Announcement

Collapse
No announcement yet.

1989 hatch autocross build

Collapse
X
  • Filter
  • Time
  • Show
Clear All
new posts

  • 1989 hatch autocross build

    I figure that I'll start a build log.

    Here are some pictures begining shortly after I got the car.
    Attached Files

  • #2
    You may have mentioned it elsewhere but here would be a great place to mention what you've done to the car. Is that an RB25 under there? Are the silver door skins for because you've gutted them for weight savings? What size tires are you running?

    I have a question about your splitter/air dam. I would have thought having more horizontal surface area would provide more downforce despite being under the overhang of the front bumper. But if you're limited to a certain area (instead of distance from bodywork like SP and SM) then I can see moving the splitter forward and covering the back side of the splitter making that area more effective. I mostly ask because in SP I'm limited to the silhouette of the body work. If that area isn't very effective, then building one for my S14 (has a similar pocket under the bumper beam also) would be a waste.

    Comment


    • #3
      It has an rb20det in it. The silver door skins are 25 thousands aluminum, for weight savings. The car is currently running 275mm a6's on 15x10 runs, this may change after seeing how well the goodyears did at nationals this year.
      XP's rules are similar to SM's, 6in past the front bodywork. My theory for closing up the front were (although they may not be right): the high pressure under the bumper support causes a force toward the low pressure regions above and below the pocket causing lift from underneith bumper support and downforce below it. I thought that this would cause a force cancelling effect, and would allow move of the air to evacuate at the lower sides without as much going over the top of the car. Plus it took off around 5lbs off the nose.

      Comment


      • #4
        Here is what I left of the doors.
        Attached Files

        Comment


        • #5
          Originally posted by josh_s13 View Post
          XP's rules are similar to SM's, 6in past the front bodywork. My theory for closing up the front were (although they may not be right): the high pressure under the bumper support causes a force toward the low pressure regions above and below the pocket causing lift from underneith bumper support and downforce below it. I thought that this would cause a force cancelling effect, and would allow move of the air to evacuate at the lower sides without as much going over the top of the car. Plus it took off around 5lbs off the nose.
          I agree about the pressure differential above and below the pignose bumper. I like what you've done. It's a little ruff, but effective. The splitter fences and the kickouts in front of the tires are a nice touch!

          Comment


          • #6
            The hood venting looks good too. My super light hood bowed up about 2 inches at the shock tower at 70mph. I like where you are going with this Josh. Good seeing you last weekend. That course was way too tight for our cars though, no place to use any power. They have way too many Miata's and Lotus'sss down there.

            Comment


            • #7
              Brian? Hutichson or Clemson event?

              Comment


              • #8
                I got around to replacing my splitter that was falling apart with a new alumilite one. Also got around to weighing the car, 2510lbs with me in it. It's a bit heavier than I thought.

                Comment


                • #9
                  yeah....that's a bit on the heavy side Josh. (not sure how much you weigh..)

                  FYI, Mikey's car (S13 coupe) weighs in around 2320 lbs without driver, and my coupe is around 2340 without driver. For a gutted hatch with gutted doors, - you should be able to get the car down below 2100 lbs if done properly.

                  Comment


                  • #10
                    Josh, Mind if I build one this winter too? I have been thinking about this for some time. XP sounds like fun. Mine might have a couple less cylinders and no hatch though. We could have fun at Packwood. Not sure yet but I am thinking so hard you can see smoke coming out of my ears.

                    MikeyV's car was down to 2280 at one point. I'm thinking 2050 is doable but not sure I want to take out the windows.

                    Comment


                    • #11
                      I was suprised that it weighed as much as it did, I was expecting a couple hundred lbs lighter. I am not that heavy. The only thing that I can figure is the rb is weighing it down. 58% of the weight is on the front wheels. There maybe some error in the weight, I used scales below there intended range.

                      Comment


                      • #12
                        Logr, the S chassis has a handful of weaknesses in the class, weight, firewall position, and track width are the biggest ones that come to mind. I've thought that the S chassis may work better in EM, where you can move the firewall and do major hacking to the chassis. I have had a great time with this build, but for what its worth I think that a miata would cost less to be competitive in xp. A light 4 cyclinder would be a way better engine than what I have. I don't think that coupe or hatch will make much difference because the hatch or trunk only have to maintain approx factory body lines, be able to open, and be able to support the wing. I think that the weight difference would be very minimal if you attempt to build the trunk or hatch to the max that the rules allow. That said, go for it!

                        Comment


                        • #13
                          I think XP will negate some of those weaknesses. A lighter shorter engine with all the stuff that can be removed from the front within the XP rules could possibly balance out the car. The 53% front is a problem in my opinion. My SX was always tail happy with relatively low power for the class, thus my bigger tire comment. I think you will need 315's to overcome that. My Z with 51% on the back tires hooks up amazingly.

                          The track width is less than an MX5 with the same wheel width. I measured Victor's car and mine. I'm also not sure it is a big issue since C5's can do pretty well. The hatch can help with f/r balance but only if you leave the glass so in XP, coupe or hatch is a moot point except the coupe may be a bit stiffer.

                          My opinion is that front track width is not as important as rear track width. Extra front width can help turn in but extra rear width runs over cones, at least for me.

                          Vitek has an MX5 with an LS7 and huge tires in XP. I am waiting to see how he does as it develops. He has had quite a bunch of issues but is working through them. He is the axware guy so we talk quite often as I do a lot of timing for several clubs. He knows his stuff. We have some crazy conversations.

                          I'm looking forward to see how you do with this car. A car that can do tight courses as well as power ones is the key BUT it needs to be easy to drive and an sx fills that bill.

                          Comment


                          • #14
                            Well, I got some good working on the car plans for the next upcoming couple of weeks. I have some 550lbs springs on order for the front, and plan to move the current 450lbs springs to the rear of the car. Getting a new front sway bar and hopefully going to be addressing the front roll center.

                            Also have some repair worked lined up as I cooked through another manifold to turbo gasket. I have some new gaskets on order, going to try the OEM ones this time. Planning on making new locking tabs to keep the nuts from backing out. When I drop the exhaust I am going to cut off the muffler and remount the exhaust to try to reduce stress on the turbo to manifold joint. Hopefully the turbo will keep the noise below 99dB.

                            Comment


                            • #15
                              You will probably need more than the turbo. I like the Vibrant resonators. Cheap, SS and deep mellow sound as well as never having an issue with noise level. I always use a turn down under the car which might help too. Supporting the exhaust and having a flex after the turbo helps keep things on the engine.

                              It is looking like I might hold off on an XP car for this year and concentrate on my Z.

                              Front roll center correction does help these cars a bunch.

                              Josh, have you considered GT2 tires. They make one under 24"tall and 10.6" wide. It is what Ron Bauer runs on his DP car using Real wheels.

                              Comment

                              Working...
                              X