@Calum said in Aprilia ETX 125 (Don't expect to go motocrossing...):
@markus-w Dude Uhhmmm yeah you're absolutely right. I feel like such a dinglebat.
Well, this is a long term project so more thinking is required! I'm not achieving very much here am I???
Thanks for correcting me, I don't know what I will do with these parts, more thought is required.
So it's still 160cc's, which is fine I am not interested in my displacement, but there are gains to having a longer rod.
Note here my failings were thinking longer rod meant long stroke which is NOT correct.
I think everyone is achieving a lot here.
Reading this thread made me remember that conrod length is a major factor in most two-stroke tuning equations as it affects peak piston speed and whereabouts in the engine's stroke it occurs. It can also alter the position of the piston in the bore at a particular point in the the stroke.
On a two-stroke this is obviously critical due to port timing. Does this big bore kit come with the 5mm longer rod? Because if that's the case, it may be that the kit manufacturer has decided the power characteristics of the original 125cc engine are best preserved by maintaining the relationship between bore and stroke when increasing cubic capacity. So after you've put it together and set up jetting etc., it should still rev like the original 125cc engine but with more power.
Conversely, the DTR 170 Athena kit has a good reputation but is not particularly "scientific" in that it just increases the bore size which is probably why users report their top-end power gains aren't spectacular. "Piston speed is higher across TDC and lower across BDC with a short rod as compared to a long rod used on the same stroke." And just bolting on an Athena 170 effectively shortens the stroke from what Yamaha intended. Considering almost all cylinder filling/evacuation on our engines occurs between mid-stroke and BDC, this can seriously disrupt this process meaning the engine is unable to rev as well as the original 125cc engine (probably why a properly derestricted KMX125 had a reputation for being faster than a KMX200 back in the day).
This article is about big V8s at the dragstrip and so of limited relevance to us in some ways, but it explains well the concepts of bore/stroke ratio, peak piston speed which is what we're interested in. Not sure I've understood it fully so bear with me; I broke both my legs in December and have had 3 hip replacements since then. Hopefully the final one last week and the anaesthetic is still working its way out of my system!