Which petrols best out of.........

Cheers!
Didn't know that VW's since '73 were designed to run on unleaded. My Bug's a '73, but I re-built the engine a few years back & got new heads and barrels & pistons etc for it just before they started fazing leaded petrol out. The car's still not finished, so I still haven't made up my mind what to run the car on once it's finished! But if the normal unleaded is fine on it, then great!

 

my engine is a 1.8 carb which has apprently been tuned to use 95 ron can you do that or is he talking poo?

 

As a general rule, the lower the RON value the sooner the mixture will pre-ignite (pink) - for the same engine etc.

A car can normally be tuned by altering the ignition timing to limit the amount of pinking at the expense of power output. On your engine, I imagine that whoever has set it up has set the ignition timing so that using 95RON fuel does not lead to pinking (by retarding the timing from what could be run using higher RON fuel.)

Some engines such as the PB Digifant engine, use a knock sensor, so they are set at a known advance on the timing and the sensor then increases the advance until the engine knocks - at which point the timing is retarded until the pinking disappears. In theory.

Some of the guys on here with 2.0l 16V engines that are going for big power output will set the timing so that the engine is just on the verge of pinking (ie. advance until pinking occurs then drop it back a tad until it stops.) This amount of advance in theory will allow the most power without risking damage due to pre-ignition.

 

is there a home test i can perform as there seems to be a lot wrong with my fuelling at the mo


it can be sound as one day and pooe the next

 

Ditto Great stuff, and if you use a lot of it, you get a special card too

Al

 

How come a lower octane rating causes pinking? i was thinking about this today. you would think that the higher the octane rate, the greater the chance that pinking will occur, because it is more explosive.

Is the fact that optimax is more expensive, you drive more carefully, and therefore get more miles out of a tank?

Im currently running normal unleaded, but in the near future im gonna run the tank down and try it out.

 

I've read before that optimax can loose its extra octanes over time, so unless you use it soon after buying it it looses octane.

 

A higher Octane rating means that there is a higher proportion of Iso-Octane than Heptane in the fuel (98 Octane = 98% Octane/2% Heptane.) The more Octane, the higher the temperature required for combustion. Therefore, hotspots in the chamber etc do not affect the mixture as much as lower Octane fuel.

Compresson ratios also affect pre-ignition - the higher the ratio, the more likely the mixture is to pre-ignite. As a general rule, the higher Octane fuels can help to combat this.

Retarding timing too much to combat pre-ignition can cause problems though as the air/fuel mixture becomes richer. This leads to several problems such as less complete combustion, lower fuel economy, increased levels of unburned hydrocarbons (which can damage your catalytic converter if fitted) and the engine will run hotter.

I can't say that this is true for me - I tend to drive enthusiatically most of the time..! And when it runs so well, I can't help myself!
Edited by: RichS

 

It's time...

This isn't entirely true. There isn't a relationship between the advance at which you start to pink and advance for max torque. Maximum torque could have been and gone long before you start to pink, and in other conditions you won't have reached max torque before pinking occurs. Adjusting the timing until it pinks then backing off is not the ideal way to achieve best torque, particularly on a boosted motor where trying to do this at boost will run the risk of killing the motor quite quickly. The only way to determine advance for max torque is on a rolling road, or by measuring cylinder combustion pressures, either directly or via a method such as ion-sensing.

OEM knock sensor systems don't advance timing at all from their base timing tables, what they will do is retard to protect the engine in the event of a fault/bad batch of fuel. By using a knock sensor the manufacturer can set a more aggressive base timing table, as without they had to be more conservative to allow for variations in conditions/fuel.

 

Cheers for pointing that out, Martyn - learn something new every day!

Too right..

 

Chhers Rich, that makes more sense.