Joe Abbin at Roadrunner Engineering has spent years collecting flowbench and dyno results
Given the fact that the first Ford Flathead engine was produced 77 years ago, and hot rodders have been tweaking them since day one, it would seem we would have learned everything there is to know about one by now. But during the time hot rodders have been building Henry's venerable V-8, more theories have been advanced and more myths have been created about them than any other engine.
One of the reasons so many misconceptions exist about these engines is that early on much of the speed equipment was developed with seat-of-the-pants evaluations, and if a certain combination won races or set records, it was deemed a success. Another reason is that many of the engine builders who have amassed a large amount of comparative data on modifications and combinations of components haven't been willing to share the information. Not many engine builders who have spent untold hours on the flow bench and in the dyno room are willing to tell all. Joe Abbin of Roadrunner Engineering is one of the few exceptions.
We first worked with Joe on our "One Last Flathead" series in 2004. At that time, many engine builders were denying the effectiveness of the classic relief, but his research convinced us that was definitely not the case--Flathead myth number one is that reliefs don't work. The second conclusion Joe came to contradicts conventional engine building wisdom, and has to do with compression ratios. With most engines, increasing compression results in more power, but with a Flathead there is a point not only of diminishing returns, but a point where power is lost--and it's lower than you might think. Flathead myth number two: the higher the compression, the more power the engine will produce.
Since we first collaborated with Joe, he's gathered more flowbench and dyno information to confirm his earlier findings, and has followed up his first book, Blown Flathead, with his new 335hp Flathead Ford V-8 Performance Handbook (available from Roadrunner Engineering). He was gracious enough to pass along some of the myth-busting information he's collected.
Optimizing The Flathead Ford
Relieving And Compression Ratios Versus Air FlowLike any engine, the Flathead will benefit from a good porting job. Increases in intake and exhaust airflow translate directly into increased power. Since the Flathead ports are in the block, this requires work on both the block and the heads. The Flathead has a very restricted flow path, with the valves being next to the cylinder rather than over the piston. This relatively complex flow path presents several areas that require attention. Airflow can be greatly improved by moderate porting and intake and exhaust airflow can be increased 35% or more with a corresponding increase in power.
The merits of relieving a Flathead have been vigorously debated in recent years. At one time, all serious Flatheads were relieved, and with good reason. My flow testing of both relieved and unrelieved blocks has clearly demonstrated that relieving substantially improves flow. Even low compression cylinder heads with very large combustion chambers and large transfer areas (between the valves and cylinder bore) will show substantial gains from a moderate, 0.125-inch deep, constant-depth relief. As noted above, mild "street" porting and relieving job can increase flow and therefore horsepower 35% or more. Since relieving provides the major part of this gain it is highly recommended. The traditional constant depth (0.100-0.125-inch) relief to the head gasket boundaries is very effective. The constant depth relief flows substantially more than the ramp type (where the relief does not go into the cylinder bore). The only bad news is that relieving does remove material from the combustion chamber and lowers the compression ratio. The good news is that not much material (about 3 cc) will be removed, which typically lowers the compression ratio only a small amount, about 0.2 to 0.3 point. Lowering the compression ratio 0.3-point reduces power about 1%, which is more than made up by the greatly increased airflow that relieving offers.
Removing material in the combustion chamber for any reason always lowers the compression ratio. In the case of relieving (area C) there is only a small penalty in compression ratio and the net result is increased power from the additional airflow.
The result of removing material at "A" is not nearly so clear-cut. Removing material in this area modestly increases airflow, but can substantially lower the compression ratio. (Cleaning up areas B and E and valves with reduced stem diameters (D) also helps airflow.) Knowing where and how to remove material in the heads for maximum benefit is covered in detail in my new book, 335hp Flathead Ford V-8 Performance Handbook.
In the Roadrunner Engineering study, published in the December 2002 and March 2003 issues of STREET RODDER article "Revival of the Fittest," the computer analysis was based on a 276 cubic inch Flathead with a "3/4 race" cam and typical intake and exhaust systems at sea level conditions. Actual flow test data and combustion chamber volumes for representative test head configurations were then used for input into Engine Analyzer Pro. The results were the higher compression engines dominated power projections at lower engine speeds, but then ran out of air at higher speeds. The lowest compression engine improved relative to the others as engine speed increased, but the improved flow never produced higher peak power than the others. Simulated and actual test results indicate that the optimum compression ratio for peak horsepower for a normally aspirated street Flathead is about 8:1 to 9:1. Going with the low side of this range has the advantage of requiring lower octane fuel with minimal loss of low-end torque and mileage. Years of street driving experience validate these results. These results were also validated in a second similar study that included the buildup of an actual engine, documented in the July 2004 issue of STREET RODDER in the article "Out with the Bad Air, In with the Good." It's also been found that supercharged street Flatheads perform best with lower compression ratios, as discussed in the books referenced above.