[ROVERNET - UK] Air filters and vacuum gauge

Fletcher gofanu at usachoice.net
Mon Feb 12 18:04:56 GMT 2007

The stock housing actually does a very good job of  distributing air, 
but it is an ugly bugger. The backplates for the stock unit are superb, 
and a very important part of the airflow. Square edges at the carb 
intake are bad, you need that big radius entry. Either fit stock filters 
in the can, or K&N elements in the can if you actually do a lot of 
miles. If it's looks you want, use the stock backplates and go find some 
suitable elements, as big as will fit in the space available, either 
paper or K&N, and make your own top plates to hold them on.
The area of the pleated paper or K&N filters is the area of the unfolded 
pleated filtering media. The area of the things you have is that of the 
holes in the chromey cover, minus any reduction caused by the similar 
perforated inner retainer and the alignment between the two, so 
Perforated things are not on, unless they are far bigger than anything 
that will fit in the space available. The restriction caused by the 
holes also vastly increases the velocity throuogh the already poorly 
filtering foam elements, sucking anything but the largest of birds right 
Be advised that the stock cans typically cause an intentional 
restriction at high speed, as a matter of reducing intake noise. Any 
change in this needs to be offset by different carb needles, or the 
mixture goes way lean at the top end, which is sure death for 
performance and pistons. For instance, I long ago learned that XKE would 
not pull the hat off your head on the top end if the filter housings 
were not in place and properly sealed. This is true of all engines with 
"airhorn" type filters, any changes require mixture recalibration and 
increase intake noise. It is actually a violation of US Fed law to 
change this nowadays, due to noise regs.
Taking vacuum signals off the vac advance port may or may not work, They 
are frequently so located that the signal is not representative of true 
manifold vacuum, but rather gives appropriate signals for the spark 
control. This varies by specific design of the advance system, and 
changes from year to year and carb to carb. I forgot about the check 
valve, yes, you have to be on the manifold side of that.

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