Friday, September 01, 2006 - 12:26 am, by: Scott Ferguson(Scott_ferguson)
I don't think that this has been covered before in too much detail, i've only seen small references to it. Maybe some of the people that have been tuning cars for ages can help me. Does a TT need some backpressure in the exhaust to operate well, i've heard the school of thought that as little restriction as possible would be best for a TT motor, but a couple people that i know that are mechanically involved with cars keep saying that you can get major turbo and engine failures if you have no back pressure at all, something about back flow. Please help me guru's of the TT world.
Friday, September 01, 2006 - 12:38 am, by: Callum Finch(Sigeneat)
I dont know much about super-airflow-dynamics, but wouldnt any form of restrictive exhaust flow be bad for an engine? Wouldnt you want to at least ensure that the exit restriction of the engine (exhaust) is less than the input (intake)?
Friday, September 01, 2006 - 01:19 am, by: Shane Ilich(Ferret)
From what I understand, the concept of exhausts is that the pulses of exhaust gas have a high-pressure area at the front, and a near-vacuum low pressure area at the back. So essentially a well-designed exhuast will allow the pulses to "suck" each other along. By increasing the diameter of the pipe too greatly, you essentially reduce this effect, slowing down the flow of exhaust pulses.
"The bigger or larger diameter exhaust pipes allow the back pressure to be significantly less than the factory exhausts system. As a result, the difference in exhaust pressure before and after the turbocharger is increased. The increase in the magnitude of the pressure difference allows the turbocharger to reach higher shaft speeds at lower engine operating rpms." - from http://www.borla.com/news/news_detail.aspx?id=25
I can see that would possibly be a problem on stock turbos?
Friday, September 01, 2006 - 01:50 am, by: Scott Ferguson(Scott_ferguson)
so the worry is then about the ceramic wheels and possibly seals? I read both those articles and the impression that i got from the Borla one is that on a turbo'd car the bigger the better. Or should i say the less restriction the better.
I wouldn't think that the turbo reaching higher shaft speeds at lower rpm would be a bad thing since it means less lag but at high rpm reaching too high of a shaft speed might, i guess it would depend how much boost creep you get i know already that i might have to get a fuel cut defender but it seems like the general consensus is that the turbos are good for about 1 bar or so
Friday, September 01, 2006 - 06:56 pm, by: Cihan Aday(Cihan)
Guys, as with any turbo car, fitting the largest exhaust system possible after the the turbine will alleviate the most power. Shane, the concept of high low pressure you're talking about really only applies to NA motors when you're after desirable effects like scavenging to increase exhaust extraction efficiency.
In essence, the exhaust turbine itself imposes a huge restriction and flow disruption. It ruins the flow streak so badly its hard to make things worse. You just do the best you can by freeing up subsequent restrictions indefinitely, so we dont add to the unavoidable problem already imposed by the turbo's being there in the first place.
Saturday, September 02, 2006 - 12:08 am, by: Scott Ferguson(Scott_ferguson)
Basically i have an exhaust mod that i plan on installing that when in use would be changing the exhaust from the stock TT exhaust to a 3" straight pipe straight off the turbos, before the cat. Do i have any need to worry about anything?
Thursday, September 07, 2006 - 07:18 am, by: Scott Ferguson(Scott_ferguson)
Do you know of any way to get in touch with them? Or is it just a car that you know of. I don't want to run into big issues because of my own ignorance.
Thursday, September 07, 2006 - 04:14 pm, by: Chris Davey(Chris_davey)
Scott: I have driven my car with just a 4" dump pipe on it. Didn't hit boost though as it wasn't tuned at the time. What do you want to know though? Cihan explained it well.
Thursday, September 07, 2006 - 04:57 pm, by: Cihan Aday(Cihan)
Scott, i run with straight pipes at calder No cat or mufflers. Next time, im just dropping the CAT converter and running like that - free to air, no restrictions imposed.
There is not a worry in the world unless you drive around town like that.
Thursday, September 07, 2006 - 11:25 pm, by: Scott Ferguson(Scott_ferguson)
Well that helps, also in regards to the last comment there, why would it be a bad thing to drive around town like that? Aside from the fact that everyone will be covering their ears as i go by. The reason that i was concerned was that a couple of people that i know had mentioned some back flow issues when coming off of WOT. Kind of scared me at first as the stories sounded pretty bad, but so far it seems like there shouldn't be too many problems.
Friday, September 08, 2006 - 04:47 am, by: Daniel Clarke(Dieseltrain)
Only thing you should probably worry about with NO exhaust, coming off of WOT is FLAMES under your car! Factory ECU's run severley RICH... I used to own a pretty hot VL turbo(a while back now) that ran a best of 11.7 @ 116mph on slicks. It used to shoot flames out the back as it had NO exhuast.
Friday, September 08, 2006 - 04:53 am, by: Scott Ferguson(Scott_ferguson)
yeah, i thought about that as i realise the factory ecu is really rich, but i'm more worried about mechanical problems. As well it should not go quite as rich as if you were shifting in an manual since it's an auto.
Friday, September 08, 2006 - 05:59 pm, by: Cihan Aday(Cihan)
Flames don't happen readily with the 1JZ, the induction metering system is MAP based.
Nissan uses Air Flow Meters to determine fuelling parameters, the AFM's are placed before the throttle body so there is a 'problem' metering induction when the throttle body snaps shut. Because the throttle body is shut, intake manifold pressure declines rapidly but airflow being metered remains elevated for some time. The ecu drives the injectors even though there's no boost, mixture runs off the scale rich and passes through the cylinder unburnt.
You can reverse engineer the common 'acceleration enrichment map' and resolve the issue to some degree, but Nissan doesn't seem to bother - presumably for safety reasons.
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