Performance numbers for Firewall Forward Horsepower Plus

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I've got a 1978 M20J with the Firewall Forward "Horsepower Plus" STC SE2741NM. This is an IO-360-A3B6D with the stock dual magnetos and no other engine performance upgrades of note. I do have GAMI injectors which got me to a spread of 0.2GPH and reduces tendency to run rough, especially at altitude. This STC limits propellers to the stock McCauley so that's what I'm running.

This engine has about 350 hours on it since overhaul by FF. No real issues with the engine so far.

The airframe only has a couple of speed mods, notably flap/aileron seals, but it still has the multi-piece belly.


Usually I fly this airplane at Economy Cruise (low altitudes, up to about 9000 feet) and Best Power Cruise above that. Ram air is open on all runs. I keep the cowl flaps closed and make sure the CHTs don't get out of hand, and in all cases below I've let the airspeed and temps settle for at least 20 minutes before recording numbers. I haven't been writing down degrees ROP/LOP. Weight on these speed runs is around 2500 pounds so I'm averaging the 2300 and 2740 speeds in the POH. RPM, MP, and fuel flow are measured with a JPI 800. I've got more runs than this but picked 4 that I could match to the POH.


The TL;DR on this performance for this STC is that starting around 9k feet, it gives me lower fuel burn, and sometimes better speed. Below that, it performs like a stock engine: at or slightly below factory numbers.


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