Dyno Time

[scubasteve]

Hey guys,
I recently moved away from the Reno area but thought Id still throw up a tech question.

I have an '05 forester xt. Sti : turbo, intercooler, up-pipe, fuel pump, cat-back. Invidia catted downpipe.

I recently moved to Fayetteville NC elevation about 300 ft. The tune I have is from Matt at K-speed, I feel like its a pretty good tune but I got it dynoed here just to see what kind of numbers I was making.



in case its hard to see: it says 226 whp/ 238 wtq/ AFRs hang out around 10.5/ boost peaks at 18.3lbs


I was expecting closer to 250-260 whp...

I was looking for issues that might be hindering my power.

My thoughts:
-I am running a tune for 91 gas
-I am tuned for a way different elevation
-Would an intake help that much? I thought subaru stock intakes werent too restrictive till bigger power numbers
-Tuner recommended a EBC to keep boost up later in the rev range
-Cat back cant be that restrictive...??

from unabomber: "Which exhaust diameter is best? Exhausts have 3 diameters:

1. 2.5": Good for 300-350 HP
2. 3.0": Good for 500-600 HP
3. 3.5": Good for 600HP + "

Thanks in advance guys!

[Kevin M]

That power is spot on for a VF39/VF43 and a conservative tune. Exhaust diameters don't make more power, they just permit it. Muffler design is just as important as pipe diameter, if not more. A 2.5" Stromung exhaust is less restrictive than a stock 2.5" STi catback for example.

I do not recommend going to an EBC at all personally, and you 100% will need a retune if you do. The reason the boost tapers off is the STi turbo isn't really big enough for the 2.5 liter engine, and there's nothing any tuner can do about that. An EBC can improve spool up at lower RPMs to a small extent, but it doesn't help the top end. You need a bigger turbo or better fuel for that.

[scubasteve]

Interesting, thanks Kevin.
I agree that switching to a 3" catback wouldnt increase power but do you feel that the Sti catback/muffler is restrictive for my setup?
Matt also mentioned that he tuned it to lower boost at the top of the rev range to keep it a little "safer" as well... Im not too sure exactly what that means
My end goal is to get somewhere closer to 300 whp, at least on the other side of 250 haha...

Are you suggesting a larger turbo as the best way to get closer to my goals? along with injectors and possibly exhaust....
thanks again

[Kevin M]

Originally Posted by scubasteve

Are you suggesting a larger turbo as the best way to get closer to my goals? along with injectors and possibly exhaust....
thanks again

Yes. If you want to approach 300whp, you need something like a 20G or larger I think. The turbo needing to taper boost is a bit complicated if you don't have a decent background in physics to understand some basic fluid dynamics and thermodynamics, but if you have studied the Law of Diminishing Returns for economics, that is a good approximation. You can keep cranking up the boost, but it gets more and more "expensive" for lesser and lesser returns.

[scubasteve]

20g sounds kinda like a build motor...hmmm not sure Im up for that quite yet

Ive read enough to get the idea that continuing to increase boost also increase heat which makes a less dense air charge = less that will fit in a cylinder....
but for the rest of it, I will take your word for it.

Whats your take on aftermarker upipe, maybe PNP headers to get a few more ponies?

[A1337STI]

Not to thread jack, but Kevin have you dynoed your Crazy/Awesome NA 2.5 build ? wondering what your whp/wtq numbers are ?

[sperry]

Originally Posted by scubasteve

20g sounds kinda like a build motor...hmmm not sure Im up for that quite yet

Ive read enough to get the idea that continuing to increase boost also increase heat which makes a less dense air charge = less that will fit in a cylinder....
but for the rest of it, I will take your word for it.

Whats your take on aftermarker upipe, maybe PNP headers to get a few more ponies?

A turbo has a range of compression efficiencies. Basically, it's the amount of air (flow) vs. the amount of boost (pressure). For some ranges of flow and pressure, the turbo does a good job of compressing the air... get outside that range and, like you said, the turbo is just a hairdryer heating up the intake charge. you can squeeze a little more power out of upping the boost, especially with a good intercooler, but really it's better to run a larger turbo in its sweet spot than to over boost a smaller turbo.

So while the 20g is a medium/large turbo, it's perfectly okay to run it at lower boost levels safe for a stock bottom-end EJ255. Because it will flow a ton more than the VF43, that same boost level is actually a bunch more power. Plus, you won't need to taper as much towards red line. I would bet a 20g at 16 or 17 psi will be over 250whp. At 18psi, close to that 300whp you're looking for. The stock STi cat-back might be a bit of a limit, but not terribly so. Your non-STi heads are far more the reason you're not getting STi power levels. Of course there will be some spool issues... the STi turbo is a wicked fast spooling turbo. So, your boost will come on a bit later with a 20g. But that can be combated with choosing the proper hot-side.

If you're not racing the car, the TD05-20g is a crazy awesome turbo on a 2.5l block. Spools like the STi turbo, but will get you to that 300whp level with proper supporting mods. However, this turbo is known to run high EGTs when WOT for long periods. Both MikeK and I have run it... it's an awesome turbo, but at the track, you'll start melting stuff like timing belt covers. On the street, it's unlikely to be an issue because you're not going full-throttle lap after lap, but it's something to be aware of.

The far more popular TD06-20g is a bigger hot-side version of the same turbo. It will spool later, make a bit more power, and run cooler. It's probably a little big for an unbuilt EJ255 w/ non-STi heads, but not by much. I bet there are tons of people running that turbo on the same block as you.

And there are probably like 40 other variations/tweaks of similar sized turbos... I'm just not up on the latest and greatest since I'm not turbo shopping for a Forester these days.

[scubasteve]

Wow, thanks for the input! That is some really good food for thought!
which company did your source your turbo from? I dont plan on tracking the car, it sounds like a really interesting idea.

What is the big difference with the STi heads? intake port size? cam size?

[cody]

No idea at all what is typical for your setup, but are your stock injectors holding you back? I know my tuner indicated that my stock injectors are hitting 100% IDC on a cool morning at sea level, which was never an issue at elevation...

I don't think it would be unreasonable for you to get a freer flowing CBE, an MBC (if your tuner is familiar with tuning for them) or an EBC and a retune, if others with your setup see better numbers on that dyno in similar weather. The boost controller won't increase max power, but it makes the car much more responsive (boost response), especially at lower RPMs.

[sperry]

Originally Posted by scubasteve

Wow, thanks for the input! That is some really good food for thought!
which company did your source your turbo from? I dont plan on tracking the car, it sounds like a really interesting idea.

What is the big difference with the STi heads? intake port size? cam size?

The STi heads flow better, have better cams, variable valve timing, and better oil breathers. They're the real difference between the 2.5L STi motors and the 2.5L WRX motors. The EJ257 and EJ255 blocks are the same for all intents and purposes, with just minor differences in the pistons and base compression ratios (which can get confusing when mating them to different heads).

[sperry]

Originally Posted by cody

No idea at all what is typical for your setup, but are your stock injectors holding you back? I know my tuner indicated that my stock injectors are hitting 100% IDC on a cool morning at sea level, which was never an issue at elevation...

I don't think it would be unreasonable for you to get a freer flowing CBE, an MBC (if your tuner is familiar with tuning for them) or an EBC and a retune, if others with your setup see better numbers on that dyno in similar weather. The boost controller won't increase max power, but it makes the car much more responsive (boost response), especially at lower RPMs.

He should have STi injectors at a minimum. Unless the FXT already came with the STi injectors. I was under the impression that the '05 FXT was identical to the '06 WRX in terms of drivetrain, which means he should have some top-feed STi injectors off something like an '07 STi.

Not only would small injectors be a huge limit on power, they'd be a safety issue unless the car's boost was drastically limited to stay away from 100% DC.

And Cory... if you're hitting 100%... get some injectors! Slap a set of deatchwerk 750's in there. It should be a simple rescaling of your injector map and some minor tweaking on-road. Not a major re-tune, since the injector latencies should be identical.

[Kevin M]

The EJ255 uses side feed injectors identical to EJ257 side feeds. Sometimes a fuel pump upgrade is a good idea for EJ255s though, and definitely if you go bigger than VF39/43.

[sperry]

Originally Posted by Kevin M

The EJ255 uses side feed injectors identical to EJ257 side feeds. Sometimes a fuel pump upgrade is a good idea for EJ255s though, and definitely if you go bigger than VF39/43.

I'm so confused... I thought the '04/'05 FXT got the setup that went into the '06+ WRX. Isn't the '06 WRX top-feed, just like the STi became? Is the '04/'05 FXT it's own deal?

[Kevin M]

I'm not 100% on the '06 WRX, but I think it's side feed and the '06+ STi is top feed. I know for sure that the early FXTs got side feeds.

[cody]

Originally Posted by sperry

He should have STi injectors at a minimum. Unless the FXT already came with the STi injectors. I was under the impression that the '05 FXT was identical to the '06 WRX in terms of drivetrain, which means he should have some top-feed STi injectors off something like an '07 STi.

Not only would small injectors be a huge limit on power, they'd be a safety issue unless the car's boost was drastically limited to stay away from 100% DC.

And Cory... if you're hitting 100%... get some injectors! Slap a set of deatchwerk 750's in there. It should be a simple rescaling of your injector map and some minor tweaking on-road. Not a major re-tune, since the injector latencies should be identical.

It's something to consider if I ever need a retune for some reason, but it's been this way for years so meh. I do have a Walbro but it wasn't enough, apparently. Who knows if my motor is still breathing that hard anyway. She's no spring chicken.