Rotary engine back pressure?

[CharlieP]

Hi all.....
 When thinking in terms of changing exhausts...
 What are the issues with back pressure? I\'m guessing not so much is needed as with a piston engine?
 
:burnout

[SiH]

If it\'s a tubby, then as littel as possible - Once you got to a FI engine, rotary or coneventional boinger, then I\'ve always been told that backpressure/scavenging effects are oft all but negated. Minimal backpressure = maximum spool.
You want as little as possible, basically.
With the caveat that on stock twins, boost creep can be an issue with a free flowing exhaust due tt lack of back pressure, so WG porting or restrictor plates may be needed.

[CharlieP]

Cool thanks, obviously with a remap after, there shouldnt be problems

[SiH]

There may be issues with boost creep on a fully decatted free flowing system - people do still experience it. I was lucky enough not to, possibly due to the combo of decat silencer and 99 spec twins, but Pip did warn me about it.

Boost creep isn\';t always something you can map out unfortuantely - it happens due to the tubby not being able to divert enough boost out of the WG.

[SiH]

In fact, this was recently posted on SXOC by Mark from TurboDynamics, and makes it much, much clearer in my head

Hi all

This is a topic that is very hard to explain without diagrams and pointing at things, however i will try my best.

lets start with the turbine side of the turbo. To stop the turbo from making more boost than desired the wastegate opens at preset level and all the gases IN THEORY should leave through the waste gate port, however this requires airflow to do a 90 deg turn through the wastegate port (please remember this point). however the design of a turbine housing is such that when air enters the housing all it wants to do is to keep going around the turbine wheel and exiting through the exducer of the turbine. Air will continue to do this and cause the turbo to contniue to make boost as it has no desire to make a 90 deg turn from its natural flow path to exit out of the wastegate this is known as boost creep.

now imagine if you put your hand over the exducer of the turbine and blocked the air that wants to follow its natural path out of the turbo, the air will now be forced to go out of the next easiest place which means leaving from the wastegate port. the same effect would be achieved if there was some backpressure in the exhaust, it would slow the exit of air from the turbine exducer and therefore making it just as easy to leave through the wastegate port as the pressures become more balanced in the housing and in the turbo elbow. This back up of pressure also keeps oil in the centre of the turbine housing and makes oil leakage less likely, which as we know oil leage would cause the car to smoke.

The design of an internal wastegate is, and was, always a comprimise. for it to operate correctly it needs back pressure which as you quite rightly stated slows airflow comprimising performance marginally. the ideal soloution if want to run with hardly any backpressure (in the form of a 3 inch straight through exhaust) and to not suffer boost creep, is to fit an un-gated turbo with an external wastegate.

As you said when a free up the flow, the car makes more boost, and when freed up too much they make too much boost, which is boost creep.

I hope this explains backpressure and its importance when applied to inernally gated turbos.

Mark

[CharlieP]

Thats good thanks....
 I\'m guessing that type of boost creep isnt controllable by a Boost Controller?!

[SiH]

Nope, it\'s a problem before it even gets that far unfortuantely - if there\'s not enough gas flowing out of the wastegate to reduce boost then a boost controller can\'t touch it

It\'s a common problem using certain size housings fo the GT28RS on an SR20 engine.

[gregmacd]

Please take the benefit of my experience. If you are determined to run a free flow get a proper boost controller like an AVCR and have it properly set up.

I ran a free flow with a good Powerfc map but no boost controller and ended up destroying my engine and I mean destroying. Both 2 dead rotors and 1 destroyed housing.

My advice: either free flow + good boost controller well set up or take the safe root and get at least a 200 cell sport cat.

If you want a sport cat give Jason a call at Super 7 Autos.

[m1tch]

Port the wastegate and install a drainpipe for an exhaust