Nick Read

I've done a bit of research into this one, asked a few people, read a few books and FAQ's and searched the forums. But nowhere can I find a definitive answer to why -or how - the classic WRX models (or indeed the UK Turbos) had such differing power outputs. Well, forget 'why' for a minute, that's not the issue I'm interested in. More like 'how'?

So we don't cloud the issue, let's forget about rolling road test results, quarter mile times, 'bum dyno', STi's etc and just look at the manufacturers quoted outputs for WRX models. According to the greatest consensus both in print and amongst those who should know, virtually all the classic wagons until the very end of the line had lower power outputs than the saloons, because they were a less overtly 'sporting' model. I can understand that as a load lugger they were designed for a lazier drive with more torque in the mid range (not outright, but in comparison with their max. power output).

The only thing that bugs me is how this was achieved. Looking at the data it seems that compression ratios were changed from year to year, which obviously has a marked effect on power/torque delivery. E.g. my MY98 wagon with a quoted 250bhp runs a 9:1 CR, more than any previous wagons IIRC. And it does feel strong low down, the turbo spools up nice and early but it feels breathless once you get up to high revs. This matches with what I'd expect from a relatively high CR and a smallish turbo. The boost target seems to be around 11-12 psi after spiking briefly to 15psi.

Everything I've heard leads me to believe that the higher power outputs from import WRX engines is not because they are built any different to the UK models, and it would seem that the wagons even run the same turbo. SO this must mean that the power advantage of say a WRX wagon vs. a UK Turbo 2000 saloon has to be down to ECU control entirely.

The Jap ECU is therefore capable of producing more power from 100 octane fuel and is mapped for higher boost and more ignition advance, whereas the UK ECU produces less because it is mapped for 95-97 octane with correspondingly less boost and advance. CORRECT OR NOT?

Anyone know the ECU boost targets and CR for each WRX and Turbo 2000 model? That would be a very interesting read.

dj219957

"The Jap ECU is therefore capable of producing more power from 100 octane fuel and is mapped for higher boost and more ignition advance, whereas the UK ECU produces less because it is mapped for 95-97 octane with correspondingly less boost and advance. CORRECT OR NOT?"

Correct.
The early phase 1 jap wagons had a smaller turbo compared to the saloons. A td04 compared to a td05. They were not tuned to run as much power.
The Uk wagons at the same time however kept the td05 and ran the same power as a saloon.

Not sure about phase 2 motors? i.e 96 on.

Andy

Nick Read

Good, thanks for the reply. The weird thing is I read somewhere else that the WRX wagons had the VF turbos, but then other people reckon they had the TD04. Mine certainly is, anyway. So the reason why mine might be holding at 11-12 psi (0.75 bar) is basically because it's mapped that way? Which is probably a good thing seeing as I always use Optimax only (no booster) and I might get some decent fuel economy if I keep the boost down at that level.

But let's say that I fancied some free power, keeping with Optimax and no octane boost. Inlet and exhaust are about as sorted as I want at the moment - STi panel filter, H&S up-pipe, GGR sports catted downpipe, H&S decat centre section, STi backbox, 1mm boost restrictor and fully cleaned out boost control plumbing. Is there any agreement over what's the maximum boost to run totally safely on a wagon? Presumably if the actual engine is physically the same as a saloon then there's no reason not to up the boost a little. But to what? Would 14-15psi be ok using a Dawes device?

And if I used a Dawes device to hit say 15psi, would the ECU recognise it as 15psi and fuel it accordingly, or would it think it was still running at its boost target of 11 psi and go very lean?

dj219957

OK, been there done that! you want the power of the saloon in your wagon!
I have a 95 wrx wagon and was gutted when i found out it had a smaller turbo compared to the saloon. Ive now changed the td05 for a td05 and changed the 3b ecu for a z4 ecu. Bingo - not quite ...apparenty the comp ratio is different too, so never quite the same.

In the case of the phase 2s i think the wagon and the saloon both use a vf turbo which is the same size - not sure - someone correct me?
Anyway i would say that it is not just as simple as adding a MBC to up the boost so that you have saloon spec. The comp ratio may be different. and you will need to want a custom map to alter fueling etc to match. No the ecu will not see the extra boost you are adding via the MBC.

Andy

wisecobandit

How much extra power did the td05 give compared to the td04 mate?Im having mine changed over next week (95 wrx wagon to ) although ive got a scoobyecu inplace for it.

I run my td04 presently at 15 psi without any problems although originally in standard ecu form it would bring the fuel cut in until i upgraded to a z4 ecu.
I also was told the wrx wagon v's the wrx saloon that its mainly the turbo and compression ratio and the inlet manifold may be different?Ive asked a few other questions before and things like injectors etc and they all seem to be the same.

dj219957

Estimated power before = 215 bhp maybee even 210 with uk fuel.


Mods

Td05
Z4 ecu
Full decat
lightened flywheel

Power on rolling road = 260.9 bhp on uk optimax.
I recon with a scooby ecu i could see 275 easy.

Car drives much beter now and pulls for longer all the way to 6.5k. Before the power wold come in and then fall off really quckly. Car feels more sporty.

Where did you get your scoobyecu from. Ive been on here for 2+ years and still havent got one for a decent price - im not paying 300+ on ebay! I would rather stick with what ive got unless i can get one cheap.
Never heard of inlet maniflds been different???

Andy

Nick Read

Yes you've hit the nail on the head! But if as I suspected a Dawes/MBC is simply fooling the turbo into running higher boost without the ECU being in on the plan and fuelling accordingly then I shan't bother. I'll wait until I get it all mapped up properly with a Power FC or something similar.

I don't know the CR's of the saloon lumps, but presumably they run lower compression and more boost. Therefore with all other things being equal, swapping a wagon lump into a saloon's engine bay and hooking it up with all the saloon's ancillaries e.g. turbo, ECU, injectors etc - would see greater power from the higher CR? ALthough once you really started to tune it you'd reach the boost limit sooner.

dj219957

Your best start would be to get the td05 and z4 in there. See how it runs. Then save up for a custom map or ecu replacement. Thats when you could see some serious power.
TBH im happy to stick with the standard saloon z4 map as it runs luvly with the decat with very little det and it doesnt put too much starin on the engine or box. Not sure about the compression ratio thing - it could be mith.
Andy

Nick Read

Well I'm thinking along similar lines but because my engine's a bit leggy (105k) I only want free power if it doesn't mean stressing the engine too much...i.e. the ecu replacement first to get it optimised in its current physical spec. Then next stage will wait until the engine needs a rebuild, at which time I might well go 2.5 litre which will need a different turbo anyway. So any replacement turbo -unless someone can supply and fit for 50p - would be a temporary measure and not worth bothering with. Looks like I'm back to plan A - get the Power FC in and map for best safe power with that.

As far as the CR goes I looked in the spec sheets at the back of the Haynes Impreza book, which is pretty comprehensive on Jap and UK models, and the differences in wagon power outputs from year to year match up with their varying CRs from year to year. Doesn't necessarily mean they're correct, but they make sense.

Explain more about the Z4 ECU. From what you're saying it's basically the ECU for the WRX saloon, which would run slightly different boost and timing?

dj219957

Yeh the z4 ecu is a stock ecu from a late 1994-1996 wrx jap saloon. It has the fuel, ignition, boost etc set up for the td05 turbo which is of course far better than that of the wagon:


WRX (1995/96) ECU Z4 - 12.71psi, dropping to 11.78psi @ 6600rpm. Fuel-cut 15.65psi.


WRX Wagon ECU V9 or 3B - 10.55psi dropping to 6.84psi (starts to drop @ 5000rpm). Fuel-Cut 13.79psi

Nick Read

Superb...these are exactly the sort of techy details I've been after, cheers. Now my MY98 wagon doesn't follow either of those profiles, which isn't that surprising considering the only standard bit of the exhaust is the manifold, with an STi panel filter although I don't know how much difference the filter makes. Anyway, what I was seeing before I drilled out the restrictor to just over 1mm (it seemed to be 0.9mm beforehand?) was a boost spike in 4th and 5th to around 17psi - presumably down to the freer flowing exhaust - then it would cut. It would then settle down to around 11psi. After drilling the restrictor a little, this improved things a hell of a lot, with the spike now only hitting 15psi max, then dropping back to 11-12psi.

Now I've not paid very close attention to the boost characteristics across the revs, but I wouldn't say I have the boost dropping to 6.84 at all, like it should with a wagon ECU. It hits the momentary 15psi spike, drops to 11 psi usually and holds there all the way to 6500 if I want. I've not ventured up to the redline yet. It's characteristics are more like the saloon ECU but just a little less boost.

Is this behaviour likely to be purely a by-product of the modded exhaust?

dj219957

The figures i gave were for the phase 1 classics not your phase 2 classic mate. So no they wont follow suit.