Re: Porting and Polishing Head and Exhaust Valves on Foce 125
I used to race motocross and did alot of my own engine work, had the case split on the kitchen table every other weekend much to my family's dismay..<br /><br />anyway, to the point.. <br />In mass produced engines, castings are often incorrect, but if they are within predefined tolerences, then the company will use them anyway.. (in fact most are incorrect to some degree).<br /><br />So, if you were to look at your transfer and exhaust ports, you will probably see inconsistances where the liner and the block casting don't line up perfectly.. and the surface of the inside of the transfer port is probably rough as guts..<br />by machining the block to perfectly align the ports with the casting behind it, and smoothing the channels in behind the transfer port.. (and to a lesser degree behind the exhaust port) you can create a smoother flow, (less turbulance) which will result in mixture getting in and out slightly faster... you can do the same sort of thing anywhere that fuel/air travels in the engine,, its called 'port matching' or 'blueprinting' depending on who you ask.. the idea being to put the engine in the state it was when the manufacturer designed it.. before they added tolerances to account for mass production inconsistances... it can get you some extra geegees, but it requires alot of work.<br /><br />Porting is actually changing the ports themselves and is best left to professionals that know what they are doing and most likely have a flowbench to test results.. A pro can get huge gains in power by changing the charactistics of the ports to get more fuel in and get it out faster.. but it isn't something where someone can tell you to take 3mm from the top of the exhaust port, its much more precise then that.. so don't try it yourself, but if your friend really knows what he is doing.. then maybe its worth a go.<br /><br />One thing I might add, even if he does a fantastic job porting, you won't see real gains until the fuel/exhaust situation is resolved as well.. if you can get more air into a motor, and it can get out as well, and you don't change your fuel jets to accomodate it, then you will lean you engine right out and probably ruin it.. <br /><br />Exhaust is one of the single most important factors in 2 stroke design.. and its also the main reason why outboards require such big engines to achieve high HP output,, simply put, and outboard leg is a very bad place to have to fit a good exhaust expansion chamber for 4 or cylinders...<br /><br />to give you an indication.. a good 500cc high performance 2 stroke dirt bike engine puts out more then 50HP.. (alot more in some cases.)<br />now double that and you have 1000cc's or one litre and 100HP..<br /><br />Now compare that to an outboard, my 100HP, (which is crank rated so its equivalent) is nearly 1.7 litres to achieve the same result.. <br /><br />and the main reason for that, is the lack of good expansion chamber, and an efficent exhaust system..<br />(it can also be attributed to the fact that outboard engines are under a more prolonged load then in a bike and a bigger engine would handly that better then a smaller one.)<br />In brief, a 2 stroke tuned expansion chamber type exhaust uses sound waves (sort of) to draw/scavange the exhaust from the cylinder (including any unburnt mixture..)<br />It does this much more effectively then a straight pipe, (which is what most outboard essentially have)<br /><br />So if you wanted to get the most bang for your buck.. you would go to this page:<br /><br />
http://beadec1.ea.bs.dlr.de/Airfoils/tpipedes.htm <br /><br />enter the relevant data, and make yourself a powerpipe for each cylinder of your outboard..<br /><br />Then pull your powerhead and drill holes in your leg so that you can route the pipes out..<br /><br />do that, in combination with the port matching, porting and polishing, and feed your engine much more fuel, (at that stage, I'd probably go for 2 stage reeds as well) and you'll have an engine with 4 times more noise, but probably twice the power it had..<br />(which would probably snap your drive shaft, clag your gearcase or get you banned from the lake due to to much noise.)<br /><br />Don't get me wrong, porting is great in the right circumstances, but its hard and in an outboard 2 stroke, there is alot more to consider..<br /><br />doing just the porting might give you an extra horse or two.. but you probably wouldn't notice the difference..<br /><br />Hope that explains things abit..<br /><br />regards<br /><br />Frank.<br /><br />PS, if your engine has exhaust Valves, then I'll eat it..
