LSPI: why does a small turbo engine need a modern oil?

It is commonly believed that a small petrol engine needs no special oil: the displacement is small, the power modest, and the API classes have long been clear. The can says SL or SM, the viscosity matches the service book, and that’s that. But a small turbo engine with direct fuel injection is a different story altogether, and far more depends on the oil in it than you might think.

Small displacement, big pressure

From around 2010 carmakers began swapping the familiar naturally aspirated engines en masse for small turbocharged direct-injection units: 1.0-1.5 litres, three or four cylinders. You know them as TSI and TFSI at Volkswagen and Audi, EcoBoost at Ford, PureTech at Peugeot, Citroën and Opel, T-GDi at Hyundai and Kia. Put simply, a small engine is expected to do a big one’s job, rather like asking a little runabout to tow a trailer of potatoes from the allotment every single day.

For the oil this means several new loads at once. The turbocharger shaft spins in bearings lubricated by the same engine oil, with a housing glowing hot from the exhaust right next to them, so the oil there runs hotter than anywhere in an old naturally aspirated engine. Direct injection produces soot, something a petrol engine practically never had before. And on short trips, while the engine is still cold, some of the fuel settles on the cylinder walls, runs down into the sump and never gets the chance to evaporate from there, so the oil thins.

The same soot hits the timing chain as well: it packs into the joints, wears them, and the chain gradually stretches. That is why a separate test (Sequence X) arrived together with API SP: a Ford 2.0 EcoBoost runs for a long time on the oil under test, then they measure how far the chain has stretched, and only fractions of a per cent are allowed. But the nastiest part is something else.

What LSPI is

Drivers who remember carburettor Zhigulis know ordinary knock well, the ringing “pinking” on poor petrol. Modern electronics dealt with that long ago: it hears the knock and retards the ignition. LSPI, low-speed pre-ignition, is the mixture in the cylinder going off before the spark plug has fired, at low revs and under heavy load. Say you are driving in a high gear, the revs are low, and you put your foot down to overtake a lorry. That is exactly when droplets of oil and fuel can ignite on their own, and the cylinder pressure jumps so violently that engineers nicknamed the phenomenon “super-knock”. Moving the spark is pointless here, because the ignition happens before it.

So where do these droplets come from? Strange as it may seem, largely from the oil itself, a film of which always remains on the cylinder walls. That is why the chemistry of the oil is no minor detail here but one of the main factors.

How the oil protects against it

The thing is, every engine oil contains detergent additives, and their classic basis is calcium compounds. Research has shown that the more calcium detergent an oil contains, the more often LSPI occurs, while replacing part of the calcium with magnesium and adding molybdenum makes pre-ignition rarer. In other words, protection against LSPI is not some separate “magic” additive but the balance of the whole additive package, chosen and proven in engine tests.

Hence a simple conclusion for fans of “improving” their oil. A can of additive poured into the engine wrecks precisely the balance that was checked in those tests, and what has actually been mixed into it, probably only its maker knows. Not a single API or ACEA class and not a single manufacturer approval provides for extra additives. I’ll say it once more: apart from oil, NOTHING needs to be poured or tipped into the engine. The article on how engine oil additives work explains why.

Which classes and approvals to look for on the can

LSPI protection arrived gradually: in the classifications of the American Petroleum Institute (API), the “International Lubricant Standardization and Approval Committee” (ILSAC) and the European Automobile Manufacturers’ Association (ACEA), and in carmakers’ own approvals.

1. API SN PLUS (2018): a stopgap supplement to SN aimed specifically at LSPI. It was later replaced by SP and SQ.

2. API SP and ILSAC GF-6 (2020; GF-6 has since been replaced by GF-7): a timing chain wear test was added to LSPI protection, and the bar for deposits on the pistons and in the turbocharger was raised.

3. API SQ and ILSAC GF-7 (2025): the same areas, but with tighter limits; LSPI is now also tested on oil that has already done part of its service life. More in a separate article on these classes.

4. ACEA A7/B7, C6 and C7. A7/B7 and C6 appeared in the 2021 edition of ACEA precisely with an LSPI protection requirement, and C7, from the 2023 edition, is built on the C6 level. But remember that in ACEA “newer” does not mean “better”: C6 and C7 are also thinner oils, and they won’t do for an engine that needs, say, C3 (more in the article on the current ACEA classes).

5. Carmaker approvals. Many of them include an LSPI test too. General Motors brought one in as early as 2015, three years before API SN PLUS, in the second generation of its dexos1 approval (the third appeared in 2021). An approval is “stronger” than a class, and “meets the requirements” on a can is only the oil maker’s word, not the carmaker’s approval.

Now about SL and SM. API SM was approved in 2004, SL even earlier, when nobody had heard of LSPI, so such oils were tested neither for pre-ignition nor for chain stretch. API itself today lists SM only for engines built in 2010 and earlier, and SL for 2004 and earlier. SL or SM oil in a small turbo engine is not “slightly worse” but a lottery in which the stake is the engine itself.

What does this mean for a turbo engine owner?

To start with, nothing new: open the service book and fill up with oil of the approval and viscosity it specifies. If the manufacturer settled for an API class, a later S class as a rule supersedes all the earlier ones, so an SP or SQ oil of the right viscosity will be the correct choice even where something older is written.

Second, short trips. If the car covers a few kilometres a day and is parked again without ever warming up, fuel builds up in the oil, and it thins and ages faster. The change interval in the service book is a maximum, and for town driving it is worth shortening (see which factors shorten the service interval).

Third, engines whose timing belt runs right in the oil (early-generation 1.2 PureTech and some 1.0 EcoBoost units). There the exact approval decides the fate of the belt itself (more in the article on the timing belt that runs in oil).

The price difference between some SM “off the shelf” and an oil with the right approval will surprise you with how small it is next to the cost of repairing a turbo engine. So an engine like this gets only oil with the approval and class the carmaker requires, and no “improvers” on top.