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Old 05-14-2001, 01:05 PM   #1
Jessie James
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Member#: 2072
Join Date: Aug 2000
Chapter/Region: MAIC
Location: Out of my mind
Question Why does HP drop off at high RPM?

Oka, I never really understood this, so someone please explain...

Why is it that as RPM increase, HP increases, until you get to the top (like, what, 6000RPM?) and then it drops off? Does the engine just stop making power?

if so, then why would anyone want to raise their redline?

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Old 05-14-2001, 01:15 PM   #2
Scooby Specialist
Member#: 1871
Join Date: Jul 2000
Location: Salt Lake City, UT
2000 Impreza 2.5RS
Blue Ridge Pearl


The engine just stops making power because it can't get enough fuel and air to make it. But, even tho you start loosing power, you can maintain a mechanical advantage by staying in a lower gear. You can also increase the upper level breathing characteristics of your car so that you have more power to use past stock redline...
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Old 05-14-2001, 01:17 PM   #3
Member#: 1904
Join Date: Jul 2000
Chapter/Region: TXIC
Location: Austin, TX
2017 Impreza Sport
E36 M3/4/5


go to www.howstuffworks.com and check out, noteably, how Cams work...that, for one, will tell you why. Also, everything the last guy said.
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Old 05-14-2001, 01:21 PM   #4
Scooby Newbie
Member#: 3844
Join Date: Jan 2001
Location: Helsinki, Finland
1993 Legacy 1.8


I think that the power beginning to drop is related to the valve system we use in our cars. The valve lifters doesn't work instantly and has a little delay caused by the mechanism. The lifters usually work with springs which isn't ideal mechanism, since the force it "transmits" is proportional to the extension. The delay prevents motor to suck air enough, which is directly proportional to the energy produced. Heh, just thinking, not sure

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Old 05-14-2001, 01:49 PM   #5
Kevin Thomas
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Member#: 110
Join Date: Jul 1999
Location: 1997 OBS, 1996 SVX, 1988 RX
1989 1989 XT6


"Why is it that as RPM increase, HP increases, until you get to the top (like, what, 6000RPM?) and then it drops off?"

I noticed this too. I believe it's the came profile of our engine. Each engine is designed for a specific purpose and usually torque rise and falls to support that purpose. Looks like torque in the 2.5 starts to fall off at around 4600rpm while the hp starts to fall off between 5600-5800rpm.

"Does the engine just stop making power?"

"if so, then why would anyone want to raise their redline?"

I don't have a clue. You wont be going any faster that's for sure. Actually, my take on this is that you'll be going slower. Why I say this? Well, when I switched ECUs from my stock 1997 OBS ECU to a 1996 2.5 Legacy ECU, my car got a little bit slower (about .1-.2 in the 1/4 slower). I tested this one night while at Atco. I just used the Legacy ECU because it got rid of my 108mph speed limiter. I'm assuming it was slower because it wasn't shifting at it's hp peak with the Legacy ECU. It shifted when my power was falling off at around 6200rpm vs 5900-6000rpm with my stock OBS ECU.

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Old 05-14-2001, 02:36 PM   #6
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Join Date: Apr 2001
Location: New Rochelle, NY
2002 WRX Sedan
WR Blue


tony you may be right about the valve train. but it has to do with the optimization of low end gunt vs uppend power. air needs time to get from one place to another and it also has mass so once it gets going it likes to keep going.

i could be wrong in a couple of these but i'm close to being right and it will explain why you lose power at the top of the rev range.
situation 1: optimum valve timing for low rpm
intake valves open after TDC (top of piston stroke) so that exhaust isn't pushed into the intake manifold.
intake valves close before compression so that the piston doesn't push the air/fuel back out of the compresssion chamber.
exhaust valves open close to BDC (bottom of piston stroke)so that the combustion can push the piston as far as it can.
exhuast valves close close to TDC so that the piston can push the exhaust out as much as possible and not suck any back in.

situation 2: optimum valve timing for high rpm
intake valves open slightly before TDC so that they are completely open for the down stroke. because of the speed of everything at higher rpms not much exhuast is pushed into the intake.
intake valves close after BDC because air has mass and therefore momentum and will keep coming into the engine evan as the piston moves up.
exhaust valves open before BDC because most power is generated in the first half of the power stroke. and this allows the expansion of the combustion to carry the spet gasses out of the engine.
exhaust valves close after TDC because the spent gasses will continue in the same direction because they to have mass.

you should notice that in the high rpm optimized situation the intake and exhaust valves are both open at certain points this is called valve overlap. if an engine that has its valve timing optimized for high rpms then un burned gases and spent gasses will get mixed to gether as well as unburned gass being pushed into the exhaust. another thing to realize is that the the length of time that the valves are open at 2000 rpm is 4 times as long as the length of time the valves are open at 8000 rpm.

you may want to raise the redline if you also switched out the cams and as well adjusting the cam sprockets for a higher rpm setup. of course if you do this you will lose low end power and possibly increase toxins in the exhuast.

[This message has been edited by TurboBoxer (edited May 14, 2001).]
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Old 05-14-2001, 02:44 PM   #7
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Member#: 1838
Join Date: Jul 2000
Chapter/Region: NWIC
Location: WA
2000 Impreza RS BRP
Vishnu Turbo/Tec3


It's also a built in safety. You could have no drop in HP right to redline but then you could over rev very easy.
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Old 05-14-2001, 06:50 PM   #8
Scooby Newbie
Member#: 5186
Join Date: Mar 2001
Location: Pittsburgh Pa USA
2000 2.5 RS Coupe


would the apexi safc help out in that case?
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Old 05-14-2001, 08:40 PM   #9
Scooby Newbie
Member#: 6604
Join Date: May 2001
2006 Impreza WRX STi
Black/Pearl Black


It is pretty typical for turbo charged cars to have this kind of an output characteristics. HP is just another way to express torque in relations to the rpm, so when torque begins to fall off in the higher rpms so will the hp. In a turbo engine, the amount of airflow depends on the turbo's ability to compress air. At higher rpm, the smaller turbo will be flowing close to its maximum, and the turbine will eventually stall out aerodynamically. I bet if you can swap in a larger turbo, you will see hp and torque gains high in the rpm range because larger turbos have a better airflow characteristic. The down side is more turbo lag because it will require more exhaust energy to drive the larger and heavier turbine. I am not sure about the Subaru engine(a newbie!), but it looks fairly squared so it should be able to rev easily. Some of the other members are right, the valve train might have something to do with it as well.

I also have a 240sx turbo charged and it does not like to rev, the power surges all the way to about 5500 and drops off sharply. The 240 has long strokes so it has more torque, but with shim buckets and hydrolic lifters it will not sustain high rpms.
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