Now, I'm stuck with a car that has a broken lump of an engine. Every day, I get taunted with the nearest thing to a Civic Type R that Honda has ever offered the U.S. market, but I can hardly afford to make payments on a new car. And my SCC frame sits, unused, on my desk at home, waiting. But soon, it will have an appropriate home. Just as soon as my new LS engine and turbo kit arrive, that is!
Thanks for all the years of great reading and the inspiration to go and try this swap on my own.J. A.From the Internet
Dear DaveDirect "Dear Dave" tech letters to dave@eyesoreracing.com. Coleman will share mind-numbing details, earth-shattering revelations and technical nerdisms in this space each month.Can you stump the geek?
Fact CheckerWhile reading the letter on injector sizing in "Dear Dave" in the Feb. '06 issue, I decided to plug in some numbers and see what crunched out. A stock WRX has 420cc/min injectors and is rated at 227 hp. According to the formula, a stocker should have 447cc/min injectors (assuming a 0.6 BSFC and 80 percent duty). STI injectors are about 565cc/min and the formula says that they need to be 591cc/min. Am I suffering from a bad case of "rectal/cranial inversion," or is your formula inaccurate?Terry CulpColumbus, OH.
Ok, you caught me. That formula comes from RC Engineering and it has been the fallback of lazy keyboard jockeys like myself for years. Instead of straining my brain to really think things through, I just fell back on the old standby. For those of you who don't carry your SCC back issues around at all times like your Binkey, here's the formula again:
The formula is right, but the constants I suggested are extremely conservative.
Russ Collins of RC Engineering suggests using 0.80 for duty cycle because injectors can't be trusted to operate consistently above an 80 percent duty cycle, often sticking open, or "going static" when pushed farther. Shiv Pathak of Vishnu Performance has done more factory-ECU Subaru tuning than anyone else in the country, and he claims even the factory calibration on a WRX pushes the injectors over 90 percent duty cycle. Furthermore, he claims he doesn't see the injectors go static until at least 96 percent duty cycle. So who's right?
Both of them.
Duty cycle is not the true measure of an injector's limitation; minimum closed time is. If you look at a one-second interval, an injector that is open for the first 800 milliseconds and closed for the next 200 is running at an 80 percent duty cycle. Another injector that opens for 3.2 milliseconds, then closes for 0.8 milliseconds and repeats that 250 times in a row is running at the same 80 percent duty cycle, but is going through a whole different kind of hell.
It takes a finite amount of time to actually pull the injector pintle closed, and at somewhere around 0.8 milliseconds it simply isn't possible to get it closed and back open again in time. This is the minimum closed time.
Collins comes from the world of motorcycles, where peak torque can come at 15,000 rpm, and batch-fire fuel injection systems that fire the injectors every revolution are not the ancient history that they are in the four-wheel world. 15,000 rpm is one revolution every 4 milliseconds. At an 80% duty cycle, the injector is open for 3.2 of those milliseconds and closed for 0.8. In Russ Collins' world, 80 percent is the limit.