1969 Tri-Power

General discussion about C3 Corvettes
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Tell us about your Corvette: 1995 Targa Top Coupe, Stock 350/300 hp LT1 Automatic
1968 Full Custom Corvette Convertible, 454/425 hp, Turbo-Hydromatic 400 Transmission with shift kit and stall converter. 1965 High Performance Resto-Mod Coupe, 350 LT-1/390 hp Kiesler 5-Speed
Location: Pinellas Park, Florida

Re: 1969 Tri-Power

Post by rbryce1 » Feb 8th, 2016 11:19 pm

I just went on line and many sites tend to say that polishing and porting will increase performance. I don't know if they know what they are talking about or Holman & Moody knew what they were talking about, but I did trust them as this was their primary business, not selling the kits to do the work with. Even the places selling the kits and providing the instructions are heavy on the warnings of difficult the job is and how easy it is to screw up. If you really realized the 20% power increase, I think the auto manufacturers would have been doing this for years now to increase performance and efficiency, but they don't. Maybe because it is too expensive, or maybe because it doesn't work on this level. I just don't know.

Not that wikipedia is an exhaustive source of information, but here's what it says:

"It is popularly held that enlarging the ports to the maximum possible size and applying a mirror finish is what porting is. However that is not so. Some ports may be enlarged to their maximum possible size (in keeping with the highest level of aerodynamic efficiency) but those engines are highly developed very high speed units where the actual size of the ports has become a restriction. Larger ports flow more fuel/air at higher RPM's but sacrifice torque at lower RPM's due to lower fuel/air velocity. A mirror finish of the port does not provide the increase that intuition suggests. In fact, within intake systems, the surface is usually deliberately textured to a degree of uniform roughness to encourage fuel deposited on the port walls to evaporate quickly. A rough surface on selected areas of the port may also alter flow by energizing the boundary layer, which can alter the flow path noticeably, possibly increasing flow. This is similar to what the dimples on a golf ball do. Flow bench testing shows that the difference between a mirror finished intake port and a rough textured port is typically less than 1%. The difference between a smooth to the touch port and an optically mirrored surface is not measurable by ordinary means. Exhaust ports may be smooth finished because of the dry gas flow and in the interest of minimizing exhaust by-product build-up. A 300 - 400 Grit finish followed by a light buff is generally accepted to be representative of a near optimal finish for exhaust gas ports.

The reason that polished ports are not advantageous from a flow standpoint is that at the interface between the metal wall and the air, the air speed is ZERO (see boundary layer and laminar flow). This is due to the wetting action of the air and indeed all fluids. The first layer of molecules adheres to the wall and does not move significantly. The rest of the flow field must shear past, which develops a velocity profile (or gradient) across the duct. For surface roughness to impact flow appreciably, the high spots must be high enough to protrude into the faster moving air toward the center. Only a very rough surface does this."

Again, I think porting and polishing is fine and almost necessary for a racing engine, but not for a street driven engine that spends 90% of it's running life below 2500 rpm and much of that at an idle. In my opinion, the increase in power is in theory and the gains you will most likely see in performance for the type of driving one normally does will never be seen, but the possible damage to the heads and the possible negative effects are not worth the risk.

In the end, you decide. let us know what you do and how it works. Interested to see.