My friend has a midpipe but he didn't put the o2 sensor in so he just has the hole open and air going out through there. My question is, is it safe for him to drive it like thay without the sensor and is it bad to have the hole just open like that.

 

It's not going to harm the engine in a direct literal sense. Only harm to the engine would be if he crashes into something from the fumes coming into the cabin making it hard for him to concentrate, or if the car catches on fire from the fuel laden exhaust igniting stuff under the car.

 

So basically he needs a cap or something to cover that hole then.

 

But thank you. I needed an expert opinion to reinforce mines

 

Most auto parts stores sell an O2 plug that he can use. Costs about $3. Alternatively, any old/dead O2 sensor could work just as well to plug it.

 

One more question, can you explain why he shoots massive flames with the hole but when I put my o2 sensor in(not connected) to cover it, he shot less flames

 

I tried it myself without the sensor n shot massive flames as well

 

Fire needs 3 things: Fuel, Air, Heat.

Plenty of heat from the engine.
Plenty of fuel from the engine.

So all it's waiting for is air.

A variety of possibilities though, from running rich due to the front O2 thinking it's running leaner, to the open hole is changing the air mix in the exhaust to make it more ripe for flames.

 

Ah okay, I thought with the o2 being in the way, it could be disrupting the airflow and making it split since its inside like that. But I like your idea with the air mix. Looks like ima just take the sensor off and buy a plug to see if that works and then tell him to use it. If flames pop out from that hole can cause some problems haha