Also loving this great info on Bullet tuning, Ace. So clearly explained too--not many can make this kind of this easy to understand.
Am I right in assuming that the hotter cam you mentions increases the overlap (or introduces overlap if there was none stock) between exhaust and intake phases to get that higher rpm efficiency?
Here's my tuning question, just out of curiosity. Say you wanted to tune the iron bullet for best fuel efficiency--I'm guessing just adding a free flow exhaust and proper jet (not too big) plus a freer intake (maybe K&N) and you'd be all the way there?
This assumes that the stock Bullet cam has little overlap and keeps a lot of back pressure in the exhaust phase so you get the most low end torque possible (at the expense of high rpm hp).
Fuel efficient tuning is tricky I think because some performance mods increase it and some decrease it.
Well, this is a pretty big can of worms to tackle on a forum post, but I'll give it a shot.
Cam/valve overlap - What is it?
Overlap is the period during a 4-stroke engine cycle where the intake valve and the exhaust valve are open at the same time.
There are two main reasons for this overlap.
One is because we cannot open valves instantly, so we must "ramp them up" with the "ramp" segment of the cam. This takes time, and so we open them earlier than necessary, so that we can have them open when we need them open.
In the case of the intake valve, it needs to be open when the downstroke of the piston occurs, so we begin to open it prior to top dead center(TDC).
Well, opening it earlier than TDC means that it is beginning to open at the end of the exhaust stroke, when the exhaust valve is open. So, that is the first half of the overlap which happens prior to TDC.
The last half of the overlap occurs because now the intake valve has been opened for the intake stroke, but we're still closing the exhaust valve on its closing ramp after TDC. We needed to have that exhaust valve open for the entire exhaust stroke before we start closing it.
The overlap period is the combination of both valves being open prior to TDC on the exhaust stroke, and after TDC on the intake stroke.
On the stock Bullet, we open the intake valve around 30 degrees BTDC, and we close the exhaust valve at about 35 degrees ATDC, so we have an overlap period of about 65 degrees with the stock cams. By the way, this is a pretty wide overlap period for an engine of the Bullet's type. So, the stock cams have plenty.
The other purpose for overlap, besides the previously stated mechanical reasons of ramping the valves, is to assist breathing of the engine, and also cooling the valves.
With an engine such as the Bullet, there is minimal piston movement within about 20-30 degrees on either side of TDC, where the piston has already reached almost full extension, so it's "dwelling" at the top of the stroke, while the crankshaft pin holding the big end of the rod is primarily moving horizontally, and not moving the piston vertically hardly at all. During this time, there is very little motion of the piston to assist breathing, either in or out.
So, at the end of the exhaust stroke, when it passes TDC, we can use this open exhaust valve, and the inertia movement of outgoing exhaust gas into the exhaust port, to "pull" fresh mixture in from the intake port, and get a "head start" on bringing fresh mixture into the engine for the intake stroke, even before the piston really starts to get going on the downward intake stroke. At least, that's the goal. During this time, some fresh intake mixture is sucked out the exhaust valve with the exhaust gas, but since that intake air is cooler, it helps to cool the exhaust valve as it passes into the exhaust port. Yes, this does waste a little intake mixture. But, it cools the exhaust valve, AND it begins an inertial movement of fresh mixture in from the intake port and gives us our "head start" in moving fresh mixture into the engine as the earliest possible time, during the "dwell period".
Then we close the exhaust valve when the piston starts taking over the job as it goes down.
That is the basic explanation of overlap.
Now, overlap works better at higher rpms than lower rpms, and it is often used to increase power levels at higher rpms, and wider overlaps are typical in performance cams for that reason. The "losses" which are caused by overlaps of both valves being open at low rpms are mitigated by the faster engine rpms, and the system works better at the faster speeds.
HOWEVER, there are factors in the Bullet engine, which enter into this equation.
And the biggest one is that our exhaust ports are oversize, and they do NOT hold a good fast exhaust speed during the exhaust event. And a good fast exhaust speed is CRITICAL for wide overlap cams to work properly.
Also, with the long-stroke/long-rod combination in the Bullet, we have quite a long period of "dwell time" around TDC, as I previously mentioned above, and if we don't have a good fast exhaust speed to cause the onset of "sucking in" of fresh intake mixture from that, as planned, then it doesn't work like it's supposed to. Then what happens is that some of the exhaust gas gets pushed back into the intake port, and causes dilution of the intake charge mixture with something called "Burnt Gas Fraction"(BGF). This is ok for pollution control purposes, but not too good for power. The BGF then gets pulled back down into the cylinder with the fresh mixture, when the piston descends on the intake stroke, and some of the intake charge is now BGF, which dilutes the incoming mixture charge, and thus reduces the amount of power that the engine can make when it compresses and burns the intake mixture. And, at the same time, the intended amount of fresh mixture to cool the exhaust valve doesn't happen as well, either. All because of insufficient exhaust speeds, caused by our oversized exhaust port.
So, "what's a mother to do"?
Ha ha!
What we can do, is limit the overlap, so as to form a "trap". This traps the exhaust gases from going back up the intake as much, and traps the intake gases from being wasted as much for very little good purpose.
So, for the Bullet, limited overlap is really the best choice. Opening the intake valve at about 20 degrees BTDC , and closing the exhaust valve by 20 degrees ATDC would be better for wide-band power production in the Bullet. Only because our exhaust extraction effects are not good, and so that overlap stuff doesn't work as well as it should in our engine. The wide overlaps cause losses in the lower-mid rpms which causes the bike to be sluggish and weak until it gets to the higher rpms.
And guess what? The British designers of the original Bullet engine had it almost just that way. The original Bullet had about 54 degrees of valve overlap in the Redditch Bullet models, compared to our present-day 65 degrees overlap. And they produced more horsepower and torque than our present day India-made Bullets. The Indian manufacturers have changed our Bullet cam timing to wider overlap with more BGF, to limit exhaust emissions, for pollution control purposes. And hence, our modern Bullet power production is down on power by a few horsepower compared to the original British Bullet design of the 1950s.
Ok, so where does this leave us with the discussion of performance cams?
Well, it seems that all the modern cam designers like to build in alot of overlap in their racing cams, regardless of whether it is really productive in our particular engine. They like a big peak hp number at high rpms, because that's what sells cams. So, all the available sports cams are wide overlap cams.
The result is that they are most often quite soft and lame at lower rpms under 3500 rpm, but the overlap can begin to get more of what it intends to do at the faster rpm speeds. So, these cams do go pretty well at high rpms. But they are not very happy at normal lower road rpms, and normally neither are the owners and riders of the bikes that these kinds of cams are installed in.
Now, here's the bug in the ointment.
The Bullet, because of the design of the long-stroke engine has very fast piston speeds. And it has an alloy con-rod, which is not as strong as a steel con-rod would be. So, the very fast reciprocation of the reasonably heavy piston causes enough stress on that con-rod to require an rpm limit of around 5000rpm, or maybe up to 5500rpm(sparingly and cautiously).
So, what happens? All of a sudden you now have a 1500rpm useful power band, and it's ALL at the top of the range.
This is why many Bullet owners become disenchanted with their purchase of sports cams available today. Many take them back out after they find what has happened to the power curve of their engine.
The older original Redditch cams would be preferred, but they are not easily found in useable condition.
There is one other option which I have found out about, from one of my colleagues, but that's a subject for another discussion altogether.
Regarding your "bang for the buck and fuel efficiency" question, you can do all those things you listed, and add a compression increase and the corresponding ignition timing change which is required by the compression increase.
No more than 8:1 compression and 7.5:1 would do just peachy.
No fancy cams.
Keep the stock carburetor.
Keep the bike very well tuned.
Keep the chains very well ajusted, neither too tight or too loose. Chains can lose alot of power in transmission losses if they are not kept properly tensioned.
Don't let your tires get low on air.
If you can accept a lower riding position for less wind-resistance, that would be a good help for economy. Or even a proper fairing of size/type you could accept.
Topic: Engine modifications and resulting improvements..... (Read 23182 times)