as luke and i were in the car looking at the logs after a dyno run he said... "notice the EMP (exhaust manifold pressure---backpressure), you can see where your wastegate is opening..."

after a bit of slicing and dicing which led to the above chart i sure would agree.

my base backpressure is 11 psi. that's at idle up to zero boost... so if we are going to make boost what's it going to cost us is the question.

if you look at the chart at 16 psi boost the additional backpressure is 16-17 psi. notice how it tracks from zero to 16.

THEN... the wastegate opens. the boost controller was set for 15.5 psi and it overshot so it opened up the wastegate. look what that does to EMP. it dives from 16 to 7 while boost drops about 1 psi.

THEN... the controller decides enough and closes to raise boost. just like slamming a door the EMP shoots up to 25 PSI.

an interesting dynamic. one that argues strongly that you want as smooth a boost as possible.

based on this i moved my solenoid tap from the engine side of the UIM to just behind the throttle body. if that doesn't do it i will tap the compressor housing. i may mess with the gain setting of course.

hc

 

^^^ Turbine wheels and turbine housings are the net result of the back pressure problems, waste gates only stop the turbo from overspeeding and flying apart, how much is bled off to control the boost is a fucntion of the pressure, the earlier the WG opens the worse the problem gets with increasing RPM and engine flow as more goes out the WG, more flow required by the compressor = more power needed to drive the turbine = higher back pressure to keep things going

[b]The only way to solve this is to run big turbines and housings and bleed of near nothing ! but then you have sweet **** all mid range as you have no boost so its always a compromise between back presure build up and early spooling (full boost)

 

the controller realizes at a certain point that the boost target (15.5) will be overrun.

as boost has been building, the controller has been cycling the wastegate a bit but now it needs to be much more aggressive...

at around 16+ this aggressiveness turns the escalating boost downward. the wastegate is full open bleeding off EMP to reduce the compressor output.

this decrease in energy results in a boost output downtick.

with the more aggressive wastegate opening the EMP drops from 17 to 7. the turbos is still making 15.0 boost from momentum and some exhaust flow. as boost drops the controller closes down to raise boost and my manifold pressure retraces mirror image on the upside.

my point is that while a modest boost variation might be considered trivial it isn't as it leads to a leveraged EMP variation whicc is disruptive.

hc

 

Howard,

Are you running twin wastegates, with two completely separate turbo runners? What wastegate(s) are you running? Sounds to me like either too small a diaphragm on the wastegate, or like you said plumbed too far from the compressors, or the controllers setttings.

I haven't logged it digitally, but you can see this easily with a analog pressure gauge. Another reason why you don't try and run a teeny turbine housing and a lot of wastegate to try and make up for it. Like Rice and I were chatting about, lets run .1 turbine housings and 4 90mm wastegates! lol...

 

sorry can you define what each axis and curve is on that chart? i'm not following it.

x axis is seconds, y axis is psi (absolute or boost?) pink curve is manifold pressure and blue curve is exhaust backpressure?

 

X is time. not in seconds.
Y is boost in PSI.
Pink is manifold boost.
Blue is boost/pressure logged from runner between engine and turbo.

as to 1Revvin7's questions:

yes i am running two separate (discrete) exhaust runners to two turbos and each turbo has it's separate WG, actuator and DP.

the turbos are internally wastegated. currently the wastegate orifice and flapper valve are stock Garrett. since the wastegate port is a part of the turbine housing location is not an issue. further, since each turbo can make 42 pounds of air at less than 25 psi, i am not attempting to run the turbos into areas for which they weren't designed. modding the WG port is easily do-able if necessary. while we are on the subject of WG flow i should mention that each turbo has a very trick ATP rear 3 inch V band high flow rear.

as to turbine housing size... they are large. T3 .82 A/R. the wheel is Stage 3.

two Stage 3s = 9.045 sq inches of turbine area

T4 O trim is 5.422

T4 P Trim is 5.894

T4 Q Trim is 6.646

i run 2 3 inch Downpipes to the midpipe where they join using a Burns Stainless Y.

DP area = 14.13 sq inches

single 3.5 inch DP is 9.62

single 4 inch DP is 12.56

Diaphragm area... i have adapted the well built OEM FD actuators. they appear to be the same approx size as the Garrett aftermarket item that can be purchased w different spring rates.

running off the spring w the controller off i get 10 psi which seems to be about right. i do plan to probably replace them this winter w the Garrett item w a touch more spring.

thanks for any and all questions, comments and advice. i am off to Beyond Redline to discuss the matter w Luke and also pull some of the Aug dyno sheets.

howard

 

Howard,

Are you recording each rotors EMP separately? If so post the same plot with each rotor instead of combined.

 

So, I am understanding this nugget of knowledge

Put into different words
Run a WG spring (close as possible) to your target boost.

Minimize the WG opening for eff. gains of the turbo (WG bleeding air off does not help your turbo eff.)

Run big A/R turbines for big power (within one's acceptable boost spool-up threshold) to minimize backpressure

More turbos/turbine wheels decrease overall eff.

What was the other one?
Power required to drive comp = flow rate+pressure+EGT?

Tony

 

i will be testing the "bigger turbine theory" sep 16. while the chart and data above is interesting (to me) my primary problem is my motor hitting a wall at 6000.

Rice's comments are on the mark.

too much backpressure.

Luke at Beyond Redline agrees. he said if i fix the boost dip i will still have the hitting the wall problem as it is a flow problem that relates to either the ports or the turbos. that's exactly what Rice posted.

thinking turbos, i called my guy Kevin Draper. Kevin said backpressure should be close to zero at idle. mine is 11 pounds. Kevin said Stage 5 turbines.

the turbos are coming off today and will be on Majestic's bench tuesday. Stage 5 wheels and back on the dyno Sep 16.

while Stage 5 are large they are only 8% larger in area than my compressors.

it will be interesting to see what the lag is. there should be more, but there should also be a major reduction in backpressure which would be a plus.

i will re-publish the boost V backpressure chart thereafter.

hc

 

rise from the dead!

Results?

 

Howard sold his setup. And it's a good thing too

-J