Editor's note: In the May issue, CT readers had an opportunity to get a snapshot of results obtained during our first engine dyno session, using the GM CT 525 project engine. Because of the abundant test and test-related information gathered, we chose to provide two levels of technical analysis; one developed as an overview (in May) and a second taken to a higher level of discussion involving reasons on which the results are based (in this article). While this latter analysis is intended to be more of a "learning experience" than observational, we believe it will provide a more in-depth understanding of the engine's performance. This is an important consideration because the next dyno session will include in-cylinder analysis of the combustion process using alternative fuels. This is another bold step toward urging circle track racers and engine builders to step into the "green racing" box with us.

What might cause a difference in brake horsepower when switching from carburetion to electronic fuel injection?

Of several reasons, two stand out in particular; intake manifold design and fuel atomization efficiency throughout the rpm range. This observation includes the fact that some degree of combination optimization will tend to minimize the differences but, from a practical and theoretical perspective, the fundamental differences should still exist. Here are some thoughts to consider.

In prior Circle Track editorial material, we've proposed that flow passage cross-section area (intake or exhaust) has a pronounced effect on flow rate. And even though both intake and exhaust systems function in the presence of unsteady, compressible flow, it's still possible and desirable (for purposes of mathematical calculations involving intake and exhaust system modeling) to deal with what we'll term "mean flow velocity." Given this notion, we can also relate mean flow velocity to engine speed.

Consider that there will be a mean flow velocity associated with any given rpm, with a value in the 240-250 feet/second range at peak torque. In fact, we've previously reviewed this concept that we'll now apply to the two manifolds in this project, essentially in terms of passage section area and length. Recall the notion that section area determines mean flow velocity (all else being equal) passage length tends to "rock" the torque (volumetric efficiency) curve about the point of peak torque rpm.

Simply put, shorten runner length and the torque curve "rocks" in a counter-clockwise direction. Lengthen it and the opposite occurs, notably adding torque below the peak and removing it from above (again all else being equal). And while it's reasonable to assume that some of this shifting in torque could be addressed by companion parts changes (notably camshaft), the purpose of our discussion is to compare fundamental differences in manifold design, single 4V carburetor to EFI, as applied to the project at hand.

You may want to refresh your memory on the comparative torque curves shown in May (Figure 1). Since the same set of headers was used for both induction system tests and applying the little equation previously shared in CT "Enginology" materials, we ran the numbers on where the header contribution to the net torque peak should have been, calculating a value of 5,216 rpm. Eyeballing the peaks shown in the graph, you can see the indicated peaks are very close to 5,400 rpm. This suggests that both intake manifold section areas are about the same, but larger than that of the header primary pipes. For purposes of this discussion, that's immaterial.