Fuel Cut

In Rob Schoen's '91 R/T TT


Fuel cut - When the ECU temporarily shuts off the injectors because at least one of the following dangerous conditions occured:

As many 3SI owners have explained, it's like you hit the brakes.  Hopefully, you don't kiss the steering wheel or dash in the process.


We experienced fuel cut on the last of several runs as we tried to find the highest boost Rob's car could support.  When the knock sum exceeded safe levels we added high octane fuel and continued.  Rob's car was making good power on this particular cold (~40 deg F) December night (12/10/99).  We didn't realize how hard we were taxing the injectors (IDC beyond 100%) until it was too late; had we known it was so high, we would have stopped sooner.


For those of you not familiar with Rob's car, here are a couple pics along with the engine mods at the time we performed the test (note stock 9B turbos, intercoolers, and injectors):



With these mods, Rob has been able to run 13.15 @ 107mph in the quarter (a 12 second run is just a good launch away).  For more on Rob Schoen's car, visit Detroit 3SI and select "Michigan Owners".  http://www.mi3si.org


As mentioned above, we made several runs , with the following results:


1)  12.8psi (.90 kg/cm2) with Sunoco 94.  1st, 2nd, and 3rd gear acceleration resulting in a peak knock sum of 18 and peak IDC of 98% (20.99ms @ 5625 rpm, near end of run in 3rd gear).


2)  14.9psi (1.05 kg/cm2) with Sunoco 94.  1st, 2nd, and 3rd gear acceleration resulting in a peak knock sum of 21 and peak IDC of 104% (22.02ms @ 5,688 rpm, end of run in 3rd gear).


3)  15.6psi (1.10 kg/cm2) with Sunoco 94.  1st, 2nd, and 3rd gear acceleration resulting in a peak knock sum of 28 and peak IDC of 110% (21.76ms @ 6094 rpm, end of run in 3rd gear).


4)  15.6psi (1.10 kg/cm2) with 50/50 mix of Sunoco 94 & 110 leaded.  1st, 2nd, and 3rd gear acceleration resulting in a peak knock sum of 2 and peak IDC of 105% (20.99ms @ 6000 rpm, 2nd gear, just before shift into 3rd gear).


5)  17.1psi (1.20 kg/cm2) with 50/50 mix of Sunoco 94 & 110 leaded.  1st, 2nd, and 3rd gear acceleration resulting in a peak knock sum of 12 and peak IDC of 108% (20.74ms @ 6281 rpm, 3rd gear, end of run in 3rd gear).


6)  17.8psi (1.25 kg/cm2) with 50/50 mix of Sunoco 94 & 110 leaded.  3rd gear acceleration resulting in fuel cut at 4281 rpm with a low knock sum of 2 and IDC of 98%.


The improvements in knock sum between 3 & 4 were due to the increase in fuel octane.  The higher fuel octane reduced knock to safe levels and may be responsible for lowering IDC (i.e. if the ECU uses knock sum as part of the equation for calculating injector pulse width - less fuel for cooling).  For more details on this subject, visit:



Run 6 is shown below:



The fuel cut is easily spotted above by the pink line (Injector pulse width) which drops all the way to 0ms just before 11 seconds.  Engine speed was only 4,281, knock sum was 2, and IDC was 98%.


Fuel cut was inevitable.  Roger Gerl and others have suggested that cars without mods will likely hit fuel cut at boost levels of ~1.15 kg/cm2 (16.4psi).  For Rob, it was a matter of the ECU learning his driving habit (boost set too high) before it nailed us.  Had we continued with the settings from run 5, we would have eventually hit fuel cut.  It will then occur sooner and more frequent unless something in the system changes such as lowering boost.


It should be explained that the stock 9B turbos can't support the dialed-in boost much beyond the mid rpm range (even with the freed up exhaust).  Recall that any IDC above 100% means the injectors never shut off.  This is dangerous for obvious reasons (computer wants more fuel than the system can deliver).  It's time to start thinking about a fuel upgrade (injectors, fuel pump, and controller) once IDC exceeds ~85%.


However, I don't believe we did damage to the engine on these runs since the knock sums remained low and O2 voltage remained high (0.94 volts - fuel rich, perhaps the upgraded fuel pump increased the fuel rail pressure resulting in more fuel injected?).  The retarded timing (from normal) suggests EGTs and the engine itself may have been safe.  Not that retarded timing is a good thing, but generally speaking results in a cooler, lower peak pressure combustion chamber.  Difficult to say why the timing was retarded.  Perhaps the ECU was operating on a low octane fuel map (could benefit from resetting the ECU and let it re-learn with better fuel).  Only a correctly calibrated and placed EGT gauge would have definitively assessed whether we were safe.  Knock sum, EGT, and O2 voltage are all excellent parameters to monitor if you want to keep it safe.


On such a cold night, the fuel system could barley keep up at 12.8psi boost.  Had it been warmer (70 degrees for example), the warmer less dense intake charge (warmer ambients render the intercoolers less efficient) would have allowed higher boost before reaching 100% IDC but may have also resulted in a higher knock sum.  To run the boost levels we did, we should of had larger injectors and a fuel controller.  Once these mods are done, it's time for a turbo upgrade!


By comparison, my car with 550cc injectors and 17G turbos reaches ~78% IDC at 17.5psi with an acceptable knock sum of 6 (from datalog with Torco 104 octane fuel & a 73 degree ambient).


Much Thanks to Roger Gerl and the TMO list for help in analyzing this page.




since August 7th, 2002


Last Updated: 08/08/02 07:50 PM