I just want to know why boost fluctuates from gear to gear, it reacts differently in different gears. And im not talking about the general fluctuation im talking about a fixed fluctuation from gear to gear. like my 2nd gear will hit 21 then bounce around between 18-20 my 3rd gear will hit like 20 and bounce around hold stable and bounce around again, fourth gear does something else, and it repeats that on every pull i do. Also it seems like it tapers down more on the lower gears 1-3rd then it does in 4th or 5th. 4th gear seems like it never drops bellow 18 psi.
Also this is a completly stock 05 evo 8. Pulls are generally done when its like 50 deg outside, thats why i see 21 even. My gauge is fully accurate tested it with 2 different vacuum pumps.

Why do the different gears change boost?

 

Good question, I'm interested in the answer also. It probably has something to do with the rate at which the RPM's increase in each respective gear. (e.g. 1st gear goes from 3k to 7k very quickly, while 4th gear takes much longer to traverse that same rpm range).

 

ya since the bleed of style boost control with the stock solenoid dosent have enough time to respond that might be why it dosent bleed off enough air in the lower gears so you get tapers that hit as low as 16.

 

My guess would be that the gearing itself creates different loads on the engine affecting the effeciency of the turbo from the spike to where it ends up.

 

DIFFERENT LOADS on the engine is correct and the stock solinoid has a programed duty cycle, it doesnt know how much boost is actualy being made so it cannot compensate

 

Actually, the stock ECU does not have a programmed duty cycle. It varies the duty cycle as needed to try to reach target loads programmed into the ECU. The ECU is just not fast enough to fully compensate in the lower gears.

 

"Actually, the stock ECU does not have a programmed duty cycle. It varies the duty cycle as needed to try to reach target loads programmed into the ECU. The ECU is just not fast enough to fully compensate in the lower gears."



Thats exactly what i suspected, cause it seems like its just sitting there going,

"Ok here comes boost, OHHHHH to much close up some bleed off, ok ok wait not enough boost open it up." And its just playing around back and forth like a little elf in your engine bay regulating it.

 

the harder I accelerate in 1st gear seams to effect other gears more for a longer time. for eg.
hard accel in first - boost drops signifigantly and lasts into 3rd gear
moderat full throttle in first - boost drops not as bad and if I take it easy wot into 2nd also it gives me full boost by the end of 2nd

the faster I roll on the throttle the more it penalizes me in lower gears. and it gives it back like a reverse decaying map

I have been trying to get a reliable and stable 20-21 psi boost using stock control and I give up for now I will be installing a mbc until some one brighter and more dedicated than me can figure it out.

 

Boost will increase as gears increase due to load. this is why if you spike 23psi in 3rd you will spike 25ish in 5th due to the increase in load.

 

if I take it easy in first I can get 20 psi in second
if I pull hard in first I get 16 psi til halfway through 3rd

 

i have the same problem too, you think i can install a mbc and that should help eliminate the problem?

 

In any system where the output (say engine horsepower) is augmented by positive feedback, it is problematic to modulate the output faster than the time-constant of the feedback. If one tries, then the feedback gain is diminished and the output is reduced. This problem gets worse as the nominal feedback gain increases.

Engine horsepower is torque X rpms. Torque is proportional to volumetric efficiency, which is proportional to engine load, which is proportional to boost (in psi absolute). Roughly.

In low gears, rpms are rising very rapidly and horsepower is being asked to rise (or "modulate") too quickly, and thus the horsepower falls short of what it should be. Since horsepower is proportional to boost * rpms, horsepower is reduced by the boost tapering.

In higher gears, the rpms rise much more slowly and thus boost tapering is diminished.

Boost tapering occurs ultimately because the turbo is a mechanical device that physically pushes air around and thus has a rather large feedback time-constant. If you ask the turbo to "respond faster than it physically can", then its gonna fall short. This description is rather imprecise but it has some truth to it.

FYI: For an evo, where you have small engine and big turbo, the positive feedback gain is rather "large."

 

Perhaps one way to get better consistency from gear to gear would be to not rely on the positive feedback loop in lower gears (effectively disable it) where it can't keep up but rely on it in higher gears where it can keep up. I haven't looked at the boost control options in the ECU recently, so I have no suggestions off the top of my head. I'll look later today.

 

the mbc should eliminate the problem I am haveing however it will not completely eliminate taper or reduced pressures based on reduced loads. projekt shocker, if you have the same issue as me when trying to control boost at higher levels with pill removal and/or modification a mbc will take the ecu out of the picture as far as boost control goes. It will still effect boost slightly though through knock control measures such as timing pull and fuel enriching
once boost control figured out I plan on going back to regain the safety net the ecu offers

 

Well I am getting the same boost that I had with the MBC (both Hallman and Forge) and same basic taper. In first gear, using the 2 step I believe I only get like 12 - 14psi, but then I am hitting like 22 in 3rd tapering down to 17/18...6th is more like 24psi, with a slow taper.....

Remember if you are racing through the gears from 1st, you will not see the boost numbers that most refer to on this site (because you are already in a higher RPM and are not doing a 2K RPM pull).....

I made the #58 gauge pill this weekend and will be installing to see if I can slightly increase my boost and improve my taper....