jlv wrote: ↑Wed Feb 24, 2021 1:49 am
That DCR dyno is impressive, but it's not exactly an unbiased source coming from their own dyno. If you can do that with just a cam, ECU and pipe why aren't the stock bikes doing that?
Do they really run 18:1 in MXGP? I thought FIM fuel was 100 octane. Hard to believe you could do that on 100 octane.
Yeah, that's why i said that dyno run was probably done at perfect conditions, but even then the gap is quite remarkable.
First of all we're talking about a stock engine, a stock engine is basically just a bunch of mass produced parts and although quality of mass produced parts is quite high these days, there's still a big difference to a proper race engine that has each and every part finely machined to match each other component. In a stock engine, you must consider every tolerance and therefore limit it's performance to a point where everything runs reliable enough. ECU mapping of a stock engine is also very conservative, because it has to be able to run at -30°C just like at 45°C, wet weather,dry weather(yes it has EFI, but a stock mx bike has a very basic EFI system so it still uses existing data independently from prevailing temperature and weather) ... fast rider or slow rider aswell as every part is unique and therefore behaves differently in a whole package. In fact each single engine is different and would need it's very own mapping to react to that. Since that's virtually impossible aswell as pretty much pointless in a production bike, they give it a map, that works good on everything. It's also worth mentioning that a stock mapping also has to be able to deal with different pipes, modifications to the intake system (yes even an air filter changes that) and mechanical wear, aswell as different oils.
It still amazes me that even the smallest changes often have a huge impact.
How do I know? I have a degree in technical engineering and i literally work in that industry, just like my whole family.
The performance gain by a camshaft is usually limited by something called "valve float". If the cam lobe profile is too aggressive/sharp or the engine revs too high, valves begin to "float", meaning they're not controlled correctly anymore and aren't dampened when they hit the valve seats. To compensate that it's important to find a good mix between lift and lobe profile, but since that's already a given factor these days, people use harder valve springs which have a quicker reaction time. It's often said that stiffer valve springs result in larger mechanical losses, but that's not always true. The engine needs more power to compress those springs, but once the valves close, the springs expand and actually somewhat help to drive the camshafts. So it evens that out a little bit. More aggressive camshafts also mean your bike will be more prone to damage through heat when idling due to "sharper" valve timing, that's why you always have to rev race engines i.o. to get more fresh air in and cool it down. A stock bike can be bought by everyone, no matter what knowledge they have so it has to withstand idling for much longer hence it's not that big of a deal on a stocker (althought I still wouldn't recommend it to run longer than 5 minutes idling).
However, those engine speeds, more aggressive cams and engine mapping has it's price, DCR and many other tuners say you can run stock springs and that their tuning doesn't take a toll on longevity and reliability, but more boom means more stress and more stress simply decreases durability.
The manufacturers usually only add power, if they were able to get more durability out of their parts, be it because of better materials or better design, in KTM's case, that's achieved by getting lots of parts produced by an industry leader like Pankl to smaller tolerances.
I don't know exactly what ratio which brand and team runs, but like I said compression usually decreases with bore diameter and the need for a reasonable combustion chamber shape. The FIM mandated limit of fuel is 102 octane(RON), which actually allows a very high compression ratio without knocking. A higher octane number is the most meaningful sign for more knock resistance, but i'd not fully rely on it either, since there are a few more significant factors aswell (combustion chamber shape, spark plug and it's position, piston crown shape, temperature ...)
All in all modern fuel and engine design is so good, that you can achieve quite high compression ratios with rather ordinary fuel.
Nevertheless, I wouldn't take those power gains too serious, peak HP is one thing, but usuable power for the average Joe a whole different story. There are GP 125 2 stroke engines which peak at 53HP @ 12500RPM, but it would be nearly impossible to ride it on an MX track. That said, realistic or not, 55HP 250Fs in this game wouldn't be a good idea in my opinion. I'd support what ddmx said.