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

Raising the compression on these engines will increase HP, however, as you mentioned above, you are absolutely correct.  It can no way increase it anywhere close to that much especially when we have to run our engines at 3,600 RPM's or less.  I also realize Ms. Snowshoveler was not implying that it would, merely some explanation as to what might be going on since Mr. Snowshoveler discovered there are two different gaskets for Tecumseh engines as shown on the Microfiche cards.   

I dug out some of the Dyno charts I had from my racing days and also spoke with a friend that was very heavy into racing with the Briggs engines.  What was interesting about the HP ratings the Dyno came up with, when you take into account all our engines are governed to run in the low to mid 3,500 RPM range, it amazed me just how much performance work must be done to these engines in order to raise the HP from say 5 HP to nearly 9 HP. 

You can gain at least a solid 1/2 HP at 3,600 RPMs merely by removing the highly restricting muffler and replacing it with a 12" by 3/4" diameter header pipe.  The down side to that is the engine becomes so incredibly loud, you would never want to use it in a populated area.  Performing a good Porting and polishing to the intake and exhaust may yield about 1/4 HP at the governed RPM, in addition to decking and milling the head to increase compression gains about 1/8 more HP.   Similar performance gains will result by changing the cam shaft and blueprinting the carburetor.   Honing the cylinder also gains you some HP, but it is not much gain at 3,600 RPMs. 

It's also important to understand that some of the modifications I mention above actually start moving the power curve of the engine, usually to a much higher RPM.  Although the HP increase may be very little at 3,600 RPMs, they actually produce much more HP at say, 5,500 RPM's. In other words, a modification like porting and polishing the intake may yield slight HP gains at the governed speed, will actually give you much more power at a very high RPM, which isn't what we are all looking for.   

With that in mind, it seriously makes a person wonder how these HP ratings are determined, regardless of that SAE J1940 standard every engine manufacturer must comply with. It makes no sense to me where all these different HP ratings are coming from and just confirms once again to me how messed up this industry really is.  The Dyno doesn't lie, that's for sure.

Back when these Dyno readings were taken was well before this HP race started in the OPE industry.  Back then, a 5 HP Briggs engine really did put out 5 HP at 3,600 RPM's.  That isn't the case now, especially with the emission carbs added into the mix.