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Losing boost at high rpm, how to be sure its the turbo's flow limit?

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Rb25 Neo, Built, HE341 holset turbo, precision wastegate, stock exhaust manifold, boost controller connected in the Tee config (Top port sees boost pressure on Boost DC% to help the spring)

Is there any way to correctly diagnose when a turbo is just not flowing enough for the engine? In this case i had the Boost DC go 95% to increase boost and it still fell, so i concluded that the turbo was just out of flow in the high rpm range.

I used to have a HE351 (bigger compressor wheel, same hot side) and i THINK i had full boost all the way, but memory could betray me.

The thing is ive read other engines, 4 cyls, doing 500hp at higher boost on the HE341, so im wondering if im really out of flow or if im missing something.....

Heres my dyno (4th gear) and log from the run. VTC range is 1500 rpm to 5500rpms, hence a switch in AFR and also it kind of matches the boost drop starting.

Attached Files

In the very simplest terms, if you are out of turbo, you'll seea sharp rise in compressor outlet temp, and turbine inlet pressure. The latter of which causes high pressure in the exhaust manifold, acting on the bottom of the wastegate, much higher than the boost pressure acting on the top of the wastegate, forcing it open and reducing turbo rpm, thus boost.

All that being said, I know nothing about that turbo. From memory it's off a cummins...? Is the wastegate vented to atmosphere? What spring have you got in it? Is there any possible restriction in the rest of the exhaust?

Externally vented gate, soft spring, restrictive exhaust. Any one, or more likely two of these together can cause boost to drop off significantly.

Hope that helps in some way.

Hmmm ive never gotten into any of those variables, yes its a dodge ram 2500 turbo, from a cummings engine i think.

Well, i do have some info on the setup and run log.

Its an externally vented wastegate, i think im using a 14-16 psi spring, the OEM RB25 exhaust manifold and i have a locally made 3.5 inches exhaust with no restrictions other than bad welds and the bends that slightly compress the tube.

I dont have a backpressure sensor, maybe i should get one to learn about these things but i do have IAT temps.

I have attached the log, IATs seem very stable all the way, but im using meth/water injection and i think its behind the IAT sensor (Nitrous active is Water/Meth) but even with its behind it should have risen still i guess.

Damn, im starting to wonder if t he Oem manifold is just not worth it. Ill get a compressor and test the wastegate to correctly detect what spring i have

Attached Files

I suspect the 'restrictive exhaust' may be more a case of the turbine and housing?

Not too much info on that particular unit, but seems you may be around the limit for it - different engine and other characteristics will affect the potential, though.

Ill be installing a backpressure sensor to see whats up, but indeed i think the 9cm2 exhaust is too small.

I did have a HE351 on it, its the same exhaust side but a bigger compressor side and i think i never had boost drops, but i suppose thats because the compressor works a lot less to make that boost pressure.

Wait a sec... reading about backpressure, if backpressure limits the intake of air, if i had 2 wastegates for example, turbo is getting the needed flow to keep it spinning, if both wastegates are relieving backpressure, wouldnt this mean i could potencionally keep the turbo spooling and eliminate backpressure?

Or would i lose the necesary flow to keep the turbo spinning faster than at low RPM to meet boost pressure demand?

Im trying to wonder if by having a bigger wastegate, or 2 of them and relieving backpressure, if i can actually let the turbo spin more and intake more air into the engine.

Edit: nevermind, saw some discussion about drive pressure and boost pressure, the answer is the latter one i put.