49cc HS exhaust length too short??

only if there is "overlap" of the durations of exhaust and intake opening does the suction wave aid in engine power (by improving delivery ratio).
I highly doubt his poo-poo engine has any overlap.
 
Good advice Old Bob.
I used to have several of Philip H Smith's books that sadly I loaned and never came back.
I built a 6 cyl header following his formula, for a Ford 250 six in a 62 Comet. Worked wonderful.
Low 14 sec 6 cyl auto complete with a single barrel carb. A Friday night money maker.

Exhaust tuning has a significant effect on 4 strokes, including length tuning.
Cone collectors and Brooklands style mufflers have the same range widening effect as tuned 2 stroke pipes.
Vizard didn't write anything about small gas engines did he? but Smith's stuff could apply to anything.

The header on this car was built according to Smith's books:
10399203_29545085802_5231_n.jpg

Ford Maverick with 351C 4bbl. Hand made equal length headers and out the back duals.
Philip Smith's theories work!

Steve
 
wow. a blanket endorsement without any care as to whether or not the exhaust and intake durations overlap. wow.
 
wow. a blanket endorsement without any care as to whether or not the exhaust and intake durations overlap. wow.

Absolutely a blanket endorsement of Philip Smith's books!
A lot more "heady" and harder to read than Gordon Jennings.
I suspect he might have been some of Jennings source material.

I have "hot-rodded" most engines I have ever owned, including my lawn mowers and rototillers.
A 12"-18" piece of straight pipe generally increased the torque of any of them.
The rototiller got an even longer pipe (24"-30"???) which helped its torque considerably.
No math theory went into this, I just kept adding iron pipe and couplers until it worked the best.
With the pipe aimed at the ground no muffler was needed. Reasonably quiet (as a muffled Briggs&Stratton).

The Briggs definitely has overlap, it is only the rare blower or turbo 4 stroke that doesn't.
Port momentum (any pipe length +6") helps any 4 stroke engine with overlap.
Longer lengths help to provide a suction as it opens, usually on a harmonic because we cannot make the pipe long enough for the low rpm of the Briggs.
 
Vizards books give you intake and exhaust formulas that apply to any size 4 stroke.
Jag, go back to the 2 smoke forum, leave us alone here you don't know what you are talking about.
 
Most of Vizard's books I read were vehicle or sport specific, so I don't recall pouring over formulas like I did with Smith. Vizard is an easier read, for sure.
 
I don't know if this is the ideal setup. The head pipe is 7/8 ID and 15" long. The straight pipe is 1 7/16" id 28" long. I'm no authority on this, but i am guessing it works buy the exhaust gases creating a draw on the head tube when they enter the larger straight pipe aiding the intake during the valve overlap. I guess that the head pipe being less restrictive than stock would aid in the scavenging as well. Bikeberry pipe.
 
Steve, David Vizards How To Build Horsepower 2nd Edition

has formulas that apply to any engine ,although somewhat basic, do the job. The important part is he explains the principles.
You can tune the header to improve power in any rpm range you desire, not just the top end.
 
Thanks all, I'll educate myself with those books when I get more time.

That kinda rings a bell Dan, thanks. Getting some 7/8ID flex sounds like a good buy for most builds.

Until then, would hole drilling into the poo poo baffle (side?) improve flow and top end performance enough to matter? Or would that tend to defeat the purpose or make worse? Assuming it didn't look like swiss cheese
 
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