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Old 08-09-2009, 03:14 PM   #1
Back Road Runner
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Default [QUESTION] Folk with modded intake (TB, manifold, heads, cams); TQ above 5.5k rpm?

Further explaination:

I'm curious what some folks have done to help extend top end power gains. I'm mainly concerned about the torque band and the typical roll off at around 5000-6000 rpm of these engines. I'm guessing this is mainly a side effect of a flow restriction, and I'm curious what changes are seen by folks who have modded the intake hardware. I know cams have been done as well as CAI. However, I'm more specifically focused on the throttle body, intake manifold, and heads.

Background:

Old Cobb tech articles on the Subaru engines:
http://www.gigafiles.co.uk/files/185....5L%20SOHC.doc

Flow of the stock system:
TB + intake manifold : 236cfm
Head + intake cam : 240cfm
Head + exhaust cam : 182cfm
Intake piping 2.5" OD, >2" ID : >350cfm
Stock exhaust : around low 200s to 250cfm (me guessing, stock piping, stock cat, stock muffler)
Aftermarket exhaust : >300cfm

A 2.5L engine 100% efficiency flow rate (just an air pump)
2000 rpm : 89cfm
4000 rpm : 177cfm
6000 rpm : 266cfm
8000 rpm : 354cfm

Scavenging:
The concept of scavenging is that excess intake air goes through the combustion chamber to clean out the chamber and ensures a full, clean intake charge. The 2.5L engine is only 2.5L from the block. The head holds 1/10 more air (10:1 compression ratio). With the head volume, the engine is effectively a 2.75L engine, or so the concept goes. It assumes extra flow actually goes through the engine, partially to clean out the exhaust charge and partially to put in more air and fuel into the chamber.

For the same rpms
2000 rpm : 97cfm
4000 rpm : 194cfm
6000 rpm : 292cfm
8000 rpm : 388cfm

VE > 100%:
As well, it is possible with pressure wave tuning that you can actually get VE above 100%, i.e. creating forced induction over a very small power band. You could effectively have a small amount of more flow capability.

Now this is what the engine can flow. Any restrictions will of course limit what the engine can do and VE drops off. It's just a big air pump, and if you can't pump the air, the output becomes limited. The intake seems to be a big part of the limitation. If the engine can only flow 250cfm, it will allow only for peak power up to around 5000 rpm and by redline at 6500rpm, VE can only be 80% and no more, given the flow limitation. In fact, this is what we see from the stock engine design if we dyno a stock car. We typically address exhaust and intake pre TB, but we don't typically address the throttle body itself, intake manifold, and in many cases the heads or cams. Normal intake mods do involve cams and sometimes head work, but the throttle body and intake manifold may never get touched.

I'm curious which folks actually HAVE modded their throttle bodies, intake manifolds, and heads and actually have dynoed the car or at least data logged and/or road dynoed their cars. I'm curious what power bands you end up seeing from these mods.
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Last edited by Back Road Runner; 08-09-2009 at 03:24 PM.
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Old 08-09-2009, 04:25 PM   #2
rivman05
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Is there a performance throttle body? I know a while back a friend took the tb off of his 05 Mustang and put it on his wrx. had to fiddle with the tps, but got it to work. Its a quite a bit bigger, than the one on it. I dont know if this helps for the tb. I have seen custom intakes on the boards, looked cool from the pics.
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Old 08-09-2009, 06:29 PM   #3
Back Road Runner
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Well, I'm curious specifically why the Subaru engine tops out at around 5.5k rpm. I'm thinking it's mainly an intake issue. Even with cams and a full exhaust, the engine still tops out around this point. The intake on our cars are largely overlooked. Some folks port the heads. Almost no one touches the intake manifold or the throttle body. I'm sure a few folks have done this however, and I'm curious if and what the gains were. I just haven't seen anyone show dyno results of any cars with a modified throttle body or intake manifold. I kind of feel it's a big deal that's largely overlooked. For example, the intake manifold is largely equivalent to the exhaust header. Subaru has revised the intake manifold some. The newer designs do flow better then the older ones although they are not a direct fit between the cable throttle and throttle by wire versions and mounting points appear/disappear/change location. The 05 intake manifold is well preferred in design, better flowing then the older ones. As well, dyno plots of the newer stock engines do also have better top end power then the older versions. I somewhat question how much a P&P gains you though over stock. It's questionable how much material can be reached and removed to improve flow. I'm somewhat surprised there are basically zero aftermarket intake manifolds. A lot of other companies do have a little bit of aftermarket selection for these parts. Subaru has zero. There are a few custom options for the turbo cars, but that's it. There has been some attempt retrofitting turbo intakes on NA and various NA years on different year cars. With enough fabrication, you can make anything fit, but I'm curious what best works.
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Old 08-09-2009, 06:46 PM   #4
BillyMagnum
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is there a wider mm throttle body out there for the imprezas?
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Old 08-09-2009, 09:18 PM   #5
Patrick Olsen
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I have a ported one, can't remember who I had do it, but it's opened out to I think 67mm (stock is 62 or 63mm for my Phase I DOHC). I can't imagine that the TB by itself is really going to accomplish anything - our stock TBs are already pretty damn big for the amount of power we're making. Consider that one of my other babies, my '89 Mustang GT, came with a 55mm TB on a 225hp engine. Even on the 5.0L, bumping the TB to 65mm was only good for a couple horsies.

I have a ported intake manifold from Grimmspeed. One of these days, if I can ever get my car to run again , I intend to dyno it with a stock intake and then again with the ported intake. I'm curious if it will help the top end at all.

It certainly seems that cams help (duh! ), although I'm curious if it's the intake side or the exhaust side that is responsible.

Here's a dyno graph showing a "stock" JDM EJ25 off Ebay with an Injen CAI and full exhaust (OBX headers, high flow cat, Stromung axle back):

Just as Back Road Runner said, at 5200rpm the torque curve noses over into free fall. The AFR is the same throughout, so it's not due to it running rich (although without my old S-AFC, that was certainly part of the problem).

And here's all the same intake and exhaust bits but on my old engine with Cobb heads and cams:

Note that the torque curve drops off, but not nearly as severely. (Sorry, the green curve against grey background is kind of hard on the eyes.)

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Old 08-10-2009, 03:07 AM   #6
williaty
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It is, at least in part, the cams. With the Delta 2000 grind cams, I saw nothing like the stock drop off at 5.5k. The airflow continues a good extra 600-800 RPM and the subjective feel of the car, as well as timing that tach needle, says the torque is flat to 7k from about 4.5k. If the feel+needle are anywhere near right, it would have to be partly from the sustained airflow (until it begins to drop off) and partly from less energy spent on pumping above the flow drop off.
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Old 08-10-2009, 10:37 PM   #7
yarrgh
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Your math is semi wrong. 99.5mm bore x 79.0 mm stroke. That turns out a displacement of 2.457 liters(given in the manual). Add 10% of that for the heads and you end up with 2.70L. Ok, not much of a difference.

Anyway, here's what I have to offer: the red line is the old reading with IM and TB done, blue is after PNP the heads. I didn't bore out the TB, just rounded off the corners on the filter side and matched it up to the gasket, and took down the throttle plate mounting screws. For the IM, I smoothed the hell out of it where it mates to the TB and as far as I could into the runners, I tried to port match it to the heads, but it's not perfect.

The car had COBB mild cams all along, which I think is similar if not equal to Delta 1000.
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