[OT] 99 Octane petrol

[Dave Plowman (News)]

The quoted stroke of the *engine* is a different matter since that's a linear measurement. The 'stroke' of the piston is fine to describe its movement from BDC to TDC.

[The Natural Philosopher]

Rubbish, Its the DESIGN of the ENGINE that determnes the compression ratio,

No, its the THICKNESS OF THE HEAD GASKET.

No. It MUST BE THE SHAPE OF THE PISTON.

Don't you love people who are arguing about something, who are just using different words to say the same thing?

[The Natural Philosopher]

Th key issue is what the engine is optimised for.

If optimised for high octane, lower octane fuel will not burn optimally

- sure the anti-knock will stop any damage, but the combustion will then be too late for optimal power. Putting higher octane fuel in will net more power and more MPG.

If optimised for lower octane, the higher octane will burn too slow, and again, the ignition timing may be automatically advanced to partially compenatee, but at the end of the day, its not likely that the engine will develp better power or efficiency, since it needs the higher comp ratio to do that as ell as the better fuel.

Ah, in the glorious days of carburettors and five star petrol, my MGs ran best on 5 star, and a damp cool misty day, and weer rough old dogs on 4 star on a dry hot day.

BTW as the formula one crowd discovered some years back when they were unrestricted on fuel, apart from it being '95 octane' or something, there are any amount of aromatic hydrocarbons you can add that will net you huge power increases in high comp engines, acting as flame retarders, and huge extra MPG, by being super dense..the fuel may have passed the test for '95 octane' but pump fuel it was not. Highly corrosive, highly carcinogenic and very very nasty stuff..

[Dave Plowman (News)]

No standard MG was ever designed for 5 Star. Early Rover V-8s were, though.

Rough running on a hot day was usually down to fuel evaporation and SU pumps. Later cars had a constantly circulating fuel rail to help keep it cool

[Rob Morley]

That's all it does - you're talking about swept volume, which is of course a volume rather than a distance, and even then takes no account of volumetric efficiency.

[Chris Bacon]

But that isn't what you were talk "The stroke of the piston and the size of the combustion chamber determine the compression ratio."

They don't, do they.

[Chris Bacon]

True for some engine designs...

True...

You cannot determine the compression ratio from the stroke of the piston and the size of the combustion chamber, letting the latter include the thickness of the head gasket (if present), for the same piston shape.

[Chris Bacon]

Erm, "yee-haw", as they are apparently fond of saying in America. Well Done.

[Sylvain VAN DER WALDE]

Correct.

You're making a fool of yourself. Just listen to Dave Plowman (and others).

It's got nothing to do with the compression ratio. (I may stand corrected). :)

Volumetric efficiency is related to the amount of air that is able to enter the combustion chamber during the induction stroke (4 stroke engine). It's helped by using better cylinder head design and construction (which includes better induction manifold design and construction). It's also helped by valve timing "overlap", and probably by improved exhaust system design. _Amongst other things_.

I used to own a BSA Blue Star 350cc OHV _single_ cylinder 1934 motorcycle. This bike had _twin_ exhaust ports and exhaust systems. The only reason for this, in my opinion, must have been an improved volumetric efficiency ( the single inlet and single exhaust cylinder head valves were the same size).

Sylvain.

[john2]

The survey was on Fifth Gear, Channel 5 a couple of months ago. I probably have a recording somewhere.

john2

[The Natural Philosopher]

You can if you know the bore..

[Chris Bacon]

[Chris Bacon]

Above is what DP says determines the CR. Do you *really* agree, or can you add something to that statement to make it right?

As it's [CR] normally measured, nothing. It was un harang rouge.

At higher RPM, having twin exhaust valves is an improvement over one, cetrainly. This is reflected in ancient engines which were constructed with performance in mind, as well as modern ones. However, tuning the exhaust system gives lesser results in comparison with the effects of tuning the input system (by quite a lot).

[Chris Bacon]

:) !

[Bob Eager]

There is just one difference - the price.

Many years ago I worked on a non self service forecourt. There was a class of customer who clearly bought 'five star' because they could afford it, and wanted to flaunt it. I doubt that much has changed.

[Bob Eager]

Not an MG Midget, I trust?

I always laugh at the memory of the Bond Bug. Clearly targeted at the very young (just passed test) kind of driver - cheap to run, etc. But also totally useless as a passion wagon, with that great engine bulk between the seats. Doomed to failure!

[<me9]

I found at one station -- Jet, that my MPG was less. Other Jet stations were fine. Sometimes it is how the pumps are set up.

[The Natural Philosopher]

Another sweeping generalisation that is only correct 50% of the time.

On any engine there will be some performance limitation areas. You work on the one that makes the most difference forest. This may be physically strength - the inability to maintain RPM without exploding, the ability of the valve gear to work at high RPM or the breathing. The breathing restrictions may chiefly be the exhaust, or it may be the induction.

Its solely down to the particular design and layout of the engine.

I once asked an F1 engine designer what dictated the way the engines went together..

"Not much. Short stroke over square because you can rev the f*ck out of them without exploding, and that gives us the maximum head area to put the valves in, then 4 valves because once you get up to 18K RPM you need all you can get, induction and exhaust.and 4 fits into a round head better than any other nimber - could use 6 or 8, but then you get complicated and heavy for marginal gains....then the devil is in the details of gas flowing the manifolds and getting the valves to work at that speed, plus the injection and ignition mapping"

No mention of one area being more relevant than another. One accepts that with turbocharging, inlet design is almost superfluous, as you an simply increase boost pressure to the cylinders by screwing down the waste gates etc..

[Dave Plowman (News)]

But a true one. Exhaust gases are under highish pressure. Inlet merely - at best - atomospheric. On the cylinder head and inlet manifold attention to the inlet tract by reducing restrictions etc that shouldn't be there but are due to the costs of removing them in manufacture, etc will pay far more dividends than the same work carried out on the exhaust ports. And most production cars are already fitted with free (enough) flowing exhausts.

[Dave Plowman (News)]

Since I had a two 1275 Midgets, I'm quite well up on that engine. I've also totally re-built several.

Neither of mine was 'better' on 5 star. Of course if you modify the engine, you're starting a new ball game. But that's not what you said originally.

By 'working' on them it would be possible to make them only suitable for aviation petrol. Or to run on 2-star. But that's not what they were designed for - my point.

Mine both ran on standard timing. And didn't pink on 4-star. Perhaps your timing technique or timing marks were inaccurate? Or perhaps they just simply needed a de-coke. Also, a slightly weak mixture encourages detonation.