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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!
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.
Ouch, a brick wall? Lay off the chronic, bro. The new Mazdas are great cars and have solid, reliable engines. A good intake and exhaust can wake up the car's performance significantly. I would recommend a proven intake from the larger companies like AEM or K&N because they have thorough research and development (R&D) programs that usually yield products that perform as promised. I can't recommend any specific bolt-on exhaust, but adding one should give you some more power and enhance the sound of the car. Factory exhausts are generally restrictive and even chopping off the factory muffler and replacing it with a high-flowing aftermarket unit is sure to give you some more power. Headers can be a bit trickier and some of the no-name eBay crap can actually hurt performance. Since I have very little experience with the Mazda6, I'm not going to recommend a particular brand or part, but talk to some other owners or visit a Mazda6 forum and ask around. Just be careful and don't believe everything you read on a forum.
For The 14.7th Time...
Why is an air/fuel ratio (AFR) of 14.7:1 called "stoichiometric", and why is it the ideal AFR? Furthermore, why are cars never tuned to run 14.7:1? Even in absolutely ideal conditions, they detonate anywhere near 14:1 under wide-open throttle.
--Ken McCollugh, San Antonio, Tex.
I'm not going to bust out on formulas here, but an air/fuel mixture is stoichiometric when the combustion of a fuel and air mixture is chemically balanced or considered ideal-think back to high school chemistry to understand a balanced equation. So for gasoline the theoretical stoichiometric ratio is going to be 14.7 parts of air to one part of gasoline to yield a chemically balanced reaction. Different fuels have different stoichiometric ratios: diesel (14.5:1), methanol (6.4:1), liquid petroleum gas (15.5:1), and ethanol (9:1). Cars are in fact tuned to run 14.7:1 on gasoline, but typically only in cruise and light loads to provide good economy and clean emissions. Most cars will actually run leaner than 14.7:1 at light loads and cruise, but emissions will skyrocket. When cylinder pressures go up (more throttle), additional fuel is required to slow the process of combustion down to prevent pre-ignition (knock) and to cool things down so engine parts don't melt. A good book on the subject for beginners is Introduction to Internal Combustion Engines by Richard Stone. Check it out and learn.
I have a '91 Honda Accord and it's a great car. The only problem is that I need a rear-wheel-drive to drift. Everyone I talk to just tells me to get a different car, but I want to convert the car that I have. If anyone could shed any light on the topic (parts needed, how to) I would appreciate it.
Honda_Tech1, Via Importtuner.Com Forums
Do it, or save yourself a ton of time and money and go buy a beat-up piece of shit S13 240SX for $1,000, and drift that into a wall. The reality is that to make the Accord drift, you will need about 10-20 times more cash than the car is worth. Oh wait, it's a '91? Make it 40 times more.
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