tech, help exhaust size

[warrenp]

ok so lets say theoretically.

 

from my header exits at 2"

but my exhaust size is 3"

 

wouldn't the header be restrictive? so wouldn't a 2" exhaust be just as good as a 3"?

[johnc]

ok so lets say theoretically.

 

from my header exits at 2"

but my exhaust size is 3"

 

wouldn't the header be restrictive? so wouldn't a 2" exhaust be just as good as a 3"?

 

Restriction (or more properly - pressure) is a result of many things - wall friction, bend radius, ID transitions, muffler type, overall exhaust construction. Gas moving from a 2" ID to a 3" ID experiences a pressure drop which may help exhaust flow depending on the transition from 2" to 3", the placement of the transition, and engine tune.

 

So to answer your question

so wouldn't a 2" exhaust be just as good as a 3"?

, it depends.

 

1. A poorly designed and built 3" exhaust would be worse then a properly designed and built 2" exhaust.

2. A properly designed and built 3" exhuast would be better then aproperly designed and built 2" exhaust.

[grumpyvette]

a correctly designed 3" dual exhaust (IDEALLY WITH AN (X) cross over , just past the collectors to further reduce back pressure will be far supperior in most cases, as its got aproximately a 14.1 sq inch cross section vs 6.3 sq inches for a dual 2" exhaust the 2.0" dual exhaust is 2.24 times as restrictive.

a recent magazine test on a SBC showed only a 7 hp differance between 2.5" and 3" dual exhaust at PEAK ENGINE HP but over the whole rpm range the 3" held a slight edge in torque and at one point (surprizingly low in the rpm range) it held an 11 hp advantage

once the exhaust exits the correctly designed headers collector the less restriction past that point ,the better.

 

I don,t see much debate room, a dual 2.5" exhaust has aproximately a 9.8 sq inch cross sectional area for flow,a dual 3" exhaust has aproximately a 14.16 sq inches cross sectional area for flow,or more than 40% less restriction.

anytime youve got more than about 1-1.5 psi of back pressure your potentially restricting power potential, the OBJECT is to MAXIMIZE the cylinder scavaging to both have a more efficient burn and a more effective removal of the old burnt gasses and a faster more effective cylinder fill with the new charge from the ports, the larger the exhaust port and header the more flow, but the lower thye velocity, a ballance must be struck to maximize velocity , yet not restrict the flow. your trying to get and need as high an exhaust flow velocity as possible with as little backpressure as possible.

a correctly designed header primairy and collector will maximize cylinder scavaging in the rpm band your trying to make the best power in, and its the headers primairy dia. and length and the collector design that maximizes the scavaging. but keep in mind the INTAKE runner cross section and length and the cam timing MUST also match to effectively use that scavaging.

when you see a case where a cars faster with the exhaust past the header collector still connected, its a strong indication that the exhaust pipe is acting like an extended collector, its not the back pressure but the extended length thats helping scavage ther cylinders and aiding power

 

http://www.ntxtools.com/fuelsystem/tu24a.htm

 

http://victorylibrary.com/mopar/header-tech-c.htm

 

http://www.engr.colostate.edu/~allan...ngth/pipe.html

 

http://www.rbracing-rsr.com/runnertorquecalc.html

the question, is will it effect the performance! its been my experiance that exhaust back pressure exceeding about 1 psi starts to reduce performance, most guys who don,t see gains from uncapping the header collectors and reducing the restriction,at the track have headers and collectors that are not scavaging correctly and the extended exhaust system behind the headers is acting like an extended collector on the headers.

it quickly becomes a trade off, do you need less restriction and back pressure or better scavaging, with correctly designed headers and collectors theres little if any increase in restriction once the exhaust is hooked up.

restriction too exhaust flow is the potential problem , and if the headers are not scavaging its not the increased dia. and low restriction of the larger dia, exhaust but the headers and collectors that are the problem, adding a restrictive exhaust can only hurt.

 

a quick check can be done by screwing this in place of the O2 sensor for a check

http://www.usatoolwarehouse.com/usat.../otc-7215.html

[Tony D]

I just want to clarify one thing:

"anytime youve got more than about 1-1.5 psi of back pressure your potentially restricting power potential"

 

Are you meaning PSI or Inches of Water (assuming Normal Aspiration).

I know Detroit Diesel gave stack restriction measurments in inches of water, and for their blower-purged 71-Series the maximum restriction in the exhaust was 3.5" H2O---which is why I'm asking.

I think the number of 1-1.5 psi is roughly equivalent to 22-30" H2O so there's a big difference there I'm wondering about.

[johnc]

Per the folks at Sunbelt, psi is pounds per square inch of air pressure measured with a very accurate air pressure gauge 18" from the final merge collector in the header.

[Tony D]

WOW! That's some restriction!

Thanks for the clarification.

 

BTW, a 'very accurate gauge' for this measurement would be a mercury manometer---this pressure would indicate as roughly 2-3" Hg... and with much resolution. Easily checked during a dyno run.

 

And yes, I went back and checked my DDA books, and theirs IS Inches H20---when I did the conversion from PSI to "Hg I went "oh. maybe I misrecalled...) Two Strokes, what can you do? LOL

[johnc]

I meant 18" downstream of the last merge collector. The numbers I quoted were for a NA engine.