14.7:1 is ideal for smog prevention.
12.5 - 13 is best for power.
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| WOT timing and fueling question. https://slantsix.org/forum/viewtopic.php?t=58163 |
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| Author: | Sam Powell [ Sun Aug 02, 2015 11:19 am ] |
| Post subject: | WOT timing and fueling question. |
Even though the engine is fuel injectedn, this is a general engine management question. When I lift the throttle a little at WOT, it seems to pull better. Short of paying for dyno time, do you think this is caused by too little fuel at WOT, or incorrect timing at WOT? I have had carbureted engines behave this way also, and never took the time to fix them. However, with EFI I can make needed changes with a few key strokes. Any ideas are welcome. I will likely pay for dyno time eventually, but for now would like to play around a bit. Any thoughts on an ideal AF ratio for a mild cam mild compression at WOT? It is currently 13:1. Thanks. Sam |
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| Author: | emsvitil [ Sun Aug 02, 2015 5:36 pm ] |
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I've heard of that symptom before. It means you're too lean. Although for a CV carb, it should apply... see step 9 http://nightrider.com/biketech/hd_cv_mods.htm |
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| Author: | Sam Powell [ Sun Aug 02, 2015 5:40 pm ] |
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Thanks Ed. That gives me some direction. Sam |
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| Author: | Josh P [ Sun Aug 02, 2015 6:44 pm ] |
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+1 for too lean. Are you measuring an AFR of 13:1 or is that a setting in the EFI? If it is the latter then your injectors might not be capable of producing enough fuel for WOT (low fuel pressure, clogged injectors, or just undersized injectors). |
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| Author: | Sam Powell [ Sun Aug 02, 2015 7:19 pm ] |
| Post subject: | |
I think the injectors are plenty big, but will check the duty cycle on the next run. These are 24 lb injectors, and am told 18s are in the stock SBC V 8s. The af ratio is set fir 13:1 at WOT, but the O2 sensor is inactive above a certain percent, which I can specify. Don,t remember what it is set at, but probably is somewhere around 80%. I raised the top row of VE values, and then did some auto tune runs with some WOT. The auto tune runs did raise the values in some cells so the assessment of two lean was spot on. More of this is probably in order. I am still wondering if 13:1 is a good WOT target. I must confess, I do not trust O2 sensors. I have had all makes and models give false or skewed readings in almost all of my cars.. Last fall while I was having trouble with the TunerStudio tuning program it was stuck in open loop for about six months, and ran fine. I think they are a necessary tuning aid, but plan on running open loop once things get refined. Thanks for the thoughts and advice. |
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| Author: | Josh P [ Sun Aug 02, 2015 10:05 pm ] |
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For WOT I would guess the best AFR would be somewhere in your 13:1 range, but maybe others can chime in with better knowledge. 24lb injectors with a max duty cycle of 80% should deliver enough fuel to support ~250hp at the crank. If you are close to that limit your injectors might not be just be starting to run out of juice right as you come into WOT. Also injector performance is heavily dependent on fuel rail pressure. If your fuel pressure is dropping a little at WOT it'll throw off the calculation tables and without O2 enrichment feedback you'll go slightly lean. Well that exhausts all my knowledge, hope it helps! |
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| Author: | Sam Powell [ Mon Aug 03, 2015 4:32 am ] |
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When running in open loop the O2 gauge would still display so af could be monitored. There is still doubt about the accuracy of any sensor. This is an essentially stock slant with oversize valves and ported head so am WELL shy of 250 HP. I will pay attention to duty cycle on injectors at WOT. I think paying attention to all relevant feedback is the best approach. So Ed's article definately offers important clues on the subject. Thanks again for thoughts. I am hoping a few of the racers will add there experience to the mix since WOT is where they live much of the time. Sam |
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| Author: | Dart270 [ Mon Aug 03, 2015 8:20 am ] |
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In what RPM or range of RPM does this happen? You might want it a bit richer than 13:1 in a lower RPM range (below 2500-3000). Still, I wouldn't expect to be able to notice something substantial on lifting, if really at 13:1. Optimum will also likely depend on ign timing. Yes, using all feedback (to you) is best. Lou |
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| Author: | sandy in BC [ Mon Aug 03, 2015 9:33 am ] |
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I vote for timing......as related to TPS. In one case I was able to keep my static and advance curves and adjust the GM TPS (Throttle Position Sensor) TPS is adjusted to prescribed output voltage, |
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| Author: | wjajr [ Mon Aug 03, 2015 11:22 am ] |
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This is a hypothetical question for my own understanding. I think Sam said his injectors are 28 pounds of fuel per hour, and were same used on a small block Chevy. 28 lbs x 8 cylinders = 224 pounds/hr. 28 lbs x 6 cylinders = 168 pounds/hr. I don't have a clue to the relative fuel delivery between the above two engines and what a given size carburetor will deliver on either engine given the same amount of crankshaft revolutions. In other words wouldn't six injectors deliver 75% of what eight injectors deliver given each engine had same number of crank revolutions per hour? Say sbc is 350ci, 75% of that displacement is 262.5 ci one would think there is enough fuel delivery capacity to operate a 225 slant. Or am I all wet, blowing smoke out my ears on this? Bill |
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| Author: | Nicademas [ Thu Aug 06, 2015 12:03 am ] |
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Someone correct me if im wrong but I thought the ideal A/F ratio was 14.7. Then again I also was told to have the A/f mix slightly rich for WOT to prevent detonation. But this is for a carbonated motor. |
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| Author: | emsvitil [ Thu Aug 06, 2015 12:15 am ] |
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14.7:1 is ideal for smog prevention. 12.5 - 13 is best for power.
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| Author: | Nicademas [ Thu Aug 06, 2015 12:32 am ] |
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Nice to see a graft. I put a wide band O2 sencer in my truck and noticed that it "feels" like it had the best power around 13.5-14. Any leaner and it loses power and struggles to get the truck moving. Any richer and it is sluggish. But at WOT it holds steady at 13.0 and runs fine. |
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| Author: | Sam Powell [ Thu Aug 06, 2015 8:17 am ] |
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Thanks Nic, That WOT observation is helpful. Sam |
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