what petrol

[scott]

what petrol do you put in your mg i put some high octain petrol in the "f" by mistake yesterday, and boy what a difference it makes the car seams to pull better and accelerate much smootherat only 5p a litre more for the high octain stuff i think i will run the "f" on it for a while what do you think

[Andrew Regens]

Yes it's the only fuel, I try and stick with Shell V power, not only runs better goes further.

[Green Squirrel]

Sorry but the *F* was designed for 95 RON and that is what I find it is very happy with so that is what I put in. Over the many years and many thousands of miles, UK & Continental, it has always run sweetly and on the odd occasion that I have put in higher octane it has not made any difference to the car - only my wallet.

Obviously if you have had the engine tuned for higher octanes that is a different matter

[David Clelland]

Got to agree with Ted, higher octane doesn't make any difference to a standard F engine (all in the mind). However, my car does so little mileage now that I tend to use Shell V Power for the cleaning properties. Mind you that's probably all hype as well and normal unleaded will do just as well. This subject has been discussed a zillion times

[James Curgenven]

It's possible that if you had never used it before the cleaning properties in the more fancy stuff may have cleaned out some gunk making the engine work better.

But you won't gain any more power or MPG from the more expensive stuff over running normal stuff with a tank full of Vpower or so, to clean it out every so often

[scott]

cheers guys thought i was on to something then

[Andrew Regens]

Over here we had until a couple of years ago ULP and PULP. The ULP was 92 Octane, PULP 98 and when SUPER was faised out we got 95 Octane. So that is why I have stuck with V power also Shell and Mobil had not diluted their fuel with ethonal.

[Debs]

Those of a weak disposition should look away now.



Can an F/TF take advantage of 97 Octane (RON), or higher, fuel as stock?


The simple answer is no! because the engine management system is mapped for 95 RON (in fact it’s mapped for slightly worse, giving a level of 'belts 'n braces' protection against detonation [pinking]). While there may be some advantages from using 97 RON or 99 RON fuels, these accrue from the fact that:

1. Higher Octane fuels tend to contain a better detergent package. These are said to keeps the engine internals cleaner and, therefore, more efficient. Ie frictional losses are reduced. This also helps prevent pre-ignition (qv later) by keeping the cylinders clear of unburnt carbon deposits.

2. Higher Octane fuels contain a slightly higher amount of Iso-Octanes (8-Carbon chain molecules) and should (theoretically) produce more combustion product leading to greater power. This is (at best) likely to be in the region of only 1 or 2 bhp and would not be felt by the driver.



Whether a knock sensor is fitted is irrelevant and is a myth that gets posted around on a regular basis. A knock sensor senses knock (detonation) and signals the ECU to retard the ignition during periods of detonation (such as when running on too low an Octane fuel). When the detonation stops the ECU will advance the ignition back to factory setting. The system will not advance past its mapped setting. Thus if you are mapped for 95RON you cannot take advantage of higher octane fuels whether you have a knock sensor or not.



To understand why this is the case we need to understand what is going on in the cylinder, why we use ignition advance, what detonation is, and what high octane fuels provide.



A 4-stroke or ‘Otto Cycle’ engine goes through four distinct 'strokes' as the pistons move up and down the bores. Simplistically it is as follows:

1. Induction Stroke: The Inlet valve opens, the piston moves down the bore causing a vacuum and sucking in the Fuel/Air charge.

2. Compression Stroke: The Inlet and Exhaust valves are closed, the piston moves up the bore thereby compressing the Fuel/Air charge.

3. Power Stroke: The Inlet and Exhaust valves are closed, the spark plug fires, the Fuel/Air charge is burnt producing expanding gasses that push the piston down the bore.

4. Exhaust Stroke. The Exhaust valve is open, the piston moves up the bore forcing the burnt Fuel/Air charge out through the Exhaust Valve.


Therefore the output (Torque) of any given 4-Stroke motor is directly proportional to the pressure exerted by the expanding gasses, produced by the burning Fuel/Air charge, upon the piston pushing it down the bore and, hence, causing the crankshaft to rotate.


Now, ideally, you want the expanding combustion gasses to start pushing upon the piston when it is at TDC. If they start pushing when the piston is still coming up the bore on the Compression Stroke (ie BTDC) then they will be fighting against the piston resulting in a reduction in torque output (and, hence, a drop in power). If they start pushing when the piston is already going down the bore on the Power Stroke (ie ATDC), then, again, there will be a loss in torque (and hence power) because they are pushing into a space that is expanding so the pressure felt by the piston will be less.

In an ideal world, therefore, you would have the spark plug fire giving an instantaneous ignition of the Fuel/Air charge at TDC, with an instantaneous burn, and a concomitant instantaneous expansion of the combustion gasses.

Of course, we don’t inhabit such an ideal world and it takes finite amounts of time for the spark plug to fire, for the flame front to expand across the piston face, for the charge to fully burn, and for the combustion gasses to exert pressure on the piston.

Therefore we fire the spark plug before TDC such that the burnt charge exerts its maximum pressure on the piston at TDC (in reality this maximum pressure usually occurs slightly after TDC for a number of reasons, not least detonation. (qv later). This is known as Ignition Advance.


As engine speed increases (ie rpm's rise) there is less time per cycle for these processes to occur so, for maximum performance, you require increasing amounts of Ignition Advance. This is known as an Ignition Advance Curve and is controlled by the distributor (early cars) or the ECU (later cars).

In fact distributors give a fixed (mechanical) advance curve, ie they are 2D and map purely Ignition Advance versus rpm, whereas an ECU controlled system can be mapped to give variations in the curve at different rpm's to take account of such things as changes in cylinder burn rates caused by differing engine loads (they will assess Throttle Position and/or Manifold Air Pressure), ie they are 3D. In fact the ECU can also alter the fuelling, thereby giving a more efficient burn throughout the rpm range than can be achieved by a distributor/carburettor set up.




OK so what is Detonation?


To understand this we have to take account of a piece of Physics known as 'Boyle's Law'. This states, for any compressible fluid such as the Fuel/Air charge in an engine, that:

[Pressure x Volume] / Temperature = Constant

That is to say that if you have a fixed volume of gas (the Fuel/Air charge), if you compress it, it will heat up.

So, on the Compression Stroke, what we are doing (aside from the heat soak from a hot motor) is heating the Fuel/Air charge as it is squeezed by the rising piston.

Now, obviously, the Fuel/Air charge has got fuel in it (petrol) so there will come a point whereby the compression induced heating effect will cause the charge to self ignite (in fact this is how diesels work – they don’t have spark plugs, but rely upon compression to ignite the Fuel/Air charge).

In cases of detonation, what happens is the spark plug fires (as we said, BTDC) and the flame front starts to travel across the piston face. If then the fuel also self ignites we have two flame fronts that must eventually collide. Now, while I said it takes a finite amount of time for a flame front to travel, the fact is that it is travelling at supersonic speed. So, when the two flame fronts collide, they do so with massive force, resulting in a sudden huge pressure 'spike'. Owing to Boyle's Law this pressure spike causes an enormous temperature spike sufficient to burn through to the piston face / cylinder head face causing pitting, or (at worst) a hole in the piston crown. On an engine such as the K Series, pitting of the cylinder head face renders it scrap.

(Note. There is another effect known as pre-ignition whereby something glowing red hot in the cylinder, such as a spark plug that is of too soft (hot) a grade for the motor or an incandescent carbon deposit ignites the Fuel/Air charge before the spark plug fires. The effect is the same – two (or more) flame fronts colliding.)



So, what puts your motor at risk of detonation?

1. Running too much Ignition Advance. If we ignite the Fuel/Air charge too early on the Compression Stroke then the charge will start burning before the piston reaches TDC. If this happens the expanding gasses are fighting against the rising piston causing increased cylinder pressure (ie more heating) to the point where detonation is likely to occur.

2. Too high a Dynamic Compression Ratio for the fuel being used. Static Compression Ratio (CR) is the ratio between the cylinder swept volume and the volume of the combustion chamber. For the F/TF this is 10.5 : 1. However Static Compression Ratio takes no account of the fact that the valves are open during much of the engine's cycle. On racing engines with long duration (ie the valves are open longer), wide overlap (ie both the Inlet and Exhaust valves are open together) camshafts, the Dynamic Compression Ratio is far lower than the Static Compression Ratio so overall higher Static ratios are used as well as greater amounts of Ignition advance. For example our racing Midget uses 11.75 : 1 Static with 29 degrees advance BTDC at 3000 rpm. However, the higher the Compression Ratio the closer you get to the detonation line.

3. Too lean a Fuel/Air charge. Leaning out the mixture causes it to burn hotter and faster. On the other hand running a slightly rich mixture will mean that not all the fuel is burnt and the unburnt fuel can act as a 'heat sink' absorbing the heat and carrying it away through the Exhaust Valve (this is why Nitrous motors are set up to over-fuel slightly to keep them safe from detonation). However, too rich a mixture will result in a drop in power, cylinder bore wear owing to the petrol removing the oil film, increased emissions and, ultimately, catalyst failure.

4. Running an engine too hot, causing excessive heat soak into the Fuel/Air charge.



So how do we prevent detonation?

1. Don’t run too much Ignition Advance. This is where Knock Sensors are helpful since they signal the ECU to retard the Ignition if detonation starts to occur.

2. Don’t run lean.

3. Don’t run hot.

4. Use a fuel that is detonation resistant.




Detonation resistance in petrol is indicated by its RON. The higher the octane, the more detonation resistant it is. In the old days of leaded fuel, Tetra-Ethyl Lead was added to prevent the fuel detonating (as well as to protect the Exhaust Valve seats). However, Lead is a toxin.

Once Unleaded fuels became the norm, initially benzo-phenolic compounds were added (although these were highly carcinogenic). Other additives included Methylcyclopentadienyl manganese tricarbonyl (MMT) (a powerful neuro-toxin) and Methyl tertiary-butyl ether (MTBE) which pollutes ground water supplies.

These days the trend is to add Ethanol (albeit at levels over 5% this can exacerbate fuel evaporation) and Ethyl tertiary-butyl ether (ETBE).



Higher RON fuels allow the use of increased Compression Ratio (CR) (we can’t change the CR of the K Series motor without a rebuild) and/or greater Ignition Advance (again we can’t change this on the K Series unless we re-map the ECU).




In conclusion then, the use of higher octane fuels is pointless in an F/TF (aside from the better detergent packages) unless the engine is rebuilt to a higher CR and/or the ECU is re-mapped to give increased Ignition Advance.

[MGF Nick]

I run mine on standard 95, brought from anywhere.