Been considering various mode to my scoob (jap 97 sti) - I've come up against something I'd really like explained....

Pretty much whenever someone asks the Q:

I'm gonna change my exhaust / intake system - do I need a remap?

Someone answers:

Yes - you'll need one to sort out the fuelling......


Whats confusing me is that the Scoob uses a MAF which measures how much air the engine is "consuming" it takes this info and calculates just how much fuel it should add to get to the mixture that it's been mapped to for - I'm sure there are a few other corrective factors but thats the basic science of it - yes?

Now if you derestrict the inlet / exhaust - sure it lets the turbo spool up quicker / allows more air in for a given RPM but as the MAF will sense how much air is going in surely it will enrichen the mixture accordingly.

I know this was the case on my '91 Saab T16 - (you'd have to maybe up the fuel rail pressure if you went too mad with the boost as you'd max out the injectors duty cycle) - and I can't beleive it's not on a car produced 6 years later AND by those chlever japs....

I can understand you'd need a remap for changing the boost / rpm / fuelling profile to get more power this way - but to need to do it as a result of derestricting the flow doesn't seem to make sense.

Can anyone please shed some light on this??

The question is, what AFR target do you map it for????

Also, the ECU doesn't always get it right (matching the target that is).

Example might be, your hitting 10:1 AFR at full boost at 6000rpm as standard, remap might tweak that to 11.5:1 and get more power.

Also, the ignition map is very important when it comes to power, a single degree more of advance might yield 6-8 ft-lbs of torque, if you can run it without detting your going to benefit. Stis tend to be quite well advanced, and may benefit from advance being removed for UK fuel.

Then there is remapping the standard ECUs boost maps, to get higher boost withouth the aid of a manual or electronic boost contoller.

Paul

As Paul says, plus on some imports (at least the STi5/6/P1 which I've mapped), when the breathing is freed up they hit areas of the map that were not originally designed to be hit and aggravate an existing leanish spot coming onto full boost. This happens to have quite a lot of timing in as well.

So if the map which is being hit in different areas has the correct ignition and fuelling then you are fine. Often they do not. On new age cars, the areas they hit with better breathing and boost can be really odd - mega rich and retarded on WRX, on STis they are rich and quite advanced and just det, particularly at 6000 RPM where they are no longer listening.

I see - so the problem is just going outside ot the current map envelope so it can't meet the necessary AFR at certain RPMs to prevent detonation?

I'm not really that fussed about making loads more power at present, just getting things to be a bit more responsive with the current ECU setup (a lot of this comes down to the cost of a remap - [new / piggyback ecu = £600 ish from what I gather]).

I've ordered a Knock link to try and be on the safe side but from what you guys are saying - it could be V dangerous for me to derestrict exhaust/inlet without getting a remap and so all the associated initial costs.

Advice - needed therefore is do I just leave my exhaust as standard until I can afford a decent remap and then do all at the same time or go for an exhaust change just check I'm not running lean (rolling road check)? - the cars already got an ITG panel filter on but I doubt this actuallt affects thinsk toooo much.

I should add that I will be able to further counteract knock with the Water injection setup I have - though not yet installed...

I wouldn't say don't do the decat but only if you have a knocklink, boost gauge and are running on Optimax plus octane booster. Then remap if you have to or when you want to?

cheers john - will go ahead with the decat then over the next month or so once I've got the knocklink in - have been running on 97 since I got it but once I've done that I'll use booster which'll take it to 99 as a matter of course - also I'll sort the W/I before the decat - that is if Graham Goodes EVER get around to sourcing a .4mm jet and some more 4mm hose (been waitin all week) - may end up going to Demon Tweeks instead at this rate - hate using them coz it's nice to support local guys but need must when etc etc etc..

The airflow properties change.
This causes the MAF sensor to "miss" some air entering the system.
And this leads to a lean mixture.

Yup - makes sense definate potential issue - must be a pretty poor MAF all in all for it to actually happen :(