Exhaust Manifold Size  By  dav3 (Read 854 times)

Make, Model, Year? Engine Size? Two or Four Stroke? Pictures?

At least in the past, most GY6 150 engines were VERY similar, though not identical.

Usually, a generic header will fit, and be useable.

On my own Xingyue 150 when the original exhaust rusted out, and I removed it, I found the header inside-diameter was nearly 1/4 inch SMALLER than the exhaust port, and, had cobbled-up welds further reducing its size.  This seems to be the "norm" and is not good for performance, restricting the flow into the header right at the port, where it is definitely NOT needed.

When replacing your header, I would absolutely recommend spending some extra on a performance header with a full inside-diameter for free-flow.

I actually made one for my Xingyue from a header from a generator, but it's not worth the effort since factory-made ones are available.  They are a tad "pricey" but not outrageous.  They ARE definitely worth the cost.  Back when I made my header, upgrade full-size headers were not easily found.  Nowadays, they are available and are a better way to go than making one yourself...

I believe Pistonguy has one on his scoot, with a matching quality muffler.  These header/muffler setups are not quite "plug-n'-play" and do usually require some minor creativity on your part to solidly mount them, but that is not difficult using stuff you can find at the home-improvement store.  I've had good results using aluminum bar-stock and modifying the muffler clamps from the original muff.

I found the larger header (probably more than the muffler) made a major difference in general performance...  Not necessarily more power, but much easier tuning, a smoother rpm transition, broader "sweet-spot" in engine rpm, cooler engine temp, etc.

The restriction of the undersized factory headers is a hindrance to good performance, and a full-inside-diameter header is a very good idea.  Also, they are usually found made from stainless-steel which is good for long-life... Not to mention they look great too... LOL!

If budget forces you to use a factory-style header, do at least "chamfer" or bevel the inside of the pipe where it mounts to the head, to ease the exhaust flow from the head to the header.  Even that little mod will make for better running.

Here is a pic of a stock header, and the one I made up to show the difference in size...




Hope this may help.

Ride safe!

Leo in Texas