# exhaust back pressure



## octave236 (Dec 4, 2002)

*exhaust back pressure*

there have been a couple of thing listed aout this but i would like to know inorder to clarify somethings ... what would be the best exhaust back pressure psi in a ga16de and a sr20de n/a of course. and could you prementaly have a gauge set up inside the car to let you know whats up. im putting new exhaust pipe in and i dont want to get to loose and mess something up or even loose hp. i have a ga16de with hs 4-2-1 headers. and a 2.5' strait thru sport muffler on my b13. could any one help .. i just wanted to know the sr's for future ref.


----------



## octave236 (Dec 4, 2002)

ok i read in mike kojima's garrage how to do the gauge thing but what's the best reading is it 10 psi or more please help ...


----------



## hpro123 (Apr 30, 2002)

You are kidding, right???

The less backpressure there is, the most HP you will get. If you can get it down to 2-2.5psi you are close to ideal.

However, note that pressure is NOT the only parameter you have to consider. Pipe diameter is also critical since it is the diameter of and the smoothness of the pipe that mainly (combined with backpressure) dictates how effective the exhaust is.

The exhaust needs to be of a diameter that allows the exhaust gasses to travel at the maximum speed attainable while having the least backpressure. Hence, you cannot go too wide a pipe since it will drop the exhaust gas speed and result in loss of torque in all the revving range except maybe the very top.

I am not familiar with GA16 engine requirements but for a n/a with less than 2 liters do not go over 2" in diameter. For the SR20DE, 2" up to 2.25" is fine.

Again, backpressure? 2psi is better than 2.5psi and 1.5psi is better than 2psi. 

I doubt that you could ever insert any permanent pressure gauge there since you would need to measure pressure at 2 point along the exhaust (so 2 sensors and 2 gauges) and I do not know of any "differential" gauge that takes input from 2 sensors.

Chris


----------



## octave236 (Dec 4, 2002)

thanks ... i just said 10 psi cause im seem to remeber a thread saying something like that but my memory sucks. i didnt know about having to have 2 sensors and 2 gauges. Kojima only said you needed one but ill look into it deeper.
Ben


----------



## bahearn (Jul 15, 2002)

It's easy to measure backpressure...with enough money. Chemical and refining plants use dP (differential pressure) cells by the hundreds. You can measure one pressure versus atmosphere, which is the requirement for this discussion, or you can measure between two points in a system (what the plants use them for). The dP cells we use are the size of a small loaf of bread and made with lots of heavy stainless steel.

Backpressure in the exhaust is relative to atmosphere. There are two competing issues here: Higher pressure means higher motive force to push the exhaust against atmospheric pressure found at the end of the tail pipe, but higher pressure means more work for the engine to evacuate the cylinder, thus less work to the wheel. Lower dP implies higher exhaust velocity, which is a good thing. Zero backpressure implies zero flow, which means all pistons are holed.

Food for thought: back in the Eighties, one muscle car magazine found that they couldn't fit too large an exhaust on a car. Conventional wisdom said 2-1/2" pipes on a dual-exhaust small block. Their testing stopped at 3-1/2" diameter and there was continual improvement in both torque and horsepower without any degradation in power curve shape.


----------



## octave236 (Dec 4, 2002)

wow that maybe a litle bit more than my brain can understand but ok ... im not the type thats good at reading and understanding but i think i know what your saying ... but i have herd befor not to go over 2' pipe many time's they say they loose a lil bit of power ... but your saying i could go as big as i want ... but wouldnt that hurt the engine in time ...


----------



## morepower2 (Apr 19, 2002)

*Re: exhaust back pressure*



octave236 said:


> *there have been a couple of thing listed aout this but i would like to know inorder to clarify somethings ... what would be the best exhaust back pressure psi in a ga16de and a sr20de n/a of course. and could you prementaly have a gauge set up inside the car to let you know whats up. im putting new exhaust pipe in and i dont want to get to loose and mess something up or even loose hp. i have a ga16de with hs 4-2-1 headers. and a 2.5' strait thru sport muffler on my b13. could any one help .. i just wanted to know the sr's for future ref. *


In testing I found that any backpressure more than 2 psi starts to degade pwer in engines with boltons including camshafts.

You want to run the smallest diameter tubing to accomplish this to keep the velocity up which is 2.25 mandrel bent in a bolt on NA motor with straight through peforated core mufflers.

Mike


----------



## octave236 (Dec 4, 2002)

thanks mike ... this is sort of new to me (the whole forum thing) and im not sure exactly who you are ... since im from tn. but from reading most your articles you know your shit. and most people never debate on something you'd say ... well at least what ive seen in last 2 weeks ... thanks to all who care to help out a guy who's just starting out in the import world ... luckly i got a nissan as my first import and im now glad i did ... there was a time i would have chose a crx but i love the sentra.


----------

