* Increased Exhaust Gas Temperature (EGT): Straight pipes lack the mufflers and resonators that help cool exhaust gases. Higher EGTs lead to more heat transferred back into the engine's combustion chamber, stressing exhaust valves. This is especially true under high load and prolonged operation.
* Backpressure: While excessive backpressure is bad, a *small* amount of backpressure actually helps scavenge exhaust gases more efficiently. Straight pipes drastically reduce backpressure, potentially leading to incomplete scavenging and increased cylinder temperatures.
* Fuel Tuning: The TBI system isn't designed for the significantly altered exhaust flow of straight pipes. Without retuning the fuel system (which is complex and often requires custom tuning), the engine might run leaner than ideal, exacerbating the heat problem and increasing the risk of valve damage. A leaner air/fuel mixture burns hotter.
* Other Factors: The condition of the engine, driving style, and overall maintenance also play a role. An already worn engine is more susceptible. Aggressive driving constantly pushes the engine harder, raising EGTs significantly.
In short: While straight pipes *won't automatically* burn valves, they dramatically increase the likelihood, especially with prolonged use and aggressive driving. It's much safer to run a proper exhaust system with mufflers and resonators. If you are determined to use straight pipes, you should be prepared for potential engine damage and the expense of repairs. A professional tune would be necessary, but even then, it's still risky.
