Exhaust back pressure video. I'm confused.
01-02-2018, 03:12 PM
#16
For your information, the Porsche PSE has not benefits in term of performance, it has been designed and sold as just an enhancement for the sound, that's it.
And please, someone, please explain to us how back pressure can improve any torque at any RPM, more exhaust back pressure will just leave more burnt mixture in the cylinder, thus leading to decreasing the VE, how could it improve the performance? it just hurts performance! Now, if you talk about emissions, yes, having exhaust gas trapped in the cylinder will cool off the combustion and reduce Nox.
And please, someone, please explain to us how back pressure can improve any torque at any RPM, more exhaust back pressure will just leave more burnt mixture in the cylinder, thus leading to decreasing the VE, how could it improve the performance? it just hurts performance! Now, if you talk about emissions, yes, having exhaust gas trapped in the cylinder will cool off the combustion and reduce Nox.
Specifically, a big cube engine with a lot of cam overlap and free-flowing exhaust with giant headers that breathes extremely well is going to make a ton of power high in the RPM range, but low-end torque will be *reduced* as compared to an engine that has smaller headers that optimize scavenging at lower RPM and at part throttle. Likewise, that restrictive exhaust is going to hurt top-end performance, but it will be easier to drive on the street, it will idle better, start easy in the cold, and not stall if you turn the A/C on.
It's a bit like a drink straw: compare drinking a coke with a tiny coffee stirrer, a regular straw, or a short section of garden hose.
A racing engine is designed to be used at wide-open throttle and make peak torque at a very high RPM, thus maximizing power and when driven slowly, it will behave poorly. A street engine sacrifices power to make peak torque at a lower RPM making it possible to be driven at part throttle (and to increase longevity).
01-02-2018, 04:29 PM
#17
Burning Brakes
Now if someone can explain how the turbo charges work in layman terms, I always thought that the exhaust gas always turn the turbine thus always increasing the MAP, how is that i have to push the gas pedal down substantially before the turbo's spool up? I'm in idiot mode now.
01-02-2018, 05:45 PM
#18
Rennlist Member
https://auto.howstuffworks.com/turbo.htm
The "lag" your talking about is a characteristic of a typical system. You can decrease lag with a smaller turbo, larger displacement, advanced timing or relevant to this conversation..... Less back pressure. Of course just like the back pressure convo, pros and cons to changing any of those.
The "lag" your talking about is a characteristic of a typical system. You can decrease lag with a smaller turbo, larger displacement, advanced timing or relevant to this conversation..... Less back pressure. Of course just like the back pressure convo, pros and cons to changing any of those.
01-02-2018, 08:15 PM
#19
Nordschleife Master
01-03-2018, 02:42 AM
#20
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This is one reason why the OEM PSE system when on Sport mode closes the valves when RPMs drop and then reopens them once they're higher again. Helps reduce one of the compromises with exhaust systems by supplying the higher back pressure at low RPMs for increased torque, and less restrictive at higher RPMs for increased flow and power, yet PSE owners continue to disconnect the vacuum actuator to prevent them from closing by themselves because they prefer the louder sound all of the time.
01-03-2018, 02:46 AM
#21
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This is one reason why the OEM PSE system when on Sport mode closes the valves when RPMs drop and then reopens them once they're higher again. Helps reduce one of the compromises with exhaust systems by supplying the higher back pressure at low RPMs for increased torque, and less restrictive at higher RPMs for increased flow and power, yet PSE owners continue to disconnect the vacuum actuator to prevent them from closing by themselves because they prefer the louder sound all of the time.
01-03-2018, 03:00 AM
#22
Drifting
It’s really pretty simple. Backpressure is always bad, but sometimes, due to the compromises made in tuning an exhaust, it is an unavoidable side-effect and the net gains from the way the exhaust is set up outweigh some small amount of backpressure at certain rpms.
There is no benefit to backpressure in a traditional internal combustion engine. If there was, top fuel dragsters wouldn’t use open headers. In fact, top fuel dragsters wouldn’t have exhaust pipes at all except they need to route the hot gas away from the engine components and driver, and the upturned pipes produce downforce on the track. The most effective exhaust is none at all, but that’s rarely practical.
There is no benefit to backpressure in a traditional internal combustion engine. If there was, top fuel dragsters wouldn’t use open headers. In fact, top fuel dragsters wouldn’t have exhaust pipes at all except they need to route the hot gas away from the engine components and driver, and the upturned pipes produce downforce on the track. The most effective exhaust is none at all, but that’s rarely practical.
01-03-2018, 09:29 AM
#23
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Iceter, also, backpressure is not always bad, especially in Turbo applications, which we're not really talking about here, but just as an example. Turbos when sized for street car applications need some backpressure to operate at their proper volumetric efficiency, balancing the intake side resistance with that of the output side impeller, which is what spins the intake side. No backpressure on the exhaust side will allow the Turbo to overspool, reducing efficiency.
In NA engines, the backpressure doesn't reduce torque, it just shifts where it occurs. Without proper backpressure, compression is reduced and the torque band shifts higher, which is not where you want the torque on a street car. On a racecar where you're running at higher RPMs, that's not what you want and why all of them are loud with straight pipe exhausts, race cams, etc making both torque and power up high, but in a street car you're balancing drivability and all out performance so you want torque to peak lower in the range and HP to peak higher in the range.
01-03-2018, 09:42 AM
#25
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01-03-2018, 09:55 AM
#26
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There are some interesting posts in this Corvette forum thread once you get down a few
https://www.corvetteforum.com/forums/c5-tech/195311-why-does-back-pressure-affect-torque.html
https://www.corvetteforum.com/forums/c5-tech/195311-why-does-back-pressure-affect-torque.html
01-03-2018, 11:16 AM
#27
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http://carnewal.com/products/P87/p97...Manual-Control
01-03-2018, 02:13 PM
#28
Drifting
Iceter, also, backpressure is not always bad, especially in Turbo applications, which we're not really talking about here, but just as an example. Turbos when sized for street car applications need some backpressure to operate at their proper volumetric efficiency, balancing the intake side resistance with that of the output side impeller, which is what spins the intake side. No backpressure on the exhaust side will allow the Turbo to overspool, reducing efficiency.
Turbos don't need backpressure any more than NA engines. Turbos won't overspool from less pressure on the downwind side, they overspool because the engine continues to provide additional exhaust gas as it revs up after the turbo reaches its optimal rotation speed and efficiency. That's why turbos have wastegates--to divert the upwind side exhaust and prevent the turbo from overspooling. In fact, in the act of preventing overspooling, a wastegate reduces backpressure in the primary exhaust tract, which is counter to your suggestion that more backpressure helps to slow the turbos.
A turbo won't spin faster than the exhaust drives it. Friction is a cruel master. Maybe back in the old days, a heavier turbine would carry some inertial rotation after the throttle is lifted, but it won't turn faster than the peak rotation the exhaust gave it. So, since the turbo's speed is entirely dependent on the gas being pushed through it from the upwind side, what happens on the downwind side of the turbo (which is always lower pressure than the exhaust on the upwind side) doesn't affect the turbo's speed.
I am open to alternative explanations, but so far, I am unmoved.
Last edited by Iceter; 01-03-2018 at 07:07 PM.
01-06-2018, 04:10 PM
#29
turbos do not require any backpressure at all because they are the restriction on the exhaust system...
and the reason a turbo will overspool is if there is too much exhaust and the wastegate is not doing its job and stays close ..
a wastegate controls boost by releasing the exhaust that spools the turbo..the more exhaust the more boost u get by limiting how much exhaust is going into turbine u limit how much boost the inducer will make..
and the reason a turbo will overspool is if there is too much exhaust and the wastegate is not doing its job and stays close ..
a wastegate controls boost by releasing the exhaust that spools the turbo..the more exhaust the more boost u get by limiting how much exhaust is going into turbine u limit how much boost the inducer will make..