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Final Drive Ratio for Road Racing an NA KA24..

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  • Final Drive Ratio for Road Racing an NA KA24..

    I have the opportunity to purchase a 4.3x or 4.6x (or both?) ring and pinion for my race car (Chump/LeMons....$500 racing?? PFFF!). The car has a basic NA KA24DE and currently has a 3.9 final drive gear ratio which is tall and makes 5th gear essentially useless.

    Just wanted to get some feedback from people who have either FD ratio in their car or who have experimented with both and what their thoughts are and which they preferred. I understand that it is very track dependent, but regardless, I would like to test either one out.

    Here's an idea of gear speeds with differing FD ratios and 225/50/16 tires. Mind you, I listed 6250 RPM because we short shift for reliability and because of the fact that the KA feels anemic past that range anyway:

    Core4 Motorsports
    CLICK HERE for Wilwood FSL6R Radial Bracket & Rear Wilwood BBK GROUP BUY
    S14 VQ AER Endurance Racing Team

  • #2
    Sorry, was working at the IndyCar race and just got home to reply..

    the main issue is that it's VERY track dependent. For example:

    MSR Houston has 2 corners that are high 2nd gear turns in the 4.1. I come out of the corner about 5500rpm in 2nd, then shift to 3rd as soon as I get the chassis settled. with a 4.6 rear end, these are now solid 3rd gear corners where you pull out just at the usable bottom of 3rd gear. that's the good thing.

    the bad thing is that I now have to shift to 5th gear on 3 straights, and 5th is so tall that I lose any advantage I gained getting off the corners. When you do the math on all of this, 5th gear with a 4.6 is taller overall than 4th with the 4.1.

    I now spend more time in 5th gear than in 2nd, so 5th gear performance (which suuuuucks) becomes the limiting factor.

    I have a 3.9 in the garage that I have yet to install, but I'm hoping it's faster overall than the 4.08 at some of the tracks. I just never got around to installing it, and not going to bother now since the car is for sale.
    Originally posted by SoSideways
    I don't care what color they are as long as they are LONG AND HARD.
    '04 G35 Sedan 6MT- The DD
    '96 240SX- The Track Toy

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    • #3
      It's lemons / chump. Put the 4.6 in it and have 5th suck as little as possible.
      '95 240sx

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      • #4
        Lemons and chump run on the same tracks as club racing around here.
        Using gears 2-3-4-5 on a single straight with a 4.6 compared to 3-4 with the 4.1 greatly increases the chance of a missed shift and trans/engine damage. Something else you gotta think about in a 24hr race.

        ...you'd think a Miata is reliable until you run it in a 24hr event.. in the last 3 races, we've blown 2 diffs and an engine, and our trans is leaking like the Valdez after this last true-24.
        Its not just about picking the shortest ratio and running it. Its bout picking the best one that will let you get out of corners and have engine and transmission left at the end of the race.
        Originally posted by SoSideways
        I don't care what color they are as long as they are LONG AND HARD.
        '04 G35 Sedan 6MT- The DD
        '96 240SX- The Track Toy

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        • #5
          Originally posted by Matt93SE View Post
          Lemons and chump run on the same tracks as club racing around here.
          Using gears 2-3-4-5 on a single straight with a 4.6 compared to 3-4 with the 4.1 greatly increases the chance of a missed shift and trans/engine damage. Something else you gotta think about in a 24hr race.

          ...you'd think a Miata is reliable until you run it in a 24hr event.. in the last 3 races, we've blown 2 diffs and an engine, and our trans is leaking like the Valdez after this last true-24.
          Its not just about picking the shortest ratio and running it. Its bout picking the best one that will let you get out of corners and have engine and transmission left at the end of the race.
          So your lap times are identical with the 4.1 and 4.6 at MSR Houston?

          The shifting is what I'm also worried about. The transmission is already the weak spot for this car so I'm not too inclined about Rally like shifting. Typically Lemons runs shorter configurations of tracks where top speeds are limited to around 100-110 for most cars and Chump will run longer/faster/bigger tracks (VIR, Watkins Glen, etc...Lemons doesn't touch those tracks). The 3.91 actually worked very well for us at NJMP Thunderbolt (2.2 miles) which has a long full throttle section after the last major turn back onto the main straight. That put me right around 6400-6500 rpm in 4th (or short shift into 5th for other drivers) at the end of the straight on a clean run. That is approaching 125mph, which is very fast for LeMons.

          But, the next race in New Hampshire is shorter and more technical with tighter turns (most Lemons races). Top speed will be no more than 105mph at this track in this car based on my experience in a similarly powered and handling Alfa Romeo Milano Lemons car. It will be my first time with my 240 in NH.

          So at this stage I think I may try the 4.3 instead of the 4.6. Get a little extra punch compared to the 3.91 without getting shift happy.
          Core4 Motorsports
          CLICK HERE for Wilwood FSL6R Radial Bracket & Rear Wilwood BBK GROUP BUY
          S14 VQ AER Endurance Racing Team

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          • #6
            Ideal final drive ratios are very track dependent. Unless you've got a lot of time to kill, don't waste it playing with diffs. Take more weight out of the car.

            My experience with ChumpCar is that the more chances you give drivers to break your transmissions, the more they will break your transmissions. Fewer shifts is better.

            Matt, are you on a 1.6 Miata diff? That one is known to be fragile. The 1.8 diff is known to be bullet proof.
            Last edited by gbeaty; 10-07-2013, 10:29 AM.
            Flatout Racing
            #23 Z32 Chumpcar

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            • #7
              Gills, my lap times are ~3sec slower with the 4.6 than with the 4.1 due to the additional shifting and stupid-tall 5th gear.

              So I was better off with the stock 4.1. I picked up a 3.9 in the hopes that I could run 2nd gear out of the corners just a bit longer, and a couple other spots on track where I bump the rev limiter for ~100' before braking for a turn. only the stopwatch will be able to tell if this is a better choice.

              gbeaty, I assume you were referring to me regarding the Miata diffs.. we have been running the 1.6 open diffs since LeMons REALLY likes to punish Miatas, and they're smart enough to know about the 1.8... After we blew our 2nd 1.6 diff in 3 races, the 'only one we could borrow on short notice' just happened to be a 1.8 torsen. The car handled MUUUCH better with the Torsen and we're shopping for a replacement 1.8T2. I would rather deal with a 10-15 lap penalty than 4hrs of downtime hunting down and swapping a junkyard diff in the middle of a race.

              The big problem with Miata diffs is putting 2-wheels off. If you're on the gas and the wheels are spinning when you come back on track, you can pretty much guarantee you will blow up the diff in short order. I think that's what happened to the last one since I was driving when it blew.. I went just a bit wide coming out of a turn and hit the leading edge of a curb (hit hard enough I was concerned about tire/wheel damage), then it blew up a 1/2 lap later.

              The previous one was just discovered to be dying after a race when we opened the plug to change the fluids and noticed chunks coming out with the oil.
              Originally posted by SoSideways
              I don't care what color they are as long as they are LONG AND HARD.
              '04 G35 Sedan 6MT- The DD
              '96 240SX- The Track Toy

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              • #8
                Based on the crappy 5th gear (0.759), which is why I switched mine to the 0.838, I'd use the shortest FD that would enable you to reach the necessary top speed in 4th. Plus the 5th gear and countershaft are weak pts. I've had the countershaft break in my trans. If you ever need any transmission parts let me know. I have many spares now.
                NASA Time Trials TT3


                BTW I work for Garrett

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                • #9
                  I don't know what speed corners you're dealing with, but our daq showed 3rd gear to be quicker than 2nd even in Sebring's hairpin. This is with a 4.08 rear, 255/40/17s and gentle, enduro shifting. I'd imagine with quicker shifting 2nd could be faster.

                  In my experience drivers of all cars tend to use too low a gear more often than they use too high a gear.
                  Flatout Racing
                  #23 Z32 Chumpcar

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                  • #10
                    Originally posted by gills View Post
                    But, the next race in New Hampshire is shorter and more technical with tighter turns (most Lemons races). Top speed will be no more than 105mph at this track in this car based on my experience in a similarly powered and handling Alfa Romeo Milano Lemons car. It will be my first time with my 240 in NH.
                    Wouldn't be surprised if it's not even 105 especially if they are running chicane-chicane instead of south oval. I'm positive my SR transmission is different, but I don't even get into 5th on the track in either configuration. You might go to 2nd in the uphill sections depending on your power and how much momentum you're carrying I guess.
                    97 Kouki SR 240

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                    • #11
                      Originally posted by 240sxTTC View Post
                      Based on the crappy 5th gear (0.759), which is why I switched mine to the 0.838, I'd use the shortest FD that would enable you to reach the necessary top speed in 4th. Plus the 5th gear and countershaft are weak pts. I've had the countershaft break in my trans. If you ever need any transmission parts let me know. I have many spares now.
                      This is how I was planning on going about it, which seems to be the 4.375 at almost every track I run at for LeMons, with the exception of NJMP Thunderbolt which was the first time they ever ran there. Where did you get the 0.838 gear?

                      Originally posted by gbeaty
                      I don't know what speed corners you're dealing with, but our daq showed 3rd gear to be quicker than 2nd even in Sebring's hairpin. This is with a 4.08 rear, 255/40/17s and gentle, enduro shifting. I'd imagine with quicker shifting 2nd could be faster.

                      In my experience drivers of all cars tend to use too low a gear more often than they use too high a gear.
                      Absolutely. I think most of the time it's always quicker to be in the higher gear and something my whole team is, or should be, aware of. I will be sure to reiterate that over the radio to the newer drivers.

                      As for pulling more weight out of the car, not sure how much more I can take out of the S13. It's at 2420lbs with 3/4 tank of gas and no driver in it's current trim. There are no windows, simple front bash bar instead of the OEM bumper, no dashboard or HVAC system, almost all sound deadening removed, hood is trimmed, door skins trimmed, no power steering. Something I haven't addressed is the rear hatch which is much heavier than I anticipated. Maybe another 10-20lbs in that?

                      Also with all this said, would you guys recommend changing transmission fluid after every race? Diff fluid? Went to empty a quart fluid from the transmission to replace with RL Lightweight Shockproof, and the fluid is dark grey with a metallic sheen to it with some small metallic flakes. The transmission was starting to get a little notchy going into 3rd when rushing it a little toward the end of the day on Sunday, but was minor. The fluid I have in there from the last race is fresh RL MT-90.
                      Core4 Motorsports
                      CLICK HERE for Wilwood FSL6R Radial Bracket & Rear Wilwood BBK GROUP BUY
                      S14 VQ AER Endurance Racing Team

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                      • #12
                        Originally posted by 0wn3r View Post
                        Wouldn't be surprised if it's not even 105 especially if they are running chicane-chicane instead of south oval. I'm positive my SR transmission is different, but I don't even get into 5th on the track in either configuration. You might go to 2nd in the uphill sections depending on your power and how much momentum you're carrying I guess.
                        I'm basing my assumption off of the Alfa Romeo Milano I've driven there which is almost identical in power to weight and handling capability. That car required going into 2nd in the right hand uphill hairpin because it's all top end and redlines at 7400 rpm. I could imagine the 240 KA will require going into 2nd often especially in traffic (98% of the time).

                        But you're right, it's probably closer to 100mph. I think NHMS might make the 3.91 FD a bit painful, honestly.
                        Core4 Motorsports
                        CLICK HERE for Wilwood FSL6R Radial Bracket & Rear Wilwood BBK GROUP BUY
                        S14 VQ AER Endurance Racing Team

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                        • #13
                          Originally posted by gills View Post
                          This is how I was planning on going about it, which seems to be the 4.375 at almost every track I run at for LeMons, with the exception of NJMP Thunderbolt which was the first time they ever ran there. Where did you get the 0.838 gear?
                          I purcahsed the gear set from Nissan. Then later I got three transmissions with broken output shafts or bad second gear. One of them had a 0.838 also. However it only works in the trans with syncro reverse (post #6 http://nissanroadracing.com/showthre...ynchro+reverse). Would sell. Shoot me a PM if interested.
                          NASA Time Trials TT3


                          BTW I work for Garrett

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                          • #14
                            Originally posted by gbeaty View Post
                            I don't know what speed corners you're dealing with, but our daq showed 3rd gear to be quicker than 2nd even in Sebring's hairpin. This is with a 4.08 rear, 255/40/17s and gentle, enduro shifting. I'd imagine with quicker shifting 2nd could be faster.

                            In my experience drivers of all cars tend to use too low a gear more often than they use too high a gear.
                            I've also found this to be the case with a 4.36 diff and 255/40-17s. Stack dash predictive lap timer always gives a big thumbs down to grabbing 2nd, even with a relatively quick shift (which always comes before completely tracking out). There's a big penalty for shifting when you could be on the power throughout the whole period, even if some of that is at a slightly slower acceleration (debatable on my car, since I was having to pedal it a lot in 2nd gear, while 3rd was foot to the floor).

                            It *feels* slow because you're not sitting there zinging off the rev limiter, but smoother is better, and grabbing the higher gear usually enables much less drama and calms the car down a lot.


                            I also notice that drivers carry too low a gear, and I primarily blame it on people learning at 4-5 tenths, then as they slowly bump the speed up they try to stay in the same gears and just don't know how much it's holding them back.
                            '18 Chevrolet Volt - Electric fun hatch for DD duty!


                            DefSport Koni Sleeve and Spring Perch Buy!!!
                            http://www.nissanroadracing.com/showthread.php?t=5902

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                            • #15
                              Originally posted by gills View Post
                              As for pulling more weight out of the car, not sure how much more I can take out of the S13. It's at 2420lbs with 3/4 tank of gas and no driver in it's current trim. There are no windows, simple front bash bar instead of the OEM bumper, no dashboard or HVAC system, almost all sound deadening removed, hood is trimmed, door skins trimmed, no power steering. Something I haven't addressed is the rear hatch which is much heavier than I anticipated. Maybe another 10-20lbs in that?
                              Well, my friends S13 is about 2,200 dry, on 17x9.75 XXRs, Z32 brakes with 350Z track rotors up front. He stripped the factory wiring and lightened the front bumper beam. He also runs without a rear bumper beam or fascia, and his (gutted, lexaned) hatch feels like it weighs 35 lbs or so. That said there is still a good amount of weight which could come out of his car (including some sound deadening), but much of it is hard to get to with the cage being in the way (and its a 1.75x0.95 cage, a bit heavier than the minimum requirement of 1.50x0.95.

                              Originally posted by gills View Post
                              Also with all this said, would you guys recommend changing transmission fluid after every race? Diff fluid? Went to empty a quart fluid from the transmission to replace with RL Lightweight Shockproof, and the fluid is dark grey with a metallic sheen to it with some small metallic flakes. The transmission was starting to get a little notchy going into 3rd when rushing it a little toward the end of the day on Sunday, but was minor. The fluid I have in there from the last race is fresh RL MT-90.
                              We drain trans fluid to see if there are any of the synchro insert springs in it. So far they always show up on the drain plug of our Z, but haven't showed up at all yet on my friends 240. The fluid never smells burnt, so I doubt it needs to be changed between every race. I think we'll keep doing so though, in order to catch broken bits which have made their way to the drain plug.
                              Flatout Racing
                              #23 Z32 Chumpcar

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