gearheadcurt

I have been getting mixed opinions on wheither or not open pipes are safe for the motor
some people say engines have to have back pressure,
some people say the lack of back pressure is just fine

reason for me asking is that I scraped over one too many bumps and my cat has come loose, and it sounds like an open header

I'm kinda worried

sanimalp

There is absolutely no problem running open header. People who claim you need backpressure have no idea how an engine works. Valve springs exert atleast 300 psi of pressure on the valve seats to "turn off" cylinder scavanging. They don't need extra help from the 1 psi of backpressure in your exhaust. The more resistance to flow you have in your exhaust, the slower you will be.

Most of the cars in the All motor forum run what amounts to open header. you get better flow that way. they may be sending it through long equal length headers and venturies, etc, etc, but it isn't to maintain backpressure.

And before anyone chimes in, yes, there were motorcycles in the 80's that experimented with variable geometry exhaust systems to maximize flow rate in the exhaust. They all stopped doing it because it was a waste of time after they discovered that straight pipes performed nearly as well at low rpm and the same at high rpm, thus making the extra cost not beneficial.

Cliffs: open header is not dangerous or bad in any way, shape or form.

mntuner2

Really, backpressure doesn't matter...? You may want to recheck your data regarding valve overlap.

When running a "high flow" exhaust you need to be careful not to overheat your exhaust valves. The exhaust valves will over heat if there is not enough pressure in the exhuast runners. Basically run your engine with a header and you should be fine.

gearheadcurt

I have the stock manifold so I guess I shouldnt be worried

sanimalp

Valve overlap is important because the simultaneous opening and closing helps scavenge the cylinder and increase intake charge using the momentum of the exhaust gas being expelled, all generally just before the cylinder starts its downward vacuum stroke. so now this is the part where you can explain why having some resistance to the momentum of the outgoing dirty exhaust gas pulling the incoming clean air makes sense in any way. If you have data that shows otherwise, I would LOVE to see it.

Burnt valves can be caused by improper valve seating, because normal solid steel valves dissipate up to 70% of their heat through the seat, and sodium filled valves were invented as a way to move some of the heat from the seat into the valve stem instead so the seat doesn't have such a big job. Bad tuning also has a significant part to play in burnt valves. Running lean and pre-ignition both can assert large amounts of heat on the valve face and combustion chamber all together. Interestingly, burnt valves can even be caused by excessive backpressure, a clogged cat, blocked muffler, etc, etc.

So, in summary, none of the scenarios you listed are made better by the application of exhaust backpressure. All of the scenarios you listed are made worse by the addition of backpressure.

mntuner2

There will always be pressure AND velocity in the exhaust stream. Pressure does not always equate to exhaust flow reversion and the velocity is created due to the difference in pressures. Basically this is not a black and white subject, many gray areas (see thermodynamics 101)

And yes exhaust valves will melt in a stock engine/electronics if there is 0 backpressure in the exhuast. I don't think dude will be reeingineering his rotating assembly anytime soon as well. remember the old adage: KISS