91diesel

I guess all those exhaust engineers are all wrong on stock street motors picking up torque when they have a little back pressure huh? You are correct in that motors will make the most HP with out a muffler, but where in the rpm range do they make it, and what does it do to the lower range? I have experience with this with my Harley in a unique way. I have a 2 into 1 exhaust that some one had whacked the back end off because his motor needed to 'breathe' better. Well after running it on my modified 100+HP bike, I made a baffle for it out of an oil filter and riveted it to the inside of the exhaust pipe. Walla! more lower end torque. Further proven when I dragged my brother on his CBR and the exhaust pressure popped through the rivets and blew the baffle out. Immediately I noticed a difference in bottom end torque, 900-3200rpm. I went and made a baffle out of stainless and welded it in to fix that problem. So how is that increasing the speed of the exhaust? It's a metal plate with holes drilled in it blocking the outlet of the pipe.

Tubbyfatty

I just drove 120 miles all on the highway going about 60, and patches of 75. Just above half a tank at 120 miles. sweet, looks like city driving kills my MPGs

nosfanatic2006

iTrader: (1)

How is it that the masses agree about backpressure? This is the FIRST time I've ever heard that back pressure is a myth. If I take the muffler off my truck its quite a bit slower (and a hell of a lot louder ), but with my Borla muffler on there it seems to run a lot smoother and quicker. So what does that say?

flashkl

Yeah, If there was more evidence saying that backpresure is really a myth I would be willing to consider the thought, but as of now I'm erring on the side of doubt. Isn't it the backpressure that causes the vacuum in the exhaust manifold that gets the stale gasses out of there quick and out the back?

ironroad9c1

I never said straight pipe makes the most power, you guys are getting scavenging confused with backpressure you cant have pressure and vacuum in the same pipe. The right size pipes and mufflers will outflow straight pipe. but do what you want, this is what i get for trying to help, I forgot you guys know it all. my 20+ years of automobile tuning and work means nothing .

So let me share this from someone else.
"Replacing the stock production exhaust system with a low-restriction, free-flow one is usually one of the first modifications made to any vehicle in the name of performance. We all know they're louder, but how much performance do they really add? We've all seen supposed dyno tests, usually run by the exhaust manufacturer's themselves on their own dyno, indicating vast power gains, and psychologically, we always equate a healthy exhaust rumble with increased power in the seat of the pants, but how much power are we really gaining? To find out, we're running a simple backpressure study, and our results will be posted here as they come. Admittedly this study is not totally scientific as there are many uncontrolled variables, but it should be sufficient to provide a rough estimate.

It is generally accepted by automotive engineers that for every inch of Hg of backpressure (that's Mercury - inches of Hg is a unit for measuring pressure) approximately 1-2 HP is lost depending on the displacement and efficiency of the engine, the combustion chamber design, etc. Our sources indicated that in the case of the L67 3800SC, 1HP per inch of Hg is reasonable.

1 inch Hg backpressure = 1 HP lost "


Another good explanation is here.
http://my.prostreetonline.com/forums...ead.php?t=1639

"Backpressure: The myth and why it's wrong.

I. Introduction

One of the most misunderstood concepts in exhaust theory is backpressure. People love to talk about backpressure on message boards with no real understanding of what it is and what it's consequences are. I'm sure many of you have heard or read the phrase "Hondas need backpressure" when discussing exhaust upgrades. That phrase is in fact completely inaccurate and a wholly misguided notion."


NOT my website and not my post, just trying to make you guys see that its not backpressure you want.

nosfanatic2006

iTrader: (1)

Thanks for the link man. I'll be reading it here shortly. Its just that this is the first I've ever heard about backpressure being a "myth" and whatnot. Do you have any other links?

ironroad9c1

Well unless you are really into performance , its not something that someone sits down and studies after dinner..lol and the internet really makes things worse cause then it spreads like wildfire .

flashkl

Thanks for the links, when I'm done with school I'll be reading them for sure. ironroad, is your avatar an animated gif or is it an optical illusion type thing??

Edit: okay, so it is a gif. Still awesome.

yogi09

92 3.0 sr5 4runner with new mastercraft a/t 31x1050x15 stock filter, new exhaust getting around 310-320 a tank,, oh yeh, manual shift also that makes a diffrence,, does better than my 98 tacoma 4cyl got.. it was horrible on mileage..

1992-4r

i got 16.5 mpg last time i filled up, i thought my gas gauge was broken because usually at around 190 miles, which is around 200 with the 31s, the needle is below the low line and almost in the red area, but i ended up with 200 miles on that tank... but normally i get 14ish

flashkl

Okay, so according to that post, I'm confusing the difference in pressure between the exhaust manifold and the tailpipe with backpressure. Certainly makes sense...

bjslade0269

I,m getting about 15mpg and that's a 50/50 mix city/highway driving. I have a 94 4runner 3.0 with a 5 speed swap, stock 4.88's, and 31" tires

TOOLMAN

I'm in the 15-17 mpg range. I get 240-280 per tank with 3.slo, AT, and 31's. I really wish I could get to that 300 miles per tank.

TransAmBandit

I'm sure its all highway drivign, right?

hosh

first off i was getting around 17 out of my 95 4runner 3.0 with 5 speed 4.56 gears and 31" tires. best was 18 in neutral going downhill coming out of Arizona.

Second
ironroad9c1 is absolutely correct. It drives me nuts when i hear otherwise knowledgeable people perpetuate this myth.

Open exhaust are not less restrictive and produce high back pressure. That's why nascar picked up 3 miles per hour when they went to mufflers. I have been wrenching since i was 9 (I'm 32 now). I am one of those people that are not able to just believe something because i was told so. I have to find out why.

The reason people believe the back pressure crap is they reduce pipe size in their exhaust and/or add mufflers and see power gains. They automatically assume because the pipe is smaller or they added a muffler it adds back pressure.
Back pressure is BAD, period. But by controlling exhaust system design you can pick where your back pressure is being made (perfect exhaust hasn't been invented yet. Variable exhaust size anyone!) and therefor where you make power.

Most people running 3" exhaust kill low end. Most people with 750 double pumpers on their 350's are over carbed. I used to run a 400 small block in a 79 short wide with a 700 cfm carb and 2" dual exhaust that would pull the front wheels off the ground (2" counts).
It ran out of steam at the upper end though because my system started to produce to much back pressure at high rpms.

Yellowtruck

Was getting 14mpg. Threw on some Toyo m/t's back in October and that dropped it down to 11mpg's around town.
I need a new cat too....and 4.88's.

ironroad9c1

Just putting this out here .....when you are checking your mileage, and you have oversize tires, you can't go by your odometer,its gonna register less miles than you have really traveled unless you change the speedo gear and make the correction. it will always show up a really low mpg numbers.

toy-keg

'88 manual 4runner with 31'' tires.
I'm getting over 27 with all interstate driving.

nevermore1701

95 4Runner, auto, 225/75 tires, 4.88 gears, city only driving, and I am NOT gentle on the gas. Right around 200 per tank, 12-13mpg. See what happens when I switch to 33s.

dromomaniac

94 xtra cab with 32x11.5 WildCountry XTX sports:

highway: 18-19, over 20 mpg once! (up 1-2mpg from the Goodyear MTRs the truck came with)
backcountry: 13-14
city: probably around 13-14, but haven't been able to isolate this mileage from other types.