Trouble holding boost to redline

On our turbo Jeep we are having trouble holding boost to redline. We will reach and hold 7 psi to about 3000 RPM then it will fall off to about 3-4 psi. Turbo is a 50 trim T3T4 with a dual port wastegate. Currently we have the port at the top of the wastegate hooked up to the intake manifold and the bottom port left open. We have tried tightening the rod to the internal wastegate which has helped some and our preload is up to ½ an inch.

I’ve read about some people hooking up the bottom port of the wastegate to the manifold so that boost helps keep the watesgate shut then running a boost controller. I’m not really clear on how to hook this up and if it would be of any benefit.

The dual port wastegate looks something like this

Typically, you would actually hook up the bottom port, vent the top port. Then boost controller "in-line."

Also: Don't hook up your wastegate to your intake manifold. (No, seriously.)

Another thing to consider is that maybe the turbo just can't handle the 4.0 up top. That turbo sounds a bit small to me.

Anyways, here's a picture.

Keep it closed with exhaust gases.... and see how much boost it makes.

PseudoSport wrote: On our turbo Jeep we are having trouble holding boost to redline. We will reach and hold 7 psi to about 3000 RPM then it will fall off to about 3-4 psi.

Okay, maybe I am confused, but boost is just a measurement of resistance to flow (how hard you are cramming the air in). As the engine rpm increases, there is less resistance to flow, so in order to maintain the same pressure level, you need to flow more air.

If your turbocharger can not support the additional air needed at a higher rpm, the pressure will taper off.

Sorry if this is all elementary to you, I guess I am just confused by your original post. It simply may be that your turbocharger can not flow enough air @ that pressure to maintain it at that high of an rpm.

Edit- Especially because 4.0L's is a LOT of volume compared to your typical 4 banger.

Well we could hook it up between the throttle body and intercooler. I don’t want to run it right from the compressor port because if we have 7 psi off the compressor then a 2 psi drop across the intercooler the intake will only see 5 psi. Boost would also taper to redline and make my current problem worse.

Edit: does a compressor map help?

Also back in 2010 we were running a single port wastegate but it was rated for most boost then we wanted to run. We would hit 10 psi and it would creep to 12 by 4200 where we then ran out of fuel.

We have since lightly ported the wastegate opening too.

If you can do it between TB and intercooler, i would do that for sure. It wouldn't make your problem any worse to run off compressor port, though... unless you were hell-bent on just running wastegate pressure for some reason.

The compressor map helps... but would be more beneficial if we had a flow for the 4.0.

It was the same turbo as 2010? What was the reason for switching wastegates?

I'd try switching to the bottom port and see what happens, first.

I could do the math out by hand but I grabbed this from the turbo calculator. I know this calculator is not perfect but it but it should give you an idea. The red line in the compressor map above should plot this out.

Engine RPM: 5000
Vol Efficency: 87
Intake Temp: 140
Expected Boost (psi): 7.6
Pressure Ratio: 1.52
Airflow (lb/min): 30.9
Airflow (cfm): 447
Airflow (m^3/min): 12.7
Airflow (m^3/sec): 0.0211
Airflow (kg/sec): 0.233

Yeah.... you're barely getting on the chart.

92CelicaHalfTrac wrote: I'd try switching to the bottom port and see what happens, first.

This is the answer...if it isn't you have a leak in your intake tract somewhere or your BOV is prematurely opening, pressurize (build a tester from a PVC pipe end and air compressor fitting tapped in, and clamp it in place of the turbo) fix your leaks...

92CelicaHalfTrac wrote: Yeah.... you're barely getting on the chart.

Yep. If the numbers aren't lying, the turbo is too big for the boost level. You need to up the boost or get a smaller compressor.

Ranger50 wrote:

92CelicaHalfTrac wrote: Yeah.... you're barely getting on the chart.

Yep. If the numbers aren't lying, the turbo is too big for the boost level. You need to up the boost or get a smaller compressor.

I don't read those things THAT well... i just know he's not anywhere close to the meat.

That said... charts are just charts.

If the turbo was behaving better in 2010, step 1 is to just get it back to that point. Step 1 swap wastegate ports. Step 2 boost leak test it.

92CelicaHalfTrac wrote: If you can do it between TB and intercooler, i would do that for sure. It wouldn't make your problem any worse to run off compressor port, though... unless you were hell-bent on just running wastegate pressure for some reason.

My preferred place to take a pressure reading was between the throttle body and intercooler. That worked best on my turbo'ed Miata running 10lbs.

1. Your turbo may be undersized in which case it can't keep up with your 4.0 motor therefore dropping boost up top. The fix for this is a larger turbo.

2. Your wastegate may not be able to hold closed against the exhaust pressure therefore bleeding off boost into the exhaust when you don't want it to. The opposite of boost creep which is when your wastegate can't dump enough exhaust pressure to control the amount of boost sent to your motor. The fix for this is adjusting your wastegate to allow more pressure at the wastergate valve. You may also need a heavier spring in your wastegate to achieve the same result if your wastegate is old and has lost the spring pressure needed to hold the wastegate valve shut against the exhaust pressure.

3. Another possibility is boost leak. Are your couplers tight? Does your intercooler have a leak? How reliable is your blow off valve assuming you have one? Is there anywhere boost could be squeaking out?

Let us know what you find...

That turbo is NOT too big for that motor. If they were really after performance i would actually say the turbine is probably on the small side as it's probably a stg 2 or 3 T3 turbine which is tiny for a 4.0.

Your turbo will EASILY hold a higher boost level to redline. I wouldnt be surprised if it could hold 25+ to redline on an otherwise stock (-flowing) 4.0.

You definitely have wastegate or charge plumbing issues.

My first step would be unplugging everything from the wastegate reference ports and seeing what it does by itself. Be ready to let off...