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Old 09-07-2005, 10:43 PM   #1
G.Subramaniam
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Default Common causes of failures of RS-T

Most failures of RS-T , based on what I read on this forum seem
to be pistons, piston rings and head gaskets

due to minor detonation at 5-10 psi

Whereas motors deliberately built for boost, seem to have more robust
pistons and piston rings

Would swapping pistons and rings to better quality brands such as
Crawford, prevent most of these failures ?
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Old 09-07-2005, 10:59 PM   #2
Wedge
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And keeping the same compression 10:1? Whats the point?

I seriously doubt it. Somethings gotta give when high comp and det happens.
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Old 09-07-2005, 11:19 PM   #3
Master2192
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Det will kill any engine. Some people are running RS-Ts on a stock motor at near 15 PSI of boost. This takes some nice intercooling and EM. Det is what kills these motors, even the turbo'd motors will die from Det
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Old 09-08-2005, 06:43 PM   #4
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Dets happen all the time, and a big Det will kill any engine
whereas it seems that even small Dets which are more common
break NA engines and I was thinking of fixing the most common NA problems
Despite the open-deck vs semi-closed deck comparisons, very few blown RS-T are blown at the cylinder, instead it seems to happen at pistons, piston rings and gaskets even on minor Dets
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Old 09-08-2005, 08:06 PM   #5
Matt Monson
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That doesn't mean that the cylinder wasn't walking or ovalizing and didn't contribute to the ring or piston failure.

But detonation is the primary killer of RS-T's on stock compression. If you are looking for a margin of safety do 2 things:
1.get good engine management.
2. Make sure you intercool it well.

Beyond that, you don't need to spend a ton of money on expensive rods, etc. But dropping the CR with a good set of pistons or an Ej257 bottom end would be a good cost effective way to approach it...
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Old 09-08-2005, 11:26 PM   #6
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Well thats exactly what I am in the process of doing right now.. Blew out one of my pistons on my 98 RS after turbo'ing (long story), at least it wasnt my fault... But I am now swapping out my 98 RS block with an STi shortblock... Not only will it be stronger, but it will be a lot safer for turbo'ing...
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Old 09-09-2005, 03:36 PM   #7
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Supply enough fuel and keep the boost down you can last on a 10:1 stock block - it's when you run into fuel starvation or too much timing advance that will get you. For RS-T, EM is just as important if not more important than fuel. Granted who am I to speak when I still have stock pump, stock injectors and stock FPR and rely on a Link-1 to pull timing and push the injectors beyond 100%... keeping my fingers crossed that my head gasket, pistons and rods are staying intact.
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Old 09-09-2005, 03:44 PM   #8
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The thing is that you can't run insane amounts of timing in these cars as well. Because of the higher compression the spark doesn't need to fire so soon. You can get away with running 11:1 AFR's for some time. I mean, you could build the engine. But why? Subaru makes affordable swapable solutions from the the factory that are completely compatible with the SOHC EJ25 head. You can do a 2.2L or a 2.5L block complete for less than what you would pay to swap out the pistons, rings, rods etc...

Most of the problems I've seen with RS-T's is lack of engine management, which in turn leads to detonation. Second would be oil line failures. Good luck. I had an RS-T for 3 years before finally doing a EJ20 swap.
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Old 09-10-2005, 12:46 PM   #9
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Definitely wouldn't spend too much trying to build on RS-T, however, if you're already down that road, just make sure you have the best EM and fuel supply. I see RS-T as the cheaper way for quick boost and not necessarily much head room for growth. As I said many times, if swaps aren't that expensive to come by when you're said and done, the idea of an RS-T would have been shot - at least up here, swaps far out-cost RS-T so unless you're looking beyond 300hp, it's just too much to swap.
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Old 09-10-2005, 12:55 PM   #10
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For street driving, RS-T seems to be the best
Off boost you get excellent torque, thanks to high CR
Even in freeway driving, how often do we exceed 3500 rpm
RS-T with 5 psi with safety measures seems to be the best for daily drivers
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Old 09-10-2005, 02:58 PM   #11
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I would tend to agree - I thought about going up in power but after getting my kit, I feel that a good tune is all I need and maybe the fueling upgrade to ensure reliability. It's not like I'm a track/drag junkie. I guess with the stock gearbox, the punch of an RS-T is good enough except when you're caught in 4th or 5th at below 3000, and even that can get up to speed very quickly. The change over at torque peak (4000rpm) is amazing, relatively speaking
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Old 09-11-2005, 12:33 AM   #12
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[quote=G.Subramaniam]For street driving, RS-T seems to be the best
Off boost you get excellent torque, thanks to high CR
Even in freeway driving, how often do we exceed 3500 rpm
RS-T with 5 psi

5psi of boost seems to be the ticket. Above it is asking for trouble. You can get away with 5psi of boost with some minor fuel adjustments.
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Old 09-12-2005, 12:24 PM   #13
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Majority of FAILED RS-T's are engines that are not tuned or not tuned well enough with engine management.
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Old 09-14-2005, 02:25 PM   #14
lloydc33
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Question

can someone explain in more detail det and also what would be considered good engine management for a putting a turbo on an RS w/o replacing pistons, rings, ect. I'm looking to shoot 9 psi into the engine. Any input welcome.

Last edited by lloydc33; 09-14-2005 at 02:58 PM.
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Old 09-14-2005, 03:22 PM   #15
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Detonation (predetonation) occures when the fuel and air mixture exploads (detonates) before the engine expects it to (or before it is timed to do so in its cycle) the explosion is trapped because the exhaust valves will not open for it to occure. You hear a knock or ping when you have detonation. This damages the pistons and can cause actual peices of the piston to break off and damage the valves and this is usually the end of your engine.

Causes:


too high compression ratio due to either increased boost or higher compression pistons, or any combination.

Bad gas

high intake air temp

Ways to reduce detonation:


Retard ingnition timing
lower boost or compression ratio
Increase Octane of gas
lower intake air temp by intercooling or relocating where the air is drawn from

I think that pretty much sums up detonation, if i missed any thing im sure people will add to that list.
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Old 09-14-2005, 04:38 PM   #16
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tuning, tuning, and tuning again
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Old 09-14-2005, 06:40 PM   #17
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We need a cost to power ratio for RS-T vs STI v4/5 vs STI 7/8
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Old 09-14-2005, 08:47 PM   #18
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Depends on what power level though - I'm sure RS-T fares pretty well below 300hp. Then of course, how you factor in reliability and all the other stuff is what matters in this comparison
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Old 09-14-2005, 09:16 PM   #19
WRC 555
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Well some of you may remember my dyno charts I put up when I ran 16psi on a T3/TO4B. I ran 16psi+ for almost 2 years. It saw almost 26K miles of over 12psi. My car was tuned as best as it could have been. In the end a rod gave out while cruising at around 25% tps!! So I knew it was not a tuning issue but just overall stress. I always predicted I would either brake a rod or blow a head gasket

So like kgb said, depends on desired power level. You wanna push over 300WHP, save yourself the time and agony and put in the EJ257 I just wanted to test the limits of the internals of the EJ251 with its high compression.

Some pics for your viewing pleasure:



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Old 09-14-2005, 09:45 PM   #20
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OUCH!!!! that definitly hurt! Did that motor knock at all?...not a detonation knock, an internal knock.
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Old 09-14-2005, 09:47 PM   #21
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hey ive been running my rs-t for about a month now and its doing great all im using is the wrx intercooler and the old school legacy turbos.. but its boosting pretty hard. so far what ive heard seems to be right... just tuning tuning and more tuning.. in total ive spend at LEAST 20 hours tuning my rst and its still has a few minor bumps
and also DONT RUN CHEAP GAS lol
thats my two cents

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Old 09-14-2005, 11:22 PM   #22
WRC 555
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Quote:
Originally Posted by Kawikid20o2
OUCH!!!! that definitly hurt! Did that motor knock at all?...not a detonation knock, an internal knock.

If you mean rod knock, no, once it blew it blew There were no warnings, just a big boom. Sounded like a gernade went off and my oil pressure dropped to around 15psi! I quickly turned off the engine and thought maybe I blew an oil line cause of all the smoke. Popped the hood and looked under the intake manifold and...well you get the picture
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Old 09-14-2005, 11:29 PM   #23
Kawikid20o2
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jeez, when my oil line blew i got this horrible rod knock at even throttle and then it seized on the highway not but 30sec after the knock started. Too bad i had no oil pressure gauge...bad mistake, could saved it.
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Old 09-15-2005, 12:59 PM   #24
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Hence people have been recommending not to run too much boost on an EJ251 - 5-6 on a daily driver on stock injectors, and I'm guessing 8-9 the max otherwise the internals just don't last all that long even with enough fuel to prevent detonation
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Old 09-15-2005, 05:22 PM   #25
WRC 555
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I think for a open deck motor with a 9.7:1 compression, on N/A internals it did a pretty damn good job! I mean my motor pushed over 265whp for over 26K miles (12K of those at 300whp)and that includes 4 track events and a few drag races. But in the end yes for the EJ251 I think 8-9psi should get you pretty far which the right tuning and EMS!
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