Perhaps A Strange Megasquirt Question

[ferny]

As I understand it, the ecu (mk1 1.6 mx5) in my car works like this;

4000k+rpm
on throttle fuel isn't controlled but on overrun fuel is cut

3000k - 4000k
fuel on throttle is controlled and fuel on overrun is cut

0 - 3000k
fuel on throttle is controlled and fuel on overrun isn't

Now, I currently get a consistent 37+mpg on a run at near enough legal speeds and seeing as the car is 46 years old (with all the sound proofing and refinement that involves) and it has a 2" straight through exhaust going much faster for a few hours isn't desirable. Now, if I fit megasquirt I'll be able to manage the fuel properly so not only will I get a more enjoyable car to drive I'll also get a couple more mpg. With my wine sponsored maths an extra 3mpg would mean the megasquirt would pay for itself in about 8k miles and then after that it'd start paying for the rest of the turbo gear... Thoughts and more importantly corrections? And no, I'm not trying to convince myself to spend money I really don't have...

The megasquirt seems the most logical thing to fit first then build up the parts before fitting once I've got used to the system and got it reliable.

[bortaf]

Any idea what system it used ? and 46 years old?
TBH i'm not sure what you meen by the fuel isn't controled ? it has to be controled somehow or do you meen it's not closed loop? as in not controled by the O2 sensor ? no car is AFAIK, a car run fully closed loop (IE at Lambda 1 ) will be pretty undrivable, most systems only go closed loop at part throttle low load IE no heavy acceleration (cruising) and revert to open loop at full throttle and low rev high load (IE you go 3/4 trottle at 50 in top gear).

[ferny]

It's the mx5 lump in my herald. As I understand it a quirk of its previous life of being a turbo engine means it stops controlling the fuel going into the engine at high rpm.

[bortaf]

That will the the EGi system, i'll have a read up on it, see what the managment manual says
TBH i cant see the fueling not being controled at all but i'll have a read up on the system in the bath

[ferny]

Happy to be proven wrong. It's one of those bits I picked up on yonks ago when doing research but can't remember where. Even if it's not exactly correct it seems to have some relation to real use. If I'm above or below the 3-4k sweet spot my mpg drops alot. Both in the herald and when it was in the original mx5.

The pops, bangs and soot when playing also suggest similar.

[goldnrust]

Firstly, 37mpg from an Mx5 1.6 engine on stock ECU is pretty good! I haven't heard of people managing much more than that on the stock ECU, with many owners with a heavy right foot getting high 20s MPG.

As Bortaf says, are you sure you're not thinking about closed loop/open loop fuelling? The stock mx5 ECU goes into open loop at 4k RPM and/or full throttle. The overrun fuel cut I believe is active above 2k rpm. Open loop is when the ECU just reads the map and puts in the amount of fuel it says to. In closed loop the ECU reads the map, starts by putting this much fuel in then looks at the o2 sensor and adjusts the fuelling accordingly till it's running at 14.7:1 AFR / Lambda of 1. The ECU only goes into open loop when it believes that you wouldn't want an AFR of 14:7:1, i.e. at full throttle when you want to be a bit richer than that to make more power.

As an aside, there's no problem with running a car in closed loop all the time, if you're using a wideband o2 sensor and an ECU with an AFR target table. That way the ECU knows what AFR value you're looking for at any given engine condition.

When you say it had a previous life as a turbo engine, does this mean you're not using the stock ECU? If it is the stock ECU a number of basic methods of adjusting the fuelling to suit a turbo may have been used. If you're having economy issues out of close loop fuelling, then have you checked you have the 1.6 injectors? Fitting the larger 'tan top' injectors from a 1.8 mx5 engine was one of these basic methods used to help provide enough fuel for the turbo, along with raising the fuel reassure with a rising rate regulator.

If you set Megasquirt up right you can gain fuel economy, and you might gain a little at the top end. The stock ECU does run a bit rich in open loop and you can gain a few HP at the top end with a slightly leaner map. The stock mx5 ECU also uses batch injection (where all the injectors are fired at the same time), with the correct configuration Megasquirt can do sequential injection, where you time each injector to match the opening and closing of the intake valve, which gives some good economy gains at light throttle and low engine speeds. It probably would effect around town MPG more than out on a run though.

Either way, I wouldn't try and justify something like Megasquirt with economy/saving money. Not only will it cost money to set-up, but it takes a lot of time to understand and lean how to get the best from it. You'll spend quite a long time with a car that's not running as well as standard till you get your head round tuning the Megasquirt properly. I've been using Megasquirt on all my cars, and helping my friends do the same, since 2009 and I still feel I've got a lot to learn……

… that said, there's lots of other ways to justify fitting Megasquirt to an Mx5 engine, first and foremost because awesome, and as you say it's better to fit it now and learn how to use it before you fit the turbo

[bortaf]

Back from the bath

I stand by my first post, fueling is controled at all times up to the rev limit where the fuel is cut rather than the spark and it's closed loop you are thinking of
PS the wife now thinks i am uber sad, taking the engine managment manual to the bath for a read, ta before she thought i was just sad, now apparently i have surpassed myself and should stick to car porn mags
goldnrust sounds like he has the gen on the Mazda system anything other than general things i'd just be quoting from a manual

[ferny]

So I was kinda right but also quite wrong?
bortaf - what manual have you been reading?

The early MX5 engine is pretty much a 323 turbo lump with bits missing and I was led to believe this is part of why the fuelling wasn't set up in an ideal way. The engine in my Herald is straight out of the MX5, complete with all the wiring. I did it that way so I had a nice working base to start with so I'd be coming from a good footing when things got interesting.

I've never really measured the around town mpg but it's around the low 20's at a guess, but that includes lots of heavy foot action. When I first bought the car I had a 200 mile drive home and as the weather was rubbish and I bought it unseen off eBay I stuck to little over 60mph for the most part and despite getting shafted on the M25 got 42mpg. Later on a 300 mile trip around 75mph got me 37mpg and the return with the roof down and a little faster got me 32mpg. Now it's in the Herald and I did a 260 mile trip the other weekend and got 37mpg at about 70mph.

All sounds a bit boring but I was using the trips to help understand the engine and also see how healthy it was. Seems like it's a good one!

Oct 25, 2014 21:16:38 GMT goldnrust said:

… that said, there's lots of other ways to justify fitting Megasquirt to an Mx5 engine, first and foremost because awesome

I think you've sold it to me now...

[bortaf]

The good old haynes emgine managment manual is a good start, had it for years as a quick referance guide then for more detailed stuff i move onto the cars specific manual, got 40 or 50 in me garage laptop and 40 odd on real paper lol some factory manuals as well but they are mainly Ford which is my main passion (and ewhy i had to have a quick read up on specifics to be sure).
Although the engine may well have been turbo'd in a prev factory instal the ECU will be differant, Mazda didn't just remove the turbo and keep the same ECU, it will have differant fuel and ign mapping software as well as differant correction factors in it, Ford used the same ECU for the 2.0 EFi sierra and the escort (small turbo) cosworth but the are totaly differant in operation and programing.

You have to take into account diff ratios and drag co when looking into fuel consumption as well, the older the body the less MPG yopu are likely to get on a run as drag will probably have been increased, for instance pinto EFi in a sierra 39 MPG on a run, same engine in a cortina with a very similar diff ratio 37 MPG at the same speed, ones a brick ones a slippery jelly mould, the higher the cruising speed the further appart the MPG figure were.
Not what you are on about but it's something to remember if you are chasing economy figures

[Hillmanjames]

The only things they kept on the MX5 lumps were the oil squirters under the pistons to keep the heat down really. Some of the older blocks have a bung in where the turbo oil feed was and i think that the newest blocks might not have had the under piston oil squirters (I'm probably wrong here) The ecu maps are different as said above, but with megasquirt and a turbo you'll have lots of fun

When you are pricing up the megasquirt remember to factor in a wideband sensor if you want to programme it yourself (or dyno time if you don't) along with anything else you need, such as a boost guage, but you'll already have 90% of the gear as its already running on the stock ecu!

a bot off topic, but another thing to bear in mind is where you buy it from. I bought all mine from DIY Autotune in the states as a kit and it worked out cheaper than buying it over here by about £50, even after taking into account the cost of import duty and shipping

My build thread is here for my hunter which has a 1.8 mx5 engine and a turbo : retrorides.proboards.com/thread/166310/1972-hillman-hunter-turbo


James