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Exhaust back pressure power loss?

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Exhaust back pressure power loss?

Postby 68corvette » Wed May 24, 2006 8:49 am

Haw you measured how much does exhaust back pressure affect to power in street engine?

I know that there are lot of variables like intake / exhaust velocities, runner lengths, seat angles, compression ratio etc..

I ported a set of LT1 heads to one guy and it made 381hp with 276hr 112lca cam, shorty headers and stock exhaust.

Now he upgraded to 2/½ dual from 3" single system and changed in longer 28" headers.
Map was also down ~92% @ 6000rpm and he upgradet from 48mm to 58mm throttle body.

The car felt a lot of stronger, but he lost ~40hp in dyno.
AF ratio was also a little lean side 800mv, but i dont think its more than 10hp. Atmospheric pressure was also a little on the low side ~98%MAP, as previous time it was ~102%

I asked him to measure cars back pressure before cats and it was quite a high ~7.3PSI.
Car is going into dyno again without mufflers and it has now ~1.3PSI back pressure with bare catalysators.

This is only article i found by googling:
javascript:Zoom('http://superchevy.com/technical/engines_drivetrain/exhaust/0505phr_exh_12_z.jpg',640,480)

Do you think it could loss 20% of HP if back pressure is 2.5PSI like showed in that graph?

here is whole article:
http://superchevy.com/technical/engines ... index.html

thanks again for help.
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Postby Cobra » Wed May 24, 2006 10:15 am

Bigger is not always better! Many cars perform better with mufflers than with open headers. When changing FI intake parts you must size everything equally or the smaller parts will limit or negate gains. Many people are surprised when they are down on power following intake and exhaust size and type increases. You must size components to the particular setup. I do not like shorty headers on anything. Your post indicates problems with the tuneup which will affect power. Do not try to speculate what power gains should occur or you will be disappointed.
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Postby beth » Wed May 24, 2006 11:03 am

When increasing exhaust size after the collector causes a power decrease (assuming it really does increase flow) it is a tuning issue, most likely fuel. A catalytic convertor can cause strange poorly timed pulses in the exhaust primary pipe and an anti reversion section before the convertor will help.
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Postby johndougherty » Wed May 24, 2006 1:05 pm

So you lost 10 hp because it was lean, and a certain percentage for less atmospheric pressure (3%?) or 11 hp. Then the throttle body and exhaust change combine for a 19 hp loss.
I would change the throttle body back and get back on the dyno.

Where are the torque and HP peaks before and after the changes?
Did the new parts move the powerband up and out of the range of the 276 degree 112 LSA camshaft?
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Postby MadBill » Wed May 24, 2006 3:04 pm

7 psi is a lot of B.P., especially once the seat-to-seat duration gets up over 280°. David Vizard says you need to have ~0.2 psi max to maintain 100% of open pipe power.
As an example, the '67 -'69 Camaro Z-28 cam made ~ 20 more HP on open headers than the milder cam fitted in 1970 and later, but at least 15 less through the stock exhaust.
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Postby needforspeed66gt » Wed May 24, 2006 8:01 pm

We have picked up 50-70rwhp just by taking off the sidepipes on 60-70's corvettes, this has happened at least 3 times in just the last year, that is with a motor only making 300 or so at the rear wheels final....have seen 50rwhp on a 489bbc in a 71 camaro going from 2.5" to 3.0" with the motor making 500 final.
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Postby Procision-Auto » Wed May 24, 2006 9:52 pm

The car felt a lot of stronger, but he lost ~40hp in dyno.


The car might be faster down the track if the power curves picked up better
numbers over the geared RPM window, so don't let the dyno fool you.

What long tube header is currently being used? The collector design
could be degrading power if the design is weak.

Another consideration is the placement of the catalytics from the collector.
As "Beth" stated, the pulse tuning may be thrown off by incorrectly placed
exhaust components.

Although emissions require the catalytics to be placed close to the exhaust
port, secondary exhaust tuning might want the catalyics placed about 12-20 inches
from the flange.

Experiment with different lengths of collector pipe (same diameter) and
even higher flow catalytics.
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Postby 67RS502 » Thu May 25, 2006 9:05 am

There are reasons why a car may pick up power with or without mufflers,
one may be what type of muffler is being used, and where its placed.
We need to first figure out whats the optimum exhaust system (headers back)
for the given combo is. Usually its a header extension, where the length is
the important factor. So lets say this engine makes the best power with
a 3 1/2" dia. x 18" lg. extension and no muffs. Now stick it in a car, where
an exhaust system is needed, and you may not be able to get a H-pipe or
X-pipe 18" behind the collectors. Now you have to compromise and will loose
some power if you have to put in 2-3ft of pipe behind the collectors to get
a H-pipe, X-pipe or something with some (plenum) volume in it. But on the
other hand if you can stick 18" of pipe and behind it have "chambered" mufflers
with some volume, the engine may see no loss as long an the pipes and muffs are
large enough, say 3 1/2", since this set up very closely resembles the open
header / 18" extensions that were used on the dyno.
67 camaro 373s
girly cams on pumpgas:
420 - 641hp BretBauerCam, 1.4, 9.90 @ 135
383 - 490hp 224/224, 1.56, 10.77 @ 124.6
502 - 626hp 252/263, 049s 1.44, 10.08 @ 132.7
62 Nova cruiser
383/200-4R/12-bolt w 373s
224/224 HR cam
1.57 10.97 @ 121.2
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Postby SUPRSLO » Thu May 25, 2006 2:02 pm

67RS502 wrote:There are reasons why a car may pick up power with or without mufflers,
one may be what type of muffler is being used, and where its placed.
We need to first figure out whats the optimum exhaust system (headers back)
for the given combo is. Usually its a header extension, where the length is
the important factor. So lets say this engine makes the best power with
a 3 1/2" dia. x 18" lg. extension and no muffs. Now stick it in a car, where
an exhaust system is needed, and you may not be able to get a H-pipe or
X-pipe 18" behind the collectors. Now you have to compromise and will loose
some power if you have to put in 2-3ft of pipe behind the collectors to get
a H-pipe, X-pipe or something with some (plenum) volume in it. But on the
other hand if you can stick 18" of pipe and behind it have "chambered" mufflers
with some volume, the engine may see no loss as long an the pipes and muffs are
large enough, say 3 1/2", since this set up very closely resembles the open
header / 18" extensions that were used on the dyno.


I'm familiar with the concept of chambered mufflers with a good amount of volume and how they essentially act as the termination of the collector if they flow enough air. What i've had questions on for sometime though is how exactly a glass pack style muffler acts? I understand it actually serves to act like an addition to the collector length, but is there any general rule of thumb to apply exactly how it will effect the collector length.
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Postby Procision-Auto » Thu May 25, 2006 2:19 pm

According to Vizard, and some other tuning sources, any reflective surface
or abrubt change in the secondary piping should be placed where the
tuned length of the collector ends.

In other words, if the collector length makes best average power at 18 inches,
place the muffler at that point.
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Postby SUPRSLO » Thu May 25, 2006 2:29 pm

Adrenaline_Z wrote:According to Vizard, and some other tuning sources, any reflective surface
or abrubt change in the secondary piping should be placed where the
tuned length of the collector ends.

In other words, if the collector length makes best average power at 18 inches,
place the muffler at that point.


Yes, exactly. As indicated before though he says that a glass pack style muffler (98% of your mufflers that flow 800cfm or more) serve to extend the lenght of the collector. He indicated that this extension of the collector can very though and was not as simple as the length of the muffler being the length of the extension or anything of that nature, but simply that the glass pack muffler serves to extend the collecotr length in an un-explained manner. Can anyone make a relation to this extension of the collector (for example a 3.5" glass pack muffler 18" long adds approximately 24" to the collector in theory or something like that.)?
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Postby Procision-Auto » Thu May 25, 2006 2:41 pm

I don't think it's the flow that we should be concerned about as much as
the inside diameter of the muffler.

Just as a stepped header, the smaller ID of the muffler will create a reflective
surface.

In addition, there is acoustic energy loss which will effect wave tuning.
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Postby MadBill » Thu May 25, 2006 2:46 pm

<EDIT> I would have posted mine first, Adrenaline, except I accidentally blew away my first attempt... :(

I would make an educated guess that a glass or steel pack would extend the effective collector length to about halfway down the core. I also suspect that it would not provide the same scavenging effect as an open header, since the pressure pulses would be dissipated in the core, rather than reflected from the open end as a negative.

Also, with the exception of Borla and a couple of others, most glass packs are pretty restrictive. If the core isn't larger diameter than the end fittings, or there are big louvres hanging into the flow path, look eleswhere...
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Postby SUPRSLO » Thu May 25, 2006 2:59 pm

MadBill wrote:<EDIT> I would have posted mine first, Adrenaline, except I accidentally blew away my first attempt... :(

I would make an educated guess that a glass or steel pack would extend the effective collector length to about halfway down the core. I also suspect that it would not provide the same scavenging effect as an open header, since the pressure pulses would be dissipated in the core, rather than reflected from the open end as a negative.

Also, with the exception of Borla and a couple of others, most glass packs are pretty restrictive. If the core isn't larger diameter than the end fittings, or there are big louvres hanging into the flow path, look eleswhere...


How would you say a Magnaflow muffler acts with it's straight through design with whole found within it? To me i'd consider them a glass pack muffler as well or atleast that's how i'd describe them.
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Postby Procision-Auto » Thu May 25, 2006 3:42 pm

MadBill wrote:<EDIT> I would have posted mine first, Adrenaline, except I accidentally blew away my first attempt... :(


Been there, done that. Good thing this place is big enough for both of us!

As a matter of fact, I like your answer better. I wouldn't konw how to figure
the pulse reaction, or effective collector length within the extension of the
muffler.

From researching Philip Smith & John Morrison, they describe absorption
type mufflers and their reaction to pressure waves and acoustics. I think
it's safe to say there were several hours involved in the lab figuring how
different materials, chamber designs, pipe length/diameters affect performance.
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