Forced induction fuel economy and durability

[Chris71]

Hi,

Out of interest, what sort of MPG figures do turbo'd or supercharged MX5s return and do they require much more looking after than NA cars?

I appreciate both will vary on how far you've taken the tuning to a degree, but principally interested in fairly mild states of tune (up to about 200hp).

Chris

[mk1salami]

The last tank I saw 308km and it took 35 litres to fill it back up again, so 25.2 mpg which is mainly enthusiastic driving. I've never really tried hard for economy so I'm not sure what it'd return. My cruise cells target around stoich so I'd expect it to be similar to an N/A car on a long run without too many dips into boost.

Regarding reliability, things do and WILL break. Expect to replace almost every component of your coolant system. Water hoses and radiators etc. If there is any part of the system that is a little bit weak, FI will show it up a lot more quickly. I've got to the position now where I've replaced so many parts that not a lot goes wrong anymore (touchwood). Besides the gearbox, which I've gone through two of. At £50 a pop and a morning to replace, it's not really a huge pain for me.

All this is based on around a year living with a 1.8 1994 BEGI'd car @ 14psi (~250bhp).

Edited by mk1salami on Sunday 15th February 22:18

[MX-5 Lazza]

I had my Mk2.5 supercharged in 2005 (215bhp) and upped the power to about 230bhp about a year ago. In that time I have had to replace the cat (I think the tuning of the EMB did it - started rich and then leaned it out - safer for the engine than starting lean but harder on the cat) and a throttle butterfly snapped (probably nothing to do with FI though). That's it. I'd consider that pretty damned reliable.

The other things I've had to change are the radiator (I fitted an FM Twincooler for track driving and mostly for the oil-cooler. If it was just road driving I wouldn't have bothered), an auto-tensioner (which is a standard part of the MP62 kit now) and a dual throttle kit (which I think is part of the standard intercooled kit from P5 now).

As for fuel economy - if you're worried about that then don't go FI. If I drive carefully I can get 28mpg and can see over 30mpg on a motorway run but that's not fun. You fit FI go get more performance and you'll use it in which case economy will drop. Budget for 23mpg and be happy if you see better than that but also don't be surprised if it drops below that if you are on a mission. I get about 10-13mpg on track for instance.

Edited by MX-5 Lazza on Sunday 15th February 23:10

[skinny]

normal road driving i will get 33/34 mpg on my mk1 1.6 turbo (around 145 - 150bhp at the mo). it will go down when i increase the boost but hopefully still above 30 on a long run with good tuning.

only thing i have to do different is change the oil more often. no reliability probs due to FI, and it's been on from the start, 160,000 miles.

[OnlyMX5ives]

200bhp SC: The only thing I'd had to do was uprate the clutch, no other probs that I remember although I have uprated other items because I wanted to.

I drive it hard and only get 200 miles to a tank (35 litres)

250bhp FM2 - loads more has been uprated and gives slightly better economy primarily due to a superior tuning.

[MX-5 Lazza]

My car still has the stock clutch. It was fitted (warranty replacement due to clutch judder) just before the SC so is pretty much the same age as the SC (April 2005) and is still showing no signs of wear despite quite a few track days and a few trips to Santa Pod. I think when it does go I might fit another stock clutch

[OnlyMX5ives]

MX-5 Lazza said:

My car still has the stock clutch. It was fitted (warranty replacement due to clutch judder) just before the SC so is pretty much the same age as the SC (April 2005) and is still showing no signs of wear despite quite a few track days and a few trips to Santa Pod. I think when it does go I might fit another stock clutch

You obviously aren't trying hard enough

Edited by OnlyMX5ives on Monday 16th February 10:33

[MX-5 Lazza]

OnlyMX5ives said:

MX-5 Lazza said:

My car still has the stock clutch. It was fitted (warranty replacement due to clutch judder) just before the SC so is pretty much the same age as the SC (April 2005) and is still showing no signs of wear despite quite a few track days and a few trips to Santa Pod. I think when it does go I might fit another stock clutch

You obviously aren't trying hard enough

Edited by OnlyMX5ives on Monday 16th February 10:33

I have to disagree considering the speed & frequency that I get through tyres/brake pads/brake discs

[Chris71]

So, two questions:

Is there a big difference between supercharging and turbocharging from an economy point of view?
Does the 6 speed 'box in the later cars make a huge difference (not strictly FI, but curious...)?

[Herman Toothrot]

Chris71 said:

Hi,

Out of interest, what sort of MPG figures do turbo'd MX5s return


Chris

More MPG than any stock normally aspirated one assuming its a proper install with standalone or even piggy back ECU, stock cars fueling is rubbish. Also in normal non fast driving you can use far less revs to make normal progress. A trackday on boost howevers another story. Bodge cars i.e. rising rate fuel pressure regulator ones will drink like a fish all the time.

Chris71 said:

Hi,
and do they require much more looking after than NA cars?

Chris

Maybe slightly more oil changes to be on the safe side in making your turbo last, more rear tyre wear, thats about it.

[Chris71]

Does sound tempting. I'm pipedreaming a little here, but if you'll indulge me, what do you reckon the best donor car and the best form of forced induction would be for a driveable, relatively economical daily driver running 160-200hp?

[MX-5 Lazza]

Chris71 said:

So, two questions:

Is there a big difference between supercharging and turbocharging from an economy point of view?
Does the 6 speed 'box in the later cars make a huge difference (not strictly FI, but curious...)?

With most Turbos it's fairly easy to drive it carefully keeping it out of boost. With SC the boost cuts in much quicker (which is why it feels like a bigger engine rather than a boosted engine) so it's harder to drive it keeping out of boost.

6 speed is nice for motorway/A road cruising but the 5 speed box has a better gear change.

Chris71 said:

Does sound tempting. I'm pipedreaming a little here, but if you'll indulge me, what do you reckon the best donor car and the best form of forced induction would be for a driveable, relatively economical daily driver running 160-200hp?

I reckon the best donor would be an early Mk1 1.8 so 93/94. These will have the 1.8 engine but with less weight.
Having said that, I deliberately went for a Mk2.5 Sport with AC, PS, EW, EM, heated leather seats, 17" wheels and have added a roll bar so weight hasn't really been something I've considered. With close to 200lb/ft it really isn't a problem

As for turbo or super... that's a massive can of worms. I like SC because I like the always-on feeling and the sc scream. Others like the power rush and pop/hiss you get with turbo. It's a matter of taste and only you can decide which is best for you.

[OnlyMX5ives]

6 speed is a waste of time particularly for FI.

You would think 6th would be the 'overdrive' that the 5 speed could do with.

You'd be wrong.

You get 6 stupidly close ratios.

[jayroberts]

Right speed, right place, right gear, right time.

[MX-5 Lazza]

I don't agree that 6 speed is just 6 close ratios. Yes the ratios are closer (that's why they call it a close ratio gearbox) but 5th is slightly lower than 5th on a 5 speed and 6th is higher. If you don't do any motorway/long distance driving then the 6 speed is pointless but on a run the longer legs are nice to have.

[MX-5 Lazza]

Filled up today. 260 miles on 45 litres so pretty much 26mpg. Not bad considering I've been "testing" my new Hankook RS2s

[Chris71]

MX-5 Lazza said:

Filled up today. 260 miles on 45 litres so pretty much 26mpg. Not bad considering I've been "testing" my new Hankook RS2s

Your car is running over 200bhp too isn't it? So, that's quite an impressive figure for a car with (realistically) more power than a base spec Griffith.

Gearbox comments are interesting. When I had my RS (with a lowered FDR) everybody said it would be screaming on the motorway, but it wasn't too bad - certainly pailed in significance to the windnoise from the hood! One of the best things about that car was its beautifully precise gear shift, so it's interesting to hear that the 6 speed isn't quite as sharp.

[OnlyMX5ives]

MX-5 Lazza said:

I don't agree that 6 speed is just 6 close ratios. Yes the ratios are closer (that's why they call it a close ratio gearbox) but 5th is slightly lower than 5th on a 5 speed and 6th is higher. If you don't do any motorway/long distance driving then the 6 speed is pointless but on a run the longer legs are nice to have.

Yes but the 5 speed is a close ratio gearbox !

So yours is a Very close ratio gearbox.

Put it this way I've had them and chose to sell them rather than install them in my car.

However just putting the high ratio diff out of a 6 speed into an FI'd car is a good idea.

[MX-5 Lazza]

Chris71 said:

MX-5 Lazza said:

Filled up today. 260 miles on 45 litres so pretty much 26mpg. Not bad considering I've been "testing" my new Hankook RS2s

Your car is running over 200bhp too isn't it? So, that's quite an impressive figure for a car with (realistically) more power than a base spec Griffith.

Gearbox comments are interesting. When I had my RS (with a lowered FDR) everybody said it would be screaming on the motorway, but it wasn't too bad - certainly pailed in significance to the windnoise from the hood! One of the best things about that car was its beautifully precise gear shift, so it's interesting to hear that the 6 speed isn't quite as sharp.

yep, over 225bhp in fact
To be fair, the 6-speed does change just as sweetly as the 5-speed and without the notchiness and noise that the 5-speed often suffers from. It's just when I'm racing it through the gears quickly I sometimes get a bit lost and end up in the wrong gear. In normal driving it's a lovely gearbox, just like the 5-speed.

OnlyMX5ives said:

MX-5 Lazza said:

I don't agree that 6 speed is just 6 close ratios. Yes the ratios are closer (that's why they call it a close ratio gearbox) but 5th is slightly lower than 5th on a 5 speed and 6th is higher. If you don't do any motorway/long distance driving then the 6 speed is pointless but on a run the longer legs are nice to have.

Yes but the 5 speed is a close ratio gearbox !

So yours is a Very close ratio gearbox.

Put it this way I've had them and chose to sell them rather than install them in my car.

However just putting the high ratio diff out of a 6 speed into an FI'd car is a good idea.

Ah, that's different, you are talking about modifying the car by mixing & matching the gearbox & dif. The original question was whether the 6-speed in later cars is worth it and comparing a 5-speed and a 6-speed with standard difs the 6-speed has a longer final drive so is better for cruising.

[skinny]

MX-5 Lazza said:

Filled up today. 260 miles on 45 litres so pretty much 26mpg. Not bad considering I've been "testing" my new Hankook RS2s

[OT]how are you finding them mate - wait until it warms up a bit outside and you get the chance to really lean on them these on your 15's? to me, they were a noticeable step above F1's and T1R's on a dry road, and fantastic on track - they *love* getting a chunk of heat into them[/OT]