Outdoor Power Equipment (Lawn Mowers, Snow Blowers, Chain Saws and more) Discussions

Borat,

   Very interesting post I read today (from 2009) was signed this way by T-Man.

P.S. I LOVE the HSK850 and TVS/TVXL840 2-cycle engine. One of the
strongest engines ever built, and very simple and powerful in its design.

   T-Man was a technical guy for Tecumseh for many years and miles above anyone on the net.  His depth of knowledge about Tecumseh engines is astonishing.

   The HSK870 seems like an interesting engine.  I've never come across one.   Have you?

   Went through exploded views of carbs 640300, 640011, 640012 and a non-EPA carb.  All 640xxx's are EPA carbs.   The 11 and 12 are used in lower HP versions of the HSK850 (lower than 7hp)   Since the HSK870 uses the 640300 I assumed it contributed to its HP boost.   The exploded views showed that to not be the case.  The differences are in things unrelated to power, more to connections and fit.  The fix idle meter jet, bowl nut jet and emulsion tube are the same in these carbs.

Well, if the carb is the same, the cylinder is the same, crank piston the same, I think it would be safe to assume that the extra power is coming from the 7 h.p. decal on whatever machine has the dubious honour to wear it. 

With all the above parts being equal, the only place the power will come from is engine speed and I don't recall seeing an increase in rpms  listed in anything I've seen for the HSK870. 

Borat,

   Saw your T-man post and posted there myself. 

    The only difference I can see in a 5hp version of the HSK850 and the HSK870 is the muffler.  The 5hp uses 390325.  The HSK870 uses 390343.  I can't find anything about these mufflers other than prices.   The difference may be the finish like galvanized or some fit thing but it could be the 343 is tuned. 

    It's too much for me to imagine Ariens reworking an SS522 body to be a SS722 and charging $2-400 more for it with an unmodified HSK850 used in the 5hp version.  Ariens in the past has used a tuned muffler to boost a HSSK50 to HSSK55.   They have made engine changes to stock versions often in the past like special cranks. 

It's possible that a freer breathing exhaust might contribute a power gain.  However, from my experience, to gain substantial power, an air intake and carburetor enhancement is usually required to go with the improved muffler/exhaust. 

I've worked on hot-rodding my Yamaha RDs and changing the exhaust alone had some, but by itself, little effect on power gain.  Improving the air intake system and increasing the size of the pilot and main jets along with the exhaust change made all the difference in the world.  A freer exhaust will expedite gas flow but only as much as the engine can produce exhaust gasses.  Without more fuel, the engine cannot provide the additional combustion required to capitalize on the more efficient exhaust.   Being that the 850 has no air intake restriction, there is nothing to be done there to improve power. 

I'm with T-Man on this one.   We're probably looking at decal power unless the 870 engine is being run at considerably higher rpm.