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.