You have questions? We don't care. Fortunately for you, Eric Hsu does. And even if it's only so that he can bust your balls in print, he looks forward to answering your toughest tech questions each month, via questionit@importtuner.com. In a bind? Drop Eric a line! Unless, of course, you're planning to ask him how to de-fog your Grand-Am's euro taillights, or what a fair price for second-hand Cavalier battle wings would be. On second thought...send any questions along those lines in, too. We could all use a good laugh!
For questions, or problems of your own, email them to questionit@importtuner.com
Compression It
Compression ratios, Eric: I understand how they pertain to engine building on principle, and that lower compression is easier to tune forced induction with; higher is better for naturally aspirated...but how are compression ratios calculated? What exactly are the two factors relating to each other in a compression ratio. And how do I calculate the compression ratio of my engine, if I should want to shave my engine's head or deck to increase compression?
--Paul Petersen, Philadelphia, PA
An engine's compression ratio is calculated by taking the volume of the cylinder, head gasket, and combustion chamber when the piston is at bottom dead center (BDC) and the volume of the cylinder, head gasket, and combustion chamber when the piston is at top dead center (TDC). The ratio of these two factors is your compression ratio. For example, while the piston is at BDC you measure 500cc and while the piston is at TDC you measure 50cc, you would have a compression ratio of 10:1. To get an exact measurement, you will need some special tools like a flat piece of Plexiglas with a hole in the middle, some silicone grease to seal this Plexiglas to the cylinder or head, and a pipette filled with fluid for accurate measurement. Then you fill the cylinder or combustion chamber with the fluid noting the volume of fluid released from the pipette. The volume of the head gasket can be calculated with simple geometry and measuring the gasket's bore, diameter and thickness-volume of a cylinder = ?r2h. You can estimate your compression ratio, but since pistons have dishes or domes in them and valve reliefs, deck heights might be a bit different between engines, so it is best to get an accurate measurement. When shaving your head or adding cams with higher lift, just be sure your pistons have enough valve-to-piston clearance!
Millenium Aftermath
After putting my old Mazda Millenia through a brick wall going 85 mph and then taking a bit of a break from the driving world, I purchased an '08 Mazda6 and am in love with it. I realize that there is not as much of an aftermarket for it as some of its other Mazda relatives, but are there any mods that you could recommend for a small budget? I am looking for some good bolt-ons that will bring power, but still easy to do without access to a shop regularly; maybe an intake, exhaust, or new header setup that you would recommend. Any advice would be nice. I am also a little weary of the warranty issues, and was wondering if you have any insight on to how modding a car might change the warranty. Thanks for the help.
--Aiden Baldwin, Raleigh, N.C.