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Best engine for an XP autocross car

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  • Best engine for an XP autocross car

    In a recent evaluation of the long term plans of my 240 I am beginning to consider swapping out my rb20 sometime down the road. The engine is relativily heavy and places alot of that weight pretty far foward in the car. From what I can come up with the rb20 weighs about 470lbs w/tranny, sr20 around 390, and ca18 around 360.

    Going with a large motor isnt feasable due to minimum weigh penlty. The rules state mimimum weight as 1200lbs + 200lbs/liter for NA motors or 1200lbs + 280lbs for forced induction motors. Assuming that I can get a s13 chassis down to 2000lbs that gives me a maximum of 2.8L forced induction or 4.0L NA motor. The rules in XP allow for any automotive derived engine.

    My thoughts are that the replacement motor should be able to deliver in excess of 400bhp with minimal turbo lag and some midrange torque. What engine do you think would best fit the bill?

  • #2
    I like where Tower is going with his VE. Depending on other mods, it would be SM legal as well.
    Chicago Region SCCA SM # 688 http://www.scca-chicago.com
    TSSCC SM # 688 http://www.tsscc.org

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    • #3
      this isn't exactly what you asked but i have personally been toying with the idea of a XP miata with something like tower's setup, except use a sr16ve crank to destroke it to 1.6l. i'll bet you could spin it pretty fast (10-11k) without piston speed problems (but i haven't actually run the numbers so i'm guessing).

      the miata chassis can hit 1800 pretty easily, and i was curious if it can hit 1650 in xp trim.

      1200 + 280*1.6 = 1648.0

      i know, different animal, but i thought i would throw it out there, at least as something for you to make sure you can beat
      Last edited by tonto; 06-29-2012, 06:59 AM.

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      • #4
        A 1650lb miata could be an awsome autocross car. Would it be feasable to build a relativily high compression / high rev ca18det?

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        • #5
          Ideally you want something with a very predictable power band and good low / mid-range for Auto-X.

          As much as it pains me to say it, an LSx is probably best fit for something like this. I would never do that, however, plus not sure if it fits in the rules (haven't had a chance to really get into it yet). That being said, I would likely go with an SR for weight distribution and the ability to build with a quick-spooling, consistent set-up.

          You could, potentially, go with a higher compression N/A RB25 set up but then you have weight distribution issues again.
          sigpic
          || Endurance Motorports ||

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          • #6
            LS's are basically out of you want to compete at the top level. The two fastest XP cars right now are a 1600lb lotus with 360whp, and a 2100lb FD with 450. Both have very wide torque curves. Assuming a 5.7L, min weight is at 2230. I can't imagine an outdated chassis competing with either of the two cars, unless sitting down at the bottom of min weight.

            I got for the VE turbo setup, maybe with antilag
            Originally posted by Jason M
            I have no chance to win without the Giken...

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            • #7
              I bet you could get a S13 coupe (isn't that what you have?) well below 2000lbs within XP rules. 1760lbs with a SR20 sounds attainable. If not, a 2.3 or something would be a step in the right direction.

              I've always thought a supercharger would make for a better autocross engine. Lower power, but instant torque. You could also run a ice-bath air-to-water for super low charge temps and you could take some weight off the nose.

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              • #8
                What's the weight of JBennets XP 240? If you're going to a class with weight-per-displacement rules you need to have a good idea how low you can go before picking a motor. You might even want to build the chassis first, then pick a motor. You don't want to pick a smaller motor than what's allowed at the lowest weight you can get the car to. I've been following another thread with a guy building an XP corvette with an Ecotec motor with turbo and anti-lag. I believe the Lotus switched from turbo to supercharger since moving to XP. The trouble with NA is that if you build a small engine to have enough top end, you end up killing the low end torque. If you go for displacment you end up being too heavy.
                Don Johnson (really!)
                Just so you know.

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                • #9
                  I think my motor would work well for your application, If anything I'd build it to 11:1 CR or 11.5:1 instead of the 10.5:1 of my setup, I have lots of torque in low revs and with 2 step set to 4K it builds a little over 12psi...and I can't make that stick in 2nd gear, I think that would be a badass
                  I am SKULLWORKS

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                  • #10
                    The LS engines have the perfect powerband, but as said before raises the minimum weight to a level that I stand no chance of being nationally competitive. I figure that I can get the car down to between 2000 and 2050 with the rb20. This is with retaining lighting and associated wiring to maintain being street legal, although not so much street safe. I think that any weight below 1950 to 2000 lbs will have to be found by reducing engine weight. BMW and Toyota both have decent sub 4L V8, but I think that they may not be any lighter that what I already have. The VE sounds like a good option.

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                    • #11
                      You're trying to make a nationally competitive street legal car? Good luck.

                      Where are you at now weight and weight reduction wise? I'm with DJ, build the chassis first and see where you're at. If you're all about getting the weight low to be competitive, get it as low as you can, then pick an engine based on that.

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                      • #12
                        If you go with one like Towers, you could always get back to SM depending how far your car is now with other stuff. Why XP?
                        Chicago Region SCCA SM # 688 http://www.scca-chicago.com
                        TSSCC SM # 688 http://www.tsscc.org

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                        • #13
                          When I started autocrossing the car at the beginning of last year I believe the sm rules required engines that were sold in us production vehicles. The car already had the rb in it so I started chucking parts. Besides xp allows you to do almost whatever you want. I figure as far as the street legal part, there isn't much sense pulling out the lighting until I am close enough the cars that are in the top of the national field, if that ever happens. There is alot more than the excess 50 or so pounds that I could remove keeping me from having any chance being competitive at a national level.

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                          • #14
                            Street mod allows plenty of modification. Imo, Sm is as far as you can go while keeping the car relatively streetable.

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                            • #15
                              Both the SR and RB are legal in SM. The engine only needs to be a production block from the same manufacturer, not necessary to be US engine. I'm sure you can lose more than 50 lbs. I'm assuming you still have stock doors, there's probably 75-100 lbs right there. The two serious local prepared guys are running plastic doors that weigh around 5 lbs each.

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