Automotive Fluids - Lubricating Oils & Greases, Fuels, Coolants & Brake Fluids

KEW Engineering




How does Oil Protect?

Base Oils Explained

Oil Aging & Degradation

Additive Formulations

Oil Viscosity Explained

Oil Quality Ratings

FAQs on Motor Oils


Brake & Clutch Fluids




Disposal of Used Oils




Copyright 2009

KEW Engineering Ltd




The usual question asked by the classic MG owner is "Has my car got an unleaded head, and if not should I fit one?"

The B Series was normally, for the UK market at least, not produced with an unleaded head.  In fact, the A Series engine only got an unleaded head in the late 80s on the A+ units.  Also, the MGC will not have a factory unleaded head. There has been some discussion about late model US specification MG Bs having an unleaded head, however. There is some confusion, too, as the V8 was reputed to be suited for unleaded whereas the B wasn't.  


The main point being whether it's necessary to have an unleaded head, that is, the question is often asked whether to get the head converted?  

It's generally not feasible to get the head converted on its own and most owners get the cylinder head converted when doing any other work on the head or engine. The conversion usually entails having hardened valves seats fitted. 

However, on an original head, lead memory is usually sufficient to prevent valve seat recession, and this is something that can be checked through regular tappet adjustment - if regular re-adjustment to take up clearance is required then there is valve seat recession. Unless you regularly drive at >3500rpm for long periods, then the consensus of opinion is that the lead memory will remain intact and minimal damage will result, if any. Sufficient at least to last the remaining life of the engine whereupon a conversion might be undertaken as part of the rebuild.

In fact the official testing done in the mid 1990s on these various additives was on an A series engine in a Metro, and the conclusion was that valve seat recession was not an issue in reality. Most treatments tested were found to add no value in terms of protection. Loss of the lead memory for any engine usually only affects the lubrication of the valves (guides and seats). 


However, the unleaded will affect performance, and ignition timing is usually retarded to meet the unleaded Octane rating. If you use standard unleaded this is 95 which does not compare as well to the original 4star 100 rating of the older leaded fuels. In reality, lead levels were being continuously reduced throughout the 80s into the 90s and so consequently many owners have been using reduced lead level fuels for a long time.

Suffice to say many owners will feel happier with the regular treatment offered by the likes of the MGOC in respect of the Castrol Valvemaster. I personally don't use this treatment, and nor do many others I know, but for peace of mind, it can't do any harm and certainly, a low mileage engine, or a low use engine will perhaps gain some benefit. 


What it won't necessarily do is improve the Octane rating, for that you would need a product with Octane booster. I know that an engineer I spoke with from the industry does not have much regard for Octane boosters commenting that deposits of ash can accumulate in the rings - his suggestion being to buy a super unleaded with higher Octane rating.


For the record, as of 2009, UK classic car owners can still buy leaded fuel from specialist outlets, albeit at a higher price.  But if your classic really does need leaded fuel (pre-war and 1950s type engines) and you are a low annual mileage owner then the cost is probably insignificant.

Look for your nearest outlet on this website:

For the ultimate in unleaded heads, Peter Burgess, a respected tuner of MGs has developed a B series head specifically for unleaded fuels, and I believe from one owner that this is a very good set-up not losing anything to the stage 2 heads etc. The problem is that the A and B series head was designed in the days of leaded fuels available in the UK  whereas modern cars have the benefit of modern head design taking into account the current fuel quality and so issues like squish etc are considered in relation to the current 95RON fuels and this is perhaps why the V8s are better at coping with unleaded and show no real improvements, and can hence run unleaded with no noticeable difference.

Engine Run-On


One problem the B Series engines suffer with is run-on. This is a combination of factors in my experience, usually a combination of a poor state of tune and use of standard unleaded petrol. However, unleaded petrol does cause the cylinder head to get much hotter than the old leaded fuels, and combined with deposits of combustion, these can glow hot and act like a sparking plug, causing uncontrolled run-on. Unfortunately, unlike a modern car that will shut down the injectors, the fuel is able to continue to enter the engine from the carb's float bowl and hence the run on. The MGOC offer an anti-run-on valve which is effectively a solenoid device that closes with the ignition on, but when switching off it opens to allow a huge rush of air into the inlet manifold that kills any run-on. My experience is simply to let the engine idle for about 15-30 seconds to cool down and then switch off, and if necessary load up the engine by engaging the clutch as you switch off. In worse case scenarios it may require engaging 4th gear and stalling the engine by partially releasing the clutch.




To improve the combustion process the fuel suppliers put an anti-knock additive in the fuel so that the flame is induced by the spark and travels smoothly across the chamber. "Lead" was the usual method of prviding the anti-knock resistance but that has been replaced by better and safer substitutes. The more anti-knock properties, or in other words the higher the Research Octane Number (RON), the less the risk of pre-ignition. This then allows greater advance and higher compression ratios. Higher compression ratios create more power, and allowing the complete combustion to occur as close as possible after TDC allows more use of the burn power, rather than it happening later when the piston is already a fair portion of the way down the cylinder.  In a B series designed for 4 or 5 star leaded petrol, then the current 95 unleaded is theoretically unsuitable. 


The US ratings are a little different to the UK. The UK uses the Research Octane Number (RON) rating, but the US uses a differing system of averaging the RON and the MON (Motor Octane Number). MON is more indicative of how the fuel acts under load.

The US numbers are typically 4-5 below the equivalent RON here in the UK. That said, 87 is very poor as that's about 91-92. Even 89 is borderline. RON 95 is about the same as a US 91 grade.

RON is a somewhat misunderstood issue as using a higher RON fuel won't necessarily deliver better performance. The head, as in the combustion chamber, must be designed to benefit from the higher RON in the first instance, plus the timing must be advanced to take advantage of the higher RON.


When cars were converting to unleaded in the late 80s/early 90s it was common to retard the timing accordingly and for the A/B series this also applies. No one engine is identical and hence timing is something that should be set according to the individual car and that is largely trial and error using the factory figure as a starting point. For those with the 25D4 distributor with vernier adjustment this is a simple enough job - test the acceleration between 30mph and 60mph in 3rd gear and listen for pinking. If none is heard then advance by a degree at a time until it is heard and then back off one degree.


The point being that timing is reset according to the fuel's RON, and thus with a head designed for leaded, whilst the car will run reasonably well on std unleaded (95) when properly timed, it may not perform as well as being re-timed and fed higher Octane (98/99). In the case of the V8, to gain from the higher Octane, the timing needs advancing, or rather, set-up to cope, and may not necessarily benefit fully.

Now as far as the more advanced fuels go today, I have been using Shell's Optimax with much success in my B series and the KV6 in my ZT.


There are several reasons, namely performance in my case, but the other benefit is the detergent nature that ensures a nice clean combustion chamber. I have used Optimax to good effect as a carb cleaner when overhauling my twin SUs, it certainly cleans up.


Now the current V-power has raised the bar on the RON to 99 with added lubricant to ensure good upper cylinder lubrication on the valves, too.


My head is a stage 2 unit so does benefit and with the mappable electronic ignition I use I can get exceptional performance from the B series. I have found that both Tescos and Shell are not that much more expensive than standard unleaded, however, the slight price hike is justifiable in performance, and fuel economy on both my modern MG ZT and MG BGT is better than on 95.


I sometimes add V-power to my wife's Rover 214 (1.4 16V K Series) but in the main, her car will not see the performance advantage as the engine does not have knock sensors so has been factory tuned to allow for RON 95 unleaded. But the odd tankful should keep the injectors and combustion chambers clean.


Contrary to popular belief, knock sensors are not fitted to all modern cars, and nor do they readjust for varying fuel/timing maps, either. Small cars without knock sensors are generally mapped for reliability and economy, and hence unleaded is the assumed fuel in use, so consequently, unless remapped by a chip-tuner, small cars see no benefit in performance or even seat-of-the-pants-feel. And even then the gains may be small as the head design may not be suited to higher Octane fuel.

Larger engines such as in my ZT and in performance cars usually have knock sensors which allow the mapping to be set for performance (albeit with reliability in mind still leaving the door open to chip-tuners). This means that the manufacturer will generally map the ECU on the basis that better quality fuel may be used, but to some extent they still err on the side of the the user buying unleaded for economy. All the knock sensor does is detect pinking and retard the timing accordingly. Hence, one often notes the drop in performance rather than the gain, something that driver's of Japanese performance imports report when having to use the odd tank of std unleaded. This is something I also notice on my ZT, the loss of responsiveness, and it can take a few tanks of super to get the performance back up again. The difference however is likely to be insufficient to note by driving alone, the main benefit being the increased responsiveness from the better burn process.

Therefore I would say use a good quality 95 from Shell or BP, but even a supermarket fuel should be adequate. But if you want performance, then get the car set-up for the fuel of your choice and stick to it.