A very basic stupid question

Okay, I've got a vintage JD 111 mower with a 28B707 engine that is supposedly 9hp. I've got a 287707 engine that is supposedly 12hp.

Now, when researching parts and pieces, more often than not, the parts are interchangeable. Visually, the engines are identical.

So how does B&S go from 9 to 14 or 15hp with the same engine? Bore? Stroke? C/R? Carb/intake?

Thanks.

The newer engines can vary hp with changes to the camshaft lift and duration, carb jet changes, etc. The older engines not so much, but during your time frame there was some label switching going on depending what the OEM wanted. the older 9 hp engines were around 21 ci, and 11 hp were 23 ci 12-13 25 ci and the 14-15 were 28 ci. So your have the cubic inches of a 14-15 but could be rated less for various reasons.

Just because the parts are the same between the engines on the IPL may not mean those are the parts it came out of the factory with. Production parts may be different than replacement parts.

I had a 1992 612 Grasshopper which had a 12.5 twin cylinder Briggs and Stratton Vanguard engine I added a bagger/blower to the mower which made her pull down on hills. My manual covered both the 12.5 HP 612 Grasshopper and the 614 Grasshopper's 14 HP which showed everything identical in the specifications so I talked to my dealer, he told me that he had changed the carburetor on the 612 to the carburetor for the 614 Grasshopper in the past to increase HP. He added that this was the only difference between the 12.5 HP and the 14 HP models, he went on to explain that it is more cost effective at that time for Briggs & Stratton to use two different carburetor's than to have two different engines. Indecently the additional 1.5 HP made all the difference in the world power wise with the new blower.

Thanks for the feedback, guys.

From what I've been able to determine, both engines I've got are 28cu. in., so it makes sense the intake might be the difference. Looks like a carb overhaul/swap might be all I need and the quickest/cheapest way out.