A bit of history (part 3) – Curse of the chocolate block

Summer 2009, lovely hot weather and a free weekend…what better way to enjoy it than a long drive in my newly completed MR2 with my lovely fiancé? Well how’s about sitting by the side of the road in a cloud of steam as my newly built engine decides to throw its guts up?

Yep…that’s more like it!

I crawl home and get on the phone to Rogue, next thing I know the car is getting loaded onto a trailer and taken back to its second home in their workshops. It doesn’t take long to diagnose, I have fallen foul of the Toyota curse that is a Revision 3 block…

[Bit of technical information at this point, cos what’s the point in breaking stuff if we don’t get to learn something from it?

In the UK we seem to refer to the mark II MR2s in five distinct revisions, often shortened to rev1 – rev5. The rev1 & 2 cars are notable by a three part rear spoiler (amongst many other things), and more importandly: the design of the engine blocks.

The rev3 cars (~1994) are notable by one key issue: an inconsistency in the casting processes Toyota used when manufacturing the engines, namely extremely variable cylinder wall thicknesses with the potential for thin/ weak points between the cylinders and the surrounding water ways. There doesn’t seem to be any pattern, some blocks let go and others don’t…and with most things like this, there’s a plethora of theories as to what causes them to let go.

From what I’ve seen, it seems to be an underlying weakness in the walls, which increases the risk of cracking when put under significant thermal strain. That could be a really hot engine suddenly seeing an influx of cooler coolant as a thermostat opens, or excessive heat in the cylinders from overly retarded ignition timing causing micro pockets of steam in the coolant on the other side of any weak/ thin areas of the casting.

Whatever the cause- the net result is that when pushed, the rev3 blocks tend to crack in the cylinders.]

Right…sorry, slight side track! Now the eagle eyed will note that part of the engine build for my car was to sleeve the block, this was an attempt to mitigate the risk of any casting defects in the cylinder walls of my block…basically the existing material was bored out, and new liners were inserted.

A cunning plan, but it had failed…why? Well looking at my block, the cylinder liners had done their job- they were still intact. The problem was a crack on the top of the block running across the deck between cylinders two and three.

Now if you cut up one of these blocks, you’ll see there really isn’t very much material between the top of the block and the waterways underneath, so my theory is that in sleeving my block, so much material was removed to make room for the sleeves, that there were excess forces for the remaining, weakened, material to deal with in trying to hold the cylinders in place.

My guess is that the cylinders may even have been moving relative to each other under force, and this combined with some decent thermal load had caused the remaining material in the top of the block to simply let go…hence the crack, and my car spewing it’s guts up on the side of the road as the coolant made a bid for freedom.

So…the upshot was I needed a new engine, and this time we decided to take a different approach- to use one of the earlier revision blocks (from a rev1 MR2) which we sonically measured the cylinder wall thicknesses in. These blocks have more material around the cylinders anyway, so should be stronger (they do have other compensating weak points, but I’ll not go down that rabbit hole just yet!)

Rogue really did me a good turn on this one, and put the new build together pretty much at cost price with no quibbles about warranties, or any other such unpleasantness. This is another foible of the tuning scene- engine warranties really are not worth the paper they are printed on: a specialist can always find a reason why the failure was not their fault. It is genuinely almost impossible to ever claim warranty, especially when we are attempting to push the limits of what these engines were designed to do.

At this stage I still believed that the specialists were the ‘ultimate experts’, and that they scientifically knew the correct way for everything they were doing…it never occurred to me that they were just like me (but perhaps with a bit more experience), and that, in fact, they are still learning on every build they do…it’s just that the customers foot the bill. Now don’t take that as me having a pop at Rogue, not at all- this is a common theme across the motorsport community, and I now know of top spec specialists in this arena doing exactly the same thing with customers footing the R&D bills in the belief that they are treading a known path, only the specialists are trying new stuff with no guarantees that things won’t go bang…and when they do, the customers get to pay again. That’s motorsport for you I guess…

So onwards and upwards, and we have a newly built engine back in my car, at minimal cost to me thanks to Patrick taking pity on me following the history to date with this car!

I was now super paranoid about my engine, and especially trying to reduce thermal loads, so I took a few additional precautions. I replaced the stock radiator with an aluminium unit for superior cooling abilities, I had both the inlet and exhaust manifolds flow tested to make sure we were getting even flow into each cylinder, and so avoiding any one cylinder running hotter than the others. Finally at Ryan’s advice, I added a water/ methanol injection kit (the idea being to inject a mist of liquids into the engine which supposedly cools the air mixture through evaporation, and also cools the combustion chambers as the mist is vapourised as part of the combustion process…more on this topic at a later date).

The next stage was to run the engine in, which we elected to do on the dyno this time- it is far quicker taking a couple of hours, and arguably gives you more control over the loads and forces the engine sees. Then it was back to Surrey Rolling Road with Ryan and a very nervous owner…

I kept an eagle eye on Exhaust Gas Temperatures (EGTs) this time, and asked Ryan to stop adding boost at 1.2bar as things were getting very hot again and I wasn’t happy to take the risk of going any further. Ryan was great to work with, and we spent a long time going through the details of the build, and he actually started to get me thinking about the design in my own terms instead of assuming that the experts had it spot on.

In hindsight I believe the HKS Turbo was simply too restrictive on the hot side, the turbine housing was too small for what I was trying to get the engine to do, and I was trying to pump too much boost through it…which in turn was creating excessive pressures and heat in the exhaust manifold. When we pulled the first built block apart the top edges of all the pistons were furred up where they looked to have gotten too hot, which bares this theory out.

If I had stuck with the car as was at 1 bar of boost, and lower profile camshafts it would have been fine, but stepping up to larger cams had pushed the peak engine torque up the rev range, and we had upped the boost at the same time. 350hp at 1 bar of boost, with a relatively early spool would have been great…instead I had something that was laggy, and needed too much boost to give me the kick in the back I had gotten used to!

Did I mention how addictive boost is?

With this in mind a new plan started to germinate in my mind…