24 April, 2020 by Simon Descoteau
Project Synthia: 1991 Eunos Roadster – The Story So Far (Part 2)
When we last left Synthia, she had been considerably altered from the almost stock condition in which she had come to me. Wider wheels and lowering were the order of the day, but this had brought along its own problems. If you haven’t already, you can catch up on part 1 of the story here.
Now as you’ll have probably realised by now, things tend to escalate quite quickly when there are cars and modifications involved, at least in my case. As I write this, I’m thinking back to the stories of how the other cars came into my possession and subsequently ended up with large amounts of work done to them in a short matter of time.
Well, what happened next was a great example of just that. The lower stance and the width of the rear wheels meant that while taking certain sharp bends the edge of the wheel arch would rub against the tyre. This was promptly fixed by rolling the lip of the arch flat to provide some extra clearance. With the car lowered on 15″ wheels, I realised that not only did the arches need lowering, but the car really needed some mud flaps as the new wider wheels were a bit too exposed from the rear for my liking. Additionally, the new wheels made the already quite small 1.6 brakes look smaller still. This simply had to be fixed, especially as the brakes were in need of a refresh.
I was able to source the full brake set from a mark 2.5 sport model MX-5 which came with 270mm brake discs vs the 235mm brake discs which were currently on the car. These brakes are a popular and direct bolt-on replacement for the stock items, requiring longer brake hoses on the front and new larger discs and pads all around. As I had to refresh the brakes anyway, it made sense to upgrade them for the significantly better performing bigger brakes.
The callipers and brake carriers that I had acquired were perfectly functional, but had some slight surface rust and generally benefited from a cleanup, so armed with my wire brushes, paintbrush and tin of Hammerite gold paint I set about making a mess of the living room, cleaning and painting the callipers ready to go on to the car.
That work completed, Joe at Wheels in Motion again was tasked with installing the brakes for me, alongside braided hoses all around, and new discs and pads, all reliably sourced from MX5Parts. Aside from looking stunning behind the open spoke wheels, the car now stopped as a sports car should. While the car was with them, they also were able to get the wheel arches rolled for me all around so that the arches would no longer rub on the wheel during enthusiastic cornering. I used this time while the car was with Wheels in Motion to source and have fitted the track rod ends from the R-Limited car which had slightly modified geometry to help counter bump steer caused by the lowering. You can read more about bump steer as part of Aethyia’s story here.
Meanwhile, with the handling and braking of the car sorted, I turned my attention to the engine tuning for a better sounding and freer breathing engine. I had found an excellent deal on an HKS 4-2-1 exhaust manifold and Cobalt cat-back exhaust so these were purchased in short order. It only made sense that since I would have to remove the air intake box to access the exhaust manifold, I should replace the restrictive standard system with an aftermarket intake system. I decided to go with the Pipercross induction kit, as it came with a heat shield. This was again sourced from MX5Parts. The car had previously had its catalytic converter replaced with a straight pipe, a common modification on these as they pre-date the catalytic converter emissions test in the UK MOT (roadworthiness) system. The catalytic converter causes a significant restriction in the exhaust system, and a less restrictive system aids a slight increase in power and throttle response. Mostly though, the car sounds better which improves the enjoyment of driving it immensely.
Cold air intake kits, or induction kits, are a point of contention amongst many modifiers and car enthusiasts, as quite often the factory intake is designed in such a way as to make the best use of the air inflow from outside the engine bay to feed the engine. Cooler air is denser and thus contains more oxygen. More oxygen means that you are able to burn more fuel and thus make more power. Conversely hot air, for the same reason, will reduce the power that your engine can make. Due to the packaging limitations of the engine which Mazda had chosen for the MX-5, the air intake box sits directly next to the very hot exhaust manifold. This problem is further exasperated by aftermarket intake kits which do not have heat shields as the air filter cone is now sat directly next to the hot exhaust manifold, taking in the hot air. In the grand scheme of things, in a naturally aspirated car with a standard engine computer, putting on an induction kit is going to make a barely noticeable difference to the performance. However, the extra noise produced by a cone filter, as with the exhaust, increases the enjoyment of driving the car, which is why for many this is the first modification that they make.
I was also really fortunate to come across a badly advertised Arc Intake Chamber on eBay. Arc is a Japanese brand of performance parts producers for MX5s amongst other cars, and they have developed a crossover pipe (that goes from the intake manifold on the left of the engine, to the air intake box on the right of the engine) which replaces the stock item, and due to it’s shape improves the mid range throttle response of the car. Due to their rarity, these are often sold for far more money than their relatively modest increase in performance would warrant, but as this one was badly advertised I managed to secure it for a very modest price.
My friends and I decided that as this was a fairly simple job, we would tackle this ourselves on one of their driveways, so the 3 of us, along with my friend’s father who is also a car enthusiast, set about raising the car and got to work installing the parts. A simple job, it was not! 6 hours of swearing, knuckle-busting and tea-drinking followed until finally we emerged victorious and with a turn of the key, the engine roared into life. If it had not been for the experience of my friend’s father, it would surely have taken us many hours more, or may even have bested us. Immediately the car had a more throaty, aggressive note to it. She seemed to bark on aggressive throttle application and pop on the over-run. A completely different animal to the timid car which had rolled onto the drive earlier that day. This would, of course, need an immediate and thorough road test.
We’d taken the opportunity while under the bonnet to tighten the throttle cable slightly. In these older cars, the throttle body is physically connected to the accelerator pedal with a cable, much like a bicycle brake cable, which pulls the throttle butterfly open as you press the accelerator pedal. Over time these stretch and have play in them so there is a lag between pressing the throttle and the car responding as the slack in the cable is taken up. This can also mean at full throttle the butterfly isn’t completely open. The adjustment is very easy and built into the cable holder. I’d strongly recommend you check this on your own car and adjust as necessary.
Once on the road, the difference was immediately apparent. Synthia was more responsive to throttle input, and the engine was revving more freely than previously. The noise, while not loud to the point of being obnoxious, was certainly more present, especially when in the higher RPM range where there was now more of a sense of urgency which was previously lacking. The joy was shortlived however, as over certain corners or bumps there was a knocking sound coming from near the transmission tunnel where the exhaust was knocking against the bodywork.
Now, Synthia is about 29 years old at this point, with over 120K KM on the clock, so the rubber engine mounts have certainly seen better days. I assumed that they had worn to a point where the now wider exhaust components were fouling the bodywork during harsh cornering. Some research on the internet brought me to BOFI Racing, a previously unknown company to me, but one that I’ve since found have excellent knowledge of MX5s and making them better. I spoke at length with Daniel about my requirements and we agreed the best choice for me would be 80A stiffness polyurethane Duraflex engine mounts.
Polyurethane, unlike rubber, does not deteriorate with age and as such is a popular replacement for rubber suspension bushes and mounts. Polyurethane engine mounts can be found in different levels of stiffness, which sacrifice vibration absorption for less play in the engine. This ultimately means that a stiffer mount will translate more of the vibrations of the engine into the chassis, but will in return prevent the engine from moving around in the mounts under acceleration so that more of the turning force of the engine will go into forward propulsion through the wheels instead of being wasted due to the aforementioned moving around. 80A was the middle ground of the stiffness ranges available, being stiffer than a stock equivalent 65-70A but more complaint than the 95A track car engine mounts. Essentially this would mean I would experience more vibrations, but not to a point that would ruin the enjoyment of the car in day to day driving.
I decided at this time to also add some bracing to the car so that the suspension could correctly do its job without having to fight the flex in the chassis. Roadsters, by virtue of not having a roof, are inherently more prone to flex as they are not closed off like a coupe would be. These cars benefit greatly from front underbody bracing between the wishbones on the subframe, as well as the same location in the rear of the car. These were standard on later cars but on the very early cars like Synthia the mounting points were there but the bracing was excluded. Additionally, I decided to add a strut brace, a rear brace that ties together the rear tie-down hooks, and thicker polymer door bushes to replace the small standard rubber items. These were all sourced from MX5Parts.
With the parts in hand, I headed off again to Wheels in Motion as the parts that required installing, on the whole would be significantly easier to install on a 2 post lift, rather than on axle stands on the drive. The outcome of that trip was satisfying yet hilarious, but as this post has again extended to brobdingnagian proportions, I will have to sign off here for the time being.
Part 3 will follow in short order I’m sure, as there is still a fair amount of history to cover before we can begin to look at the future and what it may bring. At this point, the car had been with me for 3 short months.
As always, I would love to hear your thoughts so please do leave a comment or question below and I look forward to welcoming you back for part 3, which is now live and can be read here.