Westfield build, Mazda SDV

Well, I had to do something! I’d been almost haunted by the idea of throttle bodies for a long time. The appeal for me is the noise and the throttle response. Just how much extra power they deliver on the MX5 engines I don’t know.

I nearly got a Jenvey kit a few times. At a WSCC meet last year Omex and Westfield were toying with the idea of developing a kit for the MX5 engine’d Westfield and so I offered my car as a development mule. Unfortunately though, it came to nothing.

Fast forward to now, and seeing as I’m building an engine it made sense to carry on doing stuff that’s a bit more “hands on”. After reading a few builds on mx5nutz where people were using motorcycle throttle bodies I got a bit more interested in this idea. Then, by chance someone locally had a complete set of GSXR throttle bodies for sale cheaply, and I decided it was worth a punt. They’re not the best ones you could find – being only able to split into two not four, but I thought if I liked the results I can always go out and get a Jenvey kit and sell these on.

Only split in middle, not individually

They measure 42mm internal diameter, which is 3mm less than what Jenvey supply for the 1.8. However, these are what’s recommended if you’re using bike throttle bodies and who am I to question the internet?

The first thing I noticed was the size of the throttle cam. A small movement on the accelerator pedal would result in a much larger movement of the throttle butterflies than what you’d get on an MX5 single throttle body.

Small throttle cam could result in over-sensitive throttle

There are also a second set of butterflies which I’ll remove. This should give me some space and I’ll look to retrofit the MX5 throttle cam some how. At first glance the thread on the GSXR throttle bodies is smaller but I’m sure that’s not insurmountable.

Something else to note is that these throttle bodies run two sets of injectors.

I’ll stick with the OEM injectors in the head for now, but may well leave these in-situ to save sealing the holes up.

An unexpected bonus is that this setup includes an idle control valve. Whether it will be big enough to be useful on a 1.8 engine I don’t know. I’ll have to speak to a tuner but it may be that this could save some effort in setting up the cold idle.

Idle control valve

Idle control valve opening

Lastly, as well as a throttle position sensor these bodies also have a MAP sensor included.

MAP sensor (follow vacuum tubes, on right)

Again, this should make mapping easier as won’t have to rely on air temperature and throttle position.

So, that’s another engine upgrade route chosen. Should be an exciting winter!

Decided to spend some time on the newly acquired engines recently. Nothing major just yet. Concentrating on stripping down the mk2.5 bottom end first. Planning on removing the crank and getting it measured to check all is ok, then renew the bearings.

After previously removing the exhaust manifold, engine mounts and cleaning the block a little with degreaser and a little brush, I spun the engine upside down to remove the sump. Before that though, I noticed some strange markings on the camshaft lobes. This is the exhaust cam, I can’t remember if it was on the intake as well.

I’ll readily admit I don’t know what those markings mean, if anything, but it seems the corners of the lobes are discoloured. There’s no vertical scratching so I’m not convinced it’s wear. I’m not using this head, so it doesn’t matter, just intrigued why they’d look like that.

Anyway, spun the engine over on the stand and started removing the 18 sump bolts.

It also gave an opportunity to remind myself just how much height has been removed from the sump with my custom one. Mine is flush with the bottom of the bell housing, and you can see how much the standard one protrudes. Still, sump wasn’t difficult to remove, 18 bolts and a quick cut of the sealant and away it came. No dodgy levering required.

This left the baffle plate which contrary to the Rod Grainger manual is bolted in as well as held in with sealant. My mk1 engine one wasn’t, so presumably this is a change that was made with the mk2 or mk2.5. The pickup pipe was removed then the baffle. I’ll need to swap the pickup pump with the one on my current engine when I swap the sumps over.

With the baffle removed it left the crank exposed. Before removing it I want to remove the oil pump to inspect that. Helpfully, the bolts securing that are covered by the crank pulley, which I’ve rather foolishly left attached. That didn’t want to come off with a breaker bar, so I think I’ll have to get the impact gun out. Pretty sure I had to last time as well. Ah well, good excuse to play with the air tools.

So after ruining the EBC Ultimax pads on the last track day (no slight against these pads, they aren’t track day pads after all), I set about researching what to replace them with. In the end, I settled on Mintex M1144s, which are reasonably hard to track down. Hint for those looking in an MX5 fitment, try finishline.

They arrived via first class post, with a franking value of more than I paid for p&p. That’s got to be a rare occurrence!

Last night, I managed to find half an hour spare so nipped into the garage to swap them out. All ridiculously easy of course – so much access with the Westfield! The Mintex pads don’t come with clips, but as the ones currently fitted hadn’t been in that long I simply reused them.

The rather sorry looking EBC Ultimax pads are on the top, with the new Mintex M1144s at the bottom of the photo. Interestingly, the thickness of the new M1144 pads was less than 0.1 inches thicker than the worn EBC pads. Not sure why the Mintex pads come with less material, but to be fair it made fitting a lot easier so I’m not complaining too much.

First impressions are that I’m surprised at the difference a set of pads can make to pedal feel. The pedal is now much firmer than before. You get stories of the Mintex pads liking to squeal but they’ve been 100% quiet so far. Only applied a light smearing of copper grease to the back of the pads, no fancy shims or anything. Braking performance is also considerably improved. I did a small test brake from 60mph and nearly instantly snatched a front wheel.

I’ve got a track day on Friday so it will be interesting to see how they perform, and indeed, how I adjust to the change in braking performance.

I spent a while deliberating on various ways of increasing engine power, and must have settled upon every conceivable idea going. As is the way with me though, eventually a plan sticks.

I’d quite like some more power, but at the same time keep it naturally aspirated. That’s not a common way of tuning MX5s and whilst kits are out there, you tend to have to research it all yourself. The exception to this is the Blink Motorsport 160 package, where for £1,500 you’ll get a modified cylinder head (exchange), regrind your existing cams, MegaSquirt PNP ECU, wideband lambda sender and some gaskets. Unsurprisingly, this should net you somewhere in the region of 160bhp with a 1.8.

I nearly opted for this, and I still will I suspect, but there’s something I’d like to get out of the way first. That is, building the best base I can without spending silly money. To that end, I’ve sourced two more engines…

2000 Mk2 MX5 1.8

I’ve bought this engine for the top half. The mk2 head flows ever so slightly better than the mk1 head, and has solid lifters. Unfortunately these have the shims on top of the lifters so that doesn’t translate to instant revability. I think I read that Blink tested it at the head flowed 4% more or something equally pretty small. With MX5 engine prices pretty low, it made sense. The mk2 engine had 9.5:1 compression against the 8.8/9:1 of the mk1 engine and around 140-143bhp from the factory.

2003 Mk2 1.8 (SVT)

This is the engine with the highest output (Mazdaspeed turbos excluded). It had VVT above the previous generation engine and a higher compression ratio (10:1). Unfortunately, due to the height of the VVT solenoid the engine doesn’t fit in a Westfield, else that would have been an easy 146bhp. I’m still intending to realise the benefits of this engine though by using the bottom end of it and fitting the head from the earlier engine.

Most of all though, having built the car I’ve still never stripped down and rebuilt an engine. This little project gives me the opportunity to do that,