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Thread: Help with tuning for new exhaust.

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    Help with tuning for new exhaust.

    I'll start with a little history, I have mentioned some of this in my build thread but I figure for tuning advice it's best to summarize. And disclaimer, I am slowly learning HPTuners and tuning non carbureted engines in general.
    I have a stock junkyard L61 engine from a 2004 cavalier (120k miles good compression) in a RWD car used for endurance racing, up to 24 hours straight on road courses. The harness and ECU came from a guy who supplies them for rail buggy conversions and was a very simple plug and play to get the car running.
    My team and I raced for around 12 hours in our first outing without any engine/tune issues, at that time we were running a turbo Saab exhaust manifold with a 1 5/8 pipe to a cheap Jegs resonator for the exhaust. (The Saab manifold fitted the engine bay better than the stock Cavalier manifold).
    For our next race I added a Tsudo header into a home made 3 inch exhaust with a long cherry bomb for a muffler and we ran into problems with the engine misfiring when it got up to temperature and into closed loop mode arond 4000 rpm and it would not really pull much above 4500. We thought this was a fuel delivery issue and spent most of the race swapping fuel pumps, pressure regulators, filters and rerouting the return line to the cell messing with fuel pressure by restricting the return line and did notice a slight improvement with higher fuel pressure.
    Back in the garage I started looking at data from the engine and found that the long term fuel trim would max out at 25% if I held the rpm at around 4000 for 30-45 seconds. The exhaust not smell rich and the plugs showed it was lean so the theory is that the new exhaust and ebay pod intake and filter(that we ran for the first race) has increased the volumetric efficiency so much that we need to set new fuel trims to account for this.
    To confirm the lean condition I ran the car at a trackday last friday and increased the fuel pressure first to 60 psi with a noticeable improvement, it didn't misfire but was flat and would not pull above 5000 rpm, the at 70 psi where it got better but still flat and not pulling like it should. Next I swapped in a Saab 2.0 turbo fuel rail with Siemens Blue injectors (approx 20% more flow than the stock injectors) and found that the LTFT did not max, it peaked and sat at 22% and on track it was awesome! Pulling to 6000 rpm in top gear with 3.272 gears so just under 130 mph on the longest straight.
    When I swapped the saab injectors in I didn't change the injector coefficient so effectively fooled the ECU into delivering more fuel that it is allowed to.

    So my question....Should I really need to reset the fuel trims to account for the exhaust? It seems like the ECU should be able to compensate for but everything I am seeing tells me it can't. Is there anything else in the tune I could do to tell the ECU to deliver more gas with the correct injectors or should I just stick with the bigger injectors and leave it as is? I'm concerned about lean conditions, we race WOT most of the time and don't want to destroy an engine.

    Any tips/advice welcome, I have my tune from the car if there is anything to post, I did compare it with a stock L61 tune uploaded to HPTuners and the low rpm tables are quite different but above 3600 rpm where we spend most of our race are almost identical.

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    You don't want the let the closed loop system do your correction for you. Yes, it can, BUT it is not intended to do so and if you have to pedal for another driver or turn the car doesn't understand how or why the injectors are going to react when it needs to add fuel for accel or remove for decel. The injector constant is what tells the computer how the injectors will react buy quantifying it's flow rate which is impacted by rail pressure. Ideally you would log your AFR with a wideband sensor to correct this, but this could be done a bit more crudely by logging your LTFT by cell matching your VE tables and applying the logged results to your VE in the program. Our VE tables are in percentage, and the fuel trims will log in percentage so it is more or less copy paste, don't forget to smooth.

    Something to keep in mind, when the ECU enters WOT mode, it is NOT in closed loop. The feedback loop is broken under WOT so the ECU is giving the motor whatever the table says assuming it is correct.

    Here is a link to an injector constant calculator for swapping injectors or fuel pressure. CLICK

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    I played a little with this over the weekend and am slowly getting more comfortable with the software.

    After setting the correct injector constant for the Saab injectors I mapped what I could for the LTFT in the garage and it was again maxing out at 25% in a lot of places so I just increased the high rpm VE table by 10%, smoothed and mapped again and it ended up in the 0-8% range. I think this is good enough to take it to the track and map under load without doing any damage. I plan on installing a wide band before the next track day as well so I can see what's going on on track.

    I found 28 Lb/hr injectors from a Chevy trailblazer that I may buy, these are the same flow as the Saab injectors but I can use them with my Cavalier fuel rail that has the AN fittings opposed to rubber hoses and clamps also I have a variable pressure regulator on the cavalier rail so if I need a little more flow I can always bump the pressure.

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    Check with your fuel pump manufacturer for pump specs. Usually they have a chart or table available that will show the pumps flow rates at various fuel pressures. That's good info to know when you start adjusting your rail pressure.

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    Should be good at least to 65 psi with this:
    http://walbrofuelpumps.com/GSL392-Walbro-Fuel-Pump.html

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    Does anyone know of a way to connect a wideband O2 sensor to a HP Tuners Standard VCM?
    I found this for corvettes http://www.corvetteforum.com/forums/...hp-tuners.html and assume something similar can be done with our engines, I don't have AC wiring to splice into as my harness is cut down a lot but I assume there is someplace in the ECU that I can feed the wideband signal into so it shows up in HP Tuners.

    I guess I could use the EGR valve pin since it's not used on this engine, anyone know which pin this is?
    Last edited by Brett85p; 09-15-2015 at 02:00 PM.

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    I don't think this is possible since there is no EGR on our PCM's. The best solution would be the A/C pressure which is pin 37 on the C1 connector, a red/black wire.

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    Awesome thanks!
    And this thread nicely details the pin numbers!
    http://www.ecotecforum.com/forums/sh...onnector-views
    Last edited by Brett85p; 09-16-2015 at 09:06 AM.

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    We managed to get the wideband feed wired into PIN 37 and could read a voltage between 1 and 5 in HPTuners when we displayed the AC input but when we tried to add it as a new PID with the formula it reported as not supported or invalid or something. Need to do some more research but it is promising to see the voltage feed from the wideband reported in HPTuners. Also it turns out my laptop needs a solid state hard drive as it kept rebooting from the bumps on the track.

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    Finally got the wideband feed working in HP Tuners via the AC voltage PID, so what should I be aiming for at WOT?

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