Technical 261 Complete Tune Up | Drives Like Crap

Need slight backpressure from dishonorable non-existent exhaust system. Even Confucius would tell you.

 

I have used engine vacuum in lieu of a timing light to achieve maximum setting. Proper vacuum is the symptom of a well tuned motor. 14 - 16 pounds of vacuum at a minimum and you can get to 20 depending on cam lift and duration. It is a cheap way to diagnose/tune an engine and the least embarrassing way to get the headlight doors open on a corvette.

 

I got a vacuum gauge and we'll be doing this also this weekend.

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If you insist... every exhaust system creates more or less back pressure. The more back pressure, the less exhaust gets out and the more stays in the cylinder. This can affect the mixture as it burns. Basically less back pressure = leaner mixture. Burned exhaust that stays in the cylinder dilutes and cools the next charge as it burns. This is the principle of exhaust gas recirculation, or the EGR valve that some engines used to have for pollution control.

Then again the mixture could be too rich. If unburned gas is escaping with the exhaust and burns on the way out you will get a backfire in the exhaust. Could be rich mixture, could also be lean mixture burning slowly, could be misfire leaving unburned gas in the exhaust.

A real sharp tuneup man could spend a few hours getting your engine working perfectly as it is. Then all that work would have to be done over again when the new exhaust was on.