whats up guys i just got myself a 1986 iroc. The car it self is in great shape has only 94k miles on it. Its a 5-speed also has a 4 in cowl hood(pain in the *** to drive with) but my question is when im on the highway and say im in 3rd gear and i start reaching higher rpms my engine starts making a weird noise almost like its back fireing then when i shift to 4th it goes away until i reach higher rpms. One of my friend said it might be my heat shield but i checked that out and it seems pretty tight. I really dont know what it could be my timing might be off but not to sure or i might be running on 7 pistions?????

 

Sounds like it could be detonation or pinging. Does it kinda sound like marbles rattling around in a can?

 

Are the plug wires old?

The plugs take more power to fire at high rpm/load levels since there is more air (an insulator) within the cyl.

If the wires are marginal, the power may find an easier path to ground than across the plug gap.

Check the cap and rotor too.

 

i'd make sure the tune up was good and timing and idle was set to spec.

 

yeah i think its just time for a good tune up. Thanks for all the info