Auto and Truck Repair and Advice
Serving the Gainesville and Hawthorne Florida Area
Engine Compression and Compression Ratios
Measuring your compression is done by removing a spark plug (or injector in a diesel), inserting a compression measuring gage, then spinning over but not starting the engine. Because of this, compression can vary as to how fast the engine turns over. You look more for even compression between all cylinders than a certain number on the gage, although a gas engine needs at least 100 pounds of compression per square inch (PSI) to fire.
Most modern cars turning over at a normal starter speed generate 150 PSI or more.
Compression which is too low can result in poor running, high emissions, and bad fuel mileage.
Low compression can be caused by cylinder head problems or bad piston rings (or apex seals in rotarys).
A more accurate test is the leakdown test. This blows compressed air into each cylinder. Not only can the percentage of compression loss be measured, you can determine the cause of the low compression by seeing where the air leaks out. Bad rings will leak into the crankcase and you can hear the air blow out when you remove the oil filler cap. A bad exhaust valve will blow out the exhaust pipe, and a bad intake valve will blow back through the air cleaner.
WHAT IS COMPRESSION RATIO?
The compression ratio of an engine is determined like this: the volume of the combustion chamber with the piston all the way down (bottom dead center) is divided by the volume of the combustion chamber with the piston all the way at the top (top dead center). The lowest compression ratios are around 8 to 1. The highest stock compressions (found on 60's muscle cars) are around 12 to 1. Higher compression makes more horsepower. It can also make you burn high octane, premium gas. Few modern cars exceed 10 to 1 compression ratios.
The Economechanix Cyber Mall
We also serve the surrounding communities of Alachua, High Springs, Hawthorne, and Newberry!
Thank you for visiting the ECONOMECHANIX WEB SITE. Please feel free to comment. We also serve the surrounding communities of Alachua, High Springs, Hawthorne, and Newberry! Gainesville has been my home since 1974, and I've loved Gvl and the Gators since I came here in the fall of 1974 to attend the University of Florida. I loved it so much I stayed and opened my car repair business. Originally it was out of the back of a 1963 Chevrolet wagon, but in 1977 a fellow mechanic and I opened an auto repair shop with actual walls, etc. I stayed in the same location for 26 years, and recently moved my operation to property I bought 15 miles east of Gainesville. I am doing most all the repairs myself now, having reduced my overhead from $1500 per month to practically nothing. I do work by appointment only. I mostly work only on my established customers cars, but I will occasionally take on new clients. E-mail me and I will either make arrangements to look at your car, or I will recommend you to someone who will.
George G. Scott, Jr.