Ok, Ive been on this forum for a long time now and have tinkered with my scoob but never got round to/got the money to get a remap.

Ive been pondering over a question which I cant find a definate answer for, maybe someone can help, here goes...

(Apologies as this is a real back to basics question)

I understand that the ECU is the brain in our cars and controls everything to make it run smoothly.

The bit I get confused with is WHY you need to remap your car after making modifications, for example -

The ECU depends on the MAF sensor to tell it how much air is coming into the car in order to work out how much fuel is needed. If I then go and change the way air comes into the car - eg, fit an induction kit - surely the next time I drive the car the MAF sensor says hey theres more air/colder air coming in here and the ECU increases the amount of fuel to match? Is this not correct?

Also, the exhaust system on cars are bottlenecks in order to filter out noise and gases, if we lived in a world where we had no ears or pollution we wouldnt have exhausts at all to allow the exhasut to flow completely freely. So why after removing a bottle neck such as fitting a decat down pipe do I need to get a remap? Surely its removal alone allows for much better flow.

Lastly, Ive also been reading about resetting your ECU and the car relearns everything when you drive. There is a guide on NASIOC / other US scooby forums that describe a 'fast learn trick' to teach your ECU quickly to its maximum potential. So again, after fitting a modification, why cant one just reset the ECU for it to learn the new mods that have altered air, gas flow etc...

Please please please dont reply with "mod your car without a remap and were laugh when it goes bang etc" Im interested to know WHY it would potentially go bang, is it heat, pressure, lack of air or fuel etc etc

Thanks for reading and hope you can help, yes yes Im a noob I know but just trying to learn.

Cheers, M.

 

Standard ECU's can only learn so much, they were not intended for use with an FMIC, de-cat and Induction kit....

 

I am interested as well... i have run many turbo cars without MAF and you can pretty much mod them silly with no more side effects apart from running rich. my last car (MR2 turbo) had bolt on mods to well over the 300mark and it still ran fine of the standard ecu... that ran off a MAP sensor not MAF

so my small contribution to the question is...

if you put an ecu that bi passed the MAF or got rid of the MAF completely, could you then mod the car and not incur any side effect like a normal turbo running off a MAP sensor to ecu?

 

..

 

I'm not a mapper, but I'll try to answer your post the best I can.

When you mod the intake & exhaust systems, you potentially take the parameters the ECU are used to dealing with, outside the limits it is programmed for. For example, you decat the exhaust, yes it flows more freely, but the ECU has to try & control the boost as boost spikes are more common. Also the point of a remap is to improve the performance & running, Subaru often use very conservative parameters for the knock control, therefore causing the igintion to retard prematurely & the car not make its potential full power.

With regards to the air filter, generally, the MAF needs rescaling to cope with the increased airflow, although is isn't fatal on all models, MY99/00 are probably the worst affected.

hope that helps

 

Taking the maf sensor firstly... It measures the airflow based on how quickly a heated element cools down. This is dependant on the speed of the air over the element. If you change the standard airbox to an induction kit, the size of the tubing is quite often different and therefore the speed of the air is changed. Lower if it is bigger. In this case the ecu would be thinking that there is less air going in and only add enough fuel for what it thinks is going in. Result is that the car runs leaner.

Mapping recalibrates thismaf scale and ensures safe aswell as gaining more power.

On the subject of exhausts...

The standard Subaru map are designed to build boost level by closing the wastegate on the turbo until it is close to reaching what it should be at and then gradually opening the wastegate when at target boost to keep it at the desired level. When you decat or change the way exhaust gases leave the engine it is changnng the speed of the airflow through the turbo and the speed at which the turbo builds boost. It will therefore aim for it's boost target but make boost much quicker than it thinks it would. It will go past the target before it has had time to react and then overcut the boost so you end up with boost jumping either side of the target. IF this is safe it is only by luck but at the very least it feels hesitant or throws a CEL for overboost.

Mapping sets the boost targets a little higher for more power but also the speed and way the boost control works to make it as smooth as possible.

Hope that helps.

 

still saving for your services duncan saddly wallet has took 2 bad hits lately the mrs car broke first with a nice big repair bill then my car needed work this week end that cost an arm and a leg hopefull august if your free

 

hardly ever 'free' but pretty good value

 

Thanks for the replies so far guys.

So with regards these two above and how a remap changes parameters, if the car wasnt remapped and went pop, what would be making it go pop? Eg, what does running lean and boost jumping either side of the target do to the engine that could lead to a big bang?


Interesting what Scooby_CL has said too.

 

Running lean will overheat the piston crowns, probably cause det which can kill and engine very quickly. Normally pistons, rings, head gaskets and/or big end bearings.

Overboost can do similar if the car is running off the right hand side of a map table. It will run a certain ignition timing and keep running that timing even way off the map as it assumes the final column values is what it should be doing.

The std ecu has in built strategies to help deal with det but some car's ecus are less reactive and if it is a big enough spike of det it will be more than the car can adjust within its ranges. (either way this is after the event and possibly after the damage)

The only safe way to do any modifications is to at least check fueling and det but preferably to alter the map to adjust for safety and also get the best from them.

 

Yes - that is what Simtek and Syvecs ecu run on. They still need setting up/mapping.

The MAF sensor isnt necessarily the issue on newage cars until mid 400's bhp

 

Running lean can lead to detonation *det" (very crudely put, fuel ignition at the wrong time in the combustion cycle) it puts enormous pressure on pistons/rods/bearings and a common result is a hole in one of your engine's pistons or fubared big end bearings. Bad det can kill an engine in seconds!

Overboost. If serious will overload your engine's internals and again, you're looking at a damaged piston/s, buggered big end bearings or, if you're really *lucky*, a rod leaving the block the hard way!

A principle reason for most people changing components is increasing power. At the very best, if you don't remap the car, the ECU (very clever bit of kit) will try and maintain the status quo. In the case of higher airflow, it will just try and dump extra fuel in the mix to keep the car operating within standard parameters. Result: your fancy exhaust gets you bugger all increase in power, and has a detrimental effect on MPG!

However, the ECU only has a finite range of adjustment as std and if you go too far, and the ECU doesn't know how to respond, you're looking at potential engine damage and that's MUCH more expensive than a quality remap.

If you're not going to remap, best to leave things like airflow, fueling etc.. alone! At the best, you'll not get optimal results, at the worst you'lll land up with a big bill!

Ns04

 

I liken it to having driving lessons where the instructor (ecu) has dual controls. He will help a little bit but if you yank the steering round at 60mph (det) you will crash no matter what he does with the controls.

 

very intresting read that guys.

 

What the others have said, and...

Duncan has already covered what happens if you alter the induction tract and knock the airflow vs voltage scaling out. However there's much more to this...

The ECU can do that but only within a relatively tightly defined set of limits, which you will quickly exceed.

MAF input on these is converted into an engine load figure, which, along with RPM, is used to look up a 3D map to set the mixture. So if you're doing 4000rpm and pulling an engine load of, say, 40, the ECU will look up the correct row for 4000rpm, and the correct column for a load of 40, and where they intersect it will read off the target AFR to set. It then uses that number as a basis for calculating the fuel injector opening duration.

This is fine while you're running in factory (or mildly modded) condition, as the fuel map will be scaled at a level where the engine speeds and loads the engine is capable of generating will be covered by it.

The problems start when you mod your car so that it is capable of pulling an engine load of, say, 55, but the load index of your fuel map only goes up to 42. Under those circumstances the mixture the ECU is setting will be based on the rightmost value in its map - which will be for an load some 25% lower than you are now generating. The chances of the mixture being correct at that level will be pure luck (and unlikely).

You run into the same issues with ignition timing. Timing that may be correct when your car was generating a max load of 42-43 are likely to be too advanced now your car can do a load of 50 at the same engine speed.

It's doubly critical you get this stuff right as the knock correction systems built into most of these ECUs as standard are very imprecise and likely to miss.

Not - as Duncan has already said, changes to the induction system will alter the way air flows past the MAF element, and so you can end up with a scenario where you're getting the same amount of air flowing into the engine, but the MAF sensor is actually reporting a lower number to the ECU. Hence lean running.

Because changes to the exhaust system will alter the way the turbo spools up and maintains boost, as well as potentially knocking into VE. So the boost control/error correction system, maybe mixtures, sundry other items will need altering to optimise the engine for the new configuration. Newage cars, for example, are prone to overboost after being decatted.

It does. What you then have to address are the side-effects of that better flow on the way the engine performs.

Because the ECU does not "relearn everything". The capability for learned correction varies from model year to model year, but it is actually pretty limited, and most of the self-learning functions pertain to stuff like closed loop fuel control, idle speed control, ignition control on part throttle, some aspects of boost error control, stuff like that. When you are driving the car hard, almost all of the self-learning functions are switched off and you are relying on whatever's in the ECU's main maps.

 

Very very interesting posts guys, thanks. Im glad I got the replies as it not only helps me but also anyone else reading this and has modified their scoob but not remapped it.

At this time I only have an induction kit and back box fitted that would alter the cars behaviour and must be lucky it hasnt gone bang already, I guess the best thing to do is get it remapped.

Anyone fancy doing a remap on my Scoob? How much it going to hurt my wallet?

 

I charge £350 for a full custom remap mate - where are you based?

 

Wow , I have so much to learn, this thread is fascinating and really interesting, I'm confused about ignition timing, is this happening while the engine is on, constantly? I guess this is the igniting of the plugs? Thanks

 

May I just add, that remapping also might bring to light other problems that you might not know about , you get the most out of the car if it's mapped safe

 

Yes it is the start of the igntion of the fuel/air mix.

In simplest terms the fuel/air mix takes a period of time before it combusts and creates the pressure that creates the torque to propel the engine. The ideal is for this point to be when the piston has just gone over the top and is slowly travelling back down the bore. This would create the maximum torque. If you get this maximum pressure wile the piston is still coming up the bore for example, something has to give ... pistons, headgaskets, rods etc. If you start it too late (ie have low ignition timing) you will effectively be pushing something that is moving away from you so the energy is wasted.

Obviously it is more complicated/involved than that and ideals don't happen in subaru engines.

 

a question for you duncan whats the score with 3 port boost solenoids as im told theese help with mapping but is this only for higher power levels or is good to have one fitted from the off

cheers paul..

 

Mostly it is bugeye wrx's that benefit most from them as the std actuator is very weak and they help isolate the actuator from seeing any pressure without having to adjust the wastegate arm (impossible to get to if the heatshield is on) or change pill sizes.

In essense they will enable earlier spool up and enable more boost to be held at the top end (if it is of benefit power wise). Also they react a bit quicker than 2 port solenoids so boost control can be a bit finer.

They would never be a bad thing to have (def need mapping though) but they wont always gain more power or better spool as in some cases (say with the VF35) you are limited on getting too much early boost because of compressor surge.

 

cheers duncan