Originally Posted by furball8994
Plane and simply the rec. port heads flow a lot more air. More air = more power. You could see 20 to 30% more hp with everything else the same
That's REALLY NOT a true statement. You asked for firsthand information, so here's mine. First a little background background: I have over 20 big block chevy cars, most with SHP options (high horse, high compression, solid lifter, rect ports). In addition to the 2 LS6 cars these include a factory L88 Corvette, several 427/435's in both alum and cast iron heads, and a Camaro with an LS7 crate motor (12:1 454, open chamber ZL1/second design L88 camshaft) There's also a 70 LS5 Corvette (oval port 10.25:1, hyd camshaft) and an L78 Nova (body now gone) 396/375 with rect. port.
What I HAVE NOT done is compare anything in the above list to the same engine with oval ports, BUT............ the LS5 Corvette with it's oval port heads is a torque monster, and Chevy actually published higher ratings on it torque wise 500 Ft/lbs than the other SHP big block Corvettes. From a roll in first gear it smokes the other cars in the barn, probably for several hundred feet or more. Also, I bought a 71 LS 5 (lower compression from the 70) several years back that somebody had stuck a set of rect. port heads on, and upgraded the camshaft. It had Hooker sidepipe headers on it. The guy had some time slips in the pocket on the dash where the car had run several times at the drag strip. I think they were like 13.60's if I remember, and fairly consistent over a period of multiple dates. I was gonna sell the car and wanted the open chamber rect. port heads for one of my hot roddy projects at the time, so I though it'd be a good thing to swap a set of oval ports. Long story short, made the swap, previous owner ended up with the car back about 6 mos. later and called and wanted to know what we'd done to it. He said it picked up a tenth and a half to two on a good day.
In another case, I've got a boat with 540 Big Block Chevy/Mercruiser. It's 9.5:1 with a BIG Crane camshaft. Boats of this type generally operate in relatively flat RPM ranges for extended distances and are pitched (geared) to max at 5K to 5500 for that reason. In a pinch once I replaced the open chamber square port heads with a set of open chamber oval ports. (remember now 540 inches and geared to need max torque in the mid 5's, where you'd think the rect. ports would be in the fat part of their band) The boat lost 1 MPH on top, but gained tremendous bottom and midrange, and I ended up leaving them on it until I built another motor several years later with more compression. The engine guy who builds a lot of Super Stock motors has said repeatedly that for anything under 600 horsepower and 550 inches that he can make more power with the oval ports than rectangular.
The reality is, properly designed ports increase velocity, and THAT is where horsepower is, and bigger isn't better in all cases, especially on smaller engines. I doubt seriously that there's an L78 (396/375) around that wouldn't benefit from a change to a good set of oval ports. Even many of Chevy's crate motors of medium displacement use the smaller ports, such as the ZZ502 crate motor, which has CONSIDERABLY smaller ports in the alum. heads than the old rect. port design. The Boss 302 Mustang and Boss 429 Mustang were both pigs off the showroom floor, mostly because of the huge intake ports mismatched to the other components.