Hi All,

Just wanted to get some opinions on the best way to modify an impreza if response and torque were the key goals.

I'd be working with a UK turbo engine, I don't want to rebuild it so this would essentially be bolt on mods, headline power figure of around 300bhp, perhaps a bit less, but maximum possible response and torque.

So turbo wise I think a TD04 would still be best but a hybrid version, what's out there and what sort of spec would give the quickest response/spool? I've seen Andy Forest's one, but not sure if anyone else is doing something similar or better.

I've also picked up from here that 2.5" exhausts are considered perhaps better for torque and a top mount intercooler for response, what else can you guys suggest?

 

I would consider gear ratios if youre after response.

 

good shout ^

 

Seeing torque is used to calculate BHP, you should give a power range where you want that peak torque to be produced,


For example on a car with 300bhp at 5000rpm will produce 315.12lb/ft of torque, but a car producing 300bhp @ 6500rpm will only produce 242.4lb/ft...that's fact, calculated by mathematics, and will apply to any engine, in any car on any dyno. (and also explains why a 600lb/ft Bentley Arnage only makes 420bhp...because it only revs to 4500rpm )

Thought I'd point that out...as BHP is zero without torque ( bhp = ((rpm x torque) / 5252)) - try it on a calculator .


Anyhoo, so what you really are after is a big wide power/torque band and low lag. Yeah? IN that case twin scroll. Or a ball/roller bearing turbo (VF series) as opposed to a sleave bearing that's found in the TDs. It won't be cheap.

 

Hi Ali, thanks for that, very interesting, yes the idea would be to maximise the power range I most commonly use, so i'd be working between 2000 - 5000rpm. If over 300lb/ft is possible that would be decent enough, but I'd want it to peak a little lower than 5k if poss.

Ref shorter gear ratios, I get what your saying, but generally with a torquey motor you want long gears to make the most of it, short ratios and big torque would be at odds with each other.

Ali thanks for the info on turbos, I thought any of the VF's would be too big for what I need? Twin Scroll sounds interesting but I thought these work best with plenty of revs? Whats the smallest turbo in the VF series? Also does manifold choice play a role in how the car behaves? unequal/equal etc?

 

Hi there

Speak with John@Braveheart Developments,he can billet yours existing TD04 or any other turbo,I can only recommend him

And about the getting best possible torque from 2.0L,its bit harder,on 2.1L or 2.35 or 2.5L is not too hard get higher torque figures than on 2.0L

Twin scroll is awesome,but on yours without the AVCS will never spool like on JDM STI/Spec C and conversion is not cheap too



Thanks,Jura

 

VF34 or if you really only want 300 the 28 is a superb blower

 

Vf series are a pretty old turbo and not desperately strong, as well as being a little bit over kill for 300bhp, they are more in the 330/350bhp bracket.

As Jura said above Braveheart sti is your man, and the turbo you want is a Billet TD04 hybrid, good for up to 320bhp but will have the spool and response your after, then you want Bob Rawl to map it for you as he seems to be the best mapper for extracting torque.

Your description of what you want is one of the reasons i've stuck with the TD04 and ppp map on my blob wagon, between 2500rpm and 6000 which I never need to go over, it has exactly what you need, just always pulls and once in third you can just leave it there and get on with the driving it part, between 50 and 90mph it just pulls you along at a decent rate which for me is perfect for the twisty roads that I like to play on. On and off the gas with minimal braking, using the gas to make the back end dance around the corners, and another reason mine is close to stock suspension and handling wise, too much grip = no fun, it's not the fastest way to get from A to B but it is the most fun.

 

Theres a member on here selling an Andy Forrest TD04, that wouldnt be a bad starting point for you....

 

Group B motorsport recently fitted a set of my headers to new age WRX and they got 288/315 from the td05. Ive built a few std compression 2.0's using the Vf24 and seen around 310/320. A smaller turbo will obviously work better low down but is limited bhp wise. For the record my rally car makes 330/435 at 3800rpm, very high compression, porting, valves, cams and 102 fuel. Thats with a vf34.

 

Interesting, thanks for that info, why do you think this is? is there a particular design of manifold that's better for torque? Does unequal/equal make any difference in this regard? Love those figures from your rally car!

Also intercoolers.. in terms of response/lag I know a top mount is considered more effective, certainly for my goals, but what part does surface area play? I currently have a classic STi intercooler, would a newage STi intercooler be needed or would the larger intercooler have its own downsides?

Lastly, purely out of interest, if I was to fit a hybrid version of my current TD04 turbo with no other changes (car is a classic, ppp ecu, centre decat, exhaust, green panel filter, walbro pump) what would happen? Would it drive exactly the same, smoother or cause issues? Not saying I'd do this of course! Just curious.

 

Seriously a VF28 on a 5 speed WRX is superb Add a LWFW, headers and funky uppipe and you will be getting there Mapping wise you will need the appropriate servicing and fueling mods to seal the deal Stick with TMIC as well yes

 

You'd need it mapping to get the benefit as it will have a different exhaust housing and of course a billet wheel.

I'd have a bit of a chat with braveheart sti as he will tell you what is best suited to your spec and requirements, also a lot cheaper dealing with him in person as opposed to going via specialists that supply his turbos.

 

Is it possible to upgrade just the turbine and compressor wheels in a TD04? Newer more efficient / billet compressor and turbine wheels would make it even more responsive right?

Does anyone know what combination of turbo's are used to make the hybrids like the one Andy F sells? I assume it's a TD04 exhaust housing matched with a slightly larger compressor housing and new compressor and turbine wheels?

 

The headers make a big differnece because they allow a faster flow of gasses to the turbo, the size of the up-pipe is critical also, too small and it will restrict flow, too large and it will make it laggy. The way to think about turbo responce is that the turbo is a fan and the exhaust gasses spin the fan, so to get it to work quicker (good responce) is to either blow more air through it (larger engine capacity) or flow the air faster (velocity) This is achived with either engine design (my rally car-compression, cams, porting) or with good headers and free flowing exhaust. The shorter the distance from the turbo to engine also plays a part (intercooler). As well as all this the turbo and mapping are pretty important too :-)

 

Hi there

Not sure if I would recommend on pretty standard engine and mods aftermarket headers and up pipe,in my view is not worth expense

I've been using WMS headers which doesn't have best fitment,this is not with standard turbo location,but with most of the Twisted kit like RCM or H&S,simply they didn't line up

Not always aftermarket parts are better than standard/OEM,I would stick with OE headers and if budget allow porting will help and better up pipe like Harvey Up pipe will only helps

I've been using like RCM or GT Spec headers and those headers have best fitment without the question,we are used them like on Twisted kit or with standard location turbo(MDX321T)

But agree about the using larger diameter up pipe,this can create only lag and about the exhaust,keep 2.5" for better torque,with 3" on small turbo you can loose torque

Yes I think is possible upgrade wheel on TD04 to billet spec,when I've spoke with John,he making TD04 billet or VF35 Billet

Thanks,Jura

 

forged 2.5 (single avcs wrx heads) and a sc46+ with supporting mods

 

Have you thought about running on meth? You should get an extra 20 - 30 lb/ft in mid range