Boost spike?
 

Got our 04 WRX back today with a 3" decat exhaust fitted, decat downpipe, decat centre pipe and backbox.

As expected the CEL light came on within about 30 miles as the lambda sensor has been removed but does the WRX suffer from possible boost spike as ive read with STI's?

Up-pipe is standard at the moment but we plan to have this replaced and decatted when ready for a remap later.

Just worried about possible damage but not sure if this common on WRX's with decat exhausts and no remap.

Thanks in advance.

Ideally you should drive off boost until it's remapped.

Not sure if the WRX is 'as bad' as the sti for over-boosting, but the ecu will pull loads of timing and possibly go into limp home/low boost 0.6bar mode..!

I planned mine so that I had my full decat (amongst other things:)) a few days before mapping so I didn't have to hold back too long..:D

FWIW, you won't get any performance increase from a full decat until it's mapped..;)

No performance increase at all? Surely removing two cats has some effect on performance even without a remap?

It's not usually a problem with boost spikes on the WRX as the tiddly TD04 can't flow that much air. Boost spikes more common on the VF series STi's and often need re-map or restrictor pill change.

There won't be any increase in BHP as the ECU is "blind" to the de-cat...... Spool up will be better as will the noise LOL

If you have the pennies I'd re-map (forget the up-pipe cat) for better performance and to get rid of the pesky P0420 missing cat CEL :D

Shaun

So the reduced exhaust back pressure and therefore effective pressure on the piston on the exhaust stroke as well as the reduced internal EGR (therefore increased fresh charge - measured by the MAF) will have no effect on power - MY ARSE

Simon

OK Simon, what extra BHP / Torque do you think a simple decat will get you ? 10 BHP perhaps. You will lose that standing in traffic for a cpouple of mins as the intercooler loses efficiency.

I'm also not sure that the maf will see anything different as it's ambient air that flows accross it and not "charged air" that has been through the intercooler.

TBH I don't think the OP will actually notice a startling increase in power / top end just a bit better take off and spool up.

I'm not being arsey.....just curious ??


Shaun

is it just me or has everyone failed to notice this?????

the car needs a lambda sensor as it meassures the air/fuel mixture

running without this can only end in dissaster cant it????

Your Lambda(O2) sensor isn't removed, the PE420 error is CAT efficiency below threshhold, i.e it thinks the CAT is buggered.

Removing the CAT(s) will increase power, allowing the exhaust gasses to escape quicker will increase VE and thus net you more power, a re-map will net you more power again.

Theoretically yes, you should get more power, unfortunately the newage ECU sees that the engine is 'over-boosting' and pulls it all back..! Therefor, negating any potential gains.

You could say the ECU is too clever, damned emissions control..!

IMHO fully decatting is not worth doing on a newage unless you're prepared to get it mapped..;)

PS Don't forget to get rid of the up-pipe cat..:p

The ECU won't pull the engine back(reduce timing, whether by rough or fine correction) if it's overboosting alone, it will only pull timing if it detects knock, which you won't necessarily always get if it overboosts slightly.

No, but it will try to control the over-boost, which results in the same thing..;)

Awaiting a reply from The Rookie :D

IMO Decatting a newage WRX makes a lot more noise for a minor power gain but needs are-map to make a worthwhile difference :)

Shaun

Man, i'm confused, lol. Where are the mappers when you need em'.

Probably about 10-15bhp yes (both decats)....and still 10-15bhp up on what it would otherwise make after a soak (possibly slightly mores as engine bay temps will be down with no cat in the engine bay!), just both will be down on a cool charge figure!

If more air flows in, its measured by the MAF. So if there is a smaller residual gas fraction (due to the lower exhaust back pressure) then more fresh charge enters (with manifold boost pressure the same as before), so more air through the MAF.

This has nothing to do with boost, so no increase in boost to be detected and pulled!

Simon

Thanks Simon :D

Could I ask another question........"what does EGR mean" ?

Followed by...

Those two answer actually contradict each other. I originally asked whether simply decat'ing the exhaust would increase performance, not necessarily BHP although i assumed a small gain would be had.

Boro

I should have said that there won't be any substantial increase in BHP as the ECU is "blind" to the de-cat...

Shaun

Agreed, although i understood that already. Seems to have been a breakdown in communication. I wasnt expecting substantial gains and fully understand a remap would be need to get this but i was surprised that two seperate users both said NO GAINS at all.

Seems like the mystery has been solved, lol.

Agreed, no noticeable difference..;)

Boro, have you noticed any difference from the bum dyno..?

PS don't forget to remove the up-pipe cat before mapping..;)

Hard to say really. Within 24hrs of getting the car, the clutch started to slip so we didnt give it beans incase it went completely, so havent really got anything to compare it to.

We'll be adding the decat up-pipe when money allows followed very closely by the remap.

From recent experience of our bugeye WRX PPP, the blobeye with decat seems to pull better in 3rd and spools up little bit quicker but tbh, theres isnt a massive amount in it. Will defo go for a remap in the new year for that noticeable difference on the bum dyno :D

EGR(exhaust gas reversion), basically a free flowing exhaust allows more exhaust gasses to escape on the exhaust stroke, leading to less exhaust gasses remaining in the cylinder and mixing with the 'clean' air in the intake manifold.

To be clear, a rise in power could still be attributed to the higher boost, the denso boost control is good but not instantaneous, and unlike the timing has no 'learning' when it comes to boost.

My bug-eye ran for a year at a noticeably more spritely pace, but a re-map really loosened the horses and didn't adversely affect the mpg.... so I was wasting fuel all that time.

DunxC