.10:1 diff you will need to hit 8200 rpms to see 140mph. If you go to a slightly taller tire (24.6") you will only need 7800 rpms to see 140mph (right where you idicate you want to be). Only down side to going with 225 vs 205 will be a slight weight penalty. The increase of approx 3-4" more contact with the road will WAY MORE than make up for that though.

      Just something to think about. If you're happy with your setup then you shouldn't do anything


      • #33
        Originally posted by robertm View Post
        A few things I'm not understanding. There is simply no way a 'mildly' tuned 2.0L s14 is making 222 hp at the wheels. It must be more than just mild.
        I am guessign Jake is talking about his 320is he had in the UK. Seems like most UK dynos report power at the flywheel. The 2L is rated at roughly the same 195hp the 2.3s were rated at. So increasing the power to 222 (or 27 hp) doesn't seem to be that big of a leap for a "mild tune". 222 flywheel hp - ~184 RWHP.


        • #34
          Bruce is correct. Those are flywheel figures and it was on my 320is which I only owned for 10 months. The engine was stock internally except for cams which were Evo 3 & 2 (272/264) timed to 105.5 deg (well... that's what the timing worked out to, not that it was on purpose ). Externally it only had the CF airbox, Alpha N, and 50mm exhaust. Suspension was coil overs with 5 lug conversion. Like I said, it was a fun car.

          I started out with 225/50R16 Toyo R888's. They were huge. Lots of traction, in fact, more than I needed with that engine and it was a very heavy set up. So I swapped to the 205/40R17's with much lighter wheels. It was not an M3 and it lacked tire clearance, which is why I went skinny on the tires. Not sure if you could tell, but I didn't have any traction problems.

          It was my last year in the UK so it was only meant to take me to two track events at the Ring. I only had 4 weeks to prepare it and I wasn't about to go changing valve springs so I could run bigger cams on stock pistons. At the time I had the Evo cams, Schrick 284/276, and a set of KK290s. But I lacked time to go that route so I decided to go with anything I could just drop in. In the end it was an good choice that made good power.

          I put about 120 laps or so on it and when I came back after the September event I tore it back down, reinstalled all the stock parts, and sold it. It was the best project car I ever owned to date.

          Gotta go. I'll be out of contact for a few days while I'm away on a short trip.

          Last edited by Jake; 12-16-2008, 12:01 PM.


          • #35
            The issues of applying gear ratios to a specific track are almost mind-boggling to me. What helps you in one place likely hurts you in another. And it is hard to tell what is actually "faster", vs simply a setup that for whatever reason we "like" better. We all have our own preferences, and what feels better to me is not necessarily faster.

            I like low gears. The car feels faster, lighter, and more responsive to me that way. Since this is all really just for fun anyway, I won't worry too much about raising my lap times.

            My basic plan is to gear the car to redline in fifth at the highest speed I anticipate every reaching on any track. Probably around 140. That would be a 4.75 gearset with the 265 OD tranny I am using. I currently have a 4.45 gearset and a junkyard diff that I am rebuilding with it. Down the road I might convert the stock diff to 4.75, to see if I like that better.

            The only problem is the LSD units are so damn expensive.....:sosad:


            • #36
              this has become a nice little topic..

              I now have the 4.11 fresh diff in the car

              kinda annoying to try to drive it on the street BUT .....

              I think we all need understand that the top speeds etc. are hypothetical.
              even with a nice 2.5 build when i was shifting into 4th down the main straight i noticed i didn't have the pickup i had in 2nd and 3rd ...

              it's a torque thing it is about getting as fast as you can without topping out as discussed before i believe

              my 2c
              86 E30 M3 stock
              88 EvoII resto in progress
              84 M635csi JPS replica
              Searching for an E9 for a S38 transplant

              ***to be added when i can afford it***
              Sport Evo
              3.0 CSL batmobile
              and maybe an evo2 190E


              • #37
                Not to deviate too far off track, but this thread would be perfect if it specified what car/models each ratio could be found to mate with the spreadsheet info. which is very helpful thanks Jake.


                • #38
                  US OD 5spd, 4.45 50%

                  2.3L, stock cams, Turner chip, AlphaN, 18cm airbox, 235/40/17 azenis

                  No trouble reaching the top rated speed with this combo. When you think the little 2.3 pulls good in 4th, it blows your mind shifting to 5th to see the speedo head to the last marker. Pretty good for a NA 4pot.


                  • #39
                    Good idea ///Schwartzman. I'll add the info to the spread sheet.

                    For now, here is a good link with some good info on US diff's.


                    Here is a small chart from the above link.

                    There is a German E30 M3 forum that has a chart for all the Euro cars. I have the chart saved as a word doc and it's very good, but I can't post it without losing the formatting and making it useless.

                    Hope this helps,
                    Last edited by Jake; 01-02-2009, 09:33 PM.


                    • #40
                      Originally posted by Jake View Post

                      There is a German E30 M3 forum that has a chart for all the Euro cars. I have the chart saved as a word doc and it's very good, but I can't post it without losing the formatting and making it useless.

                      Hope this helps,
                      I'd be happy to convert it and post it for people if you send me the file.

                      //Euro owner
                      1987 Euro M3, Jet Black
                      "BMW M cars: technically advanced, beautifully engineered, and the choice for total gits" -- Top Gear, Summer 2008


                      • #41
                        Hi Thor. I really wanted to try and find that German forum so we could at least give them the credit for pulling the information together. My interet is sucking tonight so it's been grueling, but I think I found it. Here is the link:


                        Here is the same info but in PDF format.


                        And again in a Word .doc format.


                        Hope this is what you're looking for.



                        • #42
                          Hey Jake and all - Here you go :cheer2:

                          90 Sterling 2.5L E30///M3 lifer
                          97 Porsche Carrera S
                          13 Audi S6
                          SOLD 03 Audi RS6 daily
                          GONE 88 E28///M5 project