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Old 02-14-2012, 03:29 PM   #1
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changing exhaust size

I've run a quick search and haven't come across the answer I'm looking for.

I'm changing my exhaust out due to rust on Monday. I'm probably going to do everything from the Y back. So, my question is what are the pros and cons of going to a 2.5" pipe over the stock 2.25"?

-Brad
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Old 02-14-2012, 04:54 PM   #3
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Thanks for the reply.

I forgot to mention what size engine and it is the 3.4 also.
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Old 02-14-2012, 11:09 PM   #4
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what monte said.... I would keep your tubing size same as stock...not sure if thats 2'' or 2.25''
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Old 02-15-2012, 04:31 AM   #6
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I have had similar results as Monte has with my magnaflow cat back system.
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Old 02-15-2012, 08:16 AM   #7
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Loss in back preassure causing the motor to not run correctly. I have 2.5'' out just from the 14'' magna flow i have and it dies sometimes at start and off of high revs.
I'm sorry but this is INCORRECT.

An engine does not "need" back pressure as you say to run properly. Back pressure is what cause inefficiency in the engine. Gases (hydro-carbons, carbon monoxide, sulfide) and all the other harmful chemicals to the air and our lungs are the by-product of engine combustion. The majority of these by-products are burnt off at the catalytic converters, but a good portion still resonates in the exhaust after the cat. So why get an exhaust? Besides the increase in sound, the purpose of going to a free flowing muffler and possibly a larger diameter piping is to allow more of these unburnt gases to escape quicker so your pistons don't have to keep working hard to push them out of the exhaust manifold.

This is the reason back pressure causes inefficiency in the engine. Unlike a header which has individual tubes connecting verses an exhaust manifold which has several ports going into one exit. It's at this point, the catalytic converters, and the mufflers where the restrictions are. It creates a "venturi," which is a restriction at the tube i.e. on an carburetor or at the end of a soda bottle. So by upgrading your exhaust, you are not "allowing the engine to breathe," you are allowing it to get the hot exhaust gases out quicker so your engine doesn't have to work so hard to push them out. There is a point though where going to the largest size diameter tubing actually hurts performance. For a Tacoma this size is a 3" tubing. Reason being is because the 2.5 gives the hot exhaust gases "just enough" room to get through the exhaust quicker than the stock 2.5, but not enough room for it to expand and cool which slow the scavenging effect.

The difference between going from a 2.25 to 2.5 exhaust is negligible is and WILL NOT cause you're engine to die out as you say. I can assure you if your engine is dying it's not because you changed the diameter of your exhaust lol.
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Old 02-15-2012, 08:40 AM   #8
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Actually dude, our motors are already almost at peak efficiency without going forced induction. even with exhaust and deck plate mod, which i have done both, almost no gains at all. And if you do some searching you can see all the proof there.

The only type of system where you want the least amount of BP is on a turbo setup so the turbo will spool faster, but thats just after there turbo, the turbo itself is creating the BP the motor needs.

and you even just stated it, going to a larger size actually hurt performance.

Im telling you that anything larger then stock..hurts performance. The 3.4l is already tuned and set up quite nicely. If you Really want more power..get a SC or Turbo..thats it. By not having enough BP when letting off of throttle the motor can stall out, this has happened to me several times, and i don't even have full 2.5, just 2.5 from the muffler out.


hence why oh idk..the THOUSANDS of ppl on here just get new mufflers and not new pipe.

It isn't worth the new pipe.
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Old 02-15-2012, 09:16 AM   #9
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Quote:
Originally Posted by Blackdawg View Post
Actually dude, our motors are already almost at peak efficiency without going forced induction. even with exhaust and deck plate mod, which i have done both, almost no gains at all. And if you do some searching you can see all the proof there.

The only type of system where you want the least amount of BP is on a turbo setup so the turbo will spool faster, but thats just after there turbo, the turbo itself is creating the BP the motor needs.

and you even just stated it, going to a larger size actually hurt performance.

Im telling you that anything larger then stock..hurts performance. The 3.4l is already tuned and set up quite nicely. If you Really want more power..get a SC or Turbo..thats it. By not having enough BP when letting off of throttle the motor can stall out, this has happened to me several times, and i don't even have full 2.5, just 2.5 from the muffler out.


hence why oh idk..the THOUSANDS of ppl on here just get new mufflers and not new pipe.

It isn't worth the new pipe.


I agree, the 3.4l engine is a very efficient, but the exhaust manifold in particular creates the most in-efficiency in this this motor. Going to a different diameter tubing helps to accelerate the scavenging effect by allowing more of these hot exhaust gases to exit quicker. Going to a larger diameter tubing will only hurt performance at a particular size, which for this Toyota engine is with a 3" tubing. Notice I said, performance, not reliability. 2.5" is the perfect mix of not going too large to as to let the exhaust gases cooled, but enough to allow more room and a higher volume exit quicker and still be hot. I have a custom exhaust setup with all new 2.5 tubing through out running an Aero Turbine muffler. Not only did I see an increase in MPG, I also saw much better throttle response on the mid-range (highway power) than with my stock setup. As for your truck dying when you let off the gas, it sounds like you have a problem with a sensor up front, maybe a TPS or A/F sensor.


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Old 02-15-2012, 09:21 AM   #10
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I do not know what the optimum size of exhaust tubing would be for a 3.4 liter engine, but I do know that an oversized pipe will hurt exhaust scavenging and thus low end performance.
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Old 02-15-2012, 09:22 AM   #11
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Well good for you i guess? You want a cookie??

You can search all through this site, and most ppl see a loss in mpgs from going full 2.5''.

And Brad isn't getting new headers which I'm sure has a lot to do with it, but spending 600 bucks on headers is just stupid for the gains you get out of them.


Either way, 2.25'' pipe with a new muffler is still the safest and best way to run a our exhausts. Plenty of proof out there for it.



Edit: Correct John, and now with my 33s, definitely seeing a loss in low end.
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Old 02-15-2012, 10:06 AM   #12
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Quote:
Well good for you i guess? You want a cookie??
Well that's the mature way to handle an informative debate. In the end this forum is about educating people and correcting misinformation.

And tell me how on earth would increasing your tubing diameter create a loss in MPG lol??? Do you even understand how an engine works? When you have a restrictive exhaust your engine has to work harder to get the exhaust gases out using more energy and fuel. Shocker I know. A less restrictive exhaust means the engine has to work less and ultimately use less fuel. The reason THOUSANDS of members run stock exhaust piping with an aftermarket muffler is simple. COST. You can easily have an aftermarket muffler welded in for right around $100. That doesn't mean it's the most efficient or safest, it just means it's the cheapest route to upgrading your exhaust. It will be minimal gains with just changing out a muffler if any, but most of us change out the muffler for sound and not performance. When you come on here and tell everyone that by increasing the tubing size on a pipe is going to make your car unreliable and die when letting off the gas, I'm sorry but that is just non-sense.
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Old 02-15-2012, 10:11 AM   #13
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Fuel economy will increase at higher rpms with an increase in exhaust diameter, but lower rpm mpgs will suffer due to the lost scavenging affect from the lack of back pressure.
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Old 02-15-2012, 12:45 PM   #14
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I never said it would make it unreliable, my truck still runs.

and the cost of a bit of 2.5'' pipe is like 40 bucks. That is not the reason why ppl don't do it.

The 3.4l can't cope well with the really open end on the exhaust due to the intake. CAI are pointless on our trucks too so you can't equalize the intake with the new free flowing out put on the exhaust...so really it does nothing for you except a loss in torque and power especially on the bottom end. And that is why FAI is the only way to really get more power in which case i would suggest getting a large exhaust. He is not going FAI so he shouldn't get the bigger pipe.

I know plenty on how motors work..
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Old 02-15-2012, 04:21 PM   #15
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Quote:
I never said it would make it unreliable
Quote:
I have 2.5'' out just from the 14'' magna flow i have and it dies sometimes at start and off of high revs. The 3.4l is very picky about the back pressure
Sure sounds like it here buddy.

Quote:
the cost of a bit of 2.5'' pipe is like 40 bucks. That is not the reason why ppl don't do it.
The cost of piping as far as materials is cheap, you're right about that. It's the time and labor the shop charges to cut, bend, fit, and weld the piping into place that makes it expensive which is the reason most people don't do it.


Quote:
He is not going FAI so he shouldn't get the bigger pipe
Why not? He already said himself he's going to replace the whole thing because of rust so why not get the added benefits of stepping up to a 2.5? You don't need FAI to benefit from having a more free flowing, less restrictive exhaust system. He's only going up .25 of an inch. It's not like he's throwing a 3.5" exhaust running a straight pipe...


Quote:
so really it does nothing for you except a loss in torque and power
So what you are trying to to tell me is that by going .25 of an inch in diameter in the exhaust tubing is all of the sudden going to loose torque and HP??? Sorry you're wrong there. Once again .25 of an inch is negligible the 3.4l responds very well to the increase in size and going to a 2.5 tubing will increase the volume in the exhaust system and higher flow capacity. A 3" exhaust would definately be overkill and running this size or anything above it would most certainly have some negative effects. The point is to find the perfect balance in which this case is a 2.5" exhaust. The stock exhaust is somewhat restrictive and 3" is not restrictive enough. The purpose of the exhaust is to get the exhaust gases out as fast as possible with the least restriction possible. Some restriction is needed to keep the gases hot enough to pass through the system, but .25 of an inch doesn't open it up enough to slow the exhaust gases to a point of cooling where the velocity slows down and the gases don't get spent. All in all you're going to continue to believe whatever you want. I'm out, it was fun but I have an exam to study for.
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Old 02-15-2012, 07:03 PM   #16
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Well in that case, I guess the same douchebag that built my engine assembled Montes as well. I did gain a bit of high rpm power and maybe mpg while on the interstate but around town it plummeted. Im not into rocket science but I can add and subtract miles on tank of fuel to get mpgs. And I did lose lower end power the minute after my install, as most other have also. Not to mention that im only 265/70s on the hoof unlike his 33x 12.5s.
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Old 02-16-2012, 12:06 AM   #17
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Quote:
Originally Posted by Yamaha Dave View Post
Sure sounds like it here buddy.

The cost of piping as far as materials is cheap, you're right about that. It's the time and labor the shop charges to cut, bend, fit, and weld the piping into place that makes it expensive which is the reason most people don't do it.


Why not? He already said himself he's going to replace the whole thing because of rust so why not get the added benefits of stepping up to a 2.5? You don't need FAI to benefit from having a more free flowing, less restrictive exhaust system. He's only going up .25 of an inch. It's not like he's throwing a 3.5" exhaust running a straight pipe...


So what you are trying to to tell me is that by going .25 of an inch in diameter in the exhaust tubing is all of the sudden going to loose torque and HP??? Sorry you're wrong there. Once again .25 of an inch is negligible the 3.4l responds very well to the increase in size and going to a 2.5 tubing will increase the volume in the exhaust system and higher flow capacity. A 3" exhaust would definately be overkill and running this size or anything above it would most certainly have some negative effects. The point is to find the perfect balance in which this case is a 2.5" exhaust. The stock exhaust is somewhat restrictive and 3" is not restrictive enough. The purpose of the exhaust is to get the exhaust gases out as fast as possible with the least restriction possible. Some restriction is needed to keep the gases hot enough to pass through the system, but .25 of an inch doesn't open it up enough to slow the exhaust gases to a point of cooling where the velocity slows down and the gases don't get spent. All in all you're going to continue to believe whatever you want. I'm out, it was fun but I have an exam to study for.
1) I told him what happened to my truck. And mine has only stalled out like less then 10 times in the last 6 months. Its not any less reliable. Can't you read?

2)Idk what exhaust shop your going to..but your getting ripped off if i can get it that cheap in the middle of no where and you can't..

3)Only .25''...

circumference of 2.25=7.065
of 2.5''=7.85

Area of 2.25= 3.97

area of 2.5=4.91


so lets say the pipe for the headers is...idk..6 feet..or 72''...


volume of 2.25=285.84

volume of 2.5= 353.52

so a 2.5'' pipe flows 67.68 cu in more..way to much..

Thats a big difference man..and it is not worth it. Yes he should still get new pipe, but not 2.5'' to keep the power on the low end good, he should stay a 2.25.

And for the last time. I did plenty of looking into this on this site and others, 2.25 is the best pipe to run as your main pipe to keep the 3.4l happy. Do some research and you'll see.

Ill leave the OP to decide what he wants. Its his truck and he can do what he wants. And i don't care what you say man, i know what i know and i highly disagree with you. But I'm not going to try and convince you anymore.

Quote:
Originally Posted by Texasdynaryder View Post
Well in that case, I guess the same douchebag that built my engine assembled Montes as well. I did gain a bit of high rpm power and maybe mpg while on the interstate but around town it plummeted. Im not into rocket science but I can add and subtract miles on tank of fuel to get mpgs. And I did lose lower end power the minute after my install, as most other have also. Not to mention that im only 265/70s on the hoof unlike his 33x 12.5s.
Yep exactly. Even with my 31s my motor from idle to 2k lost a lot. After 2k its fine and i did see mpg improvements on the highway. But damn it killed it on city which is what most of us do anyways. of course now the 33s don't help but no way in hell am i going back to 31s haha
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Old 02-16-2012, 06:03 AM   #18
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Math is hard.
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Old 02-16-2012, 08:20 AM   #20
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Almost positive stock pipe is not 2.25. I think it is an in between size like 2 3/8. That's why on my 04 Taco and my previous 2000 4runner I ran a 2.5" muffler on stock pipes and it was close enough. Measure it.

so all that math is wrong. if you get crappy muffler shop crunch bends get 2.5. It will make no difference from stock imo. Stock bends are not that bad.
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