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View Full Version : Is there any Need for any backpressure on the sr.



DETeatsvipers
03-06-2003, 03:34 PM
The reason why i ask is because i have recently gotten a y pipe on my car and i opened it up at the track. The car sounds nice and revs pretty good but doesnt like to fall back to idle with the open atmostphere bov, instead it normally stalls if i rev it up a lot and let it drop back down. Also it seems like the turbo spools up faster with my pos 2.5 inch straight pipe with resonator one then with the whole exaust unpluged from the dump pipe down. Btw im using a small hybrid t25/t28 right now, maybe the advantages are not felt until larger turbo upgrades? It just feels like the car pulls harder in the low end and before full boost with the exaust still on. It seemed like my car also ran 3 tenths of a second faster with the exaust on and not unplugged any ideas.

Hugh
03-06-2003, 05:20 PM
the turbo is enough back pressure.

d240t2
03-06-2003, 05:32 PM
No...there is no need for backpressure! Ever!

DETeatsvipers
03-06-2003, 05:35 PM
yeah thats what i thought. Wierd. haha just seems to pull a little bit better in the mid range with the exaust on. ALso with the boost settings on the avcr be affected whether i have exaust or not, as in duty cycle. adjustment.

**DONOTDELETE**
03-06-2003, 05:38 PM
It seemed like my car also ran 3 tenths of a second faster with the exaust on and not unplugged any ideas.




My car also ran better with the exhaust attached. I think having the exhaust unbolted creates less exhaust pressure in the turbine housing causing it to spool slower... Does this make any sense? I've wondered about this too...

asad
03-06-2003, 07:19 PM
My car also ran better with the exhaust attached. I think having the exhaust unbolted creates less exhaust pressure in the turbine housing causing it to spool slower... Does this make any sense? I've wondered about this too...



Not from a physics point of view. The pressure differential is one of the things that drives the turbine. Less backpressure after the turbo means quicker spool up.

What you may be feeling is a change in the shape of the power curve, but it's HIGHLY unlikely that putting the exhaust on gives more power.

Asad

Cr0usEEE
03-06-2003, 07:20 PM
No...there is no need for backpressure! Ever!



This im not so sure of, for some reason a few of the 4 cylinder rides that i have worked on that are running some sort of open exhaust or one that has very little restriction (this is on NA) the engines have gone to crap from not having what i guess is considered enough backpressure. One in question was a crx with an intake, header, and just a straight exhaust. After about 40k miles the valves were fried and the head needed a rebuild. This was at 110k on a honda (yes its a honda but they do build very reliable engines). Also i have heard of this happening on the mustang 4.6. Has anyone else heard or seen a problem such as this, ive heard of about 10 cases from people im around just curious as to others opinion.

Oh yeah, turbo should provide plenty of backpressure.

DETeatsvipers
03-06-2003, 07:32 PM
i dunno about ruining heads and valvetrains on turbo cars but it could be possibly on certain hondas or fuel injected mustangs because of some other physical or conditional factors. Not really sure on that.

and Asad thats basically what i am trying to say that my car feels like the power curve has been offset. When i have open exaust it seems like the car does better at higher rpms but tourque is lost earlier in the rpm band, or it may start to drop. I think what i need is the replace my 2.5 after the y pipe with a three inch pipe. Then go to the dyno adn see if im just not halucinating. thanks

LanceArmstrong
03-06-2003, 07:48 PM
I heard that some backpressure is good because it keeps the gas/air mixture in the cylinders when cams overlap. Is that true?

BioSehnsucht
03-06-2003, 08:14 PM
No...there is no need for backpressure! Ever!



YES EVERYONE READ AND BE AMAZED!!!

It's not back pressure you want... ever.
You want to maintain exhaust gas velocity.

HERE (http://forums.freshalloy.com/ubbthreads/showflat.php?Cat=&Board=UBB21&Number=243388#Post24 3405) is a thread in the ARCHIVE about backpressure..

the post in particular I linked to gives a pretty good explenation.. I've seen better elsewhere but I don't recall where it was, it had gfx to explain & etc.. (not on FA) so..

But in the end, even a turbo car needs a proper (for application) exhaust system, but never ANY back pressure.. turbo or NA.