Back Pressure - Can anyone explain?

BudW

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I forgot to mention in my post, they make exhaust backpressure testers. You unscrew an oxygen sensor (or pressure port bung/flange) and screw in tool, then go road testing (dyno, or whatever). No oxygen sensors? No problem, get a few of these:
O2 Bung n Plug.jpg

Make sure you get the screw in plug with them (as pictured, above), so they can be capped-off after testing. Place one after headers (or header collector) or somewhere before catalytic converter(s) – if using cats, that is, one after cat but before muffler, one after muffler but before any other restriction (tube downsizing), etc.

If there might be a possibility of using oxygen sensors in the future (fuel injection) then you might as well get the sensor flanges placed in a position as recommended by system manufacturer. Otherwise, get the flanges/bungs placed into easy to reach location(s).

The flanges/bungs and plugs are, for the most part, universal as far as the threads go. The design of how it attaches to exhaust does vary from manufacture to manufacture. A very good possibility your favorite exhaust shop may have his own to use.

There are several websites that talk about backpressure, like this one: http://www.tuneruniversity.com/blog...st-backpressure-for-smarter-exhaust-upgrades/ . the above website does a good job explaining things – but also gets into “buy this” and “buy that” so you can disregard that aspect of it.

Keep a log of the different pressures at different ports. Sense the needle does jump around some, I go by the highest-pressure mark. Might not be the best, but easier to figure out than trying to average the needle movement.
If you have enough hose, and hose(s) routed in a manor not to get melted or damaged, you can go on road test with a passenger to record your readings in real life conditions. Even better, have several pressure gauges to see all pressures at once.

I’ve used pressure sensors many a time, mainly to test catalytic converters for stoppage. Today’s cars (built sense ’96) automatically test the cats for efficiency (via engine computer), but the older cars don’t.
One (or more) cats on my ’86 Fifth Ave are (partially) stopped up – but I’m not quite to point to get them fixed, yet (got other plans in mind).
BudW
 

SixBanger

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Something that @Duke5A has done (or is doing) is to add exhaust cut-outs to his existing exhaust system. This will allow for bypassing the muffler and his car will be louder. Great for “timed acceleration trials” or to intimidate other drivers. Not so great to impress the local law enforcement or the spouse.

Are those systems legal where you live? Here it is illegally to install, only from original build like sport cars Porsche 911 etc.
 

Bruceynz

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In NZ I think illegal unless factory, if car had cats not supposed to remove, has to meet sound pressure levels and most cars have to exit over axel unless it's a coupe where there is some loop hole that lets you exit in front of back wheel. If car had anything crazy factory on it you can keep and replace but never add to a car that didn't have.

You are allowed to upgrade exhaust, make dual etc but must exit out back and meet sound levels. Weird thing is there is no ground clearance spec lol there is for body, 4" is as low as law allows. Exhaust could be 1/4" from road and that's ok
 

Rustyroger

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You achieved your goal of making the car quieter, maybe it has a bit more torque at low revs, maybe not.
Has gas mileage changed for better or worse?.
If gas mileage hasn't suffered enjoy your serene ride and quit worrying. :cool:

Roger.
 
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BudW

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The Oklahoma State law says you MUST be able to hear the horn over the engine, when it is revved to 2,000 RPM. With that said, there is no state inspections (safety or emissions) so no one to even monitor that. There are many a car here running without a muffler of any kind. I can hear ‘em racing in the streets at all hours at night.

I’m sure people are video taping them and putting some of the races on Street Outlaws (Discovery Channel). I happen to live in Oklahoma City and only a few miles from “the headquarters” but never had a reason to stop by.

I can't comment about other state (or country's) laws.
BudW
 

Camtron

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Illinois basically doesn’t want you doing anything to your factory exhaust to cause “excessive or unusual noise” or compromising exhaust emission. However this really only seems to be enforced on broken/missing exhaust systems , poorly built import tuners. If you have a nice weekend cruiser, pro street build or whatever, cops don’t seem to care here and anything built before 1996 doesn’t go through inspection for anyone to be checking.
Just depends on if the local 5-0 likes your exhaust note and car lol
 

AJ/FormS

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Just one streeters opinions;
5hp at 5000rpm means nothing to me
5 hp at stall speed is waaaay more useful.
This is why I'd rather have a 224@ cam than a 230* cam.
Having torque at my jumping off point, namely 30 mph is the single most important thing for me.Having 400 ftlbs here is more important than having 400horsepower; and here's why;
That 400hp is gonna come at 5200ish rpm, and with 3.55s, that is gonna be about 45mph., in 2.45 low gear. Of course no street tire I have ever put on my car has not still been spinning at 45mph with 400 hp.. Now,at the 1-2 shift the Rs fall to 59% or 3100, and POW! the tires have stopped spinning, and this is where those 400ftpounds are gonna be worth their weight in gold.
By 60mph where I shut down, the Rs will have climbed back up to 4000, whoopyt-do. At this 4000rpm both cams will make about the same amount of power, sooooooo I would prefer the torque of the smaller cam.
Now if you geared the car to be able to pass thru the power-peak twice on the way to 60 mph, well that would call for a different cam altogether.
But this is a what? say a one percent situation?
99% of the time you will not be burning to 60 mph. So if your combo can spin the tires to 30 mph, and hits the powerband there.... like it should with 3.55s and a modest TC, then you have enough power to ET well. More power will just annihilate the tires, to go a wee bit quicker to ~45mph. Which by itself is fun alrightee, but not that impressive ET wise.
Ok so maybe 60% of the time you will be just granny driving and the 39% remaining, you will be PartThrottle torquing through first and maybe into second.
So the question is ; why would you sacrifice 99% of your driving experience for the 1% .
Again; the 224* cam will be a far better selection than the 230* cam.
I'd say trust me since I have had both of those in the same combo; but I know how LAME "trust me" sounds,lol.
So anyway, to parrot what others may have said; if you like it; leave it alone.

Now if you had headers, this conversation does not alltogether apply.
 

AJ/FormS

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In Smokey Yunich's book "Power Secrets" on p104 he says the secondary pipes on header-equipped, circle-track cars, are typically 3.5x50 yielding a gross volume of 481 cubic inches,each.
Just to compare, mine are 3.0x~68= 481 cubic inches,each.
Now I realize my engine is not a circle-track engine, with just a 230* cam, but I can tell you it makes a truckload of torque right off idle.
IMO, no ICE (Internal Combustion Engine) engine, built for power, should be forced to endure back-pressure. And the more overlap the cam has, the more important this is.
I once watched a HR episode where they swapped the headers off a big-cam440, for log-manifolds,on a dyno, and promptly lost a truckload of power, by killing the Fifth cycle; namely overlap. I don't recall the numbers but it astonished me. I promised myself never to run log-manifolds again with any kind of performance cam.
It also revealed to me why every guy in highschool that ever put headers on his 340 car, had this great big stoopid smile on his face.
I don't know of any formula I could use, but in my estimation, for a streeter, and as for power only ; headers might be worth at least one cam size,maybe close to two.
But whereas cams have a very narrow operating window,well up in the rpm band, headers start making power very early, and never quit. And they can be tuned over an rpm range of about 1000rpm, to work with the power-curve, or not.
What this means for you Bruce is, instead of getting a cam one or two sizes bigger, (which would kill your low-rpm torque ,especially if you also had a low-compression engine) ; in your quest for performance, consider that headers will increase both torque and power with the existing engine combo; and will do it throughout the rpm range........... As would compression, and even some head-porting.
I realize on your car, this could be a challenge; In my notes I have you currently running a 262/270/110 cam(209/217@.050) which only has 46* of overlap, so not running headers would seem no biggie. However, recall that the original 340-auto cams were 268/276/114 cams with just 44* of overlap; and how did they respond to headers? IIRC 30 hp after jetting changes.And those were "just" 340s. In your chassis, you might run both pipes into a 2-into-1 muffler with a big outlet reduced down to your single 2.5 tailpipe. Or you might just terminate one side at the rear wheel, outside the bodyline, like an AAR.But I get that making these changes is like a G-note in the USA and who knows how much in NZ, so I guess it could wait until you put that 221/225 /110 roller in there,lol. With that bigger cam, the Fifth-Cycle is 56*, and so the headers will be much more important .
The trick is always, to put the power where you need it, which in a streeter, is rarely over 5300 with a 6000, 1-2 shift rpm(automatic). And usually right around 30 mph in Low.
Never agree to race a guy off the line unless your car 60fts at under 1.7 seconds,lol.
Even a dirt bike can embarrass you.
A 65pound dog can take three steps and be at 30 mph. Where will your car be? I'll tell you; not even moving yet,lol.
 
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Oldiron440

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AJ you make the assumption every time that your talking to someone who is willing to compromise performance for a daily driver. Or your only building the mildest of performance builds. The shop I'm familiar with builds maybe 1 in 25 motors like that and it's for a restoration. A guy trying to get by with a little more and call it stock. Guys are still building motors with 35 + degrees duration at 50, 400 to 500 hp small blocks and 500 to 750 hp big blocks. They are using converters and gears like they always have with the exception of the big blocks don't need 4.10s anymore, 600 lbs of torque at 2700 rpm says you don't need deep gears. You have seen the numbers for Blueprints 408, 95 percent of the total torque by 2800 rpm is what's being built today. The strokers have torque down low and power in a decent rpm band.
There are guys still building the 318 as 318s but most of them are adding a carb and exhaust to the original motor and calling it good. There are a few building 350 hp to over 400 hp but there not doing it your way. There still hot rods. I'm just curious who are you preaching to about this extremely mild combination?
I think a automatic overdrive transmission and a decent set of gears would give more zip and drivability than your mild motor with stock everything else.
 

Bruceynz

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I think he is talking to me as he knows my engine is very mild, has small cam, 209/217 and small stall 2200rpm, it was all done to keep it street able, gas here is around $9 a gallon. There are reasons.
 

Oldiron440

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I see, he has been putting the same information out repeatedly when an OD transmission and a 3.55 gear would make even a stock motor feel like a rocket for a city block or two.
 

jasperjacko

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In my experience, it' not the back pressure that has given more low end torque, but the exhaust tuning, or scavenging. Your system has probably tuned well at lower rpm. Most likely, you're giving up considerable power with your system, but that's ok. I have a header into 2.5 into 3" y Borla pro ex cess into borla round resonator between spring and tank with 3" tailpipe just beyond the bumper. Single exhaust and works GREAT! Sounds great outside, but quiet inside with no drone at all. Before that, had same system but with flowmaster delta 5 muffler and no resonator. It had drone inside, and WAAYYY too much backpressure. My car would hit a wall at 4000 rpm. I put a pressure gage on the y pipe and was getting 4 psi at 4000 rpm. My new system is plenty for the current combo and easily revs to 6000 rpm.
 

Duke5A

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@jasperjacko is absolutely correct.

Back pressure is not the goal, it is only a measurable byproduct of a properly sized exhaust. If it was the goal you could accomplish it simply by introducing an obstruction in the tailpipe.

Too big of a pipe and the flow characteristics of the exhaust is poor. The gas tumbles around and doesn't exit the system as fast. The proper amount of squeeze in diameter size of the pipe is what controls this.

Every time I hear someone talk about engines needing back pressure I want to slap them upside the head with a seabass. An engine wants to be rid of the exhaust as fast as possible and proper exhaust setup is what facilitates this.

Yes, there is more to it than this. The relation between velocity and flow, the scavenging effect, exhaust gas temperature, RPM range you're tuning the exhaust to, etc...
 

Opticon77

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Enter modern day "active" intakes/exhausts tuned to increase velocity and aid in scavenging at lower RPM then slap wide open to clear/shorten the path at high RPM.
 

kkritsilas

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Intentionally increasing back pressure should never be a goal. In a properly sized exhaust system, there will always be some back pressure due to pipe bends, mufflers and the various joints that are part of any practical exhaust system. A properly sized exhaust system will have some back pressure; a too small exhaust system will have excessive back pressure, and an oversized exhaust system will have less back pressure, but may not flow as well, please read what Duke5A wrote. If the maximum exhaust system were the ideal, people would be putting in 4" dual exhausts and calling it a day. In the real world, this is not going to give the best overall performance.

Volume is NOT the only factor. Even in top fuel dragsters and funny cars, they use zoomies. If volume was the only factor, they could just dispense with any exhaust system of any type, and let the exhaust exit out of the ports to the atmosphere. They don't have any cops pulling them over, noise ordinances, neighbors, or anything else to consider other than getting maximum performance. But they use zoomies anyway. Ever wonder why? Because they get higher performance out of properly tuned zoomies than they do without the zoomies. The tests have been done. Even though the "no exhaust system" has no volume constraints, they get more power with the zoomies (they look cool, too). This comes down to the point that maximum volume IS NOT the only determining factor in exhaust system performance.
 

Opticon77

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And while we're covering it there is one important caveat with newer tech... Turbochargers disrupt exhaust scavenging effects and any backpressure AFTER the turbine directly hinders spool speed and RPM. So the shortest, largest, least restrictive exhaust possible is the goal in that application.

Of course the exhaust design PRIOR to the turbine is still very important for velocity and focusing heat expansion into the turbine where it is recycled most effectively. Which is why rear mounted (remote) turbos are at a serious disadvantage with a very narrow window for optimal performance... It's hard to retain velocity and heat to your turbines through a wide RPM range when your turbochargers are hanging off the rear bumper.
 

AJ/FormS

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On the street
I preach mild cam and the right gear at the right time for street because passing thru the power peak twice on the way to sixty, is way more fun than, shifting into second at 50, and laboring back up to 60 on the bottom of the power curve.
The problem is the gear ratios in the TF trans are, for performance street, IMO, chit.
Most guys I know run 3.23s to 3.55s max because of the hiway-gear problem. Up here in Southern Manitoba it is a minimum 20 minutes to anywhere, and over an hour to the nearest large metropolis,lol. That means we spend most of our miles, if not time, on the hiway.
With that said,
With most performance engines, first gear is a throw away gear cuz you just can't get traction at WOT. So the fun gear is second,but finishing at 60mph=4000 with those 3.55s is booooooring. So if yur gonna finish at 4000, and first gear is a throwaway, then what is the point of having a 230@.050 or more cam in it? I mean I got a 230 and I know it's a waste, annihilating the tires. But I threw away the flipping Torqueflite, in favor of a CommandoA833/GVod combo that hits 60 at 6400 in first-over, still with 3.55s. Now, hearing a 360 at 6400, screaming thru dual 3" pipes, is real giggle excitement. Big, not-boring, fun.
Since my 360 is still spinning at that 60mph I suppose I could run a much smaller engine on these gears and probably go just as quickly. Or, I could run the same 360 with a much smaller than 230* cam. Or, I guess, I could spend another buncha money to better my 60ft time,
That's my argument.
For a streeter it's all about traction and gearing, cuz we don't have much wind to buck, which after mass,is the principle power consumer.
So for a guy with a TF trans, this is a problem that can only be corrected by more rear gear. But couple that with a 235*cam leads to an impossible situation.The 235* in an SBM, will power-peak around 5400/5500 right? and you can stretch that out to at least 5700/5800, before the power falls off .
Ok so how do you marry a 5800rpm shift-rpm to a TF at 60 mph? To use second gear the rear gear would have to be a 5.38. And instantly she needs an overdrive, and not just any overdrive but a deep overdrive as found in the Mopar A5xx series. That .69od gear will make the 5.38s into 3.71s for hiway use, really; still out of the realm of decent hiway use. But by the same token a 235* equipped SBM, even a long stroke one, is not gonna get much better fuel mileage at the rpms a 3.71 allows versus what a 3.23 would cuz that cam with a carb, is a fuel hog.
The top of the FTH performance-cam, with fuel-economy, heap, for a SBM, is about a 225/110 cam. This size of cam will still operate well at 2000rpm for fuel economy. And it will powerpeak at around 5000, which with good heads you can stretch out to say 5400. Ok so lets marry that to a TF in second gear at 60 mph. I get about 4.56s to finish at ~5200, and a 4.56x.69=3.15 final drive for 65= 2540 @ zeroslip. Cam is still too big for fuel economy.........
This is my argument.
The TF has terrible ratios for a performance streeter, looking for decent fuel mileage. And I know that with all this money tied up in my combo, I need to justify it by driving it everywhere all the time; I ain't made of enough money to have a separate DD.
If I work the formulas backwards starting with a decent hiway gear of say 65=2280, I get an A500 with 4.10s, for 65=2280@zeroslip. That could make awesome fuel economy with the right combo.
Still working backwards, to hit 60 at the top of second with 4.10s and allowing 5% slip in the TC,I get, 60=4650.... not very impressive.
Lets try the A518 with it's 1.54 second gear. I get 60=4940 a little better.
But this tells us to get to 60 quickly,we're gonna need a real torque-monster 360. Not some piss-ante smogger-teen, with bolt-ons. I'm not saying you can't have fun with a smogger-teen and 4.10s; I'm saying you won't win many trophys for low 60mph ET with it.
Ok but lets take it to the end. With a 360 to power peak at 4650 less 200/300 will take a cam in the 2487/256* range. The A999/518 second gear will like one size bigger cam. and still get great fuel economy.
So a compromise has to be struck. With an automatic,on the street, It's always about the compromises.
In contrast, even the lowly A833overdrive has a good second gear ratio of 1.67, allowing 60=5100 with 4.10s; and 65=2380 in overdrive. and that 5100 could be a nice 220@.050 cam, which combo can be tuned into the 20s mpg by the right tuner. Oh but you lose the TM properties of the big TC. Ok I'll admit that, but,firstly; the 360 don't need any TC torque multiplication off the line with 4.10s.... especially with the 3.09 low gear that is already 26% higher than the 2.45 in the automatic....... and Secondly, that extra 26% stays at 26% , ALL the way thru first gear, not diminishing from a factor of ~1.8(plus 80%) at zero mph to ~1.08(plus 8%), by around 60ft out.
The only hick-up on this combo is the 54% 1-2 split which drags 5000rpm down to 2700. My cure was just to build the engine to handle 7000, and over-rev first gear,(cuz first is a throw away gear for me) to drop into second closer to the torque peak; 6600x.54=~3560; Shazzam.
In the end I ditched this box because 30mph was; 4700 in first, and 2550 in second with 4.10s and neither was any good. No matter what rear gear I threw at it, 30 mph, my preferred jumping on it point, was just no good.
It's all about the compromises.
My final trans combo has almost no compromises. Now my 230* cammed engine has waaaaaaay too much power and the fuel-economy is atrocious, but it's ok, she hasn't been my DD since about, ohchit, I can't even recall anymore.. a long time ago,lol.
I'm keeping the Commando/ GVod/3.55s combo cuz it's geared for 93 in the Eighth @6140 in second over; and 2240@65mph in 4th/over; and 30mph = 4100first,/2540second/3190 in first over,shazzam.. the compromise, which I don't care about, is that it has no decent gear for the quarter-mile. I can cure that in about 75 minutes.
I'm waiting for the 230*cam to expire so I can go back a size or two. I've been waiting since about 2004. And now I got old so almost don't care any more,lol.

Anyway, that/those are my arguments. and based on my experiences, since this same engine has run three different cams; first the 292/292/108, next the HE2430AL which was a 270/276/110, and now finally the HE3037AL which comes in at 276/286/110.
I ran all of them, each of them, at or near the same Dcr, cranking cylinder pressure, just to be sure.
My favorite was the smallest which came in at 223/230/110@.050. She was a real sweet in like a lamb out like a lion,bruiser.
 

Oldiron440

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I still don't think adding ten or even 20 percent more torque to a 80s smog dog is as effective as a stock magnum with the OD and a set of gears. A mild smoger is a waste of money.
 

AJ/FormS

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I agree on both counts.

We both/all know that a 360 with the gears to hit 60 at the top of second, makes first gear a tire fryer. Now the cure for that is less compression or a smaller engine; maybe both.

So; say the right cam to hit 60 at the top of second is a 256, that power-peaks around 4800rpm; what would happen if we stuck that in a moderate-compression 318/5.2 with the 4.10s/ A518? Ok so now first gear is no longer a throwaway. With the right TC in there, you can have a dynomite first gear run to say 5200rpm@36 mph, then hit second and run from 2920 up to 60mph@4940.. See now we got a combo, you can actually floor the gas pedal right from zero mph all the way to 60 without a whole lotta tirespin. Ok again, it might not set any zero to sixty records, but the 5-6 seconds or so of screaming 318 is a lotta smile-factor.
And that combo will run at 65=2285, and with just a 256 cam the overlap could be around 39*, plus with a power duration of around 119*, you can be sure all the energy that was available in those expanding gasses is gonna be delivered to the crank.... meaning exceptional fuel economy would be possible. I like this combo with a 2800TC, but I would try it first with a stocker that might flash to 2200.
Ok so the final target; 30 mph in first gear would be ~4600 which sorta sucks so just reduce your jumping off point to say 24mph and the math is 3500rpm, close to peak torque.
Ok so just how much slower to 60mph will this; moderate Scr/262cammed 318/A518/4.10s combo be?, as compared to a 276cammed 360/727/3.55s ?
IDK;
line 'em side by side, and lets find out.
 
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