Back Pressure

[4CylFiero]

I ahve no exhaust on my car right now, I just finished rewiring everything and it is difficult to start I was wondering if the fact that I have no back pressure that it might be hard to start?

can anyone confirm this.

[rage2]

Define no exhaust? Is there no exhaust before the O2 sensor? Are the O2 sensors getting any exhaust gas? If not, it may be confusing the ECU.

[hjr]

i really doubt it. from what i understand back pressure is only an issue once the engine is started and there is enough exhaust flow to get pressure built up.
does the car start at all or is it just a difficult start?

[4CylFiero]

It starts It just takes about 8 seconds of cranking when the car is warmed up and about the third time I crank it for 5 seconds each time (15 seconds) it kicks over when it's cold.

Rage2 the oxygen sensor is in the manifold itself, but there is no exhaust at all, just a manifold
Hopefully Carline will help me out with that problem on tuesday of wednesday.

[95eclipsegst]

On your exhaust there could have been another O2 sensor, im not sure about this, but i do know that on my car there was another sensor behind the cat. and that could be the reason but like i said, im not sure about it?

[QuasarCav]

Originally posted by 95eclipsegst
On your exhaust there could have been another O2 sensor, im not sure about this, but i do know that on my car there was another sensor behind the cat. and that could be the reason but like i said, im not sure about it?



not on an fiero! i think it only has one.

[4CylFiero]

Originally posted by QuasarCav




not on an fiero! i think it only has one.


I do only have one, but it's far from anything Fiero now.

[Owned 2]

Originally posted by 4CylFiero



I do only have one, but it's far from anything Fiero now.

Yea, I saw this for myself this weekend. Comming along very nicely.

But really, I cannot see any reason why having no backpressure would create starting issues. Starting with NO exhaust should do nothing but maybe cause a bit of idleing problems.

You need some sort of exhaust piping, because for one, having some sort of pipe keeps the exahaust gases flowing easier, thats what makes your torque, and with exhaust just dumping out the manifold, its not going to be freeflowing, that will affect idle

[4CylFiero]

Umm who are you? If you don't mind me asking I haven't really been showing my car yet considering it just got it's tires on 2 days ago.

Of course I'm going to add exhaust to the car, it kinda sounds like your trying to convince me to add exhaust over running open headers.

[Owned 2]

Originally posted by 4CylFiero
Umm who are you? If you don't mind me asking I haven't really been showing my car yet considering it just got it's tires on 2 days ago.

Of course I'm going to add exhaust to the car, it kinda sounds like your trying to convince me to add exhaust over running open headers.

I was at your garage on Saturday, with my 2 friends. Im the guy with the red hair.

Edit, no im not trying to sway your exhaust options... im just stating the only things I could see no exhaust doing. I know your going to put an exhaust on, you were talking about mandrel bending and making an S pipe.

[4CylFiero]

OH ok cool, now I got yah, I wasn't told anything about what car you drove, I usually associate people and their cars on these forums.

Sorry bout that

[funkytuqe]

Yes this could cause hard starts. Not on all cars but on some. What some poeple have to understand is that no back pressure can be a bad thing (NOT IN ALL CASES). I assume you crank her over and over and it sputters but doesn't start??? OR maybe does not sputter at all i dunno. But my point is this: With no exhaust on your car, you are having scavenging problems and some exhaust gasses (assuming parital cumbustion) are actually going back into your cylenders and making for a shitty air/fule mix. This is caused by two things, ONE: exhaust gasses are hotter and lighter than the ambient temp air (assuming partial combustion) and getting pushed back into your cylenders. The hot gasses are trying to exit the manifold but run into a brick wall of cold dense air. If the car starts, once the air heats up inside the mani and around the mani exit (air beyond the manifold) the car will run a little better. TWO you don't have any scavenging, a vacume is created inside your stock exhaust system from the previous cylenders exhaust gasses traveling down the exhaust and actually helps suck out the exhaust gasses of the current exhaust stroke. Since you have no exhaust, this phenomenon does not occur and the exhaust gasses have trouble exiting the cylender.
That is why you loose lower end touqe when adding larger exhaust bigger or headers, your low RPM scavenging is reduced, however your high RPM flow volume is increased giving you more power.
I hope that made sense, it's kinda hard to explain.
The ideal exhaust system would have zero back pressure and maximised scaqvenging, good velocity and high rpm flow.

[4CylFiero]

An update for you guys, It now has exhaust on it and starts up just fine, has a little problem staying running but that because it has never been brought up to operating temps for all the sensors to operate and record information, until tomorrow:D Thanks everyone, I also forgot to mention that this is a fairly high compression motor 10:1 so mayeb some of you can redeem yourselfs :D