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

[Specialone]

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]

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+

[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 :p

ok so image 1 is detonation happening and image 2 is normal combustion
http://i357.photobucket.com/albums/oo16/barwell1992/dettonation.jpg

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

[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

[Paul the 6th]

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 :rolleyes:

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 :p

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

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 :batman: rar!

[Paul the 6th]

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

[Paul the 6th]

I meant I just dropped a whopping stinky brain fart lol - I already knew it was for the US emissions but went down 'lower RON rating lane' for some reason...

ah I'm not chasing big power for competition or anything, just a mate of mine had an S4 which was an ex-demo car for MRC Tuning (google them - they can do insane stuff with pretty much all of the top end VAG cars). His was around 500bhp... easily the fastest vehicle I've ever been in but you have to experience it to see what all the fuss is about. I'm not after the same thing but I fancy a re-map for mine since it's probably the least invasive of all the tuning procedures, and against my better judgement I do actually buy into the idea that it'll improve economy on motorway runs. (Peak torque being cracked up and moved further up the rev range, cruising at a constant speed on the motorway will still be at the same RPM but lower down the powerband in relation to the state of tune it's in - if any of that makes sense? lol)

I'd love one of the 4.2 v8's (b6 series) but it'd be thirstier than the 2.7 turbo (b5), the latest version (the b7) is a 3.0 v6 turbo and seems like the best compromise but they're prolly still hovering around the £30-40k mark lol..

Plus I like the fact mine looks like a common garden saloon car with nothing special to note... It's properly juvenile but there's a strange satisfaction to watching the chav'd up corsa behind you disappear when they thought you were 'just another a4' :D

[andrewsmith]

Lol the older ones are hidden with the rep-mobiles.

Its like any vehicle, 'how bigs yours wallet?'
I'd love a tuned Monaro but it'll never happen as the tune I want is £30k +

[barwel1992]

i want a tuned, mk2 escort around 650bhp full welded "cage" (no pun intended) blisteen dampers the lot :D

but keeping it low key with a chavvy exhaust so i look like a chav but go like a rocket

[Sid Squid]

Detonation is a spontaneous, uncontrolled burn, the greatest effects are, as Barwell has alluded to, the significant change in burn timing and speed, and thus timing of peak cylinder pressure. Essentially instead of a smooth flame front initiated by the plug and travelling predictably through the mixture*, the burn occurs uncontrollably and at multiple points in the gas.
Detonation is not the same as, but may be caused by the same problems as, pre-ignition, other descriptions are, (with varying degrees of accuracy), 'knocking', 'pinking' and 'pinging'.

The reason that it can vary in it's effects between being an innocuous process or serverly damaging is because there are so many variables in it's specific causes and circumstances. It's very diffiicult to predict the effect it may have on any given engine - you may have to wait until someone else finds out! The one thing that is predictable is that it's never a positive thing.

* Even though as is usual nowadays cylinder turbulence is promoted by inlet shaping and gas flow swirl intermingling between ports so that flame propogation is improved and thus burn time is reduced - but in a controlled and predictable way.

[maviczap]

I know you used to be able to buy octane boosters to bring the Ron up to a high level, which I think was to account for our unleaded petrol.:scratch:

If not fill up at an aerodrome with avgas :p

[ThEGr33k]

Wont it have a knock sensor anyway? I was under the impression most modern cars did?