port timing and possible mods???

Discussion in 'General Questions' started by emdude, Feb 25, 2009.

  1. emdude

    emdude Member

    The rant below is long but does have a point, and more specifically some questions.

    When I was a kid growing up in Germany, all we could have with reasonable "legal" ease were 50 cc mopeds. They were limited by law to no more than 1.5 HP and were not supposed go faster than 25 kph. Young wanabe gear-heads that we were at the time, we tried everything we could to squeeze every bit of performance out of these little engines we could get.

    BTW, those were the days....you could get 1:25 premix at any gas station!!!

    One of the things we would do on some engines is to ad a "distance piece" between the cylinder base and the crank case of one or two mm. Sheet aluminum of the desired thickness was cut to match the base gasket and this was then slipped under the cylinder. I remember long hours of cutting away at this with a jig saw and then filing it to proper shape with a rat tail file. This of course would raise ALL the ports in the cylinder increasing the OPEN time of the exhaust and transfer ports and of course reducing the intake timing. The piston skirt would have to be cut on the intake side by the same amount to make up for this loss.

    On some engines this made substantial difference in performance, but of course it was mainly top end that was gained. I recall one engine I had that had been "throttled" by the manufacturer to meet the 1.5 HP limit simply by changing the cylinder casting of the transfer ports. They had taken the same cylinder from what would have normally been a say a 3 HP unit and moved ONLY the transfer ports down, at least from what I could tell. If you looked at the piston at BDC the transfer ports were not fully open. The exahust port seemed as I thought it should be. I guess this was an easy way out for them.

    Adding, I think it was 1.5 mm, below the cylinder raised all the ports of course and this specific engine really went nice after that. In fact it was the single biggest improvement I made. Yes. I did cut the skirt of the piston on the intake side appropriately. I spent a LONG TIME with a mirror and lots of wet sanding paper grinding away at the head to make up for the lost compression as best I could.

    OK, as to our little Chinese engines:

    From what I found from examining mine the piston at TDC actually protrudes out of the top of the cylinder by just about the same amount as the of thickness of the head gasket. In fact I had found that the whole assembly is a bit on the sloppy side and that the head gasket has to be lined up perfectly; otherwise the piston will hit the gasket. Same thing goes for the head being lined up perfectly above the head gasket.

    The ports in the cylinder seem to be of reasonable size for what it is. I have no clue as to the timing of the ports because I have not made any form of degree wheel to try and find out.

    It occurs to me that it might be quite easy to cut a spacer as described above to fit the base of the cylinder/crank case thereby raising the cylinder just enough so that the head can be used without a gasket. If the spacer is made of the same thickness as the head gasket there is no compression loss.

    BTW we never used, nor even had any head gaskets back in the moped days. We did use some type of sealant though. It was brown a goop made for the purpose and got real hard after a while from the heat of the engine.

    Funny, but I also remember some of those engines later on coming out with NO separate head!!!. The cylinder and head were made of one contiguous piece. I wondered if they did this to prevent people from taking the heads down to increase compression or if they were just making life easier on themselves.

    The questions:

    Of course the first one is has anyone done this and with what result?

    Is there some GOOD reason I have not thought of or just don't know why the piston protrudes from the head at TDC? I do know it's not uncommon for many 2 strokes but don't know why.

    What is known about the port timing on these engines? Is it optimized as is and will any disturbance cause negative results?

    Yes, raising the ports, specifically the exhaust port will decrease the effective compression but would it be enough to loose bottom end power?

    Are specifications of the port timing available anywhere?

    Is the timing possibly conservative to the point where a slightly more advanced/lengthened port timing on the exhaust and transfer ports can be tolerated or even be beneficial?

    Has anyone modified the port timing of ANY ports, intake or exhaust, on one of these engines and if so with what result?

    I ask this because on the "throttled" engine I had it made such a huge difference at ALL rpm's. But this engine was an extreme case.

    Any input appreciated.
    Last edited by a moderator: Dec 15, 2015

  2. oldsalt

    oldsalt New Member

    I don't think I have the time to do any real port sizeing and heigth cutting. But it's a certinty that raising the barrel 40 thou, as you mention with a spacer, could not completly distroy the low end performance. I guess that's the tack I'll take. Finding what the head looks like so that a like amount, and slightly move, can be removed so that the compression dosen't suffer a loss. A guy can always use a rear sprocket with a few more teeth to make up for any loss of low end torque due to slightly raised ports. The little engine is so anemic, due to very conservative port timing and low compression, that it would be difficult to make it worse useing 1mm.
  3. spad4me

    spad4me Member

    Sounds like a nice idea.
    Last edited by a moderator: Dec 15, 2015