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Old 05-18-2009, 06:43 AM   #9
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Originally Posted by chazly413 View Post
Here's a thread by williaty which includes his conclusions regarding track pipes and HFCs on a stock engine.
Excellent post, unfortunately, as I read it, it is talking about the 2.5RS UEL + SOHC cam engine. From what I have read, the 2.5i heads flow very well, although now looking at it, we have the SOHC. Another thing I liked seeing (though I don't know if it was for the 2.5i cats):

The cat part isn't surprising. There are some flow bench testing data of various cats floating around Nasioc and Google. In stock form, the cats have little problem flowing 200-250cfm before becoming resistive to flow.
Then stealing Pat's graphs and post

Originally Posted by Patrick Olsen View Post
As a counterpoint, I'll throw up my dyno comparison of a header/cat install. This was on my then-stock 2.5GT (N/A DOHC), same day dyno, going from the stock header/cat/mid-pipe to an MRT header/cat/mid-pipe. Not only did peak power jump ~14whp, it also shifted to the right ~400rpm. I don't have the Excel file here at work, but as I recall power was up +18whp at redline. Peak torque didn't go up nearly as much, but you can see it also shifted well to the right.

I've also posted in the past this comparison of a stock SOHC 2.5RS (I think it was a 2000) vs. my stock 2.5GT.

While I think it is generally accepted that the SOHC heads flow nearly as well as the DOHC heads, and the intake manifolds and headers on these two cars were essentially identical, clearly the SOHC is much more optimized for day-to-day driving midrange. Based on process of elimination, the difference in the torque curves on the two engines comes down to (1) ECU and (2) cams. It would appear to me that the SOHC cams/tuning are such a limitation that even when you free up the intake and exhaust you just can't get the engine to breathe more. That was definitely not the case with my DOHC, even with the stock cams.

Pat Olsen
Originally Posted by williaty View Post
I think that's probably a pretty valid conclusion to draw. I knew that the DOHC engines had a reputation for breathing better but then I had seen flowbench numbers showing the SOHC and DOHC heads basically right on top of each other. That pretty much leaves just the differences in cams to account for the differences in breathing.

I'll make the first post clearer than I'm talking about the SOHC engines.
Then, my 90% asleep brain decided to make a quick math problem to figure out my necessary CFM for my engine, thus need or not need for cat upgrade. As follows:

1 HP approx equals 1.45 CFM
1 CFM approx equals 0.0745 lb of air/min
0.108 Lb/min approx equals 1 hp
1 Meter cubed/sec = 35.314 CFS = 2118.867 CFM
1 KG/sec = 132 lbs/min approx equals 1771.812 CFM

power coversions:
1 PS = 0.9859 HP = 75 Kgf m/sec
1.3405 HP = 1 KW
1 HP = 746 watts
(Source is .... )

Using this formula, and the stock hp rating of my engine, it is this: 173hp (crank) x 1.45cfm = 250.85 CFM.

So, in conclusion, not knowing if the first quote above applies to all Subaru Cats or just 2.5RS cats, the stock set up is at it's limits just at stock form. I'm no physicist, so I'm sure there are far more variables, but if you want to start getting some numbers above stock form, a cat is not as bad an idea as it may seem. Not disproving Williaty at all because he knows about 500 times more than I do about these engines, but if I'm paper racing it seems there is some leniency to the cat debate. I'll probably edit this shortly with head flow benches because this post is big enough already and my computer likes to randomly refresh pages, thus losing posts

EDIT#2: I cannot find flow specs for the 2.5i heads. They may be the same as older models, but I'd like to know, and for sure. It may very well be the heads flow less than the exhaust at this time, and I'd need to know so i can adjust my mods appropriately.

EDIT#3: Hell, I can't even find a peak CFM for our stock exhaust systems, but I found that 2.25" STRAIGHT PIPE is 490cfm.
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Last edited by ScubaruImpreza05; 05-18-2009 at 07:51 AM.
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