ouzo99max

iTrader: (2)

What kind of horsepower gains could I expect from a full exhaust on my 99 max?

Cattman Headers
Custom 3.0" Exhaust system

I am stock otherwise. No intake, etc...

If this thread is too vague let me know..

NmexMAX

iTrader: (22)

Any reason why you would go with 3" exhaust? Stick w/ 2.5" max.

My guess, 18-22 fwhp.

slinky87

iTrader: (1)

3inch is fine expect a trade in forlow end ttourqe to high end HP though if you ever plan on forced induction go for 3'

NmexMAX

iTrader: (22)

3' is waaaay big, I mean come on 36"

3" is good for F/I but not N/A.


Looks as if the vagueness is kicking in since we don't know your ultimate plans/goals.

ouzo99max

iTrader: (2)

you're right. I'm not planning to go turbo or s/c so I guess 2.5 is fine. Thanks for clearing that up

Maximus_95

iTrader: (8)

3" wont give you enough back press. 2.5" is perfect good luck and have fun.
Dyno it, so you can get exact numbers, doesn't matter what we think how much power you will get since every car is different...

NmexMAX

iTrader: (22)

Dude you've been here long enough to know that back pressure is always bad. It's all about exhaust flow velocity, a common misconception.

FantasyGod

Completely agree

willard00

iTrader: (1)

partially true. backpressure is bad to an extent. you need a small amount of back pressure so that the engine can expel the exhaust gas steadily/uniformly.

NmexMAX

iTrader: (22)

And another one Please stop saying this people, you're killing me.

mzmtg > all of j00
http://forums.maxima.org/showpost.ph...3&postcount=18
http://forums.maxima.org/showpost.ph...73&postcount=5

willard00

iTrader: (1)

alright fine, ill pull out my engines book, look it up, and take a picture for ya. its the truth.

NmexMAX

iTrader: (22)

Waiting for said pics of engine books, remember, back pressure, not velocity, is what you stated.

Apparition

iTrader: (5)

somebody is gonna get owned.. oh well.

willard00

iTrader: (1)

alright, flash was making the page bright white and without the flash my rooms too dark to read the words, so i quote:

"at idle maximum back pressure should be less than 1.5 PSI (10kPa) and it should be less than 2.5 PSI (15 kPa) at 2500RPM"
-advanced engine performance diagnosis (third edition), written by james D halderman

also, since thats all the book had to offer me (it was mostly about how to test back pressure):

"However, engines need some back pressure to work properly. A small amount of back pressure helps the engine expel the exhaust in a uniform fashion"

http://everything2.com/index.pl?node=back%20pressure

edit-i know youre going to argue with me, probably going to say something about how i shouldnt believe that website. but when you pay $30,000 to go to school for this stuff, and thats what they tell you...youre going to believe it. especially with the teacher i had for my engines class(worked for about 10 years doing specialized engine work. made 6 figures each year, built a 1200HP mustang from scratch. this guy was SMART.) im not talking about anything significant at all. just a SMALL amount of backpressure. put a 4inch exhaust on your stock car. there wont be any backpressure at all and it will run like garbage. but add a bigass turbo to that, backpressure will build a bit...and youll notice it will run better.

NmexMAX

iTrader: (22)

Another misconception here is the wording of such an argument, very fine line.

But since you paid ub3r cash to go to the uber mechanic school, you > me.

It's pointless in this thread since, well, all of these points have been proven millions of times, probably more.

willard00

iTrader: (1)

well, one last thing while we are at it. ill quote from a link in one of the threads you posted.

"Some stock mufflers have up to 18 psi of power-robbing backpressure. A well-designed performance exhaust typically has about 2 to 6 psi of backpressure. For comparison sake, an unmuffled straight pipe usually has 1 to 3 psi of backpressure."

-http://www.sportcompactcarweb.com/tech/9912scc_intake_exhaust_systems/index.html

so why wouldnt a well designed performance exhaust have 0psi of backpressure if all back pressure is bad?

NmexMAX

iTrader: (22)

Because it's not a perfect world. In a perfect world, there would be no back pressure.

willard00

iTrader: (1)

or is it because depending on the RPMs, a small amount of backpressure is needed to get the exhaust out of the cylinders?

Apparition

iTrader: (5)

Willard the thing is in order to maintain steady exhaust stream, back pressure of varying levels may be needed. But backpressure is never a good thing.

The least you can have and maintain the steady exhaust stream/ scavenge, the optimal point is reached. There is no set needed back pressure, yet there always is to some degree to keep gas ratios consistant.


In F/I cars you can run open downpipe and thats the best setup for anything IMO and theres absolutely no back pressure there.

Apples and oranges, but you want the most free flowing system possible without having stray gas bouncing around. Redundant? yes. But the point is the point.

PHAT6

Between a 10-13% power increase over the stock numbers - but how accurate are those numbers to begin with though?

willard00

iTrader: (1)

thats what i was saying.

that makes sense though. because you have the turbo/supercharger forcing the air in/out of the cylinder so you dont need backpressure to be there to draw it out.

Apparition

iTrader: (5)

Yeah its more or less you do things to keep the gas ratios right and it sometimes ends up yielding some back pressure. However, back pressure is to never be associated with being needed or being a good thing.

It's just a side effect.

willard00

iTrader: (1)

in terms of power no, i guess in that case its not. but in terms of a steady exhaust flow/gas consumption a small amount isnt anything to be terrified about. i wasnt really talking about power though, which im assuming you guys were. so i guess thats where we were butting heads.

Apparition

iTrader: (5)

:shrug: no idea..

edgar_raphael

There are so many myths out there to prove the back-pressure, and having worked on the small little 4AGE Toyota engine before, I had learnt one or two lessons about exhaust flow.

The education on comparing the myths to the like of sucking water from different diameter straws are rubbish. Why would you use 'sucking' method where in the exhaust world, we are actually talking about expelling gases? They should change the myths to blowing out from the straws instead of sucking, that is the nature of the exhaust.

Back on my experience on both NA and Turbocharged cars, it is quite simple. For forced applications, what you need that flows as easily as possible, meaning the lease backpressure your exhaust system builds, the better the exhaust housing can flow on the turbo and the faster the boost builds up.

For NA, it is a little more tricky, it requires a balance, although I would agree no back-pressure is better, but to a certain degree so to maintain the balance between the breathing side and the exhaust side of the engine.

If the engine can expel as much air out of the cylinders, it must be able to breathe (suck) in as much air for it to cycle properly. Eventhough on the air compression side of cycle may suggest this is not logical at all, but based on my experience, it does, especially when we are talking about different rpm.

On a drag application, where the engine runs on top-rpm most of the time, what you need least, is a muffler and back-pressure. On a normal driven car, you might need something to balance out the flow.

NmexMAX

iTrader: (22)

Be careful with the terminology. Backpressure doesn't draw out exhaust gases, cylinder firing/valve opening/closing forces them out, good header design helps them escape in tune (not 'on top' of each other), and exhaust velocity gets them out the exhaust stream.

edgar_raphael

and, turbocharger does force the air in but it does not force the air out, and good point to note from NmexMAX in regards to gas velocity. Good headers design helps to maintain gas velocity, but it would be useless if you have backpressure.

PHAT6

Hey Ed, the collective IQ in this forum has dropped with all the ockers and kiwis that have invaded here...

willard00

iTrader: (1)

alright, you guys are right...found it in my notes:

'there does not nessacarily need to be back pressure in the exhaust system, but in most cases it will be there. this however can help expel the exhaust gas uniformly.'

sonicrunch

30 grand to be spoon fed?

willard00

iTrader: (1)

no, 30 grand to have an associates degree. and i wouldnt exactly call it spoon feeding. everything we went over in the classroom, we did the next day in lab (43 bay garage). if we didnt i wouldnt have learned ****. cause im deffinatly a hands on-er.

FantasyGod

In a street application, this may be debatable. As I remember, an exhaust system needs to be a certain diameter and a certain length (regardless of restrictors) to acheive a maximum scavenging effect. In fact, I remember there being consequences for running open headers in a car design with an longer exhaust length in mind. If the exhuast pulses aren't in perfect sync, cold air drawn through the exhaust may crack the heads.

But to bring up a point related to exhaust flow restrictions, there seems to be some correlation between limiting airflow in both intake and exhaust systems to expedite the production of torque in the lower end of the powerband. Honda has a Variable intake and exhaust system with butterfly valves that works in junction with its Variable Valve Timing.

FantasyGod

You've already admitted fault, so just let it go. At, least you were reminded about something you've already learned and were clearly mislead on. Just read up more on scavenging effects of exhuast pulses and there won't be anymore misunderstandings on this topic

Apparition

iTrader: (5)

No, its not debatable at all.. I've been into this for a long time man. For a turbo, as free flowing as you can go is the ONLY way to go.