BOFI VVT MX5 Engine in an NA Chassis ME221

Winston has undergone many changes in the last 18 months – pretty much everything except the shell has been touched or replaced. This includes a complete suspension overhaul including NB subframes and polybushes, a full SVT brake upgrade (the brake bias still needs tweaking) and a complete drivetrain swap consisting of a ‘rodblock’ VVT and 6-speed gearbox making plenty of boost.

For this post though, we’ll focus on the engine and what makes it special.

The VVT engine currently in Winston wasn’t fresh from the breakers unit, instead, it’s an engine that has had a long (and largely static) history with us. It goes back as far as Dan’s first ever MX5. It was a 1.6 (he only buys 1.6s) NA advertised as being Mariner Blue, but was, in fact, some weird milky version of the colour. For some reason, we’d decided to take the 1.6 out and fit a set of ITBs, but before that plan could get off the ground the VVT in question was listed for sale surprisingly cheap and we snapped it up. Collecting the engine was good fun, going over tower bridge at the crack of dawn, manhandling it into the back of a really crappy Polo (a quick way of getting the stance look) and having to keep filling it back up with oil all the way back. We did eventually get it back and Dan gave the engine a good service and paint job before we got to the task of putting it in his MX5.

VVT in Crumble Engine Bay

Mated to a nice shiny set of 4-2-1 headers it looked great in the engine bay, but because we had forgotten to attach the main engine strap the MSLabs MS3 Enhanced emitted smoke as soon as power was applied. Luckily it wasn’t the end of the world as a simple wire bridge to bypass the burnt traces on the board was all it needed to get up and running again and run it did. The VVT probably ran for about 10-15 minutes in that car until rust repairs were attempted. Unfortunately, the rust was so extensive on that car, it was promptly named ‘Blueberry Crumble’, stripped and scrapped.

Dan did import a fresh MX5 after that, but it was such a nice example, the engine swap idea was abandoned and the engine languished in the corner of the garage on a stand for the better part of 18 months. It was at this point that putting the engine in Winston seemed like a good idea – even though there was nothing at all wrong with his original engine.

As I wanted to supercharge the car, I decided it would be a good idea to pull the head off and inspect the valves, piston crowns and bores for excessive wear. Yea, when I look back at this decision, I don’t know what I expected to learn either <<shrug>>. Never one to let practical nonsense get in the way of bad ideas, I stripped the engine bear within hours of getting my hands on it. Totally bare. Somewhere down the line, I had decided that I may as well put forged rods in it and skim the head – you know, while I was in there and all.

Happily, I found no issues during the strip down. There was absolutely no bearing scoring or ring ridge in the bores and all piston carbon deposits wiped off with only minor encouragement.

Clean Rod Bearings Clean Main Bearings

Clean Cams and Crank Clean Bore

My intention for this engine was to fit a Mercedes M62 supercharger with a custom pulley and overlay to get a 2.1 pulley ratio. This would be overspinning the supercharger, but should wring out every last drop of performance it could give and we fully expected this to be putting the stock rods at risk.

While it was all apart, I also decided to balance the rotating assembly to within 0.1g and to skim the head 15 thou for a slight bump in compression (the VVT already has the highest compression of any MX5 motor at 10:1). I could have gone much further with new valve guides and seats, perhaps an overbore etc, but due to the overall condition of the engine, none of that was necessary and I wanted to keep costs down.

While the big parts were off at the machine shop getting sorted, I weighed each of the stock pistons and found that there was nearly 5g of difference between the lightest and heaviest. What followed was a few hours of careful grinding and shaving of every single part to get them to the exact same weight (eg. pistons now all weigh 379.8g).

NB: Obviously these are stock pistons and they aren’t necessarily designed to be lightened in this way. I accepted that risk, however, and am happy to report that I haven’t broken a piston yet after 15 months of running this engine.

Piston before Lightening Piston after Lightening

The connecting rods I chose were the ever prestigious MaxPeedingRods, and they were impeccably balanced out of the box (I only had to shave the little ends a bit to get to my chosen tolerance). I also shimmed the stock oil pump bypass valve 1mm to increase peak pressure a small amount and to prevent dry starting, stuffed it with assembly lube.

As it was all being assembled, I performed a few other modifications to increase breather flow through the valve cover, removed the stock oil cooler, blocked off the throttle coolant ports, removed redundant coolant barbs from he head and pump inlet, and drilled the sump for a temperature sender and a turbo oil drain in case I decided to switch to a turbo later. I also gasket matched the intake manifold and throttle body, and I noticed the head casting had quite a bit of roughness in the ports where the 2 halves are joined – so I smoothed all of those too. At this point I was getting a bit carried away with the die grinder and decided to smooth out the step on the throttle body inlet side, making it buttery smooth like the rest. As you might imagine, all of this took a very very long time.

Rough Intake Port Smooth Intake Port

Throttle Body before Gasket Match Throttle Body after Gasket Match

Plenum Gasket Matched Throttle Body Inlet Ported

As I was doing a coolant re-route at the same time, I used the earlier 1.8 head gasket and not the VVT one (the VVT one has a rudimentary coolant re-route built in that doesn’t play nice with a real re-route) and I re-used the original head bolts.

NB: The MX5 head bolts are not stretch bolts, so as long as they haven’t been grossly over-torqued they shouldn’t lose clamp strength when re-used. Upgrading to ARP head studs is good insurance if your budget can stretch to it and will provide more clamping force than the OEM head bolts.

VVT in Winston Engine Bay

It turned out to be rather a pretty engine and after mating it to the 6 speed and shoving it into the car, it was just a simple matter of modifying the chassis loom to convert from CAS to separate cam and crank position sensors and adding VVT support. The ME221 had to be modified slightly to use these sensors – specifically 2 resistors had to be removed from the underside of the board (R6 and R7) and new resistors (82k) soldered across the cam and crank signal pins. I chose to solder the new resistors to the connector pins on top rather than the recommended position on the underside due to the gauge of wire I had on hand – you get exactly the same end result either way.

ME221 Resistors RemovedME221 Resistor Mod Complete

It was then simply a matter of changing a few settings in the map to accommodate the new engine and it was ready to start. I can admit to being slightly nervous about the startup, but with fuel turned off and the spark plugs removed, it cranked over and built oil pressure on the gauge very quickly. Putting the plugs in and turning the fuel back on meant it fired up immediately, so we could set the base timing and head out to bed the rings in properly. We have a set procedure we follow for that and it has worked well for us, you can see that here.

After the rings were bedded and we’d been out for a quick road tuning session, it was immediately apparent that it not only LOVED to rev, it made considerably more power than the old engine did. I had decided to run the engine NA for a bit to recover some funds for the boost conversion and to bed the new clutch in properly. It is quite hard to describe how different the entire character of the engine is compared to the original 1.8. Most of this I put down to the balancing work done on the rotating assembly. It is especially evident under load and on track, with downshift blips being where it really shines – orders of magnitude better than what I had previously.

In fact, the engine felt so nice I soon came under fire from increasing amounts of peer pressure over the supercharger plan. There were so many good reasons to just go for a turbo that my only reason against it was noise – I really wanted the supercharger sound. I didn’t hold out for long though and soon succumbed. I couldn’t use just any turbo, if I was going to buy a turbo it’d have to be a good one. I’d had my eye on the Garrett GTX2867R for a while and was pretty convinced that I needed it. That was until I saw this (poor quality) video:

I looked into the EFR range after that, and the EFR 6258 soon caught my eye. As it comes shipped with a recirculation valve in the housing and a boost control solenoid bolted to it, it didn’t take much effort to man-maths my way into the decision.

The supercharger plan was dead, and the EFR 6258 was ordered.

Major Parts:

  • Standard VVT block with stock 83mm bore
  • Stock Oil pump shimmed 1mm
  • OEM crank with King Racing bearings
  • MaxPeeding Connecting Rods and ACL Race Bearings
  • Stock Pistons – balanced with NPR rings (Gaps: 1st-17 thou, 2nd-19 thou)
  • Standard VVT Head, standard cam and valve setup
  • 640cc Injectors

Machine Work:

  • Block decked and honed
  • Crankshaft, flywheel and pressure plate balanced
  • Head skimmed 15 thou

If you’re wanting to do the same thing, know that building an engine down isn’t overly difficult. As long as all the parts are kept in order, labelled, everything is loosened/tightened in the correct sequence (spiral – outside in to loosen, inside out to tighten) and everything is kept clean, there is very little to go wrong. Some specialist tools are required for some stages of assembly and to take all the necessary measurements. We’ll be putting out a full engine build manual in the very near future – so look out for that.

 

Links to parts used in this build are below;

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