Sub-Subaru

I am fitting an Sti TMIC to my 2004 Blob WRX soon but intend to keep the stock WRX bonnet scoop. I prefer the look of the WRX scoop and I am also keeping the WRX spoiler to match. I am only running a TD04 Hybrid so not too concerned about the cooling disadvantage of the WRX scoop.

When I was doing some research a few years ago, I found a thread where someone suggested the Bug Sti undertray would fit/almost fit with a Blob WRX scoop. Can anybody confirm this or give guidance?

Henrik

iTrader: (5)

I've ran both the bug sti undertray and blob sti undertray on my WRX.

The bug undertray fits into the scoop without any modifications, BUT there will be an approx 1cm gap between the top of the tray and the top of the scoop.

If you use the blob tray, you will need to do a small amount of metal work. I basically drilled out the spot welds on my WRX undertray so I could lift the scoop section off, then bodged it into the sti undertray . It needed an extra strip of metal IIRC.

Both setups worked well for me. I have a small suspicion that the WRX undertray mod might be "better" as the fit into the scoop is better, but I also don't think it's significant enough to worry about (probably), at least not until power levels go higher.

My car also runs a hybrid td04. What kind of spec are you getting?

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Henrik

iTrader: (5)

This is how the bug tray fits inside the wrx scoop.. now that i've had a look again, there's actually a pretty big gap between the scoop / tray.



Think I might try to plug it with some foam of some sort, however to date I have not experienced any issues with it in its current form.

Sub-Subaru

Thanks very much for the comprehensive reply. I am struggling to find any Bug Sti undertrays anywhere and it doesn’t seem like something that comes along often.

However there does seem to be a lot of Blob Sti undertrays about so it would be easier and quicker to modify and join as you say. I haven’t actually ever dealt with one.

When attaching the top WRX part to the Sti bottom part, does this create a bigger restriction than the Bug Sti one or is it pretty much the same then?

With regards to my setup. I am still collecting parts but really want to get them fitted and the remap done ASAP. I bought the TD04 hybrid off someone five years ago but didn’t have funds to do it then. Although the guy was a nice guy and in the local club I didn’t know how to check if it was 100% hybrid so need to get it out and make sure it is.

I am doing the following mods with the hope of making a brisk street car. TD04 hybrid, pink injectors, Sti TMIC, panel filter, new plugs, Walbro 255, Prodrive sports cat downpipe, decat up pipe, de resonated centre section 2.5”. This will be the first remap but I have really been wanting to do it for years now. I have most of the main parts now.

What is your spec and what figures are you getting?
Also out of interest why are you now running the Bug Sti undertray rather than the modified Blob one?

Any suggestions on further parts I should consider adding before the remap would be Cool.

Henrik

iTrader: (5)

Yes, they do seem a lot rarer, however having had a look at my own again, I would say the blob undertray (modified to fit a wrx scoop) would probably be "better" anyway (at least without sealing the tray to the scoop), as there's a lot of area for the air to escape straight into the engine bay, bypassing the intercooler.

Given that the STI bug isn't prone to heat issues (when moving etc etc), I would say the blob WRX scoop is adequate given that it is bigger than the bug STI scoop anyway.

I will also say that I had a bug estate for a while, which was rebuilt by Engine Tuner, and it ran about 350hp with an STI8 TMIC, modified undertray, on a standard bug WRX scoop. Given this, I don't think that even the bugeye wrx scoop is too small for reasonable power levels. My WRX was "fine" on a relatively large turbo (td05-20g) even on track etc (then again, I wasn't measuring intake temperatures to be fair, but I never had any issues and when I came in after a session the intercooler always felt cool to the touch (until it started heat soaking from the engine when stationary).


A TD04 hybrid makes for an extremely nice road car IMO, and works well on track too. I used to be all into big turbos and lag etc, but now I prefer less lag and less power. My own hybrid is something I cobbled together myself, but I think it makes north of 300hp (although not dynoed), and it feels just as fast around track as my old td05-20g. I have power available under 3000rpm, which means that tightish corners (~40mph?) is manageable in 3rd gear, where-as before I'd need to be in 2nd and immediately change gears on exit. Not as fast on the straights, mind, but the corners definitely make up for it.
One thing to bear (bare?) in mind is that the engine will still feel like it's running out of puff at about 6k RPM - I find myself shifting at about 6000-6200 when out and about, because revving higher just feels pointless. This was not the case with a bigger turbo, when I'd regularly go to 7k and still have power. I actually prefer the lower shifting point - I think it's less stressful on the engine (probably). Before going hybrid, I found I naturally shifted at about 5500.


My current spec is much the same as yours:

- forged 2.1 (though this is not necessary for the hybrid power levels, mind)
- td04 hybrid (mine's a TD04 HL exhaust wheel, 19t compressor wheel and upgraded thrust bearing)
- sti8 tmic + bug tray
- blue hawkeye injectors (much the same as your pinks at this level)
- walbro 255
- k&n panel filter
- ported exhaust manifold (diy) + decatted up-pipe (diy...)
- some form of sports cat downpipe - i think maybe scoobyworld?
- miltek deresonated centre + backbox (2.75")
- TGV deletes (diy, what else?)

If you can stretch to it, or can do it yourself, it's probably worth porting your standard exhaust manifold. I don't know if the TGV deletes are actually worth it at this power level, but I had already done them, so... I think my next "engine" type mod is a resonated centre section or new backbox.. I'm at 98db static, which is too close for comfort for noise tests on track days. I used to have a PPP backbox and that was the nicest backbox I've had I think.

Regarding all the above, and all my other posts, please remember that I am just a "bloke on the internet", so please take any recommendations/musings with a pinch of salt

Linksfahrer

At the risk of being just another bloke on the internet ,( I like that Henrik )

Personally I would not be so bothered about the undertray.
Subaru went to the lengths of putting some heat shielding on the original TD04 , that wont fit back ontop of the aftermarket Turbos so then it seems logical to me that this area
needs some attention.
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As most of us are worried about heat soak from the Turbo and go and put some aftermarket wrap over the housing it seems
logical to get some air flowing down to cool off the area close to the TMIC , Its obvious to me that while sitting at the lights this is a big problem for the WRX / Sti
dynamically of course a lot of heat can be blown away from the air passing into the scoop. So why not use it ?



I just cut the baffle area on the WRX to leave this area on the left open.



I lifted the Bonnet a little here so you can see the slot , its free cooling that would otherwise not be used on
the left side of the Sti IC In this setup you really only miss the air coverage on the right which you could rectify
if you were to use the Sti Rubber channel.



So I would leave it at that , If you are concerned about the IC intake temperature then you can monitor it and the mapper can integrate its temp output for the Map
( I have a sensor on the IC which you can see on the top picture ) but actually I don't use it yet , I doubt its really very relevant at 330 bhp with a SC36
but if you push high boost or look for more power then of course you can use it for a bit more margin of safety / map control.

Sub-Subaru

Thanks very much for all the info. When I looked into upgrading things a few years ago I realized the TD04 Hybrid was what I was after. I don’t do track days and it osn’t Something that really appeals to me. I like quick acceleration and being nimble. Yeah, at the moment there is no point going beyond 5.5k and you feel when it is out of puff, but I’m not desperate to rev it to the max to get power so that’s fine.

I did think that the Blob WRX scoop can’t be too bad if the Bug Sti undertray fits inside. I will have to work something out with mine and a Blob Sti one.

I had to look up what the TGV delete is as i’m Not familiar, seems like a bit of a hassle so I don’t think I will do anything with that. I’m not too bad at doing things myself and will be doing as much as possible myself before the remap. I have heard and read about porting before, but perhaps you can clarify it a little.

Is porting the exhaust manifolds folds just smoothing the first two inches inside the manifold pipes or is it about enlarging them. I have always been so surprised how much of a deal is made about it, I’m suprised it makes such a difference?

The only thing not standard on my car is a pro drive back box with an oval exhaust tip. I have recently picked up a Cobra centre non-res section which came with a Cobra back box and I’m wondering if there will be any difference in sound or performance between the two.

Sub-Subaru

Thanks, this has given me something to think about. The problem is I have not seen and inspected an Sti undertray to compare it to the WRX one so I don’t understand where the differences are. Can you tell me the difference between them, are they not wider and longer?

Linksfahrer

As I say the WRX Newage / Blob curtain covers an estimated 80% of the Sti IC.
The Sti curtain ( which I don't have a photo of ) is just slightly deeper and is perhaps 12-15 % wider (across the width of car ) that the WRX one. Perhaps someone reading this can measure the inside dimension of the Sti 7/8 IC curtain and jump in here ?

I have now have my WRX into Plymouth Engine tune for some more work and a remap so I can't measure it out for you now.
Once Lockdown 2 is over I will look to install a IC Temp guage and log the IC Temps as I do have a sensor in the Sti IC.

My gut feeling is unless you only drive around built up areas on heavy throttle or go out for a blast around towns in +30 deg ambient temperatures, that the IC temps are not likely to become a power limiter in sub < 350 bhp installations
however you would need a mappers opinion on that theory , and a mappers input as to what IC intake temperature triggers a reduction in timing/boost/fueling etc.

However as I said before, the decision to go with better IC cooling has to be taken in context of the level of modification and target BHP involved,
if the mapper cant get the map done without heat soak on the rolling road , then you will have spent your money for little gain.

You could have course do the road mapping ( which is the real world on the day ), from my experience the Road map that I had done from JGM , it was very careful development with a lot of focus on the slow speed pickup in 2nd / 3rd gear and also
focus on boost tail off in 5th. The down side of a pure road map is you do not know the power figures, you rely on the mappers experience and your Bottom meter.
Whilst you can equally argue that it is perhaps unsurprising that RR mappers don't / can't get the same smooth results whilst doing just 3rd gear pulls on the RR with only a blower in a box for cooling. I will however vigorously defend the RR mappers who do the ground work in the RR box and then confirm it on a road test ( this is what Plymouth Engine tuner do ).

The brilliance of the best mappers like the late JGM is/was they had so much experience in both Road and RR mapping and car knowledge in depth , that they often pin point technical problems in the car and advise after having reviewed the laptop values
where you bottle neck in tuning was likely to be.

Henrik

iTrader: (5)

Here are some shots from my car just now. Regarding the skirting size, the skirt measures 46x16cm (inside measurements). Not sure how that compares to the WRX skirt, but my feeling is that it's a significantly larger area (but maybe not).

Also the bug tray has those little slats to the left which I presume are there to cool the turbo slightly (or maybe it's a vent for when stationary?).

Linksfahrer

Originally Posted by Sub-Subaru View Post
Thanks, this has given me something to think about. The problem is I have not seen and inspected an Sti undertray to compare it to the WRX one so I don’t understand where the differences are. Can you tell me the difference between them, are they not wider and longer?

I still have the Std WRX intercooler in the garage, so I measured its vane core area at 41,5cm x 11.5cm ( 477,25cm sq. )
The exposed frame around the WRX vanes on which the skirt sits upon add appx 1,5 cm. So that's Approx. a potential cooling flow area 43cm x 13 cm ( 559cm sq. ) available from a WRX Scoop skirt.

Henriks Sti IC cooling skirt internal measurements are 46cmx16cm ( 736cm sq) So the WRX skirt provides just 559/736*100 = 75.95% air flowed area of the Sti IC.

The point about the Sti Scoop of course that it is also has larger Scoop area to catch more air. I would "guess" that Sti Engineers also gave it the same increase in volume in order to maintain the flow rate /sq cm area
at the same speed as the WRX , but only someone with access to both scoops could confirm that calculation.

In my particular Frankenstein hack modification to the baffle on the left of the WRX scoop , I have opened another flow area of appx 8cmx13cm , ( 104cm sq )
although clearly this is not directly funneled efficiently into the IC, and while only 50% of it falls over the extra width of the STi IC. My purpose here was to get more air down over the Turbo.
Never the less if you take just 40cm sq of that flow area as effective (559+40)/736*100 = only 81.3% of the STi IC area can ever be cooled by a WRX Scoop/Skirt.

Doing some real rough and ready math's on this ,, bearing in mind the larger volume of the Scoop , the potential of the Sti Scoop/Skirt IC cooling at any given speed is likely to be in the order of 40% more, ( assuming the bonnet is down)

While doing Rolling Road Tuning the bonnets are normally up , so it becomes clear that a FMIC with a blower placed directly in front of it will be far more effective than either a Classic / Blob WRX or Blob Sti TMIC .
So all the TMICs are going to perform less efficiently in a RollingRoad box than on a Road Tune , therefore what you really achieve when you take a Sti IC into a rolling road is a initially slightly higher mass which will heat up a
bit slower than the Classic/WRX IC and of course the larger Sti IC will re-radiate heat /cool down a bit quicker "if a front blower is running".
The mappers job then becomes a bit easier and he will have and potentially need less cooling time to get the tune optimized / or to wait before a final power run.

So in conclusion if you hybrid a Sti IC undertray to a WRX scoop you will still lose some IC cooling potential , but probably only Approx - 20% of a full Sti Scoop/undertray/IC .
Doing it with just the Sti IC and WRX Scoop/Undertray (like I do) maybe as much as -40% on the road.

At what point /ambient temperature/speed at what Bhp does that become a critical factor on the road...... who knows
for that you need constant IC temp monitoring which I as yet I don't have.

I can say on my WRX IC @ 315bhp tune level it "starts" to preclude further RR runs without extended cool down periods
on a PPP exhaust TD05 16G pushing 1.5 bar. at 82% Duty cycle with WRX 06 Dark blue injectors.

Henrik

iTrader: (5)

Who knew we could have such an interesting and nerdy thread about undertrays 👍

Regarding the size of the scoop, I think you're right Ralph, in that the total flow of air is bigger through the sti scoop, but I think this is mainly a low speed improvement. What I mean here is that as speeds increase, the WRX scoop would be "good enough" anyway even if it flows x% less. At higher speeds the amount of air flowing through would be enormous anyway, so the extra size doesn't really matter. E.g the newer STI has a much smaller scoop (although to be fair it looks more like a NACA duct), and that presumably has enough flow.

Now, going back to the area of the intercooler:
If we think that the WRX is good for about 315hp (estimates seem to vary from 300 up to 330, so I've just picked the middle value), if we scale that up by the area to the sti size, we get:

315*(1/0.76)=414

From people like Andy Forrest and Simon back in the day, it does seem like they were under the impression that the sti IC was ok up to low 400s.

I think, based on the above, that actually the WRX and sti intercoolers are equally efficient (ish), except the core area is larger on the STI.

Caveat: not sure if the larger area of the sti means that at the same airflow, it would have less pressure drop than the WRX item.

Sub-Subaru

Thanks guys for all the input, I have also had a look at my WRX one today to see what it is like. I understand your point about there being only a little extra benefit of the Sti undertray with the WRX scoop. I think though that I will try and do something with the undertray.

It is interesting that the area on the left has that cross hatch plastic that doesn’t open up fully, I wonder why it is not more open there and whether many other people have trimmed and cut this area open like you have.

With regards to porting the headers, can you tell me anymore about this and what is involved?

Henrik

iTrader: (5)

my ported headers / process can be seen here: https://www.scoobynet.com/projects-4...l#post12063750 A couple of posts further down in that thread is something about the up-pipe etc as well.

Again, I must stress that I don't actually _know_ if this is beneficial at all, but my car at least responds well enough for me. Basically I smoothed the bumps out as far down as I could get with my die grinder, and tried to ensure that there wasn't a big step between the exhaust ports and the manifold (important note - you cannot use the gasket for this, because the gasket is much bigger opening than the exhaust ports themselves).

If you have OEM "everything" before the turbo, I think the biggest spool increase likely comes from decatting the up-pipe, but the ported headers probably also help (many small streams make a river, and all that).

Linksfahrer

Henrik's project post shows the grinding steps to open / flow match the WRX casting. Which I think he has done pretty well in his search for strong low rpm Torque.

I was thinking of doing it myself but decided to have that work done by Plymouth Engine tune, In the past I have always done my own head flow work, which I started on Mini 1275cc 12G295 heads
so I know a fair bit of theory on this but in the case of the WRX iron cast Its actually not that easy to do well, as the material is very hard and to do it well a fair amount of material from deeper into the passages need to be smoothed/removed.
Additionally its not ideal just to match the head port to the header casting, but to be take into consideration a a slight lip from the cylinder head that needs to be maintained to resist exhaust gas backflow,
In order to confirm your work, ( how well that your grinding has been carried out) Unless you have of course access to a flow bench you have to wait until your new modification has been dyno'd.

But as it interests me and perhaps others on here with similar tune level, I will be putting up a Dyno graph on a new post before and after this single modification, by about Dec 5.

Henrik

iTrader: (5)

I am very interested in this for sure - please post when you have them 👍