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Old 02-09-2009, 04:09 PM   #1 (permalink)
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Dumb Idea of the Day(TM)?: removing a compression ring to mimic the XFi 1-ring piston

Whenever someone asks if it's worth swapping in the XFi 1-ring (compression ring) pistons in place of the garden variety 2-ring style, the opinion seems to be: probably not worth (it in terms of fuel economy gains), unless you're taking the motor apart anyway to do something else.

The implication is that the gains are probably pretty insignificant.

Then Peter7307 posted some interesting figures about the relative amount of friction from bearings, pistons & valvetrain for a Mercedes 2.0L gas engine operating at 2500 RPM and medium load (in the thread about valve spring strength as a possible efficiency mod).

Total engine friction losses were estimated at 1.5 kW:
  • 42% - piston assembly contributing
  • 39% - bearings
  • 19% - valve train
(Source: Engine friction lubricant sensitivities: A comparison of modern diesel and gasoline engines)

(Anybody know approx how much power a 2.0L gas engine would be making in those conditions?)

Anyway, the relative amount of piston drag got me thinking about the XFi pistons again, and thus the Dumb Idea of the Day(?) TM:

Would removing the bottom compression ring be feasible for non-XFi owners?

Here's the XFi piston. Anyone got a pic of a regular piston to compare to it?


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Old 02-09-2009, 04:18 PM   #2 (permalink)
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you can either collapse the second ring or leave it out. We did this on qualifying motors where we didn't want as much drag on internal parts, racing oil filters are another trick (i.e. NASCAR restrictor plate tracks, less filter media to push through, less horsepower)loss
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Old 02-10-2009, 01:09 AM   #3 (permalink)
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Not really worth changing the pistons. Just leave the second ring off
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Old 02-10-2009, 01:41 PM   #4 (permalink)
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You would have to be hardcore about economy to do this.

The gains could be noticeable, but at the expense of increased oil consumption and blowby. Does the crankcase ventilation go directly back into the intake on this engine? If not, the blowby, consisting of unburnt hydrocarbons, etc, could cause more damage to the environment than you would save in fuel consumption reduction. If you reduce the engine compression sealing ability far enough, you'll end up with lower power and worse economy.

As for the overall engine output at this condition, with only 1.5kW of total friction loss, it wasn't making much... Total engine friction power loss at low loads and speeds is often half the engine output. Your link is dead though so I'm not sure I have the whole story.
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Old 02-10-2009, 02:18 PM   #5 (permalink)
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Quote:
Originally Posted by Deezler View Post
You would have to be hardcore about economy to do this.
Sort of like the Suzuki/GM engineers who designed the XFi engine with one compression ring?

Yes, the engine has positive crankcase ventilation back to the intake, so if there is any additional blowby, it's going to get combusted.

Quote:
If you reduce the engine compression sealing ability far enough, you'll end up with lower power and worse economy.
Of course. But again: see the XFi example.

Another potential downside is faster ring wear. Though if I recall, Coyote's experience with these motors is they pretty much require a rebuild at 150k miles regardless of XFi vs. garden variety. At the rate I drive, that means I should set aside some time to tear it down around 2055.

Yet another potential downside someone has pointed out privately to me: the ring spacing may be different between the 2 piston styles, to keep the piston properly aligned in the cylinder vs. thrust angle. If the piston is "unbalanced" it could cause scoring.

Quote:
Your link is dead though so I'm not sure I have the whole story.
Hmm. Works for me. But it's just an abstract. There's really no more useful info than I posted.
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Old 02-10-2009, 02:19 PM   #6 (permalink)
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PS thanks for the pic, Coyote
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Project MPGiata! Mods for getting 50+ MPG from a 1990 Miata
Honda mods: Ecomodding my $800 Honda Fit 5-speed beater
Mitsu mods: 70 MPG in my ecomodded, dirt cheap, 3-cylinder Mirage.
Ecodriving test: Manual vs. automatic transmission MPG showdown



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Old 02-10-2009, 02:37 PM   #7 (permalink)
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Quote:
Originally Posted by guudasitgets View Post
you can either collapse the second ring or leave it out. We did this on qualifying motors where we didn't want as much drag on internal parts
OK, so the obvious question: why didn't you do it on the racing motors as well?
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Honda mods: Ecomodding my $800 Honda Fit 5-speed beater
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Ecodriving test: Manual vs. automatic transmission MPG showdown



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Old 02-10-2009, 07:47 PM   #8 (permalink)
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I had a picture I found from teamswift of both piston types side by side and the top ring is in the exact same position on them both. The only difference is the xfi piston doesn't have a second ring groove cut into it. So really just leaving the second ring out on a metro will work fine with no trouble.

If I had a base metro I would tear it down and take the second ring out but I don't really think any normal person would be willing to do that much work for what would probably amount to 1mpg. For a metro needing a rebuild there is no question I would say leave out the second ring.
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Old 02-11-2009, 12:37 PM   #9 (permalink)
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Oooh. Thanks for the info, Coyote. I went searching for said photo @ TS last night with no luck. Comparing your pic to mine of the XFi piston, the oil ring looks set a bit higher up on the XFi, but it's nearly impossible to tell from 2 different pics.

Anyway, I wouldn't tear the motor down just to pull off a compression ring, but I'm considering shaving the head as well, plus swapping in an old (potentially softer) set of valve springs. Time to start making use of that old 1.0L I pulled from the blue ForkenSwift.
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Project MPGiata! Mods for getting 50+ MPG from a 1990 Miata
Honda mods: Ecomodding my $800 Honda Fit 5-speed beater
Mitsu mods: 70 MPG in my ecomodded, dirt cheap, 3-cylinder Mirage.
Ecodriving test: Manual vs. automatic transmission MPG showdown



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Old 02-11-2009, 07:54 PM   #10 (permalink)
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don't shave the head unless you want no end to pinging and general trouble

I am going to swap mine back to a stock head once the weather gets nicer. If you want to do any head work the best bed would be just a mild deburring/smoothing and put some stainless exhaust valves in it. A 3 or 5 angle valve job might be something to try. I am really not sure if any of that would give you much though but it would probably run smoother. And if you are going that far into the engine a crank scraper and screen windage tray would be worth adding at the same time as well as deburring the rods and bottom end.

Also I think that piston was out of a 4 cyl so the rings wont quite line up even if you could get them side by side. I will look around some and see if I can find that picture. Maybe I will get lucky

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