MY1PATH Posted December 2, 2008 Share Posted December 2, 2008 (edited) Determineing Octane Requirement If you scroll down to "7.4 What is the effect of changing the ignition timing?" You'll see what I'm trying to get at. Some time ago I installed a 55,000v ignition coil and moved my spark timing form 12 deg before TDC to 15. I noticed little impovement in power but a definate increase in MPG. with the current drop in fuel prices I decided to experiment some more based on the article linked above. I filled up with premium and set my timing to about 18. I then drove it in multiple RPM ranges up steep hills and put it under a few hard accelerations trying to see if i get a knock or hear any pining. So far no issues, 4th and 5th don't seem as weak as they used to be. It also feels like a can shift earlier and run lower RPM arround town. I didn't expect big results and was surpised I could notice any differance right off. I'll give a few more test drives see what comes maybe leave it for a while and see how it does on mpg. the floor is open; Ideas, Questions, Concerns, Warnings etc... Edited January 24, 2010 by MY1PATH Link to comment Share on other sites More sharing options...
Precise1 Posted December 3, 2008 Share Posted December 3, 2008 Ok, I'm no expert on this but I recall reading an article that basically stated that octane requirements were primarily determined by cylinder compression ratio. That putting too high an octane into a motor actually reduces it's efficiency. This article basically states the same but notes the spark advance to offset the slower combusting higher octane gas. My understanding is that the ECU uses the knock sensor to adjust (retard) the timing when that condition occurs, that's why you aren't hearing any pinging or knocks. The computer won't let it... Therefore, you can only advance the timing so much, probably an unknown amount. My questions: is the MPG difference (you said minimal power gain) worth the difference in price? Does the engine run hotter. I look forward to your observations... B Link to comment Share on other sites More sharing options...
Kingman Posted December 3, 2008 Share Posted December 3, 2008 Ok, I'm no expert on this but I recall reading an article that basically stated that octane requirements were primarily determined by cylinder compression ratio. That putting too high an octane into a motor actually reduces it's efficiency. This article basically states the same but notes the spark advance to offset the slower combusting higher octane gas. My understanding is that the ECU uses the knock sensor to adjust (retard) the timing when that condition occurs, that's why you aren't hearing any pinging or knocks. The computer won't let it... Therefore, you can only advance the timing so much, probably an unknown amount. My questions: is the MPG difference (you said minimal power gain) worth the difference in price? Does the engine run hotter. I look forward to your observations... B What does the ECU do to control the timing? Being advancing or retarding the timing is a manual, physical thing, I'm interested in what the ECU does Link to comment Share on other sites More sharing options...
MY1PATH Posted December 3, 2008 Author Share Posted December 3, 2008 Ok I turned up to 20(farthest I'm gonna go for now) drove it quite a bit more here and there and longer streches. Temps seem pretty much the same. MPG will be high this tank because where I've been driving so I'll have to wait till next tank to see any actual mpg benifit. I don't have any proof but I'd say it seems to be a little peppy on the highway compared to what it used to be. as far as ecu managed timing how much control over other engine functions did the TBI ECUs have? Ok, I'm no expert on this but I recall reading an article that basically stated that octane requirements were primarily determined by cylinder compression ratio. I think octane rating depends on allot of things and yes I agree a vast majority of it has to do with compression ratio but heres some food for thought... some old carb'd 350's can't run pump-gas on 11:1 CR but the LS1 (350) will run on midgrade if you tell it to But allots changed between the 2; fuel delivery, head design, combustion effeciency, ecu controlled coilpacks and fuel maping etc... and some sport bikes have like a 13:1 CR and still run off pump-gas. Link to comment Share on other sites More sharing options...
bitemedoughboy.com Posted December 3, 2008 Share Posted December 3, 2008 Ok I turned up to 20(farthest I'm gonna go for now) drove it quite a bit more here and there and longer streches. Temps seem pretty much the same. MPG will be high this tank because where I've been driving so I'll have to wait till next tank to see any actual mpg benifit. I don't have any proof but I'd say it seems to be a little peppy on the highway compared to what it used to be.as far as ecu managed timing how much control over other engine functions did the TBI ECUs have? I think octane rating depends on allot of things and yes I agree a vast majority of it has to do with compression ratio but heres some food for thought... some old carb'd 350's can't run pump-gas on 11:1 CR but the LS1 (350) will run on midgrade if you tell it to But allots changed between the 2; fuel delivery, head design, combustion effeciency, ecu controlled coilpacks and fuel maping etc... and some sport bikes have like a 13:1 CR and still run off pump-gas. im using the MPI from a 300zx on my pathfinder and the base timing is 15-20 degrees per the FSM for the z31 .... and as far as how much the ecu controls the timing... have you looked in the FSM for the tbi pathfinders? Link to comment Share on other sites More sharing options...
unccpathfinder Posted December 3, 2008 Share Posted December 3, 2008 sport bikes have like a 13:1 CR and still run off pump-gas. I bet the cylinder/block is a lot hotter which allows the fuel entering the engine to further vaporize or reach a higher temp allowing for a more complete combustion; thats just a WAG (wild ass guess) being that i never got into the efficiencies of different grade fuels...One potential hazard that will cause knocking is getting into the region of compression ignition or spontaneous combustion because the fuel has an increased temperature but I guess if you can balance out the timing with the ideal temperature of the octane that you choose to use you could increase efficiency http://www.eng-tips.com/viewthread.cfm?qid=142307&page=7 I did a quick search for efficiencies of the otto cycle you may be able to find more information if you want...One big thing to understand is the differences in the ideal cycle and the actual cycle and trying to hunt to find the fine line between theory and reality (as you can see in figured 3.8 and 3.9) P vs T http://web.mit.edu/16.unified/www/SPRING/p...tes/node25.html Link to comment Share on other sites More sharing options...
MY1PATH Posted December 3, 2008 Author Share Posted December 3, 2008 I bet the cylinder/block is a lot hotter which allows the fuel entering the engine to further vaporize or reach a higher temp allowing for a more complete combustion; thats just a WAG (wild ass guess) some good insight, I think I heard the LS1's run hotter too. Link to comment Share on other sites More sharing options...
MY1PATH Posted December 4, 2008 Author Share Posted December 4, 2008 tonight I ran some errands, My pathy started up slow. HUH? after my first stop It started even slower and pulling outa the lot It suputterd @ 2800RPM. I pulled it over and free-reved it and it was as if it had a rev-limter set to 2800. to lay waste to my first suspision I pulled out the timing light and set it back to 15(what I'd been running for months) and tunred it off for a few min. It started slow but not as slow. I stopped by japanesse auto and the guy there said (inspite my experiment) it sounds like a MAF sensor. (he owns and loves an 89 HB, same motor). b4 replacing anything he said to set the timing back (done) and unplug it for a few hours to re-set it.(In progress) It started up normal after that but still won't rev over 2800. I'm starting a new thread "wont rev over 2800" incase the probem is unrelated. but I'm done with my Timing experiments untill I can prove its un-related. any ideas reply to new thread thanks, -jacobs Link to comment Share on other sites More sharing options...
unccpathfinder Posted December 4, 2008 Share Posted December 4, 2008 hmm that sounds like the typical TBI problem (i think its maf I know theres been tons of threads on it) Link to comment Share on other sites More sharing options...
MY1PATH Posted December 4, 2008 Author Share Posted December 4, 2008 hmm that sounds like the typical TBI problem (i think its maf I know theres been tons of threads on it) If thats the case maybe I'll splurge for the PYTHON MAF to go with my python injectors. I cleaned it a few months ago so I know it can't be dirty. Link to comment Share on other sites More sharing options...
94extreme Posted December 4, 2008 Share Posted December 4, 2008 the floor is open;Ideas, Questions, Concerns, Warnings etc... watch out: http://npora.ipbhost.com//index.php?s=&...st&p=354617 Link to comment Share on other sites More sharing options...
BoneZ Posted December 4, 2008 Share Posted December 4, 2008 some good insight, I think I heard the LS1's run hotter too. work for a GM OEM yes the ls1, lq4, ly6, lsa, and lst, (all versions of 6.0-6.2l) all run 15-20 degrees hotter then the older engines like the 5.7(350) 4.3(262) and such. Link to comment Share on other sites More sharing options...
tekazgtr1984 Posted December 6, 2008 Share Posted December 6, 2008 Just going over this thread and a lot of what I read is very interesting. I've been researching performance modification applications for my 98' R50 in the hopes to increase the output of its VG33E, an engine which, to me at least, is grossly underpowered. It's pretty much stock at the moment, running Bosch Platinum spark plugs and an HKS Super Hybrid intake. I'm running 91 octane fuel with Lucas Octane Booster/Fuel Conditioner additive, and have noticed my rig in considerably more peppy and efficient. I can manage over 400km/tank in 2WD (without hubs) and around 360km/tank in 4WD. Once I've installed manual locking hubs I'd imagine those numbers will increase somewhat significantly. As for other output increasing applications, what is next is beyond me. But with running 91 octane and octane boost, could this have adverse effects on my engine long-term? Sorry for being such a newb... Link to comment Share on other sites More sharing options...
MY1PATH Posted December 7, 2008 Author Share Posted December 7, 2008 (edited) A Higer power ignition coil and enen an MSD ignition module would do some good. If your exploring the area of timing to get more efficiency out of your higer octane you do so at your own risk. I ran 15 degrees BTDC on my TBI motor (factory is 12) without an issue and I had good results and no drawbacks. I think pushing it farther may have hurt it. (went to 20) your MPFI engine runs 15 BTDC from the factory I would not push it more than 3 Degrees beyond factory specs (18 BTDC) Edited December 7, 2008 by MY1PATH Link to comment Share on other sites More sharing options...
bitemedoughboy.com Posted December 7, 2008 Share Posted December 7, 2008 A Higer power ignition coil and enen an MSD ignition module would do some good. If your exploring the area of timing to get more efficiency out of your higer octane you do so at your own risk. I ran 15 degrees BTDC on my TBI motor (factory is 12) without an issue and I had good results and no drawbacks. I think pushing it farther may have hurt it. (went to 20) your MPFI engine runs 15 BTDC from the factory I would not push it more than 3 Degrees beyond factory specs (18 BTDC) have you tried using an timing light with the advance knob to see if there is any change in the total advance? if i remember correctly when i still had my tbi it didnt matter where the timing was set at the total advance was always the same.... you might check the total advance @ 12 degrees and @15 and see if there is a difference... Link to comment Share on other sites More sharing options...
bitemedoughboy.com Posted December 7, 2008 Share Posted December 7, 2008 have you tried using an timing light with the advance knob to see if there is any change in the total advance? if i remember correctly when i still had my tbi it didnt matter where the timing was set at the total advance was always the same.... you might check the total advance @ 12 degrees and @15 and see if there is a difference... one more thing, the FSM says 12° +/- 2° for the base timing, also the timing is based off of readings from: crank angle sensor, AFM, CHTS, & TPS i dont think the computer will allow it to advance too far if your off by 3°-5° as long as your total advance isnt over 34-36° you should be fine. my 280z is at full advance before 3k and has a base timing of 17° and thats with mechanical advance only. Link to comment Share on other sites More sharing options...
MY1PATH Posted December 7, 2008 Author Share Posted December 7, 2008 have you tried using an timing light with the advance knob to see if there is any change in the total advance? if i remember correctly when i still had my tbi it didnt matter where the timing was set at the total advance was always the same.... you might check the total advance @ 12 degrees and @15 and see if there is a difference... that is a good question, I set timing to 15 when I installed the coil (negbor reccomended 3 degrees with any HEI coil) so I don't know if the advance, the coil or both helped my mpg. I used a very basic timing light but I should be getting my own one soon, and I think it has a digital advance as well as a tach so I can ajust for variances. I will have to try that out when I get it. Link to comment Share on other sites More sharing options...
MY1PATH Posted January 24, 2010 Author Share Posted January 24, 2010 (edited) have you tried using an timing light with the advance knob to see if there is any change in the total advance? if i remember correctly when i still had my tbi it didnt matter where the timing was set at the total advance was always the same.... you might check the total advance @ 12 degrees and @15 and see if there is a difference... I just thought to check this today (2 years later lol) no matter where the timing is set it advances +30degrees @ 4500rpm (VG30i) 15BTDC->45 @ 4500rpm 12BTDC->42 @ 4500rpm 10BTDC->40 @ 4500rpm I did some research and the vg30e and vg33e ecu will comensate if the base timing is not 15. So by the time you reach your power band the advance will be the same unelss you have turned the cap beyond the ecu's ablity to compensate (which may cause other problems) Edited January 24, 2010 by MY1PATH Link to comment Share on other sites More sharing options...
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