Safe Airflow on Stock fuel system

[Artago]

Hey all,

Quick question. How much airflow can I safely run on a stock fuel system?
The reason I ask is because I have a bigger than stock turbo but haven't gotten around to upgrading the fuel setup. So in the mean time I need to run a Conservative boost setting so I need to know how much airflow is safe keeping the IDC under 85%. 25lbs/min? 30lbs/min? Also could someone take me through the calculation so I know how to do them myself?

Thanks,

Tom

 

[tkelly27]

Why don't you get yourself a logger. It's a lot cheaper than doing a rebuild.

The factory computer should give you fuel cut before anything bad happens. I think it might be around 15-16psi. Get a logger though, seriously.

 

[2gGSX]

Why don't you get yourself a logger. It's a lot cheaper than doing a rebuild.

The factory computer should give you fuel cut before anything bad happens. I think it might be around 15-16psi. Get a logger though, seriously.

Absolutely correct.

If you want a solid number though, 85% IDC on 450's at 43 psi base fuel pressure is 2.56 lbs/min of fuel. With the factory tune, you'll be at 9.5:1, and support 24.33 lbs/min of airflow. This is true across all the load maps you should be seeing, assuming you have a normal airflow curve with consistent boost and don't spool above say 4500 or so.

 

[Rob10_99]

Im at about 95% IDC on stock injectors/fuel system at 12-14psi.
That's with a B16g and MBC that doesnt always stay where its set.
Fuel cut is right at 14psi+ I have a new fuel rail, fuel filter, just need to get injectors and a pump/apfr to complete the 16g setup.

IMO I wouldnt go over 12psi on the stock fuel system. As always check you plugs if you think your running out of fuel.
At 12psi @ 7k rpm @ 95% IDC, thats about 11:1 afr's

 

[Artago]

I have Pocketlogger and I just got back from doing some pulls. I ran 8-9 PSI of boost and here are the numbers.

<table width="760" cellpadding="0" cellspacing="0"> <col width="50" /> <col width="42" /> <col width="55" /> <col width="148" /> <col width="35" /> <col width="46" /> <col width="49" /> <col width="63" /> <col width="53" /> <col width="42" /> <col width="77" /> <col width="109" /> <tr> <td width="54" height="20"><strong>Time</strong></td> <td><strong>Load</strong></td> <td width="64"><strong>Coolant</strong></td> <td width="72"><strong>Short Term Fuel Trim 1</strong></td> <td width="67"><strong>RPM</strong></td> <td width="41"><strong>Speed</strong></td> <td width="46"><strong>Timing</strong></td> <td width="71"><strong>Air Temp</strong></td> <td width="63"><strong>Airflow</strong></td> <td width="73"><strong>TPS</strong></td> <td width="84"><strong>O2 1 Bank 1</strong></td> <td width="85"><strong>O2 Trim 1 Bank 1</strong></td> </tr> <tr height="20"> <td height="20" align="right">02:46.8</td> <td align="right">53.44</td> <td align="right">192</td> <td align="right">0</td> <td align="right">2392</td> <td align="right">30</td> <td align="right">24</td> <td align="right">54</td> <td align="right">5.04</td> <td align="right">67.19</td> <td align="right">0.9</td> <td align="right">0</td> </tr> <tr height="20"> <td height="20" align="right">02:47.7</td> <td align="right">55.8</td> <td align="right">192</td> <td align="right">0</td> <td align="right">2572</td> <td align="right">31</td> <td align="right">19</td> <td align="right">54</td> <td align="right">5.74</td> <td align="right">78.98</td> <td align="right">0.92</td> <td align="right">0</td> </tr> <tr height="20"> <td height="20" align="right">02:48.7</td> <td align="right">60.51</td> <td align="right">192</td> <td align="right">0</td> <td align="right">2760</td> <td align="right">33</td> <td align="right">22</td> <td align="right">54</td> <td align="right">6.83</td> <td align="right">80.55</td> <td align="right">0.9</td> <td align="right">0</td> </tr> <tr height="20"> <td height="20" align="right">02:49.7</td> <td align="right">64.44</td> <td align="right">192</td> <td align="right">0</td> <td align="right">3036</td> <td align="right">37</td> <td align="right">21</td> <td align="right">54</td> <td align="right">8.03</td> <td align="right">83.3</td> <td align="right">0.9</td> <td align="right">0</td> </tr> <tr height="20"> <td height="20" align="right">02:50.6</td> <td align="right">67.58</td> <td align="right">192</td> <td align="right">0</td> <td align="right">3332</td> <td align="right">39</td> <td align="right">20</td> <td align="right">54</td> <td align="right">9.25</td> <td align="right">83.3</td> <td align="right">0.92</td> <td align="right">0</td> </tr> <tr height="20"> <td height="20" align="right">02:51.6</td> <td align="right">74.66</td> <td align="right">192</td> <td align="right">0</td> <td align="right">3644</td> <td align="right">42</td> <td align="right">16</td> <td align="right">54</td> <td align="right">11.14</td> <td align="right">84.09</td> <td align="right">0.92</td> <td align="right">0</td> </tr> <tr height="20"> <td height="20" align="right">02:52.6</td> <td align="right">86.84</td> <td align="right">196</td> <td align="right">0</td> <td align="right">3988</td> <td align="right">48</td> <td align="right">15</td> <td align="right">54</td> <td align="right">14.15</td> <td align="right">84.48</td> <td align="right">0.92</td> <td align="right">0</td> </tr> <tr height="20"> <td height="20" align="right">02:53.6</td> <td align="right">89.98</td> <td align="right">196</td> <td align="right">0</td> <td align="right">4392</td> <td align="right">51</td> <td align="right">14</td> <td align="right">55</td> <td align="right">17.15</td> <td align="right">84.48</td> <td align="right">0.92</td> <td align="right">0</td> </tr> <tr height="20"> <td height="20" align="right">02:54.5</td> <td align="right">92.34</td> <td align="right">196</td> <td align="right">0</td> <td align="right">4836</td> <td align="right">58</td> <td align="right">12</td> <td align="right">55</td> <td align="right">18.99</td> <td align="right">84.87</td> <td align="right">0.9</td> <td align="right">0</td> </tr> <tr height="20"> <td height="20" align="right">02:55.5</td> <td align="right">95.87</td> <td align="right">196</td> <td align="right">0</td> <td align="right">5260</td> <td align="right">62</td> <td align="right">10</td> <td align="right">57</td> <td align="right">21.37</td> <td align="right">84.87</td> <td align="right">0.92</td> <td align="right">0</td> </tr> <tr height="20"> <td height="20" align="right">02:56.5</td> <td align="right">94.3</td> <td align="right">196</td> <td align="right">0</td> <td align="right">5672</td> <td align="right">67</td> <td align="right">10</td> <td align="right">57</td> <td align="right">22.79</td> <td align="right">84.87</td> <td align="right">0.9</td> <td align="right">0</td> </tr> <tr height="20"> <td height="20" align="right">02:57.4</td> <td align="right">93.12</td> <td align="right">196</td> <td align="right">0</td> <td align="right">6056</td> <td align="right">74</td> <td align="right">11</td> <td align="right">57</td> <td align="right">23.86</td> <td align="right">84.87</td> <td align="right">0.9</td> <td align="right">0</td> </tr> <tr height="20"> <td height="20" align="right">02:58.4</td> <td align="right">93.12</td> <td align="right">199</td> <td align="right">0</td> <td align="right">6432</td> <td align="right">78</td> <td align="right">9</td> <td align="right">59</td> <td align="right">25.24</td> <td align="right">85.27</td> <td align="right">0.9</td> <td align="right">0</td> </tr> <tr height="20"> <td height="20" align="right">02:59.4</td> <td align="right">89.59</td> <td align="right">199</td> <td align="right">0</td> <td align="right">6772</td> <td align="right">81</td> <td align="right">10</td> <td align="right">59</td> <td align="right">25.5</td> <td align="right">85.66</td> <td align="right">0.88</td> <td align="right">0</td> </tr></table>

Airflow is maxing out 25 lbs/min. Looks like my TPS needs to be adjusted and my rear O2 has a bad connection. O2's look ok.

My only question is about my timing. Shouldn't my timing start around 8-10 and go up to 20-24 and not the other way around.

Thanks all,

Tom

 

[2gGSX]

Airflow is maxing out 23 lbs/min. Looks like my TPS needs to be adjusted and my rear O2 has a bad connection. O2's look ok.

Don't worry about the rear O2, as that's only used for emissions purposes.

My only question is about my timing. Shouldn't my timing start around 8-10 and go up to 20-24 and not the other way around.

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Unfortunately, you're leaning out and probably knocking. This is confirmed by your front O2 value steadily going down.

The expected timing and knock are estimates are off because I didn't interpolate any of the data, but the overall shape of the curve is the same, as is the conclusion that you're knocking.

The stock fuel *should* be able to support this, with you roughly being at 90% IDC and the fuel pump at roughly 85% duty cycle. It obviously isn't though and my first guess is that the fuel pump is not keeping up, especially given the front O2 trend.

Check the connections/contacts for power and ground to the fuel pump, as they're probably corroded and causing you to lose voltage to the fuel pump, which could cause you to lean out. You could also check that the alternator is functioning correctly, and that you're getting enough voltage at high RPM. A more expensive option (but inevitable, given your growing mod list) is to put an aftermarket fuel pump in there and/or rewire it.

Avoid going into boost until you can fix this. You're getting enough knock to do some real damage.

 

[Artago]

^^Yeah, I thought so. For the time being I should probably rewire the stock pump and check the alternator. Correct? I've had some bucking and misfire issue which could be a bad alternator. Also, my voltage when the car is running is low (I'll post the exact numbers once I test it).

 

[2gGSX]

While the voltage that you can actually log is what the ECU sees (not necessarily the same as the fuel pump), a low reading is a good indication that the fuel pump itself isn't getting enough voltage, which would result in your lean condition.

As for now, yeah, check/rewire the pump, and make sure the alternator is working correctly (especially <6000 RPM).

 

[Artago]

What kind of O2 values should I be looking for, 0.92 V?
Can you recommend a method of testing the alt? I've found several ways to do it, I was just wondering what the easiest is.

 

[2gGSX]

It's hard to say what O2 values should be since they're not accurate, especially between different cars. With my car on a completely stock tune, it was about .98V throughout the whole pull. In general though you don't see .88-.92V, especially on a 2g, as that's what a lot of people see after they tune their AFRs.

Most places will actually test your alternator for you, but if you want to do it yourself look through this-- Testing Alternators and Car Batteries

I've never actually directly measured battery voltage while revving the car to ~6000 and up. I don't suspect anything dangerous could happen, but you have liability for your own safety. Worst case scenario, log voltage through the datalogger and rev it to 6,000.

 

[Artago]

It's hard to say what O2 values should be since they're not accurate, especially between different cars. With my car on a completely stock tune, it was about .98V throughout the whole pull. In general though you don't see .88-.92V, especially on a 2g, as that's what a lot of people see after they tune their AFRs.

Most places will actually test your alternator for you, but if you want to do it yourself look through this-- Testing Alternators and Car Batteries

I've never actually directly measured battery voltage while revving the car to ~6000 and up. I don't suspect anything dangerous could happen, but you have liability for your own safety. Worst case scenario, log voltage through the datalogger and rev it to 6,000.

I checked my voltage at idle and it's 14.1 V. I have pocketlogger and there doesn't seem to be an option to log voltage on it. Am I missing something? Can pocketlogger log voltage?

 

[2gGSX]

Ignore me, that's not a loggable parameter. Is there any way you could get someone else to hold the gas pedal or rev it via the throttle body?

For the record, the voltage under load (fuel pump running full bore, lights, radio, etc) is what would be most desirable, along with at a higher RPM since that is where the alternator is prone to not keep up to begin with.

 

[Artago]

^^ Yeah I got someone to help me. I'll give it a shot and let you know how it goes. I probably wont get around to doing it until the weekend however.

Tom

 

[Kevin Jewer]

Front O2 voltage will often go down over the duration of a pull just because of the heat put into it. A WBO2 sensor further from the turbo is able to maintain a steady temp and isn't subject to this additional assclownery.

A rewired 2g pump will support 30 lbs/min at 25 psi and 35 lbs/min at 20 psi, assuming fuel with a specific gravity of .75 and a target AFR of 11:1. Stock 450 injectors at 43 psi base pressure will support 32 lbs/min with the same conditions. 25 lbs/min is a safe number, with a decent tune. Your timing at higher RPMs is low though, as 2gGSX points out in his chart, so something isn't happy. Probably the fact that your pump is not rewired, it makes a huge difference in fuel flow.

 

[Artago]

Front O2 voltage will often go down over the duration of a pull just because of the heat put into it. A WBO2 sensor further from the turbo is able to maintain a steady temp and isn't subject to this additional assclownery.

A rewired 2g pump will support 30 lbs/min at 25 psi and 35 lbs/min at 20 psi, assuming fuel with a specific gravity of .75 and a target AFR of 11:1. Stock 450 injectors at 43 psi base pressure will support 32 lbs/min with the same conditions. 25 lbs/min is a safe number, with a decent tune. Your timing at higher RPMs is low though, as 2gGSX points out in his chart, so something isn't happy. Probably the fact that your pump is not rewired, it makes a huge difference in fuel flow.

Hmm thanks. That is all the info I could ask for and more. I definitely agree that something isn't happy. I get P0300 (Random misfire) and P1400 (MDP) CEL codes. I have to figure out why I'm misfiring. Any thoughts? I though about checking the Ignition coil, alternator, MAS and MDP. I've taken my car of the road in the mean time for diagnosis. Any help would be appreciated.

Thanks,

Tom

 

[spyderturbo007]

I wrote this up awhile ago that should help you pin down airflow at different boost pressures.

 

[Artago]

I wrote this up awhile ago that should help you pin down airflow at different boost pressures.

Wow that's a wicked write up. Thanks for the link and the write up.

Tom