2 stroke moped compression question

teasser · Derestricted Danger Joined: 04 Jul 2009 Karma :

I was reading up a on topic the other day and wondered if i took the bore off my 2 stroke moped. and skimmed about 3 - 4 mm off the bottom. Making the compression more would it make the ped quicker and faster. But then i may need to extend ports and they would be open and closed for shorter times.
Would this be worth doing or should i just leave it alone.

prawny1 · World Chat Champion Joined: 28 Jun 2005 Karma :

lowering the ports won't help in gaining power and at 3-4mm will likely cause piston to head contact. you would do better to have a nice exhaust system fitted and a rejett tbh.

you only really want to skim the head on a two stroke, and thats only really if you are raising / reprofiling the ports and spacing the cylinder up from the crank case etc (all of which need to be calculated and balanced the the exhaust/intake pulsed etc).

A two stroke doesn't respond to head skims like a four stroke does, once the comp ratio has been corrected the actual compression ratio of pretty much all two strokes is low really,

bladerunner · World Chat Champion Joined: 09 Sep 2006 Karma :

As prawny said. If you machine the material from the bottom of the barrel then you lower the ports so thus droping the power lower down the rpm. Remove it from the top of the barel or indeed the head and you keep std porting..but doubt you can remove anywhere near 3-4mm without MAJOR piston/head hitting each other issues! Usual trick is to measure your squish and set that to around 0.8 mm or fit thicker base gaskets to raise the ports and machine the heads to suit....hardly worth the effort on a ped though to weld the head and machine a better squish and head profile and the such Confused

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hmmmnz · Super Spammer Joined: 22 Aug 2006 Karma :

take it off the top, not the bottom, or just remove the gasket,

as has been said, more than anything the exhaust can give or take the most power
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teasser · Derestricted Danger Joined: 04 Jul 2009 Karma :

True i can see what you are saying now
thanks for the info
what do you mean from taking it off the top
Also my ped has a sports exhuast and other mods

teasser · Derestricted Danger Joined: 04 Jul 2009 Karma :

Oh you mean my lowering the cylinder head

stevo as b4 · World Chat Champion Joined: 17 Jul 2003 Karma :

I had 0.5mm skimmed from the head face of my old speedfight 100, ages ago. You do need to properly measure the squish before hand, to make sure that there will not be any piston to head contact. Also increasing the compression ratio to far will likely cause detonation, and as you cannot alter the CDI advance curve, you would need to try crude methods, such as using a 1grade cooler plug, and super unleaded fuel and or octane booster.

My SF100 did go alot better off the mark with the compression ratio increased from 6.7:1 to 7.2:1 (corrected), but it was no quicker flat out, (maybe even felt a bit flatter) but was just better at picking up from low speeds. I still had 1.3mm of piston to head clearance after the head was fitted, but i used a cooler plug and 98RON fuel soon after, as i seized it on the second ride using std 95RON fuel.