Boost spike, how to fix?

[A.J.]

Ever since I installed my SRT-4 2.4 I've had a boost spike. With my 2.5 is was rock steady and now that 2.5 in my Dad's '85 wagon it holds steady. It'll spike to a certain psi and then drop and hold steady at a different psi. I thought it might be my home-brew 3 stage boost controller: http://www.turbo-mopar.com/forums/sh...oost+cotroller so I by-passed that, no change. I installed my new air/water intercooler to replace the one I had with a 3 psi pressure drop, no change. A friend suggested I use the barb on the turbo outlet instead of the barb on my intake behind the throttle body, no change. I was told my wastegate spring is too weak. I got a new wastegate with a 15 lb spring, no change. Another friend suggested that the back pressure in my exhaust manifold may be too high. At some point I want to remove the manifold so I can get a bung welded on it to check the back pressure. I'm thinking it might be that the wastegate hole is too small and I need to open it up because I'm using a TU 3" swing valve, I haven't done that one yet. Does anybody have any suggestions or experience in dealing with this? Once I get this resolved I'm going to hit the dyno again to do some tuning.

[glhs727]

if you are using a wga with a 15psi spring, you will never get it to stop spiking! That spring is way too stiff. What brand is it?

[A.J.]

My old wastegate had a 10 lb spring in it but only got about 7 psi out of it.

My new one is a Forge that I got off of a customers car. A customer came in with an Neon SRT-4 and it turned out the diaphragm was torn. I got him a new one but kept the old one because it looked cool. I recently dug it out and took it apart, got a new diaphragm, spring assortment, took apart an old T-II wastegate to harvest the adjustable rod and built myself a $180 wastegate for $50.

[wheming]

What boost are you expecting? And what are you spiking to?

If you have that MBC still it could be the orifice is too small before the bleed.
Did you try the softer spring in your wastegate?

[A.J.]

Right now I have the 14-19 psi spring in the Forge w.g. The boost spikes to just over 15 psi and drops to about 12 psi. And that's with the turbo outlet barb going straight to the w.g.

[turbovanman²]

Sounds like it could be too stiff or the wastegate hole is too small.

[Tempted]

Could be a leaky BOV.

[Force Fed Mopar]

Stock turbo?

[wheming]

My PT GT with stock turbo and computer would spike 3 psi before boost settled. I'm not sure you need to worry about this being abnormal.
If the wastegate hole was too small what you would also see would be boost creeping, not being able to maintain enough flow through the wastegate to limit the boosting.
maybe your signal line to the wastegate actuator is restricted? Or too long? Going to a MBC that is remotely mounted either in cab or on the inner fender adds length to that signal line that will slow that wastegate actuator response, thus increasing the potential for spiking.

[GLHNSLHT2]

a WG hole that is too small usually causes boost creep, not a spike. I would say the spring is too stiff. Factory setting is 5-7psi with a direct line from the intake to the WG with nothing in between. Orifice holes in the grainger valve that are too small or clogged will also cause a 3-4psi spike. Is it just the whole turbo is so small that the 2.4 spins it up faster than a mitsu would on an 8v causing the spike? Maybe. But if your boost holds steady after the spike all the way to redline then I'd say your WG isn't opening quick enough.

[Force Fed Mopar]

Stock turbo 2.4's spike like that w/ MBC. Also, the closer to redline, the more it drops, turbo can't keep up..

[GLHNSLHT2]

but he's running a log header and an S60 turbo. Also on stock 2.4's the turbo can't push enough air to get through the crappy intercooler. Upgrade the IC on an SRT4 and it'll hold full boost all day long.

[A.J.]

Next week I'm going to pull the turbo to open up the wastegate hole. While I have it off I'm going to have a bung welded onto the exhaust manifold to check/measure back pressure and I'm also going to ceramic coat the manifold. The turbine housing is already ceramic coated.

Could be a leaky BOV.

Nope, checked it. It holds 20 psi and then starts to open.

Stock turbo?

Chrysler flanged S60 - .63 turbine, .50 compressor w/stage 1 wheel.

My PT GT with stock turbo and computer would spike 3 psi before boost settled. I'm not sure you need to worry about this being abnormal.
If the wastegate hole was too small what you would also see would be boost creeping, not being able to maintain enough flow through the wastegate to limit the boosting.
maybe your signal line to the wastegate actuator is restricted? Or too long? Going to a MBC that is remotely mounted either in cab or on the inner fender adds length to that signal line that will slow that wastegate actuator response, thus increasing the potential for spiking.

Vacuum line to the actuator is 3/16" and less than a foot long, when bypassing my home made boost controller.

[A.J.]

Harry just reminded me, when we were initially tuning my P.E. table, we unhooked my wastegate and I still got 10 psi of boost. It wasn't instant but it did eventually get to 10 psi. So I think my waste gate hole is too small.

[GLHNSLHT2]

yep, or the turbo is too small

[A.J.]

yep, or the turbo is too small

As far as the turbo goes, it may be too small but it feels good on the street. My .48 Garret that I had on my 2.5 was awesome on the street and the .63 was a bit more laggy. If I never went to the track I would have been perfectly happy with the .48. Now I'm at the same point with the .63 on the 2.4, do I stick with it because it feels good on the street or do I go bigger and deal with a little more lag to get better track times.

[turismolover22]

If you adjust your arm on your wg can to make it longer, it will help reduce the ammount of total boost you see, as well as help with spike issues.

[A.J.]

If you adjust your arm on your wg can to make it longer, it will help reduce the ammount of total boost you see, as well as help with spike issues.

Maybe on other application but not mine. I played with different arm lengths and it didn't make any difference.

[A.J.]

I got my turbo and manifold off today. I took the TU 3" swing valve off and you can see by the carbon stains that it could use some porting.

[A.J.]

I ported my wastegate hole. I gained 1/8" up and down and 1/4" side to side. I couldn't do a perfect circle because the hole was already close to the ceiling of the turbine housing.







I also took off my exhaust manifold and had an O2 bung welded on so I could measure turbo to head backpressure. I also had a flexable oil line made for the turbo. A lot of people suggested I do that.



My backpressure measured 25 psi when I have 20 PSI going into the intake. Everything I've found on the internet says 2:1 is average. I'm 1.1-1.2:1 so I'm doing good in that department.

I'm still getting a boost spike but the 14-19 lb spring that was geting me 15 psi of boost is now getting 20 psi of boost. The only other thing I did was grind down some sharp edges in the manifold. My turbo Volvo friend suggest I do that because it could cause some goofy turbulance. It only took me an hour of grinding so why not. I'm getting a boost spike to 25 and it's bleeding down to 20. I haven't had time to mess with it too much. I'm going to try the lower lbs springs and playing with the wastegate arm preload.