Signs of excessive backpressure?

[brandon19]

Hi all, not sure if I am seeing signs of excessive backpressure or a restriction elsewhere. After 6000rpm or so the car feels like it has a big power drop, it was more noticeable when I was revving out to 7000RPM in 4th gear and honestly after 6500rpm, it was struggling to even move the rpm needle, or is this normal at such RPM on stock camshafts etc.

I have a Mazda 6 MPS, semi-built motor (rods + pistons) with the following mods:
- B5855 turbo (G30-770 equivalent)
- CP-E EWG manifold with v-band welded on for turbo
- EX44 GFB EWG
- 4" intake
- Upgraded HPFP
- Stock intake manifold ported and VTCS deleted
- AEROFLOW 600x300x76 race series intercooler
- 3" downpipe no cats
- Stock exhaust modified at the Y split in the rear to a single exit 2.5" (I think this might be my issue)

I have provided a log of my latest run, which my MAF sensor shorted out right after it, so I wasn't able to test further.

Also in the virtual dyno I posted, you can see the power falling.

282kw at 5800RPM dropping to 260kw by 6500RPM. 7000RPM will just be more of a drop off if it was logged.

 

[Enki]

Edit:
The power numbers are 378 HP @ about 5300 RPM and 357 ft-lbs @ about 5700 RPM
...
And ~38 PSI of boost!?

Also OP if your tuner says this tune isn't done yet, you need to slap yourself and have some patience.
Also #2: yeah of course if you neck down the exhaust you're going to lose out on top end. Hell, you shouldn't even be on stock exhaust if you're pushing for more than 400 WHP, which at almost 40 PSI I assume you are.

 

[brandon19]

It is 38 psi absolute pressure, that includes barometric pressure, so minus 15.0 for where I am, that is around 23-24psi.

I am self tuning the car, tune definitely is not complete because I had to call it due to the MAF issue.

I have since asked a mate if I can borrow his 3" cat back to see the changes, so I will update later today.

 

[Enki]

Go back to a base map or risk overspool. Please post logs in boost PSI and dynos in HP and TQ

 

[brandon19]

Can you define what you mean by overspool?

Another member told me to use 'absolute pressure' over 'boost psi' and do the math after because it logs with better resolution due to 'boost PSI' being calculated by VersaTuner.

On a base map, what sort of timing would you suggest running at this sort of load level?

 

[Enki]

Boost pressure doesn't matter, it's just a number you can use to compare good health to bad later on down the road...Load is the real thing you need to worry about, because that's the measure of airflow vs cylinder fill rate.

As for overspool, if you just open up the fart side on the same map, the pressure ratio and thus force against the turbine will significantly increase. This will cause the turbo to spin up faster and harder, and you WILL overboost (if you've increased flow with that change).

 

[brandon19]

Yeah correct, boost is rather a reference of restriction and yes I will be lowering the WGDC back down and work my way back up again.

With a higher flowing exhaust, odds are I'm going to have a setup that will exceed 400hp easily. When tuning on the stock DI, when do I decide it isn't worth chasing power anymore.

How much can I lean out AFRs within reason to open up room for IDCs. If I'm not mistaken lower timing allows leaner AFRs to be ran but to a certain degree, because it is all about controlling EGTs. I think at the moment near peak torque I am around 2-3* and by redline it flattens around 8-9*.

 

[Enki]

With DI, I wouldn't risk going above 100% IDC (on stock cams) or above 11.78 ish :1 AFR.
Well, unless you're doing class racing, don't care if the motor lasts and want every 1/10th of a HP you can get.

 

[brandon19]

Alright, I'll see how things go then.

Edit:
What sort of TQ / load numbers should I try to stay below, being forged rods, forged pistons but stock head bolts (risk lifting the head).

I have always tuned around 2.2-2.3 load at peak on a stock motor but now seeing loads in the 2.4-2.5, it feels odd. I'm sure it will be fine, but I wasn't sure if you had a general consensus of what you try to stay below.

 

[Easter Bunny]

You will run out of injector before you should have to worry about the motor

 

[Enki]

Exception being trying to make all the torque as soon as possible on a big turbo that spools like a much smaller turbo.

 

[Easter Bunny]

Exception being trying to make all the torque as soon as possible on a big turbo that spools like a much smaller turbo.

nobody wants to talk about your stupid EFR that doesn't run.

 

[Enki]

nobody wants to talk about your stupid EFR that doesn't run.

 

[Crazycanadian]

It is 38 psi absolute pressure, that includes barometric pressure, so minus 15.0 for where I am, that is around 23-24psi.

I am self tuning the car, tune definitely is not complete because I had to call it due to the MAF issue.

I have since asked a mate if I can borrow his 3" cat back to see the changes, so I will update later today.

Absolute Pressure is what the ECU uses for all of it's calculations. While you are tuning, you want to log in absolute pressure.

If you want to reference gauge pressure for Boost. Its Absolute Map - Baro.. You can log baro and do the math if you want.. Or just look up your local baro on a weather app and do the ruff calculation.

 

[Enki]

I don't know why you want to use exclusively absolute pressure, when ultimately it doesn't (or at least shouldn't) matter. What *should* matter are your trims, ignition timing, and AFR...Boost is just a number you look at for turbo efficiency and longevity, IMO.

 

[Easter Bunny]

Boost is just for bragging. You should be tuning for load

 

[brandon19]

Is it normal to misfire anything above 100% IDC, or is every car slightly different? As soon as I touch around 101-103% IDC I will just get misfires, keeping in mind this is above normally above 6500RPM if that matters.

 

[Enki]

Yes, I assume you are running at least some corn mix and have quite high ignition timing. In this case, you need to back that timing or fuel flow down a bit, as you're effectively spraying fuel out the exhaust.

 

[brandon19]

I'm still on pump gas, only around 8 degrees above 6500RPM so using that graph you sent, I should be good up until around 102% which does seem to line up with my misfire.

 

[Enki]

I think you have a cam or crank timing issue, as you should be able to push it much further before misfires. On stock map timing, I've seen logs get close to 150% IDCs before breaking up.

Keep in mind you should avoid going over 100% anyways, as DI is not like PI and there are consequences for this.