I looked into the cam timing and duration on the D675's.
Another piece of info to clarify to folks considering removing their Exup.
Question: Why does the engine run lean in the low RPM band when EXUP removed and stock tune used?
A possible answer (Dyno testing would be the proof)
Has to do with air charge scavenging. On an engine with moderate to high overlap and low RPM, there is a phenomenon called "blow through". This is where the exhaust pressure is somewhat low. In fact, it is typically lower than the intake pressure coming into the cylinder. When this happens, the cylinder achieves high VolEff (maximum air charge) because gas flow continuity actually causes intake air to be pushed out of the exhaust valve when both are open at the same time. This can sometimes be tough to model s it can only be done with open loop control. The added unburned oxygen can affect closed loop A/F control. Manufacturers incorporating variable valve timing actually adjust the exhuast valve timing to allow for late exhaust valve closing in order to take advantage of this "supercharging" effect and achieve the highest possible level of brake torque whe the customer requests max performance.
As the engine speed increases and exhaust pressure builds (IE from turbo wastegate or restrictive catalyst/exhaust) then the "blow through" phenomenon gets reversed (higher exhaust pressure vs intake pressure) and now the problem becomes an issue with burned gas residuals not being scavenged properly out the valve. This causes reduced VolEff and less power. This is why the exhaust valve closing is made earlier at high RPM, as it prevents reduced VolEff.
My theory is that the exup increases exhaust pressure and thus prevents the blow-through from occurring. As a result, the open loop tuning requires less fueling since VolEff is reduced (Basically there is less air to burn). So when you remove the exup, the open loop control doesn't know this, and thus is sending less fuel to the injectors than the air requires. If these bikes used mass airflow sensors, this would be less of an issue. But since they are "speed density" (Based on MAP, throttle plate angle and air temp), the ECM simply doesn't know how to compensate. As a result, you get a lean condition. This will not happen during closed loop control, contrary to opinion. The fueling will compensate and should only happen during open loop operation (Typically above a certain throttle opening when max torque is desired by the customer)
Exhaust wave theory could play a role as well, but as I said before, I don't believe the EXUP is being used to promote exhaust scavenging. I believe it is an easy way to tune the bike for different markets with different drive by noise laws, yet not have to deal with a restrictive exhaust at high RPM. I guarantee the exup cal for EU bikes differs from the cal for US bikes.
Anyway...Just a thought.
I was just talking with my fellow "engine buddy" at work the other day about the overlap on these bikes... According to MacBandit, the D675 has 50.75 degrees of overlap and the Street Triple has 25.35 degrees of overlap.
It would make sense to use a variable restriction in the exhaust to reduce "blow through".
Then again, on a race motor, the only time it becomes a concern is during the specific RPM range that the engine is operating at on the track. They could care less about "Blow through" at idle and lower RPM's.
On a race motor (on the street), perhaps it can be tuned to reduce any negative effects of blow through.
But keep in mind that there's only one EXUP valve.. for all 3 cylinders, all at different points in the combustion cycle. If one were to use a valve to control blow through, it makes more sense (in my twisted head anyway) to have one for each individual cylinder. I know, I know, that adds a ton of complexity and potential for malfunction (especially if they build them like the standard EXUP valves).
I could see if the timing of the 3 cylinders was "juuust right" where a single EXUP valve could be of great help to avoid overlap.
Now that I'm really starting to think about it, Do you ever see a valve in the exhaust of any other engine (factory produced, race derived, street legal) besides those in motorcycles? Do you ever see it emblemized on any type of performance vehicle the same way it's shown off on the graphics of some motorcycles *cough"* honda&yamaha *cough* No. You see "EXUP" graphics all over a Honda CBR the same way you see "VTEC" graphics all over Honda's cars. But you don't see exup systems in cars, trucks, ATV's or anything else (If anyone can find other examples, please point them out because I could be wrong, I'm making an assumption based on my memory of vehicles I can recall at this instant.)
We see an Exup valve in all sorts of motorcycle engine configurations such as I-4's (Even the R1's flat plane crank motor), I-3's, I-2's, V-2's, and so on. These motors all have a wide variety of firing orders, overlaps, and other variables. Regardless of the engine configuration, there's probably at least 1 common effect of a single exhaust valve downstream of the collector, such as noise reduction and any "general" benefits of backpressure that aren't related to firing order/timing.
But, given the wide variety of engine configurations, firing orders, cam, overlap & timing specs, and other cylinder/timing specific information of all these different types of engines, I'm not sure how much an effect the EXUP valve will have on "Blow through" due to cam overlap, or any other scavenging type effect... IF it is highly dependent on firing order or any of the criteria I mentioned at the beginning of this paragraph. Rather, I think it could be of great benefit if it is designed specifically with the engine configuration, firing order, and timing of all the relevant events that take place in the combustion cycle.
Furthermore, you also don't see EXUP valves in aftermarket headers. Maybe they choose to not include them to reduce costs, or because they feel the amount of R&D they'd have to put into it would be cost prohibitive, or whatever. Or maybe you don't see EXUP valves in aftermarket exhausts because the guys who designed them (the real experts on exhaust design) don't see any sort of performance benefit to using them, as opposed to more basic things in exhaust design such as length, pipe size, type & location of the colletor(s), etc.
I would really like to see aftermarket header manufacturers take the things you mentioned into account, such as blow through, scavenging, etc. etc. I'd like to see a 3 cylinder header that's designed specifically for our engine that uses design characteristics such as unequal length pipes, crossover pipes of certain sizes at certain locations, X shaped crossover pipes instead of H shaped crossover pipes. and other things that are beyond my knowledge as an enthusiast that I would expect an exhaust engineer to know about.
Blow through is a big deal, and it's something I've been thinking about a lot when I just tuned by bike to run on E-85. Without getting into it in great deal, the engine runs great on E-85. Just so you all know, the stock fuel system is capable of providing enough extra fuel for a stock motor. But it got me thinking about whether or not there's a relationship between blow through and the "injector pulse width".
Using only TuneECU to convert the bike to run on E85 without any actual physical modifications, the software simply increased the injector pulse width, or the amount of time the injectors stay open. By lengthening the time fuel is sprayed through the injectors, fuel is now flowing during a time in the cycle before top dead center when it didn't flow before. Because I've now increased the duration of the fuel spray cycle, I'm not sure specifically what effect that has on blow through and combustion as a whole. (I can say that it cools the intake valve, and the net result is positive in this case, but it may be allowing more unburned fuel to flow out the exhaust valve if the injector is now firing during the overlap cycle). Gasoline might not have been as forgiving.. Not sure.
I did this E85 conversion on my stock motor as one of many tests for my other motor that's being built specifically to take advantage of E-85 with the highest possible compression I can get, specifically with a shaved head, thinner head gasket, and perhaps different pistons if I can find them. But I also plan on doing some more complex things with regard to the dynamic compression that require a lot of testing and research beforehand with regard to the the cam selection, duration, overlap, and cam timing. Without going on too much (more) of a tangent, I'm deciding on whether or not it would be worth it to run a different exhaust cam with less duration to increase dynamic compression and reduce blow through.
But also, trying to figure out when overlap takes place compared to the injector timing to understand if blow through has an effect on the air fuel ratio (or wasted fuel) if the pulse width is increased by a significant amount.
Of course, there are ways around this. Rather than increasing the pulse width, I could increase the fuel pressure, and/or the injector size. Both of which would allow me to shorten the pulse width or "injector timing" so "Blow through" doesn't waste fuel or adversely affect the air fuel ratio.
Coming back to the exhaust valve, I'm going to do some tests this afternoon to see whether or not it can help me improve the idle quality and this awkward spot right after idle when running E85. Doubtful. But worth a short amount of time.
Does anyone know of a header for our bikes that's a lot different than most designs? One that actually uses the bike's firing order, engine configuration, and other things to its own advantage, rather than just a fancy piece with bigger pipes?
Sorry for the long post. I think out loud. I hate reading long posts as much as you all probably do.
Homer
