I did some reading up on this exact question. This is what I came up with.

500hp 2 1/2 pipe is fine with non restrictive mufflers.I guess with 2.5 inch pipe the mufflers are something to consider.Ls1tech had a big thread on exhaust and it's effect on hp.

 

mustangs love back pressure

 

I hope your joking! If not then you should be running a 1 inch exhaust. That should create more backpressure (that mustangs love). :rofl:

 

No way in hell i'd use 2 1/2" with a blower :shakehead

You need headers with a 3" collector, and 3" all the way back with some "breathing" mufflers..

With a larger exhaust comes cam timing, and tuning. You can't just bolt on 3" and expect it to make more power...The more "off" your tune, and cam timing is, the more a better breathing engine will uncover those issues.

Good luck

 

Go with the 3" if you where to lose 15hp big deal as the exhaust has room for you to grow ie a 331 , 347 or a big bore 363 motor in the future the exhaust system is not that cheap if done right .

 

DO A 2 1/2

 

:blam:

Why...Because back pressure is a good thing?? Get away from the magazines

 

on my blown 331 i use 1 3/4 mac long tubes with a 3" collector, 3" pro-chamber and a 2.5 flowmaster catback.

 

It amazes me that when someone suggest 2.50" of diameter for exhaust tubing, they automatically assume that they are suggesting backpressure. Sometimes people look for a fight. Ignore these types.

A 2.50" or a 3.00" exhaust will do fine for you.

A buddy of mine stopped by the house last night just after putting in some 1.75" primaries into 3.00" exhaust on a N/A 346. No loss of low-end power was noticed, yet he gained 4 tenths and 4 MPH at the track over the stock exhaust. This was an LS1.

 

My point is, if you're going to spend the money...do it once, do it right the first time!..Why on earth would you recommend someone PAY for a 2 1/2" exhaust??

BTW, If 2 1/2" is OK by your standards (And this whole corral boards mentality)...Then why do OPEN HEADERS (with a good long tube header) ALWAYS run faster down the 1/4??? Unless you just can't fit a larger exhaust, it is ALWAYS the way to go...

And don't mention DYNO numbers...A dyno does NOT measure acceleration..

Everytime someone tries to shed some light to most of you people, you say..."Well my car made 2000 Horsepower with 2 1/2"....Well dude, your combo is capable of 2500 if you unbolted that junk..

 

my vote is for 3in.

3in - forced
2.5in - natural

 

If you plan on running the stock block.....both are capable of supporting block splitting horsepower. My 2.5" doesn't seem to be holding me back much...and I'm running full length pipes.

If you already have a 2.5" exhaust....keep it. If you need to upgrade regardless, you might as well go big (3").

 

judgement call, personal preference or whatever you want to call it. if you are planning on making serious HP someday you might as well go with the 3".

 

thanks guys i have the car torn apart right now was switching from shortys to long tubes and already sold my old exhaust to help balance cost for swap ive decided to go with 3" exhaust now with the better flow what kind of muffler do you guys recomend

 

Smart choice.

 

If the idea of a better flowing exhaust doesnt appeal to you, and quicker acceleration is not what you want then stay 2.5. Otherwise My Vote is for the 3in.

MODULAR: Whos picking a fight? These are just poeple stating their opinions plain and simple. Quit being so sensitive. You remind me of my girlfriend.

To the OP. A bigger, free flowing exhaust (3in) will almost always be the best bet. The faster the engine can expell spent exhaust gases, the faster it can draw in a clean air/fuel charge. Plain and simple.

 

I'm going to mirror Millhouse,

If you already have 2 1/2, There's no reason to step up with the current combo..

If your going to spend money, do it on something that will give you better results.
Good heads, matched cam, meth, tune, tires,
A short block that will live behind 450+ rwhp.

Over 450, I don't doubt there's something to be gained.
Question is, Is that the best bang for the buck at that number.

Also, if you run tailpipes, you my find 3in kits don't quite fit fox bodies without hitting something.

 

You should also invest in a straight thru muffler.

 

An open header does not always run faster. If someone does, that tells me that you had a positive pressure wave up into the combustion chamber (contamination) when a midpipe was connected, which is BAD. Unbolting the midpipe gave you a better tuned length (not perfect), out of the 3-4 you can choose from with the exhaust length. It does not have to do with the diameter alone, as many believe.

You CAN lose HP/TQ, IF your combination is not put together with the exhaust size in mind. The blower combination will help band-aid this, even if the combination is not planned with the 3.00" in mind.

This is why you see some gains with larger exhaust and some enthusiast get slower with larger exhaust - luck of the draw.

With some early exhaust valve timing events and longer exhaust duration, the large exhaust can show a loss in HP and TQ. You want cylinder pressure, right?

HP is a mathmatical function of RPM and Torque. You mess with one, you will see a change in HP/TQ together, throughout the RPM range.

Despite what some may want you to believe, it is the ENTIRE combination, not one part that makes or breaks you. The intake packaging has to match your exhaust packaging.

You do not just throw on a 1.75" header and expect gains on a 302/306/331/347. You also do not go from a 1.75" header and expect gains with a 1.625" header on a 302/306/331/347, etc.

Liljoe07 - What is your experience with 2.5" to 3.0" exhaust swap? I have a couple of examples.

 

.

Liljoe07 - What is your experience with 2.5" to 3.0" exhaust swap? I have a couple of examples.

I want to hear the examples............

 

You are correct, the combination does need to be matched..

A custom camshaft designed to take advantage of the larger exhaust, along with timing and fuel adjustments...The larger/ and or open header is faster.

Let me ask you this, in your "examples"...Was each combination with both 2 1/2 to 3", or open header tested at the track. With each combination testing different camshafts, fuel, and timing? I can bet NOT...They had a combo with the 2 1/2" (And that combo was probably FUBAR)...Bolted on the 3" with no other changes made. And they did not respond

With a good Long tube header, and a good collector. Timing, fuel, and valve timing events to take advantage...It's faster, period...It has been tested

 

Only on 3 different cars. And each one showed promise once the swap was tuned and cammed accordingly. Simply put, the harder the piston has to push the spent gases out the more power is wasted. And a free flowing exhaust helps aid in this. While the diameter of the pipe is not the only factor, the design of the system does come into play(ie. bends and turns) but on the larger diameter exhausts the radius of the bends are generally less due to the larger diameter of the pipe. A 2.5 pipe can have a tighter radius that a 3.0inch. The straighter the exhaust(given the correct valve events. timing and fuel are present) the quicker the engine will be able should be able to rev and therefore moving the car quicker. This is Track Tested results. Agree or not. I will agree that it is the combination of parts thats important. And having a 3in exhaust in mind when putting things together helps. But to achieve the most out of any combo is gonna require tuning. Mutiple things must be aproached when changing out certain aspects of a combo. The exhaust is one of them.

And your right in some cases people have shown losses in the HP and TQ. But theres also been cases when people have shown losses in both categories and still went faster at the track.

 

2.5 inch is not a restrictive setup. If your getting into 600-700+, then maybe. But if your mufflers aren't restrictive, your not gaining much from going to a 2.5 to 3 in.

 

His

 

Liljoe07 said, "the harder the piston has to push the spent gases out the more power is wasted. And a free flowing exhaust helps aid in this. "

This is also known as pumping loss.

If the valve events are designed around a smaller or larger set, is the when/if pumping losses will occur. With larger exhaust, the exhaust events can be later, but with a smaller exhaust, the events can be earlier because at the end of the power stroke the pressure inside the cylinder is greater than atmospheric. By opening up the exhaust early, the exhaust gasses start flowing out. With a large exhaust, you can open the exhaust valve closer to BDC than with a smaller exhaust. One does not NEED a large exhaust, nor does one NEED a small exhaust. These are just some of the factors that can be worked out with the camshaft valve timing.

I listed one example above. The other most recent one dealt with an old GM 350. He "uncapped" his headers from the mid-pipe and lost ~2 tenths and nearly 2 MPH. He thought he "lost" backpressure, but he actually gained backpressure.

I have removed my H-pipe quite a few years ago to hear it. Ran it on the street and the low-mid range was hurting a bit. No track data on this one though, just the highway where I lived.

An open header does not guarantee a faster time at the track. I have seen both sides of this.

 

Liljoe07 said, "the harder the piston has to push the spent gases out the more power is wasted. And a free flowing exhaust helps aid in this. "

This is also known as pumping loss.

If the valve events are designed around a smaller or larger set, is the when/if pumping losses will occur. With larger exhaust, the exhaust events can be later, but with a smaller exhaust, the events can be earlier because at the end of the power stroke the pressure inside the cylinder is greater than atmospheric. By opening up the exhaust early, the exhaust gasses start flowing out. With a large exhaust, you can open the exhaust valve closer to BDC than with a smaller exhaust. One does not NEED a large exhaust, nor does one NEED a small exhaust. These are just some of the factors that can be worked out with the camshaft valve timing.

Let me see.... so camshaft valve events, since apparently you have reached that level...YR!..... are now solely dictated by the exhaust ID/length?.... cylinder head exhaust configuration/capabilities do not have a say?..... You mention camshaft design.... what about us simple earthlings, that work based on results rather than forums theories (for years I might add)..

How do those theories explain the facts that a 302 with a B-303, 195 cc inline cylinder heads, box style EFI intake and a 3" full exhaust upgrade grassroots setup, is now performing N/A at the same performance level it was with smaller heads, 2.5" exhaust and long EFI intake runners with a 100 HP NOS shot?

Will all setups run better with open headers w/out looking at the other peripherals (aka: intake, pistons, fuel delivery system, compression, etc, etc)?...... DUH! ..... isn't the full combo what matters? Make a run with open headers and experience (key word) a drop in performance, not HP or TQ......ET's (as I have worked on)..... $10 and 1/2 hour later (jets change, or add more "contamination").... and win a 5-3 grudge match.... and this was 18 years ago!

Making generalizations based on exhaust ID is not correct..... or isn't the whole combo what matters? Is a bigger, free flowing, correct length, full exhaust better?..... obviously!...... specially in a setup with forced induction....... or is it not?

I listed one example above. The other most recent one dealt with an old GM 350. He "uncapped" his headers from the mid-pipe and lost ~2 tenths and nearly 2 MPH. He thought he "lost" backpressure, but he actually gained backpressure.

I have removed my H-pipe quite a few years ago to hear it. Ran it on the street and the low-mid range was hurting a bit. No track data on this one though, just the highway where I lived.

An open header does not guarantee a faster time at the track. I have seen both sides of this.

You are correct.... "An open header does not guarantee a faster time at the track".... IF you don't know what you're doing.

Again.... you cannot change one aspect of the full combination, w/out setting/reconfiguring the rest to support the new capabilities (ie. exhaust) and draw conclusions based on that single change..... it is not an "apples to apples" comparison, but rather biased.

Once you set a specific component in your combo as a constant, you need to set the others "around it".... when and where applicable. Exhaust sizing, even with the prehistoric alphabet cams, like a better sized (aka: 3") exhaust system..... or aren't they (alphabet cams) not exhaust biased?....... they need all the help they can get.

You mention the "everything needs to compliment/be compatible clause"...... but as soon as a change on the the "other side" of the combo is brought to light...... the "non apples to apples BS" is also brought up. So which one is it?...... the combo needs to compliment itself and plan for the best?........ or should you limit yourself to a 2.5" exhaust, even though the OP specified forced induction, and "crutch" the rest of the combo based on that limitation?

 

93 coupe 302 TW heads stock bottom end ect.ect. 2.5 exhaust

first and best run with 2.5ex

7.7x @ 88mph 1.70 60

two weeks later same set up, just 3in staight pipes, timing up 2* and slightly different cam

7.20 & 94mph

not good enough?

How about

306 gt40s explorer intake basic mom and pop setup, with 2.5 ex

first run

8.35 @ 81mph

after just a 3in exhaust nothing else

8.10 @ 85 the 60' was off a tad on this one.

Now with a different cam, some more
timing( the fuel was already there, just needed some tweeking) and this car went high 7s. 7.9 something. I wasnt there on this run, but there is no reason for him to lie.

Difference between my examples and yours is pretty easy to see.

In your examples nothing else was done to compensate for the open headers. Or you didnt list them. But im assuming since you dont know what brake grease is that you didnt change a thing.

Im my examples both owners with my self, attempted to try and optimize for the larger exhaust and were rewarded with the faster times. No driving down the street for us, We took it to the track.

My current ride has 3in ex but i have yet to take it to the track, as i have alot of work to be done before hand.

 

I'm happy you were called out with the misinformation...And it was corrected

 

If I might throw my $.02 out there.
I have spent plenty of time on the dyno tuning and trying different things.
My car is what I would consider a bolt on car with a blower.
I have a 2.5 inch exhaust with 1 3/4 shorties. I have made over 600 to the tires with a 306 with a T trim and small heads
My previous combo was a stock 99 Explorer short block with AFR 165's and a smaller cam and an S trim. At the time I had 1 5/8 shorties with the 2.5 inch exhaust. I made a tick under 560.
From my experience header type is not very critical on a forced inducted car. Heck, we have had blower cars make over 800 through shorties and 2.5" pipes.
The exhaust diameter does add a very small amount of power For a street car I prefer the smaller diameter as it keeps it quieter and fits better around the rear.
Guys go splitting hairs about this stuff. If you like it loud then go 3", but just let it be known there are cars making a lot of power with small stuff.
Its just like the long tube header theories. We have cars making a ton of power on shorties.

 

I will say it again, if you're starting from scratch, the larger exhaust is the best way to go..

A dyno does NOT measure acceleration....A dyno is a tuning tool, nothing more, you can see 4 HP gain on a dyno, but pick up 2 MPH in the 1/4....Do we race dynos? Do we race flow benches?...NO

 

No but we have quite a few 9 second cars with shorties and 2.5" so what is your point?

 

Sure maybe all those cars could make more power. But they arent race cars. They are street cars and they work great the way they are. Why go to the extra expense and make extra noise??
I would say save the 3" for the dedicated race cars and keep the 2.5" for the street cars.

 

Yes, I will agree...That if the primary concern is ground clearance, noise (use mufflers), if you find it easier for YOU to use the smaller exhaust, and shorty headers...That is entirely up to you.

But, with the correct set up..you are leaving ET and MPH on the table. And that's what this thread was about