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deleted mufflers=power loss???

10K views 68 replies 19 participants last post by  MC94XR7 
#1 ·
I finally cut off my mufflers off today and put straight pipe in. It sounds awesome and I love it!!! The only thing is, it seems like I've lost some power. It will still break the tires loose, but I have to get on it harder to get it to do it. I would've thought that the opposite would've happened. Has anyone else experienced this??

Mike
 
#27 ·
Wow, the myths are rampant aren't they?

Any back pressure in the exhaust is bad, any.

The goal of all exhaust systems is to remove the exhaust. The goal is not to play with it, speed it up, slow it down, or twirl it around etc.

The goal is to get it out of the engine, an engine is just an air pump. The more restrictions are in the exhaust(for any reason), the less air is pumped through the engine. Less air coming out means less air going in, less fuel is needed, and less power can be made.

The bottom line is simple, and this is what people do not get. Any defined air flow through the engine needs a specific amount of fuel to make the most power.

If you decrease(smaller pipes, mufflers) the airflow with the same amount of fuel, you get less power(running rich).
If you increase(bigger pipes, mufflers) the airflow with the same amount of fuel, you get less power(running lean).

The answer is always to correct the air/fuel ratio. You have altered the A/F ratio by removing the mufflers. The power is down not because back pressure is needed. Power is down because the engine is running leaner, it needs more fuel.

The only part of the exhaust which critically needs to be a certain size for the given application is the exhaust manifolds, the headers. Those need to be sized for the rpm band of the specific engine. The entire rest of the exhaust can be made as large as you wish, the bigger the better. If you tune the engine(PCM or carburetor), then no power is lost at any rpm, none. There is not any power lost at lower rpm's if you properly tune the A/F ratio. In fact, the power goes up at all rpm's.

Why is this stuff so magical to people, doesn't anyone spread this knowledge around, instead of letting the myths grow? Regards,
 
#3 ·
I did... i dont know if its the placebo effect or what, but I immediately noticed a difference in low end torque.

I think somewhere on this site there is a chart showing the differences of mufflers and no mufflers and power loss, etc...

What size tips did you use btw? mine were 3.5x22 and i LOVE the low, hollow sound lol
 
#6 ·
Borla XR1's look pretty nice. I'd suggest those b/c of the torque which is great for shredding tires. :D Now if only I can scrounge up some $$$.......
 
#16 · (Edited)
On a basically stock engine. The point is moot. Exhaust is one of the last mods you should look at making. The torque differences between Thrush, Dynomax and Borla would be minimal on a stock engine.
The sound difference would be the defining determining factor.

I went with the Borla Pro XS mufflers for my new set up for the following reasons:

  • I always wanted Borla :D
  • They're the quietest of the choices out there (according to the chart I cited earlier)
  • They're 4th highest rated in terms of HP gains (according to This post)
  • They rated highest in power gains in six RPM ranges on the PDF spreadsheet.
  • They're reasonably priced. I purchased mine from Wicked Nicky's Exhaust in the 2.5" size in the offset in / center out configuration for only $84.00 each. They're also available in 2", 2-1/4", and 3" sizes.
  • OX said he regretted selling his Borlas.
  • The Great Obucina recommended them and I heard a set on his F150 (Very Nice Sound!)

Torque is not lost.....it is now in a different place in the power band....in the higher rpm's.
Agreed. Torque loss due to loss of back pressure is an urban Legend/Myth.

Here's another good explaination from a fellow member here.

ARGGH! The whole "backpressure in the exhaust" thing is another one of my pet peeves!

You never want backpressure in the exhaust, plain and simple. If you had any pressure in the exhaust at all, your power would drop like a rock. Ever drive a car with a plugged-up cat? It probably wouldn't go more than about 30mph, right? THAT is backpressure in the exhaust, and as you can tell, it didn't help the power of the engine at any point along the way.

You want that vacuum I was talking about for the scavenging effect, and vacuum is the opposite of pressure. The thing is that vacuum is only created by the velocity of the exhaust gases leaving the pipes. If you open up the pipe without increasing the quantity of exhaust gas that is going through it, you have just decreased the exhaust gas velocity. Decreased velocity causes decreased vacuum for scavenging, which is technically the same as increasing the pressure in the exhaust. So going to too large a pipe is technically increasing the pressure in the exhaust system. I bet nobody on the internet has ever told it to you that way before! Increased pressure (or decreased vacuum, since they are really the same thing) means that the exhaust has to be pushed harder to get out of the engine. This means more of the power the motor made is wasted pushing the exhaust out the tailpipe instead of pushing the car forward.

So, ANY pressure above ambient atmospheric pressure is going to drastically hurt your power and torque curve all the way across the line. While it is true that putting too large an exhaust on a car can hurt low end, the reason is not because the stock system has backpressure, but quite the opposite it is because the stock system has enough velocity at the lower rpms to create a scavenging effect. On a high perfrormance car, it might be beneficial to you to sacrifice a few lb-ft of torque down low to gain a few hp up top, and that is what the larger diameter pipe does for you because it moves both the rpm at which the scavenging occurs and the rpm at which the exhaust becomes a restriction up higher in the powerband. If you have a completely stock engine, and you don't rev it higher than the factory set rev limiter, and you go put dual 3" exhaust on it, you have just lost some low end torque, but you haven't gained anything up top because the operating range of your engine and the operating range of your exhaust system are now nowhere even close to each other.
 
#11 ·
So are borla turbo mufflers the same as XS that's on the chart? I went to borlas site and they don't list an XS and they are more than I wanted to spend. I cut mine off for sound and cheapness :tongue: BUT, I have noticed the shift in the power band and I want it at lower rpms. So to achieve the no muffler sound with lower torque, which muffler would be better? I know I don't like the sound of flowmasters on a 4.6. Thrush turbos don't sound too bad and are cheap but will I get the performance out of them that I'm looking for?

Thanks
Mike
 
#13 ·
IMO if you want to get a good amount of performance and sound you should run a true dual setup with either a mid-mounted magnaflow or a set of borlas out back, if your not looking to spend a ton of money then I would just throw a cheaper set of mufflers out back, something like dynomax or anything along that line.
 
#14 ·
That's what I was thinking. I figure a cheaper set like Thrush turbos which are dynomax would keep me around factory torque so I'm leaning towards those for now. Am I right, would a cheaper set like that bring my torque back to the lower end?

Mike
 
#18 ·
I've read (here) that using slightly smaller diameter pipes in place of the mufflers will allow the exhaust velocity to remain the same as with mufflers, hence not losing any power, but as mentioned above, I don't think you really do lose power. I haven't noticed anything less, and I'm using straight pipes too.

I love the sound, but I don't think it's loud enough :)
 
#20 ·
I didnt notice/feel any loss of power with either of my cars when they were straight piped. With my tbird I did notice a slight increase in throttle response (or so I thought), but on my cougar I havent noticed the slightest difference except the exhaust tone. Thats not saying much though since the blowmasters that were on there when I got it were completely rotted out.
 
#26 ·
Nope. There's nothing you can do to protect them. If you absolutely have to have that sound just replace them with another set when they go bad or switch to another brand. FYI the Borla's are stainless steel.
 
#25 ·
Most auto parts stores sell hi-temp paint. The stuff I use is the "ceramic" and about $8 a can. It holds up good. As for some good cheap mufflers, try Summit Racing Mufflers. I have these:
http://www.summitracing.com/parts/SUM-638222/ They are chambered similar to Rustmaster and have a similar sound without the (annoying) ping. They come dressed in primer, so paint is a must. If anyone uses them, the inlets and outlets are 2.5 inside diameter and it makes getting a proper fitting tip tough. As for performance, they are identical to the Summit Fully Welded, but are not bi-directional. I asked on line about the fully welded and was told these are identicle in performance but would give a lower tone (and they do!). Here is a copy (cut and paste) from Car Crafts comparisons a couple years ago:
I have listed here a muffler shootout test done by CAR CRAFT, they show the best bang for the buck, and show the db's

The Mufflers
MAKE MODEL Part No. COST
SUMMIT Turbo 630125 $14.75
THRUSH Magnum Glasspack 24214 $16.50
THRUSH Boss Turbo 17718 $23.95
HOOKER Competition 21006 $25.95
DYNOMAX Super Turbo 17733 $28.50
DYNOMAX Race Magnum 24215 $31.95
HOOKER Super Competition 21106 $35.95
SUMMIT Fully Welded 630325 $38.69
FLOWTECH Afterburner 50322 $39.95

MUFFLER FLOW TEST

MUFFLER Flow at 28-in H20
DynoMax Race Magnum 528.64 cfm
Thrush Magnum Glasspack 507.40 cfm
Summit Fully Welded 343.38 cfm
Flowtech Afterburner 342.20 cfm
DynoMax Super Turbo 333.94 cfm
Hooker Competition 232.46 cfm
Hooker Super Competition 320.96 cfm
Summit Turbo 331.16 cfm
Thrush Boss Turbo 297.36 cfm

MUFFLER Idle dB WOT dB
DynoMax Super Turbo 89 123
DynoMax Race Magnum 94 133
Flowtech Afterburner 92 124
Hooker Competion 92 122
Hooker Super Competion 90 125
Summit Turbo 89 124
Summit Fully Welded 92 125
Thrush Boss Turbo 90 123
Thrush Magnum Glasspack 92 128

DYNO TEST
All mufflers were dyno-tested on a 355-cube SBC with 10.0:1 compression, Air Flow Research 190 aluminum heads, a CompCams 292 hyd. a Victor Jr. intake, a Holley 750-cfm double-pumper, and 1 5/8 Headman headers.


MUFFLER HP TORQUE 2,500-6,000rpmAverage
Hooker Competition 397.4 381.1 286.8hp/351.9 lb-ft
Thrush Boss Turbo 407.1 384.9 292.1 hp/357.5 lb-ft
DynoMax Race Magnum 409.5 394.3 298.8 hp/366.9 lb-ft
Flowtech Afterburner 409.7 391.2 294.8 hp/361.7 lb-ft
Thrush Glasspack 409.5 389.8 297.7 hp/365.3 lb-ft
Summit Turbo 411.5 386.3 291.5 hp/357.4 lb-ft
DynoMax Super Turbo 412.7 387.2 292.6 hp/358.6 lb-ft
Hooker Super Comp 413.8 387.2 292.8 hp/359.0 lb-ft
Summit Fully Welded 415.4 390.7 295.6 hp/362.4 lb-ft

These aren't too shabby!
 
#29 ·
No, you didn't cover it in post #16, no one here did prior to my post.

You did post several quotes, many of which erroneously support the myth that low end torque(or power) is lost, again with no explanation of why.

The A/F ratio is critical to power, so is the ignition timing. A stock tune is not the best for maximum power(and torque) of any engine when you start altering the exhaust. Read it again, read yours and your quotes. Then you may see that no mention is made about the air and fuel ratio except by me.

I'm not trying to claim sole knowledge of these facts, I don't care who does or when it was discovered. I just want the truth to be posted, all so that anyone can understand how to get the most power out of their exhaust.

The key should be to go bigger with the exhaust after the headers. Always go bigger, but always be sure to retune the engine with more fuel to correct the A/F, to gain power. Regards,
 
#37 ·
No, you didn't cover it in post #16, no one here did prior to my post.

You did post several quotes, many of which erroneously support the myth that low end torque(or power) is lost, again with no explanation of why.

The A/F ratio is critical to power, so is the ignition timing. A stock tune is not the best for maximum power(and torque) of any engine when you start altering the exhaust. Read it again, read yours and your quotes. Then you may see that no mention is made about the air and fuel ratio except by me.

I'm not trying to claim sole knowledge of these facts, I don't care who does or when it was discovered. I just want the truth to be posted, all so that anyone can understand how to get the most power out of their exhaust.

The key should be to go bigger with the exhaust after the headers. Always go bigger, but always be sure to re-tune the engine with more fuel to correct the A/F, to gain power. Regards,

Well.. we're both arguing the same point here. We're both on the same side of this issue. I agree with what you're saying.

The whole air/fuel ratio issue was covered in the linked article that was part of my post #16. Under the sentence. "Torque loss due to loss of back pressure is an urban Legend/Myth." which clearly explains why.

You may not have noticed the link or bothered to follow it to the supporting article.

Here, I'll post a copy of the article here.

Destroying a myth.

Some say that "an engine needs backpressure to work correctly." Is this true?


No. It would be more correct to say, "a perfectly stock engine that cannot adjust its fuel delivery needs backpressure to work correctly." This idea is a myth. As with all myths, however, there is a hint of fact with this one. Particularly, some people equate backpressure with torque, and others fear that too little backpressure will lead to valve burning.

The first reason why people say "backpressure is good" is because they believe that increased backpressure by itself will increase torque, particularly with a stock exhaust manifold. Granted, some stock manifolds act somewhat like performance headers at low RPM, but these manifolds will exhibit poor performance at higher RPM. This, however does not automatically lead to the conclusion that backpressure produces more torque. The increase in torque is not due to backpressure, but to the effects of changes in fuel/air mixture, which will be described in more detail below.

The other reason why people say "backpressure is good" is because they hear that cars (or motorcycles) that have had performance exhaust work done to them would then go on to burn exhaust valves. Now, it is true that such valve burning has occurred as a result of the exhaust mods, but it isn't due merely to a lack of backpressure.

The internal combustion engine is a complex, dynamic collection of different systems working together to convert the stored power in gasoline into mechanical energy to push a car down the road. Anytime one of these systems are modified, that mod will also indirectly affect the other systems, as well.

Now, valve burning occurs as a result of a very lean-burning engine. In order to achieve a theoretical optimal combustion, an engine needs 14.7 parts of oxygen by mass to 1 part of gasoline (again, by mass). This is referred to as a stochiometric (chemically correct) mixture, and is commonly referred to as a 14.7:1 mix. If an engine burns with less oxygen present (13:1, 12:1, etc...), it is said to run rich. Conversely, if the engine runs with more oxygen present (16:1, 17:1, etc...), it is said to run lean. Today's engines are designed to run at 14.7:1 for normally cruising, with rich mixtures on acceleration or warm-up, and lean mixtures while decelerating.

Getting back to the discussion, the reason that exhaust valves burn is because the engine is burning lean. Normal engines will tolerate lean burning for a little bit, but not for sustained periods of time. The reason why the engine is burning lean to begin with is that the reduction in backpressure is causing more air to be drawn into the combustion chamber than before. Earlier cars (and motorcycles) with carburetion often could not adjust because of the way that backpressure caused air to flow backwards through the carburetor after the air already got loaded down with fuel, and caused the air to receive a second load of fuel. While a bad design, it was nonetheless used in a lot of vehicles. Once these vehicles received performance mods that reduced backpressure, they no longer had that double-loading effect, and then tended to burn valves because of the resulting over-lean condition. This, incidentally, also provides a basis for the "torque increase" seen if backpressure is maintained. As the fuel/air mixture becomes leaner, the resultant combustion will produce progressively less and less of the force needed to produce torque.

Modern BMWs don't have to worry about the effects described above, because the DME (car's computer) that controls the engine will detect that the engine is burning leaner than before, and will adjust fuel injection to compensate.

So, in effect, reducing backpressure really does two good things: The engine can use work otherwise spent pushing exhaust gas out the tailpipe to propel the car forward, and the engine breathes better.

Of course, the DME's ability to adjust fuel injection is limited by the physical parameters of the injection system (such as injector maximum flow rate and fuel system pressure), but with exhaust backpressure reduction, these limits won't be reached.

- Adapted from Thomas V.
 
#30 ·
Mufflers aren't the restriction for 1. Its the cats and compression bends all over the place.

All you get from straight pipes in place of mufflers is a swarm of bee's sound. I had the ART true dual system on my car with high flow cats and it was an improvment over stock definitly but now im running NO carts midmount magnaflow and 3 inch piping. Talk about flow. And the sound is incredible. And of course low end power isnt noticeable for me with the stall im running and supercharger. If anything I have to much low end power for the street.
 
#31 ·
Trunk Monkey, this quoted section below is part of your post #16 quote above.


ARGGH! The whole "backpressure in the exhaust" thing is another one of my pet peeves!

You never want backpressure in the exhaust, plain and simple. ...


If you have a completely stock engine, and you don't rev it higher than the factory set rev limiter, and you go put dual 3" exhaust on it, you have just lost some low end torque, but you haven't gained anything up top because the operating range of your engine and the operating range of your exhaust system are now nowhere even close to each other.
There is a little truth in there, but this last sentence is really bad. That is at the core of myths about exhaust.

That one sentence is wrong badly on two points. It states that you lose low end torque with a bigger exhaust(stock engine). It follows that error by saying that above the low end there is nothing gained up top.

I call that BS, period. There is no mention of the air/fuel ratio, which is absolutely critical. No one should be altering the inlet and outlet restrictions on any engine without consideration of the tune of the A/F ratio. The A/F ratio must be adjusted, without it you might as well be swapping injectors without adjusting the PCM.

If you bolt on a dual 3" exhaust onto a bone stock Mustang 302, and adjust the PCM, the power will go up from idle to maximum rpm's. All of the power at every single rpm will be higher, but only if you adjust the air/fuel ratio properly.
 
#32 ·
I dont see the air/fuel needing to be adjusted just for an exhaust upgrade.Only time a retune would be reqiured for air/fuel adjustment is when you change injectors and or MAF.
 
#33 ·
I also just read this on another thread. Thanks Driller.

Here is a list of what DOES NOT require a reburn.

•Cold Air Induction system, as long as there's no bends right before the MAF, 86 - 04.
•Exhaust
•Headers, Shorties and Mid Length
•Transmission shift kits, as long as they don't increase line pressure all the time
•Pulleys, as long as they aren't effecting your alternator's ability to charge at idle
•Intake, as long as it isn't a radical change, like changing to a box manifold
•Light intake and head porting
•PI cam upgrades on the 4.6L engines
•MILD camshaft upgrades
•Different throttle bodies
•Rear gear changes, in non-electronic transmission vehicles as long as you only go 2 steps. Example 3.08 to 3.55

Here is a list of things that WILL require a chip reburn.

•Injector upgrades
•High lift cams
•Major head porting
•PI head swap onto a Non PI engine
•Cold Air Induction on 05+ Mustangs, Trucks and Crown Vics using the new style blade type MAF sensor.
•Different MAF; such as a Mustang MAF on a Tbird, Marauder MAF on a CV
•Supercharger or Turbo
•Nitrous
•Engine swap
•Rear Gear changes, in electronic transmission vehicles
 
#34 ·
Sorry again, more myths. Every thing that you change for the engine, inlet and outlet restrictions, they all affect the amount of air going into and out of the engine. That alters the A/F ratio needed, that is, available by the existing PCM tune. The PCM is not keeping up with the new air flow.

Modern PCM's do a fine job of adjusting timing and fuel, up to a point. They are not perfect, and actual on road retuning of the programming is the ideal way to get it right.

Yes of course minor changes do not require a new tune, but that does not mean that the existing tune is as good as a new tune. It only mean that not much is being left on the table, potential.

Back to the topic, the OP said he has less power now with the mufflers removed. I believe him, he is not lying or making that up. He is down on power because he has altered the exhaust enough that it's running leaner than the PCM can compensate for.

That is the point, the "magic" charts you quote suggest that he is wrong, that he doesn't need A/F ratio adjusting. I am saying that that is wrong, he is having a loss of power, and it's because of the A/F ratio need(change) of the engine.
 
#35 ·
lol If he was at his limits for richness on the engine he would throw a Check engine light. Just an fyi. This isnt a pre OBDII mustang. I believe these cars will add fuel up to 25%?
 
#36 ·
There are tens of thousands of people with flashers. Those people all gained a good chunk of power, yet they are not all highly modified vehicles.

The fact that you can gain something like 10-25hp in any stock Ford OBDII application with a flasher, that tells you that the OEM programming needs help. The stock PCM programming is not that good, if it was then there would be no gain from an SCT Excal on a stock vehicle.

Let's not derail this too far. The original poster of this thread brought up an excellent question. He has lost a little grunt by removing the mufflers, which reduced his exhaust restriction(leaner) somewhat. How much isn't important, that he can feel the difference, it's noticeable, that's important. I'm not suggesting that he immediately spends $400 on a flasher just to get back the power and more from changing only mufflers.

Most exhaust changes people make are not one time changes. People change often, to get a better sound or to alter the tip appearance etc. I think that he should think about his exhaust and decide what he wants it to sound like etc. Make those decisions and get it done to suit him, and then think about a tuner.

Everyone has a different budget, with the kinds of modifications possible these days we all need to spend wisely. Set a budget, plan it out, include PCM programming and try to do that last. Regards,
 
#39 ·
That's what I was thinking, that the A/F ratio has been affected by opening up the exhaust flow by removing the mufflers. And you're also right, I'm still toying with the idea of different mufflers I just haven't decided which ones to go with since I'm on a tight budget. I was looking for cheap good sound which I have and now I've lost some off the line grunt. I also don't have that kind of money for a flasher to adj the A/F ratio so I guess I'll live with it for now until I can decide on the exhaust, unless there is another cheap way to adj the A/F ratio. I've read on another post about turning the TPS to make the voltage at idle higher, .98 volts, instead of the factory setting of around .75 volts, to make it run richer. I don't know if this is true, but I am replacing my TPS because the idle voltage is only .63 and WOT is 3.63, but that is an entirely different thread I started.:D
 
#38 ·
I did glance at that link after you mentioned the #16 post again. I read the first sentence that ends with "...needs backpressure to work correctly...." and didn't see a need to read much further. He is saying there that a stock engines needs back pressure, which is wrong.

I agree that we are on the same page, that link due to the quote I posted is not nearly as good as it could be. I am no writer by any means, I often use words that don't exactly say what I mean. The "Adapted from Thomas V." link should not use those words that I quoted. He essentially says that the back pressure is needed with his words, though we know that isn't what he meant to say. He should have worded that differently. Regards,
 
#40 ·
doesn't our cars computer adjust the A/F ratio automatically? I've covered my intake with my hand(partially) before and the engine will slow down and almost quit then it will adjust and come back to regular idle, when I remove my hand it will idle a little higher and then come back down again.

is that not the computer adjusting A/F to compensate for the lack of air when covered, and brief lean condition when uncovered again?
 
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