I am asked this question on a continual basis and thought I would share my observations with the forum. Much of this will be old news to many of you, but I hope some of this information will prove useful. This is taken from an article I wrote a few years back, but the principles remain constant.
Cylinder head flow is directly related to engine performance. At low rpm, little benefit will be realized to increasing volume, since it is velocity that promotes crisp throttle response, but when matched with a higher lift/rpm camshaft, the differences do become apparent. I have been blessed with the opportunity to use a flow bench and a dyno almost as often as I would like to. It pays to have friends with this equipment when you are doing this kind of engine work.
To that end, I have found no need to port heads at horsepower levels under 400.
We have dyno'd numerous engine combinations and so far, a ported/polished cylinder head has always yielded a performance increase. However, it is often at a much higher rpm range than the average person will operate their engine!!!
Some heads, by design, flow less than others and no two heads yield identical flow numbers. Although you may not bolt on a set of ported heads and 'feel' the difference, it is there. I have installed headers on several cars that never gained 1/10th or 1 mph in the 1/4 mile. I have installed headers on other cars that made a significant difference.
My 429 Ford, with 11.25:1 compression and big solid lifter camshaft, ran noticeably stronger across the entire rpm range with a set of 'hand' ported heads. These heads were flowed and showed over a 20% increase in exhaust flow compared to a stock version. However, that was a 7500 rpm engine and I doubt that any real benefit was to be had in the lower rpm ranges.
In comparing a 426 Hemi to a 440, the answer is simple; the Hemi has a soggy bottom end and needs a low axle ratio to offset the low velocity of the intake port flow. My 440 was much, much stronger, crisper and made better power (overall) under 3500 rpm. As such, I prefer the 440 to the hemi in street applications.
Kent Ford, Cylinder Head R&D for Bill Elliot's NASCAR winning team has gone on record to say little, if any benefit can be found when porting a cylinder head that (by design) is made to produce higher flow numbers. (Circle Track - Oct. 1990). This would encompass most of the aftermarket heads available today.
So, what really works and is porting worthwhile? That question can best be answered by using techniques that have yielded proven results under 'real world' driving conditions. If you study the principles of hydraulics and movement of a fluid medium through various fittings, you will notice that smoothness is a key factor to flow. Like a fluid medium, but on a lesser scale, air also responds to a smooth surface. Cylinder heads are not 'rough cast' to promote flow or increase throttle response, but because the unmachined surface is cheaper to replicate.
The greatest benefit is seen when the bowls directly under the valve seats are smoothed and blended into the port throat, without alteration of the short turn radius. Matching the entry of the intake ports to the gasket is beneficial, but the port exit of the intake should be left as cast. Subsequently, I leave the port exits on the exhausts as cast to promote their flow into the headers.
In closing, I have to say that porting yields its greatest benefit on engines with limited airflow potential. Even the alcohol burning 5 HP Briggs & Stratton's we build for Junior Dragsters are treated to a full port & polish before they leave the shop. In the case of engines with very limited airflow characteristics, like the 21 & 24 stud Ford Flathead, porting and polishing makes a difference you can see (and feel) on the dyno.
On OHV cylinder heads, which are superior in terms of airflow by design, the difference is not so readily apparent.
On aftermarket performance heads (like Edelbrock)I doubt any benefit, except in very high rpm applications, could be realized by porting/polishing because of the flow characteristics imparted by the cylinder heads' design. This is based on the theory that most people never run their engines that hard on the street and the biggest benefit from increased airflow is had at or around 3000 rpm.
If you smooth away restriction without altering the basic shape of the port and match the ports to their respective manifolds and/or headers, I believe the benefit, though small, is well worth the effort.
In closing, I can attest that we made over 400 horsepower with a 9.50:1 440 and stock '915' closed chamber heads. Using that as a performance baseline, I see no reason for the average enthusiast to port/polish on any street engine designed to operate in the 1200-5500 rpm range.
I hope this information is helpful...Robert
