Dumb question, I know but I can't find it by searching.. Maybe this belongs in the newbie forum Move it at will!

So the problem/question is:

If I had my boost controller set up for lets just say 20psi boost (Ran it at night.. 60 degrees f) I then trimmed some of the boost controller vacuum line ... about 1 foot from both nipples on the MBC. So total of 2 feet less vacuum line.

Now.. I ran it with the same boost setting at slightly different temperature (80 degrees f)

Its now running 2 lbs LESS than before the trim..

Should this happen?

I don't want to raise my boost now and then later on when its cold it rises to an undesirable level.. unless the only reason its running less now is because the length of vacuum line.

Thanks for the help

 

Guess no one knows for sure ? I'll just re-set it i guess

 

A number of things may have happened when you trimmed down the hose. I would just make sure you have an accurate boost guage and turn up the boost a little again!

 

Bringing back from the dead....I have a similar question related to this one.

I decided to ditch the hard vacuum line from intake manifold (the one that passes over and up along the driver's side of the valve cover. I have always used this line for vac/boost signal for both my MBC and BOV (via T-fitting). I swapped out the hard line because its ugly and it clutters things up in that area with all the brackets its attached to. I replaced it with a high pressure vacuum line...not your avg vac line. It has very thick walls, is quite rigid and dense, and fit over the nipples VERY snugly...all connections zip tied.

My question relates to the OP's since I have shortened the original line a bit, and have replaced the hard metal line with a rubber line (albeit a very strong rubber line)....and now I am seeing about 2 psi less in spike and hold. Car still behaves the same...very responsive, does not feel like power has been robbed. I've been trying to wrap my head around the logic of the pressure signal the mbc receives from the IM...but can't come up with any reasons why a shorter or possibly different inner diameter line would change boost pressure. I know this forum is littered with engineers and automotive geniuses who are just dying to impart their wisdom upon a dim-witted medical student.....thanks

No boost leaks btw...first thing I ruled out

 

The vacuum line that feeds into your boost controller contains the air coming from the manifold. The boost controller is in line with the wastegate. Let's for a second assume that you have no boost controller and the vacuum hose goes directly from the intake manifold to the wastegate.. as the turbo spools up, the 'vacuum' inside the hose changes into 'boost' and slowly starts to press against the wastegate spring... now the spring, depending on your wastegate, can handle a certain amount of boost pressure before it starts to open. Let's say, 14psi for example.. as the pressure in the manifold gets to 14psi, that pressure is also pressurizing the line to your wastegate, which now has enough force to overcome the spring and pop open the gate.. you now have 'wastegate' boost level, or the level of the spring.

Let's now put the boost controller back into the mix. Since it's 'in line' between the intake manifold and the wastegate, it can now manipulate the amount of boost the wastegate is "seeing" from that hose.. it can reduce that amount, "tricking" the wastegate into acting as if you have not achieved 14psi yet (remember we have not changed the spring) and therefore the gate won't open. Finally, let's say you have your MBC set enough turns to get 20psi in the manifold (on your boost gauge) what you're doing is using the MBC to keep the wastegate from seeing 14psi on the hose, until you actually have 20 in the manifold.

Now, there is a delay in this whole equation, that is, the turbo is spooling up with the wastegate shut and filling up the manifold with higher pressure air, when the wastegate finally starts to see target boost and starts to open... but it does not happen simultaneously, there is a delay due to the pressure having to travel from the manifold through the controller to the wastegate... hence, the "spike" is born. Next thing that adds to the spike, how tight you have the wastegate preloaded.. if your wastegate arm is preloaded really tight, it will delay the opening of the gate as the arm is already "pulled out" somewhat and the internal mechanism inside the wastegate needs to "catch up" with where the arm is before it can actually push it open... so both of those things can affect how big of a "spike" you see when you hit peak boost. (Also, how much torque hit you get out of it to the wheels)

With all this being said, you can see how vacuum hose length and wastegate preloading can effect how your turbo spools up, and on a stock turbo, this is huge, since the engine can eat up more air than what's being fed into it, that boost spike is sometimes the peak power you get, or the peak acceleration of the car. You just have to be careful how much of a 'spike' you allow, since this is where you'll push the engine/fuel the hardest too.

 

Nice, this helps explain it....

I was thinking along the lines of pressure and volume relationships when trying to rationalize, however they usually have an inverse relationship, all other things remaining unchanged in the system.

This makes sense to me tho...shorter vac line = less distance for the boost signal to travel = earlier wastegate opening or less spike in this case...thanks Liqquid

 

So make things short, shorter vac line = quicker spool?

 

no I didn't say that, my experience has never been that.