Casting Flaws

[bofforider]

I have a buddy who is crazy with a dremel. He has ported and polished his 250 2 stroke and insists that there are many casting flaws left behind in all bikes. He claims that by cleaning up the casting flaws there would be a significant performance increase. He has already taken another guy's 426 and cleaned up the flaws and they both claim it would now easily match the power of my 450. Not that I necessarily believe them, but my questions are whether cleaning up these casting flaws would deliver a noticeable increase in power and are there really that many flaws left behind in these bikes? Any insight would be greatly appreciated.

Thanking you in advance,

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BoffoRider
YZ450F

 

[john3_16]

I hope he knows what a casting flaw is and what's really supposed to be there..

 

[Rich Rohrich]

, but my questions are whether cleaning up these casting flaws would deliver a noticeable increase in power and are there really that many flaws left behind in these bikes? Any insight would be greatly appreciated.

Casting flaws on the surface of a long flat stretch of port have no impact on performance. To prove this to an annoyingly vocal disbeliever, I once glued M&Ms in the port entrance of a DR350 and did back to back flow bench tests with and without the M&M power enhancers . The flow bench couldn't see them and neither would the engine. Expansions and contractions in the port will seriously effect flow, surface finish does not. The funny part is, most garage porting specialists work so hard at smoothing the surface finish that they end up making the wall wavy, which introduces random port expansions and does effect flow, in a NEGATIVE way. :yikes:

In a modern four-stroke single the space from the valve guide to the valve seat, especially the floor or short side radius is critical. This area is commonly called the port throat. The area from the seat to the chamber enterance is also critical. There is some airflow to be gained by carefully working this area, but it's just as easy to wreck the flow then it is to improve it. The area ratios between the port throat and the valve seat are sensitive and blowing it will totally flatten the air flow in the port. Even if you get it right it won't make a 426 as strong as a 450. Anyone who thinks otherwise is deluded. ;)

Basic rule is, you can't easily see or touch the important parts of the port with the valve in place.

Same basic rules apply to two-strokes. The area at the port exit and the long radius of the port turn are important. The Time/Area of the port is far more important than anything in a two-stroke.

If your buddy thinks I'm just blowing smoke have him review the fundamental concepts of boundary layers in his high school physics book. He'll soon find out that the air running right up against the port wall is essentially stagnant and doesn't really see surface imperfections as a result. :cool:

 

[DEANSFASTWAY]

Boffo Does your buddy also wear a tophat and carrie a magic wand? if hes such a genius maybe someday well call him Mr president .On his 2 stroke Ask him how many degrees his intakes open/close or his exhaust if he gives you an honest answer he might have an idea as to what hes doing Did you drag race him with your 450? in the porting world im afraid that the polishing part that really helps to sell however. its moot in my opinion except on exh ports or cyl dome face .sometimes ports are better off being smaller not just hogged out Do some reading here my friend and youll find yourself more informed .you might want to search(use search function) for California Posers or something like that, .. It was an old thread .Id put my money on your 450 all other things being equal . Good Luck