I've got 2 exhaust questions...

[jefpar1]

I've got a 430hp ls3 with a rear torque arm set up in a mini tubbed 69 Camaro. I want to run 3" exhaust, but I know that building 2.5" exhaust would be easier. Will I loose any hp using 2.5" pipe instead of 3" pipe? The car will be mainly for the street, very little if any track time. Also, can anyone recommend a magnaflow or borla muffler part number? I see there are different options and I'm not sure which one to go with. Any info would be greatly appreciated.

[65 drop top]

I've always thought that 2.5" dual exhaust was good for up to 500 hp or so. Over sized pipe will hurt low end torque.

[Ron Sutton]

If you tell me what the max rpm is, that the engine sees during your driving ... I'll run a quick exhaust calc for you & tell you the optimum size.

[jefpar1]

I'm not sure what max rpm will be. Normal street/highway driving with a 4L60E transmission, 3.73 rear gear and 335x30x18 rear tire. Sorry to be vague. I hope this helps. Thank you for any info you can provide.

Jeff

[Ron Sutton]

I am pretty confident that 2-1/2" exhaust will be fine for your application & 430 hp. But to run the calculatoions ... I need an rpm that you feel will be your maximum usable rpm. I can guess, but it's your car.

[sam 74]

i know a few guys that run 3 inch to the mufflers and then 2 1/2 out back, i'm useing 3 inch all the way out on my a body with a 600 hp big block, i also use the Borla XR1 mufflers that are straight through and still pretty quiet.

[Rod]

2 1/2 is fine

[Ron Sutton]

Jeff,

Just so we're not all over the place. The optimum diameter of tubing depends on power output & rpm. For an LS engine @ 430 hp, 2-1/2" should be your target. When guys build more power & run higher RPMs, more flow is needed, just like it is in the heads.

When I'm designing an exhaust system, the diameter is the first thing to figure out of course, because you need to buy tubing & bends. We build our exhausts "tuned" to the engine, which requires a little understanding of sound wave harmonics & all the details on the engine, heads, induction system & cam specs. We can then place everything ... H-pipe, X-over, mufflers, collectors, merge collector, interior cones, etc ... where it matches up with the natural exhaust pulses. This squeezes every last bit of power out of an expensive engine.

It sounds like that is more detail than you desire. Again, for your LS making 430hp, 2-1/2" will be good.

Best wishes.

[jefpar1]

I want to thank each of you that responded to my questions. Looks like I will go with the 2.5" pipe. I've also decided to go with the Borla XR-1 muffler. I see that one option is 4" longer than the other. Any recommendations on the longer or shorter one? Thanks again.

Jeff

[sam 74]

when i orderd mine i went with the 17 inch sportsman muffler, its pretty quiet especailly when compared to the old Flowmaster 3 chambered mufflers that i used to have. i think a larger muffler would be a little to quiet for my taste.

[sccacuda]

I've always thought that 2.5" dual exhaust was good for up to 500 hp or so. Over sized pipe will hurt low end torque.

I don't think an oversize pipe has any effect on torque and HP. The primary tube, collector length/size does. Undersized exhaust can cause a restriction affecting both.

[sccacuda]

I don't think an oversize pipe has any effect on torque and HP. The primary tube, collector length/size does. Undersized exhaust can cause a restriction affecting both.

So I guess I'm asking a question as well. Ron, I thought the optimal power output would be dictated by the collector length and size for properly sized primaries and engine combo. I was under the impression that past this point of the tuned collector you wanted a larger pipe as not to affect the collectors tuned length. Your saying that your tuning all the way back. I wasn't aware velocity was sufficient to have any effect unless you were undersized. I would think that an undersized exhaust would be a tuning crutch? I'm asking because I'd like to understand.

[Ron Sutton]

Hi Craig,


So I guess I'm asking a question as well. Ron, I thought the optimal power output would be dictated by the collector length and size for properly sized primaries and engine combo. I was under the impression that past this point of the tuned collector you wanted a larger pipe as not to affect the collectors tuned length. Your saying that your tuning all the way back. I wasn't aware velocity was sufficient to have any effect unless you were undersized. I would think that an undersized exhaust would be a tuning crutch? I'm asking because I'd like to understand.


The optimal power output & torque curve is only affected by the primaries (diameter & length) & the collector (diameter & length) for a specific engine combination ... if it is an open header race engine. If it has an exhaust system past the collector, that assembly, its components & their exact placement will affect the power output & torque curve also.

There are really 3 things to keep in mind:

1. If the tubing size is too small, it will act as a restriction & reduce flow at higher engine rpms ... affecting (reducing) upper rpm power the most. How much depends on how small is the tubing compared to what is optimum. If optimum is 3" ... 2.5" tubing will cause a small loss in upper rpm power ... while 1.5" tubing would cause a substantial power loss at upper rpms, even limiting the rpm potential of the engine.

2. If the tubing size is too big, it will act as an expansion chamber, slowing the exhaust air speed (but not the volume). This MAY reduce top end power A LITTLE or A LOT depending on what it does to the harmonics. It WILL reduce low rpm engine response & low end torque. How much loss depends on how large is the tubing compared to what is optimum. If optimum is 2.5" .... 3" tubing will cause a small loss in low rpm engine response & low end torque ... while 4" tubing would be more substantial.

3. Exhaust flow harmonics is a big deal. Since we're all PT here, I'll keep it to V-8's. You have 8 cylinders pressurizing the exhaust system at different times. This creates exhaust pulses. The camshaft duration, centerline & overlap ... along with over key engine criteria such as how long the pistons "rock" at TDC, rpm, etc ... define the pulse pattern. I cant' say this strongly enough ... for optimum power you need to get the exhaust pulses working in harmony.

As long as the pulses are NOT running into each other ... the exhaust flows out smoothly & at a high rate of cfm. If the pulses run into each other ... the exhaust gases slow & "stall up." It's like comparing a smooth flowing group of traffic on the freeway ... to ugly, stop-n-go traffic. When exhaust pulses run into each other, the exhaust flow speed slows & exhaust cfm is reduced ... wasting power.

Engines are basically air pumps. To build power, you need to get the air (and fuel) into & out of the engine ... smoothly ... and in high volumes.

Several things in the exhaust affect the exhaust gas pulses, including:
a. Primary tube diameter ... or diameters if it is a "step-header" ... and of course length.
b. Collector style ... be it the traditional 4-1 ... or more developed 4-2-1.
c. Collector diameter and length.
d. If merge collectors are used ... and if so ... exactly where & how small.
e. If Vortex cones are used ... and if so ... exactly where & how restrictive.
f. If a H-pipe is used ... and where it is placed, exactly.
g. If a X-over is used ... and where it is placed, exactly.
h. Where the mufflers are placed, exactly.
i. How long the exhaust pipe is after the mufflers.

Oops! I left off a key item. The diameter of the tubing used to connect items "c" through "i". Which, surprising to most, is going to be the same diameter as the collector, assuming the collector has been made the optimum diameter .

Getting the placement of these exhaust components correct ... to create a harmonious freeway for the exhaust gas pulses & flow ... is key to getting optimum power out of a performance or race engine. I've seen street cars with 450hp pick up 40 hp when the exhaust system was corrected. Meaning they were losing 40 hp before.

Going back to #2 ... if the exhaust tubing is too big ... it changes the speed of the exhaust gases ... affecting the pulse timing. If it is off a little, and your engine made crazy big HP, you may not notice. If it is off more, the exhaust gases run into each other, slow & "stall up" ... and exhaust flow cfm is reduced ... hurting power output.

If you're looking to get the optimum power through out the rpm range, getting the exhaust system designed correctly is just as important as selecting the right carb, intake, heads or cam. it's all part of the big "package" picture.

I feel optimizing the exhaust is very important & often overlooked. We invest the time, effort, expense ... and design every piece of our exhaust system ... with the same care as building the engine ... to optimize total power output & torque curve.

[69cutlassrkt]

Hey Jeff, I don't know if you have a crate motor or what, but to get the most out of your exhaust it would need to be tuned. By tuned I mean like dynotuned.

[sccacuda]

As always Ron, the info is greatly appreciated.

I've played with some resonator models for air delivery but never considered it after the collector.

Keep the tech info coming!

[77tony]

Hi Ron, Running a new LS376/525 injected crate motor in a 1980 Porsche 928 for the street. Plan is to run stock manifolds, no cat single all the way through (if it fits). What size pipe would you recommend ? If I were to run duals with stock manifolds, an x-pipe up front and rear muffler, what size pipes would you recommend ? TIA. Pics of a Porsche 928 (with Porsche stock motor) below for reference of underside. TIA. 77tony "Just Peachy" build

[Rod]

Whew! that's a lot of writing but I often notice that many run headers with header tubes that are to large in diameter also, I don't have any wiz bang data to back that up, I just know from what we have done on the drag cars and now for the last few years on the autocross cars that manyof the cars we built gained HP and torque with smaller shorter headers and smaller diameter tubes than many thought, I agree with Ron and its well explained by him, thanks Ron


Hi Craig,




The optimal power output & torque curve is only affected by the primaries (diameter & length) & the collector (diameter & length) for a specific engine combination ... if it is an open header race engine. If it has an exhaust system past the collector, that assembly, its components & their exact placement will affect the power output & torque curve also.

There are really 3 things to keep in mind:

1. If the tubing size is too small, it will act as a restriction & reduce flow at higher engine rpms ... affecting (reducing) upper rpm power the most. How much depends on how small is the tubing compared to what is optimum. If optimum is 3" ... 2.5" tubing will cause a small loss in upper rpm power ... while 1.5" tubing would cause a substantial power loss at upper rpms, even limiting the rpm potential of the engine.

2. If the tubing size is too big, it will act as an expansion chamber, slowing the exhaust air speed (but not the volume). This MAY reduce top end power A LITTLE or A LOT depending on what it does to the harmonics. It WILL reduce low rpm engine response & low end torque. How much loss depends on how large is the tubing compared to what is optimum. If optimum is 2.5" .... 3" tubing will cause a small loss in low rpm engine response & low end torque ... while 4" tubing would be more substantial.

3. Exhaust flow harmonics is a big deal. Since we're all PT here, I'll keep it to V-8's. You have 8 cylinders pressurizing the exhaust system at different times. This creates exhaust pulses. The camshaft duration, centerline & overlap ... along with over key engine criteria such as how long the pistons "rock" at TDC, rpm, etc ... define the pulse pattern. I cant' say this strongly enough ... for optimum power you need to get the exhaust pulses working in harmony.

As long as the pulses are NOT running into each other ... the exhaust flows out smoothly & at a high rate of cfm. If the pulses run into each other ... the exhaust gases slow & "stall up." It's like comparing a smooth flowing group of traffic on the freeway ... to ugly, stop-n-go traffic. When exhaust pulses run into each other, the exhaust flow speed slows & exhaust cfm is reduced ... wasting power.

Engines are basically air pumps. To build power, you need to get the air (and fuel) into & out of the engine ... smoothly ... and in high volumes.

Several things in the exhaust affect the exhaust gas pulses, including:
a. Primary tube diameter ... or diameters if it is a "step-header" ... and of course length.
b. Collector style ... be it the traditional 4-1 ... or more developed 4-2-1.
c. Collector diameter and length.
d. If merge collectors are used ... and if so ... exactly where & how small.
e. If Vortex cones are used ... and if so ... exactly where & how restrictive.
f. If a H-pipe is used ... and where it is placed, exactly.
g. If a X-over is used ... and where it is placed, exactly.
h. Where the mufflers are placed, exactly.
i. How long the exhaust pipe is after the mufflers.

Oops! I left off a key item. The diameter of the tubing used to connect items "c" through "i". Which, surprising to most, is going to be the same diameter as the collector, assuming the collector has been made the optimum diameter .

Getting the placement of these exhaust components correct ... to create a harmonious freeway for the exhaust gas pulses & flow ... is key to getting optimum power out of a performance or race engine. I've seen street cars with 450hp pick up 40 hp when the exhaust system was corrected. Meaning they were losing 40 hp before.

Going back to #2 ... if the exhaust tubing is too big ... it changes the speed of the exhaust gases ... affecting the pulse timing. If it is off a little, and your engine made crazy big HP, you may not notice. If it is off more, the exhaust gases run into each other, slow & "stall up" ... and exhaust flow cfm is reduced ... hurting power output.

If you're looking to get the optimum power through out the rpm range, getting the exhaust system designed correctly is just as important as selecting the right carb, intake, heads or cam. it's all part of the big "package" picture.

I feel optimizing the exhaust is very important & often overlooked. We invest the time, effort, expense ... and design every piece of our exhaust system ... with the same care as building the engine ... to optimize total power output & torque curve.

[Ron Sutton]

Hi Ron, Running a new LS376/525 injected crate motor in a 1980 Porsche 928 for the street. Plan is to run stock manifolds, no cat single all the way through (if it fits). What size pipe would you recommend ? If I were to run duals with stock manifolds, an x-pipe up front and rear muffler, what size pipes would you recommend ? TIA. Pics of a Porsche 928 (with Porsche stock motor) below for reference of underside. TIA. 77tony "Just Peachy" build


Hmmm ... just over 500 hp is on the cusp of a size step. If you can provide me with an accurate max rpm redline you'll run ... and at what rpm the engine builds peak torque & what rpm it builds peak HP ... I'll run the calc's both as a single & dual & post the results.

Otherwise it's a SWAG.

[Ron Sutton]

Whew! that's a lot of writing but I often notice that many run headers with header tubes that are to large in diameter also, I don't have any wiz bang data to back that up, I just know from what we have done on the drag cars and now for the last few years on the autocross cars that manyof the cars we built gained HP and torque with smaller shorter headers and smaller diameter tubes than many thought, I agree with Ron and its well explained by him, thanks Ron


Yup. yup. See it all the time.

[Todd in Vancouver]

Wow that is a ton of info Ron and your knowledge is always much appreciated by those of us without a ton of experience. For the garage home build guy like myself would you recommend going with the pre bent systems available from some of the aftermarket performance companies like Magnaflow?

For example on my own car I was looking at the following;

SBC dyno at 476hp at 6800rpm (I may squeeze a little more out of it around 7K'ish)
TCI built headers 1 3/4" primary & 3" collector
I was looking at a 3" exhaust with an X pipe
I want to run full exhaust out the back and probably a 40 series type muffler

I would think a first gen Camaro like mine is typical of most builds that get done at home and most guys usually end up purchasing a kit from one of the performance suppliers. Sadly there are so few shops left anymore that can build performance systems that most of us have to install what we can order online.


Also, what are your thoughts on oval 3" pipe? I saw a builds with it and it looks awesome and gives a little more ground clearance.

[Ron Sutton]

Hey Todd,

Hang tight until I get Tony's exhaust worked out & I'll have some answers for you.

[Todd in Vancouver]

Hey Todd,

Hang tight until I get Tony's exhaust worked out & I'll have some answers for you.

My car is hanging on a rotisserie so I will need an answer by the summer of 2015, or so. :ssst:

I would think that mine is typical of what the majority of guys build at home.

[Ron Sutton]

My car is hanging on a rotisserie so I will need an answer by the summer of 2015, or so. :ssst:

I would think that mine is typical of what the majority of guys build at home.


Whew! I better hurry then ! :lol:


Just as a precursory primer ...
Of course the exhaust size matters ... so we'll work that out for your application. But it is pretty simple, based on horsepower level & rpm. For an engine to make Xxx HP ... it has to flow Zzz cfm of air into the engine & Yyy cfm of air (exhaust) out of the engine.

Muffler placement matters ... and affects power ... but not to the point of killing the power if they're not in the "happy spot."

H-pipes and/or X-overs ... can really help a high power performance or race engine to achieve optimum performance ... if they are placed correctly. On the other hand, they can hurt the engine's power output significantly is placed wrong. Adding H-pipes and/or X-overs to an exhaust system is a potential power gain ... with a potential risk of power loss.

It's all about the exhaust pulses. If the H-pipes and/or X-overs are placed correctly for your engine's unique pulse pattern ... the exhaust pulses on one side are "relieved" at the point of the H or X ... and that pressure slips into a "traffic opening" on the other side. On the other hand, If the H-pipes and/or X-overs are placed incorrectly for your engine's unique pulse pattern ... the exhaust pulses on one side "start to be relieved" at the point of the H or X ... and that pressure runs into "exhaust pulse traffic" on the other side ... causing both sides stall up. Just like on the freeway. This reduces exhaust flow ... sometimes drastically ... and hurts engine power output on a level equal to the flow problem.

Muffler placement does become more important when H-pipes and/or X-overs are utilized, because the muffler acts as a "Yield" sign ... slowing up exhaust flow traffic to a degree. Tubing size becomes more important too ... as size of the exhaust tubing plays a role in the velocity of the exhaust gas pulses.

-----------------------------------------------------------------------------------------------------------------

I am a big fan of H-pipes and/or X-overs ... as long as they are placed correctly for the specific engine.

The formula I utilize to calculate this requires:
Peak HP
RPM at peak HP
Bore
Stroke
Rod length
# of cylinders
If odd fire, specific firing degrees of the cylinders
Compression ratio
# of intake valves
Intake valve diameter
Intake valve stem diameter
Max intake valve lift
Intake cam lobe duration @ .050"
# of exhaust valves
Exhaust valve diameter
Exhaust valve stem diameter
Max exhaust valve lift
Exhaust cam lobe duration @ .050"
Actual degreed cam centerline
Lobe center angle
Exhaust Port centerline length

Then, I need decisions on plans for ...
Header primary tube: standard, 2-step or 3-step
Header design: standard or tri-y
Collector: Standard or merged (reduced)
If for racing, ran as "open headers" with no mufflers or anything past the collectors
If for racing, ran with mufflers right out of collectors
If Vortex cones are used for sound suppression or not
If an H-pipe is to be utilized
If an X-over is to be utilized

With this info, we can spec every detail of the headers & exhaust system for optimum performance. To me, this is just as critical as all the time & detail that went into he engine build. Building a killer engine, then just throwing on an exhaust "that fits" would be like throwing on any old carb that's laying around.

-----------------------------------------------------------------------------------------------------------------

This is too involved for most PT builds.
I think it only makes sense when we're talking extreme PT machines, Track Cars & Race Cars. I provide this service for race teams I consult to.

Here is an example of optimum & worst placement for the PT/track car I'm building for myself. All dimensions are from the end of the header primary tubes to the center of the H or X.
H-Pipe: Best power = 15.7" / Worst power = 23.5" **
X-Over: Best power = 62.7" / Worst power = 47.0" or 94.0" **

** This "worst" dimension creates the worst exhaust pulse traffic jam, therefore kills power the most. You'll notice the H-pipe dimensions aren't that far apart, because the closer to the primaries you work, the more sensitive the exhaust pulses are. I've seen shops place the H-pipe where "it fit well" ... and reduce the engine power output ... a lot.

---------------------------------------------------------------------------------------------------------------------------

I hope you found this interesting, but again, I think it is too involved for most PT builds.

[Todd in Vancouver]

That's amazing Ron and I agree its way past what the majority of guys would do. I do find it amazing to try and understand though and do you have any recommendations on where a garage hack like myself can do a bit of reading and research to try and understand this a little better?

Also, here is the website for our local supplier and they have a pretty good section on calculators.

http://www.mopacautosupply.com/calculators.html

[Ron Sutton]

That's amazing Ron and I agree its way past what the majority of guys would do. I do find it amazing to try and understand though and do you have any recommendations on where a garage hack like myself can do a bit of reading and research to try and understand this a little better?

Also, here is the website for our local supplier and they have a pretty good section on calculators.

http://www.mopacautosupply.com/calculators.html


Unfortunately, I do not know of any books on the subject.

[IMPALA MAN]

In most cases (not always) you can typically pick up some high end HP by going larger on the tube size. This of course is at the expense of giving up some low end TQ in the lower rpm ranges. It's (almost) always a trade off. The opposite is usually true for the opposite...smaller tube, more low end TQ, at the expense of less high end HP. So why do so many people go too big on the tube?
I will share two scenarios and let you answer.
Scenario 1: A guy walks into his engine builder's shop after he is told he can pick up the engine because it is complete. What is the first question EVERY customer asks as soon as the builder tells him the dyno process is complete? HOW MUCH HP DID IT MAKE. Does anyone ever ask how much TQ did it make? Seldom, but at best it's the second question. So back to the scenario. So the engine builder knew he would be asked this question. So he spits out a true number of 515 HP. He then tells you some additional specs like header tube size (which seems a little large to you). You go away happy.

Scenario 2: Same scenario except this time the engine builder put a header on it that would best fit the applications demands. Some low end for stoplight to stoplight driving and not so focused on the high end, high rpm number that you may use at say 7000 rpm. So when you ask the HP number he spits out and accurate 505 HP. Now you are disappointed because your buddy had he same engine built down the road and it got 515 HP!!! The next engine build you take to the guy down the road.
Now who's fault is it that everyone goes too big on the tube size?

Engine builders get put in a tough spot by customer demands of HP. If they design the engine to give you the best OVERALL PERFORMANCE, you go down the road. My best advice, listen to your engine builder, tell him what you are going to do with the car and give him some freedom to build you the best engine for the given demands. AND DON'T get so wrapped up in the HP number.

[Ron Sutton]

Too true. That's why I often use the term "power" ... instead of horsepower ... because in our racing engines we're looking for the most torque under the curve of usable rpm. Who cares what one peak number is.

On the street ... I feel more emphasis needs to be on power in what I call the "most likely rpm curve" ... which for most is somewhere around 1800rpm to 3500 rpm. They young guy I was helping work out engine combinations on here recently made a smart choice. I showed him two engine combinations ... with two different powerbands ... & a predicted dyno curves. (I attached it).

It's hard to see an engine with 30 more horsepower & 10# of extra torque & say "no." But he wisely chose to go with the package that made more power below 4500 rpm ... where he will enjoy the power more. Unless you're seriously tracking your car often, that is where most PT & Muscle cars spend their daily lives.



80542

[SSLance]

It's always fun to listen in when Ron's talking... :worship: :hail: :secret:

I ran what I always supposed was an ill-advised H-pipe position on my exhaust for a lot of years...because that's what was on my car when I bought it. Ran 1.5" long tube headers for the same reason, always wondering in the back of my mind how much power this was robbing.

Had to change headers after my T56 install, ended up with a set of 1 5/8s" Hooker shortys and changed to an X-pipe after my cross member at the same time, mainly due to fit reasons. Wasn't sure how that was all going to affect my power range but didn't have much choice at the time. Thankfully it added power in the right places (according to my butt dyno) and sounded better as well. The car picked up about 50 hp at the rear tires the next time I had it on the chassis dyno, but I'm certain the T56 addition plus some tuning changes helped in that regard along with the exhaust changes.

Had I known Ron at the time I would have spent the time to get him the info he needs like noted above and found out for sure what was the best exhaust scenario for my set up.

And BTW, coming from someone that has a 340 hp "truck engine" that puts 400 ft lbs of torque to the rear tires at 2000 RPM, I can vouch for "power" over horsepower in the fun factor. :drive:

[77tony]

Thanks for getting back to me Ron. Dyno chart for the LS376/525 injected motor. The plan is to lay out the exhaust configuration tomorrow similar to the bottom pic but without headers. Running with the stock manifolds to a 'Y' pipe and onto a 3.5" ? single PIPE back to a magnaflow muffler where the "Y" pipe is now located, or all the way back as in the pic below. Would like for the system to sound very much like the video link below (scroll to .25 seconds-on) TIA. 77tony

http://www.youtube.com/watch?v=PVtXzsCBMH8

[Kennymax]

I bought my 67 camaro with a 327 that has a very large cam. My "very young" cousin put a 3" pipe with racing flowmaster mufflers cut at the muffler down ward under the seats. My result was a very loud drone in the car obnoxious to myself and others. I know that loud is the way to go but this was annoying. So went to the muffler shop where the guy is a master at classic cars he went down to a 2.5 inch pipe and some mufflers." Forgot the kind" now it sounds like a POS. I can't stand it. So my question is? Do I keep the 2.5 inch and change my mufflers? If so what's the best to be heard before your seen in a cool classic sound "cam American muscle popping crisp and deep"? Or should I have just kept the 3" and changed the mufflers and ran them out the back? Thanks I'm new to all this

[pro68chevelle]

Hi Ron,

This is Scott from the aero tread, I had a question about the size exhaust I should use 2.5 or 3.0. I am using a GM LS376/480, it makes 480HP @ 5,750 RPM and 475 LB. @ 4,500 RPM, max recommended RPM is 6,600 with stock GM exhaust manifolds. I will be using a set of Kooks LS swap long tube headers with 1 7/8 primaries and 3" collectors and a Magna flow exhaust system with an x pipe. I may add power at some point in the future but should never be over 600HP.

Thanks Scott

[MonzaRacer]

Here are some tip I learned over many years, 6k or lower rpms, street driving, limited track time, keep primaries on smaller side like 1 1/2-1 5/8, run 4 into 2 into one (Flowmaster has great set of collectors) take them, extend them ,if needed.
Upon final tuning of the engine paint cheap, flat white paint stripe down side. then make hard full throttle pass and where paint quits burning cut there and install reducer to exhaust pipe size.
Most mid level and lower performance cars, etc 2 1/2 in pipe is fine, duals should use x pipe or h pipe. Larger mufflers usually deeper and quieter.
If engine is gonna go over 6k add 1/8in for every 1k rpm over in general. Same collector tuning as before.
3 in exhaust flows more free, but you will get more resonance at idle also.

Up to around 350-375 cid 2 1/2 is fine UNLESS you exceed the rpm rule, rpm needs free breathing. lower rpm needs more exhaust velocity, hence smaller pipes.
Only reason for smaller tail pipes after mufflers on stockers is noise reduction.
These are just some of the observations of a mechanic/hot rodder/drag race engine builder for budget minded folks.
Kind of develop and perfect theories over period of time building a guys car up a little more every year from stock to totally off the chain full blown race engine. Took a guy from stock rehoned mild cam 350 all the way to a full on 350 billet crank, h beam,forged piston,aluminum head roller engine. He went from high 9s in 8th on mild cam to high 6s all in same car over 7 yr period of time, then sold that engine for much more than he had in it, and built a full on race engine. THEN car was not up to it so he get s race chassis and retires car,,now son has soon to be cool car for school!
He still has old 2 1/2 duals and flow *******s in attic, the next incarnation with 3 in pipe, dynomax race mufflers, and full on $1000 set of stainless headers from full blown engine.
Lucky kid, dads car, and all left over parts.

[socoken]

I have a question about exhaust cutouts. I realize that open headers on a street car is likely to cost power, but I like the cool factor. My question is, does having that split just after the header affect flow when they are closed? It just seems like a split with a dead end would act like a resonance chamber and interrupt flow as it passes.

[Ron Sutton]

Hey Scott,


Hi Ron,

This is Scott from the aero tread, I had a question about the size exhaust I should use 2.5 or 3.0. I am using a GM LS376/480, it makes 480HP @ 5,750 RPM and 475 LB. @ 4,500 RPM, max recommended RPM is 6,600 with stock GM exhaust manifolds. I will be using a set of Kooks LS swap long tube headers with 1 7/8 primaries and 3" collectors and a Magna flow exhaust system with an x pipe. I may add power at some point in the future but should never be over 600HP.

Thanks Scott

You are on the right track, but your two situations ... 480 hp now & 600 hp later ... will have different optimum set-ups. For what you're doing now, 2.5" would be the optimum exhaust size. This size would help build a good torque curve with no sacrifice in power in the upper rpm range.

But the headers with 1 7/8 primaries and 3" collectors are larger than optimum for the 480hp. That size of header ... although you didn't state the primary tube length other than "long" ... is more in line with your future 600 hp needs & will provide a less than optimum torque curve with your current engine combo.

When & if you build up this engine to make 600 hp, the headers will be spot on & the 2.5" exhaust would still work well, providing a great low-to-mid-range torque curve ... but giving up some power above 5500 rpm. If that upper rpm matters to you, you would want to increase the exhaust size to 3".

[Ron Sutton]

I have a question about exhaust cutouts. I realize that open headers on a street car is likely to cost power, but I like the cool factor. My question is, does having that split just after the header affect flow when they are closed? It just seems like a split with a dead end would act like a resonance chamber and interrupt flow as it passes.

First, open headers will produce more power, if designed correctly.

Second, your question of "will the turn out act like a resonance chamber and interrupt exhaust flow pulses as they pass?"
I "think so" ... but I do not know "for sure" ... as I have never done this. Most of my advice is hands-on, been there-done that and know the results kind of advice. I've never done what you're asking, so I don't really know.

Maybe someone else on here has tried this & will share their experience. Otherwise, if you try it, you can let us all know.