On 23 Nov 2002 at 8:09, Eric Conrad wrote:
> I have disabled my air pump by removing the belt...
And Hugh Fader asked:
> Does [the air pump] increase exhaust backpressure?
Here is another data point and opinion. As with the TR6, the
Spitfire's air pump is driven by a single-purpose belt which can be
removed easily. On many cars, the pump is siezed so you either
remove the belt or spend big bucks on a new pump. If you're going to
remove the belt, you might as well remove the pump to save its weight
and plug the injector tubes with the appropriate bolts (or weld (or
braze or whatever) them closed), easier to do wih everything
disassembled.
As for its contribution to cleaner emissions, its purpose was to
provide oxygen for the catalyst. But the catalyst is not likely to
be chemically functional after all this time. By today's standards
these cars weren't especially clean anyway. The engineers were
scratching to get what they could and their target was to meet the
federal specs for idle. The big problem was that low CO and low NOx
were conflicting requirements. Unless you have all of the original
pieces and settings in place, including gas composition and EGR and
PCV stuff, you won't meet those specs. Even so, the big win was
while idling in urban traffic, something I'll bet you don't do much.
If you're really worried about air quality you shouldn't be driving a
25 year old TR in the first place!
As for performance, I can speak only from memory of prevailing
opinion "back then" and from general principles of what "should"
happen. The air pump did rob a lot of power but mainly due to its
mechanical load, a bigger deal on smaller engines like the Spitfire
than on the TR6. Exhaust backpressure shouldn't be affected much
because the amount of air it moves is quite small compared to the
exhaust itself. Manifold temperatures were higher than pre-emission
controls, but this is partly due to the leaner mixture and other
tuning details. (High catalyst temps were a big problem. I was
flagging a race at VIR once when things had to be stopped briefly
while the fire crew put out a grass fire caused by the catalyst on a
Peugeot parked by a spectator in the infield.) The catalyst itself
was a also big restrictor in the exhaust system. Finally, overall
power loss could be attributed to the fact that the presence of the
catalyst upset the traditional exhaust system tuning.
The bottom line would seem to be that the AP isn't a big win today,
except in originality points. :-)
Just another opinion.
--
Jim Muller
jimmuller@pop.rcn.com
'80 Spitfire, '70 GT6+
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