Posted Tuesday, April 24/07 in Mods & Tests

Is the camshaft a car's heart? Maybe that's the wrong metaphor - it could be the fuel pump. Oil pump? Hmm... Maybe there's no perfect camshaft bio-medical analogy. I came up with the heart comparison because I've had valves on my mind lately.

Valves were also on the minds of the engineers who designed the uber-efficient US-only Metro XFi. Their "economy" camshaft was one of several mechanical improvements that increased the XFi's EPA ratings by 15% (city) and 18% (highway) above its thirstier siblings (the garden variety 3-cylinder Metros).

In pursuit of better fuel economy, a few American TeamSwift.net members have already transplanted XFi cams into their regular Metros. Recently, I followed in their pioneering footsteps, thanks to a friend who performed a cam-ectomy on a junkyard XFi in Utah and sent the prize north across the border on a medevac flight... I mean UPS.

Camshaft 101...

First off, let me be the first to admit I'm no expert on the intricacies of camshaft design, tuning, and its impact on fuel economy. Everything I know on the subject I learned over the past few months right here on the old Interweb.

(If you need a detailed introduction or refresher on what a cam is and what it does, start here: Camshafts - at How Stuff Works.)

• Essentially, a camshaft is a compromise between a number of factors, including sometimes "opposing" characteristics like low RPM torque and high-RPM performance. A cam that favours one factor over another gives an engine a distinct character. Most of the time, engine designers seek to balance these various characteristics to leave the majority of drivers happy with their engine's smoothness, driveability, fuel economy & performance.
• More and more, engines are using variable cam profiles to try to get away from this "compromise" situation. With greater valve control, engine performance can be optimized across a wider range of speeds and demands from the driver.
• Swapping or modifying cams is nothing new. Gearheads have been doing it for generations - although mostly in pursuit of more horsepower and high RPM performance. Thus we have the "hot" cam.

XFi camshaft vs. non-XFi - what's the difference?

The gearhead engineers behind the XFi cam went in the other direction, and came up with a "cool" cam. Its profile favours low-RPM performance to maximize low-speed torque at the expense of high-RPM horsepower.

The XFI's economy cam differs from the regular cam in several ways:

• less lift (how much the valves open)


• less duration (how long the valve stays open)


• more advance (when the valve opens, relative to piston position)

The differences in numbers (from a 1994 service manual, for those who like the nitty-gritty):

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).

And - according to the resident cam guru (superf1y) at teamswift.net, the XFi cam is "6 degrees advanced from the stock cam, based on measurements of intake centerline."

Not GM's first "cool" cam...

The XFi engineers weren't exactly blazing new trails. Decades earlier, GM offered a "turnpike cruising package" in its 1967 Oldsmobile Cutlass model, which was meant to improve cruising efficiency so that "fuel consumption at 80-90 mph would be what owners now expect at 50-60 mph."

1967 Oldsmobile Turnpike Cruiser - the gas guzzling era's version of an XFi?

They modified several of the same things that engineers would later do in the Metro to create the XFi, including using a taller final drive ratio and - surprise, surprise - a special "efficiency cam".

Noting that "internal engine friction increases roughly as the square of rpm, and is relatively unaffected by the load on the engine or the degree of throttle opening," the goal of the Oldsmobile's engineers was to create a car with good torque at low RPM - just like the XFi.

To permit loafing the engine along while maintaining acceptable driveability, the Olds engine included several other improvements from the stock motor, but of all the mods, "the camshaft is the key."

Like the XFi's cam 25 years later, the economy cam in the Olds used lower lift, shorter duration, and advanced timing. As a result of the increased lift rates, both the Olds and the XFi also have somewhat less peak power compared to their less efficient siblings (because the redline is lowered due to the potential for valves to float at high RPM).

Baby's first heart transplant ...

XFi camshaft - the Actual Unit that went into the Blackfly

I'm happy to report that the Blackfly's cam swap went quite easily.

Someone with more experience than myself (XFi owner Rick) said it would be a 30 minute job. Not quite - it took me more like 2 hours. It'd take me 30 minutes to do it again, though.

Some tips for other Suzukiclone owners who may go down this road:

1. I cut my timing belt cover with a dremel so I could remove just the upper portion (the bit that covers the cam sprocket). This was to avoid having to remove both the pulleys from the water pump and crankshaft to get the entire cover off. (I was careful not 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.

The operation was a success ...

Immediate observation from the first test drive: WOW! I like it!

• I was worried I wouldn't be able to detect any difference, or it would be so subtle that I wouldn't be sure if I was just imagining it.
• But the car definitely has more low end cojones than it used to. (OK, it may be more accurate to say it has "a cojone" now, because it had none before. Let's just say it still poses no threat at the local tractor pull competition.)
• I can upshift at least 5 km/h sooner than I used to, and the engine just pulls with no complaint: smooth, smooth, smooth.
• I'll have to back off the ignition timing. On the test drive, I heard some ping when I tried to open the throttle as much as I usually do for pulse and glide driving.

• There's a tiny amount of valvetrain noise that wasn't there before. But hey - what could I expect from swapping out a cam with barely 12,000 km on it for one from a car of unknown condition and year (though we know it has to be at least 4 years older), and with enough miles on it that it was obviously ready for the junk yard.
• Fuel economy is bound to improve: 1) I can shift sooner and cruise at lower RPM's, so losses to internal friction are reduced, 2) I get to open the throttle further (once ignition timing is sorted out) and reduce pumping losses, and 3) with lower valve lift, less energy is expended compressing valve springs (OK, that's a stretch).

6 weeks later ...

The XFi cam suits my econo-driving style much more than the stock unit. The engine is more laid back - perfect for short-shifting, no muss, no fuss. It seems quieter, too, with a deeper sound. (I could be just imagining that part - it's pretty subjective.)

Just as with the longer-legged 1967 Oldsmobile, I think it's safe to say this cam is a perfect match for the taller final drive transmission I put in last year. Like peanut butter and jelly!

Just the facts, ma'am ...

There's one small conundrum: figuring out empirically whether the camshaft has made any difference to fuel consumption. It is, after all, somewhat difficult to do a roadside cam swap on a hot engine for the purposes of immediate before & after comparisons.

But, I will attempt to give it a try, once the weather warms up. Of course letting the engine cool enough to do the swap between runs will allow more variability to creep into the results (there's also the possibility of changes in ambient temperature over a longer test period). I'll just hope that if there's any improvement to be seen, it's large enough to be obvious above any experimental noise.

Resources ...

• Turnpike Cruiser: Oldsmobile Designs a Long-Legged, Strong-Willed Gas Miser - Car Life, 1967
• Discussion of the XFi cam swap - at Ecomodder.com
• Similar discussion - at Teamswift.net
• Project 'nerd gear': taller tranny transplant nets +5.2% MPG - MetroMPG.com
• The holy grail: 82 mpg in a Geo Metro XFi - MetroMPG.com