Isuzu D-Max hater! bakit angdami nila?

**arsen** · October 18th, 2012 11:38 PM

Originally Posted by d_mac

By the way Chief Arsen, you may also know that i'm fond of motorbikes - but of course, i'm speaking now in terms of gasoline engines

This bike that i have runs in carburetor and it is premixed with 2-stroke oil. It comes with a long-stroke configured engine running in bore x stroke of 66.4 mm x 72.0 mm. Yamaha redesigned this dirtbike's engine in 1999 from its old configuration of 68.0 mm x 68.8 mm.

I had my squish tightened on the 2008 model YZ250 in the US (head modification), didn't have the dome cc'd since i wanted an increased compression ratio. From the stock squish somewhere like 1.55 mm squish i had it slightly tightened to 1.20 mm.

The consequence was, i rejetted the carb (leaned-out both my pilot and main jets and fatten slightly my needle) but i am now using a higher-octane gasoline than before (Petron Blaze 100-octane) to compensate for the increased compression ratio. If there is still a bit higher octane pump fuel the better sana.

The result was more power everywhere along the rev-range but seems slightly not as much overrev like it was in stock compression ratio, more snap off-the-bottom useful for upstarts in hillclimbs, i can run 1 gear higher in the mountain trails, relatively less clutching, sounds crisper, burns cleaner, and it consumes lesser fuel than before.

i know this is an unrestrained performance bike i'm talking about, not a road-legal bike, rather different in demographics, but it just a scenario wherein increasing the compression ratio in a long-stroke engine may not necessarily mean that it will result into inefficient fuel combustion. Whilst in carbureted motorbikes and vehicles you have to adjust manually the settings, in EFi's though (direct diesel injection and gasoline EFi) fuel and air adjustment is done automatically.

Cheers!

Not much knowledge about motorbike but wiki says that your YZ250 is liquid-cooled so good thing that it is already equipped with a cooling system that is more effective in dissipating the additional heat due to the increase of compression ratio.

Besides you mentioned that you also restrict the fuel. Since less fuel, kung meron man increase ng heat, it should be minimal.

Saka mga motorbike sa offroad naman eh kaya pitik-pitik lang naman lagi. So mga transient heat lang yan na madaling eh cool down ng liquid cooling system.

But in-case you use it in the highway (of course not you because kinakarga mo sya sa bed ni DMAX heheh), wherein continuous high speed, high rpm, high fuel delivery and continuous excessive heat (more than the capacity of the cooling system) the compression chamber could become hotter and detonation/pre-ignition can occur. The engine may become so hot na bago pa man mag fire ang spark plug, nag self combust na ang fuel. AKA pinging.

Engineers usually has no choice but to go in-between and consider the "what if" in their mass produced products. Good thing though that there are end-users like you that knows the modifications which suit fits your intended use. Just don't forget the limitations once you did some modification para tuloy-luloy ang ligaya...