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joey.youssef

2.5" vs. 3" comparison

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Trying to figure out if I should go 2.5" or 3" for my build.  I have lots of mixed opinions, some saying 2.5" is more than plenty and others say 3" is the way to go.  Has anyone with a higher compression (10:1 or greater), N/A & hot cam set-up have done a dyno comparison of a 2.5" set-up and a 3" set-up and seen any difference?  

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2.5" is more than enough for any NA L28 engine on the street. Even on a stroker. You will see minimal gains in HP between a 2.5" exhaust and 3.0" on a NA engine. Much more important on a Turbo engine. What you will see ( or hear ) is a LOT more exhaust drone  and noise from a 3" system.

 

FWIW. I ran a 425HP Camaro Hillclimb car on a single 3" system. Under chassis design on Third Gens  limited exhaust choices. The minor amount we lost on top end HP was more than off set by the Torque we retained down low. ( Important for Hillclimbs ) . Also as the car was driven on the street, a 3.5" or 4" single system ( 355 ci or 5.7 liter ) would have been excessively noisy. I held the SP3 Hill record at Knox Mountain Hillclimb from 2000 to 2005. So engine made plenty of HP.  Now you need over 550 HP in a SBC to be competitive... in a Street Class!!   

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Not Z specific, but clos-ish. I've run many Porsche 944's on my dyno, for a loose spec race series. They put out around 140HP at the wheels for "stock" wink-wink engines. There were many different exhausts among them. And the ones that were the noisiest and most piercing were the smaller diameter ones. I was surprised by that. Exhaust pipe diameters ranged from 2.25" to 3."

 

Back pressure is bad. If the engine is viewed as an air pump, you'll see that a larger exhaust is always better for power at any RPM. It simply doesn't have to work as hard to exhale.

 

Now, practically, a 3" exhaust is more expensive for pipe and muffler, heavier, harder to fit, and will sit at least half an inch lower than an ideally tucked 2.5 inch exhaust.

 

Motor Trend Engine Masters Episode 9 features a 2.5 vs 3 inch exhaust comparo on a 454. 300ish crank hp per bank. Worth a watch.

Edited by bradyzq
clarity

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You have to take Motor Trend Engine Masters with a big grain of salt. They are Drag racing oriented and only tend to concentrate on overall HP figures. For Road Racing , Auto cross or even general street driving, some of their videos are full of omitted data. Lots of holes in their testing parameters in some cases. 

 

Their test of Mechanical vs Electric Cooling fans was a complete joke as they never tested the most important aspect of cooling fans. CFM.  Also, the mechanical fan they tested was an old design. With  heavy metal blades and low efficiency blade design. It was a full lockup design, not a governed RPM design likie Nissan uses and not high efficiency,  curved 7 and 8 blade designs with lightweight Plastic Blades .  Again Drag Racer oriented and Detroit specific. 

Edited by Chickenman

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If you're going to actually use the engine at peak power regularly and chasing every HP, go 3". If you are just building a fun street car, go which which ever sounds better. 2.5 will sound brighter/higher pitched over tones while 3.0 will be deeper but as noted more prone to drone. If you get drone with any setup a well placed resonator in the middle of the longest straight piece will help wonders.

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Yes I have dynoed exhaust before and after. I don't feel it is apples to apples, though.

 

I have 13:1 compression, .565" lift cam with 324/316 duration (Sunbelts Race Cam #2), and 45mm DCOEs.

 

I have compared 2.5" to 3" with the same cam and block, but the DCOEs were 40mm at the time.

 

My 2.5" exhaust were MSA 6-2-1 headers and non-mandrel bends with lots of wrinkles.

 

My 3" exhaust is from Stahl with 1 5/8" primaries (not 1 3/4"), with header collectors (3 primaries) measuring 2.5" each. I have a 20 degree merge collector that cones these two down to a single 2.5" and then flares out to 3". I have custom 3" mandrel stainless steel all the way back. This setup, including headers, merge collector, header coatings, mufflers, vband clamps and Tig welding labor cost about $5k... it's a huge investment relative to the street setup.

 

What did this absurd investment buy me? A shit-eating grin and compliments of best sounding car at all autocross races, for starters (even 300 car fields!). "What kind of V8 ya got, bud?" I love that question, despite that I can't figure out why people think it sounds like a V8. Significantly more torque down low for sure, horsepower increase marginal. The tone now has a higher pitch when the cam has kicked in, and the idle sounds more like a popcorn machine (lopey cam) rather than a less choppy hum (smoother). Even with a radical cam, I launch my 245mm Hoosiers at about 2,000rpm and minimal pedal. I'm able to spin these tires very easily with torque. Credit to the stroker and many other details, but this was not possible with my street exhaust -- previous launches were at 3k+ rpm and significant pedal.

 

I can't praise the impact of a merge collector enough and the mandrel bends downstream... the engine is much cleaner at changing RPMs and throttle positions. Torque is now a major asset during a race. In terms of drivability, having tuned headers (Stahl) and a merge collector really helps this car change throttle positions smoothly and therefore deliver changes in torque more smoothly. (Intake velocity stacks, venturis, etc. must also be tuned to get it to *really* sing.)

Edited by zredbaron

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Hey Mark, it may not sound much like a V8 to you; but I can really hear it...I remember the first time seeing your car at ACU-4, and thinking to myself "cool, another V8 Z."  I was wrong, of course, but that memory has always stuck with me.  Cheers!

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