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Old 07-30-2004, 09:52 AM   #1
Corn-Picker
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Default Why does the USDM STi have a 7000 RPM redline and not an 8000 RPM redline?

I've come up with a few possibilities.

The 2.5 liter engine has a larger bore to stroke ratio than the 2.0 engine. Perhaps Subaru was worried that 91 octane wouldn't combust fast enough? Any engine design and its operating parameters are always based on the fuel you're burning, thus diesel (slow burning fuel) engines have extremely long strokes relative to their bores.

Perhaps the extra weight of the larger pistons in the 2.5 were too much for reliable operation over 7000 RPMs?

Maybe the Subaru engineers decided the car didn't need to operate over 7000 RPMs, they had enough power at that level and didn't want to make the car seem too high strung to the American buying market?

It just seems a little wasteful to me that a transmission balanced for 8000 RPM operation is limited to 7000 RPMs. I might want to work around this limitation one day, but not without knowing why Subaru engineers put the redline at 7000. Thanks a lot for any suggestions
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Old 07-30-2004, 10:16 AM   #2
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Larger pistons, same size rods. Also, bore to stroke ratio doesn't affect overall rev limit, really that much. You have to look at more of the piston speeds and the interia caused by those heavy pistons having to move so fast. Anyways, 91 octane combusts faster than 93 or 100 or C16(reason why you can run more timing advance with those higher octance gasolines). I have to call mechanical disadvantage from longer stroke and heavier pistons.
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Old 07-30-2004, 10:24 AM   #3
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Quote:
Originally Posted by eightballrj
Larger pistons, same size rods. Also, bore to stroke ratio doesn't affect overall rev limit, really that much. You have to look at more of the piston speeds and the interia caused by those heavy pistons having to move so fast. Anyways, 91 octane combusts faster than 93 or 100 or C16(reason why you can run more timing advance with those higher octance gasolines). I have to call mechanical disadvantage from longer stroke and heavier pistons.
I think we have a terminology difference, When I say combustion, I mean how fast the fuel burns once it's ignited. Ignition delay is how long it takes the fuel to begin combustion after it's been sparked (or compressed in a diesel).

Coal is an interesting extreme example of these properties, it has a very long ignition delay compared to gasoline or diesel, but explodes more than it combusts when it finally ignites
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Old 07-30-2004, 10:27 AM   #4
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Bingo... got ya.
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Old 07-30-2004, 11:06 AM   #5
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I would guess that because the loads induced by engine speed increase exponetionally with RPM and the VF39 is out of breath at 5300 they figured no need to put the extra stress on the rods and they could use the same ones they already had in the parts bin.
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Old 07-30-2004, 11:17 AM   #6
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i vote cams...
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Old 07-30-2004, 11:58 AM   #7
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I also vote cams....and valve spings.
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Old 07-30-2004, 12:39 PM   #8
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so if one were to go with a later spooling turbo, what would it take to increase the redline 500-1k rpm?
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Old 07-30-2004, 12:54 PM   #9
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The USDM STI uses higher lift cams than any other model, so the valvetrain may be one reason. I think the larger pistons may be more of a reason, although I think Jeff Sponaugle is running his hybrid setup to 8300 rpm.
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Old 07-30-2004, 12:57 PM   #10
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Arent the ports on the cylinder heads deeper? And camshafts are different on the RA Spec C...?
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Old 07-30-2004, 06:27 PM   #11
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I'm going to say the extra side-load from the larger bore causing increase wear on the rings and cylinder walls. I haven't checked the rod to stroke ratio, but if it's not terribly great with the 2.0L, increasing the bore size would exacerbate the problem. So more of a warranty concern than anything else.
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Old 07-30-2004, 06:52 PM   #12
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typer,
You mean longer stroke causing side loading and wearing on the rings. Bore size shouldnt have anything to do with side loading since that is caused by the angles at which the rods operate at inside the cylinder pushing the pistons against the walls. That is the reason that people look into R/S ratios a great deal for race motors that run at high rpm level constantly.
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Old 07-30-2004, 07:06 PM   #13
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A longer stroke will cause more side loading, but bore size counts too. The larger the bore the more the piston is inclined to rock about the pin, and a bad R/S ratio will make things even worse.
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Old 07-30-2004, 07:10 PM   #14
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Ok I see what you are saying... longer stroke causes side loading but the bore size and tolerances and ring spacing and all allow it to be more of a problem? I could agree with that
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Old 07-30-2004, 11:54 PM   #15
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Exactly.
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Old 07-31-2004, 12:10 AM   #16
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Well, the power curves I have seen for the car seem to start droping off
before 6,000RPM -- so I'd ask why you'd want to go to 8K ? At 7K you
are already beyond the power peak.

From what others have written it seems to be the turbo running out of
steam that causes the power drop. Perhaps a larger turbo and better
flowing heads would lift the top end of the curve and give a reason to
bump the limiter.

I do know that if the limiter was set, maybe, 200RPM higher then the STi
would totally trash the Evo in the 0-60 sprint. That 2-3 shift then back
on the gas has to be worth a good 0.5 seconds....

-Brian
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Old 07-31-2004, 10:16 AM   #17
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lol I was about to say the same thing, we dont make hp much over 5500rpms why in the world rip it up to 8k... when you shift you're way past your torque/power range, hence why people suggest shifting before 7000rpms and more like 6500 rpms... now if you have a huge turbo, thats different.

Robert~
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Old 07-31-2004, 11:16 AM   #18
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Of course we would need a bigger turbo. The attraction of 8000 RPMs is that you can get more power with the stock driveline. Torque is what kills transmissions, not power. We can get more power with the same torque if the redline is higher. I was curious as to why the redline was set at 7000, and there are a lot of probable answers in this thread.
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Old 07-31-2004, 03:35 PM   #19
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What do I have to do to rev like the RA Spec C at 8,000rpm..?
If I have a USDM 04 STI.
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Old 08-02-2004, 02:03 PM   #20
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Quote:
Originally Posted by FLEMON
What do I have to do to rev like the RA Spec C at 8,000rpm..?
If I have a USDM 04 STI.
i don't think there is an easy answer.
swap the cams to spec c, lose your avcs, plop on some sort of piggy back, set higher rev limiter...
see what implodes, spit, rinse, start again...
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Old 08-02-2004, 03:26 PM   #21
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Its piston speed

The USDM sti heads are the same shim set up as ej207 that revs the 8k

USDM sti cams have less lift than the Spec C cams by 1mm thats why it drops power after 6k. The Ports are also smaller on the USDM cars.

Last edited by bikerboy; 08-02-2004 at 03:31 PM.
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Old 08-02-2004, 03:27 PM   #22
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Quote:
Originally Posted by FLEMON
What do I have to do to rev like the RA Spec C at 8,000rpm..?
If I have a USDM 04 STI.

Buy a V. 8 spec c motor and put it in your car
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Old 08-02-2004, 05:38 PM   #23
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i think one of the main reasons would be because the japanese manufacturers know that americans love displacement. also, all they really did was get the 2.5L blocks from the 2.5RS's. fortify them change some of the internals and there you have the 2.5 STi block. not much of an engineering feat as the JDM 2.0L motors. so the extra 2.5L enabled engineers to get that little extra power from the .5 liters in each cylinder. although not a fully built from the ground up motor is the 2.5 block, it still can prove to be quite strong. in my opinion though, i would rather make more hp/torque with the 2.0L JDM block w/8000rpm redline than anything else. imagine the possibilities and wider power band. also, those JDM engines are built tough, just take the supra 2JZ engines, DAMN!!!!!!!!!!!! okay, you guys can flame me now. haha, i can take it.
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Old 08-02-2004, 08:00 PM   #24
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Were can I buy Spec C cams. Spec C cylinder heads etc etc?
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Old 08-03-2004, 12:44 PM   #25
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agreed
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