Do I get myself a Subaru Impreza or Subaru Forester?

Hello JM,

I sold my old Mark X last week (The one I told you had an airbag issue some time last year. I fixed it before sale). I’ve some money and I want to buy a new car, not a Japan import. I’ve therefore been doing some searching, and have learnt that the Mazda 6 or CX 5 have a “breakthrough” in internal combustion engines (ICE) in nearly 40 years. I’m reliably informed that this is not a simple thing to achieve since ICE have been around for 100 years. I want a fuel-efficient car, but not a hybrid. Here are my questions.

1. What, in layman terms, is SkyActiv? I learned there is SkyActiv G (for petrol) & D for diesel.

2. Are the 30 per cent fuel savings and 30 per cent more torque claims true?

3. Is this a new badge of Toyota’s VVTI Tech?

4. What is your opinion on Mazdas?

Moses

Hello Moses,

The thing about announcing having a “breakthrough” is, shall we say, a case of overselling a product. There may have been advancements made in new Mazdas, but they’re not as earth-shattering as they may sometimes sound, but, credit where it is due, advancements were made and their effects are tangible. So tangible are they that Mazda is experiencing an unprecedented demand for this innovation of theirs.

1. SkyActiv is mistakenly taken to imply engine development only, but it actually stretches beyond that to include suspension, transmissions and coachwork. The idea behind the concept is to optimise fuel efficiency and engine outputs beyond “standard” levels.

It’s easy to see where the hype comes from when it comes to engine development because despite not literally reinventing anything, Mazda did pull a rabbit out of a hat with its SkyActiv engines. For starters, the SkyActiv G has an insanely high compression ratio (CR) of 14:1. Most other petrol engines have CRs in the 8:1 to 10:1 region.

The downside of having a very high CR is that it increases chances and instances of pre-ignition/detonation, or what we call “knocking” or “pinging” (odd how one phenomenon can have so many different names)

Mazda engineers countered this by almost literally bending physics: they use a tuned 4-2-1* exhaust system to reduce residual gases; there is a cavity in the piston to “readjust” the compression ratio and fuel injection is highly “optimised”.

A nightmare to reassemble

The heavy use of quotation marks in that preceding sentence is because I’m using single words to describe events that would require an entire textbook to fully explain.

Airflow is ramped up and injection pressure is increased, alongside using injectors with multiple nozzles. This, naturally, makes for a very complex engine that is a nightmare to reassemble once disassembled, but the upshot is, you’ve an engine with a very high compression ratio (good for clean burning and with better thermal efficiency) without the risk of knock (bad for engine life).

Now, here is the thing about engines: petrol engines have lower CRs compared to diesel engines. Mazda decided to flout convention with the SkyActiv G by increasing the CR in a petrol engine, a risky manoeuvre. They then got a diesel engine and flouted convention again by decreasing the CR. If you thought the SkyActiv G is an over-engineered wonder, allow me to introduce the SkyActiv D.

Diesel engines usually have CRs between 18:1 and 23:1. Previous research indicates that lower CRs in diesel engines greatly reduce emissions especially oxides of nitrogen and particulate matter (hence no need for a DPF) and they run quieter (no posho mill clatter).

However, the price you pay for this is hard starts especially from cold. The SkyActiv D goes for the benefits of lower CRs (14:1) and counters the hard start by using piezoelectric** injectors with multiple nozzles that change spraying patterns depending on prevailing conditions. The exhaust valves also hang slightly open to allow exhaust gases to be sucked back into the cylinders during the intake stroke so as to warm the engine up faster.

Again, this kind of over-thinking does not come without hardship. In 2016, Mazda had to recall 130,000 SkyActiv D engines after discovering an error in the settings that led to soot building up in the exhaust valves, which, in turn, would lead to noise and vibration followed shortly by engine failure. This has since been resolved.

There is an upcoming engine called the SkyActiv X that now takes a petrol engine and gives it the combustion abilities of a diesel one. Like I said, Mazda is not exactly reinventing the engine, but they sure are flouting convention.

The rest of the SkyActiv suite is not as elaborate as the engines. SkyActiv-Drive is a chain of autoboxes that avoids the fashionable and contemporary dual-clutch setup and sticks to a conventional automatic arrangement but it instead reduces torque converter duty by adjusting lockup control via a series of clutch plates. SkyActiv-MT is the manual transmission with lighter shift action, a short-throw shift and reduced weight.

SkyActiv Body is a lighter (8 per cent) and stiffer (30 per cent) body shell with higher crash safety characteristics, all while looking snazzy with it.

2. Those figures seem massaged because my own research shows economy gains are in the region of 15 per cent. For this tech to actually take proper effect, one has to drive properly, which is not a universal occurrence, unfortunately. I hear people complaining of thirst in unlikely vehicles then observe them driving in ways suggesting they don’t possess any form of mechanical sympathy. This type of behaviour can easily undo the gains Mazda engineers so painstakingly tried to achieve.

3. No it’s not.

4. My opinion on Mazdas is that I like them. I owned one some few years back, which I sold to buy something bigger, louder and more robust (a Subaru). Mazdas look good and feel good, they're really nice to drive. The company has a tendency to do things that nobody else wants to do, such as the opposite-world SkyActiv magic described in 1 above and making rotary engines with full knowledge of all their inherent flaws (they have since sobered up and stopped rotary production).

*(A 4-2-1 exhaust system is one in which the 4 headers come out of the cylinder head. These are then conjoined into two big headers, which are, in turn, joined into one as opposed to a 4-1 exhaust that starts with 4 headers, which are then joined into a single one in one step)

**(piezoelectricity is electrical impulses generated from impacts or pressure.

The curious case of a Corona Premio that billows smoke on acceleration…

I once sought your help on a CPS dysfunctional sensor that almost wrecked my car. I appreciate your advice. My question this time round is about the same beloved Corona Premio with 7a fe lean burn engine that billows smoke especially on hard acceleration after coasting down a slope. To keep it to the level, I add 500ml of engine oil for every 1,000 kilometres driven. Despite this, it achieves good fuel economy of 13-14 kilometres for every litre of petrol. I’ve sought advice from mechanics and a number advise replacing the engine with a fairly used one to overhaul.

What is your take on this? How long can one drive a car whose engine smokes but the level oil is maintained as recommended? I look forward to your response.

Half a litre of oil per 1,000km, huh?

The mechanic may be on to something. Run-of-the-mill engines for small Toyotas are fairly cheap, and buying a replacement may turn out easier on the pocket and less of a headache compared to overhauling the current one.

Now, I know we live in hard economic times and the typical Kenyan protocol of squeezing the last ounce of life out of something before ultimately abandoning it would dictate that you drive that engine into the ground before sourcing a replacement, but... Don’t do that.

It’s burning oil, which means it’s a loss-incurring pollutant, and if it keeps sipping oil, whatever the cause of that may be will keep getting steadily worse until the day you suffer catastrophic engine failure.

Buy a replacement engine

This may occur at a most inopportune moment, such as when you are on the open highway or in the middle of nowhere. Just replace the engine as soon as you can; it’s not that expensive.

In the interest of full disclosure, let us briefly explore the source of the oil consumption. You say it mostly burns oil under load, yes? The prime suspect here is worn out or damaged piston rings — other causes of burning oil such as worn out valve seals or a bad PCV valve present differently, such as smoke on cold starts or on deceleration rather than acceleration, but not under load. To confirm whether it’s the rings, do a compression test.

It’s not open heart surgery to replace piston rings; some car owners even do it themselves, but from observation, a sizeable portion of sufferers, some who have instigated correspondence with Car Clinic over the years, report that following the opening up of their engines, their cars were never quite the same after that. Unless you have a competent mechanic, this is an operation that you may perhaps want to eschew.

Buy a replacement engine and recoup your costs by breaking down the ailing one and selling off sundry parts.

Do I get myself a Subaru Impreza or Subaru Forester?

Good morning Baraza

I’m thinking of buying myself a car. I’m a Subaru fan, though I am used to driving my brother’s Toyota Fielder 1800cc. I was thinking of getting a Subaru Impreza gh3 2011 1500cc, but I know I will feel it sluggish when I decide to carry things in the trunk when travelling to uphill areas like Kinungi on the Nairobi-Nakuru highway. With this in mind, I decided to get a Subaru Forester SG5 instead due to its size, speed and ground clearance. I want a car I can use for years without considering resell, but before I make up my mind, I need to know which is most reasonable to buy between the Impreza or Forester. I live and work in Nairobi.

Kind Regards

Eliud Githinji

Hi Eliud,

You'll be fine in the Forester, trust me. In fact, you will thank yourself for getting a Forester rather than an Impreza given that:

a) You are graduating out of a Fielder. Getting an Impreza is downsizing, literally.

b) You mention lugging things around. A Legacy wagon or Outback would be even more ideal for this given their truck-like dimensions, but you don’t seem interested in them, so we go with the next best thing on your list: the Forester

c) Speed and ground clearance: well, duuh...

As for being manageable, it all comes down to something I’ve repeated for so long, it’s starting to feel embarrassing now. Drive your car gently and maintain it fastidiously, you’ll do swimmingly. Avoid short unnecessary trips in it, that is an enemy of engine life. Other than that, all the best in your new Forester.