I've seen more people screw up engines with a die grinder than help them. If you have an old, crude engine--like a Briggs or a garden variety pre-80's clunker, yes you can help it with some intelligent grinding. I don't know about 20hp. Hard to say. But if you take an old 460 and stick your finger in the exhaust port and though the port looks big, you can't get your finger in--grinding out that big fawking air-pump lump has to help. Those old motors weren't designed very well so your guess is as good as Fords.
But on a modern engine, the factory does not have the luxury of slapping any damn thing together like they used to. They are trying to just certify the motor. Poor designs don't burn clean or efficiently. They cannot produce a really shitty head, one like a side-valve briggs, and expect it to meet the emissions and fuel economy standards--not to mention make competitive power.
Let's take an example piece of trash motor--a 1977 Camaro with a 350 4V. It made 170hp at 3800rpm and had 8.5-1 compression. If we plug that into our handy BEMP calculator you get 101psi of Brake Mean Effective Pressure. That's basically a yardstick to see how well an engine is using its cubic inches. Now take a pushrod 4.0 with 9.0 compression making 160hp @ 4200rpm--that works out to 124psi. The 1977 motor is in the junkyard because it was inefficient. You aren't going to make the pushrod 4.0 better with your grinder.
In fact, Ford didn't make it better with a grinder when they decided they needed 50 more horsepower to be competitive. They threw the pushrod heads away and built a cammer out of it--sort of like they did with the 429 way back when. The 207hp @ 5250rpm SOHC 4.0 posts a little better BMEP with 127psi, but they could not make the pushrod motor spin fast enough with the old ports--there was no way to make that old casting flow enough air. They had to enlarge and straighten the ports to much--they had to move the pushrods out of there. You could not do a valve pocket job on an already near perfect motor. That's why I never believe people that claim to have getten very much of an improvement out of a 4.0. Where did the gain come from? Why didn't Ford do it? Soup up your pushrod motor, claim it makes 50 more hp and then get your ass kicked by a SOHC 4.0.
Even the 127psi of that SOHC motor pales when you put it against an even more modern 4-valve variable cam timing, variable geometry intake manifold. Those motors are putting out BMEPs of 165psi. It wasn't that long ago that a factory turbocharged motor was putting out similar numbers.
The whole point being, in the past, they didn't need to make anything very good. Now, they have to make everything perfect. A team of 30 PhDs is responsible for developing just the controls technology for an engine manufacturer. Thousands of engineers, test cell operators and skilled craftsmen who can whip up an experimental part with their bare hands in a couple of days to experiment with a new twist or turn of a design all work together to produce the motor under your hood. If the bleeding phenolic spacer was going to do anything, it would be there already. A table spool of aluminum removed form 400,000 4.0 engines would have save Ford a bundle--and they suffer losses every year. A couple clicks of a mouse would have removed that metal--it's not there. It was there in the 1977 Chevy 350 because motors back then were shit.
Even Briggs and Stratton has to work now. They have overhead valves and such. Kohler has fuel injection now. Briggs is using plastic carbs and electronic ignistion. Nobody can build shit anymore. That's a good thing. But die grinder specialists will go out of business.
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