Here is a discussion regarding some of the issues involved in selecting a new camshaft for your TR4/4A engine. The same issues 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 mixture 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 combustion 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 after all!!), 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?
N.B. Send me more if you have them! Webmaster
|"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|
|Derrington||280||30-70/70-30||0.435||Near limit for street use (see Elgin 7010-9|
|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||?||0.408||Mild Cam|
|BFE #148||282||?||0.425||Slight lope @ idle, but low lift rate, so not as hot as others|
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 hits 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 modeling 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 coincidence 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 modeling allows designers to discover the optimal timing 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.Starting Up a Long Dormant One
|P.S. I was also amused at the fellow in the video puzzling over the eMog badge (minute 8:00) and what it was about! The intention behind the eMog badge and pins was merely to allow emoggers to discretely recognise each other without seeming to advertise for more adherents at club events we were at. We hoped that without anything written on this insignia, they wouldn't act as a billboard. In the internet age, we were concerned about the survival of the traditional clubs. That is the reason the eMog Moderators never encouraged Meets of our own,. though some did, indeed happen. :) Sadly, we were prescient. In many cases forums have virtually destroyed their local clubs. No secret in that. The marque has become much less known to the young because of that. We must all come up with a format that allows both to flourish. Those "old days" were great! I will now get off this podium!|
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