I'm looking for answers as to why my sbc loses so much power when there is a seemingly small exhaust leak at the manifold exhaust tube junction? I always thought more flow=more power right? Open headers make more power. My dad used to run stock cars and I remember him talking about back pressure on the valves being necessary but I don't understand why. Is it because the exhaust and intake processes overlap a little? A sincere thanks to those who can shed some light on this for my edification (and perhaps that of others.)
Borrowed from another site: "THE MYTH OF BACKPRESSURE" …is probably the most widely misunderstood concept in engine tuning. IMO, the reason this concept is so hard to get around lies in the engineering terms surrounding gas flow. Here's the most impotant ones you need to be aware of to understand the things I'm about to say: BACKPRESSURE: Resistance to air flow; usually stated in inches H2O or PSI. DELTA PRESSURE (aka delta P): Describes the pressure drop through a component and is the difference in pressure between two points. One other concept needs to be covered too, and that's the idea of air pressure vs. velocity. When a moving air column picks up speed, one of the weird things that happens is it’s pressure drops. So remember through all this that the higher the air velocity for a given volume of gas, the lower it's internal pressure becomes. And remember throughout all of this that I’m no mechanical engineer, simply an enthusiast who done all the reading he can. I don’t claim that this information is the absolute truth, just that it makes sense in my eyes. Ok, so as you can see, backpressure is actually defined as the resistance to flow. So how can backpressure help power production at any RPM? IT CAN'T. I think the reason people began to think that pressure was in important thing to have at low RPM is because of the term delta pressure. Delta pressure is what you need to produce good power at any RPM, which means that you need to have a pressure DROP when measuring pressures from the cylinder to the exhaust tract (the term "pressure" is what I think continually confuses things). The larger the delta P measurement is, the higher this pressure drop becomes. And as earlier stated, you can understand that this pressure drop means the exhaust gas velocity is increasing as it travels from the cylinder to the exhaust system. Put simply, the higher the delta P value, the faster the exhaust gasses end up traveling. So what does all this mean? It means that it's important to have gas velocity reach a certain point in order to have good power production at any RPM (traditional engine techs sited 240 ft/sec as the magic number, but this is likely outdated by now). The effect of having larger exhaust pipe diameters (in the primary, secondary, collector and cat-back exhaust tubes) has a direct effect on gas velocity and therefore delta P (as well as backpressure levels). The larger the exhaust diameter, the slower the exhaust gasses end up going for a given amount of airflow. Now the ***** of all this tech is that one exhaust size will not work over a large RPM range, so we are left with trying to find the best compromise in sizing for good low RPM velocity without hindering higher RPM flow ability. It doesn't take a rocket scientist to understand that an engine flows a whole lot more air at 6000 RPM than at 1000 RPM, and so it also makes sense that one single pipe diameter isn't going to acheive optiaml gas velocity and pressure at both these RPM points, given the need to flow such varying volumes. These concepts are why larger exhaust piping works well for high RPM power but hurts low RPM power; becuase is hurts gas velocity and therefore delta P at low RPM. At higher RPM however, the larger piping lets the engine breath well without having the exhuast gasses get bundled up in the system, which would produce high levels of backpressure and therefore hurt flow. Remember, managing airflow in engines is mainly about three things; maintaining laminar flow and good charge velocity, and doing both of those with varying volumes of air. Ok, so now that all this has been explained, let's cover one last concept (sorry this is getting so long, but it takes time to explain things in straight text!). This last concept is why low velocity gas flow and backpressure hurt power production. Understand that during the exhaust stroke of a 4 stroke engine, it's not only important to get as much of the spent air/fuel mixture out of the chamber (to make room for the unburnt mixture in the intake system), it's also important that these exhaust gasses never turn around and start flowing back into the cylinder. Why would this happen? Because of valve overlap, that's why. At the end of the exhaust stroke, not only does the piston start moving back down the bore to ingest the fresh mixture, but the intake valve also opens to expose the fresh air charge to this event. In modern automotive 4 stroke engines valve overlap occurs at all RPM, so for a short period of time the exhaust system is open to these low pressure influences which can suck things back towards the cylinder. if the exhaust gas velocity is low and pressure is high in the system, this will make everything turn around and go the opposite direction it's supposed to. If these gasses reach the cylinder they will dilute the incoming mixture with unburnable gasses and take up valuable space within the combustion chamber, thus lowering power output (and potentially pushing the intake charge temp beyond the fuel’s knock resistance). So having good velocity and therefore low pressure in the system is absolutely imperative to good power production at any RPM, you just have to remember that these concepts are also dependent on total flow volume. The overall volume of flow is important because it is entirely possible to have both high velocity and high pressure in the system, if there is simply not enough exhaust piping to handle the needed airflow. It’s all about finding a compromise to work at both high and low RPM on most cars, but that’s a bit beyond the scope of this post. All I am trying to show here is how the term backpressure is in reference to a bad exhaust system, not one that creates good low RPM torque. You can just as easily have backpressure at low RPM too, which would also hurt low RPM cylinder scavenging and increase the potential for gas reversion. And understand that these tuning concepts will also affect cam timing, though that is again probably beyond the scope of this post. At any rate, hope this helps, peace. " -here's a reply to the above post- "I've been seeing a resurgence of the backpressure misnomer, but didn't have the time or inclination to write it up. So, again, thanks. There is one thing I'd like to add to texan's work: Exhaust Scavenging In essence, this is the opposite of the exhaust reversion that texan describes. Reversion: at the beginning of the intake stroke during cam overlap, exaust gas in the header is under high pressure (negative delta P) and is pushed back into the cylinder, diluting the new air/fuel charge. Scavenging: at the beginning of the intake stroke during cam overlap, the momentum of the exiting exhaust gasses creates a brief vacuum (positive delta P) in the header, pulling out the remaining exhaust gases from the combustion chamber, and allowing the new air/fuel charge to be full-strength. Scavenging is also the reason for differently shaped headers (4-2-1, 4-1) and collectors. We use the momentum of exiting exhaust from one cylinder to scavenge exhaust from another that is next in the firing order! The different shapes allow for this to happen at different airflow velocities thus at different RPM bands. Scavenging takes advantage of the momentum of the exiting gasses. In essence, the fast moving exhaust pulse pulls a vacuum behind it. Momentum is mass times velocity. So not only do we need to keep the velocity high to prevent reversion - but it greatly improves the scavenging effect. Thus we have a balancing act (as others have pointed out). We want to minimize friction to lower the backpressure as much as possible - larger pipes have less friction because they have less surface area per unit volume. But we want to increase the delta P as much as possible to prevent reversion and increase scavenging effects - smaller pipes increase delta P because they increase velocity. There are lots of tricks to try to widen the useful RPM band (stepped headers) or to increase the overall effiency (ceramic coated exhausts), but it's still subject to this basic tradeoff: Friction vs. Velocity AKA: Backpressure vs. Delta Pressure You want low friction and high velocity. You want low backpressure and high positive delta pressure. "
2 or 3 times I felt like low rpm power or at least throttle response went wonky when a fully muffled system broke completely. I mentally wrote it off to reflected pressure waves messing up the carb during overlap. Reversion and double or triple trips thru the carb + super rich. That theory doesn't work so well for a modern Fuel Injection car
gimpyshotrods, excellent post. It has amazed me for years how many of my gearhead friends are under the false notion that "some back pressure is good". Grab a book on thermal dynamics and we soon discover how complicated exhaust flow can be. The least restrictive is the best but also the loudest. I started building my own exhaust systems at 16 on my dirt track 2 stroke motorcycles. I learned a lot fast. After experimenting on the cars I quickly learned how complex a V8 system becomes. Like all things automotive hi performance there are compromises to be addressed. Getting the 2 race cars to pass sound is easy but it sure hurts the top end power. Straight through packing style muffler has not worked for me in killing the sound at the race track ( 103db ). So after two failed attempts with packed style mufflers I installed some used deflection or friction style (Flowmaster) I passed sound by a large margin but they killed the topend of my engine. On my old Buell street bike I was sick and tired of the aftermarket mufflers breaking mounts and blowing out the packing . I installed a SpinTech muffler of the appropriate diameter and length. A lot quieter and now I noticed at wide open throttle the float bowl would dry up. That's because the engine breathing was increased. Cure was quick and easy petcock change. Made a believer out of me. So now the race cars get SpinTech's and my newly acquired Buell will be getting one also. I prefer the sound of a straight through packed style muffler. They also have the advantage of being quieter inside a car or truck. For me this only works on street cars because of room for a long muffler. To address the OP, with an exhaust leak at the head or manifold to pipe joint the laminar flow is broken and the scavenging reduced so you experience less flow through the system.
Slack, complicated subject but a small leak shouldn't cause a large power loss. I assume you haven't dyno numbers, have you compared 1/4 times or anything definite?
Back pressure on the valves is not necessary. You need length to take and vantage of low end torque. the reason they run zoomies on real race cars is to make them lay down on the low end so they can get out of the gate, then when they get revved up good and tight they can take advantage of the shorter length. Open headers don't make more power just more noise. On a car that is properly tuned and the header and collector is properly tuned then open headers make more power but it is more then just the headers that are working to do that.
The people, "They" have ignorantly and wrongly named the entire concept as "BACK PRESSURE" The concept is without flaw, but its not called back pressure. Open headers will only make more power at a very specific rpm that they are tuned for. Every other rpm will be down on power.
Thanks for the great write up Gimpyshotrods! Maybe going slightly off the topic: In this video on the building of Chris Rusch's 30 Sedan I noticed that the primaries are made of two cones - to form some sort of expansion chamber(?) Never seen anything like this and finding it hard to understand how it would benefit - I'm keen to hear if anyone can explain the thinking on this?
As an expansion chamber opens into larger diameter it actually sucks the exhaust gases out of the cylinder and also some of the fresh charge along with it, then when it hits the reversed cone it stops the flow and will essentially stuff some of the exhaust back into the cylinder to prevent pulling out too much of the fresh charge and lose what you had drawn into the cylinder. Expansion chambers are very commonly used with a 2 stroke engine, not 4 strokes.
Thanks Dick - kind of what I thought, though I'd wonder how one would arrive at the numbers especially when coupled with a full system. If there are real benefits to be had I would have expected to see the idea in race/high performance applications - maybe I just haven't been looking closely enough?
It's not a large loss of power but it is significant and very noticeable (and not a little annoying). I would guesstimate at least 10% loss. From what I've read, this is due to the fresh mixture being sucked out or "scavenged" due to overlap in the exhaust to intake portion of the cycle. I'm sold on that, especially due to the proximity of the leak. I was just really curious to know why. I know I'm talking over my head here so bear with me and save the smart aleck comments. All this discussion leads to the obvious observation (based on what I've read here) that exhaust is fixed and/or sometimes tuned to a desired optimal point for power in one part of the RPM range. So if you had something like a conical iris or turkey feathers on a fighter jet you could (theoretically) adjust for optimum power throughout the RPM range? Does that make sense? Yes, no, too much? I'm just thinkin out loud here (and way off the original topic.)
Like I said, it is used all of the time on 2 strokes because they don't have the flexability of cam timing to work for them, so the expansion chamber sort of helps alleviate that shortcoming. Once in awhile you will see reverse cone megaphones on 4 stroke motorcycles but it really doesn't benefit their performance like it does on 2 strokes. They have been used on racing 2 strokes, including competition chain saws for many years.
I hope some of you guys are reading this. I can't explain why, but the more stock the motor the more it relies on the back pressure of an exhaust system to perform. That's why I see cut up hot rods that weigh less than HALF the car their motor came out of and don't have the power to pull steam off a hot dog. I'm talking ultra light fenderless, hood less, and running board less 30's cars with Hemis, Nailheads, and vintage Caddy motors that came out of 4000 pound cars that can barely spin ONE 4" wide bias tire. LOL. Short, open headers don't seem to work on stock stuff.
What's missing there is velocity of the exhaust gasses. I can almost guarantee that the primaries are way too big, way too short, and open into a velocity killing cone. Looks cool You can not build velocity removing containment of the pressure.
I didn't look at the vid, but is it possible they're playing with divergent cone collectors? By necking the collector down at a specific length past the primary tubes and going back to the full diameter velocity is increased. Like a venturi but going out vs in. There was an obscure mention of such a thing in an older GM Performance engine book. Expansion chambers are essential to 2 stroke power for the reasons stated above. I happen to believe there's nothing to ever be gained in a 4 stroke application with conventional engines. Surely someone can come up with some odd 3 cylinder 4 stroke industrial gig but for our stuff, nuttin. I too hear all the "...need back pressure..." wives tales. I find that even with a stock engine you can get cleaner combustion by making the manifolds larger than the ports. The smaller port into the larger exit creates a boundary layer within the flow during overlap and less spent gasses find their way back into the chamber. The higher the RPM and overlap the better the engine will respond. Going out on a limb, I might think that the OP's initial question and effect may be perceived vs a measurable reality. Unless it's fuel injected and depends upon an O2 sensor the odds are that it's the nasty sound covering up what's normally sensed as a smooth power delivery. That's not an insult, actually an example of someone that's as well tuned to their car as their engine is. Just sayin...
Coanda effect used in 4 stroke exhaust, basic cone with in a cone..Was an article in HRM years ago, if any one has access to HRM archieves..searching Coanda effect did not turn up the exhaust work, I believe it was done in UK...X2 with Highlanders post..
Somewhere I have a book that I used back in the days I raced motorcycles, to build my own expansion chambers, that gives the formula for calculating the sizes and lengths of all of the elements that are in the entire exhaust system. Although I haven't used it for about 40 years, I'm sure I still have it in my collection somewhere.
This appears to give a fair overview into expansion chamber design. http://froggy.se/skalmanmc/rd350/tuning/expan1.htm
Not a scientist nor did I sleep in a HI Express last night, but my first exhaust build was a hot rod Harley Sportser/Buell motor. With open pipes it ran like crap and had little power except at WOT. Added baffle and suddenly it had usable power throughout the rpm range and still pulled up top. So slowing down the flow a bit helped out a bunch. Can't tell you why? And my CZ 2 stoke will not even start with our an expansion chamber. Just puffs and drools.
When you open the pipes up, you need to make the mixture richer by rejetting, especially them harleys. So instead of re jetting richer, you changed the circumstance for needing a rich mixture by adding in the baffles. Wide open drag pipes on Harleys only work when DRAG racing harleys at WOT high rpm.
Exactly, when you make any changes that affect the breathing characteristics of any engine, you have to tune it properly or it will not make the most power it is capable of doing.
Have this exhaust system on my Harley Davidson 113 cubic inch JIMS Engine. https://www.rbracing-rsr.com/exhausttech.htm Might be voodoo science but I assume the same theory applies to any 4 stroke internal cumbustion engine. Yeah and I'm a doctor specializing in wart removals and meat injections.
While we're talking exhaust, the recommendation on installing a crossover pipe, paint the new pipes and where the paint stops burning off, install the crossover a little forward of that. Anyone ever followed that? I always put mine where I can still access shift linkage or trans pan. For most street cars a 2 1/2 inch system minimize bends and make the bends as large as possible to prevent crushing the pipe and a set of turbo mufflers is gonna be about the best performance. We don't have the tools to science it out and not many guys wanna buy multiple exhaust systems to try different ideas. The theory is great, I've read a bunch of the articles, I'm a mechanical engineer and am capable of calculating all that shit out, I choose to call Hooked and have them send me a set of headers. Unless you have a tube frame car it's difficult to get a true equal length header under the car, with equal length you want same bends, then decide which pipes need to be parallel or across from each other to give the best scavenging effect. Anyone cut their collectors off to try different cone shapes inside the collectors, and different collector lengths? Some of this stuff just isn't applicable for full body vehicles, it only works on cars that are dialed in for a specific power band---drag strip, nascar, salt flats. I still like talking about it though
http://www.jalopyjournal.com/forum/threads/heres-some-really-good-exhaust-info.737874/ 2 very good links within
There is no way to make blanket statements about tubing size. Doing so reveals an absence of understanding of this science, and yes, it is a science.
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