SStrokerAce

iTrader: (2)

I've read alot lately from people on here that say we need backpressure to get HP from these engines, when it is shown that when we decrease the backpressure and add better flowing headers our cam swaps make much more power! It's also the reason that after you do the Intake mods and a Cat back it's stupid to change Cam or anything else internal untill you go to headers. I'm not going to go on and on about this because who am I? My name or experince means nothing to people. So instead I want to parapharase Dave Vizardfrom his book "How to Build Horsepower- Volume 1"

"Backpressure vs. Power

Over the years, I've heard many strange claims from so-called experts. One I've heard more than once asserts that a performance engine or any conventional four-cycle engine for that matter-needs some exhaust backpressure to optimize efficientcy. This pronouncement was almost certainly made by someone who had never used a dyno or carefully evaluated how exhaust system flow affects performance. I have no doubt thtat someon, someplace, at some time produced a power curve that "absolutely proved" that an increase in backpressure resulted in an increase in power. My years of dyno experience tell me that this test must have been done under questionable conditions. I have found that reducing exhaust backpressure always improves power and fuel economy, providing, of course, the air/fule ratio and ignition timing are carefully optimized both before and after exhaust system backpressure is increased. Once in a while there will be a specific test or engine that upon first examination appears to defy logic (there always is). But let me assure you that if you minimize exhaust system backpressure and optimize the engine for those conditions, 999 times out of 1000 you'll find more horsepower aat th rear wheels."

Then he later goes on to explain why computer controlled cars don't always get gains from changing the system.

"Many late-model cars use a sophisticated engine management computer that monitirs several engine variables, and many times per second instructs the ignition system how to advance or retard timing and/or direct the injectors to vary the quantity of fuel introduced into the air stream. Needless to say, a system this complex must be carefully crafted by engineers well versed in both engine devlopment and computer programming. It should come as no suprise to anyone that abitrarily changing some part of this complex system may cause an unwanted related change, resulting in poor driveabilty and reduced performance.
Under thee conditions, installing a low backpressure exhaust system without making any other changes may not improve performance."

So as you can see, once we get in there and change lid, MAF, T/B, headers, cats, y-pipe and cat back LS1 Edit or custom programing is the best alternative to make you LS1 achive higher HP numbers than previously.

Bret

gwj

I've never been a fan of the "backpressure is good" theory either. But, like the man said, it won't necessarily give you any gains if you can't bring the other parameters in line.
Still, I think these pcms we have are pretty good at dealing with changes.

ShadowZ28

Oh I have a major problem with this I got the heads and cam done before the headers and I made 378rwhp and 373rwtq with the stock manifolds. Yes if I had headers it would bump my numbers up to probably a little over 400 but back pressure is a necessary thing in the LS1. When I went to the Vette Doctors to pick my car up I talked to Carmen, Dennis and my dad (who came with me) about the headers and and my dad said something about openning up the headers at the track and Carmen and Dennis both said that doing that would lose me like 10hp and they have proven it with dyno testing. These engines do need a little backpressure if you want the most hp you can get from them.

Joe <img border="0" alt="[Burnout]" title="" src="graemlins/burnout.gif" /> <img border="0" alt="[guns]" title="" src="graemlins/gr_guns.gif" />

wizkid

iTrader: (3)

I have heads/cam/headers and I will be measuring the back pressure on my new exhaust system in a week or so. I opened my cutout and gained both HP and torque across the board. I just need to see what the closed and open BP's are.

BP stinks.

Eric

BadMonkey

iTrader: (1)

I say yes and no. Alot of it depends on what you do. I lost 15HP and 20TQ, when I dropped my muffler and went with the Cats and the LT's (pretty much a cut out)Yes this was dyno'd and I can show you the graph. But I honestly think that due to so many variables between different cars and how they respond to different mods is what alot of us really need to think about, because with what happen to me looks as if me losing backpressure by removeing my muffler cost me power (what it looks like and what it really is confuses alot of people). Engines respond differently to different mods and changes. If you do not chaneg anything like your A/F ratio or reprogram the ECU then ya back pressure will gain with it because your car is trying to run optimal which was the way before you added that particular mod. My .02. I do not think back pressure alone will cause you to lose power but somewhere it will cause something else to adjust and that will cause it. Back pressure is the root cause. Not the only....

Fulton 1

iTrader: (9)

Nice thread. I've heard the "you need to have some backpressure" argument time and time again over the years and have never bought it. None of my experience has ever supported a power loss due to a reduction in backpressure (with proper tuning of course).

RPM WS6

iTrader: (3)

Our PCMs can learn quite a bit. Most LS1s respond well to upgrades with stock tuning even. Unless you have a FI car, or you’re A/Fs are seriously messed up, you won’t pick up gobs of power from tuning on stock ci from what I’ve seen. Sure, tuners make claims of 20hp gains on stock motors, but those claims are <img border="0" alt="[bullshit]" title="" src="graemlins/gr_bs.gif" /> for 99% of the mostly stock cars out there.

I did a cam before headers. I feel that this was a good decision. I knew just what cam I wanted, but I have yet to choose headers, so why wait? LS1 manifolds aren’t like the old SBC manifolds that didn’t flow worth a crap, they are actually pretty good as far as manifolds go. Granted, I’m sure I’ll pick up power with headers, but they are not “required” to see gains from a cam or heads/cam. My exhaust is pretty free flowing, 2.5” ORY with a very good flowing 3” catback. I will gain only 15rwhp at peak adding headers in my current application, IMO. I plan to eventually, but in the mean time I’m sure my stock manifold/B1 cam car can spank some header/stock cam cars... <img border="0" title="" alt="[Wink]" src="gr_images/icons/wink.gif" />

RPM WS6

iTrader: (3)

</font><blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr /><font size="2" face="Verdana, Helvetica, sans-serif">Originally posted by SStrokerAce:
<strong>It's also the reason that after you do the Intake mods and a Cat back it's stupid to change Cam or anything else internal untill you go to headers.

</strong></font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">Also, I disagree completely with this statement. It is never "stupid" to upgrade internals. Headers are NOT a requirement for gaining power in an LS1 past a lid and catback.

BadMonkey

iTrader: (1)

All mods and the order they install are up too the individual. I really don't see how one way is dumber than the other. But your right though the headers I put on SLP's and had 338HP and 358TQ. That was roughly 16Hp gain and 20 TQ gain not much but depending on the cam you may not get more than that either. I pulled more torque than my buddy witha hotcam. I do my mods in the order of cost normally, not best to worst. I did headers now, not just for power but for the sound to. With the NOS it should be interesting, and eventually I will get a cam. All in due time. I agree with everyone so far, kinda like listening to my wife though, I opnly hear (see) what I want to and ignore the rest <img border="0" title="" alt="[Smile]" src="gr_stretch.gif" /> .

<small>[ May 02, 2002, 04:21 PM: Message edited by: bd mnky ]</small>

TimZ28

Any backpressure in the exhaust system is going to work against the engine in terms of physical force. If a car experiences a power gain with an increase in back pressure it means one thing and one thing only... the air/fuel mixture is not optimum. Burnt valves were a result of too lean of a mixture.

ShadowZ28

Headers totally aren't a necessity with H/C package. I made 378RWHP and 373RWTQ without headers and yes I know that I will make more with the headers but that's not because of backpressure it's because of better flow.

Joe

Sears

Also remember when changing your axhasut system your air/fuel ratio can be affected. So to say you lost 15 rwhp when runing open headers was most likely to to bad tuning. And you should have extensions on the headers if running them "open", otherwise outside air will alter your O2 accuracy.

Ragtop 99

iTrader: (7)

</font><blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr /><font size="2" face="Verdana, Helvetica, sans-serif"> ...said something about openning up the headers at the track and Carmen and Dennis both said that doing that would lose me like 10hp and they have proven it with dyno testing. These engines do need a little backpressure if you want the most hp you can get from them.
</font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif"><img border="0" alt="[bullshit]" title="" src="graemlins/gr_bs.gif" />
It is exhaust gas velocity and scavenging (Sp?) you want, not back pressure. Tuned properly, open headers will yield more power. However, the header collector is part of the flow equation and if it is too short, as may be the case with non-race headers, it reduces the system effectiveness. You may need a small extension to the collector and you'll see that open headers will rule.

You don't see race cars adding exhausts to create backpressure; they run open headers if permitted to do so.

Backpressure reduces efficency. End of story. <img border="0" alt="[judgement]" title="" src="graemlins/gr_judge.gif" />

BTW, the graemlin refers to what you were told; not you or your dad as individuals. <img border="0" title="" alt="[Smile]" src="gr_stretch.gif" />

jmX

</font><blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr /><font size="2" face="Verdana, Helvetica, sans-serif">Originally posted by SStrokerAce:
<strong>it's stupid to change Cam or anything else internal untill you go to headers</strong></font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">I totally disagree with that bit of logic. My car makes 345rwhp through 100% stock exhaust, stock cats, stock catback with a B1 cam. Now, no, thats not overly impressive, however, if I'd had a stock cam and done headers instead of the cam I'd have a noisy obnoxious car that'd obviously not sound stock at all.

If ever given the choice again, I'd definitly do a cam before headers again...not to mention a cam can be done cheaper than some of the really nice longtubes. The stealth factor is just too cool. Nobody could ever tell I have a cam and everything in the engine bay looks stock except the air filter assembly.

Anyways, sorry for the tangent post.

SStrokerAce

iTrader: (2)

Thanks for some much discussion on this.

Jud Massingil of SAM even said that after 350rwhp you would gain power by the install of headers.

Yes our manifolds are better than they were. I just belive in doing as much to the external system first to get power up and then diving in to internal parts.

That line is just my opinion, but I still think you make what you have to start with most efficent and then you go on to the internals to make power. Most of the guys who are running 10's and low 11's on stock bottom ends would back this up. (I said MOST, not all)

I just thought I would post this because it will help a good majority of people. If you don't belive it then that's up to you. Even though Ice-T is a thug gangster he still speaks in front of thousands at Harvard and Stanford and other campuses every year, he has a great quote "Smart people learn from others mistakes, dumb people learn it themselves," I'm not inffering that anyone is dumb here, but if you actively pursure the exhaust system and backpressure on the LS1 on a dyno and flowbench then you will find that it is not a positive for increasing HP. That's what I think, and i'm sticking to it. (because it's right.) BTW if someone told me that a engine needs backpressure to run, i'm never going to let hem touch my car or any friends car of mine ever again.

Bret

I just posted another topic which I think is important to real power production.

DriveATransAm

backpressure helps keep fuel economy up <img border="0" title="" alt="[Big Grin]" src="gr_grin.gif" /> thats all i gotta say <img border="0" title="" alt="[Big Grin]" src="gr_grin.gif" />

i get like 10 mpg if i leave the cutout open through my off roads and lts <img border="0" title="" alt="[Big Grin]" src="gr_grin.gif" />

feels torqier on the street with it closed- but maybe im afraid to test it out fully because its so loud <img border="0" title="" alt="[Big Grin]" src="gr_grin.gif" />

jimmyblue

iTrader: (11)

Back pressure is the poor man's EGR. Back
pressure means more exhaust remains in the
cylinder and dilutes the input charge.

The motor probably doesn't want back pressure,
when you see it respond to increased exhaust
length you're really seeing the improvement
from resonant/inertial scavenging - the moving
exhaust gas column "pulls" the next puff out.
Open headers see atmospheric pressure, a tube
in resonance can give you a lower pressure
right when the exhaust is opening (although
mean pressure is higher). Getting the right
resonant frequency to "peak out" the motor
is the name of the game in selecting header
collector length and the location of H/X/Y
pipe features. Long tubes build torque lower.
Short tubes may require too high an RPM to be
usefully resonant, depending on where you like
to rev (they still may provide the benefit of
reduced restriction).

Looking at mean backpressure does not tell the
tale. It's port backpressure during the exhaust
stroke that matters. I've never seen anybody
instrument this up, but I bet it'd be interesting.
You'd need a port-local pressure bung, a fast
pressure sensor (with minimal dead air space)
and an oscilloscope synced to the cam timing.

<small>[ May 03, 2002, 09:40 AM: Message edited by: jimmyblue ]</small>

Slowhawk

iTrader: (12)

Nice thread <img border="0" title="" alt="[Smile]" src="gr_stretch.gif" />
I agree and disagree with different statements here.
The way I see it headers are the way to go.I't letting the motor breath better.A cam your mechanically feeding the motor more air ect.while putting more strain on the components while doing so.This puts the exhaust as the major restriction to your setup.WHY???,Do the headers first and uncork the restriction before making mechanical changes.
I personnaly gained 20hp/20tq by just letting the engine exhale better without changing internal parts.That,I think should be the first thing you do before going internal to get the most out of your setup.

RPM WS6

iTrader: (3)

</font><blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr /><font size="2" face="Verdana, Helvetica, sans-serif">Originally posted by SStrokerAce:
<strong>That line is just my opinion, but I still think you make what you have to start with most efficent and then you go on to the internals to make power. </strong></font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">Why not do it the other way around? Make your internals produce more power first, thus maxing out the stock exhaust setup, and then worry about headers?

I say this because, if LS1 manifolds were already maxed out with a stock motor, then things like N2O, blower, H/C, etc. would show no gains over stock. But the fact of the matter is, they DO show gains. Therefore, stock manifolds are capable of supporting more power than the stock motor can give them. That’s not to say that headers won’t give gains on a stock motor, but even still the manifolds are not maxed out just because those gains occur. So what’s really the difference which one you upgrade first? Who cares? Either way you’re going to see gains. Many LS1 headers available cost more than a new cam & valvetrain would, so price isn’t really an issue. Yeah, maybe warranty concerns, but I don’t really care about my warranty (in my case). Driveability? Well, headers have issues too on some cars (banging, leaking, ground clearance, etc.). So really either mod (headers or internals) has down sides, and both are expensive. So who cares which you do first? They both offer similar gains for the cost, so I don’t see the issue here? <img border="0" title="" alt="[Confused]" src="images/icons/confused.gif" />

But I do agree 100% that less back pressure will always = more power as long as it's tuned in right. That should be well known by now, and I can't understand why anyone would argue that point when so many have proven it to be true.

SStrokerAce

iTrader: (2)

RPM WS6,

Hey you think what you want and I do the same thing. I just feel that external mods are our modern day tweeking or "tune up" we now play with some of the same things but in a different way, and even though our headers are expensive, external mods are almost always cheaper. I'f I'm going to spend alot of time and money on a H/C install then I want to make sure the car is ready for it so I get the most out of the time and money.