I have been wondering about the possible bad effect of using a turbo with a restrictive exhaust. In my mind, a small stage I wheel and/or a .48 A/R could cause excessive backpressure.
Excessive backpressure, if my thinking is correct, will cause more hot exhaust gasses to remain in the cylinder after the exhaust valve closes. This seems like it would fight the incoming charge and add heat to it as well. This is obviously bad.
Any thoughts?
Small Turbine = Detonation???
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Swedish Mike
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Re: Small Turbine = Detonation??? (Edub1)
Correct, small turbines make both manifold and the rest of the engine warm.The heat will go backwards into the cylinders and help you getting detonations. The EGT will drop with a bigger turbine and the turbo will last longer.Edub1 wrote:I have been wondering about the possible bad effect of using a turbo with a restrictive exhaust. In my mind, a small stage I wheel and/or a .48 A/R could cause excessive backpressure.
Excessive backpressure, if my thinking is correct, will cause more hot exhaust gasses to remain in the cylinder after the exhaust valve closes. This seems like it would fight the incoming charge and add heat to it as well. This is obviously bad.
Any thoughts?
This is also one of the reasons big turbine guys can use non natium cooled exhaust valves without problems even at high hp levels. No risk for melting as long as the fuel is there.
I think .48 is ok for most people but there are even smaller turbins out there and I wouldn´t recommend it for boost junkies.
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C-Kwik
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This depends on the boost levels and how well matched the compressor is to the turbine. The turbine only takes as much energy as it needs plus the inefficiencies of the compressor and turbine at any given pressure and flow. The rest is vented out of the wastegate.
That being said, smaller turbines and compressors tend to have their peak efficiency areas in lower flow areas for any given pressure ratio than a larger turbo (generally). It's tough to directly compare though as smaller turbos tend to boost in lower engine RPM's where larger ones can't, but a smaller turbo would likely be more efficient at those lower RPM's where larger turbos can't make boost, if a larger turbo were to actually be able to make boost there.
However, larger turbos will operate much more efficiently than smaller turbos at higher RPM's as a smaller turbo's compressor and turbine efficiency starts to drop off forcing more air through the turbine and less out of the wastegate.
Now of course all this also depends on the boost level you run, as the efficiencies of both are affected by this.
But you are quite correct that excessive backpressure will put additional heat which can reduce an engine's ability to discharge the exhaust gasses from it's cylinders. It could potentially lower the ceiling before you start getting detonation as well, but there are a lot of factors that associate with that as well.
This is something I've said here many times before and a big reason I tend to like larger turbos and don't care much for T3/T4 hybrid turbos. These turbos have their place, but it's likely that a T3/T4 has a much narrower overlap of efficiencies between the turbine and compressor. It's a pretty good compromise between spool and power, but still a compromise nonetheless. At a track, there should be few reasons to be at the lower midrange RPM's where a T3 Turbine would show it's positive traits over a T4 turbine. And as long as you keep your RPM's up above the boost threshild, turbo response is still plenty fast.
That being said, smaller turbines and compressors tend to have their peak efficiency areas in lower flow areas for any given pressure ratio than a larger turbo (generally). It's tough to directly compare though as smaller turbos tend to boost in lower engine RPM's where larger ones can't, but a smaller turbo would likely be more efficient at those lower RPM's where larger turbos can't make boost, if a larger turbo were to actually be able to make boost there.
However, larger turbos will operate much more efficiently than smaller turbos at higher RPM's as a smaller turbo's compressor and turbine efficiency starts to drop off forcing more air through the turbine and less out of the wastegate.
Now of course all this also depends on the boost level you run, as the efficiencies of both are affected by this.
But you are quite correct that excessive backpressure will put additional heat which can reduce an engine's ability to discharge the exhaust gasses from it's cylinders. It could potentially lower the ceiling before you start getting detonation as well, but there are a lot of factors that associate with that as well.
This is something I've said here many times before and a big reason I tend to like larger turbos and don't care much for T3/T4 hybrid turbos. These turbos have their place, but it's likely that a T3/T4 has a much narrower overlap of efficiencies between the turbine and compressor. It's a pretty good compromise between spool and power, but still a compromise nonetheless. At a track, there should be few reasons to be at the lower midrange RPM's where a T3 Turbine would show it's positive traits over a T4 turbine. And as long as you keep your RPM's up above the boost threshild, turbo response is still plenty fast.