Some people are hard on brakes, some are hard on gearboxes. I'm hard on
valves, and exhaust valves in particular. My MGA 1500 tows a luggage
trailer for more than half of its miles, last year the trailer went
10,000 miles. And the car is not timid either, it's typically running
70-75 mph with the cruise control on (4000-4500 rpm), towing or not, and
hot summer weather is not much of a deterrent. It also goes autocrossing
in a big way, enough to wear out a set of race tires every year just on
the track.
When towing at these speeds, the gas mileage is down to about 22 mpg,
sometimes 20 with a head wind, maybe 26 without the trailer. At a steady
60 mph without the trailer it can get 30 mpg. So there's a big
difference in heat generated here.
30 mpg at 60 mph = 2 gal/hr (nice and cool)
22 mpg at 75 mph = 3.4 gal/hr (lots of heat)
Result is shortened life for the exhaust valves. I've been running a
no-lead head since 1989, hardened steel seats and silicone-bronze guides.
After 80,000 miles the intake valves are OK, the guides are still very
good and the seats are absolutely like new, but the seating surface of
the exhaust valves really takes a beating. Standard exhaust valves burn
to the point of loosing compression after a year (15,000 miles) and need
a regrind. Second time around they need to be replaced.
A couple of summers back I treated it to a set of stellite exhaust
valves. 30,000 miles later (half with the trailer) still running OK
(good news), I just pulled the head for a look-see before the upcoming
15,000 mile Alaska trip. The exhaust valves were noticeably worn, a
little concave on the seating surface, and one in particular (#4) was
nearly beyond regrinding, but still sealing OK. So I removed the carbon,
refaced the valves, installed new valve seals (rubber & teflon seals were
still OK), and slapped it back together.
Now a couple of questions. Can someone tell me if the "stellite" is only
skin deep like camshaft hardness, or if is a solid material. Also, could
any of you racing buffs out there recommend an exhaust valve that may be
more burn/wear resistant than stellite valves?
Just thinking ahead,
Barney Gaylord
1958 MGA with an attitude
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