vim LT4

So I got my car dynoed yesterday and for both runs, I had an air/fuel ratio of 14.7 average (it was between 14.6 and 14.8).

I don't know much about AFRs, but I've been trying to look up the archives about that topic and I understand it doesn't look too good. The car runs great though and puts 300hp/310tq on the rear wheel.

Does it look normal to you? Shouldn't it be around 12.something?

ChrisB

hmm, do you perhaps have the dyno runs to scan in - those numbers do sound very lean for WOT. Just to verify, these are WOT numbers and not idle/part throttle number? What do your O2 sensors read at WOT - they should be around 450, probably 350-600.

vim LT4

Yup, WOT numbers] the a/f ratio curve looks flat accross the whole RPM range, for both runs.

I'll try to figure out what my O2 sensors read at WOT tomorrow, and will post the results here.

DON M

That ratio is stoichiometric (proper balance of oxygen to gasoline molecules for complete burn) which is the "Perfect" ratio. The best horsepower is usually made slightly leaner than that. BUT, unless you like holes in your pistons it should be slightly richer at WOT, for safety.
Good luck,
Don

Aaron's 87

Best power AFR depends solely on the engine, but 12.5 to low 13s is a general rule.

What were they using to measure - a wide band O2? Did they put it in before or after the cats? I'll bet they put it in after the cats.

ChrisB

You can't really say what the perfect A/F ratio is for power, but NA it is normally close to 13:1. My car liked 12.9:1 at the torque peak, leaning out to 13.3:1 at 6800.

I asked about O2's since if that is the correct a/f ratio they will be fairly accurate and give us additional verification. I am guessing you still have catalytic converters on your car, and wideband was a tailpipe probe? If this is the case then your readings are skewed by the converters.

vim LT4

ChrisB, yes they used a tailpipe probe to measure the air/fuel ratio while doing the runs, and I still have my converters.

I plugged my Hypertech powerscan in the computer and recorded some values. I got in 2nd gear, low RPMs, started the recording and went WOT up to 6000RPM. I did it twice to record different sets of parameters. Here are the results (o2 sensors are in run #2)


Run 1:
TIME____RPM____AFR____Retard____Spark___ _Loop____TP(%)____
______________(:1)____(Deg)_____(btdc)__ __________(%)____
0.00_____1068____14.7_____0_____34____Cl osed______7____
0.10_____1105____14.7_____0_____35____Cl osed______7____
0.20_____1126____14.7_____0_____35____Cl osed______7____
0.30_____1163____14.7_____0_____35____Cl osed_____14____
0.40_____1179____14.7_____0_____30____Cl osed_____18____
0.50_____1265____14.7_____0_____27____Cl osed_____19____
0.60_____1318____14.7_____0_____25____Cl osed_____25____
0.70_____1358____14.7_____0_____19____Cl osed_____50____
0.80_____1436____14.7_____0_____17____Cl osed_____86____
0.90_____1554____12.1_____0_____18____Cl osed_____87____
1.00_____1661____12.0_____0_____22____Cl osed_____82____
1.10_____1763____11.9_____0_____29____Cl osed_____82____
1.20_____1857____11.8_____0_____29____Cl osed_____84____
1.30_____1993____11.6_____0_____29____Cl osed____100____
1.40_____2125____11.6_____0_____30____Cl osed____100____
1.50_____2238____11.6_____0_____31____Cl osed____100____
1.60_____2350____11.6_____0_____33____Cl osed____100____
1.70_____2439____11.6_____0_____33____Cl osed____100____
1.80_____2527____11.4_____0_____33____Cl osed____100____
1.90_____2691____11.2_____0_____34____Cl osed____100____
2.00_____2769____11.0_____0_____34____Cl osed____100____
2.10_____2874____11.2_____0_____34____Cl osed____100____
2.20_____2979____11.4_____0_____34____Cl osed____100____
2.30_____3117____11.6_____0_____35____Cl osed____100____
2.40_____3234____11.7_____0_____35____Cl osed____100____
2.50_____3295____11.7_____0_____35____Cl osed____100____
2.60_____3457____11.7_____0_____35____Cl osed____100____
2.70_____3570____11.7_____0_____35____Cl osed____100____
2.80_____3592____11.7_____0_____35____Cl osed____100____
2.90_____3691____11.7_____0_____35____Cl osed____100____
3.00_____3891____11.7_____0_____36____Cl osed____100____
3.10_____4018____11.6_____0_____36____Cl osed____100____
3.20_____4029____11.7_____0_____36____Cl osed____100____
3.30_____4160____11.7_____0_____36____Cl osed____100____
3.40_____4350____11.7_____0_____36____Cl osed____100____
3.50_____4421____11.8_____0_____36____Cl osed____100____
3.60_____4565____11.8_____0_____36____Cl osed____100____
3.70_____4674____11.9_____0_____36____Cl osed____100____
3.80_____4741____11.9_____0_____36____Cl osed____100____
3.90_____4899____12.1_____0_____36____Cl osed____100____
4.00_____4859____12.1_____0_____36____Cl osed____100____
4.10_____5059____12.1_____0_____36____Cl osed____100____
4.20_____5257____12.1_____0_____36____Cl osed____100____
4.30_____5285____12.2_____0_____36____Cl osed____100____
4.40_____5257____12.5_____0_____37____Cl osed____100____
4.50_____5382____12.5_____0_____37____Cl osed____100____
4.60_____5523____12.5_____0_____37____Cl osed____100____
4.70_____5421____12.5_____0_____37____Cl osed____100____
4.80_____5704____12.5_____0_____37____Cl osed____100____
4.90_____5794____12.4_____0_____37____Cl osed____100____
5.00_____5946____12.3_____0_____37____Cl osed____100____
5.10_____5994____12.4_____0_____37____Cl osed_____83____
5.20_____6043____12.3_____0_____38____Cl osed______0____
5.30_____5564____14.7_____0_____37____Cl osed______0____
5.40_____5041____14.7_____0_____37____Cl osed______0____


Run #2:
TIME____RPM____AFR____Ho2S1B1____Ho2S2B1 ____Ho2S1B2____Ho2S2B2____
______________(:1)_____(mV)________(mV)_ ______(mV)_______(mV)____
0.00_____1208____14.7_____43____529_____ 75____580____
0.10_____1218____14.7_____39____404_____ 59____576____
0.20_____1246____14.7_____43____239_____ 55____569____
0.30_____1262____14.7_____39____180_____ 51____565____
0.40_____1265____14.7_____43____149_____ 63____561____
0.50_____1303____14.7_____39____137_____ 39____557____
0.60_____1362____14.7_____35____118_____ 31____541____
0.70_____1427____14.7____682____129____7 02____525____
0.80_____1491____14.7____718____294____6 59____525____
0.90_____1555____14.7____659____329_____ 75____522____
1.00_____1676____14.7_____51____227_____ 71____514____
1.10_____1724____14.7_____47____122_____ 71____490____
1.20_____1888____11.7____792____349____7 96____596____
1.30_____1966____11.7____800____557____7 88____647____
1.40_____2086____11.6____784____616____7 84____663____
1.50_____2250____11.6____780____631____7 76____671____
1.60_____2335____11.6____776____635____7 76____667____
1.70_____2451____11.6____686____612____7 10____635____
1.80_____2601____11.4____678____596____7 14____616____
1.90_____2703____11.2____659____588____7 10____604____
2.00_____2833____11.1____749____604____7 53____608____
2.10_____2958____11.3____757____608____7 33____608____
2.20_____3062____11.5____675____592____6 90____592____
2.30_____3141____11.7____565____580____6 67____580____
2.40_____3248____11.7____537____565____6 31____569____
2.50_____3351____11.7____545____553____6 63____565____
2.60_____3519____11.7____675____549____6 16____557____
2.70_____3557____11.7____737____549____6 39____553____
2.80_____3654____11.7____690____541____6 63____549____
2.90_____3850____11.7____655____541____7 02____545____
3.00_____3980____11.6____624____549____7 25____541____
3.10_____4062____11.7____749____549____6 94____541____
3.20_____4189____11.7____678____561____7 18____545____
3.30_____4343____11.7____682____573____7 45____549____
3.40_____4376____11.7____694____580____7 18____553____
3.50_____4579____11.9____671____573____7 25____549____
3.60_____4644____11.9____741____573____7 33____549____
3.70_____4741____11.9____737____580____6 27____541____
3.80_____4867____12.0____745____600____7 25____545____
3.90_____5016____12.0____729____612____7 22____549____
4.00_____5156____12.2____722____612____6 94____545____
4.10_____5229____12.3____698____596____7 29____541____
4.20_____5343____12.4____604____584____6 82____537____
4.30_____5266____12.4____714____584____7 06____537____
4.40_____5617____12.5____745____592____7 29____541____
4.50_____5596____12.5____725____588____7 14____541____
4.60_____5704____12.5____698____592____7 41____569____
4.70_____5805____12.4____729____608____7 33____596____
4.80_____5863____12.4____737____600____7 25____580____
4.90_____5946____12.3____753____620____7 49____604____
5.00_____6068____12.3____706____624____7 53____620____
5.10_____6183____12.3____710____624____6 82____620____
5.20_____6196____12.3____812____682____8 27____714____
5.30_____5863____14.7____345____694____5 80____718____
5.40_____5411____14.7____522____698____6 35____725____
5.50_____4948____14.7____533____706____5 22____729____
5.60_____4324____14.7____376____710____4 00____733____
5.70_____4415____14.7____314____710____6 82____725____
5.80_____4382____14.7____663____706____4 86____729____
5.90_____4363____14.7_____20____698_____ _8____722____
6.00_____4324____14.7______8____667_____ _4____706____
6.10_____4421____14.7______8____318_____ _4____678____
6.20_____4343____14.7______8____102_____ _4____153____
6.30_____4243____14.7_____12_____71_____ _4_____82____
6.40_____4262____14.7______8_____47_____ _4_____51____
6.50_____4177____14.7______8_____31_____ _4_____31____
6.60_____4177____14.7______8_____24_____ _4_____20____
6.70_____4090____14.7______8_____16_____ _4_____16____
6.80_____4068____14.7______8_____12_____ _4_____12____
6.90_____4068____14.7______8______8_____ _4______8____


Comments welcome

Vince.

ChrisB

Ouch, you are *definitely* running lean at WOT. If there is one thing stock O2's are good for it's telling you if you are very lean - and you are.

1) Do you have a ported MAF sensor. IF so I would switch it out for a stocker.

2) Do you have a FP gauge - your FP might be falling under high fuel demands (this seems likely given the disparity between actual and calculated A/F ratio

3) And type of custom programming, etc. on your computer?

vim LT4

Thanks Chris!

I don't think I do, but I'm not sure. I didn't touch it but the previous owner might have. The MAF doesn't have a screen, so I don't know if it's stock w/ no screen or if it's different altogether. I've never seen what a stock MAF looks like.

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:[/color]<HR><font face="Verdana, Arial" size="2">
2) Do you have a FP gauge - your FP might be falling under high fuel demands (this seems likely given the disparity between actual and calculated A/F ratio
[/color][/QUOTE]

I'm ordering one which hopefully will come in this week. It looks like a tool that one needs to have around

How can you tell the disparity between actual and calculated A/F ratio?

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote][/color]<HR><font face="Verdana, Arial" size="2">
3) And type of custom programming, etc. on your computer?
[/color][/QUOTE]

Yes, it's programmed with the Hypertech PowerProgrammer+. I remember that Hypertech said somewhere (website or doc) that with a 160 stat, they could be a little more agressive in their programming because the cooler engine was less likely to detonate.

Would it be usefull to do similar runs with the stock computer programming?

Vince.

ChrisB

You can tell your calculated and actual a/f ratio's are so far apart with the actual O2 readings. Now you can't discern from about 800+ how rich you are - the numbers aren't precise (transfer function is to steep). But once you get to the leaner range (where you are 500-750) you can be fairly sure you are around 14:1 a/f ratio.

You can tell if your MAF sensor is ported by looking it at. No screen is fine (actually probably a good thing). Look at the sensor and see if there is a big metal fin (divider) running down the middle, or just the wires. If there are only wires it's ported, if there is the metal fin chances are you are fine.

It would definitely be a good idea to try stock programming and do the same thing. Your O2's (pre-cat) should be greater than 800 while at 100% throttle.

Another thing you might check is the fuel filter - try changing it.

vim LT4

Chris,

My MAF is a round piece of metal with a horizontal divider (kind of thick) and the vertical wires, so I guess it's stock with the screen removed.

I changed my fuel filter three days ago (just before dynoing), and the old one wasn't too clogged either.

I'll do the run with stock programming soon then. But now I'm getting worried with my lean-running motor!

Vince.

silver & red CE

Why do you think you're running lean? Your WOT a/f measurements look right on to me. Are you using the factory sensor's reading from the HyperTech Power Scan? The sample rate of the PowerScan isn't the greatest, I don't think the actual reading sensor reading (in mV) can be used as a reflection of the acutal A/F reading because they jump all around. A better scan tool might work, but I'm not sure if the data port can output the data quick enough.

Eric

ChrisB

He used a wideband sensor and read A/F ratios of 14.6-14.8. Now this was throught he cats which skews them, but not *that* much.
Are you looking at his calculated A/F ratios? Those are what the computer thinks he should be getting based on the MAF, injectors pulsewidth, and known inj. size. Look at his O2 readings - yes you can not tune by stock O2's, but his values are far enough to the lean side that we can tell just that (that they are lean). 400-750 MV is not going to give you a 12:1-13]1 A/F ratio.

silver & red CE

Got it, thanks.

Eric

ChrisB

Silver & Red CE
I seem to remember from your ignition post that you are an engineer? Though this is a bit more Chem. than Electrical it gives some good info on the stock O2 sensors - you might appreciate it.


http://shared.slowcar.net/o2_volts.xls

Rich_01659

Vim LT4, do you have any other LT4s to compare too. i had my LT1 dynoed at a local dyno day ,only LT1 there, and also ran AFR of 14.7 at WOT. with no other LT1 to compare to i suspect that the mixture is lean. I took my car to the shop and had the fuel system checked for flow and pressure, no problems were noted. I use an auto x-ray and my 02 at WOT was always 930mv. I tested a LT4 and it was the same 930mv. My point is find another LT4 get a comparison. also the o2 readings look good although a bit lower than my buddies LT4. any thing below 500mv is concidered lean at wot you were around 600/700 mv not as rich as 930 but i would like to see more LT4s compared.

good luck

Mez

If I am reading your runs correctly, it looks like you are NOT running lean.

Run #1: For the first .80 seconds, you were at 14.7 AFR then drop to 11.6 > 12.3 Since it takes time for the exhaust gas to get to the AFR pick up at the muffler tip, this appears normal. Then at 6000 rpm, the throttle closes and then the AFR goes from 12.3 to 14.7. Again, because of the delay in exhaust gas moving to the pick up, this appears normal to me.

Logically, the AFR measurement delay is longer when you go from closed throttle to WOT because the exhaust is moving thru the exhaust system slowly at only 2000 rpm. The delay is short at WOT at 6000 rpm because the exhaust is moving much faster. That is why you are seeing a .8 sec delay in the AFR reading at the beginning of the run and only .3 sec delay at the end.

On your run #2, I see a similar result except for the operator must have let up the throttle after peaking at about 6100 rpm because you see the rpms dropping slowly and AFR goes to 14.7. This also looks normal to me.

By the way, I had a dyno run on my stock LT4 and got 300 HP and 307 lbs torque on all my runs with AFR averaging 11.6 to 11.8 at WOT. I see the 14.3 at the beginning of the run for about 1 second. I do not see 14.3 AFR at 6000 rpm because the operator shut it off at the peak rpm.

Am I looking at this wrong??

ChrisB

The values you are looking at are NOT actual AFR measurments (taken by a wideband) Rather they are the calculated AFR parameter that the computer logs, based on MAF flow rate, injector pulsewidth, and injector constant. The numbers themselves would be fine except they are an indirectly calculated value, and not measured directly.

1) A true Wideband meter gave a 14.7]1 reading. Though skewed by the cats this is still lean

2) His stock o2's read from 600-700 at WOT. Again, stock O2's aren't precise enough to get a A/F ratio from, but they are excellent at telling us when we are lean - and this is lean.

Mez

Chris,
Oh, I see now. The posted measurements were recorded by the Hypertech later. I thought they were from the dyno run.

Sorry if I caused any confusion.

YLO Z06

Per my 94 Factory Service Manual, the ideal AFR for an LT1 (and I assume LT4) is 14.7:1. So why is everybody saying he's lean?? The manual also says that at WOT, the PCM will momentarily increase the fuel ratio to 20:1 to compensate for the extra air in the cylinders when you press the gas pedal. But 14.7:1 is dead-on perfect.