25 Ways to Live Like a Car GuyThe "25 ways to live like a car guy" article in the January issue was great. After reading it I realized I had already accomplished 5 of the 25, (buy a beater, powerslide, watch a short-track event, modify your car the day before an event, and last but certainly not least, jump your car) all just because I had the chance. Needless to say, when I heard about the "The Eve of Devastation" event at our local track, I had to enter. I took my '88 MX6, gutted it, and then removed the exhaust and spray painted it. It was the most fun I've ever had, competing in the forwards/backwards race, spectator race, and the jump contest. It was the first time I've ever been to any track, let alone raced on one. Just wanted to let you know the other 20 ways are now on my to do list, and keep up the great work.Matt HeimlichStevens Point, WI
Love Letter of the MonthFirst off, I have to say that you guys are a godsend. You're the one thing that keeps me going at work and the always-needed bathroom read. I have learned things from SCC that you can't in a classroom. Just want to thank you guys for being the best friend I always wanted but never had. Keep up the good work!Jonathan MarionOrlando, FlFrom 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?
Is that a Bernoulli in your pocket?A few days ago, I got into the age-old argument that backpressure (meaning small diameter exhaust pipes) equals bottom end torque. I was well prepared for this argument with Bernoulli in my back pocket. I tried to explain that the small diameter tubing would increase the exhaust velocity meaning there would be less backpressure at low RPM, but would become a bottleneck at high RPM and increase backpressure.
The inverse goes for larger pipes, at low RPM the velocity is down so backpressure builds until higher RPM where the larger pipe with more gaseous fumes will increase velocity and lower backpressure. I realize that this has probably been addressed a thousand times, but this old wives' tale has to be killed off.Rob KingPittsburgh, PA
I can't sort out which side of the argument you and your pocket Bernoulli come down on this issue, but backpressure does not increase torque.
Here's the phenomenon in a nutshell: Exhaust doesn't go out your exhaust pipe in a steady flow. At the tailpipe, after several cubic feet of pipes and mufflers, it's reasonably constant, but in the exhaust port it's a different story. Every two revolutions, the exhaust valve opens, some air is pushed out, then the valve closes and the air has to stop. During the closing of the exhaust valve there is an opportunity.
If you imagine the cylinder full of air that just got pumped into to the exhaust as a solid, it will be a big tube, or column, of air. This air column has some inertia, so it isn't terribly thrilled about having to stop when the valve closes. As the valve starts closing, the air column tries to keep going. This inertia can be used to help suck the last of the exhaust out of the cylinder. And because the intake valve is starting to open as the exhaust valve closes, this sucking alsohelps start the flow of fresh combustibles into the cylinder.