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let's discuss backpressure!

bcrazy

New member
Joined
Feb 13, 2003
Location
Str?mmen Norway
I know that if you have a too free exhaust you can drag the fuel/air mixture out of the cylinders and thereby reducing performance. Let's say you wan't to build a free flowing exhaust for a NA volvo red block. All the cams for the red block are mild designs with little overlap. (except perhaps the H and K) On the B230FB that i will soon build an exhaust for the exhasut duration and lift is less that that of the intake. So if you have an extremely good flow exhaust with a good header with good scavengin you still wouldn't pull very much of the air/fuel out of the cylinders because there is so little overlap? In my experience wiht vw aircooled engine i have learned that as little backpressure as possible gives the best power. This applies to totally stock beetle engines with less than 60 hp and a tuned 2.0L producing 150 hp. More backpressure produces more heat and less power. At least this i what i have experienced over the years. Anyone who has any input? If I am correct when you look at NA racing engines the all run free exhaust, or close to it after the collector?
 
Well, it depends on where you want to make your power, and how concerned you are with torque down low. If I made exhaust for an NA car (trying to design one for my friend's Altima right now), I would run relatively small pipe size (2.25" for a red block?), no cat (God bless Florida emission regulation.. or lack thereof), and a high-flowing dynomax turbo or magnaflow muffler. This setup should yield decent low-end power on red blocks, and good mid and upper range... Low turbulence and high velocity due to the lack of major obstructions and smaller piping size... Though match your cam profile, if you run a K cam or similarly aggressive, higher-revving cam, aim for making the power higher in the rev range, i.e. bigger piping etc...

There's some basic stuff, I'll let the more seasoned people delve deeper into things.
 
Not sure how you are dragging the fuel air mixture out of the cylinders as a result of a free flowing exhaust. I think there is some confusion with improperly tuned manifold or header scavenging and cam design - rather than an actual exhaust issue (and I think for purposes of aftermarket exhaust applications for most on the board we are talking 'cat back' scenarios).

The issues addressed in both Bcrazy and Erics posts seem to be more cam and manifold related specs than they are exhaust design, which is pretty easy to confuse - not slagging anyone here :wave: .

A short thought on the matter of the need for back pressure: I would like to see ANY dyno chart that demonstrates back pressure being beneficial in any exhaust design scenario that we are likely to be employing.

The need for back pressure is one of the great folk-wisdoms of tuning.

"Back pressure in an exhaust system is evil" - Corky Bell.

JB
 
[quote:07fffe8f12]"Back pressure in an exhaust system is evil" - Corky Bell. [/quote:07fffe8f12]

Remember, Corky was writing strictly with regards to turbocharged engines, after the turbo. That's not what we're talking about here....

Backpressure and scavenging ability can easily get confused. If you build a system with good scavenging ability, you'll have a system with some backpressure. If you build a system with no backpressure, you'll end up with a system with no scavenging ability. The trick is to find the fine balance between the two. The bottom line is on a normally aspirated engine, too big results in poor performance....practical experience has told me that.

I'm sure everyone has differing opinions on this one....
 
Fair enough Dale.

Can you speak a bit about the scenarios where you have found oversized exhaust (cat back) degrading performance? Think your experiences would be welcome additions to the thread - at least from this end.

Best

JB
 
i have a red block na. i have tried many setups but found keeping the stock piping size, gutting the cat, piping through the first muffler and putting a high flow muffler on the rear with the stock tail pipe too work quite well. i used a summit muffler as it was cheap and i perfer the slightly louder tone. but that is just my opinion.
 
One of the T'Bricks members here in Calgary (Volvo88) has a beautiful '87 244. Shortly after he got it he put a high flow down pipe and full length 3" exhaust system. His first complaint was that he didn't think it had as much power as the original factory system although it was free flowing and relatively loud. Fortunately (for him) a shop crushed the system when they put it on a hoist. He got it replaced with the factory downpipe and a 2 1/4" (maybe 2 1/2"?) system. He was amazed at the difference it made. Much more power. Even more so than the factory system. I may be a little wrong with my dimensions simply because it wasn't my car but I do know that when he went to the smaller system his performance improved drastically. It didn't take any more evidence than that to convince me that bigger isn't necessarily better in a normally aspirated application.

There may be a certain point in the length of the system where the size will no longer be a factor. Scavenging has much to do with size and length of header runners but the size of the system beyond that may have some substantial impact as well. I'm sure there are others who will have some valuable input here as well....
 
Cylinder Scavenging, backpressure etc all has to do with manifold/extractor/header design, once your past that point, you want as much flow as possible even in an NA car.
What some seem to overlook though, is that often header design needs to extend to well under the car depending on where in the rpm band you want to make good power.
 
Ya - Bishop is echoing my perspective as well.

I can see where going with too large a pipe straight out of the NA manifold may impact scavenging and alter the OEM exhaust tuning (I think I recall that OEM Volvo exhaust mani and related initial piping is fairly well engineered for a production vehicle), but I agree with Bishop - once you get to the cat flange and back I think it would be tough to find a need for any sort of concern for a 'tuned runner' type of scenario, and any sort back pressure on the exhaust in general - doubt that even if there were to pulse tuning on the exhaust itself, that the wave/pulses could get past the cat.

My guess is that the large pipe straight out of the manifold was probably impacting the scavanging capacity resulting in the decrease in performance rather than limited 'back pressure' from the large exhaust diameter per say. Pulse tuning is a bit different than just adding back pressure, and I would venture to guess that a more restrictive exhaust is going to have an impact on the scavenging effect of the manifold and downpipe/primary pipe as well.

I agree that different diameters should be considered when it comes to the downpipe in NA vs Turbo, since the scavenging is going to be less of a concern in the Turbo app a larger downpipe is the way to go. After the primary exhaust stage however I think the bigger the better.

Just some thoughts there - appreciate the dialog.

JB
 
I simply suggest that the evil gremlin of backpressure in the exhaust system (the exhaust system = anything added after the optimum header or optimum charger outlet plus a possible optimum downpipe) will, and does, in general, limit the overall possible performance potential, unregarding if it might be a NA, or a boost application.

And I also suggest that whatever features/specs that might be added behind the/a optimum format header on a correctly combined, built & set engine, you seldom find any gains...
The key issue is simply to have a design that allow for the least overall losses.

[Even the sort of conservative Volvo oem engineers suggested that a 60mm exhaust on a NA-mode 112bhp B23A was worth +10bhp some 10-15 years ago...]

In normal NA-race applications we're in general quite happy when our customers accepts to build a as straight as possible 100mm x100mm (4"x4") "tunnel" after the header (adapted in the apropriate manner) which, after the rear end, exits into a carefully designed 2 x 3" pipes with equally carefully selected 3"muffler (100dB noise race regulations are common here in Sweden...) and then finally exiting the exhaust system.
This format has proven to yield small-to-none torque/bhp losses, and when trying with, and without, the exhaust system are the needed/optimum a/f & ignition settings typically unchanged.

In a boost application I simply suggest that as soon as the last "wheel" has been passed in the charger has the "exhaust system" started.
Hence does the format of the exit of the charger matter as well as both the radii as well as the dia of the "down-pipe" matter, as well as the rest of the system.

So, in general, and obviously unregarding such practical issues like noiselevels, heat and exhasut fumes, etc; after the best format of the charger exit and the needed (?) optimum format of dia & radii of the added downpipe there are rarely no gains to be found, mainly losses.

Obviously with the proper settings of fuel & ignition vs load & boost in EACH comparisson case.

And the noble art of avoiding this correct m.o. comparison procedure is often the case when oem or smaller (read "too small") exhaust systems seemingly produce better performance than systems with less pressuredrops...

Best regards

Mike

M.Aaro@mail.bip.net

Lule? (northern part of Sweden)

NEW sept '03 files: Gr10/Engine-kit, Gr11/Head & the EMS/T-file [Zip + PDF]
 
I agree with most of you guys. If you have to large primary pipes in the header you loose velocity and therefore there is less vacum created in the collector at low revs. So if you have a header that is well matched to the cam and rpm range of the engine, any backpressure you add after the collector will hurt performance. So what you want to to is get the noise level down without adding too much backpressure. When working on my 1600 cc beetle engine i noticed that running a 2.5" straight trough mufflers right off the collector reduced combustion temperatures and allowed me to run more compression and more igniton advance. Performance increase was very noticeably improved over running a chamber style muffler, even before i upped the compression or altered ignition timing. I expect the same applies to all 4 stroke engines.
 
But we shouldn't get confused by automatically assuming that bigger pipes bring lower backpressure. Too-high gas speed will cause more backpressure in a constant flow situation, but since we're talking about a pulsing flow, anti-reversion properties play a role in the consideration of pipe size. Sudden changes in pipe diameter, or inappropriately sized pipe, can bring about high-pressure pockets that will reduce flow potential.

Managing the importance of flow with anti-reversion is the key. As Mike rather confusingly illustrated, NA race engines like to see a gradually decreasing pipe cross section, and the reason is because as the exhaust cools, the gas becomes more dense and requires less cross-sectional area to maintain velocity and pulse strength. With a turbo engine, anti-reversion (for the purpose of scavenging) properties are less important and bulk flow becomes the focus, so a larger exhaust is necessary, but I still believe that incorrectly sized pipe WILL increase backpressure, even if the pipe is on the large side of "correct".

Most of the myth that backpressure is REQUIRED to make an engine run right comes from the "carburated" era. Increasing exhaust flow brings about an increase in intake flow, and that requires different carb jetting and ignition events. Most people just toss on a bigger exhaust and when it doesn't improve power or when it brings about driveability problems they assume it doesn't work, but they don't play with mixture or timing to optimize the package. Believe it or not, the factory optimized all the systems to work properly with one another, with the compromizes assumed. When you mess with one aspect, you've got to mess with ALL of them.

Of course, one other thing that reinforces the need for backpressure are two-stroke engines - since by nature the overlap of the transfer ports and the exhaust ports are HUGE (by 4 stroke standards), and the air/fuel charge is actually supercharged by the crankcase, a certain amount of backpressure is required to keep the charge in the combustion chamber. The expansion chambers make power by allowing the high-pressure blowdown gasses to exit smoothly, but when the exhaust pulse hits the back wall (10-15 degree converging cone) it reflects back towards the exhaust port, "shutting off" the flow at a certain speed. When a 2 stroke comes "on the pipe", the exhaust system is tuned so the pressure wave hits the exhaust port just in time to maximize pressure within the combustion chamber. Since the crankcase compresses the air/fuel and forces it into the combustion chamber, exhaust scavenging isn't as important on 2 stroke engines, provided they have single or divided cylinders.

Supercharged engines are also very sensitive to backpressure - because all camshafts have overlap, added exhaust backpressure will always increase boost pressure. Whether or not this increases horsepower depends on how much boost is gained by how little backpressure. Radical cams have huge overlap most of the time, and that overlap allows plenty of air/fuel to flow out the exhaust ports, wasting fuel and horsepower. A little backpressure keeps the air/fuel in the CC, and it also allows a certain amount of pressure to develop in the chamber while both valves are open.

If your exhaust is TOO open, and your engine runs with an O2 sensor, you may get some fresh air drawn up the exhaust pipe, which will completely change the reading going to your ECU. Or, as has happened to me, the O2 sensor can run too cool which basically shuts it off. This is more evident in low-speed operation, but can certainly be confused as a lack of low-end torque.

Now, what pipe size is right for our engines? Your guess is as good as mine! :wink:
 
It is really a balancing act. You want a good smooth flow but if you reduce back pressure to much tourqe will suffer.
I was always looking for a way to get next to zero until we dynoed a harly I helped build.
We put several different exhaust comvos on it an found that some of them flowed to much and we lost low and mid range tourq.
We tinkered with it and ended up threading a 1/2 inch bolt into the tail pipes, which created just the right amount of pressure.
 
[quote:0710a74178]Too-high gas speed will cause more backpressure in a constant flow situation[/quote:0710a74178]

Need that one explained just a bit more if you could Matt.

JB
 
Before I explain, I should say that I see what you're saying, Jerome - once the exhaust gas exits into the cat, the effectiveness of the tuning pulse is lost and there's no reason to attempt to "tune" anything past the cat. I gotta believe that you're wrong, but I have to admit that the POWER gains to be had by tuning the rest of the exhaust are minimal. I believe that by tuning the lengths of the pipes between cat and muff #1, and between muff #1 and muff #2 (if used), and between muff #2 and the end of the exhaust, you can increase the effectiveness of the mufflers and quiet the system without adding backpressure. However, I have NO IDEA how to calculate this!!!

All I'm saying in your quote is that it takes a certain amount of energy to force gas down a pipe, and the faster you try to drive that gas the more energy is needed. Increasing it beyond a certain amount (some say 200 ft/min in an exhaust pipe, some say 350 ft/min) drives up the energy required sharply. I don't know how you'd derive what velocity is high enough to keep the gas moving in one direction while still keeping backpressure to a minimum, but I also don't know how to derive how much exhaust gas volume is coming from a given engine at a given time, so I guess it doesn't mater... Hey, it's just a discussion, right?

So let's toss this out for discussion: Why are collectors necessary on normally aspirated racing headers (where they exit to the atmosphere right after the collector) but zoomies are effective on supercharged motors? If collectors are necessary on NA engines why wouldn't they be needed on supercharged motors, and vice-versa? For now, forget about the need to connect the header to any exhaust system...
 
collectors help scavenge exhaust out by putting a vacum on the other runners( if properly designed) think about the venturis on a carburator.

on the supercharged car, the volume of gases exiting the chamber is usually larger than that of the NA car, but as for the lack of a collector, I would assume that in that case it creates some sort of back pressure problem or perhaps the duration on the cam is such that a collector isnt needed (i.e. with a little bit of blow thro in mind)
 
Thanks Matt. Shaping up to be a fun discussion.

Well let me make sure that in the course of this discussion of NA exhaust systems we aren't confusing "back pressure" with "exhaust tuning" or designing the manifold and related primary exhaust components to maximize the effect of the high pressure waves that you are trying to take advantage of in the NA exhaust scenario. They are two very different things, and I wonder if they are getting confused.

It may be better to split the discussion into two seperate focuses: what happens UP TO the end of the header/manifold and related tubing; and what happens AFTER that point.

Back pressure, per se, is probably an issue that we on this board concern ourselves with AFTER the tuned exhaust components (unless we are talking about designing headers and manifolds etc...which very few of the folks on this board are probably considering). Point being that AFTER those tuned components the least amount of back pressure the better [running with open pipe would be best performance scenario if it were legal and tolerable from a sound and emmisions stand point], and pulse tuning as it applies to the cat-back components are a pretty moot point.

If we are talking about the primary NA exhaust components (from the end of the header or manifold and related tubing forwrd to the head) then we are talking about pulse tuning to maximize the effect of 'finite-amplitude-waves' as they act to draw out exhaust gasses in the combustion chamber. But these waves are not 'back pressure' they are a very different animal.

I mention that for clarity only, not to suggest that no one in this thread understands the difference.

Regarding the supercharger [we can toss in T-chargers too for this] and the reason you don't rely on collectors etc - is that you are dealing with an intake charge that is already compressed by the s or t charger - as such you have a pressurized intake charge before combustion so as soon as you crack the ex-valve the pressurized exhaust is ready to blow out of the chamber, so the pulse tuning is a moot point - that is the reason that you design the cam differently as well - with very little overlap to prevent reversion when the intake valve opens while in an NA app you WANT the valve overlap to use the intake charge WITH the high-pressure waves to clear the cylinder.

That is why I think that the problems Dale mentioned in Volvo88's scenario were the result of pulling that tuned pipe from the manifold rather than too large an exhaust. Changing the pipe diameter at the manifold reduced the capacity of the high pressure waves to clear the cylinder, resulting in degraded performance.

My thoughts, Long winded as always - apologies. NEXT

JB
 
With a long exhaust system, the gases naturally lose energy as they travel away from their poiunt of origin- in terms of kinetic energy as well as thermal. This causes a loss of velocity both due to the exhaust volume decreasing as well as loss of kinetic energy due to friction, turbulence, etc.

What this means is that what might have had good velocity in a pipe with a 3" cross section right after the collector will be essentially at a standstill halfway down the system. Now you've got gasses that have actually stopped inside the pipe (well, it's all dynamic, more exhaust stacks up behind it and pushes it through so notheing actually "stops" except in an instananeous, mathematical sort of way). But basically you're back to forcing exhaust out, rather than it carrying itself out via its own energy.

So, apart from possible benefits from pulsetuning on the exhaust side (which is indeeed acheivable by some sort of complex calculation most likely similar to the inverse of intake runner tuning) the goal is to decrease the cross-sectional area of the pipe in PROPORTION to the overall energy loss of the exhaust, therefore keep velocity CONSTANT, since a change in velocity respresents a pressure change/drop which is as already stated, undesireable.

Almost like the good ol' days....
 
May I?

like JB, I await with bated breath for clarification on your too high speed statement, Matt...that should be good...

I will agree that the cat acts as what is sometimes referred to as a pressure wave terminator...meaning that the cat basically kills the pressure wave aspect of pipe tuning ...

...if I understand Mike's comment on the 4x4 pipe, it sounds like that is being utilized as both a pressure wave terminator and as a pressure recovery accumulator...to basically result in a quieter exhaust noise level...[interesting layout]

I would agree with the assertions that the old adage of too little backpressure hurts torque is from the carburetor era; and that that is because of messing with only certain parts of the factory system, and not sufficiently matching the rest of the components to work with those that were changed...it does become a bit of a vicious circle at times...
and I would also agree that reducing backpressure can still have negative effects in FI apps for the same reasons: mismatching of components...

what is the best pipe size? that depends on several things...it keys off the diameter of the exhaust valve; and proceeds from there: NA has its complicating factors; boosted has its own factors...

JB, I agree that from what was described, Volvo88's probs stemmed primarily from a mismatch of components...

...and unfortunately, I have to agree that most do not concern themselves with backpressure issues before the DP [turbo], or with the proper sizing of the primaries of the exhaust system at the ports [NA]...for the reasons you gave...[and JB, I am in the process of designing and building a turbo header in 2 configurations: stubby S/R; 4 tube pulse T4/T3]

I have been experimenting with both 2.5 and 3 inch systems; NA and turbo...with some interesting results...2.5 works well on the NA apps with stock exhaust manifolds; and I suspect that 2.5 would still be fine on those apps if a tri-y header were installed...

...Garrett equipped turbos appear to derive more benefit from 3inch than do those with the mitsu's...I suspect that is because the mitsu's are so restrictive on the hot side to begin with...in fact, I just ripped off a 3inch from a mitsu equipped turbo, installed a 2.5 system, and the power output and performance improved considerably: much smoother acceleration....[subjective observations, admittedly; but the car's owner heartily concurs]

sidenote: on a 21FTi with Garrett .63T and 3inch, I just installed a 90+ manifold opened up to flow well into the garrett housing...the results were impressive: boost threshhold lowered about 100rpm to below 2400rpm; 5psi reached easily by 2450-2500rpm; manifold and turbine housing glow after a hard pull was reduced in size and duration; and the limitations of the T cam are now very apparent: the engine runs out of cam well before 4000rpm...

I have noticed one complicating factor in the pipe sizing situation that can seriously affect effectiveness: size; design[configuration]; and location of the muffler(s)/tailpipe...I have observed considerable variations in performance just by changing the muffler/tailpipe [the location was not changed]; I was surprised at how much of a difference that made on a turbo-with-3inch system...and suspect similar variations would also result on a 2.5 system...so you can't always blame it all on the pipe size...

back to component mismatch: too often, people get things backwards: they throw in a cam, then try to match the rest of the car to it...the cam is the last item to be decided upon...

...and on pipe sizing: for NA, I have not seen any benefits with pipes over 2.5; and on turbos, 3inch does not like tiny turbines...YMMV...
 
Kenny - You've got the NA aspect right, and you're touching on what I understand to be the reason on the supercharged aspect of things. The supercharger is all the scavenger the engine needs - it creates a higher pressure in the intake than is in the exhaust, so during overlap when a conventional engine would rely on a good header to scavenge the exhausts, the supercharged intake just "blows" the exhaust gasses out. Like a NA engine, a turbo has higher pressures in the exhaust than in the intake, so it must rely on scavenging (if possible) to clear the cylinders.

Other than that, I'm not exactly sure why I brought it up... I was going to compare the importance and function of the collector to that of the exhaust pipe, pointing to sufficient gas speed being required to keep the exhaust moving at maximum energy while not creating so much pressure drop that the engine loses power, but I think the focus has shifted more to "after-cat" exhaust function and backpressure versus velocity versus pipe size, so the header and collector isn't really a valid arguement.

Jerome - Good explanation WRT "backpressure" and "exhaust tuning", however I believe they *sort of* go hand-in-hand. A strong tuning pulse comes from a pipe that is sized small enough that there is SOME resistance to flow velocity changes, and that seems usually to be small enough to bring a little backpressure as well. Notice that in some SBC applications, a 1 7/8" header outperforms a 1 5/8" header almost everywhere on the dyno, but the 1 5/8" header will outpull the bigger one on the track. Major generalization, and brings up the discussion of dyno testing versus real life plus the importance of torque versus all-out horsepower, but it suggests that the smaller pipe, which doesn't "flow" as much as the bigger pipe, makes a stronger pressure pulse. The fact that the exhaust starts moving through the primary pipe during the blowdown phase, when it's under several hundred PSI and is able to send the exhaust ports to near (or up to) sonic velocities, means that the "resistance to flow velocity change" of the smaller pipe is not a big deal, and it pays off later in the amplitude of the pressure pulse. Did I explain that right???

Now: with your supercharger and turbocharger header explanation... I wouldn't be so hasty to lump the turbocharger engine in with the supercharger engine like this. The turbocharger engine can still benefit from a scavenging exhaust pulse before the turbo, while the supercharger can't. The only problem with a turbocharger header/collector arrangement as I see it, is that instead of a vacuum, the header/collector might only be able to create a lower pressure pulse, and the mathematics involved hasn't been well-documented. Pressure's up in the manifold, which increases the wave speed of the pulse, temperature's up, which lowers the wave speed (I think...), etc. Anyway, I'm sure it can (and has) been done.

Cappy - pretty much what I've been sayin, but can you help me explain WHY keeping gas speed up helps reduce backpressure? Oh wait - I think you did.

Tom - I tried explaining it to Jerome earlier, and I'm not sure it worked out so well. Are you just busting my balls here or do you disagree? :wink: If so, speak up!

I agree - Mike's exhaust sounds like there's more trickery in the 4x4 section than just flow - probably allowing the pulses to cancel and soften so the muffler can easier deal with them, plus it simulates an "open header" design by providing a huge volume for the header to dump into. Very interesting and innovative, and a good catch on your part - I'm not sure I would have noticed the significance of the noise requirement.

The cat being a pressure wave terminator - well, I guess so, but there are still pressure waves coming out of your tailpipe, aren't there? (I, admittedly, haven't had a catalytic converter on any car I've owned in at least a decade... maybe they affect more than I remember.) Maybe if the body of the cat were redesigned it would allow the pressure waves to continue on downstream, but the problem is that the pipes entering and exiting the cat are usually so abruptly transitioned, they don't do anything good for pressure pulses OR flow.

I think the rest of your post backs up what I'm saying fairly nicely, thank you - NA engines don't like exhausts that are too big, restrictive turbos don't like exhausts that are too big, freer-flowing turbos appreciate the bigger exhausts. Pretty much boils down to how much HP (and therefore exhaust gas) the engine is making and how much energy is there to drive it. As well, turbos don't seem to like chambered mufflers, but they like the Ultra-Flow or Magnaflow style just fine. I believe that you've designed and tested these exhausts fairly - now we should be examining WHY you found what you found.

Sorry guys - I don't mean to be dominating the discussion, especially since I don't have nearly as much practical experience as a couple of you do. I'm also not as familiar with physics as I need to be to be an expert in this field - I'm just arguing my position as best I can, and I'll be willing to listen to (and hopefully learn from) other arguements. In my heart of hearts I believe that high-but-not-too-high gas velocity is important even BEHIND the cat, but I am missing one or two key pieces of knowledge to effectively prove it. However, I still could be wrong - I have been before!
 
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