tony de wonderful

Ok so I have a ver 5 Sti but I'm new to Scoobs.

So my car is stock but I want to free up the air flow.

Intake: Should I just put an K&N filter in the existing intake or go to a POD? Which offers less restriction? Will the later screw up the flow sensor even if done properly?

'Zaust: I want a cat so what else do I need? Can I get a 3" dump, then Cat, then rest of the 'zaust? Or is the Cat intergral to the dump pipe? What are my options? I don't want a droney 'zaust making a racket at idle. I've got HKS silent high-power on my skyline but they don't call it Hi-kost-Spec for nothing. The HKS has a mid-muffler.

Oh yeah do I need a new fuel pump? Will do the ECU/re-map at some point.

What about BOV's? Do they offer any advantage on a lightly tuned Scoob? I know on Skylines the existing return flow valve is good until crazy power levels so only people wanting a cool sound (in their own mind ) go for BOV's.

Splitpin

Inherent in that is an assumption that the airflow is "not free" at the moment.

The former, unless you're talking about a significant package of mods designed to up the power up around or beyond 400bhp.

Define "done properly". Under most circumstances you'll either cause the airflow sensor to misread, or you'll set up a chain of events that will damage it, therefore causing it to misread. Given that there's no real advantage in fitting an induction kit the most sensible approach is not to bother, until you're actually finding it a bottleneck - which you never will as long as you're running the standard turbo.

The catalyst is in the downpipe (or dump pipe as you call it), so if you want to keep one but with better performance than the standard setup, a good open neck 100 cell sport cat DP.

The standard one, when new, should be more than up to the performance of the turbo when remapped, but the pump in your car, assuming it's standard, is now something like 10-11 years old. This alone might

No, usually a disadvantage, in performance terms, as long as you're running the standard management.

tony de wonderful

Ok but so would a K&N filter give me the same dyno power as a POD is basically my question?

Ok do you know why that is? Does the turbo spool back up quicker with the stock return flow type valve?

Thanks for the rest of your answers!

Splitpin

There are two aspects to the answer to that question. The first thing to say is that from a point of view of intake efficiency, yes, a good (K&N/Green/Cosworth/etc) panel filter should give you the same dyno power as any drop in induction system replacement, at least up until you're well beyond the capacity of the VF28 turbo.

You will only "gain" from the fitting of an induction kit if the standard setup is a bottleneck. At 330bhp or so, it isn't, and so even if you map around any MAF scaling issues it's unlikely to generate any inherent advantage - and you still run into side-effects.

However, there are circumstances where you might find an aftermarket replacement might deliver a handful more "dyno power". However, if this occurs, it will be because the induction kit is either skewing the airflow readings (and leaning the mixture) or is transmitting elevated levels of vibration into the airflow sensor, which will, again, lean your mixtures (and the more vibration the sensor is exposed to the more seriously it will under-read).

Leaner mixture = more power, but uncontrolled, unmonitored leaner mixture eventually = expired engine, so if you get more power via this route, t's not the sort of power hike you want - especially on an STI5/6 which is already very marginal on UK fuel.

It's primarily because the ECU calculates fuelling over a throttle lift on the assumption that the standard recirculating valve is fitted. If you fit an aftermarket one with either atmospheric dump and/or a non-standard opening characteristic you throw the ECUs calculations out of whack - primarily (on a VTA) by dumping significant amounts of metered air without the ECU knowing. Cue over-rich mixture on reapplication of throttle with consequent poor response until it clears.

And that's before you consider the consequences of aftermarket valves sticking, being set wrong, etc. etc.

tony de wonderful

Ok thanks. That kind of confirms what I suspected. I know on my skyline the stock airbox with a high flow filter is as good as a POD, plus it doesn't suck in hot air from the engine bay...just like the one on my Sti also...it sucks air from behind the front 1/4 panel? I probably won't **** with it now and just run a high flow panel filter. I'm new to MAP sensors 'cos my skyline has just a MAS.

Can't the MAP sensor detect when the BOV vents and then meter out fuel accordingly?

Splitpin

No. The standard Subaru ECUs don't use the MAP sensor to calculate fuelling, and even they did, the MAP sensor has absolutely no idea what the dumpvalve is or isn't doing as, for obvious reasons, it monitors manifold pressure. During a lift the inlet manifold, and its pressure sensor, is obviously isolated from whatever's happening further back in the inlet tract by the closed throttle plate.

Fuelling on these ECUs is based on mass airflow measurement. Because the ECU is designed with a recirculating dumpvalve in mind, it always knows how much air is in the system as it all gets measured on the way in. If you fit a vent to atmosphere dumpvalve, there is no possible way of measuring the amount of air dumped out of the inlet tract, and thus there is no way for the ECU to calculate/estimate the amount of "missing" air for the purposes of its fuel calculations.

tony de wonderful

I thought the BOV or return flow valve takes air from between the turbo impelor and throttle body? It's only isolted from the upstream side by the impellor which has obviously got large spaces between the blades.

If you quickly vent the air out downstream surely the MAP will recorded a fall in pressure also. Obviously is is all complex and dynamic since the turbo still spins for a time on throttle lift and the engine sucks in air on overrun.

Splitpin

Correct.

Correct. However the MAP sensor is sampling the air pressure in the inlet manifold, and this is separated from the upstream side by the throttle body.

Yes and no. As above the Manifold Absolute pressure sensor is monitoring the pressure in the inlet manifold, not the induction tract. When you close the throttle the MAP sensor does read a huge fall in pressure. The throttle shuts and the engine continues to try and suck air in on overrun, so the MAP sensor sees vacuum relative to atmosphere. As such, you are perfectly correct to suggest that MAP drops on a closed throttle.

However, the MAP sensor will see the same pressure drop irrespective of whether you run a VTA or recirculating dumpvalve - because, as above, once the throttle shuts, the inlet manifold (and pressure sensor) are effectively isolated from the upstream side of the induction tract.

Therefore, your suggestion is wrong, there's no way for the MAP sensor to differentiate between air being dumped out to atmosphere and air being recirculated through the turbo. And in any case, I've already said more than once, these ECUs don't use the MAP sensor as a direct input to fuel calc anyway.

The ECU does use mass airflow (and engine speed) as the primary inputs in setting a target mixture. However, again there is no way for the ECU to magically tell the difference between the standard recirculating bypass valve and "something else" (whether a VTA or a DV delete).

When you shut the throttle under the standard recirculating setup, the ECU will see a sudden and large drop in MAF reading (to, pretty much, zero) - firstly because you've shut the throttle, and secondly because most of the turbo's ongoing air requirement is being delivered by accelerated air being recirculated through the dumpvalve and back into the compressor. Any continuing reading on the mass airflow sensor under these conditions will be air actually going into the engine via the idle control valve. The ECU is programmed to calculate the correct mixture under these conditions.

However, when you shut the throttle on a car with an externally venting dumpvalve, the compressed charge that would have been recycled back into the inlet tract and through the turbo is now no longer present. As a result, large quantities of fresh inlet air will continue to be drawn past the mass airflow sensor and out of the dumpvalve by the decelerating turbo.

Because the ECU is programmed to expect a recirculating dumpvalve, and has no magic way of knowing that you've changed it, it will continue to assume that any air passing the MAF sensor is actually going through the engine and will calculate fuel on this basis. It has no way of knowing that the vast bulk of this measured air is actually being vented straight out to atmosphere. You have effectively introduced a large leak into what the ECU thinks is a sealed system.

This therefore creates a transient discrepancy where the ECU believes a huge amount of air is running through the engine, when in fact this isn't the case. This knocks into fuelling (and most probably ignition) map lookups, and will gradually dissipate as the turbo continues to slow.

It's this discrepancy that causes overfuel, which leads to the increased popping and banging often heard on VTA-equipped cars, and which will deaden throttle response for a short period if you go back on the throttle immediately after lifting, until the ECU's understanding of the airflow passing through the engine actually matches reality. See it now?

There are several theoretical ways to solve this. You could fit a second MAF sensor the the exhaust port of a VTA dumpvalve, use it to measure the air going out and have the ECU subtract this number from the inlet value before calculating the load. Or go to a blow-through system with the airflow sensor after the dumpvalve, or just go to a MAP-based ECU.

However none of those situations are relevant to your original question, which is a standard-ish car running the standard engine management through the standard airflow meter in the standard location. As I said a few hundred words ago, under these circumstances, aftermarket dumpvalves are usually a disadvantage, in performance terms. Hopefully now you know why.

Quite, there are a couple of other complex dynamics not even touched on yet (like the effect of charge air being reintroduced to the compressor via the bypass valve on its rate of deceleration). Hopefully now you "get" it?

tony de wonderful

Yes I think so.

BTW can I fit an induction kit w/out screwing up the air flow meter? I know it may not be 100% neccessery but I kind of like the sucking noise of a good POD filter.

Splitpin

I suppose someone should ask one of the mods to sticky this somewhere, as you won't find a much more comprehensive explanation than that as to why VTA dumpvalves aren't ideal!

The short answer is no. The long answer is yes, provided you choose the right one, mount it properly (i.e. isolate it from vibration in the same way as the standard airbox), have any changes in the air:sensor voltage relationship checked and mapped into the ECU, and monitor the car afterwards on an ongoing basis for any deterioration in sensor performance (although that latter step's worth taking even on a regular setup).

If you depart from the above to any degree, then you're playing russian roulette.