I have an 89RS 2.8L 173 with a 2.5" manifold to cat: 3" cat back exhaust system and I am having back pressure problems due to the massive volume of air being pulled away from the motor compared to what is being put into it. Now I want to do two things that may help my problem:

1) direct airflow via two plastic tubes from the front, lower grill to the aircleaner assembly(create more inward air pressure)

2) a throttle body from a larger v6 or even a v8(someone once told me that the v6 and 305 TB are similar, i dont know for sure)

Now before I do any of this, I was wondering what would some of you recommend doing in this situation and is it possible, or even good. I know the larger throttle body will decrease fuel efficiency but I am not too too worried about that.

Thanks,

Dave Marsh

 

well, the most restrictive part about our motors are the exaust manifolds, its really quite useless to do any other mods because they will do close to nothing if you still have stock exaust manifolds.

What goes in, must come out. There is no use having the best intake system in the world if your exaust system cannot flow as much air (or more) air than the intake.

I would wait for the Pacesetter headers to go on sale, and then grab some of them, it will be a good bang for the buck. And then then you can start tinkering with your intake setup and porting heads and stuff if you want to.

 

well the exact problem im having is too much air flowing from my engine and not enough coming in....so that is why im proposing doing something that would increase the air intake of my engine

 

oh yeah...but i do plan on putting on pacesetter headers when they finally come out

 

i highly doubt you are getting "too much" air into your system and not enough out. The stock exaust manifolds and Y-Pipe are very restrictive, and even the stock intake system can outflow them... unless you still have your baffle box thingy installed.

The baffle box doesnt really limit the airflow, but it does slow it down alot, making a very crappy throttle responce, (bad throttle responce feels like there is no power) i would eiether replace the baffle box with some 3" PVC, or just do a CAI setup with a K&N filter and such, this would be great for when you get your pacesetters, and also help in throttle responce without them.

http://thirdgenmods.firebirdv6.com/coldair.html

this site also has bigger throttle bodies if you are interested:

http://www3.sympatico.ca/fjpeters/throttlebodies.htm

 

i think u are missreading my posts..I have too much air flowing from my engine...as in my exhaust pumps out more air than my engine is taking in so i have a back pressure problem, which in turn causes my air sensor to think that thereis more air coming in than actually is, and it is causing poor throttle response for that exact reason. Ive brought it to two auto shops and they have told me the same thing..in a few words or less.

 

backpressure is when your engine pumps out more air than your exhuast system can handle, not as what your describe it.

And, an engine can only pump out as much air as it takes in, so you cant pump out more air that you take in, which would tottaly negate what you are trying to say.

An engine is basically a giant air pump. What goes in, must go back out, and what goes out, must have went in.

If the auto shops are saying you have too much backpressure, they are correct. Crappy exhaust manifolds and Y-Pipes can cause backpressure, and is exactly what happens on our engines.

 

if u want to get down and dirty into physics and such then we can. The size of my exhaust system creates a larger area for the outflow of air to fill, and being that the car is moving there is a pull of air through the entire system. Its like a fuel pump operating with very little fuel pressure on the intake end, it has to work much harder to match the output demand on the other end thus creating a pull. Air will go in and will go out...but its the fact that there is a lack in equal pressure on the output end than on the input end that is causing the backpressure.....so if u put more fuel into the fuel pump you solve the power problem...so i need a way to put more air into my engine so it can put more out...

 

*sigh* no...

if you knew how engines worked, you would see that the actual backpressure problem is the Y-Pipe and Exaust manifolds.

if your intake system could not flow as much as your exhaust system, you wouldnt be having this problem. The exhuast system does not "pull" air thru the intake as you are discribing, the pistons do. The pistons push the same amount of air out the exhaust as they pull into the intake.

You have the same engine as almost 99% of the people on these V6 boards, we all know the problems of this engine design, and finally the solution is in the works.

Alot of people have modded thier cars, and seen very very little improvements because the one thing they didnt change, the number 1 thing they should have changed, was not changed. THE EXHAUST MANIFOLDS AND Y-PIPE.

I know what your problem is, because i have that problem, infact mostly everyone with a stock (or mildly modified) 2.8L has the exact same problem.

 

you still dont get it...i know of the restrictive manifold problem, but my exhaust is not stock all the way back...in fact the 3" cat back system is too large for the engine size. Exhausts dont pull air out.....a moving system does. It could be fixed by forcing more air into the chamber, therefore there would be more air to force out, filling the extra space in the exhaust which causes air to move around and not flow where it is supposed to go.

Anyway..i know what the exact problem is..yes there is a problem with the manifold and the y-pipe, i have read up on that...but other than that i was just trying to find out if replacing my throttle body would be a good idea..or even the air induction system you recommended(which i am going to put together tomorrow) would be a good idea. Anything to increase the air intake..especially if it forces more air into the system, and fuel if necessary.

So to end this discussion, thanks for your recommendations, when I get the time I a m actually going to get a new throttle body from the site u gave me and implement the cold air induction system you pointed me to..with a few mods of my own ofcourse.

I will let you know how it turns out..pictures and all..thanks for the help though..and I can't wait for the pacesetter headers and the y-pipe they intend on making.

dave marsh

 

Lee is right, you cant flow more than you can take in. having a larger cat and catback wont matter. your manifolds and y-pipe are restricting the possibility of having the cat and catback flowing too much. improving the intake system wont do a thing. you have to open up the whole exhaust system.

you do have a larger exhaust, but not the WHOLE thing. at the beginning, its stock... restrictive. so youre not going to get more flow out of it. once you open up the manifolds and y-pipe, then you can get better flow. think of it as a bottle neck the the bottom cut off, you arent going to get better flow b/c the bottle neck is restricting it from doing so. thats where your whole problem is.

 

while the CAI and the Larger throttle body is a good idea, its not going to give you that much extra HP, so dont expect it yet.

When the Pacesetter headers are available, that is when the Throttble Body and CIA will really start to shine.

The Pacesetter headers (or any good headers for that matter) will enable our V6's to outrun Stock or Mildly Modified 5.7L's, they also have a very restrictive exhuast system, but a killer intake (like our V6's)

GM did a very nice job on our Intakes, the exuast is where they fukt up.

 

Exhaust is way easier to fix then an intake

 

3" catback is way too big to run on a stock v6 (i assume you're runnin stock internals). i am running a 3" catback on a highly modded 3.1. the large exhaust without being needed will hurt your throtle response. the biggest i would go on the exhaust on a stock motor would be 2.5"...and that's pushin it. lee7 is right, you cannot flow more exhaust than the air you are taking in. i bigger throttle body will also hurt throttle response...may help a tad bit on top end but will hurt your low end. a good CAI will help your top end power.

 

ok, you obviously have no clue on what your tring to say...

1. there is no way in hades that your engine is putting out more air then it is taking in. PERIOD. Won't and more importantly CANNOT happen. That pesky thing called combustion takes care of that. Mass/Volume is deminished by 30-50% after the mix is burned. Hense why EVERY SINGLE car exhaust valves are smaller then its intake valves.

2. What you ARE dealing with is tring to fill a bigger space then your engine can fill. The biggest a V6 should go, is 2.5". What you are running into is a pressure wall that is occuring when too little mass moves into a larger opening.
Flow dynamics calls this increased flow rate at decreased velocity.
If you were to go from say a 2 1/2" to a 2 5/8" ect ect slowly spanning several 3-4 feet between increases, your problem wold go away, as the area would increase slowly, helping keep velocity at peak.

 

Sell the 3in cat back and get a new one. Unless you are gonna go V8 at some time. What you have is a lack of back pressure issue, the pipe is so big the flow out never fills it and creates any "back pressure".

Matt

 

is it possible to use the tpi throttle body on our mpfi systems after freeing the exhaust up (just a thought) and if so will the prom have to be reprogrammed?