Back pressure? How does it feel?

Slug · May 20th, 2003, 08:39 PM

Hey Im still pretty new to all this and I was wondering how you could tell if you have too much back pressure?

Can you just "feel" it or is there some way to measure this (cheaply)?

the reason i ask is because i just installed HS header along wiht CAI and a new cat to my existing Greddy exhaust and I feel no increase - just sound which is actually too loud for me.

I have heard talk of back pressure that maybe i dont have enough, or that i have too much. Could it be the cat? the resonator?

Thanks to any who can help!!

fastpakr · May 20th, 2003, 10:31 PM

You NEVER ever want backpressure. That being said, pipe that is too large will create backpressure because velocity drops at low RPM, allowing the exhaust to cool and slow down in the pipe. You need to figure out what RPM you're tuning the exhaust to be most efficent for and then do the calculations for the right pipe size.

Myetball · May 20th, 2003, 11:22 PM

Check out this post .

May 21st, 2003, 07:32 AM

Please site your source for this statement...

Quote:

Originally posted by fastpakr
That being said, pipe that is too large will create backpressure because velocity drops at low RPM, allowing the exhaust to cool and slow down in the pipe.

fastpakr · May 21st, 2003, 08:48 AM

What is your argument with it? It's a simple principle that as volume increases, pressure, velocity, and temperature decrease. When that happens, backpressure is created. As long as there is enough volume being exhausted from the engine to limit temperature drop adequately, pipe size is fine. What is it you disagree with?

**chimmike** · May 21st, 2003, 09:23 AM

Quote:

Originally posted by fastpakr
What is your argument with it? It's a simple principle that as volume increases, pressure, velocity, and temperature decrease. When that happens, backpressure is created. As long as there is enough volume being exhausted from the engine to limit temperature drop adequately, pipe size is fine. What is it you disagree with?

hhaha so you're trying to tell me wiht a larger pipe you get MORE backpressure???? HAHAHAHHA OK buddy. Try and tell that to me with a straight face. Or any turbo guy, or any N/A V8 guy. lol

fastpakr · May 21st, 2003, 09:30 AM

Yes, that is EXACTLY what I am telling you. IF the pipe is too large for the motor, it will create backpressure. There is a proper tube diameter for any motor, and going too far on either side of that will create backpressure. A turbo motor is a little bit different because most backpressure is created by the turbocharger itself, going large post turbo isn't that critical.

May 21st, 2003, 09:41 AM

This is what I disagree with... state your source for this please.

Quote:

Originally posted by fastpakr
Yes, that is EXACTLY what I am telling you. IF the pipe is too large for the motor, it will create backpressure.

dschrier · May 21st, 2003, 09:48 AM

Well I would agree with the fact that increasing the diameter will decrease the temp and velocity, but I'm not sure about the backpressure part, but I do not know the true answer myself so I won't dispute the post. From what I've always been told about how the exhaust works and the right size etc is to look at it like a straw. Too big of a straw and you won't be able to blow, or suck the exhaust out unless you have more strength, which a 4 cylinder car doesn't have much of for over a 2" exhaust, but the turbo generates more power so you can go 2.5" or more. Too small of an exhaust and you can't get the exhaust out quick enough. My analogie could be completely wrong as well, that's just how I look at it though.

fastpakr · May 21st, 2003, 09:48 AM

I will try and dig up some articles on it - I don't have any links offhand. What is the question though?
> pipe volume = < exhaust velocity
< exhaust velocity = more temperature drop in pipe
more temperature drop = denser air, which the next cylinder's exhaust slams into. at that point, backpressure is generated because the hot exhaust from the next cylinder is being restricted by the overly cooled exhaust from the previous.

If the volume exhausted is sufficient to keep it from cooling excessively in the pipe (because velocity remains high), then the pipe size isn't too big. For this reason, an engine turning high RPM can use a significantly bigger pipe than one built for towing, even though they're the same or similar displacement (Ford 4.9I6 vs. a 5.0HO for example).

zeno · May 21st, 2003, 10:17 AM

Here we go on another physics debate

The point is, on a NA motor:
-too small of a pipe (in this case <2") BAD
-too large of a pipe (>2.5"), BAD
-between 2"-2.5", we have a winner!

Turbo: need 3" or bigger. Bigger is better (not how you use it...).

I think I'll go back to college and major in physics just so I can resolve this stuff.

Slug · May 21st, 2003, 10:34 AM

Im glad there is a great debate going on here! Now I dont know half as much as any of you on this matter, so can someone tell me exactly HOW you can tell/feel this backpressure? or do you just assume it is going on when your car doesnt go as fast as you thought it would...

May 21st, 2003, 10:48 AM

You can roll your eyes all you want but that solves nothing. You need to prove your point stated below or just admit that you are just "jawboning" us...

Quote:

Originally posted by zeno
Here we go on another physics debate

The point is, on a NA motor:
-too large of a pipe (>2.5"), BAD

fastpakr · May 21st, 2003, 10:49 AM

BGriffey, explain your disagreement. I have posted a very basic physics explanation of it and you haven't made any attempt to explain why you disagree. At this point, you're accomplishing nothing more than being a troll unless you have a reason for disagreeing.

zeno · May 21st, 2003, 10:52 AM

Quote:

You can roll your eyes all you want but that solves nothing. You need to prove your point stated below or just admit that you are just "jawboning" us...

What do you mean prove my point? I don't have to, thousands of people have proven this point by simply trial and error. This is standard info. on exhaust recommendations for the sr20de engine recommended by NPM, se-r.net, Mike Kojimo, and thousands of members on Nissan Forums and sr20de forums.

This is silly, I'm not saying you guys don't have great info. but the guy wants to know about why he doesn't feel a difference w/ his header and exhaust not a physics lesson.