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Thread: Building an Exocet for ST2/3

  1. #61
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    I think you need to escalate the arms race.
    This would involve staking out YET ANOTHER SoCal freeway off-ramp and spending a lot of time there with my collection jar. This and another 10 years or so and maybe I will be in the arms race. But, of course by then it's gonna be a Duracell versus Eveready race.

  2. #62
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    I haven't updated this in awhile but progress on the Exocet build continues. Last weekend we completed the fuel lines and oil lines (oil cooler and accusump) and made good progress on understanding what we were going to need for the coolant lines. (Honda engine and Miata aftermarket aluminum radiator.... and "interesting" in and out ports on the engine.)

    We also started laying out the wiring which we are more or less doing from scratch except for the engine harness. Everything we have read suggests that the K24A2 breaths best through a full 3" exhaust. I tried (unsuccessfully) to convince Ryan that 2.5" would be fine.... but he won the argument and now I have 3" SS plumbing pieces laying around the garage. Holy cow, this stuff is expensive! A couple of 4' straight sections, a couple of 15-degree bends, couple of 90-degree, a u-bend, v-band clamps, and a ss muffler and the tab before shipping was $500!! Ouch!

    I'm thinking that there's some potential for a first start this weekend and I'd be really disappointed if we didn't achieve this during the long weekend coming up. It's great to see some light at the end of this tunnel.

    Our home track (Spring Mountain) is shut-down during August for construction (probably hot as hell anyway), so I think we are targeting September for on-track dialing in sessions. Thinking this will be a dedicated 2-3 day effort and this is where it helps to have virtually unlimited track time.

    But, before the track time, we'll need to spend time on the dyno to get the engine management dialed in. We want to have two tunes. One tune being a max (safe-for-endurance) output and another tune to be in the ST3 class-compliant arena. From what we have read, the all-out tune should be in the 220ish RWHP and assuming the car is around 1750 with driver, we'll likely target the ST3 tune at ~ 175 RWHP. (We'll address the rule "subtractions" with ballast.)

    Anyway, I hope to post up a few pics during the weekend. I have so much respect for the other build threads where they have a ton of pics. I, on the other hand, have the best of intentions but when working on the car I tend to get narrowly focused on working on the car! LOL

  3. #63
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    Achieved a milestone last night with the first start of the K24 in the Exocet. We have the Honda engine harness but ALL of the wiring on the body side is custom. We wired the push-button start to be hot with the main battery switch on (but ignition not on) so priming is now trivial. We did a couple of 12 second bursts to get oil pressure and then switched on the ignition and she fired up!

    Attaching a video here of the second start. We need to tidy up a lot of the wiring today as we intentionally left it un-bundled to aid in trouble-shooting which we ended up not having to do!

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  4. #64
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    Finally got the Exocet out to the track for the first time! Spent a lot of time on the dyno prior to this developing a tune for the K24. This is our first experience with tuning a Honda motor and proved to be a lot more challenging. We developed an unrestricted tune at 220 RWHP and a "restricted" tune for ST3 class compliance at about 180 RWHP. (The Exocet is going to be around 1800 lbs with driver so we're in the ballpark now after we make some further rules-based adjustments.) We currently have 450/336 springs on it which with only about 40 minutes of track time still seems too stiff. We had some electrical nuances (may have been sending the ECU some unexpected signals as we botched a home-made circuit to adapt the Miata's VSS to something the Hondata wanted.)

    Anyway, we're looking forward to having it in a NASA (ST3) sprint race - maybe March if all goes well. I think it will take a season or so to get the suspension all dialed in.

    img_5563.jpg
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  5. #65
    http://www.trackhq.com/Banners/yellowsitesponsor.gif emilio700's Avatar
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    F&R ride height, measured at the little welded on jack pad tabs?
    Which shock?
    Wheel offset and width?
    Tire model and size?
    F&R sway bar diameters?
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    But obviously I just dont get it. -fatbillybob

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    Hi Emilio. We are at our very infancy in setting up the suspension... we will be trying some different set-ups as we get some more track time. Your feedback and that from others may help to kick us into the right direction.

    We are using the Ohlins shock with 450/336 spring rates. At the moment we are utilizing the Exocet-sourced shock spacer on the front and a 1/4" spacer at the rear.

    Current ride heights are 5.25" at the jack pad tabs. I think it still may be a bit low.

    15x8 0 offset wheels. Toyo RR 205/50

    Currently have OEM (1999) F&R sway bars.

    Again, this is just our "out of box" setup. Car was relatively neutral - no crazy push or oversteer. Cool morning (~55F) so we had pressures at 22 to get a little heat. Ran only with the 180 RWHP map. We have a long ways to go!

  7. #67
    http://www.trackhq.com/Banners/yellowsitesponsor.gif emilio700's Avatar
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    That shock has very limited stroke. At that ride height with the 1/4" rear spacer, you will likely be topping out the rear suspension frequently.. we have done some development work with the Exocet. You must run the Exomotive supplied rear spacer which is closer to 50mm.
    The Exocet chassis is not symmetrical with the Miata chassis so you have to run the rear about 12mm lower than the front to match a Miata that is level. Like most good suspensions, the Miata is highly sensitive to rake and will strongly affect the resultant balance. You don't want to run with the incorrect rake and attempt to Band-Aid the balance by putting funky springs or sway bars on it.

    The next issue is your wheel offset. A 15 x 8 needs to be no lower than 25 mm offset. At +0 you are effectively changing your motion ratio, which significantly lowers your wheel rate which makes your Springs actually too soft. That the car feels too stiff indicates you are running out of travel, probably on both ends. The stock NB1 sway bars are perfect what for what you are doing so I would stick with those.
    I would suggest pulling the springs and measuring your bump and droop travel front and rear at the target ride height and rake. Since those shock are stroke limited you want to make sure that you're running about 50 to 70% sag. IIRC, the rear Ohlins DFV has something like 85mm stroke. Our Xida XL in comparison, about 130mm. Your spring rates are about what they should be for a racing focused set up.
    Last edited by emilio700; 12-31-2017 at 04:02 PM.
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  8. #68
    http://www.trackhq.com/Banners/yellowsitesponsor.gif emilio700's Avatar
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    Typed that last post from my phone, sorry for the typos. Back to the shop, looked up the specs on the Ohlins. Assuming you are running the Japanese made length adjustable Ohlins, the specs are as follows
    DFV
    F 96mm
    R 76mm

    PCV (same cartridge front and rear)
    F 84mm
    R 84mm

    If you are using custom built TTX, they might be longer. I'd suggest determining stroke on the shocks so you know what you are working with. If you decide to start over with shocks actually designed for the car, give us a call
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    But obviously I just dont get it. -fatbillybob

  9. #69
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    If you decide to start over with shocks actually designed for the car, give us a call
    As we have only started the set-up process, not quite ready to pitch the Ohlins!

    Are you still recommending a frame height of 8-11" as you were in Post 26? (We are quite a bit lower than that.)

    In that same post 26, you said you were also developing a 15x8 0-offset wheel "specifically for the Exocet". Seems you are no longer recommending?

    The shock body on the Ohlins has a LOT of adjust-ability. Running a 1/4" spacer with ~35mm extended shock body (without exceeding limit lines) seems the same as running with the Exocet spacer and a non-extended shock body?

    The Exocet chassis is not symmetrical with the Miata chassis so you have to run the rear about 12mm lower than the front to match a Miata that is level.
    Can you elaborate on how you measured this?

    I would suggest pulling the springs and measuring your bump and droop travel front and rear at the target ride height and rake.
    Seems like good advice, thanks!

  10. #70
    http://www.trackhq.com/Banners/yellowsitesponsor.gif emilio700's Avatar
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    The Exocet community begged us for a 15x8 +0 because they "fit perfectly".
    We announced we would be making the size, going so far as to make prototypes. We discovered that the Exocet community was ah, mistaken. The correct offset is closer to +25, so that's what we put into production. Should be here in 6-8 weeks.
    Our Exocet specific 15x9 +12 started shipping a few months ago.

    We measured subframe heights then correlated those numbers to repeatable frame height measurements. Well, as repeatable as you can get with an Exocet. They seem to have a rather wide tolerance for frame dimensions. US made Exocets have their rear subframes lowered relative to the front. That unfortunate design choice requires running the rear ride height lower than the front to maintain the roll center axis the chassis was designed around.
    It also puts the rear suspension into the steeper part of it's camber curve and motion ratio. Also not good but it's what we have to work with.

    Lengthing the rear shocks to compensate for having the wrong shock spacers is one of the "band-aids" I recommended against

    Ride height recommendation remains as stated.Read this if you get a moment.
    Miata Race alignment
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    While I agree that a +25 offset would be preferred, I don't think that running that offset will be realistic for racers - specifically those cars required to run the requisite fenders. The fender support themselves (especially from Exocet) are junk and making/triangulating them from scratch is a PITA. Frankly, making those to work on a +25 offset wheel and insuring no interference (especially with the frame) through the suspension range of motion would be a real feat. Most likely, someone opting for a +25 offset will likely have to incorporate a fixed fender solution.

    I too want to measure subframe heights and compare to one of the Spec Miatas we have here and see if I can corroborate your assertions regarding subframe height differences. It could very well be the case but I need to see for myself.

    Still not sure why you want to refer to taller rear shock as a band-aid as physically it seems like the same thing?

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    Curious to see what numbers you come up with on your chassis. Equalize subframe heights from ground plane on both chassis. Then compare pinch weld/jack tab heights.

    Lengthening shock body doesn't create more stroke. Do the bump/droop check I suggested. Then reference it to your desired ride height. The set up restrictions dictated by the lack of stroke will be apparent. Adjusting shock length won't fix that, thus the "band-aid" comment.
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    But obviously I just dont get it. -fatbillybob

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    Duh, I realize that changing shock length doesn't change stroke - any more than adding a spacer won't change stroke. Increasing shock body length or adding a spacer is the same thing - hence my question of why you called it a band-aid. Aesthetically if nothing else, I guess I prefer to run without the funky caged spacer - as long as I'm not compromising geometry.

    I'll let you know what measurements we come up with - it will probably be next weekend.

  14. #74
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    see what numbers you come up with on your chassis. Equalize subframe heights from ground plane on both chassis. Then compare pinch weld/jack tab heights.
    Spent a couple of hours swapping the Exocet and the Spec Miata onto the lift and doing some comprehensive measurements.

    We were largely using a tape measure and not a mircometer - so I would venture that our degree of accuracy at about 1/16".

    Like so many inaccurate assertions that you have made in the past about the Exocet, the allegation that subframe heights (relative to the chassis) are different is also incorrect. Our measurements show that they are identical.

    Having the opportunity to understand more, we now know what Exocet jack pad heights equate to specific a-arm angles and how these jack pad heights relate to Spec Miata pinch weld heights yielding the same a-arm angles.

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    Quote Originally Posted by CoolTech View Post

    Like so many inaccurate assertions that you have made in the past about the Exocet, the allegation that subframe heights (relative to the chassis) are different is also incorrect. Our measurements show that they are identical.
    Is it possible that factory has made adjustments? Since I haven't built one, I don't know but also could it be an issue created by individual builder/assemblers?

    I've always been intrigued by the Exocet, in fact I started a thread about them many years ago on this site I think before they were available in the US. Hopefully all of the issues or perhaps inaccuracies can be fixed going forward.
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  16. #76
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    Is it possible that factory has made adjustments? Since I haven't built one, I don't know but also could it be an issue created by individual builder/assemblers?

    I've always been intrigued by the Exocet, in fact I started a thread about them many years ago on this site I think before they were available in the US. Hopefully all of the issues or perhaps inaccuracies can be fixed going forward.
    I really believe that since launch in the US (by Exomotive) the chassis have all been the same. All of them are made on jigs using pre-fabricated parts (tubing, notches, gussets, etc.). I simply can't imagine the variances that have been alleged. I know that Emilio and perhaps others were working with some early-adopter owners and were literally surprised to "catch" the shock tower issue and this no doubt pissed some folks off. (It would me!)

    In complete transparency, I will tell you what i believe is the totality of the frame geometry issues - in order of importance.

    1. The design brings together the main tubing at a point just in front of the rear wheels and where the main roll bar hoop extends vertically. While I'm not making excuses for Exomotive, I'm old enough to know that there are always trade-offs. In the case of the Exocet, an -25 offset 8" wheel may just fit if you are running a 205 tire. A 225 tire may rub (under load). In our case, we are using a 0-offset 8" wheel because we needed a bit more room to fit the support rods for the obligatory fenders. I wish I could use the -25 offset wheel as the roll ceneters would be closer to ideal and certainly closer to a Spec Miata where there is a wealth of knowledge.
    As you can see, this "shortcoming" is not the biggest deal for us - but maybe a bigger deal for those peeps wanting to use a much wider wheel/tire for aesthetics or whatever. For road racing, given the weight of the Exocet, I think a 205 or at most a 225 tires is going to be just fine.

    2. Exomotive followed the MEV kit insofar that they came to market with a low-cost kit where you could take your Miata parts and put them onto the Exocet chassis. In their infinite wisdom, they placed the shock towers ~40mm higher than the OEM Miata - the effect of which is that those peeps moving their suspension over would achieve a lower ride height without having to purchase coil-overs, etc. As such, early Exocet adopters (like one of Emilio's customers) who purchased performance suspension for the car really got a bad surprise. These suspension kits (and I'm oversimplyfying) were already designed to lower (and stiffen) an OEM Miata and when put onto an early Exocet chassis, they just didn't work. (Combine this with wheels and tires that might otherwise work and they don't fit either - and I can understand people's initial reaction that the Exocet was a POS!) But, the reality is that Exomotive came out with an elegant simple fix which consisted of ~40mm shock spacers which essentially serve to shorten the top hat of the shock tower. Both simple and effective.

    That is really it on "geometry issues" of the Exocet. I will say, however, that as a low cost kit targeted at lower budget car peeps (as opposed to avid racers, for example), almost all the cars I have seen are set-up improperly. Most are just way too low on ride height and the a-arm angles (and ball-joint stress) are just ridiculous - let alone the cars will not handle. Compounding this, many folks opted for wide tires and therefore used crazy spacers and really whacked offsets. As such, the car's performance on the race track likely reinforced some of the early-adopter opinions that the car was a POS!!

    To summarize;

    A. The shock tower height is no longer "an issue". Be aware of it and use the spacers and/or long shock bodies and there's no problem.
    B. You can likely use a 15 x 8 -25mm offset wheel if you are a better fabricator than us and can squeeze in the requisite fender supports. Otherwise, like us, you can decide to settle for 15x8 0-offset with either a 205 or 225.
    C. The Miata is a unibody and the Exocet is a tubular frame structure - there's no doubt the jack points are at different heights. With some rough level of accuracy, an Exocet that is 3/4" higher than a miata at the jack points will have nearly identical static a-arm angles as compared to a competitive spec miata. Not 2", not 6", but 3/4" will put you right in the ballpark.

    Finally, I have assumed you interest are similar to our own - wanting to use the Exocet to road race. (All of my comments apply.) However, if you are like MOST Exocet buyers, you may have more interest in getting an SB100 and running around on the street with a unique vehicle. In that scenario, I don't think the shortcomings/trade-offs necessarily apply!

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