Last update a month ago? Crazy how time flies.
After the last update, I had an eventful day out with a van, heading down with
Dan_betG to meet
RobinJI to grab a whole heap of standard mx5 bits I needed.
Sadly once I'd got home with the engine, which both Robin and I knew was a gamble, it turned out to be a lemon. When I started stripping it down it became apparent that somewhere along the line, it had ingested a whole heap of aluminium metal fillings. The intake manifold was full of them and I could see piles on top of the pistons when looking down the plug holes.
Robin was very generous and refunded me what I'd paid for the engine without me asking him to, can't fault the guy.
At this stage, with another dead engine in the pile and needing to start the hunt all over again, I lost patience with it. Essentially I just shut the garage and walked away from it for a couple of weeks…
Still feeling despondent, I started pulling the engine to bits so that it was in bits small enough for me to carry to the car and take to the tip. When I took the head off I could better see the fillings that had got inside the engine. They were aluminium, and to be fair it looked like there was no damage to the bores and the vale seats looked no worse than mx5 ones normally look (i.e. pretty poor!). At this stage I figured for the cost of a head gasket, it was worth a shot at just cleaning it all out and chucking it together. If it worked great, if it didn't I'd only lost £30.
So onwards with the engine swap…
I collected my old original gearbox which had been patiently waiting to be useful again in my parent's garage. It was looking pretty sorry for itself and it's probably a good job I rescued it, another couple of years and I think the light surface rust that had started to form on the input shaft could have been the end of it.
As it is, it cleaned up ok, and I soon got the standard subframe back in the car along with the original gearbox, some wiring modifications and fuel lines routed back to their original path.
The head gasket arrived and was fitted without drama, and soon there was an engine back in the car. The big question is, would it work???
Before that could be tested, I needed to sort the intake. I already had a set of CBR600f4 38mm throttle bodies, left over from the Lancia project:
… and I bought some CBR600 inlet rubbers, velocity stacks and some thick wall aluminium tubing from eBay:
There was some considerable luck going on here, following a few calculations from the book on 4 stroke tuning I've got, I figured 38-40mm throttle bodies would be the right size and the intake needed to be 34-37mm diameter and 320-330mm long to make the most of using ITBs.
I already had the 38mm throttle bodies, which actually taper to 36mm where they join the inlet rubbers. The inlet rubbers were designed to fit over a ~41mm OD pipe. I was able to buy the tubing off the shelf at ~41mm OD (well 1"5/8) and 36mm ID. The stock mx5 ports are oval but are the same cross sectional area as a 36mm diameter circle. There was also just about enough room, with a small tilt up of the ITBs, to get 315 and 335mm intake lengths (CBR600s use twin length velocity stacks!). It was all just working out!
The tried and tested method to do DIY throttle bodies on an mx5 is to start with a cut down the standard manifold, leaving the mounting flange, fuel rail and injectors. So that's what I did.
Here I've cut down the intake manifold and tacked in place the first of the aluminium tube sections. I squeezed the one end of the tubing oval in the vice, and it matched up with the original manifold nicely. Cylinders 2 & 3 were easy with straight runs, but due to my throttle bodies being slightly to narrow spaced, and not the type that can be re-spaced, cylinders 1 & 4 have an S shape to the aluminium tube sections. I know this isn't optimal, but this is very much a budget build so I just did the best I could with what I had available.
I MIG welded the lot together, MIG welding aluminium isn't pretty, and is a pain to do, but I'd been perfectly functional for me in the past and seems to have done the job here too. I then spent a good while with the die grinder making sure the transitions were all as smooth as possible. While I had the grinder out I went over the exhaust manifold. The eBay stainless exhaust manifolds for mx5s are notorious for not matching up well with the head, this one was no exception.
Sadly I wasn't really thinking and didn't take any pictures of this port smoothing, on either the intake or exhaust. While I like keeping this thread as a good record of what I've done on the car, I really don't want to disassemble it all now just for a picture
Here's some pictures of the finished engine bay to make up for it!
Anyway… with that done it was time to start the old girl up and hit the road. It turned over nicely on the starter, I had good clean signals from all the sensors but no spark. Hmmmm. After a day or two of chasing it round in circles, I borrowed the ignition amplifier off my mums mx5, and it fired into life straight away and set there idling just lovely! A quick eBay session and the next day the post man brought me a new ignition amplifier and I was all set to go for a drive!
It's slow! People say rotaries have no torque; they have a lot more torque low down and midrange than a 1.6 4 pot! They've got a lot more top end power also! haha. Once I got over the drop in power, things started to look up
The engine is easier to deal with around town, partly this is going back to a standard weight flywheel I think, it's happier pottering about than the rotary was. When giving it more throttle, the ITBs sound great and because it's quite a soft tame sortta feeling engine its easy to keep it singing near the redline and have fun thrashing it without it being intimidating. I'd say there same about the handling too, the rotary did change the balance of the car, I think it felt more like a mid engined car. Going back to the 1.6 it feels more predictable on poor surfaces and again easier to handle. It seems like the car's more interested in just having fun no matter how fast you wanna drive it, whereas the rotary conversion was a much much more focused drive requiring concentration to get the best out of it.
One thing that was lacking was an exhaust note. The large rx8 silencer I'd fitted to get the rotary engine down to sensible volumes was now too quiet, Dan said it reminded him of a standard BMW exhaust in it's kinda wafty quietness! To make things worse, the piston engine moves about more than the rotary (narrower spaced engine mounts) and the rx8 silencer was now rattling against the underside of the car at times. So I've reinstalled the original 5x9" oval silencer, along with a small internal baffle, as a centre exit. This meant cutting rear bumper, but I've always liked that kinda track car look thing anyway
It's a good compromise of volume now, still fairly subtle, but enough to hear and enjoy
Finally, I need new rear tyres as mine were getting edge worn (alignment booked for tomorrow to stop this!), but I saw a pair of 14" Watanabes going cheap on eBay, chucked in a cheaky bid and ended up with a pair with nearly new tyres for £50, total bargain. For now I've got my old front wheels on the back and the Wats on the front.
Despite being the 14x6 'standard' fitment, sitting on my 25mm spacers, I'm happy with how they sit. I'm really so pleased with them, the car rides better, the steerings lighter and it's a look I've always liked. Just gotta find another pair now! I'd really like to find some in 14x6.5 or even 14x7 for the rear, but I know I'll have to pay a lot more than £50! hahaha.
There we are, up to date. It's ended up being a monster post! I need a few months break from car drama I think! I'm hoping all I'll have to do for a few months now is do some tweaks to the interior and cosmetic stuff, mainly I just wanna go drive it
That said there's a dark cloud on the horizon, in the shape of a knocking noise I'm struggling to find there source of. It's a short nose, but the keyway is in good condition, I've locktite-ed it and carefully torqued the bolt. Hot compression is 180-185-175-180, so nothing showing up there. The water pump is past it's best but removing the belt doesn't stop the noise so it's not that or the alternator. It's only really apparent on idle and very light throttle up to about 2.5k, I fear it may be a knocking rod. Maybe the engine hadn't survived ingesting all those filings after all…