TESCO 99RON PETROL

[Del_GTi]

Fair enough on a turbo'd or twin turbo'd car with a lowish compression ratio and some nice boost you can get decent gains with a higher octane fuel but the GTi engine doesn't have high enough compression to generate the heat to make the most of the 98-99 octane.

I ignored this coment earlier cos it didn't seem important to correct anyone as the thread is about Fuel octane and benefits but.....

That's right mate, don't correct it, cos the comment isn't incorrect.


So what if its' 'fairly high compression', it' still not high enough. Your '2nd Point' supposedly confirms what high compression is.

I'm also aware it's not just about compression. However since higher compression provides more heat , and a higher octane fuel requires more heat to make the most of the potential energy it provides, clearly compression is an imprtant factor.

Also so what if compression gives a bigger bang? We don't want a 'bigger bang, it's a slower burn and more controlled burn which is required.

Unless you think the GTi engine is a diesel then no i'm pretty sure we aren't talking about diesel.

[Del_GTi]

Also the Gti runs at 11.5:1 which is also not the limit for 'pump' fuel. Seems strange if your 'track' car is running 11.3:1.

[bstardchild]

Also the Gti runs at 11.5:1 which is also not the limit for 'pump' fuel. Seems strange if your 'track' car is running 11.3:1.

Thanks for finding out the GTi Compression ratio - in any 4 valve engine that is very high...

To cover the "Seems strange if your 'track' car is running 11.3:1" coments.....

Every engine has a design limit on compression ratio maximum - factors like the bore size, piston design, combustion chamber shape etc etc all affect the flame speed. This limit is also very hard to calculate - you tend to find it when you go too far!!!

My track day car has a 95mm bore piston and a 4 valve head but the combustion chamber shape isn't perfect and neither is the piston shape but at least the spark plug is mounted centrally between all four valves.

If I go above 11.3:1 on this engine the level of activity on the knock sensors goes ballistic (detecting pinking/knock) - the ECU sees this and changes the ignition settings to stop it - this effectively wipes out the advantage of running a higher compression and give as a result lower torque and bhp results

How do I know this - well I've now built four engines in the last 8 years from scratch - on one engine I went to 11.5:1 and no matter what i did with ignition timing or fuel grades I couldn't stop the knock activity so I ended up removing the cylinder head and machining some material out of the combustion chamber to lower the compression - I dropped the compressin down to 11.3:1 and I got zero knock sensor activity on 98 RON fuel - I do get some knock activity if I put 95 RON fuel in it but the ECU changes the ignition map and a reduction in bhp and torque is in line with the fueling change

I had found the limit (for this engine 11.3:1)- I won't go above that again

Notes

Pinking/pinging/knock etc is when the fuel burns too rapidly in the cylinder producing a explosion rather than a controlled burn. Can be heard as a strange rattling ticking tinkling sound.

Causes

- Accelerating from low revs

- using a fuel that has too low an octane

- too much advance on the ignition.

- Air leaks on the induction side

It is very bad for the engine as it can and will do internal damage.

Prolonged knock causes erosion of the piston and cylinder and eventually results in catastrophic failure of the engine. In turbocharged engines, knock can destroy an engine within seconds. For these reasons, all commercial engines are designed to operate without knock. Knock restricts the performance of an engine chiefly by placing an upper limit on its compression ratio. A higher compression ratio yields more efficient conversion of the fuel's chemical energy into useful work to drive the piston. Unfortunately, increasing the compression ratio also directly increases the engine's tendency to knock and so to subsequently destroy itself....

Every manufacturer has to allow for an awfull lot of different fuel grades (esp in cars that go to a lot of different markets) so it can be run on anything from 85 to 98 RON

Whether a naturally aspirated engine will knock at a particular compression ratio depends primarily on the type of fuel used. The ability of a fuel to resist knock is specified by the fuel's octane number. A fuel with a high octane number is more knock resistant and allows the use of a higher compression ratio before knock occurs.

[bstardchild]

So what if its' 'fairly high compression', it' still not high enough. Your '2nd Point' supposedly confirms what high compression is.

So what is high enough for a GTi? At what point would you say the compression is high enough to justify use of 98 RON fuel - I'm interested in the number?

2nd point answered above

I'm also aware it's not just about compression. However since higher compression provides more heat , and a higher octane fuel requires more heat to make the most of the potential energy it provides, clearly compression is an imprtant factor.

higher compression generates more heat I agree - not wrong there

higher octane fuel does not require more heat to make the most of the potential energy it provides - I'm really not sure what you mean by requiring more heat.

Also so what if compression gives a bigger bang? We don't want a 'bigger bang, it's a slower burn and more controlled burn which is required.

OK - Rephrase "Bigger bang ignited by a perfectly timed spark in controlled manner"

How slow a burn do you think you can get away with at 7000 rpm?

Unless you think the GTi engine is a diesel then no i'm pretty sure we aren't talking about diesel.

The diesel bit was there to illustrate the point that if you allow knock to take control of the ignition - ie self ignite the fuel air mix - you will end up with a engine that continues to run....... in a petrol engine this would be catastrophic and be a melt down.

In my younger days often when you turn of the ignition on a car it would run on - carbon deposits in the engine providing hot spots for ignition - still full carburetors allowing a dribble of fuel into the air flow providing all an engine needed to run - On it's own!!

[Si_GTi]

In my younger days often when you turn of the ignition on a car it would run on - carbon deposits in the engine providing hot spots for ignition - still full carburetors allowing a dribble of fuel into the air flow providing all an engine needed to run - On it's own!!

My old Mini, which ran on four-star and then LRP, used to do that sometimes, usually when due for a tune-up or a service. You'd turn the ignition off and she would overrun a little, just for a couple of seconds. Pinking was an irregular problem too, a quick adjustment from my local mechanic sorted that out for a bit until it started again (needed tuning up every few months to keep her running sweetly). Thankfully the Polo is a lot easier to live with and I have never noticed any pinking or any other irregular engine activity

Incidentally, I seem to one of only a small number of owners running on BP Ultimate regularly, but I can tell you that it certainly seems to be doing a good job! Unfortunately the nearest Shell Optimax garage is 12 miles away so I have to make do with the one and only BP Ultimate pump in this town... bugger! I still haven't persuaded my 'rents to fill with diesel Ultimate (they own a TDI), they just won't have any of it and tend to go to Tesco for their fuel

[Del_GTi]

Mate it's getting a little sad here. And to be honest i'm getting a little tired of all the quoting and now your even adding notes and asking yourself questions as you write. Let's not get into a more heated discussion as that's not what having a Polo or this site is about.

Sorry if i'm offending your intelligence but it's all getting a bit 'blah blah', especially when you started going on about piston sizes and chamber shapes etc.

The main point of the topic was which is a good fuel?

At the end of the day i keep using optimax cos i feel the engine pull a little smoother and they say that it cleans the deposits off the intake valves. I don't know cos i've not taken it apart to look.

I say Optimax but if others, such as Grum find another which they feel is better then go with that.

So lets just get back to talking about that.

[bstardchild]

Ok I won't quote in this reply

It's not getting heated and you aren't offending my intelligence either (very little of it there to offend or I wouldn't spend as much money and time on cars)

look back up the topic and you will see Optimax is top of my list - it's really desparate times if I have to fill up with anything else.

As far as it's cleaning properties go - that I have validated myself and found to be totally true - it's been the subject of several studies in the motoring press too and they have reported the same thing....

interestingly enough I am also old enough to remember "formula shell" and that didn't get a good reception

[Del_GTi]

Well not too heated, yet. Also you clearly know your stuff when it comes to motors. I also saw your top 5 were.

I don't remember Formula Shell though the name does sound familiar.

Aren't they supposed to be stopping Optimax here and giving us V-Power. I'm sure i read somehwere it has an even higher octane rating than Optimax.

[bstardchild]

Well not too heated, yet. Also you clearly know your stuff when it comes to motors. I also saw your top 5 were.

I don't remember Formula Shell though the name does sound familiar.

Aren't they supposed to be stopping Optimax here and giving us V-Power. I'm sure i read somehwere it has an even higher octane rating than Optimax.

Not heated at all - lively discussion - hopefully resulting in me learning something too - I've been working on my own cars and motorbikes for 25 years - not just maintenance but improving performance from the engine you pick a few things up over the years

Formula Shell was the name given to a "performance" petrol in the 80's - it resulted in a huge amount of litigation against Shell for damage caused to engines due to excessive wear caused by the lack of or in some cases excessive amounts of detergents - less detergents equalled higher octane more detergents meant cleaner engine (ish)

Unfortunately it all went horribly wrong for Formula Shell the 'magic' additive was added by the delivery driver at the petrol station, probably because the petrol didn't all come from Shell refineries. Problem was they didn't always get the quantity right. The result of overdosing was that it collected on the valve stems of engines, and those with closer valve/guide clearances (predominantly Vauxhall, but others were affected) ended up with sticking and burned out valves.

The Police suffered with their cars, but were able to prove it was Shell petrol as they bought petrol on contract. Some other users were not so lucky and had to pay for repairs. Not surprisingly Formula Shell was quietly dropped.

It particulary affected Vauxhall Cars!!!

[bstardchild]

Interestingly enough Optimax was/is marketed as Formula Shell in countries where the original "Formula Shell" wasn't used in the 80's in order I assume to assciate the brand with Formula 1 and Ferrari.

If you search the Shell UK site you'll find no mention of it although internet searches result in a lot of hits directing you to the Shell website pressumably cos they had a lot of questions related to it from worried consumers

Some people I know (my age group) still won't use shell petrol regardless of how good it is

Think Persil and how it puts holes in your clothes and other seriously good own goals....... Free flights with hoover etc etc

It takes a long time to get rid of a bad reputation in a market place and a very short time to earn one

One thing I did find out is that Shell Did learn from the original "Formula Shell" fiasco and that is it's only refined at one location so they can get the additives right

[Del_GTi]

There were actually a few concerns when Optimax came out aswell and it was the same reason as what you mentioned with the Formula Shell. They were never backed-up though.

Now that i think about it this was probably due to what happened with Formula Shell.

Coincidently, this was on a Vectra website.

[dubpolo]

One thing I did find out is that Shell Did learn from the original "Formula Shell" fiasco and that is it's only refined at one location so they can get the additives right

the only place in the uk that makes optimax is Shell Stanlow Ellesmere port.

[waj787]

Guys, on Fifth Gear tonight they're gona be doing tests and stuff to find out if expensive petrol is actually worth it. Definately worth watching!