SiHethers

Any technical explanation welcome.

Had a 3 inch GGR exhaust with sports cat fitted. Logged on delta dash and now showing more knock correction(-2 degrees) between 3-5000 rpm with same boost(1.1bar). Previously registering -1 degree at most. I thought reduced back pressure would reduce charge temps etc and reduce chance of det?

Maybe I got a bad dose of optimax

Simon

Bob Rawle

Its running a bit leaner hence more knock correction, you need to use a bit of booster as well, -2 degrees is not a major problem though, -4 would be. Keep an eye on it.

SiHethers

Bob

Thanks. My AFR's are still >0.9V though so this should be pretty rich still I think. I've turned the boost down a little on my Dawes Device and filled with a new tank of optimax. The garage had disconnected a pipe coming off the MAP sensor pipe and so I wasn't getting fuel cut, so boost was peaking above 17-18psi without me realising (I try and keep my eyes on the road at WOT ) after I refitted the Dawes. I'd removed the Dawes this time cos after I got my brakes done they'd fiddled with it and increased the boost. You'd think they'd know better.

What booster would you recommend?

Ta

Simon

MorayMackenzie

As I understand it...

Fitting a more efficient exhaust system should improve engine breathing, reducing the amount of unscavenged gas left in the cylinder and reducing reversion, thus making for a higher density, more "pure" charge (less exhaust gas mixed into the fuel air mix and more fuel air mix in the cylinder)... both these conditions will lead to a reduction in the spark advance necessary as the flame speed is increased in this improved density/purity charge.

I would expect (to a point) the MAF and lambda sensors to adjust the level of fuelling in line with the increased exhaust efficiency in order to maintain the same air/fuel ratios. This may be why your fuelling appears unaffected whilst the spark is being retarded a couple of degrees. If this is the case, I would say it shows that the exhaust is doing a good job.

The long and the short of it is that a more efficient exhaust should, in theory, lead to more power for the same boost and less advance. Reduced advance fhen modifying the engine system is not always the bad thing (tm) that people seem to think it is. When you raise engine efficiency, you simply dont need the same spark lead time as you did when it was less good at cramming good cool clean dense air fuel mix into the cylinders.

Moray
(Looking for asbestos jacket)

Andy.F

Flame flame

I can see this being a good one

Moray I agree with some of what you say regarding a more efficient engine generally requiring less advance......however my experience has been that reducing exhaust backpressure allows more timing advance before det. Er, that's not strictly true what I found was that the octane requirement of the engine was less if the EGBP was reduced (ignition timing staying the same)

Andy

MorayMackenzie

Andy,

Could this be because the increased efficiency of the exhaust is getting more of the exhaust gas out and therefore leading to lower temps in the charge mix before it is lit up... thus reducing the required octane level a little as the cooler charge is less likely to preignite?

Moray

Andy.F

Yes but if I retained the higher octane, I could have advanced the timing as I reduced EGBP (if I had an adjustable ECU) instead I just reduced the octane additives as they were no longer required.
Still the opposite of the issue above though

Andy

MorayMackenzie

Maybe exhaust has improved cylinder filling, you have more mix going in and the ecu sees more air mass going through and richens the mix? If you run the mix richer (or leaner, IIRC) than ideal, you will need more spark advance in order to get the mix burning well by the appropriate time. Your OE ECU may well be clouding the issue.

Lame answer, I know, but if we had wideband closed loop lambda control and logging of AFRs on the engine in question, it would be easier to see what was going on when we changed the exhaust.

nom

I'd have thought it depends on too many other things to be able to make a single 'correct' answer? Anyway, wot I fink...
Less exhaust gas (so a cooler charge 'cos less heat from the old exhaust kicking about) and plain old more new charge (cooler again) so denser charge, so faster flame front, so less advance needed makes sense to me.
Hmm, not a lot else to 'go the other way' is springing to mind at the moment (things don't spring very often anyway, so that doesn't mean much...)
Which means I'm joining the first flame-suit camp
Not that that helps much.

[Edited by nom - 3/26/2003 5:11:19 PM]

Andy.F

Where's John Banks when you need him to throw up some graphs

Where's Adam when you need the whole of the web searched for pertinent data

I can only assume my engine must be very inefficient then as I run 'loads' of advance to achieve my meagre output

So will fitting a 2" exhaust allow me to retard my timing and gain loads of power ?

nom

Naa, what you want is one of those curly exhausts that Pat came up with (here about 2/3 of the way down). I particularly liked the curly-wurly.
But I bet they're both good .

MorayMackenzie

Andy,

I stated that, in my understanding, increasing exhaust system efficiency should improve scavenging and reduce reversion, which in turn should yield a cooler, denser, cleaner charge that doesn't require as much advance as the flame front will travel faster.

I was hoping for a serious discussion here. For some reason it seems I drew a short straw and got you instead on this occasion!

David_Wallis

Andy.F

Moray

The last 'serious' discussion we had was regarding centre diffs, you backed out early and left Adam to take it on the chin when you were wrong

Andy (never wrong)

[Edited by Andy.F - 3/26/2003 5:55:10 PM]
Edited to correct where I was wrong

[Edited by Andy.F - 3/26/2003 6:11:54 PM]

MorayMackenzie

Andy.F

Moray > speachless ? That's a first

I now look up to Adam for his bold stance in that discussion

[Edited by Andy.F - 3/26/2003 6:10:49 PM]

MorayMackenzie

Andy,

I was trying to offer the guy an answer to his question.

You appear to have been after an argument for the sake of it.

It seems you have a need to hear this, despite its irrelevance here :

Andy, I believe that you were, inf fact, mostly right about the centre diff. I was thinking in terms of the steady state operational condition, and got lost there. Well done. (y) I "ducked" out early because I was A) not as sure about it all as Adam and you both seemed to be and B) had more pressing things to be doing at the time. I did not make any wager on the issue.

Now thats out of the way, any chance of moving on with trying to answer this guys question?

Moray

MorayMackenzie

PS: I don't suppose you ever found a compressor map for the 18G wheel you could forward to me? It seems as though at least one person aluded to having seen the map, but no-one was actually able to find one to forward to me when pressed.

Andy.F

I was the one offering the benefit of my experiences which I find are often of more use than theory

Sorry, I have been unable to find an 18G map. However 'in theory' it's just what you want

[Edited by Andy.F - 3/26/2003 7:33:17 PM]

MorayMackenzie

In theory its just what I want? Do you mean the wheel or the map for the wheel? Is this another its not worth discussing unless you have some practical expirience of it sort of thing?

SiHethers

I'm back. Thanks for the interesting (but not necessarily relevant) replies.
I may need to explain a little more. The car is MY99, K&N filter, Samco hoses, Dawes Device (peak 17psi held 16.5psi pre exhaust) ae800 ECU, standard map.
Pre exhaust I saw a max of -1 degree knock correction. After exhaust I am consistently seeing between -1 to -2 degrees retard between 3-5000 rpm. I take this to mean the ECU is sensing more knock.
It is, however, not that simple as the mechanics at GGR had disconnected a pipe from the MAP sensor meaning the ECU was reading only 11psi boost instead of the new boost levels post exhaust of 18psi peak 17psi held and no fuel cut.
I think Moray is right about cleaner charge etc but also better pressure ratio across turbine meant more boost for same setup on Dawes after exhaust.
Now my imagination takes over - if the ECU uses any element of MAP sensor reading to calculate load, it would have thought the engine was under less load than for the rpm point and possibly applied too much advance causing the neg knock correction.
Anyhow, I've reattached the pipe and reduced the boost slightly. Not had a chance to delta dash it yet though. Otherwise exhaust has transformed the car

Sorry to hijack my thread back

Simon

PS If anyone wants the delta logs I can email them to you.

nom

Well, anyway, as Bob said yonks ago (it seems now), a couple of degrees is no problem. It's just running more efficiently so needs less advance.

SiHethers

So, in summary, it's not a bad thing, but if I want to suppress it, add booster

Cheers guys

Simon

SiHethers

Re-logged the car after fresh optimax, bottle of Millers and an ECU reset - hey presto - no more KC. I would still like a universally accepted explanation, but beggars can't be choosers

Cheers

Simon

swan

Bob, where is the split between "ok" correction of -2 and "problem" correction of -4? IOW, why is -4 bad but -2 acceptable?

Pavlo

I would expect more airflow to equal higher dynamic compression ratio. Higher airflow would also put you on a new area of the map, which may or may not adjust the fuelling/ignition accordly.

Reduced cylinder polution often reduces cylinder temps which is good, but a little polution can slow the flame speed, not that the flame speed is always a direct cause of det.

Do you have any DD logs that show a change in either load or airflow?

Paul

john banks

The differences would probably be statistically insignificant unless you could control a lot of variables.

Det threshold and ECU knock correction (which aren't always the same thing) vary if you sneeze on these cars.

Pavlo

JB is the 99-00 ecu expert, not me

I've seen another car which seemed to det a little more with a good exhaust.

Depends on the individual car. Usually more fresh charge equals lower cylinder temps and so on, but can also increase dynamic compression ratio (CR based on the ACTUAL inlet charge going in).

Paul

john banks

Just found this thread, only go to Gen Tech when Drivetrain gets boring

Suspect everyone is right and that there are superimposed interacting factors involved.

Certainly a big power big turbo big intercooler big exhaust map is often not as advanced as a low power small turbo interheater small exhaust map on full load. Multiple interacting factors at work.

At the end of the day if the car is not actually detting then fine, and most of us agree that it is difficult to reach minimum timing for best torque on our turbocharged engines and that detonation is the limit, so that is where we tune our safety margin from on boost.

[Edited by john banks - 4/9/2003 5:59:33 PM]

SiHethers

Paul

How would I assess load using deltalogs? My impression was that the my99 ecu mainly used MAF for this, but I would expect throttle position to play a role also +/- MAP. I haven't looked that closely at the logs. Would, for example, I need to find an rpm point before and after where boost is similar then look at the MAF voltage? The o2 sensor certainly wasn't reading lean.

John, very diplomatic

Simon

Edited after re-reading JB's post

[Edited by SiHethers - 4/9/2003 6:48:43 PM]