Subaru Legacy Forums

Subaru Legacy Forums (http://legacygt.com/forums/index.php)
-   Logan's Run (http://legacygt.com/forums/forumdisplay.php?f=57)
-   -   Tune with a downpipe.....why? (http://legacygt.com/forums/showthread.php?t=188611)

turbodog 07-02-2012 07:43 AM

Tune with a downpipe.....why?
 
Can someone please offer a convincing technical explaination of why you would need a tune when installing an aftermarket downpipe. From what i have seen personally, the only difference is that reduced backpressure may result in difficulty with boost control, which can be fixed without a tune. I read for a while and ol / cl delay was mentioned. Please advise.

Lardo420 07-02-2012 07:46 AM

i thought you knew everything about everything?

cypher0117 07-02-2012 07:47 AM

if your car suddenly has less back pressure on the turbo you will probably overboost which can cause lean conditions and knock and then motor death.

laff79 07-02-2012 07:48 AM

dammit. Lardo beat me to it

turbodog 07-02-2012 08:03 AM

Quote:

Originally Posted by cypher0117 (Post 3963138)
if your car suddenly has less back pressure on the turbo you will probably overboost which can cause lean conditions and knock and then motor death.

Correct, there is no question this can be a problem, but it can be solved by changing restricter pills, running wategate boost, an ebc or mbc. Or even porting the wastegate in extreme boost creep cases.

Is there another reason? Is it a problem with boost spiking in closed loop fueling?

GTTuner 07-02-2012 08:05 AM

The timing needs to be backed down also.

paintpollz 07-02-2012 08:21 AM

Quote:

Originally Posted by turbodog (Post 3963157)
Correct, there is no question this can be a problem, but it can be solved by changing restricter pills, running wategate boost, an ebc or mbc. Or even porting the wastegate in extreme boost creep cases.

Is there another reason? Is it a problem with boost spiking in closed loop fueling?

andddd how much of a pain is it to get a $100 OTS tune?

KCwagon 07-02-2012 08:48 AM

Quote:

Originally Posted by turbodog (Post 3963157)
Correct, there is no question this can be a problem, but it can be solved by changing restricter pills, running wategate boost, an ebc or mbc. Or even porting the wastegate in extreme boost creep cases.

Is there another reason? Is it a problem with boost spiking in closed loop fueling?

I would choose to have a $100 off-the-shelf map, rather than changing restricter pills, using a MBC and porting the wastegate.

If you think all that work is worth it, and are confident that your car will operate safely, then go for it and report back.

SeeeeeYa 07-02-2012 09:01 AM

Quote:

Originally Posted by turbodog (Post 3963129)
Can someone please offer a convincing technical explaination of why you would need a tune when installing an aftermarket downpipe. From what i have seen personally, the only difference is that reduced backpressure may result in difficulty with boost control, which can be fixed without a tune. I read for a while and ol / cl delay was mentioned. Please advise.

I am curious about your thought process... what advantage to you believe an aftermarket downpipe gives that justifies a workaround, as opposed to tuning for the downpipe and reaping the normal stage2 rewards?

My first mod to my LGT was to gut my stock downpipe. The second mod was to gut my uppipe. I "noticed" a difference, and a dyno said I made 230 or so AWHP... puny and insignificant for the work those actions took. Within two weeks I had a Cobb DP on and Stg2 AP tune... now THAT made a difference worth the effort and money. I still got great MPG, but the car became alive with response and power.

If you have no interest in stage2, and tuning, you can get some gains out of merely gutting your stock downpipe... which is a snap to do, taking less than an hour.

Edit:

I don't think merely changing the pill will do what you believe. Different pills change the rate of boost development, but don't limit it, per se. And, if you chose an aftermarket downpipe without a cat, even directly connecting your compressor to the WG would still likely result in overboost under certain circumstances. Retention of the mid-pipe cat would mitigate, but not eliminate, such boost creep under all conditions. Removal of both cats would certainly be an issue. Maintaining both cats would help, but again begs the question: why the AM DP.

You better have the means and inclination to maintain continuous touch with your ECU via logging. That goes without saying with even the simplest of mods, but especially those that may cost a motor.

sumfoo1 07-02-2012 09:06 AM

boost comes up earlier too so timing needs to start retarding a few hundred rpms earlier.

Qckslvr 07-02-2012 09:19 AM

Yikes,

Ive been trolling this thread because I recently purchased a Tactrix cable, and am in the process of determining whether i want to take the plunge directly to stage 2.

Only now do I realize how amateur my understanding of the variables involved with forced induction tuning.

I feel like I know so little that I couldnt possibly undertake this jump as of yet. If anyone wants to keep explaining some of the direct implications of reducing restriction at the downpipe, you will have an avid reader following...

Lardo420 07-02-2012 10:00 AM

look at your stock downpipe and then look at the aftermarket one. if you cannot figure out what this will do, you need to immediately give up all hope of tuning your car yourself.

manletGT 07-02-2012 03:04 PM

OEM DP'on the left' vs CNT DP'on the right' (Gutted OEM DP, are you even trying?)

I gutted my stock DP, but after installing an aftermarket CNT DP...I noticed a difference in performance and sound. (Standard Stage 2 Cobb AP map)

http://i.imgur.com/O79v1.jpg

dr_sharp 07-02-2012 03:21 PM

Quote:

Originally Posted by turbodog (Post 3963129)
Can someone please offer a convincing technical explaination of why you would need a...

tl:dr
Because racecar

BAC5.2 07-02-2012 03:30 PM

Simple - Unpredictable boost control


All times are GMT -6. The time now is 04:36 AM.


LegacyGT.com