2013 Corvette Procharged - power drop above 5,500 rpm

[Chevy406]

I've been doing some tune work on a manual 6-speed 2013 Corvette Grand Sport with BTR blower cam, headers, ~90 lb/hr FIC injectors, and Procharger P1SC. This car is making good power until a point between 5,300 to 5,800 rpm where there is a step drop in power, airflow, injector pulse width, and injector duty cycle. Roughly a 100 rwhp drop that maintains through the rest of the pull. This power drop has been consistent on 5 or 6 dyno pulls.

From my data log I do not see any change in throttle pedal position or voltage, ignition timing stays 17? advance, none of the torque management pids show any sign of activity, and AFR is steady between 11.7 to 12.0. I can correlate the drop in power on the dyno graph with a drop in MAF, MAP (unfortunately this car still has the stock 1bar MAP but reads up to 120 KPa) injector duty cycle, injector pulse width, and can see RPM curve flatten out.

Observations during testing:

• Car has a twin disk clutch and there does not appear to be any sign of clutch slip (no RPM spike)
• No sign of throttle blade position change during the pull
• Blow-off valve appears to close and stay closed during the pull
• No obvious sign of belt slip like dust or excess belt heat
• Boost gauge rises to a peak of 5 or 6 psig then drops when power drops
• I do not know what spark plugs are used or what the gap is
• I do not know if the crank pulley was pinned - need to verify that

Anybody want to take a crack at trouble shooting this power drop? I feel like it is definitely airflow related, and then fuel flow (injector pulse and duty cycle) drop as a result of less airflow. But what is the root cause?

Clyde

[Ben Charles]

Since boost drops something is happening... I bet belt is slipping or something to that nature

[GHuggins]

It actually starts happening at around 5100 according to your dyno graph and even starts to show around there in the log if you watch your cylinder airmass numbers dancing up and down... Like Ben said tighten the belt more or boost leak check your hose connections - may just be a loose clamp letting off pressure after it sees a certain pressure...

[Chevy406]

Thanks for the input. I agree that it really appears to be an airflow problem. In 4 pulls it was quite repeatable and steadily dropped about 100 rpm from 5,800 to 5,600. On the last couple pulls I removed all smoothing in the dyno graph and thats when I saw what GHuggins is talking about the instability before the power drop. I will be sure to update this thread for the record.

Clyde

[Chevy406]

3/12 Update:

Ben Charles and GHuggins were correct - this loss of power was an airflow problem. The mechanic that installed the system corrected a boost leak and the car eventually put down 690 rwhp.
I appreciate your feedback gentlemen. You both continue to contribute greatly to this forum.

One tidbit that I logged from last night - methanol injection appeared to increase peak power by 8-9 rwhp. I lost a coke bet that methanol injection would not increase power without the benefit of more ignition timing. So maybe there is an added benefit of greater air density with the cooler intake air charge?

Clyde

[JekylHyde]

What boost did it make? I only saw like 120 KPA which is just shy of 3 psi right?

[Ben Charles]

That?s Bc the factory 1bar map on Gen4s read up to that point

[JekylHyde]

That?s Bc the factory 1bar map on Gen4s read up to that point

Yeah I didn't even notice he had that at end of description. Thanks.

[Goatmandan10]

I'm having a very similar problem. 04 gto 408 f1a 2700 cfm treadstone intercooler. Big red race valve. No obvious signs of belt slippage. McLeod twin disc. Peaks around 5700 at 15psi then drops to 7psi by 6500 rpms. Cam was spec'd by tick performance when Martin was there to match my setup. Any way to check for valve float? Without just swapping springs. Tired of just throwing money at it. Lol I'm nee to tuning so not sure if theres a log that I can see it.