2 Stroke vs 4 Stroke

Two strokes in general will always have a size/power/weight advantage, it's inherent in the design - but, the biggest "issue" with the four strokes is that to my knowledge they're simply not readily available as a "kit" engine, or designed specifically for fitting in frame.

There's a huge diversity in engines ofc, but most of the four strokes you see for bikes are utility or lawn care engines, there's also the commonly available moped/pit bike motor. They're all fine ofc, but as they're all retrofits there's some problems. Utility engines aren't engineered to be lightweight, small or performance oriented, lawn care motors seem best suited for rack mounting & the pit bike motors are all horizontal in the displacement we're after... but the only reason those are "wide" is their intrinsic gear boxes w/clutches.

Basically the problem isn't about the two stroke's advantages, but the lack of a vertical 50cc four stroke engineered specifically for bicycles like the Chinese in frame two stroke.

In fact, a quality 49cc four stroke such as these: http://motorbicycling.com/f37/alternate-4-stroke-engine-list-19630.html have at least equal to or greater power output than the 66cc Chinese two strokes despite being a smaller displacement, solely due to the primitive design of the Chinese two stroke - which negates much of the two stroke's normal power to weight advantage. To get that you'd need to match or exceed manufacturing & design quality, which means using a Morini or it's equivalent, which clearly illustrates a two stroke's true potential.

The Chinese in frame two stroke kits really only have three things going for them, their sheer simplicity, the fact they're specifically designed to power a bicycle and their ludicrously low cost - combined, those advantages are hard to beat... BUT were there a vertical 50cc four stroke designed for in frame mounting I suspect there'd be far more folks using four strokes due to their overall reliability and efficiency (both of which could be negated were they as cheaply manufactured as the Chinese two strokers ofc).

So... while I believe strongly that "a well engineered 4 stroke engine kit could be made smaller, lighter, and narrower to compete with the China 2 stroke" - much depends on what is meant by "compete". Compete in price, simplicity and/or weight? No way in heck - how do you compete against a $80 motor with so few moving parts? Compete in actual output, efficiency & reliability? Absolutely, that's already been done.

TBH I believe the two vs four stroke debate boils down to "apples and oranges" - there's no real way to directly compare the advantages/disadvantages as it's about the rider's priorities far more than the engines themselves, the Chinese in frame kit being the most popular choice simply due to it's relative ease of installation and extremely low price. Aside from that I believe it's all about the right tool for the job & what that "job" might be is completely personal preference

Again I think it boils down to what the definition of "better" is as it pertains to the specific user - if by "better" it's simply a matter of higher quality and the most power in the smallest package (design), it already exists in the readily available two stroke Morini (as well as others ofc, like pocket bike motors), it's going to be hard to compete with such a well established brand that has a preexisting support structure with plenitude of repair, maintenance and aftermarket performance parts and still remain competitive in price.

If by "better" it's simply an improvement of the Chinese engine's manufacture to improve reliability, that's possible - but any improvement will result in a increased price, it would be critical to keep the price no more than say roughly half of what another, higher quality motor of a superior design as the primary selling point of such a primitive engine is of course - price.

Four strokes are simply different - there's no "better" between the two save what the end user decides. It has many "advantages" but they may not be what you personally consider "better" or worth the "disadvantages" as again, it's about preference.

If you're trying for the utmost in acceleration and speed performance - it'd be a quality two stroke.

If you're trying for the most economical build with the least investment of time and money - it'd be the Chinese two strokes.

If you're after quiet reliability and utmost efficiency - it'd be a four stroker.


I personally think if there were a readily available four stroke as ascetically appealing and easy to mount as the in frame two strokes for an even roughly equivalent price (meaning no more than twice the cost) it'd most likely be in great demand - it would also not need to be a completely "new" design as there's already proven examples. While I love the two strokes, they do have some disadvantages in today's market - specifically with those interested in efficiency and environmental concerns, like many bicyclists.

I started with 2-stroke chinagirl kits in '06, and I think they are great for what they are. Cheap, light, compact, easily rebuildable, lots of mods. They are fast, and lots of fun - when done right!

I did the 4-stroke conversion last year because I'm a long-range spontaneous type. Sure, most my riding is short-range under 20 stuff, but I got itchyfoot and might just undertake a 300 mile ride with a trailer on the whim. I'm not the typical sheep type justifying him/herself with a 4x4 SUV, but I am very likely to just take a big ride. I bet a lot of yous feel that voice callin And filling up my 1+ gal tank at the station takes under 1 minute pumpin.