So I was recently in Europe on business for 10 days, and before I left I was looking for some reading material to get me through the plane rides. I'm a member at Speedtalk.com, and although I rarely post anything there, I do get notifications from the web site now and then. A few weeks ago I got a notification that there was a new book for sale called The Horsepower Chain, subtitled Racing Engines Explained Through Hardcore Math. Well, I'm a math guy from way back, so I thought it would be an interesting book to read.
When I went to buy it, I was shocked by the price; 158 pages, and $100! Makes my book look like a bargain LOL! I almost didn't buy it, but after looking at the information about the book and thinking about how much money I've got in many of my engines, I decided what the hell, I'd give it a try.
Turns out that the book is a reprint, under a different title, of a 2007 book called "Engine Pro - the Book". The reading is somewhat tedious, but I've read through it completely once now, and have to say it has some interesting information. The premise of the book is that there are seven cycles that an engine goes through each time a cylinder fires, and these have to be optimized individually and with respect to the previous and next cycles in order to get the most performance out of an engine. The seven cycles are:
- Intake pumping, where the piston going down into the bore creates a vacuum and pumps air/fuel charge into the cylinder.
- Intake ramming, which starts after BDC , where the piston is coming up in the bore and is no longer pumping, but the momentum of the column of air/fuel going through the port allows it to continue to fill the cylinder. The book claims that this is the process which allows very high volumetric efficiencies in some racing engines.
- Compression, which starts when the intake valve closes and the pistons starts compressing the air/fuel mixture.
- Fuel burning and expansion, which starts when the plug fires and continues to TDC and through when the piston is pushed back down the bore.
- Exhaust blowdown, which starts when the exhaust valve opens and the higher pressure in the cylinder blows out through the exhaust valve.
- Exhaust pumping, which starts at BDC of the exhaust stroke, when the piston starts to move up and pump the exhaust out of the cylinder.
- Valve overlap, when the exhaust valve is closing and the intake valve is opening near top dead center.
The book says that the author is a former pole winning NASCAR engine builder, the owner of Speedtalk.com, and is the person who developed Engine Pro simulation software. Most of the formulas in the book seem to come directly from that software package. Here are some claims made in the book that I think are pretty interesting:
- The intake pumping cycle is the most important cycle for making horsepower, and the intake ramming cycle is the next most important.
- Referring to the diameter of the port throat under the intake valve, the area of this throat can be used to calculate optimum bore size. He says that in the best performing engines, and he uses Pro Stock drag race engines as a typical example, the engine bore should be about 4.2 times larger in area than the area of the throat under the valve.
- Looking at the graphs in the book, the ratio of the throat diameter to the valve diameter should come in between 87% and 93%. In other words, if you have a 2.19" intake valve, the diameter of the throat should be in the range of 1.905" to 2.036". My porting guy always uses 90% for this measurement, smack dab in the middle of the range.
- Again looking at the area of this throat, the perfect port will flow 145 cfm per square inch of this area, at a depression of 28" of water. I guess this is something SuperFlow published with their flowbenches quite a while back. Unfortunately, frictional effects from the a/f charge flowing by the walls of the port makes this number impossible to reach, but the author says state of the art engines are running at about 133 cfm per square inch.
- The most critical valve event is the point where the intake valve closes. I think this is because if the intake valve closes too soon you can cut off the intake ramming process, and if it closes too late you can have reversion of the intake charge back into the port, but the book doesn't directly say this.
- Running an individual runner intake manifold, like a Weber setup or the Hilborn setup on my SOHC, you need about 6.25 times more flow through those eight butterflies than you would need if you had a standard throttle body arrangement (like a carburetor) on a plenum. The plenum helps a lot with sharing the intake charge between cylinders.
I think these are very, very interesting rules of thumb, and most of them generally correlate to what I've found over the years. The interesting thing to me is that in many cases the author puts numbers on this information, rather than just making a general statement. I think this is very useful.
I applied a few of these rules of thumb to my high riser engine just to see how it looked. That engine uses a 2.300" intake valve with a 90% throat diameter, or 2.07". Area of the throat is 3.365 square inches. The bore is 4.39", for an area of 15.136 square inches, and the ratio between bore and throat area is 4.498. So, according to the book I need a bigger throat to take advantage of the bore size, for example a 2.375" valve with a 90% throat diameter would get that ratio to 4.22.
With the existing valve and throat size, optimal flow should be 133 X 3.365, or 447 cfm. I was pretty happy with the 401 cfm I got from my high riser heads, but obviously I'm way down on flow compared to a state of the art racing engine.
One of the things that the book leaves out, which I thought was disappointing, is the relationship between the cross sectional area of the port and the valve diameter, or throat diameter. The author makes the general statement that you should minimize the port size while still retaining the best flow, because if you don't the velocity in the port will be too slow, and will limit the amount of intake ramming that you can accomplish. But he doesn't put any numbers on port dimensions, and he also doesn't talk about things like the taper of a sheet metal intake runner. Since I've built a few sheet metal intakes this would have been very interesting to me, but so far anyway I haven't found anything about it in the book. Also, I have a suspicion that the ports of the high riser engine are too big, leading to a less effective intake ramming process. But the book doesn't help me figure out how much oversize they are. If I reduced the size of the ports, I'd also reduce the flow somewhat, and there is nothing really in the book that directly addresses where the tradeoff is.
In light of all this, though, some interesting dyno experiments come to mind. For example, if the intake valve closing point is really the most important valve event, then advancing or retarding the cam will allow tuning of that parameter. With the Danny Bee belt drive on my high riser, it wouldn't take long to adjust the cam position and try some dyno pulls. That would be fun...
In any case, despite the outrageous price I could recommend this book if you want to go seriously in detail on basic engine parameters to maximize performance. There is no formula for building an engine that maximizes horsepower, but if the information in this book can be believed, following these rules of thumb will get you close.
