AFR suggestions for max mpg at cruise

[volkaholic1]

I did a search and did not find much, although I am certain this has been disucssed to death, I can't find it. Anyway,
I am running the CB quicktune FI on a 2 liter type IV in a 68 bug. Engine is rather mild, 7.43:1 CR,webcam#73 426" at 224 dur, 44X36 valve heads, good for 85hp. I run an actual 36hp shroud, late model wide fan, venturi ring, stock oil cooler with custom ducting from the fan. Custom made cylinder tins. Oil temp never gets over 200F, cylinder heads never get over 275F even when it is 95F outside. I drive 100 highway miles round trip to work, sending 90% of my time in 1 V/E zone. I have leaned out that zone to a target AFR 15:1. I get 20 mpg. I was really hoping for 25mpg. I realize that every engine is different, but how much leaner can I safely go? My plan is to keep raising the AFR 0.1 at a time and watch head and oil temp. I feel 350F is as hot as I want my heads to get. 220F on the oil is my upper desired limit. Is that a reasonable approach? Any other suggestions on a better way or a better indicator? I really do not want to melts a spark plug or a piston.

[miniman82]

Raise the compression ratio 2 points, put in a better cam and try again. Compression that low is what's limiting my mileage. Come to think of it, a better fix for you would be going turbo. Seriously, the spinning compressor wheel actually adds MPG.

[Steve Arndt]

What is your timing at during your cruise conditions? Try bumping it up a few more degrees when you further lean those map points.

[miniman82]

I'm not sure it will work, but you can try 16:1. For me at the same AFR it was right on the edge of bucking/lean surging, I suspect you'll experience the same.

[supaninja]

What are you using for ignition? Your mileage makes me suspect you are using a 009?

[Steve Arndt]

I'm not sure it will work, but you can try 16:1. For me at the same AFR it was right on the edge of bucking/lean surging, I suspect you'll experience the same.

That must be the Siamese intakes battling for charge (charge robbing). Are you running batch fire still? With a proper manifold design and sequential you can lean it out more without the roughest cylinder limiting you as quickly. I can run 17:1 without a lot of hunting, but 16:1 is smooth.

[supaninja]

I got ITB's and it gets rough right around 16:1 too, when I have some time I'll try leaning it out more, and bump the spark up, maybe add a little dwell.

[Steve Arndt]

I got ITB's and it gets rough right around 16:1 too, when I have some time I'll try leaning it out more, and bump the spark up, maybe add a little dwell.

Matched injectors? The matched "gami" injectors for aircooled aircraft make all the difference. I can actually run 18:1 and it drives smoother than it does at 12:1 under cruise.
I do have high tech lean burn happy super squish pistons though and over 12:1 compression (91 octane) setup.

[Piledriver]

Oddly, I have found ~no measurable change in fuel economy between targeting 13.1:1 of 17:1 (even with slight egt based timing optimization)

Very smooth at 17:1---she'll run at 18:1 but I can't really reliably "see" that with the current calibration setup, the old JAW can read out to 22:1 but I'm using the LC1-default cal table.

Relatively low CR running 87 octane doesn't help.

lean misses show up clearly as single event misses on my setup.

My deck is not optimal, and the plug gaps that may allow higher AFR may hurt running at WOT or under boosted conditions.

Everything's a compromise.

[miniman82]

That must be the Siamese intakes battling for charge (charge robbing). Are you running batch fire still? With a proper manifold design and sequential you can lean it out more without the roughest cylinder limiting you as quickly. I can run 17:1 without a lot of hunting, but 16:1 is smooth.

Sure, but it's a VW turbo with single TB not a Honda.

The biggest thing keeping anyone from reliably running stable high AFR numbers is the cooling system, you simply can't keep the engine at an even enough temperature for it to work. It always works better on a water cooled engine.

[volkaholic1]

OK - guilty as charged on 2 counts:
1 - I built the engine like 12 years ago. At that time I was planning on adding a turbo later, so that is why lower CR and mild cam are OK,,, well here it is 12 yrs later, and still no turbo. When I got the pistons, they showed up dished. I was told that ALL bus pistons are dished. Turns out that is a LIE, but I did not know that. That 15.5cc dish volume dropped my CR from what would have been 9.22:1 to the current 7.53:1. So, if I swapped out to flat top pistons, and kept the 0.050" deck, I'd be at 9.22:1, which seems kinda high for a street engine. I could play the shim game or mill the pistons to get something around 8.5:1, but that is definately a winter project.

2 - I paid for the programable ignition with the MSD dis and super gee whiz coil as part of CB FI kit. I had NOTHING BUT PROBLEMS! I burned up 2 rotors, had all kinds of electrical interference, and finally got zapped so hard my arm when numb. I said "no thank you, I would rather not get killed" so I yanked it all out and went with 009 & blue coil. Yes, less than ideal I know, but it works. I sold the distributor. I would consider upgrading to a SVDA if it would help. I would love crank fire, but I don't have the funds for that right now. I am currently running 8' BTDC at idle. I would have to check my total this weekend.

[miniman82]

The distributor is a big part of the problem, you should get either Megajolt or the CB crankfire kit. That alone should put you at 25mpg easily. If I'm nice to my car I can get 25-26mpg, but most of the time my foot is all the way down and i end up around 18.

[volkaholic1]

OK - double check synco and linkage, then checked timing, here is what I have
rpm advance
800 6
1000 6
1500 10
2000 12
2500 25
3000 27

I am going to bump it up 2 and see. Like I said crankfire would be cool, I have the CB map in the program, but don't like getting nearly killed by the MSD super coil. Should I put on a flash suit and try it again?

[Piledriver]

Your RPM-only curve is probably ~OK for WOT, but you may be 15-20 degrees retarded at cruise.

That typically leads to higher EGT and CHT, and poor economy/drivability.

[supaninja]

9.2:1 is not too high for a street engine, not even close. The tighter the deck clearance, the more the squish which equals more threshhold till the motor knocks/detonates...which means in plain English you can safely run more timing and a leaner mixture.

A SVDA will give you more mpg's right out of the box since it bumps your timing up when in cruise (009 doesn't do that). Of course the best ignition is a computer controlled one, take your pick of the flavor of poison but I have had great success with my MS1 unit controlling spark. It can be done for about the price of a brand new SVDA unit if your creative and patient. MS1 unassembled is 2 bills, trigger wheel, and I am using GSXR coil packs ($10 on ebay), I made my own coil driver ign box for about $40. That's about all you need, you can go fancy and add a coolant and IAT sensor so the MS will pull timing when it gets hot, but that's up to you.