>Flat tappet cams are alot cheaper to produce by manufactures than roller
>cams. There's a lot more cost involved in a roller setup due to the
>component parts being more complex. There's not a lot that can go wrong
with
>a good flat tappet cam setup so, I doubt that manufactures would switch to
a
>roller cam for warranty reasons.
Flat tappet cam lobes will wear prematurely if not broken in properly,
which takes some time and expense. A roller cam requires no break
in so doesn't have this problem.
>I would expect that emissions had more to do with it than anything else you
>mention.
That's the big driver. OEM's have to meet emissions regulations over
longer time and mileage intervals now.
>Manufactures use these (roller) cams because the motor can breathe as well
>as a motor that uses an aggressive (flat tappet cam) profile) while still
>maintaining the smooth idle and road manners of a motor that uses a stock
>profile cam.
My '87 Mustang has an OEM hydraulic roller cam with max lift of 0.444".
It quits making meaningful power around 5000 rpm. It is by no means a
match for an aggressive flat tappet cam. In fact, it's timing events are
very similar to the flat tappet grind in my truck. If the OEM roller were
anything special, you'd see stiffer valve springs. No matter what the
tappet, the quicker the valves are opened, the stiffer the springs must be.
Plus, springs for a hydraulic roller have to be stiffer to begin with to
offset the extra weight of the roller lifters.
By comparison, the hydraulic flat tappet cam in my '66 Mustang has
moderate duration (225 degrees duration @ 0.050) and 0.550" lift.
The springs are substantially stiffer and the lift rate is higher.
Coincidentally, those specs are very similar to Ford Motorsport's
X-303 grind, one of the hottest they offer for the 5.0.
The smooth idle you mention is due to wide lobe centers (116 degrees
for the OEM roller I mentioned above) and short duration which
together limit the overlap when both intake and exhaust valves are
open. Narrower lobe centers will make better power at the expense
of idle quality.
When duration and lift go up, flat tappet lobe life goes down. A roller
cam has a substantial advantage there. Recently, I've been looking into
cams for the next engine for my Pantera. I'm looking at a cam in the
neighborhood of 0.600" lift and perhaps 240 degrees duration @ 0.050".
Because of the lobe life issue, I wanted to go with a roller lifters but
after discussing the issue with cam grinders, it looks like I'm stuck with
a flat tappet. The hydraulic rollers won't support my desired max rpm and
require too much spring for an aggressive profile due to lifter weight.
Solid rollers are another option. Since they are lighter and don't pump up,
they can rev higher (or support a higher lift rate) at the same spring rate.
The racers I've talked to love them but their reliability on the street is
suspect. Apparently, any clearance in the valve train beats up the needle
bearings. So I'm back to flat tappets.
>The extra lift would allow time for the cylinder to fill more completely
>over a longer time period (at max flow) and would certainly not be wasted.
The additional lift will not increase peak flow. That part will be wasted
on heads that peak below the maximum valve lift. You are correct that the
area under the curve will increase but, with the off-the-shelf grinds I've
looked at, the difference hasn't been that great.
Dan Jones
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