cygnusx1 Posted July 16, 2007 Share Posted July 16, 2007 We all know that NONE of us can leave well enough, alone. My Z is running spectaculary after a few weeks of street tuning. But of course I can't leave it alone. I have the "tweak gene" in my blood. So I decided to see if I could improve my fuel mileage by leaning out the AFR's in the cruise areas of the map. So far I have 15:1 in the 1500-2000RPM bands at 40-70Kpa. I can definitely feel a "hole" in that part of the map when I drive. How lean has anyone gotten the L28ET in the highway cruise range? I don't feel I can go much leaner unless I try a different load range or something. I want to see if I can beat the factory EFI in MPG's. What's the best way to handle ignition timing for good lean burn? Quote Link to comment Share on other sites More sharing options...
woldson Posted July 16, 2007 Share Posted July 16, 2007 Hee hee, you might want to use the search function, hee hee.... Sorry could not resist. There is a posting on this matter. Can't remember if i saw it under afm or something. Quote Link to comment Share on other sites More sharing options...
cygnusx1 Posted July 16, 2007 Author Share Posted July 16, 2007 Silly me.... http://forums.hybridz.org/showthread.php?t=111088&highlight=lean+cruise+AFR Where is the "I slap myself" icon? I guess the short answer is...how lean, depends on how un-driveable can you tolerate. It's all for experimentation anyhow. Not really trying to make a hero out of the MS. Just playing Quote Link to comment Share on other sites More sharing options...
jgkurz Posted July 16, 2007 Share Posted July 16, 2007 cygnusx1, I too am one of those people who can't seem to leave well enough alone. The leanest cruise AFR I have been able to run on my L28ET without a stumble or hesitation is 15:1. I have an FJO WBO2 integrated into my Tec3 so I think the reading is fairly accurate. I've tried as much as 15:5 to 16:1 which becomes annoying on the freeway from the stumble. Interestingly, I have a datalog from a friends VG30DETT setup and his AFR under cruise gets as high as 16:1. The car cruises perfectly on the freeway. Quote Link to comment Share on other sites More sharing options...
cygnusx1 Posted July 16, 2007 Author Share Posted July 16, 2007 Cool you pretty much corroborate my experience of 15:1, I know it's all about combustion chamber design and atomization/distribution so I guess the "poor" lean-burn characteristics of the L28ET are a limiting factor. Quote Link to comment Share on other sites More sharing options...
randy 77zt Posted July 17, 2007 Share Posted July 17, 2007 lean running has something to do about swirl in the combustion chamber-newer cars are designed for this.i usually run about 14.3 to 14.7 for freeway.i heard long term lean running is hard on valve seats but everybody has thier own opinions on this. Quote Link to comment Share on other sites More sharing options...
Tony D Posted July 17, 2007 Share Posted July 17, 2007 Remember the Chrysler "Lean Burn" engines of the 70's and early 80's before EFI? Some cases they were 17 and 18:1.... Iron Block, Iron Heads, hardened seats... Industrial Stationary engines routinely run 22:1 with stochiometric precombustion chambers for consistent ignition. Stellite is your friend in those engines. Stellite Valve Seats, and Valve Faces. Anybody up for recutting a head to optimize lean burn abilities? You got my attention. Quote Link to comment Share on other sites More sharing options...
Matt Cramer Posted July 17, 2007 Share Posted July 17, 2007 Remember the Chrysler "Lean Burn" engines of the 70's and early 80's before EFI? Some cases they were 17 and 18:1.... And Chrysler didn't, as near as I can tell, redesign their combustion chambers for this on most of their engines. Some of their Lean Burn engines ran 1960s vintage open wedge heads. BTW, my slant six Megasquirt project is using a hybrid of Chrysler Lean Burn and GM HEI parts. Quote Link to comment Share on other sites More sharing options...
BayAreaZT Posted July 17, 2007 Share Posted July 17, 2007 I don't run any leaner than stoich. Any higher than that I get the annoying intermitant stumble to. Quote Link to comment Share on other sites More sharing options...
Bernardd Posted July 18, 2007 Share Posted July 18, 2007 have you tried to reduce the timing when it starts stumbling? Quote Link to comment Share on other sites More sharing options...
jgkurz Posted July 18, 2007 Share Posted July 18, 2007 have you tried to reduce the timing when it starts stumbling? I agree that reducing timing would probably smooth out the stumbling but in my case I really didn't think I needed to run leaner then 15:1. Plus, I'm still unsure whether an extremely lean cruise AFR has long term effects on valves and valve seats. Do you think running say.. 5* less timing and adding full point in AFR would net better mileage overall? Quote Link to comment Share on other sites More sharing options...
Brad-ManQ45 Posted July 18, 2007 Share Posted July 18, 2007 It is my understanding that you need to ADVANCE the timing in order to ge tthe lean mixture fully ignited (on time). I would think that particular attention to the "lean cruise range" for both timing and accel enrichment (@kPa points/rpms) would help a lot. I'll be paying attention to this thread,because my engine is out and I'm starting the MS process... Quote Link to comment Share on other sites More sharing options...
Tony D Posted July 18, 2007 Share Posted July 18, 2007 Leaner Mixes burn faster than rich mixes. It's why explosives are the way they are. If you have a homogenous mixture (high-swirl helps with this) then you can run leaner with less advance and have identical cylinder pressures. Problem is we usually get two flame fronts going, one from one AFR, and another with a much leaner AFR---probably something closer to the lower explosion limit for the fuel being used, and when they collide "Ping"... If you take advance out on a lean engine, the burn starts later but is faster, and hopefully the peak pressure is not reached before the crankthrow has swung to the 'power production side' of the rotation. If you're too lean, and run the advance to high when this occurs your peak pressure still occurs on the engine's compression stroke...this is usually evidenced by a noticable buck of the engine---one cylinder burns faster and tries for a second to run the engine backwards. Same reaction as too much advance when starting---engine is turning slower than the flame front takes to reach peak pressure and if peak pressure occurs before TDC Swingover of the throw, it tries to run in reverse. Richen it up to a point, and it slows the burn, allowing more advance and smoooooother rise to peak pressure. Run methanol and you can watch the flame front move it's so slow...and you can throw in even more advance because it's peak pressure comes on nice and smooth. So with a lean surge, you take timing out and hope your faster burn will reach peak pressure at the right time. For the cylinders not having the fast burn, you start seeing higher EGTs because the fuel in them is still burning at a regular rate. This is why the High Swirl is important, you need that homogenity not only for consistent burn in the cylinder to prevent ping, you need it homogenous cylinder to cylinder to prevent variation and smooth running overall. Quote Link to comment Share on other sites More sharing options...
cygnusx1 Posted July 18, 2007 Author Share Posted July 18, 2007 So maybe my manifold, heated up so much from all this playing around that I warped it or burned the gasket and that's why my motor is chirping like a bird under load....Oh well. Time to track down the chirp and probably buy some new gaskets. I am trying to find that Grafoil gasket for the exh. manifold but am having no luck. Quote Link to comment Share on other sites More sharing options...
Bernardd Posted July 19, 2007 Share Posted July 19, 2007 I agree that reducing timing would probably smooth out the stumbling but in my case I really didn't think I needed to run leaner then 15:1. Plus, I'm still unsure whether an extremely lean cruise AFR has long term effects on valves and valve seats. Do you think running say.. 5* less timing and adding full point in AFR would net better mileage overall? it might, what tony says is basically what happens....it's up to you to find the sweet spot. Quote Link to comment Share on other sites More sharing options...
Brad-ManQ45 Posted July 21, 2007 Share Posted July 21, 2007 Thanks for the info Tony! I remember some old Honda Engines - and maybe some Mitsu's that had a small chamber with a regular A/F ratio opening into the bigger combustion chamber running a lean mixture- CVCC and MCA-Jet I believe were the designations, but I could be wrong.... Anyway, there is a thread here which is where I obtained my info: http://www.msextra.com/viewtopic.php?p=164025&highlight=lean+cruise#164025 Quote Link to comment Share on other sites More sharing options...
WizardBlack Posted July 22, 2007 Share Posted July 22, 2007 I never leaned the car out that low. I usually start at about 2250 or higher and taper it leaner so it runs that A/F without the closed loop system. If you have it tuned for stoich and then closed loop pulls it leaner it will cause some issues. Make sure you are smooth with your A/F targets. As you move leaner, you eventually start to get cooler, but I am not sure if you would get it that far. I would tune some turbo cars for about 15.5:1 and they usually were fine. Again, the car needs to be tuned to hit that without the closed loop system. Also, if the steps the closed loop system takes are too big it will stumble and judder badly. It has to be a tad slower and smaller steps so it doesn't overcorrect into stoich or richer and suddenly get more power, etc. Quote Link to comment Share on other sites More sharing options...
twoeightythreez Posted July 25, 2007 Share Posted July 25, 2007 I had mine running 16.5:1 when cruising cross country. 30mpg. Yes it did stumble a little bit but as soon as I saw any load it would richen straightaway. It was pretty neat seeing the car accelerate up hills without touching the gas pedal. I have since been running it richer. It still goes to around 17:1 when decelerating. Quote Link to comment Share on other sites More sharing options...
Administrators RTz Posted July 26, 2007 Administrators Share Posted July 26, 2007 If you have it tuned for stoich and then closed loop pulls it leaner it will cause some issues. That's contrary to the OE's. They are normally tuned to 'fail rich'. I too share the opinion that closed loop should be pulling fuel, on a leash of course. I'm curious what issues you have run into with this method? Quote Link to comment Share on other sites More sharing options...
hughdogz Posted July 26, 2007 Share Posted July 26, 2007 ...I am trying to find that Grafoil gasket for the exh. manifold but am having no luck. I got mine at the local Nissan parts department for $38...actually $18 with the "Z-club discount". Make sure it is for an L28ET. I think Tony D mentions this in a different thread too. Quote Link to comment Share on other sites More sharing options...
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