single exhaust system

[Owen]

Hey everyone,

My motor is a 383 and my muffler is a single

2.5 inch OD. Should I go with a 3inch system

or 2.5inches?

What are the pros and cons of each?

Thanks,

Owen

 

 

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http://www.homestead.com/s30z/index.html

[BLKMGK]

If you can only run a single exhaust make it a 3inch. You need only look as far as the SLP exhaust for the Camaros to see that on a stock 350 an upgrade is 3inch exhaust. My understanding is that a single 3inch flows better than 2 2.25 pipes but that dual 2.5's beat the 3. However due to fluid dynamics, friction on the pipe walls, and other stuff I barely understand I'm not positive the dual 2.5s beat a single 3.

 

Bottom line - a single 2.5 is probably way too small if that motor is making much power. If you decide to go 3inch take a look at what the "roundy round" racers run. I spent some time at a friend's chassis shop the other day and he's found cheap OVAL tubing including adapters in some of the catalogs that cater exclusively to circle track racers. Unfortunatly I didn't get the name of the catalog. Oval tubing will get the pipes up further and give you ground clearance - he's using it in pro street cars that sit way low....

[pparaska]

Blatant opinions and armchair engineering follow... this is the thought process (i.e;, brain fart) I had on this topic:

 

I don't want to debate frictional wall losses now, but dual 2.5s have considerable more cross sectional area (39% more). But as noted, more wall area for frictional losses (it's more involved than that - where is Michael Ol - he's the aerodynamicist Ph.D. around here ). The ratio of wall area between dual 2.5s and single 3 is 1.67:1, so yeah, there are some drawbacks.

 

Personally, I wanted to do a single 3.5" system, which would be lighter (a plus) and flow about the same as dual 2.5s. But...

 

Something to consider is the type muffler available for your use. Most mufflers that handle 3 and 3.5 are race type or too loud for my tastes, in general, although I haven't heard one of the new delta 50 series Flowmasters in the 3 or 3.5 inch single on a V8. The regular or old series Flowmasters in that size are too loud for me. Call me a wuss.

 

Anyway, that, along with the ability to tuck dual 2.5s up better (for ground clearance) than a single 3.5 and the muffler issue is what drove me to dual 2.5s. Also, I've learned that (in general) to lower noise without increasing backpressure, you need alot of muffler volume. That's another reason why I chose dual Dynomax Super Turbo Hemi mufflers. LOTS of volume there. And a sweet sound (on other V8s I've heard - mine is still in the incubator ).

 

So muffler choices play a big role if you want the system to be quiet and flow well.

 

Another thing to consider is resonant frequency of the pipe wall. The larger diameter pipes have lower resonant first frequencies, for the same material type and thickness. Generally, lower resonances are more powerful and can create more discomfort than higher frequency ones with the same sound power.

 

Oops, there I go again engineering the damn thing. Sorry. I can't help myself - please send money for the shrink!

 

[This message has been edited by pparaska (edited July 05, 2000).]

[pparaska]

Hey Fast Frog, thanks for the vote of confidence!

 

I forgot to mention the mandrel bent issue. I just saw a page today about press bent versus mandrel bent pipes.

http://www.extrememotorsports.com/mandrel.htm

The cross sectional area reduction for the examples it gave, work out to around 20-30% depending on the angle of the bend, etc.

 

The other thing to mention is that the buckling of the inside wall of a press bent pipe adds a bunch of turbulence to the flow, which is not so good.

 

BTW, my entire system was hand fabbed by me and my mini-mig using a slew of 180 degree 2.5" mandrel J bends (cut into pieces) and a stick and a half of straight pipe, an X-pipe, and 10 3-bolt flanges. This took over 70 hours (I work REALLY slow). I made the angles as few and small as possible, and the exhaust is up VERY tight under the car. The blowproof bellhousing and subframe connectors hange below the entire exhaust.

 

You can see the results under "Exhaust" on my site. Well, I need to get the car on a lift to take some installed pictures some day.

 

I still think a cool system would be an single oval pipe with a big quiet muffler. I wasn't about to research all that and took the easy way out . Plus that oval pipe is usually expensive!

 

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Pete Paraska - 73 540Z - Marathon Z Project - pparaska@tidalwave.net">pparaska@tidalwave.net -

[Owen]

Thanks everyone for the info. I already

placed my order for the 3 inch piping. Mandrel bends, of course, those were in the plans before I even had a car! One place wanted $30 for one 90-degree bend, so I bought 2 180-degree U-bends instead at $25 each (I needed 4 anyway).

Hope to get some cutting and tack welding done this weekend. I've finally decided to give up using the flux core wire for my MIG welder, and go with a shielding gas instead.

Flux core sux.

Lates,

Owen

 

 

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http://www.homestead.com/s30z/index.html

[Guest Fast Frog]

Before I start a car project, I try to research as much as I can to see if I'm on the right track and to see if the project is doable. 6 yrs ago I dreamed of installing a sbc 383 into my 76Z. But first I started researching engine mechanics, flow characteristics, VE, valve train timing events, BSFC, etc. I checked out a couple of good books on basic automotive engineering and ran across several great articles in many hotrod mags. I was surprised when one great article likened the gasoline combustion engine as a thermo pump! Basically, an engine breaths in the right mix of air and gas, combustion occurs, and the spent exhaust gases pass back into the atmosphere. The more efficient that the air/gas mix is brought into the combustion process, ignited, and relieved back into the atmosphere; the more efficient is the thermo pump-which translates into more power!!

 

Many of us spend tons of time brooding over our intake process, the flow character of our heads, and the VE/BSFC/etc of our engines before and during combustion. But tend to spend much less time with the process once the gases leave the cyl head.

 

Now cometh the musings and armchair engineering comments of one Pete Paraska.

Gentlemen! Don't take Mr. Pete's comments lightly-he's right on! Without going into great detail which is prickly with mathematics, exhaust flow thru a pipe and into a muffler is very much like water flo thru a PVC pipe into a retention pond and back into a pipe for runoff control. Vol of a substance (gases or liquid)= velocity of the substance X the cross sectional area of the containment devise (pipe). Cross sect'l area is A=pi times r squared. A 3" pipe has about 31% more capacity for flo than a 2.5" pipe. However, two 2.5" pipes do not quite flo 39% more than a 3" pipe due to (what Pete mention regarding pipe surface area vs cross sectional area) pipe friction. In other words, a 3" pipe will flo more than two smaller pipes whose xsect'l area equals the 3" pipe because the two smaller pipes have more pipe surface area for frictional drag. That reduction is about 7-10%.

 

Now use this concept and math on dual 2.25" pipes and you'll find that one 3" pipe will flow slightly more gases than two 2.25" pipes. Two 2.5" pipes flo better than one 3" pipe-ALL other things being equal!

 

When designing your exhaust sys, make sure that all your constrictions, twists, and bends are minimal. If you're doing a 2.5" dual sys, make sure your all your connections from the hdrs to the muffler are 2.5" and use mandrel bend pipe!

 

(Wow, this sounds like I'm lecturing. Didn't mean to do that-just wanted to help and maybe save someone some money and a lot of cussing!)

 

[This message has been edited by Fast Frog (edited July 06, 2000).]