Is there any performance hit in taking an open dump (externally wastegated) and routing it back into the exhaust system?

There will be some, but it can be minimized if merged downstream and at a shallow angle.  If it's a relatively lower horsepower setup, then the change will probably be pretty minimal as there isn't a ton of exhaust flow in the first place.  Of course, this assumes there's a decent sized exhaust on the car.

 Nah.... 3" is plenty big for 400whp.  I want to say the Evo guys didn't see any benefit to going bigger until around 600whp.  Heck, some S2000s are making almost 400whp on the stock exhaust which is 60mm I think.

I think recirculating the dump will barely make a dent in your power, just make the merge angle as shallow as possible.

Posted By Lessendz on 12-04-2011 06:27 AM
my opinion is that judging exhaust pipe size by wHP levels is waaaaack
a full run of 3" mildsteel or stainless helps to put you Where? on a 400whp turbo's compressor map??

recirculating the W/G isn't suppose to increase power..
its suppose to improve response? right?

It's actually not that wack.  Horsepower is a function of mass airflow; the more mass airflow, the more power.  That airlfow goes out the exhaust.  So for the most part, any car making 800whp is going to flow about the same mass flow of air and therefore exhaust (assuming similar fuel and BSFC).

Exhaust needs to be sized to match the exhaust flow.

Recirulating the W/G is for noise and emissions reasons.  For an OEM, all the exhaust flow has to go through the CAT for emissions.  From a track car perspective, it's just loud as hell dumping.  Dumping makes more power though as that effectively increases the exhaust size; all the exhaust flow getting dumped reduces the mass of exhaust gas going through the main exhaust and therefore reduces the backpressure.

OMG..  i'm not interested in turbo's & backpressure..
Are you?? 

i'd size a turbo exhaust by space..  got the space? run the damn size!!
smaller(ish) exhaust piping from the turbo won't help you achieve favorable Turbine Pressure ratio's 

BTW:  you shouldn't  "assume" anything bOss...
i'm talkin Response & yer talkin Power

Dumping doesn't increase the exhaust size..
it reduces the exhaust pipe length/volume thus changing the pressure ratio..

the theory about  W/G recirculation is for 1 continous exhaust flow   &   "steady"  turbine pressure ratio

Posted By Lessendz on 12-06-2011 04:58 AM

OMG..  i'm not interested in turbo's & backpressure..
Are you?? 

i'd size a turbo exhaust by space..  got the space? run the damn size!!....."

 

/\ That makes me think of this for some reason \/

Posted By meisterschutze on 11-29-2011 02:51 PM

Saw this in a lowes parkin lot in Indiana, PA.  I swear its NOT photo-shopped!

 

www.facebook.com/photo.php

Posted By Lessendz on 12-06-2011 04:58 AM

Dumping doesn't increase the exhaust size..
it reduces the exhaust pipe length/volume thus changing the pressure ratio..

 

Actually it does effectively increase the exhaust size.

Scenario #1:  Recirculated.  All the exhaust flow (70% through the turbine, 30% through the wastegate) goes through the exhaust 3" exhaust, about 6' long.

Scenario #2:  Dumped.  The 70% of total exhaust flow going through the turbine goes through the 3" exhaust.  The other 30% goes through the dump, say 1.5" tube a foot long.  So now the total exhaust is the 3" diameter/6' plus the 1.5"/1'.  Sure, the 1.5" tube is small diameter, but it's also very short so the pressure drop isn't that bad.

oh Boy
forget the pipe size bOss ya confusing yerself
oversimplification of a complex subject isn't scoring you MotoIQ engineering points right now
Pipe = thick walll
Tube = thin wall
i already stated 3"(O.D.) stainless or mild steel are both rather Small (I.D.)
once you subtract out the wall thickness of whichever material u choose to use for the 400whp turbo exhaust

the purpose of the W/G is to divert SOME of the flow in an attempt to stabilize boost pressure
DUMPing causes exhaust gasses to suddenly wanna take the Easiest unrestricted route out of the system
that's Mos def NOT thru the turbine blades, nor the full exhaust system following it..
The likely situation is likely TOO much exhaust gasses will take the easiest path & skip out on driving your turbine blades
thus resulting in a W/G "hunting"? to try and stabilize boost..

remember kids..
chasing TaiL is only good if its in a Short Skirt !!!

Posted By Lessendz on 12-08-2011 09:34 PM

oh Boy
forget the pipe size bOss ya confusing yerself
oversimplification of a complex subject isn't scoring you MotoIQ engineering points right now
Pipe = thick walll
Tube = thin wall
i already stated 3"(O.D.) stainless or mild steel are both rather Small (I.D.)
once you subtract out the wall thickness of whichever material u choose to use for the 400whp turbo exhaust

the purpose of the W/G is to divert SOME of the flow in an attempt to stabilize boost pressure
DUMPing causes exhaust gasses to suddenly wanna take the Easiest unrestricted route out of the system
that's Mos def NOT thru the turbine blades, nor the full exhaust system following it..
The likely situation is likely TOO much exhaust gasses will take the easiest path & skip out on driving your turbine blades
thus resulting in a W/G "hunting"? to try and stabilize boost..

Eh?  Wastegating is to regulate turbo speed and therefore boost which you appear to understand.  Who the hell cares about the definition of 'pipe' and 'tube'?  That's irrelevant to this discussion.  We're talking about recirculating the wastegate flow back into the exhaust system post turbine or just 'dumping' it to atmosphere and whether or not there's an associated power loss with recirculating as opposed to dumping.

If you recirculate, then all the exhaust gas has to go through the single exhaust path out.  Guess what, more mass flow = more backpressure.  If you dump the wastegate flow, then there's less mass flow going through the main exhaust resulting in less backpressure that the turbine wheel sees.  As you now have two exhaust paths by dumping, the original plus the extra for the wastegate flow, the overall exhaust is effectively bigger.

The original question was whether or not recirculating at this current power level, and therefore mass flow rate, level would hurt power.  It's my estimation that the affect will be minimal based on other people's testing in the 400whp-700whp range using 3" and 3.5" exhaust systems.

On an engine making 100hp, going from 3" to 4" will do practically nothing.  On a 1000hp engine, going from 3" to 4" will significantly reduce exhaust restriction.  Run the numbers if you don't believe me.

For max power on a turbo car, there'd basically be no exhaust to have no backpressure on the turbine.  Of course, on a street car, that's not really practically as you'd go deaf.  So there's the compromise of noise level vs. exhaust restriction and therefore power output.  Look at the big V8 drag cars making 3000hp, and they have just enough of an 'exhaust' to get the exhuast flow outside the bodywork.  And they also dump the wastegate flow.  I've seen some of their datalogs, they have no issues with wastegate hunting; it's a non-issue on a proper wastegate setup taking into account flows and components.