boost drop off?!

[spooled240]

I am running a treadstone cast iron manifold, a t3/t4 turbo and an external 38mm wastegate. The problem is that the boost drops off like 2psi as i rev out the engine. IE: I hit full 8psi of boost at 3K rpms, then the boost drops to 6psi at 6K rpms.

Could this be a boost leak? I never had this problem with my old setup using the SSAC tubular manifold, but I did have a long vac line going to the wastegate before...now the vac. line is about a few inches.

[480sx]

This really isnt uncommon for a wastegate only controlled boosted car. If you were to get a EBC or a MBC you'd be fine.

[nelson8708]

it would be a good idea for you to make a boost leak tester and check it out.

[480sx]

Wait.. I skimmed your post. Where was your wastegate sourced before? Now its coming directly off the turbo?

Im guessing before you had the wastegate vac source hooked up to the manifold. Now you have it hooked up to your turbo outlet. Whats happening, is your getting a two psi boost spike, then leveling off to 6 psi MANIFOLD boost pressure. Your IC system is causing your 2 psi loss, which is not uncommon.

[spooled240]

thanks guys =]

here's what happened, I had an ebay tubular manifold before on a solid 10 psi and the wastegate was hooked up directly to the turbo with a longer vacuum line(different angled fittings)

then i got a cast iron treadstone mani and the wastegate is still hooked up to the turbo but with a much shorter vacuum line(better, more convenient-angled fittings)

after this manifold swap is when it would hold a solid 8psi at 3K rpms and constantly decrease to 6 psi as the RPM's increase. <as if my turbo was too small to feed the engine constant boost, which i know isn't the case with my t3/t4.

But NOTHING else was changed from when I was running 10 psi with the revhard mani and the 8->6psi I'm running with the treadstone. FMIC setup is identical/untouched. Even the treadstone manifold is somewhat identical to the revhard manifold.

I'm thinking is has something to do with the wastegate signal, would a longer vacuum line possibly off the intake manifold help keep a stable boost level? This would be an interesting experiement, but I can't see it making much of a difference

PS** I got a MBC recently but it didn't make a difference...I'm running 13 psi at around 3-4K rpms, but 10psi at around 6K rpms as if the ka didn't already fall on it's face at 5700rpms, my power "curve" is most likely a straight line lol

[nelson8708]

did you have to clock your turbo when you changed the manifold setup? Possibly tore the compressor seal? Did you change the diameter of the wastegate tubing?

[WDRacing]

Get a T fitting and reference your boost gauge and wastegate signal from the same source.

[spooled240]

what is really confusing me is that it never did this before but weird stuff happens with cars so w/e

I was reading up on different wastegate sources before and after the intercooler and from the intake manifold. It seems like tapping the intake manifold for the wastegate signal wouldn't be ideal

If you [tap the intake manifold for a vacuum source], the wastegate will be closed damn near all the time. When you're at part throttle, just tooling along at 3 or 4 PSI up a hill, the wastegate will be closed and the turbo will be spinning like mad, putting as much pressure into the intake piping as it possible can, trying desperately to get to a limit it'll never reach, and making gobs of heat in the process.

Reading up on this particular thread with turbo miatas, it seems like tapping the cold pipe would be ideal but tapping into the intake manifold would be dangerous. I sorta understand his concept of the throttle body leaving the wastegate closed and having the turbo spool way hard climbing a hill..but having the throttle partially closed would also limit the amount of exhaust gases spooling the turbo which in turn limits the amount of boost the turbo will create..

here's the rest of the thread:http://www.miataturbo.net/forum/t32479/

hm, maybe my air filter is just dirty lol

[WDRacing]

If you're at 3 psi...then the turbo isn't spinning like mad...lol. it's spinning enough to make 3 psi. When you're not at full boost the wastegate is SUPPOSED to be closed. If it was partially opening it would create boost loss and lag.

I used to read that forum...not exactly what I would consider a wealth of knolwedge

I've had 5 cars with turbo's on them, all have been run like I suggested.

BTW, got any pics of your manifold mounted up?

WD

[spooled240]

that's what I thought, just didn't make any sense to me really being at half throttle and the turbo spooling 30 psi in the intercooler piping lol

I think i'll try and tap the boost gauge line like you said, should be the most accurate signal.

I don't have any pictures but lemme tell you that I had fun modifying the brake master cylinder so the turbo would clear. I had to cut off one of the brake lines and tee it in to the other. In the end everything worked out great and the brakes feel perfect, I'll see if I can get those pics up.

[spooled240]

alright so I got the wastegate line tee'd into the boost gauge line and it's holding boost pretty straight. It spikes to 11 psi for a short while then holds 10 psi all the way to redline

I was noticing that my car would hesitate everytime I "re-floor" it after every power shift.so it's like:WOT -> *shift* -> WOT w/ short hesitation -> WOT hauling azz

could this be due to the wastegate closing really fast from engine vacuum? Or maybe my clutch is slippin a bit =/

[Chris28]

Quick question: Where are you getting your boost reference from?

Right now since I'm N/A but still have my boost gauge in it's hooked up to my bmc vacuum line, but in between the check valve and the bmc. I can read vacuum, but it's always vacuum. I don't feel like searching so I'm asking/assuming that the check valve closes when it seems positive manifold pressure, and in order to read boost from the bmc line I would have to tap in between the mani and the check valve. Please set me straight lol.

Good to hear that you got your problem fixed. About the hesitation issue, if it's the clutch then the engine would still rev, the car just wouldn't move. Is it an engine hesitation or an actual acceleration hesitation?

[spooled240]

I looked for vacuum lines that come RIGHT off the intake manifold. On my car, there are 2 good lines that come off the intake mani at the bottom of the #2 and #3 runners. If you're using the BMC vac line, you'll have to tap your intake manifold vac line BEFORE the check valve because boost will not pass through the valve, only vacuum.

It actually seems like engine hesitation rather than the clutch because the rpm's don't rev in between shifts. The rpm's just sit there and I'm at WOT but the car doesn't pull at all for a full second at the most. Then it starts pulling hard again.

It never did this before I tapped my intake manifold for the wastegate line, so I was wondering if my car is experiencing some sort of a lean condition or something due to the new wastegate setup I'm gonna check it out again tho.

[spooled240]

weirdest thing-I filled up at the gas station and now the stuttering between hard shifts at WOT doesn't happen...I had JUST installed a walbro fuel pump and my gas was pretty low(at or below empty) when the stuttering occurred.

the new walbro pump strainer is shorter and doesn't go as deep into the tank like the OEM strainer so it is most likely sucking some air when the gas sloshes around between those hard shifts causing a momentary fuel cut and a short stutter.

As for the boost, it spikes a little when the flash spool first hits but levels off and delivers a constant 10 psi now so I'm happy

[DevilMB3017]

weirdest thing-I filled up at the gas station and now the stuttering between hard shifts at WOT doesn't happen...I had JUST installed a walbro fuel pump and my gas was pretty low(at or below empty) when the stuttering occurred.

the new walbro pump strainer is shorter and doesn't go as deep into the tank like the OEM strainer so it is most likely sucking some air when the gas sloshes around between those hard shifts causing a momentary fuel cut and a short stutter.

As for the boost, it spikes a little when the flash spool first hits but levels off and delivers a constant 10 psi now so I'm happy

I've noticed that my car acts funny sometimes if I have less then a 1/4 tank of gas, like 1/8 or so - according to my gauge. When I have time I want to drop the tank and clean it out, but I'm too busy doing other stuff to it and find time to not be lazy enough to drop the tank... lol

[spooled240]

yeah that sounds like a daunting task, it might just be your fuel pick up being dirty or not being long enough (that's what she said lol)

I don't know it the factory fuel pickup/strainer would work with the walbro, but that would be a nice setup.

[480sx]

Thats really the worst thing about the standard ka-t FP upgrade.. You really shouldnt let your tank get lower than an 1/8th tank. Fuel pumps need fuel for lubrication and cooling, when they pick up air their life expectancy goes down.

[C-Kwik]

I just read through this thread a bit and will just interject a few thoughts.

As for the boost, it spikes a little when the flash spool first hits but levels off and delivers a constant 10 psi now so I'm happy

If you are seeing constant boost now from changing the source of the boost gauge signal, then its likely just that the wastegate is doing what it should. But, if the source of the wastegate signal is at the compressor, then its not indicative of what is actually occuring at the manifold (which for purposes of actually making power is what is important). With yourwastegate signal at the compressor and the boost gauge signal at the manifold, if you were seeing a pressure spike and then a loss of pressure, it may be indicative of a pressure drop. What kind of intercooler are you running? It might be possible that you have a one-way boost leak (coupler). Naturally, a small pressure drop should be expected across the I/C core as airflow increases but a 2 psi variance on top of the base pressure drop (you'ld be able to check this by checking pressure at the manifold and the compressor with your boost gauge) seems kind of high.

This problem would not be apparent while you had the wastegate running off the manifold pressure as the wastegate would compensate for any pressure drops or leaks by allowing the turbo to spool more. But its certainly not ideal either way as it requires more boost at the compressor to reach a given manifold pressure with bigger pressure drops. This in turn means there is more heat in the air being compressed by the turbo. This not only results in hotter air at the TB, but can also diminish your I/C's heatsink and heat transfer capacities. Such a set up might be okay for the street, but can become problematic at the racetrack where an intercooler will see more frequent boost and longer boost durations and run into problems with heatsoak.

I would look into this regardless. If its attributed to a boost leak, then it should be an easy effective fix. But if it's an I/C problem, then of course you would have to decide if its worth the additional expense to remedy or not.

I was reading up on different wastegate sources before and after the intercooler and from the intake manifold. It seems like tapping the intake manifold for the wastegate signal wouldn't be ideal

Reading up on this particular thread with turbo miatas, it seems like tapping the cold pipe would be ideal but tapping into the intake manifold would be dangerous. I sorta understand his concept of the throttle body leaving the wastegate closed and having the turbo spool way hard climbing a hill..but having the throttle partially closed would also limit the amount of exhaust gases spooling the turbo which in turn limits the amount of boost the turbo will create..

This is correct, but doesn't account for all of it. With some smaller turbos (relative to motor size/airflow), its possible there may be enough exhaust energy to actually pressurize the I/C piping. It won't be an extreme amount by any means, but any additional pressure will translate to more airflow passing the throttle plate. However, there are 2 feedback loops that prevent constant acceleration. They ar ethe BOV and the driver. The BOV will open to allow excess air out of the I/C pipe. The driver will likely adjust throttle position to achieve their desired level of acceleration (or constant velocity). This may actually sub-conscious involvement that could be simply detected as normal throttle adjustment. Eitherway, its definitely not something to be concerned with.