Update on the MX5

MX5 with GSXR 600 throttles
MX5 with GSXR 600 throttles

Just a quick update on the MX5. The GSXR 600 throttles have been assembled and attached and are now on their way to running pretty nicely.

The physical side of the install was pretty simple, mostly a case of bolting stuff up. The only real issue I faced was the routing and length of the throttle cable, which took a few attempts to get right. After a lot of trial and error and the use of some mountain bike parts to help the cable get round some tight bends, the cable now fits fine. The ratio of throttle plate movement for a given amount of cable pull is now a little higher that it was before, making the throttles very sensitive to small pedal movements. This can currently make the car a little difficult to drive smoothly in traffic and when parking. I have a plan to move the cable mounting point on the pedal to resolve this, but it’s not an urgent modification.

On the ECU side of things, the car is tuned and running well under most conditions. Startup and idle are smooth (the car sailed through its MOT last month) as is 90% of driving whilst the car is moving. Increasing the revs off idle is still a little rough, things go lean for a split second and stumble, which again makes driving the car in traffic a little laborious. Once moving, the fuelling is smooth and wide open throttle sounds great.

There are a few things that I rushed to get the car ready for the summer which I’d like to go back and revisit. For now I’ll just enjoy the sunshine though, when autumn starts to arrive I may take it all apart again and make a few small improvements. Amongst other things I think the head would benefit from a small skim. We’ll see.

13 Replies to “Update on the MX5”

  1. Hi ya

    I have seen you zipping about mk, and on the mx5 forum, enjoying reading what your doing with the gsxr bodies. What ecu did you run in the end

    1. Thanks, I’m running an MS2 ecu, which seems to be doing the job well. Hopefully some more updates to come in a month or so.

    1. Yes, I’m using the standard TPS that comes attached to the Suzuki throttles. It’s easier that way as it’s already connected nicely to the butterfly spindle. I just had to cut the standard plug off and wire on a new plug to connect to the Mazda loom.The signal the Suzuki TPS generates works perfectly with the Megasquirt ECU.

      1. Great the wires on mines are black yellow and blue u wouldn’t happen to know which is which from your build lol

        1. Actually, I lied! I actually cut the Mazda connector off and used the Suzuki connector that came with my throttles, wired in place of the old Mazda TPS connector. The photos below might help. There are three wires – from memory I think they are, 12v, ground and signal. You should be able to see what’s what with a multimeter and see the voltage on the signal wire going up and down as you open and close the butterflies. I vaguely recall that if you get things slightly wrong then you get an negative instead of positive signal, but this still works fine, you just calibrate in Tunerstudio accordingly.
          These photos might help…
          Photo 1
          Photo 2
          The wires going off to the left go to the TPS (you can just about see the yellow and blue wires). The ones to the right are then wired into where the old Mazda TPS plug was.

    1. No, no IAC installed. I do have one in the garage – If I ever did fit it, I’d attach it to the vacuum block as that is connected to the throttles, post-butterflies.

      1. Thanks for the info. My setup is on a 1970 Porsche 911 (2.2L). I’m using six ITb’s from a Suzuki gsxr 600 also. I printed a vacuum accumulator where a hose from each throttle body joins into the small chamber and is then off to the MS2. Each of the ITB’s used has a second port (currently capped) where I am thinking about making another block to use as an IAC. I am concerned that the MAP circuit is too close to the IAC circuit, but perhaps that won’t matter if its used for startup/warmup only. The other horizontally opposed engine setups, Porsche anyway, I’ve found don’t use an idle control (they either use the manual high idle or feather the throttle), but I feel like I may as well use all the capability the MS2 has to offer.

        1. Mine also have twin ports per throttle. I guess a second vacuum block would probably be a little neater, although I’m sure it would work just fine off the existing one, especially as it’s really only for a few minutes while the car warms up. I keep my second ports capped and use them occasionally for attaching a carb/throttle balancer to make sure everything’s still balanced (once a year or so). The ITB setup on the 911 sounds very interesting – got a link to any more info or pictures?

          1. Good point.
            This is the first project of this caliber for me and I still have a lot to figure out. The goal is to implement all the advantages of modern EFI, all the while being able to return the car to original condition if needed (valuation is the only reason I can think of). The stock Weber manifolds and throttle linkage are used. I’m going to make an enclosure (rear seat delete), also removable if needed, to house all the electronics. The speaker holes in the firewall were cut by the previous owner. I will repair them, but as you can see in the video, for now they are used for wire loom routing.

          2. Looking pretty cool, nice one! I’d love to do what I’ve done on the MX5 to a 914 or perhaps a 924 buy sadly prices seem to be heading skywards these days.

    2. I have an IAC there where to mount it I’m in two minds at the moment I was going to make a bracket that would attach to the tapped holes from the original fuel rail on the throttle bodies and have it placed in front of one of the velocity stacks mines are 90mm but I will have a think about it the head is with my dad being skimmed at the moment playing it safe with a 40 thou skim I wanted 60 but he said no lol just doing a small bit here and there

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