Boost drop / boost fall off

[Silverbullet91]

I'm currently using a DNA manual boost controller with my stock t-25. I've set the boost at 12 psi but in the lower gears it falls of down to 9 psi at redline. I know this is very common with mbc's and small turbos. But what I don't understand is WHY this happens? I know that boost creep happens when the wastegate can't flow enough gas around the turbine, correct?

Also, if anybody has any suggestion for helping me to maintain boost I'd appreciate it. (But I can't run right out and get a ebc and bigger turbo). Thanks.

S13 redtopt-25greddy fmicgreddy type rsdna mbcgreddy downpipeapex n1 cat-back
Modified by Silverbullet91 at 3:20 AM 11/10/2005

[Silverbullet91]

bump

[95lstegman]

where is your MBC and therefore WG hose plumbed into? i mean, where along the intake tract? it should be on the cold pipe or even the throttle body. if you've got it on the hot side of the intercooler or something it will have very adverse effects, as well as it will register, say, 12psi, but you'll only be getting 8-9psi in the manifold, so your "12psi" numbers on the dyno will suck compared to other people's 12psi numbers.

[Silverbullet91]

Hey man, whats up. Thanks for the reply.

Yeah, I got my boost controller in the line coming from the throttle body to the wastegate. I also had to hook up my bov into that line (before the boost controller) since my greddy pipping and stock turbo don't have any places to attach a hose.

[95lstegman]

there are 3 vacuum ports that come off the throttle body. run a line from one to a T, and one side of the T goes to the fuel pressure regulator and the other side goes to your boost gauge. for another port, run it straight to your MBC, through that and into the wastegate. for the third port (bottom of TB, not top like the 2 obvious ones), run it straight to the nipple on the BOV. trying to run too much stuff off one port is not a good idea, and will probably result it poor performance from the items on that port.

[Silverbullet91]

I thought that the bottom left nipple on the TB was for the carbon canistor and should not be used?

[95lstegman]

i run my WG off the stock cold pipe nipple, the bypass valve off one of the top ports on the T/B, and the boost gauge and FPR off the other top port on the T/B. i think you're right though. i plugged the port on the bottom.

[Silverbullet91]

Sounds like you've got all your vaccum llines connected correctly. I didn't want to use the same port for the bov and wg but I didn't really have much of a choice unless I drill and weld a nipple onto my pipping.

[95lstegman]

well the BOV and WG are the two biggest users of vacuum i think, at least they are both VERY important and should get their own ports. i think you really should think about drilling a hole in the cold pipe and bolt a vacuum nipple into the hole. i think your problem comes from poor vacuum/boost feed to the WG. it's the least you could try. i mean, we're talking <1 hour and <$5 to make this up, from the time you pop your hood to the time you've cleaned the hose of metal shavings, gotten it installed again and are ready to test.

[Silverbullet91]

I don't think thats whats causing my boost drop at higher rpm. If the wastegate was getting a poor signal than then it wouldn't open, thus causing my boost level to increase, not drop off. Right? I think its pretty typical of cars with small turbos and mbc's to lose 3 psi or so near redline such as I am doing so there probably isn't a lot I can do about it.

I had actually started this thread in the hopes that somebody could give me a technical explanation as to what causes boost fall off in cars with small turbos and/or manual boost controllers. Unfortunately, it looks like you are the only one with an interest in this post so I guess we're on our own with it.

[95lstegman]

i don't think the MBC is the problem, and boost fall-off isn't going to happen really. i that's a thing of the past for all intents and purposes. i've seen lots of cars running MBC and they don't seem to fall off.

[Silverbullet91]

What do you mean by "bolt a nipple into the cold pipe"?

[virus77]

he means add a nipple attachment to the coldpipe so you can run a vacuum line from it. You can do it buy drilling a hole in the cold pipe and screwing in a fitting, which is what he meant by "bolt a nipple". Or you can jbweld like I do

[Silverbullet91]

Isn't an ic pipe kind of thin to screw a fitting into? Also, I don't understand how the bov would interfere with the wastegate in a way which would cause it to open more/for longer and make my boost drop off. It seems like the worst the bov could do is decreae the pressure going to the wastegate causing it not to open and making the boost spike.

[95lstegman]

no, it isn't too thin. it's metal if it's aftermarket or it's thick plastic if it's stock.

just because the BOV isn't opening doesn't mean it isn't stealing vacuum. just trust us and run another line from the cold pipe to the wastegate and report back. it won't take you long or cost much.

[Silverbullet91]

Okay, I'll look into it and probably give it a shot. Thanks.