Optimal exhaust pipe 2

[LJW]

I am looking for information about the design of exhaust systems. My goal is to improve MPG. I want to keep modifications limited to exhaust pipe and muffler without the use of tube headers. The vehicles in this experiment are a 1994 Ford van (300ci 6cyl)and a 1995 Geo Metro (61ci 3cyl). Is there a point when the reduction of back pressure becomes detrimental? Will lower pumping losses force changes in A/F ratio?

Thanks,
LJW

 

[MaxRaceSoftware]

with the correct diameter and total length exhaust system,
you can improve torque/hp in approx. a 1500 rpm or greater
RPM curve, this will increase MPG if its correctly sized for the RPM range you need

i can't see anytime when "BackPressure" would benefit

if you lower pumping losses on the exhaust stroke
you could sometimes see a "leaning" of A/F Ratio
"inside cylinder" , as small part of fuel is wasted out
exhaust port during overlap .

you could try to purchase/find a copy of
Scientific Design of Intake and Exhaust Systems
by Smith & Morrison


Larry Meaux (meauxracing@mindspring.com)
Meaux Racing Heads - MaxRace Software
ET_Analyst for DragRacers

http://www.mindspring.com/~meauxracing/

Support Israel - Genesis 12:3

 

[americamper]

while I only know enough to be dangerous.most exhaust use some type of scavenge effect where alittle back pressure helps torque.I believe flowmaster uses this type principle in there street systems.

 

[gyjoe]

for maxracesoftware

i have only stumble onto this forum like just now, and was going through few pages of posts.

>>i can't see anytime when "BackPressure" would benefit
i have been reading your posts on exhaust pressure etc and begining to understand how "backpressure" is not beneficial. question is i have some friends who reported an improvement in low-end torque after they have put back their catalytic converter (which they have removed and put in a straight through pipe). an you enlighten me pls?

>>if you lower pumping losses on the exhaust stroke
you could sometimes see a "leaning" of A/F Ratio
"inside cylinder" , as small part of fuel is wasted out
exhaust port during overlap .

How would you actually reduce pumping losses? also, what are the effects of the leaning of the A/F ratio as compared to running rich. correct me if i am wrong, but does this occur due to low pressure when the exhaust valve is open, thus leading to wastage of fuel?

 

[MaxRaceSoftware]

Exhaust "Back Pressure" = benefit to Torque and HP ???

in my opinion , and from all the scientific data
i've read on exhaust systems, and from my dyno testing experiences over the years ...i believe the
"concept" of increasing exhaust system BackPressure
as to increase Torque/HP is a Myth .


Another concept or Myth i've seen on my dyno
is racers painting a long line across length of collector
and where line discoloring stops, they cut the collector length to .....so we REMARK the collector again with the paint mark and make another dyno test ....this time the discolorization mark has moved upwards...so the racer cuts
the collector again at that point..and we again
remark the collector with a fresh mark ...again we redyno test , again the discolor mark moves upwadrs again....
..all this time the engine is losing Torque/HP and
if you keep doing this, you will have no headers left
and no Torque and HP


"use some type of scavenge effect where alittle back pressure helps torque" - americamper

using "scavenge effect" and "back pressure"
together seems a little contradictory

you get the most "scavenge effect" when exhaust system pressure
goes below atmospheric pressure during overlap
----------------------------------------------------------------


Temperature greatly influences the speed of sound

The speed of sound greatly influences the wave-tuning effects during overlap period

1-Catalytic Converter runs hotter , keeps speed of sound higher
the Factory "tunes" the original OEM exhaust system's total length
with the effect of the Converter in place

There exists temperature differences
with the Catalytic Converter -vs- the straight pipe
in the total exhaust system length which affects the speed of sound
and wave-tuning during overlap period.


2-There exists volume and diameter differences between Converter
and straight-thru replacement pipe
which also affect the speed of sound and wave-tuning during overlap period .


i can't see anytime when increasing the actual PSI pressure
(BackPressure) inside an 4-cycle exhaust system will increase Torque or HP
Are you sure you are using the term "BackPressure" correctly ?
...... or all you are really seeing is the effects of
changing wave tuning during the overlap period
by making changes that affect exhaust system's length, diameter,
and operating temperature .

From a maximum Torque/HP point of view, the whole goal of an exhaust system
is to get rid of ALL the burnt exhaust gases with least pumping losses
and to cause a negative wave to be centered on the overlap period
to increase intake system velocity in the desired RPM-range.

---------------------------------------------------------------------

"How would you actually reduce pumping losses?"---gyjoe

you can reduce pumping losses a few ways by ;
reducing ring friction (ring tension , oil)
reducing intake flow restriction
reducing exhaust flow restriction

on the exhaust side by reducing exhaust flow restriction
and increasing vacuum or below atmospheric pressure
during the latter part of exhaust stroke till end of overlap.

you use some of the exhaust vacuum during overlap to start
and move the intake mixture before the piston moves downward.
--------------------------------------------------------------------

you can waste some air/fuel mixture out exhaust port
during overlap period .

when the intake valve closes , the air bounces back to plenum , but some small part of
fuel wets out , and stays behind intake valve....when intake valve 1st opens
during a very low pressure during overlap period , some of it can go right
out of exhaust port.
-------------------------------------------------------------------------


another thing to think about is ;
why doesn't the incoming air/fuel mixture instantly catch on fire
during overlap period ????
on a typical engine , if you put in an exhaust temperature probe,
you might see reading between 1200 to 1500 degrees F or so .

even with a large roller cam with a lot of overlap period ,
you don't see this engine constantly shooting out flames out of the carb
under load even at a relatively low rpm range

so this tells you that after the initial exhaust valve
opening and exhaust blowdown , there exists cooling effect from expansion
and quick reduction of pressure inside the cylinder , along with
further reduction of pressure during overlap period if
exhaust system is designed correctly .

if this were not actually occuring, then it would be quite impossible
for the internal combustion engine to cycle/run/work/function !

------------------------------------------------------------------------

"What are the effects of the leaning of the A/F ratio
as compared to running rich." ---gyjoe

approx 14.7 A/F ratio = stoichiometric
if you look at it correctly like IvyMike does
and ask a typical racer whats lean , they might tell you
13 or 14:1 ...but its still on the "rich side " of stoichiometric ratio

or you could say below 14.7 is rich
and above 14.7 is lean

there have been times on my dyno where a racer brings an engine to
tune and it might be making good or OK HorsePower and Torque,
but its using a lot of fuel (Hi BSFC) , and you can't cure it
or stop it with the parts the guy has , its basically a pig !

then there have been times when you put 2 heads ported by 2 different
people on the flowbench, they might have the same exact flow numbers
but on the dyno , one set makes more Torque/HP and makes it with
a bunch less fuel than the other set of heads .


Maybe Greg Locock can add some more info ??
Anyone else ?

Larry Meaux (meauxracing@mindspring.com)
Meaux Racing Heads - MaxRace Software
ET_Analyst for DragRacers

http://www.mindspring.com/~meauxracing/

Support Israel - Genesis 12:3

 

[americamper]

?maxracer your right I was putting backpressure with sound waves.I learned on another forum about the differences in normally aspirated and turbocharged.turbo's want as big a exhaust leaving turbo as possible and straight thru exhaust where as normally asperiated street vehicles can use the scavenge effect(this is as I understand it).also regarding the intial flow of intake air when intake opens the mixture would have to meet the minuimun lel to flash.On the ported heads it would seem to me that the cylinder head that was the most consistent in diameter with the straightess runners in carb version how it is received by the chamber and piston crown.one more,when you decrease the primary tubes are for that matter putting any tube causing intial back pressure.I know you want to maintain velocity but that depends on your usable rpm

 

[gyjoe]

wow, that's a whole lot of info to digest...

regarding reducing pump losses via reduction of exhaust restriction:

what do we mean by tuning of exhaust? is it varying the length/diameter of primary header so that like u said there is low pressure when the exhaust valve open? if this is the case, i have some questions that i can't really figure out.

1. the pressure transient at the exhaust valve caused by the closure of the valve is like a square wave (let's ignore losses for the moment) of high and low pressure with a period of 4L/a (L: length of primary, a: speed of sound). how do one tune the exhaust such that the low pressure coincide with valve opening? do we want the valve to be opened again in the same period of the wave, or do we want it to coincide with the low pressure in the following periods? cause i can see how one can specifically go for a particular pressure when the distribution of the low/high pressure over time is like 50/50. am i making sense? or have i over simplified?
2. comparing 4-2-1 and 4-1 headers, how is 4-2-1 better for low-end compared with 4-1. if assuming 4-1 has a longer L than 4-2-1, how would the shorter period (pressure transient) of the 4-2-1 help?

pls correct any mistakes

 

[evelrod]

Larry, a simple case where I increased backpressure to good effect---a high compression racing engine used in a street car and pump gas. Lots of pinking and knocking, even with the highest octane fuel available. I altered the camshaft timing and ign timing a bit and put a couple of smallish diameter rather trashy mufflers (made it REAL quiet) on the little beast and, voila, knock all gone. Power was probably adversly affected but, with all the extra (surplus) horsepower he had, who could tell!!! If I had it to do over, I could possibly hook up some type of EGR to accomplish the same thing. It was a LONG time ago and EGR's weren't around.


Rod

 

[MaxRaceSoftware]

Rod, you probably saw the same effect as an EGR-valve .

i've had a few experiences with Ford's EGR-valve
usually if you disconnect it on an OEM engine,
it will "ping" .

Rod, you probably were just getting exhaust gases
pushed back into cylinder/chamber during overlap

i guess there are a few circumstances when backpressure
might help you ?? or is it really the correct "fix"
to solve the problem ?

wouldn't it make more torque/hp , if every variable was
engineered correctly to work together ?


Larry Meaux (meauxracing@mindspring.com)
Meaux Racing Heads - MaxRace Software
ET_Analyst for DragRacers

http://www.mindspring.com/~meauxracing/

Support Israel - Genesis 12:3

 

[MaxRaceSoftware]

what do we mean by tuning of exhaust? is it varying the length/diameter of primary header so that like u said there is low pressure when the exhaust valve open? --gyjoe
----------------------
you want the low pressure at overlap period when the intake valve is starting to "open" and the exhaust valve is
"closing"



like a square wave --gyjoe

more like a "decreasing amplitude sine-wave"



how do one tune the exhaust such that the low pressure coincide with valve opening? --gyjoe

predominately by "Pipe LENGTHS" for desired RPM ranges
and
predominately by "Pipe SIZE" for engine size CID

so what you windup with is a combination
of pipe lengths and diameters for desired RPM-ranges
and engine size CID

you can change the lengths of exhaust system
and cause exhaust to be pushed back into
chamber/cylinder ...you don't need "BackPressure psi"
to accomplish this , you can achieve it with mostly wrong exhaust system lengths.

4-2-1 should have 3 negative wave reflections back to chamber ..one when 4 goes into 2 , and another when 2 goes into 1. the exhaust gas speed should be higher
in the 4-2-1 system

4-1 should have 2 negative wave reflections
should have slower exhaust gas speed in collector than 4-2-1
along with collector temperature differences


usually trying the racing headers with both the
4-2-1 collector -VS- the 4-1 collector
on the dyno or track ..it depends upon what the collector
length was originally .

most of the competion headers made have 10 to 12 inch long collectors ....and street type headers have very much less length than this , sometimes as little as 3-4 inches length

so typically most headers , the collector length is too short..so when a racer tries a 4-2-1 header its usually longer in the collector length ...then they say it picked up torque/hp ....but if they would have compared it to a highly scienced out , correct 4-1 collector length,
there would be less gains or differences .


Larry Meaux (meauxracing@mindspring.com)
Meaux Racing Heads - MaxRace Software
ET_Analyst for DragRacers

http://www.mindspring.com/~meauxracing/

Support Israel - Genesis 12:3

 

[gyjoe]

ok, the square wave (theoretical) analogy is what i retrieved from from undergraduate notes

more like a "decreasing amplitude sine-wave" -- Larry

Is this sine-wave at the exahust valve be due to a single closure of the exhaust valve? or super-imposed from many opening and closures?

4-2-1 should have 3 negative wave reflections back to chamber ..one when 4 goes into 2 , and another when 2 goes into 1. the exhaust gas speed should be higher
in the 4-2-1 system -- Larry

I still don't understand why a 4-2-1 would be slightly better than a 4-1 for low-end power. are you saying the when the exhaust valve closes, a negative pulse is created which goes down the collector, reflect back (i know some will carry on) to the exahust valve which will by this time open again? Since the pressure transient is like a sine wave, isn't it likely that the exhaust valve will open right when the pressure is +ve and peaking? Or is this negligible as the 3 negative pulse would have a greater effect?

Why would the exhaust speed be higher in a 4-2-1 headers? smaller diameter of collectors than a 4-1?

and why would there be 3 negative pulse reflection for a 4-2-1. i counted only 2 (+1 when 4-2, + 1 when 2-1)

sorry for the whole stream of questions. i really appreciate ur answers. i can't seem to find similar sort of expertise available in my country (maybe i have been looking at the wrong places)

 

[Whittey]

On the turbo/scavenging thing..... Even with large amounts of backpressure (created mainly by the turbo) you can get the scavenging effect. 15psi of exhaust backpressure would be insane for a N/A vehicle (*shudder*) but what if you were pushing 15psi on the intake? What if you were pushing 20psi? And even if you had 12psi of backpressure on an N/A engine, you'd still have some scavenging effect from the sound waves, though you'd lose alot more than you'd gain.

As for putting headers on the EFI300, I wouldn't bother unless you put a cam in there (and got a computer change and/or swapped in 19lb injectors and an adjustable fuel pressure regulator). The stock dual cast manifolds flow well enough for any stock to near-stock 300 so the only thing you'll gain from headers is the scavenging and a ration of **** from the smog nazi's.


-=Whittey=-

 

[LJW]

Tube headers are not in the plan, just a "cat-back" modification. I have been checking various locations at the side and rear of both vehicles with a manometer, looking for a low pressure area to exit the pipes. Also plan to install pressure taps at a few places on tailpipe asembly.Anyone have a formula or ratio for intake manifold vacuum/exhaust manifold pressure targets?

Thanks Again and Again
LJW

 

[MaxRaceSoftware]

Is this sine-wave at the exahust valve be due to a single closure of the exhaust valve? or super-imposed from many opening and closures? ---gyjoe

The Sine-wave effect is due primarily to
the exhaust valve "opening" and "tunes" to exhaust system length.
---------------------------------------------------------------------------------


I still don't understand why a 4-2-1 would be slightly better than a 4-1 for low-end power. are you saying the when the exhaust valve closes, a negative pulse is created which goes down the collector, reflect back (i know some will carry on) to the exahust valve which will by this time open again?

Since the pressure transient is like a sine wave, isn't it likely that the exhaust valve will open right when the pressure is +ve and peaking? ---gyjoe



---(probably not the best visual example)
Like dropping a rock into a still pond
you will see wave peaks and valleys spreading out from source ,
dissipating over time

When the exhaust valve opens , a "Positive-pressure" wave is started
then like a sine-wave , it turns into a "negative-pressure" wave
then if exhaust system dimensions are correct,
this returning negative-pressure wave is centered on the overlap period

if exhaust system dimensions are wrong at that point,
there is a positive-wave occuring during overlap period.
the postive wave can push some of burnt exhaust gases back up chamber,
back up head and intake ports , and seen as
sooting intake port bowls, runners, and intake manifold runners and plenum.

ideally if everything is correct, you should be able to
disassemble cylinder heads and look in the intake bowl areas ,
and they should look like the day they were ported
........and not sooted-condition .

like i said, if its wrong, then you can take the carb off the intake
and you can see evidence of exhaust soot in plenum if its severe enough.
....sort of the similar visual-effect of an EGR-valve stuck or defective


You can "see" this efect on the SuperFlow dyno sheet in the
Ve% and SCFM columns
when exhaust system dimensions are wrong for a certain RPM,
then the postive pressure wave can "sometimes" make it all the way up
and out of carb before the exhaust valve closes ,
and the SF-Air_Turbine measuring device will record this action easily !

in fact, the positive pressure wave doesn't have to even make it to
the combustion chamber/cylinder , for it to hinder/hurt Torque/HP.
this can still be measured by SuperFlow Air_Turbine, along with TQ/HP/BSFC/BSAC
----------------------------------------------------------------------



Why would the exhaust speed be higher in a 4-2-1 headers? smaller diameter of collectors than a 4-1?---gyjoe

4 small pipes go into 2 pipes ...you will have more velocity this way
than if 4 pipes dump immediately into 1 large pipe

--------------------------------------------------------------------------------


and why would there be 3 negative pulse reflection for a 4-2-1. i counted only 2 (+1 when 4-2, + 1 when 2-1)---gyjoe


4-2-1 = 1 negative wave reflection from 4 to 2
1 negative wave reflection from 2 to 1
1 negative wave reflection from 1 to atmosphere
----------------
3 total (but could be more)

4-1 = 1 negative wave reflection from 4 to 1
1 negative wave reflection from 1 to atmosphere
------------------
2 total (but could be more)

Larry Meaux (meauxracing@mindspring.com)
Meaux Racing Heads - MaxRace Software
ET_Analyst for DragRacers

http://www.mindspring.com/~meauxracing/

Support Israel - Genesis 12:3

 

[MaxRaceSoftware]

Sorry LJW Industrial
for straying off your original question !



Larry Meaux (meauxracing@mindspring.com)
Meaux Racing Heads - MaxRace Software
ET_Analyst for DragRacers

http://www.mindspring.com/~meauxracing/

Support Israel - Genesis 12:3

 

[gyjoe]

Larry, thanks for the explanation. i will spend more time digesting, and will create a new thread instead if needed.

LJW, sorry for getting a bit carried off...

 

[Rob45]

To LJW:
Having done much of the original exhaust development work on your 300 C.I. Ford engine, I can tell you a couple things.
First, you aren't going to make a significant difference in anything other than your bank account with only "cat-back" modifications. There's just too much restriction in the converter, and that engine is not enormously sensitive to backpressure anyway. For best fuel economy, you want to run it at as low an RPM as you can under all conditions; for example, 1200 - 1300 RPM would be just about right on the highway. This will have you running near WOT and will minimize pumping losses, while the low RPM will reduce frictional losses. Needless to say, you want to minimize losses in the transmission: if your cruise RPM is below your torque converter lock-up speed, you'll get less than optimum economy.
To dispel one myth, however, let me tell you that there is no "optimum" backpressure for any engine. A colleague of mine did an experiment wherein he tested an engine on a dyno (a 351W) with a variety of backpressure levels, from about 1.5 atmospheres of pressure to about a half atmosphere of vacuum. Decreasing backpressure always increased output - as long as the fuel mixture was adjusted to compensate for the changing WOT airflow. This testing was documented in an SAE paper which is out there somewhere; I think this was done maybe late '70's.
So here's where that old myth started: back in the days of carbureted engines, you could completely ruin an engine's driveability by making large changes in exhaust backpressure without making corresponding changes in the carb jetting. This led some folks to thinking the backpressure had some magical effect.

 

[evelrod]

My favorite new word---SHEESH---To think we put dual exhaust systems on our cars to make them SOUND GOOD!!!!!!!"we've come a long way, baby"---Have we?


Rod

PS---Do you realize how rediculously cheap motor fuel is in the US? It was approx $.32/gal in 1960 when min wage was $1.00 and it is less than $1.70/gal in 2002 when min wage is $6.00!!! My son paid $1.70/liter in Paris last year!