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International heart transplant: the Blackfly gets an XFi cam
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.
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.)
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:
The differences in numbers (from a 1994 service manual, for those who like the nitty-gritty):
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."
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 ...
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:
The operation was a success ...
Immediate observation from the first test drive: WOW! I like it!
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.
darin AT metrompg D-O-T com, or here