is there a hp diffrence with tailpipes vs. turndowns

[93calcobra]

i today just added some tailpipes definitely sounds different are there any hp advantages or disadvantages with the tailpipes on ?

[Dark_5.0]

You definitely make more torque with tail pipes. It really all depends on the engine. 5.0's usually run better with a lil' back pressure.

[Ron1]

During a chassis Dyno session last year, one of the things measured was exhaust back pressure. With a full exhaust system in place it measured 0.018. The object (as far as I understand it)is to get as low a number as possible. I have had a number of people tell me that a little bit of back pressure is good, but I have yet to find an explanation as to why it is good. Let me qoute from an article on exhaust technologies.."During the exhaust stroke, a good way for an engine to lose power is through back pressure. The exhaust valve opens at the beginning of the exhaust stroke, and then the piston pushes the exhaust gases out of the cylinder. If there is any amount of resistance that the piston has to push against to force the exhaust gases out, power is wasted. Using two exhaust valves rather than one improves the flow by making the hole that the exhaust gases travel through larger."
And from an article by Dr.Gas on back pressure.."Re-synchronizing the exhaust pulses reduces back pressure, cancels sound, and provides additional scavaging." again talking about improving power by reducing back pressure.

Ron

[Coupe50h]

I saw no losses or gains, i just put my mac chrome's back on for looks, went to track and still ran in the 101.30 range, without them i went 101.40, but i doubt the difference was enough to feel.

[93calcobra]

thanks for the info just a matter of getting used to them

[MiracleMax]

The only way to lose power with zero back pressure is to over scavenge the cylinder.

The exhaust system works like an intake manifold with resonant tuning. so whenever you merge the exhaust pulses and/or change the dia of the exhaust you generate a different amplitude of pressure wave based on its distance from the exhaust valve

[Chris_H]

I have been seeing a lot of messages lately with people talking about turndowns. I have been thinking about it a lot lately too, especially since these new slicks I have rub the pipes. My question is this: do you put the turn down right after the muffler, or do you shorten your exhaust some first? the reason I ask this is because my mufflers are about a foot at the most from my differential, and so I wouldn't have much pipe from the muffler before it has to turn down. What has everybody else done with theirs?

Thanks
Chris

[Ron1]

MiracleMax is exactly right. That can cause lean outs and if running N2O it can cause...well you know. On my exhaust, right behind the mufflers, I have a turn down that sits at a 45 degree angle, and is aimed directly at the slicks. When I do a burn out, it blows the tire smoke right of the slicks and out the back. Works really well. Never have a car full of smoke.

Ron

[93calcobra]

i miss my turndowns

[Dark_5.0]

The need for back pressure on a 5.0 is the reason you dont see people putting 3 inch exhaust on a stock stang. It would slow you down.

Originally posted by Dark_5.0
The need for back pressure on a 5.0 is the reason you dont see people putting 3 inch exhaust on a stock stang. It would slow you down.

I agree. I used to run turn downs for a while...when I was younger, dumber & wanted to have as loud of exhaust as possible. I reinstalled the tailpipes to help quiet it down...& gained a bunch of low end torque back. However, my 1/4 mile time didn't really change but there are so many variables when running at the track that that small amount of torque I gained back may not have shown up on my time slips due to track conditions, weather, inconsistency in my driving, etc.

[Ron1]

I will ask again...what makes a "lil" bit of back pressure a good thing? You tune your exhaust for what you have. Any resistance in pushing the exhaust gasses out, robs power. And how do you build back pressure? Start putting some crimps in the tail pipes until you measure a "lil" bit?
Do you try 5 different exhaust systems until you get what you think is an adequate amount of back pressure? And if so, what is that? A "bunch" of torque? Dyno sheets by any chance?

Ron

[Chris_H]

Ron,
Since it seems like you know quite a bit about the turn downs, here is my question to you. If you are familiar with the Fox bodies and their exhaust, like I stated in my last message on this topic, I don;t have much room after my mufflers for a turndown. Is there a certain length that they probably should be, or can about 6-8 inches of pipe be enough for the turndown? I don't think I can get that much pipe in there with how close the mufflers are to the differential. I have debated doing turndowns for a while, but now with these bigfoot tires I have for slicks, I need the pipes taken off. I hate tying them up at the races, looks all ghetto.

Thanks
Chris

[84LX89GT]

This is the reason i've always heard that a little backpressure is good.
It's the same reason you don't throw a Tunnel Ram Intake on a stock engine or a 290* duration .550" lift cam in a stock non-HO F250.
Resonance tuning, and torque range is the key reason. That's why it's recommended to have a crossover pipe as well. It matches the tuning and scavanging frequency of the engine. While one engine may pick up top end with little to no back pressure, another one may not have enough cylinder pressure and airflow to make up for the scavanging losses from having scavanging pulses eliminated at low rpm's.
That's why you hear that on a stock engine it's better to have smaller primary headers. It changes the rate and strength of the exhaust pulses and also the retention of heat within the exhaust helps keep the exhaust moving as well (loses less energy through cooling the exhaust).
If you stick a restrictive single exhaust on a vehicle and design the rest of the engine around that, the operating range will be at a lower rpm band because of the rates of scavanging in both the intake and exhaust. The will provide a wide torque range, but with the top rpm's suffering. If you were to open one side up excessively and not the other, chances are you will kill any kind of lowend the vehicle has because it cannot keep up enough airflow even at WOT to have proper reverberation rates. That's why you hear of people throwing on 1 3/4" long tube headers with 3" pipes and having a SLOW car. Or why a stock honda civic with 3" single exhaust can't get out of it's own way.
This is also an argument for proper cam selection. if you have a long overlap, you're better off on an engine that can move lots of air with opened up intake system and exhaust. If you have little overlap, the torque band occurs earlier and you want to match the airflow on both the intake and exhaust to the camshaft.

There is no set rule for how much back pressure you need, but if you're operating your engine at 4,000+ rpm at the dragstrip, you really don't need the torque range provided by lower flow components.
I hope this makes sense and is accurate, can anyone back me up on this?

[Ron1]

6 to 8 inches is plenty before the turn down. I looked at mine and it's about 3 inches before it makes the 45 degree bend. As far as back pressure, any, will rob power. What we should be talking about is velocity. The larger the exhaust, the velocity is absorbed by the larger pipe and slows things down. Does that create back pressure? I don't know if that can truly be defined as such. I'm not an expert but I do know this, the engine builders I have been around and some really good ones, believe that you want it freeflowing with VERY low or no backpressure. I think about it in very simple terms by visualizing what is happening inside that air pump. If it takes force to scavenge the exhaust, that is power being lost.
I think that this is why we use crank scrapers and windage trays, titanium keepers and valves, ultralite lifters and knife edge our cranks...to get weight out of the rotating assembly. It will RPM quicker and get into the power band quicker. The last thing I would want is to make some of the pistons work against a force that will keep them from reaching TDC. It can't be good for the motor.

But I have been proven wrong before...but let me leave you with a quote from David Vizard (Volume 1 ) "How to make HP: " Over the years I have heard many strange claims from so called experts. One I have heard proclaim more than once that a performance engine or any convential 4 cycle engine-needs some backpressure to optimize efficiency. That statement was obviously made by someone who had never used a dyno or had not carefully evaluated how exhaust systems flow affects performance. Let me assure you that by minimizing exhaust sytem backpressure, 999 out of 1000 times you will find more power. Keep in mind however that the air/fuel ratio and ignition timing are carefully optimized"

If you don't know who David Vizard is...well never mind.

Ron

[84LX89GT]

You may find top end power...but will it reduce low rpm torque. Are you talking about just race engines? On a race engine you are tailoring the power band for maximum high RPM power and torque and not worrying about gas mileage or engine efficiency at lower rpm's. backpressure brings the exhaust pressure waves closer together and can be used to tune the engine to a certain power band. The effects of backpressure as a positive are overruled when you go to WOT with a high flowing engine at higher RPM's.
You can make cars go much faster by almost eliminating back pressure, but some heavier cars and low hp engines don't respond well to eliminating all backpressure. I don't think backpressure is near as important as pipe diameter, tube length, crossover locations. I'm not talking about having enough backpressure to push exhaust back into the combustion chamber (like log style exhaust manifolds to), but rather a slight opposition to absolute free flow that can be used as an aid in tuning the airflow/scavanging of the engine.

[Ron1]

Race engines only? No. If you read the quote from David Vizard, he said all 4 cycle engines. I think what you are talking about is what he calls resonant tuning (not backpressure). That occurs in the headers, when it creates a low pressure pulse. Quote.." This reflected wave-flow initiates air-fuel flow into the cylinder through the open intake valve before the piston begins the intake stroke."
You may want to do some reading on the subject by picking up his books. He has been doing this since 1968 and has written 100's of books engine performance including extensive testing on exhaust systems.

Ron

Ron1-
It looks like your Mustang is a track only car, am I right? Well the majority of the people on this site...I would say...have street/strip stangs. Of course if you are on a dyno and want to acheive maximum horsepower, you wouldn't want much back pressure, if any. But that is at wide open throttle. I for one have a Mustang that goes to the track maybe 6-8 times a year & sees a lot of street driving on the weekends. No I don't have dyno sheets to prove that my tailpipes gained me back some low end torque (now remember...that would be at lower rpms...not WOT), nor would I spend the money to do a dyno tune just to see the difference in turn downs or tail pipes. It seems very easy to understand how an exhaust system with some backpressure could benefit a street driven Mustang. Because...if you didn't realize...when you are driving on the street you are not always at WOT. I'm surprised you press this issue so. If you were correct than why isn't every Mustang at the track running open headers (hopefully long-tube ) to gain their combinations full potential??

[Ron1]

Pressing? Who pressing? Just a lively discussion. The FB is a track car, yes. But my Coupe was not, daily driver. Mandrel bent pipe, 2 1/4 exhaust, flow masters and long tube headers. When you ordered your exhaust did you request mandrel bent pipe? All I am saying is that if you only do the exhaust you are looking for trouble. Every thing changes or has to change. A/F ratio, jetting, timing...
And as far as me being correct? I am just quoting some passages from someone whose opinion I respect. I am NOT an expert. You can write DV through his web site directly www.davidvizard.com and tell him what you think.
My last question is how do you adjust back pressure? They measure it. Oh, oh..too low. Now what?

Ron

[Nixon1]

I've read up on a long discussion on this stuff on my old board for my old car..ThirdGen.org... From what I've heard and thought up myself, here's my best guesses on this. Open headers is the best exhaust system. They've always got good diameter to them, and allow the exhaust gases to rapidly escape. Here's WHY an exhaust SYSTEM with too large diameter doesnt work good. What happens when air hits a pocket? It expands, obviously. When it expands, it becomes less dense. Denser air moves quicker. Thus, air in large tailpipes slows down...which also causes it to cool down more in the pipes. This creates an obstruction to additional airflow, thus by INCREASING your tailpipe diameter, by my estimation, you're actually INCREASING your backpressure as well!! So...open headers, you're good to go if you can get past the backfiring issues! Exhaust system..keep the diameter limited...2.5 seems to be the best for almost any car out there, whether it be V6 or V8. 3 in. is good for heavily modded cars...otherwise, I wouldn't try it..loss of bottom end torque is too bad to justify it.