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Old 06-19-2010, 09:38 AM   #1 (permalink)
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Real world results from exhaust and boost change

Once upon a time I had a Legacy GT. Not the best car for economy, but it had enough grunt to tighten my seatbelt on a hard launch. And that's why this post is in the off topic section.

Up a certain hill near a certain town in NZ it goes from a 50km/h speed zone to 100km/h, up a steep hill and merging onto a highway (three lanes wide at the merge point). The average car can get to 100km/h by the top of the hill. My legacy could top 135 out of the box when I changed up at 6000rpm (for other reasons, but the redline was at 7000).

I then got a boost controller, and O2 sensor reading kit, and boost guage. So naturally I turned the boost up.

Going from 8psi to 14 resulted in a speed increase to 145km/h with no other changes. So I gained 10km/h.

Then I came across a cheap exhaust system. $100 for a 2.5" turbo back exhaust with a single resonator and no cat (not required under NZ law at the time). So I bought it and installed it. And my top speed rose another 10km/h to 155...

It's worth noting that the boost increase was really felt, wheras the exhaust had comparitively little (noise was another story, but I'm talking about power). I'm putting this down to the fact that boost affected peak torque soon afer I put my foot down, and it was unlikely that the exhaust was restrictive at 3000rpm. However once you got near 5000, the story would have been different.


Just thought I should share my story.
Ben

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Old 06-19-2010, 10:23 AM   #2 (permalink)
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The boost increase will bump up your power of course, the exhaust just lets that power come in sooner.

This is where the line is drawn between turbo and N/a gas engines. These performance upgrades can help your mpg with a turbo, but usually not with n/a. More torque down low can help mpg.

Of course thats only if youre driving for mpg lol.
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Old 06-19-2010, 01:41 PM   #3 (permalink)
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...acceleration is greatest at time zero, then tapers off to zero.

...velocity is zero at time zero, climbing towards an asympothic "limit" (HP vs aero-drag).

...hence, the quicker (and more) torque you can get sooner (turbo!), the faster the acceleration, but likely little more velocity at top end due to same aero-drag.
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Old 06-19-2010, 06:57 PM   #4 (permalink)
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Make of it what you will, those are the results I got. I'm assuming that the boost affected peak torque and the exhaust simply kept the peak going for longer. Which makes sense...

Incidentally with my boost controller, the exhaust raised peak boost, but only by 0.5 psi or so. This had to be put down to to pre-exhaust levels to aviod hitting the boost cut.
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Old 06-19-2010, 07:46 PM   #5 (permalink)
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...don't get me wrong, I'm not disputing your results!

...just saying that increased boost, sooner in the RPM curve, produces BIGGER results sooner, but typically doesn't change top-end velocity...unless the additional boost is also maintained higher up the RPM-curve (dangerous).

...available HP and vehicle aero-drag are what limit top-end velocity. But, if the engine can produce more HP at a higher RPM, then the upper MPH limit will be increased, but only slightly because aero-drag is a cubic-functon of velocity.
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Old 06-19-2010, 08:58 PM   #6 (permalink)
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Thanks to the disign of the boost controller (it's a pressure reducing valve, not a bleed valve, and so has a small amount of 'feedback') the boost curve was flat. So my 14 psi was seen at the top end as well as when boost initially hit.

Oh, none of the speeds were power limited top end, they were acceleration runs that cut of after a defined distance that was also uphill. With my boost controller set to 4psi (wastegate pressure on most electronically controlled cars) it would hit its 180km/h limiter given a half decent straight road, but up the test hill the car struggled to top 110...
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Old 06-19-2010, 10:03 PM   #7 (permalink)
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...curious, do you work in ballistics by any chance?

...I'm thinking: BLSTIC = ballistic.
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Old 06-19-2010, 10:17 PM   #8 (permalink)
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That's what it means allright. I chose it because 1) It's unique in forums (I have the same username on perhaps 15 different forums) 2) I saw it on a highly modified 260Z when I was 12 and 3) It means hard and fast, explosive, and totally isn't me...

No relation to the field of ballistics at all. Except I made a PVC cannon that fired mangos once...
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Old 06-19-2010, 10:39 PM   #9 (permalink)
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the subes love the changes...
the EJ engine does not last as long, but hey..get what you want while you can.

N/A and change, oddly enough, was a must do for my own subaru. Not often does these respond to it, the subes really do. they are a conservative bunch.

Unless reminded of EA series and trochoid oil pumps. Little monsters squashed into runts for reasons I have yet to decipher.

I had to make a 90hp climb a short city onramp onto american lunacy highway..and it did it. Gave it some old fashioned american hot rod ideas. there is extra to be had from them. My own is over double what they claimed for mpg, and I am a heck of alot more power as well.

My locale hacks the hell out of the subes. Occasional exhaust troubles with laws, other than that, they just go...alot better than where it came from.
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Old 06-19-2010, 11:06 PM   #10 (permalink)
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Dunno about not lasting. I had mine from 27500km to about 295000. She had her quirks, but it held up to my thrashings allright. I know it hadn't had an engine swap because it had no major issues from when it was imported (at about 90,000km)...

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