Backpressure effect on wastegate
Backpressure effect on wastegate
I have tuned several PSA xud9t engines, the most recent set at 20 psi on stock turbo, and it maintains full boost 20 psi all the way to governed redline of 4800rpm. If I turned it up to 25 psi would it still make 25psi at over 4500?
I know that too smaller an exhaust housing makes too much backpressure at high rpms, but what are the symptoms other than high egts?
Thanks, Jack
engineengineer
09-24-2018, 04:41 PM #1Hi, want to ask a couple of questions relating to turbo backpressure. Is a wastegate designed to open from pe turbo exhaust backpressure to alleviate it? Here's a hypothetical scenario- om606 with stock turbo- starts coming on to boost low rpm, say below 2000rpm, is making full boost by 2500- still making full boost at 3000- at the point where the exhaust housing becomes too small, would you see a drop in boost pressure as the wastegate opens from backpressure or will it keep increasing turbo rpm and backpressure to maintain boost?
I have tuned several PSA xud9t engines, the most recent set at 20 psi on stock turbo, and it maintains full boost 20 psi all the way to governed redline of 4800rpm. If I turned it up to 25 psi would it still make 25psi at over 4500?
I know that too smaller an exhaust housing makes too much backpressure at high rpms, but what are the symptoms other than high egts?
Thanks, Jack
85 300D om617: 8mm M pump 175cc 5200rpm, holset he221w @ 30psi, large A2W ic, compounds on the way.
most stock internal waste gates don't have the area to allow the back pressure to push them open against the actuator spring unless you are running very high pressures. they are for the most part purely governed by the boost reference line feeding the actuator. there are some installs the use exhaust pressure to blow the gates open but most of these are compound turbo sets running 50+psi of boost and 70+psi of back pressure in the exhaust manifold. that plus most compound set ups are using external gates much larger than the internal ones in most OEM turbos. to use your OM606 example as the rpm increases the compressor speed must increase to both handle the increases in mass flow and maintain the pressure ratio the spring in the waste gate actuator is targeting (technically its not targeting a PR its targeting a specific PSIG figure but precluding dramatic density elevation changes it works out to a nearly steady PR). as for your turbo being able to maintain 25psi at higher rpm that is dependant on the VE of the engine, the compressor map of the turbo, your local density altitude, and a few other things. excessive back pressure can cause high EGT but the main issue is the reduction in volumetric efficiency and the increase of pumping losses.
85 300D om617: 8mm M pump 175cc 5200rpm, holset he221w @ 30psi, large A2W ic, compounds on the way.
Baldur Gislason
All wastegates are affected by the exhaust pressure, some more so than others. For the most part it does not matter because you tend to run electronic control that compensates for this, but it means you cannot run a spring soft enough to allow the wastegate to open at 0.5 bar boost and then expect to be able to run 3.0 bar by controlling the air pressure alone. Drag racers use CO2 for this purpose, to generate more pressure on the back side of the diaphragm than can be had with boost pressure alone, to positively keep the gate shut for 3 bar boost downtrack but still be able to run low boost in first gear.
Baldur Gislason
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