Effects of 'knocking' or detonation in air/fuel mixture?

[Paul the 6th]

Properly geek question here - what are the potential effects of detonation in an air/fuel mix within an internal combustion engine?

If you take an engine which requires super unleaded (98 RON petrol) and then put some regular 94/95 unleaded in there, what are the possible problems you could come up against?

For those who are curious: http://en.wikipedia.org/wiki/Engine_knocking - the article says "Effects of engine knocking range from inconsequential to completely destructive" but then stops there, hence this post.

n.b. - I'm looking for info referring specifically to detonation and not pre-ignition. Cheers

I would expect premature wear, lack of performance, rough running, bad fuel economy, all the usual.
On older engines this used to be really bad, but they are made so much better these days.

[andrewsmith]

is this on the S4 paul?

If its like the subsequent hotrod Audis the motors are tuned to run of stupidly expensive in europe and 93 RON everywhere else (to my knowledge). This does depends upon age as the newer VW/Audi's have ECU switchable maps to run on any grade fuel

Its premature wear to the motor and a over efficicent burn (higher burn temps) and the knock ons valve seats etc...

[Paul the 6th]

cheers guys - yeah it's with regards to the s4 when it gets re-mapped it'll need running on 98 RON, but I was just pondering the possible side effects out loud.

I think you're right with the slightly capped performance figures for rest of world - the american S4's are listed as 250bhp but euro spec are 265... Guessing they knocked down the compression/reined in the ignition curve slightly to allow the engines to run lower grade fuel without knocking?

[andrewsmith]

Originally Posted by Paul the 6th

cheers guys - yeah it's with regards to the s4 when it gets re-mapped it'll need running on 98 RON, but I was just pondering the possible side effects out loud.

I think you're right with the slightly capped performance figures for rest of world - the american S4's are listed as 250bhp but euro spec are 265... Guessing they knocked down the compression/reined in the ignition curve slightly to allow the engines to run lower grade fuel without knocking?

In theory yes.

If it was a US listed model is should be able run lower grade after a remap.

Its common for manufacturers to re-tune the motors to suit local fuel grades makes them more saleable, but the Europe and Japanese markets run the super explosive 95 RON+

can damage the rods,crank,bearings but thats extreme case's

basically if the combustion takes place before it should and the rods are dead straight to the crank then the downward pressure is exurted straight through the rod to the crank and then to the bearings and casing

where in a normal scenario the spark plug fires just before tdc and combusts just after tdc forcing the piston down with the crank at a slight angle forcing the crank round rather than just straight down.

hard to explain as im cr&p at English

ok so image 1 is detonation happening and image 2 is normal combustion


and yes reducing compression will solve the problem, then just stick a full system on and remap to gain back the lost HP

All I can tell you about is going from 4 star to unleaded, the issues then was valve seats which had to be hardened to take unleaded to stop premature wearing

[Paul the 6th]

Originally Posted by andrewsmith

In theory yes.

If it was a US listed model is should be able run lower grade after a remap.

Its common for manufacturers to re-tune the motors to suit local fuel grades makes them more saleable, but the Europe and Japanese markets run the super explosive 95 RON+

Brain fart = I already knew it was because of North American emissions regulations but didn't patch the 2 together

Originally Posted by barwel1992

can damage the rods,crank,bearings but thats extreme case's

basically if the combustion takes place before it should and the rods are dead straight to the crank then the downward pressure is exurted straight through the rod to the crank and then to the bearings and casing

where in a normal scenario the spark plug fires just before tdc and combusts just after tdc forcing the piston down with the crank at a slight angle forcing the crank round rather than just straight down.

hard to explain as im cr&p at English

and yes reducing compression will solve the problem, then just stick a full system on and remap to gain back the lost HP

nah that bit in bold makes absolute sense mate - that's exactly the sort of detail I was trying to get my head around but couldn't fathom it out.

It'll be getting aftermarket downpipes with sports-cats and then a map to suit. Many people just get them mapped (from 265 upto around 300-330bhp depending on other bits in the setup; air filter, dp's, larger intercoolers from rs4) and then do the DP's later but the K03 turbo's are known to fail when put under the extra pressure of running more boost/fuel than standard, hence getting rid of the stock cats to try and alleviate some of the restriction in the system....

Anywho, later on I'll just get some k04 turbo's from an rs4 (££££££££ wonga) and push it to nearer 400bhp rar!

[Paul the 6th]

Originally Posted by hardhat_harry

All I can tell you about is going from 4 star to unleaded, the issues then was valve seats which had to be hardened to take unleaded to stop premature wearing

Yeah I've heard that the conversion on leaded petrol engines was basically all about fitting hardened exhaust valve seats, otherwise the knock from unleaded would burn them and knacker the cylinder head.. The wiki article mentions that lead and lead based salts help to reduce the temperatures during combustion so it's all making sense now

[andrewsmith]

Definite Brain Flatuence on my part!

I thought you were thinking tuning, stage 1 is my recommendation through a guy a worked with who had several Audi's tweaked and a hardcore P1.

If you were thinking 400 brake buy a modern RS4 the 420 brake 4.2 l V8 be cheaper in the long run. I stuies with a guy who had one outright loved it raced a viper for a laugh and won, sold it for a Exige in Orange