Why am i getting excessive boost spike with my setup?

[mhuffman]

1g FWD, 14b, stock smic, 2g ex. mani., 3" DP, 2.5" cat, 2.5" catback, stock o2 housing, CI MBC.

I fixed a whole junkload of boost leaks, and i found i was hitting higher psi so i dialed the MBC back.

The problem is that steady state i am right around 13psi, but between shifts it spikes to 17ish. Why does it only hold 13ish to redline when it spikes to 17ish between shifts?

PS~ my boost gauge is tapped correctly and my MBC lines are extremely short and they're not leaking.

RESOLUTION: See last post.

 

[TurboAnything]

Could be the 3in downpipe on the stock O2. make sure you have no exhaust leaks before the turbo.

 

[BoostFrenzy]

Is anything on that list ported? You are saying that it spikes between shifts? Only thing I can think of off the top of my head would be that your BOV isn't releasing the air as fast as it should. So when your TB plate closes and there is still pressure coming from the turbo it reads higher. I could be completely wrong though. If you have some sort of A/F gauge, what does it say during this spike?

 

[mhuffman]

My fault in poorly explaining. The boost spikes immediately after the shift when i am wot again to 17psi and falls to 31 after about 1-2 seconds. i have stock 1g bov but i have a forge on the way.

 

[zippyshoe]

Where do you have your MBC tapped? You will want the boost reference port of the MBC connected as close as possible to the turbo compressor housing (such as at the j-pipe). You definitely do not want to use your BOV line as the MBC's boost reference. If your MBC is connected correctly and you are running short vacuum line connections, then you may want to try and swap-in another MBC.

 

[mhuffman]

Where do you have your MBC tapped? You will want the boost reference port of the MBC connected as close as possible to the turbo compressor housing (such as at the j-pipe). You definitely do not want to use your BOV line as the MBC's boost reference. If your MBC is connected correctly and you are running short vacuum line connections, then you may want to try and swap-in another MBC.

I might do that because i have the reference end of the MBC 0.5" from the j-pipe (turbo outlet) and the line from the mbc to the wastegate is scarcely 5".

 

[98eclipsekidd08]

can long mbc lines cause this mine are not extreemly long but there not no like couple inch,but im having this same issue.

 

[mhuffman]

can long mbc lines cause this mine are not extreemly long but there not no like couple inch,but im having this same issue.

Yes, the longer the lines the longer it takes for the wastegate to respond.

i kept my lines as short as physically possible, literally.

PS, what're ## mods and what MBC are you using?

 

[98eclipsekidd08]

its a Turbonetics MBC good to 25psi, the only other mods i have so far are intake and 1G BOV, new plugs. I have no boost leak, Plugs are good, all gaskets are good, no shaft play in turbo,no cracks in manifold but I am also having this problem i spike 16-17 but actually am holding around 12 haha it sucks. I guess ill try making my MBC lines like 1cm, thanks for that little bit of advice i did not know that.

 

[mhuffman]

its a Turbonetics MBC good to 25psi, the only other mods i have so far are intake and 1G BOV, new plugs. I have no boost leak, Plugs are good, all gaskets are good, no shaft play in turbo,no cracks in manifold but I am also having this problem i spike 16-17 but actually am holding around 12 haha it sucks. I guess ill try making my MBC lines like 1cm, thanks for that little bit of advice i did not know that.

Ya this sounds like my problem as well. i have one small boost leak at TB shaft seals. I spike to 17ish and fall to 13ish.

Anyone?!?!

 

[98eclipsekidd08]

This is my thread about almost the same thing http://www.dsmtuners.com/forums/showthread.php?t=270219 that may help out maybe not. I shortened up my MBC hoses to about 3-4in max alot shorter than they were earlier i mean alot ha. Im still having my problem tho . If i were you I would fix that gasket at the TB and go on from there. Check your spark plugs to. If you figure out your problem pm me cause i would love to know this to, thanks

 

[mhuffman]

This is my thread about almost the same thing http://www.dsmtuners.com/forums/showthread.php?t=270219 that may help out maybe not. I shortened up my MBC hoses to about 3-4in max alot shorter than they were earlier i mean alot ha. Im still having my problem tho . If i were you I would fix that gasket at the TB and go on from there. Check your spark plugs to. If you figure out your problem pm me cause i would love to know this to, thanks

Thanks for the link to your page, ill do a couple of checks.

 

[mhuffman]

my exhaust manifold wasn't tight at one corner and was leaking. it had previously been stripped out and i had repaired it but didnt wanna torque too tight. i re torqued and the boost spike is much less, i spike to 17~ish and it falls to 15psi all the way to redline. does this sound more 'normal'?

 

[1bad92awd]

Ya this sounds like my problem as well. i have one small boost leak at TB shaft seals. I spike to 17ish and fall to 13ish.

Anyone?!?!

Could potentially be a boost leak, but more likely the un-even piping in the exhaust to me seems like the problem. On my old talon, I ran a HKS cat-back exhaust with stock boost and i noticed a boost spike as well. The spikes were also worse when the air was cooler and more dense outside while I was driving. Its a common problem, but I wouldnt expect all of your spike to disappear until you have a downpipe at least and all the piping for I/C, exhaust and everything is all even.

 

[Bmxr152]

You might want to try porting your 14b. I know when i had my 14b i had alot of creep issues until i ported it then it was pretty much no creep at all.

 

[Black_Bullet]

The spikes were also worse when the air was cooler and more dense outside while I was driving. Its a common problem, but I wouldnt expect all of your spike to disappear until you have a downpipe at least and all the piping for I/C, exhaust and everything is all even.

Because more air going into the compressor when its colder outside, denser air.
The more air that gets into the compressor in the first place the more pressurized air there will be in the system once it gets into the intake tract.

My car is set at 20lbs and holds 20 in 2nd, and 3rd gear and would get 21-22psi spikes in 4th and 5th because theres more load on the engine in the higher gears causing the turbo to get pushed slightly harder before your mbc has a change to respond. Sort of like a power surge.
I then did a cold air intake and now 2nd gear sees 20psi the same, but 3rd gear up sees 23 psi spikes before settling back down to 20 in the higher rpms... Now mind you this is with only doing a cold air intake and nothing else... Much denser air ( like 40 deg temp drops while driving after the cai.) Causing more air to get packed into the intake system building more pressure, and even flowing more lb/min of airflow...
My only concern with my boost spikes is whether or not the colder air offsetted the heat gained by the added pressure. In the end it comes down to whether or not your knocking or not bottom line. Log your car and see what it does at the spikes. Mines sees 3-4counts when it hits 23psi. Nothin harmful, but better to tune for no knock...

Hope this was of any help.

Edit:: ( OOps mistaked the date on the thread, a couple months old. But ill leave my post for any future reference.)

 

[mhuffman]

Update:

Since my previous post, i broke a piston so i rebuilt the engine. Also, i replaced the leaking throttle body shaft seals.

I have no boost spike, but i have bad boost creep.

Mods: 2g ex mani, stock O2, 3" DP, 2.5" cat and catback.

I am running a line straight from the j-pipe to the wastegate. No MBC. I get 9psi around 3k rpms and it creeps almost to 15psi by redline. With the MBC set at 14psi, it creeps almost to 20psi at just 5k rpm, which is why i am running stock boost. It creeps FAST and high.

I checked the wastegate actuator. It opens around 11psi.

After researching, i think ill pull the O2 hosuing and port it and port the wastegate, maybe even get the 34mm flapper.

 

[Black_Bullet]

So you get 5 psi worth of creep.. Yea port the waste gate hole and it will cure it.
Once you take the o2 housing off you will see a small hole in there. Port it as big as you can without going over board and you shouldnt get more than a 1 or 2 psi creep at most. It worked for me on my old set up.

 

[mhuffman]

Well i ported the turbine housing and the wastegate to the very edge of the stock valve. boost response is much better, and the car can hold 15psi at redline without going over, so i'd say it's a success!

ported 14b's are awesome!! more spool and good mid to high range power improvement over unported!

 

[Black_Bullet]

Well i ported the turbine housing and the wastegate to the very edge of the stock valve. boost response is much better, and the car can hold 15psi at redline without going over, so i'd say it's a success!

ported 14b's are awesome!! more spool and good mid to high range power improvement over unported!

Glad to hear!
You definitly could increase the flow by porting the o2 housing and porting the waste gate hole does for sure help solve boost creep issues on internal w.g. set ups.
But on the contrary, for a fast spooling high boost/high rpm car that doesnt need to see lower than 20 something psi, I wouldnt recommend porting the o2 of an internal w.g... It can possibly limit some of your overall max psi, high rpm boost holding due to more overall exhaust gas able to escape reguardless of the flapper position... Theoretically.