I’ve had a few comments and questions sent to me recently about my throttle body setup and it occurred to me that the most recent post and photos that I uploaded here are quite out of date.
There’s not a great deal to add, but I’ve now fitted an airbox (Pipercross 600 series) over the throttles, tidied things up a little, given the cam cover a lick of black crinkle paint and refined the fuel and ignition maps plenty more.
I was having some issues with rough running at very low TPS and RPM values, I think the very short intake trumpets I’m running due to lack of space was causing turbulent airflow at low revs. The airbox appears to have the effect of lengthening the intake tract and smoothing things out a little. The car definitely runs a little smoother with it fitted, although that great ITB sound is muffled a little unfortunately. Whether it robs the car of any power at the top end is up for debate. Perhaps a bhp or two, but I can live with that.
I’m also running some more aggressive, track-biased tyres now. As the car only gets used in the dry or for the occasional track day, I figured I may as well get something suited to those conditions. In the dry they’re great, plenty of grip and very controllable at the limit. I wasn’t so keen at Snetterton circuit earlier this month though in cold and soaking wet conditions. Part of the problem was that I forgot my tyre pressure gauge and was running far too much pressure. Either way, there was lots of this going on…
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.
Over the last couple of weeks I’ve been taking lots of the MX5’s engine compartment to bits in order to fit a set of individual throttle bodies from a Suzuki GSXR 600. Over the winter, while the car was in the garage, I’ve been collecting parts here and there for the conversion whenever I see them come up cheaply on ebay. Now the weather has improved and the evenings are getting lighter, I’ve started putting it all together.
To the right is a picture of most of what is going into the car. The manifold is an old MX5 inlet manifold that’s been cut down very short and had four small aluminium flanges cut out and welded on to mate up to the Suzuki throttle flanges. The throttles themselves have had the injector ports sealed up as I shall be using the original injector locations in the inlet manifold. On the underneath of each throttle is a vacuum port which shall be connected to a common vacuum block to provide a vacuum feed to the ECU, brake servo and fuel pressure regulator. The standard MX5 throttle cable should attach easily to the whole assembly and the Suzuki throttle position sensor should connect into the MX5 loom with only a small amount of rewiring. Other than that, I’m hoping it’ll all be a fairly simple process.