Here is a discussion regarding some of the issues involved in selecting a new
camshaft for your TR4/4A engine. The same issue apply to the earlier engines,
but variation in intake port design may alter the balance point for trade-offs.
First, do you really need to change your cam? Maybe not. The engine designers
did a good job with balancing the trade-offs of the camshaft. Certainly they
knew how to make a "hot" cam, but chose the one they did for very good reasons.
A "hot" cam works at higher RPM, but sacrifices low RPM flexibility (i.e low
idle speed and grunt at low rpm in higher gears. However, the hotter cam
allows more power at the upper rpm range. Unlike some modern short stroke
engine designs, our Triumphs are not able to get safely past what is now
considered a mid-range rpm range. So, the really hot cams that allow good
power over 6000 rpm are not really useful here. The cams we will be working
with are in the range of stock to mild to moderate. The choice of cam will
also be influenced by what other modifications have been made to the engine
to allow the cam to work the way it was intended. It is almost like buying
a person with a bad heart a racing bicycle. Even though the bicycle can go
very fast, the person using it can not sustain the effort to make it go!
What does a "hot" cam do differently from a "stock" one? It does nothing
differently at all, it just does it at a different time. Time is of the
essence, particularly with cams. The cams open and close the valves which
allow the mixture to be drawn into the engine and the spent exhaust to leave
the engine. At low rpm, it is not hard to get the misture into, and the
exhaust out of, the engine. As the rpm gets higher it is progressively more
difficult to get the gases in and out due to drag throughout the system. At
high rpm you have to wait proportionally longer to get the gasses in and out.
This wait is the time that the intake or exhaust valves are open. That is
the entire issue. Higher RPM requires longer valve duration for good
performance than low RPM. In addition, when optimized for one range, it is
not as good for the other range.
As I said above, the use of the hotter cams requires other modifications
to the engine to put the cam to good use. Specifically, you may need a
"better flowing" head. This requires that the ports and valves be treated to
reduce low resistance. This will allow the gases to move more easily so you get
a larger volume in and out of the engine in the same period of time. Again,
there is a trade off here. Very large ports allow huge slugs of gas to be
expelled without having a lot of drag induced by very high flow speeds.
Unfortunately, the flow can be too slow as well, so large ports can allow
the gas to flow so slowly that problems can occur. When these problems
occur it may be impossible to obtain a low idle speed.
Another modification that a hot cam often requires is a higher compression
head. With excessively low compression it may not be possible to get the
compustion efficiency that the engine requires.
So, is it reasonably possible to get a "better" cam without having to modify
the rest of the engine? The answer is a qualified yes. Certainly you can
get some more performance from the engine, but will the cost of the cam removal
and reinstallation be worth the 6-10hp difference? Only you can answer that
one. I, unfortunately, subscribe to the "more is better", or "in for a penny,
in for a pound" philosophy.
I have not yet made the dive into the engine, but since my engine rebuild
will require at least new valve guides, why not shave the head to increase
the compression, and explore the cost of "porting and flowing" the head? Once
I spring for that cost, it would make sense to install larger intake valves.
Larger intake valves require that the head gasket and block be relieved around
the edge of the valve orifice. Since the engine has quite a few miles on it,
and the pistons and liners are aging fast, maybe I should put in 89mm pistons
to increase displacement from 2138cc to 2187cc. Certainly after all these
modifications the carburetors themselves are causing a restriction, and some
other carbs would be necessary. Maybe 2" (instead of the stock 1.75")
SU's, but Webers or Weber-look alikes are better. Webers make sense here
because I have lost some of the low-end anyway with a "hot" cam. Needless
to say, I now need a better exhaust manifold.
You can see where this is going. Since I am not looking for a ton of
horsepower (its not a Corvette afterall!!), I will stick to a more mild cam
where I do not have to worry too much about porting, flowing, new manifolds
and the like. I will run a moderately increased compression, and moderately
improve the head to go with the moderate cam I will install. This will be
only moderately expensive (I hope).
Which cam is which?
VINTAGE/Old Style cams
TYPE Duration Inlet/Exhaust Lift Comments
Stock 254 17-57/57-17 0.375
"D" 284 33-71/71-33 0.393 Race/Street 10:1 compression required
"F" 300 39-81/81-39 0.432 Full Race, 11.7:1 comp., 4000-6000
G-3 309 51-79/79-51 0.499 Racing only, 11.7:1 comp., 4200-6500
SAH #26 264 22-62/62-22 0.388 Mild Cam
Piper 268 24-64/64-24 0.387 Mild Cam
Derrington 280 30-70/70-30 0.435 Near limit for street use(see Elgin 7010-9)
MODERN Cams (none listed here are asymmetrical)
(probably because they are regrinds of the stock symmetrical cam: $100)
Elgin 6710-18 268 24-64/64-24? 0.404 No major Mods req. Pulls Hard, good power (effectively a "1/4 grind")
Elgin ????-? 274 27-67/67-27? ? No description. Effectively a "2/4 grind"
Elgin 7010-9 280 30-70/70-30 0.375 "3/4 grind", Streetable with 87mm and header
BFE #260 260 ? - ?/? - ? .408 "mild" cam
BFE #149 282 ? - ?/? -? 0.425 Slight lope @ idle, but low lift rate, so not as hot as others
The following may not be available for the 4 cyl. TRs
Elgin 71508-18 286 33-73/73-33? 0.436 Req. stronger springs, higher comp, header
Elgin 7508-12 300 40-80/80-40? 0.461 Race only. Prepared head, 12:1 comp, etc.
Elgin 7706-9 308 44-84/84-44? 0.446 Race only, best over 5000 rpm
How do all these variations make a difference? Well, they vary the time that
the valves open and close. To understand why the timing is important, you have
to understand a little of what is happening inside the manifolds and cylinder.
In a model of engine function where the intake and exhaust gases have no
momentum nor and drag, then all you need to do is open the intake valve at the
beginning of the intake stroke and then close it at the end of the intake
stroke, then compress the mixture, fire when the piston hits the top and then
open the exhaust valve when the piston is at the bottom.
In the real world the gases do have momentum and drag. We can use momentum
to our advantage so the engine does not have to do all the work pumping
the gases in and out. Using momentum, the gases will (to a certain extent)
blow themselves in and suck themselves out.
For the intake stroke, we can use the partial vacuum created by the exhaust
gases as they are shooting out of the cylinder to help suck in the new
mixture. In order to do this, we need to open the intake valve before the
exhaust valve is closed, and we can even open it before the piston has come
to the top of the exhaust stroke! At higher revs we need to open the
valves earlier and keep them open proportionally longer in order for the same
amount of gas to be moved. At low revs, prolonged valve overlap will cause
some if the new mixture to be sucked out with the exhaust, causing no end
of problems, and reducing performance. This is the first instance of why
a cam timed for high revs does not work well at low revs.
Now a little later into the intake stroke the exhaust valve has closed and
the mixture is being sucked into the cylinder as the piston moves downward.
When the piston hits the bottom of the intake stroke, the incoming mass of
mixture has momentum driving it into the cylinder, so we can keep the valve
open even after the cylinder has begun its upward compression stroke and allow
momentum to drive even more mixture into the cylinder. The length of the
intake runners will help in this matter as the echo of the intake valve
closing on the last stroke can come back and compress even more gas into the
cylinder. The echo is a wave of gas bouncing back from the carburetor end of
the intake runner just as a wave bounces off a wall in a swimming pool. Again,
at higher revs we need to keep the valve open proportionally longer in the
cycle to get the full charge of mixture. The added charge of mixture
entering the cylinder is a poor man's supercharging.
On the compression stroke the valves are closed, so there is no magic here
as far as valves are concerned until near the end of the firing stroke.
After the spark has fired and the mixture combusted, the piston is driven
downward and eventually it is time to open the exhaust valve. We can
open the exhaust valve before the piston hist the bottom of the stroke
without losing much power. The early opening of the valve gives more time for
the exhaust to leave the cylinder. The early opening also allows the exhaust
to be shot out of the cylinder with a little more force and speed. This is
important later in the exhaust stroke. Again, at higher revs it is necessary
to open the exhaust valve proportionally earlier, but at lower revs, that
will result in noticeable loss of power.
Now the piston is moving upward expelling the exhaust gases. THe length
of the exhaust runner is important since the blast of exhaust has momentum and
can help suck the exhaust out of the cylinder at the end of the exhaust stroke,
and can even help suck in some of the new mixture. The length of the runner is
important since if it is too short it will develop less suction (extractor
effect) than it should. If it is too long then it will cause excessive back
pressure. This is how "tuned pipes" work, and why they are a mixed blessing.
If you are going the right (high, usually) RPM, then the extractor effect is
very useful (as long as your cam is timed appropriately). At lower RPM, the
pipes are too short and you lose the extractor effect.
Now we have been through the entire four strokes of the engine, and it is more
clear why cam selection is important and such a difficult decision.
What is an "asymmetrical" cam? In the old days, and even today, cams
were usually symmetrical. (E.G. the 17-57/57-17 stock TR4 cam) This means
that the intake valve opens 17 degrees BTDC of the intake stroke, and closes
57 degrees ABDC of the intake stroke. In this case the exhaust valve opens
57 degrees BBDC of the exhaust stroke and 17 degrees ATDC of the exhaust
stroke. Why are the numbers the same? I am not sure, but I suppose it
was easier to machine that way (before computer controlled machines) and with
an infinite number of possible asymmetric settings, they stuck with the more
limited choices of the symmetrical style. Nowadays we have computers!
This allows us to make any cam profile we want with very little extra
effort. We just type in a different set of numbers into the computer and
let it make the machine run. In addition, we have supercomputers to do
advanced modelling of the complex flows in the engine. In my description of
the 4 strokes above I never said anything about the exact time the valves
were supposed to open. There is no special reason why you should open
the exhaust valve 17 degrees BBDC just because you opened the intake valve
17 degrees BTDC. As a matter of fact, there is every reason to suspect they
should NOT open at the same number. It would be a mighty coincidince if the
optimal numbers were the same. More of a coincidence than you having the same
phone number (except area code) as your sister living in another state!
Advanced computer modelling allows designers to discover the optimal timimg
of all valve opening and closings. Actually, the most advanced engines
have no cam at all, instead, the valves are solenoid actuated and can be
opened at varying times based on engine demands. We do not have that luxury.
Note: I have found that the asymmetry can be in the ramp profile of the lobe.
it can open and close at different speeds. I.E. It can open fast with a steep
ramp, then close more slowly to minimize valve bounce by using a more shallow
ramp angle.
Well, that is it. I hope this explanation helps you understand cams and
cam selection a little better. I still have not decided which cam will
be the best for me. Probably unless I make a BIG mistake, any cam I choose
(even the stock one) will keep me happy.
Anthony Rhodes
August 31, 1999
Other sources
Elgin Cams
Ashford Performance Cams
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