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Camshafts durations and profiles.

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Camshafts durations and profiles.

Post by LhYnxz on Tue Sep 23, 2008 7:56 pm

"Cam(alot) of power!"Cam profiles and cam durations.

The cam shaft controls the inlet and outlet ports. A pear shaped lobe
on the shaft keeps the valve closed until the pointy bit rotates
producing lift and opening the valve. Double overhead cams have
separate profiles for inlet and outlet ports and are more tunable than
single overhead cams.
The timing of the opening and closing of the port can greatly affect engine
characteristics. Manufacturers profile the cams to give good economy
and reasonable performance through the low to mid rev range.
Some high performance cams come marked for competition use only.
This is important as a competition car is stripped and rebuilt
regularly and the competition cams can wear much more quickly than
standard cams completely stuffing any lift benefits you were hoping to
gain. Running a fingernail over the lobe will show if there is any wear
particularly in the center where contact is made with the lifter - if
there is any more than 1mm wear it is time to replace the cam. A longer duration means more time to suck air and fuel into the engine = more power.




Different engines have different timing needs and a
manufacturer has to fudge things to take a Friday engine into account
along with an engine that has done 70,000 miles so you can see there is
already vast room for improvement.



Cams help create more power (HP) in the mid to high rev
ranges and come in the following types. (Its worth noting than in most
applications high range Torque is unchanged but low range torque can
suffer. The degrees figure relates to the duration of the cam and a
full 360° would not close the valves at all! This implies that ignition timing
must also be reset for cam durations of 260° and above. A longer
duration means more time to suck air and fuel into the engine.)


Mild Road Cam 257° – 260° Gives low end improvements
and used in unmodified engines suitable for all car types and gives a
smooth tickover. Fuel economy may actually improve and power and
responsiveness will be greater.

Fast Road cam 270° – 285° Good compromise for a road
car the tick over is still fairly smooth and emissions are within legal
limits. A good choice for a car used daily and for weekend track use
and perfect for a hot hatch. This profile still gives reasonable fuel
economy and a choppy tickover.

Competition cams 290°–320° The best performance profile
for an engine which works continually over 3000 revs. The tickover is
very lumpy and irregular and the car is lumpy to drive round town. Fuel
economy is pathetic and it is unlikely that a car with this fitted will
pass any kind of emissions test.

Developments – Rover
devised the VVC cam where the cam rotates within a rotating holder and
creates a marvellous tunable engine. The cam durations and lift can be
adjusted to match the engine speed – higher revs can give a different
lift and duration to a slower engine speed.

Originally devised for economy the VVC engine has
become a tuners dream and in its 1.8 form the engine can be tuned to
deliver up to 220 bhp (were not just talking cam modification here).
Not surprising then that this engine is fitted to Lotus and MG's.

Hondas
VTEC valve timing system is equally impressive and provides two stages
of valve timing. A gear chooses the profile with rpm so effectively
there are 2 cam profiles to each valve and a lifter is made rigid so it
picks the faster cam at a predefined rev range.

From the drivers point of view, as the revs climb the
second stage is reached giving explosive power and an aircraft style
scream. You get two cars for your money – an economical round town
supermarket cruiser and a track day screamer. The short cranks in Honda
engines give a lot more rev range and redline at around 7000-8000 rpm a
lot higher than engines of a similar size.

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