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MetroMPG 11-28-2007 01:28 PM

Blackfly XFi? Geo Metro economy cam shaft swap! Update: swap completed
[ A full report of this thread appears here: International heart transplant: the Blackfly gets an XFi cam - ]

Thanks to a certain website overlord who shall remain nameless, I now have in my posession a camshaft from a Metro XFi (year and mileage unknown).

This cam will find its way into the Blackfly's engine compartment in the not too distant future, replacing the extravagantly wasteful OEM cam that came with the car.

Metro aficionados know that the XFi cam was one of several mechanical differences between that model and the garden-variety Metros, which cumulatively gave it a significantly better EPA rating:


In 1994, the XFi was rated at 53/58 US mpg city/hwy - that's an astounding 15%/18% better than a regular '94 3-cylinder, 5-speed Metro (46/49 mpg) . -
How much of a difference the cam will make by itself is anyone's guess.

I'd really like to do some A-B-A to get an actual number, but cam swaps at the side of the road on a hot engine may be problematic. Besides, the return of testing season is 3 months away at best. So I'm just going to install it and revisit the experiment question later.

MetroMPG 11-28-2007 01:30 PM

Won't the car also need an XFi computer?

No. It's a mechanical thing: the different cam profile - lower lift & duration - raises the cylinder pressure and shifts the torque peak downwards, so less throttle is required at low RPM.

It lowers the XFi's HP rating from 55 to 49, but the torque value is unchanged (the RPM at which the max occurs is lower). (EDIT: The cam alone may not be the HP limiting factor - that could be ECU-related.)

A number of people have done similar swaps including GeoMetry on this forum, and 3 that I know of on I don't think any of them also swapped ECUs.[/quote]

MetroMPG 11-28-2007 01:31 PM

Reconsidering the ECU issue...

One thing the XFi ECU may do to compensate for the different cam profile is adjust timing differently. Higher cylinder pressure would mean more chance of pinging under high load / high temps. So a guess is there may be less advance in the XFi ECU under those conditions.

Something to keep an eye on. Or an ear.

MetroMPG 11-28-2007 01:32 PM


The following is from my 1994 metro service manual:

XFI Model
Standard camshaft lobe height is 39.628 to 39.788 mm
(1.5602 to 1.5665 inches).
Wear limit 39.528 (1.5562 inches).

Base and LSI Models
Standard camshaft lobe height is 40.415 to 40.575 mm
(1.5911 to 1.5974 inches).
Wear limit 40.315 mm (1.5872 inches).
Here's some more info about the XFi cam from the member who makes the "economy grind" cams:


You don't want to advance that xfi cam anymore than about 4 degress. They are ground on a different centerline, and coupled with the shorter duration, you might have problem with high dynamic compression, and loss of midrange power.
What that tells me is that in addition to shorter lift & duration, the XFi cam timing is advanced compared to stock.

MetroMPG 11-28-2007 01:34 PM

Note that I've swapped transmissions and have a much taller final drive. So the cam swap with lowered torque peak should be a better match to the gearing I have.

MetroMPG 11-28-2007 01:35 PM

The Actual Unit - 6 lobes of fury:

I'm itching to get this done, but it'll have to wait until winter retreats a little. It's -22 C / -7 F outside. That's frostbite weather, not Blackfly fixin' weather.

MetroMPG 11-28-2007 01:38 PM

One potential effect of raising cylinder pressure too high is pinging/detonation.

RH77 offers some detailed info about the implications:


The more dramatic term for "ping" is "detonation" -- basically the fuel igniting at the wrong time and not having the piston in the right spot to move (some people call it "spark knock". Constant ping will destroy an engine as Ryland put it: extra, undue forces on internals. Rare ping is normal, but I'm like you -- if the car idles funny or pings, I immediately begin the diagnosis and treatment. A common fix for the troubled pinger, is to do a carbon cleanse, adjust that timing, or go up a grade in octane rating. But if you have it happen occasionally, then that's entirely normal. We're talking over 50,000 miles of serious, constant ping to destroy a well-built engine.

Causes range from poorly tuned timing to a abnormal build-up of carbon or even high intake or coolant temps. Ford Crown Vics/Grand Marqs were notorious for building up so much carbon that the engine would ping itself to death. My Father-in-Law's old G-Mq had so much carbon in the intake manifold that it split, spewing raw fuel all over the engine. No fire was started, but at the time, he did trade it on a new 2005 Civic LX Sedan upon the advice of an unnamed son-in-law (and he loves it). Take a ride in a cab and you'll see what I mean. Law enforcement agencies use seafoam or pure water to clean the carbon out of their Crown-Vic units to get them to last forever, so they ping less commonly.

Moral of the story: unless you experience ping every time you drive the car, then I wouldn't worry about it. Otherwise, perform some of the above maintenance.

MetroMPG 11-28-2007 01:39 PM

March 11/07...

Swapped cams this afternoon. Warmest day of the year so far: + 6C, sunny & calm. OK for driveway tinkering!

Haven't driven it yet (just started it up - sounded the same). I'll take it for a spin after supper and will report back...

MetroMPG 11-28-2007 01:43 PM

How, exactly does changing cam lift & duration improve low end torque?

I'll be the first to admit I'm no a cam expert. Everything I've learned, I learned online... recently. Such as:
  • Lower lift & duration, reducing overlap, and advancing cam timing for more low end torque has apparently been understood for a long time.
  • Here's a description of an "economy cam" (designed like the XFi's to shift the torque peak to a lower RPM) in a special "efficient" 1960's era Olds 4-4-2 engine. It has the same effect: lower HP, shifted torque peak (so you can operate the engine more usefully at lower RPM), better fuel economy:

...the camshaft is the key. Intake duration is reduced from 286 to 250', and from 286 to 264' on exhaust. Overlap is reduced from 58 to 36', and lift is reduced from 0.472 in. on the 4-4-2 camshaft to 0.435 on the special ... camshaft.

The vital point of maximum torque is dropped from 3600 to 2600 rpm. - source

I can't defend the decisions of the engineers who made the XFi cam. But the proof is in the pudding - the XFi gets better fuel economy. And I suspect the laws of physics were respected.

MetroMPG 11-28-2007 01:44 PM

A Metro expert suggested it would be perhaps a 30 minute job. It took me maybe 2 hours. It'd take me 30 minutes to do again though

1. I cut my timing belt cover with a dremel so I could remove just the top portion (the part that covers the cam sprocket). If you don't do this, you have to remove both the water pump pulley and the crank pulley to get the cover off (careful you don't cut the timing belt too!).

2. I was prepared for the challenge of getting the valve cover off. There are some spacer/gasket-y things under the nuts. They need to be spun off or the cover won't budge.

3. I had to go to a friend's house to use his vice (to hold the cam sprocket so I could get the bolt out and back in, tight enough). Need to buy a vice. Can't believe I've gotten away without one for so long.

Having said that...

WOW! I like it.
  • I was worried I wouldn't be able to detect a difference, or it would so small that I wouldn't be sure if I was just imagining it.
  • But the car definitely has more low end cojones. (Maybe more accurate to say it has "a cojone" now, because it had none before and I'm still not going to sign up for any tractor pull competitions)
  • I can shift at least 5 km/h sooner than I used to, and the engine just pulls smooth smooth smooth.
  • I'll have to back off the timing though. I was hearing some ping, and on my test drive I couldn't open the throttle as much as I usually do during P&G. But I was told in advance to expect this - since the cam effectively raises the cylinder pressure, uncontrolled combustion is more likely when you tune it near the edge (timing advanced beyond stock).
  • There's a tiny amount of lifter tap that wasn't there before, but hey, what could I expect from swapping out a cam with 12,000 kms on it for one from a car that is at least 4 years older, with who knows how many more miles on it (obviously enough that it was ready for the junk yard).
Anyway - on the test run, even without being able to use as much throttle as I would have liked, I was able to pull off a 84.8 mpg (US) P&G round trip - 9.8 miles, 1 hr EBH assisted start, 3C temp.

So it certainly hasn't hurt!

In fact, I think it's safe to say it's a perfect complement to the taller tranny. Like peanut butter and jelly!

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