minesfaster

iTrader: (24)

When i had my 1994 firebird formula i gutted my cat, and had a flowmaster muffler and right before the muffler i had a cutout. When i would have it open i had a major loss in bottom end.(Couldnt even peel out). But when it was closed it all came back.I could light em up all down the street. Does this have anything to do with backpressure ? Why did that happen ?

Also on my setup i have now i have a 2000 z28 with gutted cat's and a dynomax bullit in the i pipe do i have enough backpressure ?
Sorry im new with the whole backpressure issue.

blkZ28spt

Quoted from the third post.

spy2520

iTrader: (3)

thats funny i have no cats and LTs going through a cutout and with it open i can burn tires til they're bald, closed cutout for me is much more tame, might be because peak torque RPM is close to 4000 and i can get it revved up so quick (i'm guessing due to less restriction).

Newbiewar

well, all in all, back pressure is unwanted... although that is how the 4 stroke engine is based upon, so we must work with it... this is the reason different cams give you different power bands, because they are manipulating the back pressure to where you want it.

Back pressure, if you took off your muffler, you have less back pressure and more horsepower, because your engine has to do less work to get the exhaust out of the cylinder. if you take the cat off as well... again more power, less work... easier to get the exhaust out of the cylinder...

Back pressure is unwanted, but essential in todays vehicles...

300bhp/ton

This is technically true to an extent.

First off, to understand the WHY about back pressure lets start at the beginning.

HP is only an expression of a measurement of torque.

hp = torque (lb ft) X rpm / 5252

So this means there are two ways to increase HP.

1. Increase the amount of PEAK torque being produced at the same rpm. Larger capacity or FI can acheive this easily.

2. Move the PEAK torque higher in the rpms, but with out the actual number changing. Cams and bolt ons will often affect this.

As a rule both factors will influence the results, dependant upon which modifications are carried out.

This is an important factor where exhaust tuning is concerned, so that you can understand the results of any changes.

Back pressure is a cuase of a restriction in the exhaust system. i.e. it can't flow enough so a build up of pressure is greated upstream (towards the engine). This back pressure restricts the breathing capabilities of the engine, limiting the air intake.

If you remove some of the exhaust restriction, thus lowering back pressure you will generally move the PEAK torque higher in the rpms. It will often feel as a lack of low end grunt, but usually it is just a re-adjustment of the actual PEAK without actually lowing anywhere in the rpms.

But by reducing backpressure and allowing the engine to breath better it also alters the fueling. So any drastic changes to the exhaust can produce negative results on a stock tune. If you recalibrate the fueling requirements to match the less restrictive exhaust you will increase BHP output.

In addition, is radical changes are made to the exhaust, then the intake system will also need to be modified to ensure the intake flow is also increased, else you will just have wasted potential.

So the answer is YES in some cases, back pressure is a good thing.

Now going back to the equation for HP, if you want to keep a good low end and lots of torque, your best bet is to keep the PEAK torque low in the rpms. This of course will affect the HP reading as well. A more restrictive exhaust with greater backpressure will help promote a strong bottom end. It will however then to run out of breath in the higher rpms.

So match the exhaust for the requirements, if it's a truck and is used for towing, then low down grunt is often more useful than high rpm power.

Conversly, if an exhaust is to free flowing you may still lose out. As the lower end torque will suffer as a result. Also if you are running stock heads and/or cam(s) your engine may not be able to take advantage of the free flowing setup.

In order to produce a free flowing low loss (potentially zero loss) exhaust, use this equation as a guide:

2.2cfm for every 1hp

So, if you engine is making 500bhp, you'll need to have a minimum of 1100cfm flow capacity from your exhaust. Any more than that and it potentially will be over kill and may not produce as good results.

LS1-450

iTrader: (33)

[QUOTE=BackinBlack02SS]I think you are confused[Quote]


I'll take the HP/TQ gains that resulted from making no sense & being confused over being an expert any time.

SouthFL.02.SS

More restrictive exhaust: Less peak torque, yet peak available lower in the rev range (feels stronger- 'cause the tq comes on sooner, but it's slower).
Less restrictive exhaust: More peak torque, yet peak is available higher up in the rev range (faster, yet butt-o-meter doesn't seem to think so).

bigradical

iTrader: (3)

When I did my headers and ORY I lost a good bit of bottom end but man does she pick it up when she gets rollin'.

DaddySS

Well I'll have to disagree.

What you're saying is let the exhaust compensate for the duration and overlap in your cam at the expense of the higher RPM peak Torque and HP.

I say size your cam correctly and then your exhaust correctly for good velocity and exhaust pulse characteristics. If you want better down low torque, go with a shorter duration cam.

I have never read from any of the exhaust "experts" that back pressure is good. They all say that the proper sizing in both capacity and length is important for flow, velocity, and exhaust pulse timing.

300bhp/ton

2 points.

1. I never mentioned cams? That's a whole different area, for a basic bolt on car work within the tollorances of the stock unit unless you already know what you're going for.

2. Backpressure is only good on a STOCK or near STOCK tune. If you mess with the fueling then the motor probably won't run as good, which is what a free flowing exhaust 'can' do - simple.

Get a tune and it'll be fine and make more power than stock.

If you want performance, you want low back pressure, but if you don't have the rest of the car working with it, it will only produce minimal gains or even degrade performance.

I see it a lot, most Chav's have no idea about cars and only ever mod them badly, usually lowering performance, instead of imporving it.

300bhp/ton

here ya go, from Random Technolgy:http://www.randomtechnology.com/universal.html

minesfaster

iTrader: (24)

NIce there is a lot ot learn in this thread about backpressure. ALso (BIGRADICAL) i hear ya about losing it down below and when she gets going oh yeah.

I1ULUZ

iTrader: (1)

"Before we delve into the dark art of exhaust theory, let's take a quick journey through the exhaust system from the perspective of the exhaust gases.

As the piston approaches top dead center, the spark plug fires igniting a fireball just as the piston rocks over into the power stroke. The piston transfers the energy of the expanding gases to the crankshaft as the exhaust valve starts to open in the last part of the power stroke. The gas pressure is still high (70 to 90 p.s.i.) causing a rapid escape of the gases (blowdown). A pressure wave is generated as the valve continues to open. Gases can flow at an average speed of over 350 ft/sec, but the pressure wave travels at the speed of sound (and is dependent on gas temperature). Expanding exhaust gases rush into the port and down the primary header pipe. At the end of the pipe, the gases and waves converge at the collector. In the collector, the gases expand quickly as the waves propagate into all of the available orifices including the other primary tubes. The gases and some of the wave energy flow into the collector outlet and out the tail pipe.

Based on the above visualization, two basic phenomenon are at work in the exhaust system: gas particle movement and pressure wave activity. The absolute pressure differential between the cylinder and the atmosphere determines gas particle speed. As the gases travel down the pipe and expand, the speed decreases. The pressure waves, on the other hand, base their speed on the speed of sound. While the wave speed also decreases as they travel down the pipe due to gas cooling, the speed will increase again as the wave is reflected back up the pipe towards the cylinder. At all times, the speed of the wave action is much greater than the speed of the gas particles. Waves behave much differently than gas particles when a junction is encountered in the pipe. When two or more pipes come together, as in a collector for example, the waves travel into all of the available pipes - backwards as well as forwards. Waves are also reflected back up the original pipe, but with a negative pressure. The strength of the wave reflection is based on the area change compared to the area of the originating pipe.

This reflecting, negative pulse energy is the basis of wave action tuning. The basic idea is to time the negative wave pulse reflection to coincide with the period of overlap - this low pressure helps to pull in a fresh intake charge as the intake valve is opening and helps to remove the residual exhaust gases before the exhaust valve closes. Typically this phenomenon is controlled by the length of the primary header pipe. Due to the 'critical timing' aspect of this tuning technique, there may be parts of the power curve where more harm than good is done.

Gas speed is a double edged sword as well, too much gas speed indicates that that the system may be too restrictive hurting top end power, while too little gas speed tends to make the power curve excessively 'peaky' hurting low end torque. Larger diameter tubes allow the gases to expand; this cools the gases, slowing down both the gases and the waves.

Exhaust system design is a balancing act between all of these complex events and their timing. Even with the best compromise of exhaust pipe diameter and length, the collector outlet sizing can make or break the best design. The bottom line on any exhaust system design is to create the best, most useful power curve. All theory aside, the final judgement is how the engine likes the exhaust tuning on the dyno and on the track.

When considering a header design, the following points need to be considered:

• 1) Header primary pipe diameter (also whether constant size or stepped pipes).
• 2) Primary pipe overall length.
• 3) Collector package including the number of pipes per collector and the outlet sizing.
• 4) Megaphone/tailpipe package.

There are many ideas about header pipe sizing. Usually the primary pipe sizing is related to exhaust valve and port size. Header pipe length is dependent on wave tuning (or lack of it). Typically, longer pipes tune for lower r.p.m. power and the shorter pipes favor high r.p.m. power. The collector package is dependent on the number of cylinders, the engine configuration (V-8, inline 6, etc.), firing order and the basic design objectives (interference or independence). The collector outlet size is determined by primary pipe size and exhaust cam timing.

For more detail on the specifics of header theory read ‘The Scientific Design of Exhaust and Intake Systems' by Phillip H. Smith’ "

Then go read this: http://www.popularhotrodding.com/eng...exh/index.html

DaddySS

Ummm 2 points:

1. These guys sell converters. No reflection but I hardly expect they are going to say no cats are good.

2. If the argument is that because of backpressure the car is tuned accordingly, it still doen't mean backpressure is good, it just means that as you said, if you're going to improve the exhaust, improve the intake to match.

mrr23

iTrader: (3)

sure it is. look at all the NHRA cars running open headers and blowing the superchargers right off the motor. there's also the theory of burning up the exhaust valves if you run without any type of exhaust manifold/header. you know, the cold air warping the valve.

mazdized

Back pressure is not wanted but velosity is good. I used to play with rotary engines and since they don't have valves they are very touchy on exhaust set up. Small exhaust will have good velocity at lower engine speed which helps torque and economy but often times it become restrictive at higher engine speed. So if an exhasut system is design for lets say 5k rpm and if you stay with in that range it is fine, but any where above that back pressure may start to increase. Now on the opposite side if you have a exhaust system designed for 8k rpm plus and the highest engine speed is under 5k rpm then the motor will most likely never get into the range of having good velocity going. I think exhaust needs to go with the engine combo and operating range just like intake and cam. Warping valves... well think about it a drag race engine does not see vacume until shut off so exhaust ports always see pressure and no vacume. Now if you street drive a car you will see a lot of off throttle and possible low pressure in the exhaust ports so they may suck some cold air back in if the cold air is close to the exhaust ports.

spy2520

iTrader: (3)

so how the hell can i find out my exhaust flow rate?? i've determined based on 310 rwhp that i need 682 cfm or something like that. How can i find out the flow rate of my exhaust??

Stark

iTrader: (25)

300bhp/ton

easy way. 310rwhp is going to be circa 350bhp at the engine.

so 2.2 x 350 = 770cfm flow rate.

Now lets assume you'll add a few more mods and get it upto ~400bhp, thats about full bolt on for an Fbody.

400 x 2.2 = 880cfm

You don't want to go too big, but allow room for future mods.

Or an easier way.

3" Y setup for 500bhp or 2.5" dual.

So you should know what size tubing you have and whether its a Y or dual.

Find out what muffler you have and speak to the manufacture and ask them what it's flow rate is in cfm, or more precisly scfm (use Google if you want to know more about the rating).

If it's a Y setup, then all the gases go thru 1 silencer so it must flow at the minimal of the total flow rate. Here 880cfm.

Lots of people make silencers which are good enough, Borla, dynomax my fav Powerstick. But remember not EVERY muffler from one company flows the same.

The Flowmasters people complain about only flow under 300cfm (if memory serves) which is why they rob power. A Hooker aero chambers (from the cat back) is good for upto 375bhp, so only a few bolt ons will max it out, with a Y setup.

Duals are different, as each bank only has to contend with 4 cylinders, so simply half the required flow capacity. In this case 440cfm would be sufficent.

This is why the same Flowmaster work fine on Mustangs, they have duals. So the muffler only deals with gases from 4 cylinders and Mustangs generall produce 230-290rwhp, so less than your typical Fbody.

If you have too much flow from a muffler it will just make the exhaust louder and yield no extra power.

A 3" Powerstick flows around 2000scfm (YES it is that good), so these can be used in a Y pipe no problem. In a dual setup it would be wasted potential and you'd be better off with the 1000scfm 2.5" muffler. As it wouldn't loss any power on a 500bhp motor, but would more affectivley muffle the sound.

SouthFL.02.SS

Trusty old list:

Heres HP/Noise Level:

The results in HP order:
1.Flowtech Warlock (open) 374.2 hp/ 333.1 tq
2.Bassani Real Street 373.7 hp/ 333.8 tq
3.Hooker Maxflow 373.5 hp/ 333 tq
4.Borla XS 373.3 hp/ 332.6 tq
5.Magnaflow 372.8 / 332.5 tq
6.MAC 372.3 hp / 331.5 tq
7.Flowtech Afterburner 372.3 hp/ 330.1 tq
8.Hooker Aerochamber 372.1 hp/ 330.4 tq
9.Bassani 372 hp/ 333.5 tq
10.Spintech 371.6 hp/ 332.2 tq
11.Edelbrock Performer RPM 370.9 hp / 331.3 tq
12.Borla XR1 370 hp/ 334 tq
13.Flowtech Terminator 369.5 hp / 331.3 tq
14.Dynomax Ultra Flo 369.4 / 333.2 tq
15.Flowmaster 369.4 hp / 331.8 tq
16.Flowtech Warlock 366.3 hp / 325.3 tq
17.No muffs 365.2 hp / 330.1 tq

Sound Off in Quietest to Loudest (Decibels):

1.Borla XS, 80db at idle, 90db at 2,000 rpm, 110db at WOT.
2.Hooker Maxflow, 80db at idle, 90db at 2,000 rpm, 120db at WOT.
3.Magnaflow, 82db at idle, 91 at 2,000 rpm, 114db at WOT.
4.Flowmaster, 82db at idle, 93 at 2,000 rpm, 115db at WOT.
5.Flowtech Warlock, 83db at idle, 92 at 2,000 rpm, 112db at WOT.
6.Dynomax UltraFlo, 83db at idle, 94 at 2,000 rpm, 113db at WOT.
7.Borla XR1, 83db at idle, 96db at 2,000 rpm, 118db at WOT.
8.Bassani Street, 83db at idle, 96db at 2,000 rpm, 120db at WOT.
9.Edelbrock RPM, 84db at idle, 93db at 2,000 rpm, 118db at WOT.
10.Bassani Real Street, 84db at idle, 96db at 2,000 rpm, MAX. (120+db).
11.Flowtech Afterburner, 86db at idle, 94db at 2,000 rpm, 115db at WOT.
12.Flowtech Terminator, 86db at idle, 94db at 2,000 rpm, 119db at WOT.
13.Hooker Aerochamber, 87db at idle, 94db at 2,000 rpm, 114db at WOT.
14.SpinTech, 87db at idle, 97db at 2,000 rpm, 116db at WOT.
15.MAC, 87db at idle, 98db at 2,000 rpm, 119db at WOT.
16.No Muffler, 91db at idle, 103db at 2,000 rpm, MAX. (120+db).