It should be fairly straightforward to figure out if it's the turbo. Make a pass at lower boost like 15psi and see if it pulls cleanly to 7k in higher gear. If it does than its likely turbo related if it's not spark blowout. If it still starts actin up in higher gears at lower boost it's probably not the turbo.

 

I agree, but it might be tough reaching 6000+ in high gear without the high boost. When you drive a turbo out of it's happy place enough to lose performance you are normally in high load with flow going way over capacity.
Sounds like he has strange controller issues, might be something causing issue in that system.

Kurt

 

I can turn the boost down easily enough, but I won’t make the power to push it past 6200 rpm in the ¼ with the current gearing. We don’t have a very long shut down area, but I can try to stay in a little past the finish line. Guess I could pop the chute if I needed it. (still never used it). I have a 3.64 gear I bought for the 30” tire. Puts me right at 160 at 7k without any slippage on a 28” tire though, so I think that’s a bit much. Looking for a 3.50 to try.. (3.25 currently).

Don’t know of anyone running a larger T4 frame turbo I could swap with. I could try out the new VS racing billet S480 for $790. It will drop right in my current T4 housing. Think 4mm is a worthwhile upgrade? Upgrading my current turbo with Forced Inductions V3 83-85mm w/ race cover would end up costing quite a bit more. At least this way I’d have another turbo I could sell. Hard to give up what I know is a genuine BW CHRA turbo for a china clone!

Using an MS3X ECU.

This ecu logs any cam/crank sync issues. I’m logging zero sync losses..

I log fuel pressure, and the plugs/AFR look good. Don’t think it’s fueling. This 4303 pump is a monster with the boost-a-pump at 17ish volts. Pretty rock solid 11.0 AFR above 15lbs.

Thanks for the suggestions all!

 

I would swap in a different engine controller if you can as that weird calibration change is alarming. Swapping your box into another car running same system might tell you something also.
If the new turbo will supply more air at your current pressure ratio I would swap that also, but it's surprisingly easy spending your $790!

Kurt

 

That's a good point about different compressor efficiency. What if he went to a smaller tire again just to get it where the rpms will reach in high gear. My money is something electronic as well like you mentioned.

 

You say AFR's remain good....are injector pulse widths also remaining stable, or are they doing anything strange ? Are they comparable to previous runs etc ?

 

I’m sure the china 80mm will flow a bit more, but can’t imagine it will be a night/day difference. $790 isn’t bad on the grand scale if I knew the turbo was the issue. I was quoted over that amount to have mine upgraded to a "name brand" 80-85mm. I’m sure I could sell my current turbo, so I won’t have much invested. Will be a nice comparison for guys running the china 76-80mm billet S400 compressors anyway.

Unfortunately my MS3 ECU is an oddball Canadian model without standard harness plugs on it. So that limits my swap-ability… Going to dig into the harness and check a few things.



Yes, DC and fuel flow are inline with previous runs. It’s in closed loop the whole pass and made very little changes 2-3% here and there. My required fuel parameter had to be skewed a crazy amount to make the AEM inj driver happy with these huge injectors and E85. So my DC and fuel consumption numbers would look like gibberish if I posted them.

 

Im not saying this is the issue, but what I have seen with MS/AEM being odd with timing does not show as a sync issue, because it doesn't think there is an issue (if that makes sense) its simply firing the plug at the wrong time. IE- it thinks you at 15 deg Adv. but because of latency with something its firing at 5, or 0. So you have some issue that does not show up on a log that a new ECU or firmware randomly fixes.

 

If you have eliminated valve spring pressure, converter slippage, ample spark energy and your back pressure reading is accurate and there is no tune/electrical issues, you have probably reached the compressor limit at the pressure you are using. The engine is consuming more air at higher rpm than whats available from the compressor and past a certain boost your just heating the inlet temps reducing power.

 

I still stand by my 2 theories.

Mid-range converter slip or hotside choke. Its possible you are topping out your turbo, but people have ran almost 20 mph faster on a similar turbo. Since boost is still increasing without an IAT spike, I would still shy away from that idea. You can measure boost post-compressor and pre-throttle to give you an idea of how hard the turbo is really being worked.

If your converter is slipping excessively when you turn up the boost, but coupling up top, you'll still make more power and trap the same, with a similar % slippage across the line. My Nova is guilty of this. 4-7% slip up top depending on boost. At peak torque it slips as much as 27%.

Hotside choke doesn't always manifest in backpressure spikes.

 

Not saying It’s not the turbo, just doesn’t make sense. I’ve run a lot of setups that were turbo limited. All of them would lose boost as RPM increased and spike IAT’s. I’m getting none of this. IAT is steady and I can run 20 or 28lbs easily without IAT’s going out of control with no loss of boost with RPM rise. Getting into more boost in the lower RPM picks up on the first half or so, but makes no difference thru the traps.


I still haven’t gotten to try a pass without water, but originally when I put this engine together I did back to back passes with it on/off and saw zero difference. That was gaped at .024 they are now at .019.

Hot-side thing still makes me wonder. Spoke with a few big names and they say you will see a steady rise with RPM in back pressure if the hot side is too small and have experienced this on some large cube builds with small diameter piping. Many others say the math doesn’t jive with the RPM I’m at and a 370 shouldn’t be maxing out 2 fully divided 2” pipes at 6k and 1000 crank hp. Can’t imagine I’m choking out the 1.25 housing and 87/96 wheel… We know what kind of power that’s good for. Sure wish I would have went with 2.125” or 2.25” at this point thou!

I have a VSS, so it should be pretty easy to check my slippage to the ¼ slip. I’ll do that when I get home. It’s a pretty tight converter. Can’t imagine I make enough power to slip a 14 stator 9.5 PTC, especially at lower boost levels. Zbrown doesn’t seem to have any issues, same converter/stator combo making a ton more power. More boost defiantly picks up on the first half… Just does diddly on the top half.

Only thing I can think of I did differently when I assembled this motor was installing new lifter trays with a new set of gen4 gm lifters. I’ll check lifter preload when I pull it apart to see if I bent any push rods… I didn’t do that with the new lifters.

Assuming everything else checks out good, I’ll drop an S480 into my t4 housing and see what happens. Should be able to see either unit easily enough if it doesn’t help.

 

High backpressure is more often a sign of a choking turbine than small diameter piping.
With a small turbine and larger piping, you have less velocity with a high pressure.
With a large turbine and small piping, you have a much higher velocity. Pressure readings can be very sensitive to how they are measured this way.

Also, the closer your backpressure probe is to the turbine, the more its likely to read lower. The way the probe is mounted can also effect the reading.

 

I still want to see you swap to a shorter tire again, hell borrow a buddy's. Pull it to 7k in high gear and see if it does it cleanly.

 

Mine is mounted off the #1 runner. Where the emission stand pipe deal is on the fbody manifolds.





It is still on the short tire. Was on a 30” now it’s on a 28”.

 

When the compressor is done it will still make the drive pressure:boost ratio explode. Since compressor can not driver any more air flow. Can keep driving the turbine harder and harder and making drive pressure rise but boost will not

 

If you want to eliminate turbo from the equation, get the VS billet 80 with standard turbine and run a T4 to T6 adapter flange. If it still chokes, it's your hotside. If not, it's the turbo (assuming the problem is one of these 2). This way you won't have to modify anything other than maybe the downpipe.

 

Crap I can't edit posts lol.

I was gonna say, worst case, I buy the used billet 80 from you lol.

 

Sorry, I should have clarified. You said if you turn down the boost you won't have the power to rev past 6200rpms in the 1/4. What I'm suggesting is you put a 26 or 27" tire on it and turn the boost down so you can Rev it out in high gear and see if it pulls cleanly to 7k at lower boost now that you've sorted out your springs.

 

Before they bump up to $850 I went ahead and ordered the VSracing S480 to try. Can't hurt anyway...

 

damnit man! Throw some short tires on that B and turn the boost down.