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Old 09-12-2001, 01:15 PM   #1
Jaxx
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Default Adjustable Cam gears for SOHC

sooo.. we have been hearing rumors that aem will make them ...

is this usefull..with adjustable gears on a SOHC exaust and intake be retarded/advanced at the same time ...


help here little lost?

-j
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Old 09-12-2001, 06:59 PM   #2
yamarocket630
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typically retarding the cam will increase peak power, and move the peak higher in the revs (not a good thing unless you can increase the rev limit safely, ie better valve springs etc) advancing the cam does just the opposite. could be usefull for eliminating lag on a turboed car
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Old 09-13-2001, 10:56 AM   #3
Jaxx
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ooookay ...


so what would it do for a N/A engine


- it seems from their (AEM) web site that they are in fact not making cam gears for he 2.5 but for the DOHC wrx instead .. which i understand.
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Old 09-13-2001, 12:39 PM   #4
AllWheelDrift
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It has the same effect on a N/A or Turbo engine. As yamarocket630 mentioned, retarding cam timing moves the power band up (resulting in more peak bhp but hurting the bottom end.) Advancing cam timing moves the powerband down, which results in more low end at the expense of the high end.

Since the SOHC ej25's powerband begins to drop off well before redline, a small amount of retard could probably increase performance without a need to increase the rev-limiter.
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Old 09-13-2001, 12:40 PM   #5
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Are you sure about retarding adds more power?

By advancing the cam you are making all the ports open and close earlier in the cycle. In case you don't know, cams are usually timed such that the inlet port is open before the engine hits top dead center (TDC) and closes after bottom dead center (BDC). Yes, this is correct, the intake port actually closes after the pistons has begun its travel back up the cylinder. This causes the potential for some air/fuel mixture to be pushed back out of the cylinder, decreasing power.

By advancing timing you aleviate this somewhat by having the intake port closer earlier on the upstroke, lowering the potential for air/fuel to be expelled back out. More air/fuel = more power.

Also, the exhaust valve is timed to open shortly after combustion on the downstroke of the piston. This is done so that the piston does not have to push the gas out of the cylinder, this would reduce power. Instead the pressure of the gasses inside the cylinder are utilized to help the evacuate from the cylinder, which is why the exhaust port opens up well before BDC on the combustion stroke.

By advancing the timing of the exhaust valve you can potentially allow more gas out of the cylinder before the next upstroke, leaving less for the piston to push out and increasing power. Obviously you don't want to do it too early or you'll rob the cylinder of its gasses that are expanding to push the piston down too early.

Also, by closing the exhaust port earlier, you decrease the amount of exhaust gas that may be sucked back into the cylinder for the next cycle. This gas is high-temp and so would encourage detonation, and also ocupy volume that incoming air/fuel mixture could take up.

So advancing cam timing should provide performance gains assuming the manufacturer hasn't already pushed the envelope, which I don't think many have. I think that retarded cam timing helps emissions (possibly by "reburning" more exhaust??), which seems to be more important to most manufacturers than top performance.

Comments, questions, flames? I think I'm right in my reasoning here...
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Old 09-13-2001, 04:02 PM   #6
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Also, not sure if this is true or not, but doesnt the ECU take the timing measurement for the spark from the cam position? would this also move the spark in relation to piston position?
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Old 09-13-2001, 04:21 PM   #7
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I know Unorthodox racing is developing them now for our 2.5 lt engine.

http://www.unorthodoxracing.com/subaruts.html
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Old 09-13-2001, 04:31 PM   #8
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That has no information... just that they are making a subaru app. I bet it will be EJ20.
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Old 09-13-2001, 04:31 PM   #9
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I'm guessing that if the spark is cam triggered, it's probably done from a sensor on the cam gear itself. The adjustability would come from moving the camshaft in relation to that gear.

So the gear would be installed such that the spark even thappened at the correct time relative to piston position, and then you could adjust the camshaft to provide the advance/retard that you want.

I guess you could probably also use the adjustable cam gear to dial up ignition advance/retard if you wanted to. Just install the cam gear advanced or retarded from the crank gear, then dial in the camshaft adjustment to account for that too.

Of course all of this is if the spark is cam triggered. I seem to remember something about a crank trigger, but I could be thinking of a TEC-II.
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Old 09-14-2001, 12:10 AM   #10
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oook

still confused .. sounds like every one is...


i understand the theroy of opening intake valve early ...
but if you also opened the exasut valve early woun't that be bad ...

ok where are the experts?
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Old 09-14-2001, 04:10 AM   #11
iodine23
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Like I said, it actually helps to open the exhaust ports somewhat early. That way you use the pressure of the gas to let it expel itself from the ports rather than having the piston do the work of pushing it out. The piston is rising both from it's rotational energy, but also from another firing cylinder. So you'd be using some of that other cylinder's combustion energy to push gas out of the previous cylinder in the firing order rather than push the wheels faster.

At some point you'd advance too far and rob the current cylinder of its energy to drive the piston because you'd just be blowing the inital rapidly expanding gases out the exhaust rather than using them to drive the piston.

If you retard too far you end up blowing more of the air/fuel mixture out of the late-closing inlet valve, robbing the engine of power.

BTW, I'm getting a lot of my information from "Modern Engine Tuning" by A. Graham Bell. It is a very good book about the effects different tuning parts can have upon an engine. It is kind of dated, though... so some of the information may not be quite up to snuff.
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Old 09-15-2001, 12:20 AM   #12
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Sean I was unaware you were so informed. I can see where his logic is at.....I thought Alex said the gears were going to be available for the RS also

Andres
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Old 09-15-2001, 05:01 PM   #13
iodine23
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I've been doing a little bit of reading lately.

I want to be sure that I know exactly what's going on before I go and get crazy tuning the car. For like $50 you can get Modern Engine Tuning and Maximum Boost from Amazon and learn a lot of stuff like this.

And then you don't have to rely on coming to the i-club and having people say "I thought it did this" or "I think it does that." It also lets you see through any of the smoke that some shadier vendors may try to blow your way. Though it seems like most of the subie people are good folks and aren't gonna jerk you around.
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