OK, I ran at the track the other day with the wideband hooked up. Initially, I ran with an 11.5 average a/f ratio. Then I changed it up to a 12.5 and maybe 12.8 way up top rpms. I had no change in mph or trap speed. Why would this be so? I even checked that I actually changed the a/f with the wideband. I subtract and add fuel in the percent change to PE vs rpm. Any suggestions would be appreciated.

 

Couple ideas:
You will get to a point of where the engine is just making all it can. At that point nothing will seem to make a difference.

What Duty Cycle are you running?.
If your in the 95-100% range the injectors are in a state where they are errratic, and making changes in that area just aren't going to make sense.

 

Check out this site: Air Fuel Ration and the SRF . It explains that in the area you are running, the power change is pretty small with fairly large changes in A/F ratio.

 

OK, well it seems that I am seeing a 9.2 ms pulse width at 6000 rpms. I believe that this is 92 percent duty cycle. I still don't buy the math but as far as I know this is correct. This doesn't explain to me how ski, 11sec91z and others have smaller injectors then me and are a few mph faster. I guess I could buy a pressure regulator and bring the fp up and the pulsewidth down. Think this would help?

 

Ohhh, my hypothesis is that I am getting poor air distribtution and when I lean out all the cylinders, some get too lean but some make more power because of a more appropriate a/f mix. This also happens when it is too rich in that the cylinders that go lean are in the proper range and the ones that are usually properly distributed go rich. But that link above did say that lean effects power much more than rich.... I dunno

 

You might be right at the DC that the injectors are going erratic.
Might try adding 15% up top and see if you go richer. Going that much richer should put them fully static.

What's not to buy about the math?.

 

LOL, sometimes that's just the plain-old-fashioned truth. There's a vette guy w/ a 406 that could adjust his fp about 8psi either way and it wouldn't effect his times one bit, neither would larger headers. I think the same could be said for his engine. Gofaster, I guess your tune is very detailed and dead-on like you've said.

 

Go faster...

Ok. Let me first say thanks for being honest with your finding. This is exactly what I have been saying all along with respect to AFRs.

This graph which I posted a while ago explains EXACTLY what you are experiencing.



Its in the nutshell telling you that on the RICH side of AFRs, changed in AFR will make little difference in power. Conversely, on the lean side of the spectrum, small changes will yeild HUGE power swings.

In my own experience which is detailed with numerous dyno runs, richer AFRs will not yeild huge power swings. Maybe 10-12 HP max. Barring any grossly rich conditiion....ie <10.5. Do not look to gain 1 sec by going from 11.9=>12.8AFR. Its just not going to happen with that alone.

If I were you I would start looking at your spark advance. Playing with that CAN/HAS for me, yielded very nice gains. 2* intervals starting low and working your way up, till power drops off.

As a side note this also applies to the controversy of the MAF and 255g/sec, but that is another topic....

PS: Are those MPH at the track or with a gtech? If they are official track MPHs, then for *** sakes man, put some tires on that car. You have to be spinning 40% of the track away!

 

Just cruised your web site, and see your running a LT1 Intake.
Just going from memory, I only remember the individual cylinder fuel trim was less then 2%.
The K+S Knockguard web site had some info., on individual knock control, which would run hand in hand with what your suspecting on your car.

 

Cool graph, Ski. I think that helps me understand more. Was it one of your friend's car that put on the bigger headers and didn't gain any ET? I can't remember who it was know. I also think Gofasterbird posted his time in the TPI board, I think he trapped around 117mph, six less than the G-Tech said.

 

What cam are you running?

People often forget about overlap. Big overlap can mean that oxygen and fuel are being brought in, not used, and pushed back out the exhaust. On an NA engine ... I have seen cases where RICHENING a mixture at 12.2AFRs actually produced more power and a HIGHER reading on the wideband (12.9's). Crazy - but true. Overlap can really screw up what you THINK the sensor reading is. Keep in mind that with a lot of overlap the wideband sensor is not accurate in terms of telling you what AFR the motor is consuming.

... this probably isn't what you are experiencing but it's still important to mention. Overlap is one of the less explored parameters in EFI cam selection. It can create huge power gains - but most people don't know how to tune them so they say to stick with 112 or higher LSA cams.

t

 

Oddly thou, Motec, FAST, Accel, haven't mentioned this.
Hmm, and in that oem's engine powertrain lab I went thur the Horiba's were still using the NTK WB, and they were running some NASCAR engines there.
It just seems odd, they all missed that.

But, if your tuning and measuring results, and not tuning to an AFR it's all rather mute anyway.

Of course reading the works of Glass and others might shed more light on it anyway.

 

Camerojoe,

Yeah Corky and I did an experiement at the track, where he made a few baseline passes with his 1-5/8" headers, then we put on my 1-3/4" headers and made a few more passes.

With his weather station etc, he gained about .06sec is all. So basically here was NO difference in 1-5/8" =>1-3/4". Mind you that was on a pretty beefed up motor, so I doubt a 406 and less CI motor is going to see anything different.

At 117 MPH trap speed he should be at about 11.5-6. Sounds still like he needs some serious help with traction.

Another good example of AFRs not effecting power is my setup the last time we ran it, since the addition of the walbro pump. It not runs PIG rich 11.9AFR. I tried to trim back the fuel pressure, but I have a ****ty breathless performance regulator and the lowest I can put the pressure is ~43psi. The last time at the trap I got two new bests...1.49 60' time(down from 1.53), and a new MPH in the 1/8 mile. And my ETs stayed right at 11.1@123. Now I might have been able to mustar an 11.0x or possibly a 10sec run if the AFR would have been ~12.8, but I am not going to get overly excited thinking that is the case. Just as the graph shows, little difference in power, when in the rich zone. Conversely, had I been seeing AFR of 13.6 and ran a 11.1@123, then I would be excited thinking that more power could be had, and hence a better ET. But as it stands, that is not the case.

I burned a new chip with a few more tweaks, if the weather holds out for tomorrow, then I will have some more results. Picking up a .05sec at this level is next to impossible without more mods etc.

 

They never missed that. They just don't talk about it. Tune an NA 350 with a 108LSA cam on an EFI engine and you'll see what I am talking about. What I have said is true - and you know it. Bigger overlap mean more time with BOTH intake and exhaust valves open. Thus, more of the intake charge (including the fuel) gets pulled straight out the exhaust (at specific RPMs). This is nothing new. Carbed setups have been doing this forever. Fact is - throw an O2 sensor on one of these setups and the AFR you read will NOT be the AFR that the engine is actually burning. The AFR that the wideband is reading will be a combination of what the engine is burning combined with the fresh intake charge being pulled out the exhaust. Things get a lot more complicated when you leave both valves open for a LONG time. Change the LSA by 4 degrees and you get 8 degrees of extra overlap.

Tim

 

But isn't this typically true at low RPMs, i.e. not operating RPMs? I would think that the RPM range that exhibits a lot of overlap would have very low cylinder pressures and low torque? Seems like this would be the RPM range that you'd be flashing through with a torque converter, to get to the less overlap high RPM area where the torque output is good...I guess what I'm saying is, if the car is configured properly, this would be a bit of a moot point, except for closed-loop idle (good luck with that).