I have a 71 442 with a 455 with J heads. I bought the car 3 years ago with the engine already rebuilt and modified but i have no idea what was done to the engine. I do know that it has J heads but everything I have read states that J heads are not all that great. The car is very fast so I am wondering is it possible that the J heads were modified, basically my question is are heads modifiable?

 

Well of course, you can get things like hats, haircuts, and sunglasses.

 

Well I had a 455 that I rebuilt 17 years ago with bone stock J heads and it went just fine. I'm sure if I had either done some porting on them or gotten different heads like C, E, or G I would have been better off, but the thing was just a bit of a stump puller as it was.

 

J heads are fine for the street. Plus, any Olds head will flow the same if you modify it. The thing about the J heads is they have small valves and bosses for AIR injection in the exhaust ports.

 

so is it possible to change out the valves for bigger ones, or are they unchangeable?

 

Yes, you can have the larger valves put in J heads. It requires new valve seats too.

 

Compared to what? Folks who have only driven econoboxes think any big block car is "fast". As for mods, yes, anything is possible. This is obviously not the original motor, so who knows what was done. In fact, porting a set of J heads and installing larger valves makes for a nice sleeper.

On the other hand, stock small port J heads will probably provide plenty of torque at low RPMs until the ports and valves run out of steam. That's why I asked "fast" compared to what?

 

actually never really raced it against other cars nor had it dyno'd. It definitely has low end torque but never have gone over 45 mph because it currently has no windshield. Actually not even sure if the speedometer is working correctly either.

 

The smaller ports of the J heads means that performance will be limited at higher RPMS -- let's say over 4500. Note that I didn't say "at higher speeds." As long as you don't put stump-puller gears in the car, you can go over 100 mph easily without exceeding 4500 rpm.

If you don't want a screaming, high-rpm engine, there is no reason to replace the J heads. If you keep them, just make sure you don't get stupid and start putting other high-rpm components in the engine and drivetrain. Everything's got to work together as a system.