Boost drop on dyno

[scottodog]

What would cause boost to drop from 7 psi at around 8200 to 5 psi and stay there. My bike made only 198 hpr with a stage 1 Hahn. It has a 10 psi internal waste gate, water/meth inj. at 5.5 psi and full exhaust. The tune center thought I was losing boost but they checked all hoses and they were good. Stock 2006 hayabusa, 40psi fuel idle, 85psi fuel on boost, plugs gapped to 25 as per Hahn. I disconnected the water inj. and it made no difference. When I got home I tried to adjust the W/G up and it maxed out within 1/2 turn. Thanks Scott

 

[TurboSN95]

Load from the dyno has a HUGE effect on turbos. My turbo'ed Mustang would not load correctly on a dyno, only making 16 psi. Got it on the road and it was making 24 psi.

 

[Mr Bogus]

boost will surge and then drop on many apps as the gate finds its "happy" zone (hysteresis of sorts).. how are you controlling boost pressures? bypassing exhaust/ dumping charge air?

quality of control devices is also pretty important, a simple BOV can cost a pretty penny but you normally get what you pay for..

almost every system I have seen will overboost at some point (and this is hard to replicate on a dyno as you are providing a stable load) On the street the load is not stable and engines can overboost pretty easily waiting for waste gates/ bov's

 

[scottodog]

The waste gate routes through the exhaust. I put a digital boost gage with recall on and it said I make 6.7 psi on the road. Is 198 hp correct for this amount of boost? Should I gap the plugs less than 25?

 

[TurboSN95]

On my turbo cars, going smaller on the gap never hurt my numbers... i was running .024" gaps when I used to run .028". No difference other than it gave the plugs more time if they wanted to open up on their own, and prevented blowing out the spark a little more.

 

[Commuta_Busa]

sounds like your gate is partialy open and bleeding boost. chenging your spark plug gap wont make a difference in the amount of boost.

 

[cdhoroho]

exhaust leak
that and possibly a boost leak out the charge side
but the exhaust leak is what it points to

 

[scottodog]

wouldn't I hear an exhaust leak with a full exhaust system?

 

[Mr Bogus]

first off the recall feature is only partially worth a hoot... it will pick up a max reading but be aware that these readings can be a bit misleading.. some systems will show an extra few pounds as wastegates/BOV's take some time to operate and they can cause an uneven boost reading.. (real time recording is a better option when available) They (wastegates) will also allow too much boost to bleed for a short period.. Dyno runs can miss this depending on how the load is applied IME..

Gapping your plugs tighter would only be done if you were say dropping holes at boost.. this just makes up for a weak ignition system.. better option is to upgrade the ignition system if the motor is missing at full boost. It is just masking a problem with a bandaid in other words..

If you had an exhaust leak, I would think you would already know it.. (they can be pretty destructive on the hot side of the turbo too)

on the wastegate: if it is an integral gate, they tend to be a bit unstable at higher boost levels and I suggest sealing them off an purchasing a "good quality" aftermarket wastegate (larger diaphragm, more consistent spring rates, and better valving)

I also add BOV's on dry systems as a further safety backup should the wastegate hang or malfunction..

 

[cdhoroho]

Ive had exhaust leaks that were never heard I.E. cracked headers and many problems of the sort
IMHO there is an issue with a leak weather its the intake or exhaust side you have a leak somewhere
internal gates tend to boost creep not lower boost

 

[Mr Bogus]

internal gates tend to leak with age (just nature of the design with small springs and flappers) and also provide too little surface area to bleed sufficient heat/gas to keep the turbo from over boosting.. So you end up with an internal leak at low boost and not enough leak at high boost levels.. (read stock internal gates are typically not adequate for big boost systems)

If the system is more than 5-7psi.. we have always welded them up and re-vamped the boost controls.. (cheap compared to a set of pistons or lifted heads)

I have never had a system with a hot side leak that was not obvious on the dyno..

 

[bubba_zenetti]

if the gate is not leaking exhaust, i would have to say it is an exhaust leak. pull the turbo off the flange and see if the gaskets show any signs of leakage (soot on the gasket surfaces flowing outward from the exhaust piping) also check at the head. they do come loose from the head. also many of the flanges on a lot of kits are not flat to begin with or have warped and will leak from there.

inspect the header for leaks around the welds. sometimes you see obvious leaks, other times you wont see them and they leak more as they get hot as the metal expands. to test for leaks, seal the header with rubber balls on the small elnd, then fab a metal plate to bolt to the flange and drill a 1/2 (i think) hole in it then stick a tire valve stem through it. seal it with a gasket made from an old inner tube. fill it to about 10psi and stick it in a tub full of water. if you see bubbles, you got leaks.

 

[MET]

Look for the easy first. If the bike was running fine then all of a sudden the boost went down. Check that you did not blow the plenum partial off of the T bodies.

 

[turbobusa2000]

My plenum had a hair line crack in it, didn't even notice it till I had it off the bike. I could barely see it without some light on it. There are lots of places for it to leak, just start checking.

 

[Mr Bogus]

My plenum had a hair line crack in it, didn't even notice it till I had it off the bike. I could barely see it without some light on it. There are lots of places for it to leak, just start checking.

far more likely than a hotside leak for sure (I bet that one was a joy to find).... Another tough one to find can be ruptured silicone boots between plumbing items...

Normally takes a pretty large leak on the hotside to loose boost

FWIW.. if you lost 2 psi and the gate is maxed out, you may want to take a peek at the spring itself.. they have been known to go south or even break.. Make sure the gate is sealing as well (valve leaking)

I have for testing only, locked or even blocked off wastegates when they are suspect.. (our off shore boat motors were notorious for stuck gates from salt)