Exhaust Backpressure

modi***- a small degree
a limited number of
a moderate amount
"do it like this baby, you will get lots modi***"= "it's just a little bitty thang, don't worry about it"

 

Read this test done by Ted Eaton and see what different systems do for power. And yes, bigger does not always mean better. That is true on most engine components.

http://www.y-blocksforever.com/forums/Topic55379-3-1.aspx?Highlight=exhaust+test

 

A stock engine needs "something" in the way of exhaust. It's arguable just what, but needs something. That's why all these jalopy type cars around here can't pull steam off a hot dog with their abbreviated open headers. I've seen Nailheads, Caddy's, and Hemi's that once powered 4000 pound cars, now in an 1800 pound car that can barely burn rubber with 4 inches of tread on the ground. An engine...and the closer to stock it is needs some kind of exhaust. Even 1200+ HP Pro Stocks don't run straight Zoomie type exhaust. They need something that involves science, not just an open door

 

How to calculate muffler size pipe diameter, complete web site address shown below.

http://www.exhaustvideos.com/faq/how-to-calculate-muffler-size-pipe-diameter/

Exhaust Pipe Size Estimate: A good section of straight pipe will flow about 115 CFM per square inch of area. Here’s a quick table that shows how many CFM each common pipe size will flow, as well as the estimated max horsepower for each pipe size:
<TABLE style="TEXT-ALIGN: center" cellSpacing=0 cellPadding=5 border=1><TBODY><TR><TD align=middle width=116 height=38>Pipe Diameter (inches)</TD><TD align=middle width=73>Pipe Area (in2)</TD><TD align=middle width=78>Total CFM (est.)</TD><TD align=middle width=95>Max HP Per Pipe</TD><TD align=middle width=132>Max HP For A Dual Pipe System</TD></TR><TR><TD align=middle height=17>1 1/2</TD><TD align=middle>1.48</TD><TD align=middle>171</TD><TD align=middle>78</TD><TD align=middle>155</TD></TR><TR><TD align=middle height=17>1 5/8</TD><TD align=middle>1.77</TD><TD align=middle>203</TD><TD align=middle>92</TD><TD align=middle>185</TD></TR><TR><TD align=middle height=17>1 3/4</TD><TD align=middle>2.07</TD><TD align=middle>239</TD><TD align=middle>108</TD><TD align=middle>217</TD></TR><TR><TD align=middle height=17>2</TD><TD align=middle>2.76</TD><TD align=middle>318</TD><TD align=middle>144</TD><TD align=middle>289</TD></TR><TR><TD align=middle height=17>2 1/4</TD><TD align=middle>3.55</TD><TD align=middle>408</TD><TD align=middle>185</TD><TD align=middle>371</TD></TR><TR><TD align=middle height=17>2 1/2</TD><TD align=middle>4.43</TD><TD align=middle>509</TD><TD align=middle>232</TD><TD align=middle>463</TD></TR><TR><TD align=middle height=17>2 3/4</TD><TD align=middle>5.41</TD><TD align=middle>622</TD><TD align=middle>283</TD><TD align=middle>566</TD></TR><TR><TD align=middle height=17>3</TD><TD align=middle>6.49</TD><TD align=middle>747</TD><TD align=middle>339</TD><TD align=middle>679</TD></TR><TR><TD align=middle height=17>3 1/4</TD><TD align=middle>7.67</TD><TD align=middle>882</TD><TD align=middle>401</TD><TD align=middle>802</TD></TR><TR><TD align=middle height=17>3 1/2</TD><TD align=middle>8.95</TD><TD align=middle>1029</TD><TD align=middle>468</TD><TD align=middle>935</TD></TR></TBODY></TABLE>
NOTE: These numbers are just estimates. All pipes are ***umed to be 16 gauge steel.
The table above is probably over-estimating pipe size, but you can see that a 400 hp vehicle with a dual exhaust system only needs 2 1/4 – 2 1/2 inch pipes. Anything larger is overkill.

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Mark, every time i have worked on one of those engines with the exhaust pipe running across the front of the engine, i find the exhaust valve is burnt on the rear cylinder on the driver side. i think that cylinder has all that extra problem getting the exhaust out. run the dual exhaust!

 

The single best, simplest and most cost effective upgrade for a Y-Block is an exhaust upgrade. I've had good luck with rams horns, and a dual 2" system, with an x-pipe.

 

This link has the clearest explaination to my question I have found yet.

http://www.miata.net/garage/KnowYourCar/S4_Back.html

 

There are many to lie with statistics so be careful out there.

I've actually seen dyno results where an enginge gained power when a pair of mufflers were bolted on [ the dyno tests were done by the muffler manufacturer ]

What wasn't told was the engine had a special cam ground with the exhaust timing altered, so it relied on exhaust backpressure to prevent over scavenging of the intake charge.

"Yeh mate,Seeing is believing! I Just bolted these mufflers on and she picked up 20 horsepower"

 

NO engine "needs" back-pressure. For maximum power, an engine needs the exhaust gases to exit the combustion chambers as quickly as possible. Pipes that are too large reduce the "velocity" (speed) of the exhaust flow, and can actually reduce performance. While you don't want too much restriction, you also want the exhaust to flow through the pipes at the maximum velocity possible. Bigger is not always better. Anyone who thinks a set of 4-inch pipes are gonna make more power on a small-block Chevy than a set of 2-1/2" pipes, is seriously misguided...

 

Damn i guess i wasted a bunch of time perfecting and matching the bends those 1 3/4 zoomies to go on my mild 302 but 1 1/2 looked so puny guess ill throw them away (smh)

 

In my past studying of this topic, I learned that one of the biggest restrictions in exhaust flow is the drastic change in temperature. Hot g***es into cold air+brick wall. There's testing that shows how the mean temp of the system is as important as the size in certain applications. The popularity of ceramic coatings these days owes much to this knowledge. It works. Some older cars had insulated exhaust systems for that purpose.

FWIW, zoomies are intended for WFO only operation. Definately not scavengers in the most obvious sense but they can scavenge. I'd think cam timing has as much to do with that abilty as the diameter. You can run 2" zoomies on your 302 SBC and get the same job done since getting the spent g***es out as fast as possible are the only concern. They really have little in common with the topic. And for you real technical types, yes indeed I over-simplified the theory. Feel free to babble on in deeper tech about it...

 

Heresy, ain't it all about the LOOK??

 

Ok, here is a set of pipes with a***tude, but they worked, and worked TOO well.





In the Book "We Were The Ramchargers" the story of these pipes was told - the megaphones were engineered to match the tube length and create a scavenge. As I said, it worked too well and there was a hole in the fuel metering curve they could never fix - the cylinders were over scavenging at around 3000 rpm making it go lean and stumble, then it would come on like a freight train after that and wind right out. They learned to drive around it and never were able to fix it before they were on to the next race car.

this thing must have been loud as F@#K!

 

See if the above edited link works better. The YBF site changed servers which made the old links inoperative. Ted.