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Old 05-01-2021, 09:09 PM   #11 (permalink)
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I suppose I'd like to ask, what are the goals of your build? Here in the US at least, BMWs have a reputation for being relatively expensive to own and maintain, and an inline 6, though smooth, is never going to be a highly efficient configuration.

Is it to have the lowest running costs possible with your BMW? Highest distance per dollar of fuel? Highest distance per energy content of fuel? Or really just to push the limits of combustion?

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Old 05-02-2021, 01:23 AM   #12 (permalink)
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Quote:
Originally Posted by cRiPpLe_rOoStEr View Post
You mean something comparable to the Thermac valve?
Yes, something like that.

@Ecky - I'm trying to improve engine's efficiency to lower the fuel consumption with E85 fuel.
My optimist goal would be to have the same or better fuel economy than with gasoline.. even tho it'll be a bit cheating going for a 6spd swap.

I'm quite confident, it won't be as difficult as it may sound like.
Since I have a little bit experience of a high compression engine built just for E85 fuel, and it was a lot more fuel efficient compared to factory condition even without lean burn and "enough" ignition timing while cruising.. and no variable valve timing, plus worse aerodynamics than the BMW E39 plus 500cc bigger engine and even shorter gear ratios (not for street use) which aren't really helping the fuel economy.

Yes, BMW's are expensive to own, atleast if they are not well maintained.
Which is exactly what my (cheap) BMW is.. over 20 invidual faults on it , and I've fixed quite a lot of faults already.
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Old 05-02-2021, 06:53 AM   #13 (permalink)
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You'd need to gain around 33% more economy to make up the BTU difference with E85 vs gasoline (though I'm sure it's cheaper per unit volume). I've personally found gearing to be much lower hanging fruit, and lean burn to be more for the last few percent. I increased the numerical ratio in my car's top gear by approximately 40% and found that to be worth something like 25-30% improved economy.

Which of course doesn't mean I'm not all running lean, or that I would discourage your experiments.
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Old 05-02-2021, 10:23 AM   #14 (permalink)
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I agree.
The engine would have run quite a lot more lean to have more effect on fuel economy.
Better improvement could be found with optimizing variable valve timing and ignition advance.. and as you said, even better with longer gear ratio.
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Old 05-02-2021, 11:04 AM   #15 (permalink)
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I once had a 1972 air-cooled VW Super Beetle that I increased the compression ratio from 7.3:1 to 10:1 and ran it lean, around 16:1AFR (I'm not sure what that equates to lambda since I just went by an AFR gauge. The car ran que well.

One big thing that was necessary was to tweak the ignition. Lean burn needs more advance. So yes, I had an air-cooled car with a high compression ratio running lean and had an ignition timing that was quite a bit advanced more than stock mainly driving up and down steep mountain passes. I didn't run any ethanol in the car though. But I did average 30mpg.

I would think if you have sudden changes in acceleration it's due to ignition timing.
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Old 05-02-2021, 02:42 PM   #16 (permalink)
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Quote:
Originally Posted by Ecky View Post
You'd need to gain around 33% more economy to make up the BTU difference with E85 vs gasoline (though I'm sure it's cheaper per unit volume). I've personally found gearing to be much lower hanging fruit, and lean burn to be more for the last few percent.
This really depends on the car, sometimes mechanical friction is the main source of inefficiency (taller gearing helps more), but other times it could be low combustion efficiency or high throttling losses.

Mechanical friction reduction from taller gearing has diminishing returns. If you have like 50-60% mechanical efficiency like my car at cruising conditions, slightly taller gearing drastically cuts down the friction and raises mechanical efficiency a lot. If you're at 80%, the gains are much much less, probably far less than half.

On cars with engines that are tasked with higher average loads, lean burn can be a tremendous help.
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Old 05-02-2021, 02:59 PM   #17 (permalink)
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Quote:
Originally Posted by serialk11r View Post
This really depends on the car, sometimes mechanical friction is the main source of inefficiency (taller gearing helps more), but other times it could be low combustion efficiency or high throttling losses.

Mechanical friction reduction from taller gearing has diminishing returns. If you have like 50-60% mechanical efficiency like my car at cruising conditions, slightly taller gearing drastically cuts down the friction and raises mechanical efficiency a lot. If you're at 80%, the gains are much much less, probably far less than half.

On cars with engines that are tasked with higher average loads, lean burn can be a tremendous help.
Taller gearing doesn't only lower friction, it also increases load. Load reduces throttling losses.
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Old 05-02-2021, 07:52 PM   #18 (permalink)
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I plan to tweak my lambda and timing tables tonight and try to target 1.25 lambda (around 18.5:1) in closed loop, drive around with that for a few tanks. I can't go any leaner than that in closed loop with my current engine management.
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Old 05-03-2021, 01:49 AM   #19 (permalink)
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Quote:
Originally Posted by Isaac Zachary View Post
Taller gearing doesn't only lower friction, it also increases load. Load reduces throttling losses.
Yes, pretty much any car can use taller gearing.

The question is, do you get more out of lean burn? In many cases, it's extremely expensive to make the gearing taller. In other cases, the taller gearing has diminishing returns, while lean burn can add substantial efficiency.
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Old 05-03-2021, 02:53 AM   #20 (permalink)
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Quote:
Originally Posted by serialk11r View Post
...while lean burn can add substantial efficiency.
True. Most all gasoline engine vehicles try to run at stoichiometric for the most part, which isn't ideal. Of course an obvious disadvantage is the greater NOx emissions.

The question is how lean? Lean helps burn up all the fuel. It also reduces throttling losses like EGR, but burns up more fuel. But the leaner you go the slower the flame, which hurts efficiency. You can counteract that to a degree by advancing the timing.

This is why a stratified charge would be ideal. Or maybe multiple sparkplugs.

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