Does Your Header Choice Matter

we are on the exhaust stroke now what's happening is the exhaust of course is being pushed out the exhaust here we have this residual these exhaust molecules left over in the cylinder here all this burnt exhaust the exhaust valves has started to close but now the intake valve oh the exhaust system over here has an incredibly powerful draw or at least it's supposed to so the Scavenging effect on the exhaust pulse is very powerful if the exhaust is designed right now one of the things that can diminish this and I see guys do this all the time is they put too big of an exhaust on oh I got a three inch exhaust you know on my 305 or whatever you know with a big long tube with with Shorty headers and big tubes well restriction to flow creates velocity over here if your exhaust system is too big it actually creates less draw on the exhaust side now I'm not saying that bigger exhaust is not going to help your engine because generally the exhaust systems on the factory exhausts are generally very restrictive so usually we can go a little bigger but you don't want to go too big on your exhaust especially if you have a smaller cubic inch motor because that is going to diminish the amount of draw that you have here if you don't believe me um uncork your pipes from your exhaust manifolds on your stock car and drive around with just exhaust manifolds you have no Scavenging whatsoever and that thing is going to run like dog poop so the air fuel is drawn in by the exiting exhaust pulse or velocity and the incoming air and fuel that's coming in here helps to evacuate the exhaust the residual exhaust out of the cylinder so everything is moving from the intake over to the exhaust and this is before I've even gotten to my this is before I've even gotten to my intake stroke now camshaft selection really has an effect on this because you can get a camshaft that has a lot more overlap but if I get too big on my overlap and I am going to get a choppy idle with that what that does is it moves the Power Band up higher in other words the camshaft is really not gonna if it down there where that engine is running I idling choppy it's not going to really make any power right so overlap you gotta you gotta really consider this and a better flowing your head is the less overlap you need so really what we're trying to create here guys is we're CR we're trying to create cylinder pressure we're trying to build a lot of cylinder pressure well the bigger the overlap the more that the overlap event is going to interfere with the combustion process so at low RPMs when this piston is moving relatively slow if I have big overlap it's going to bleed a lot of pressure out of that cylinder because the valves are open for so long here right and that's why the engine runs choppy because the valve opening during overlap is interfering with the combustion process and it causes low compression on the cylinder and it basically runs rough right so overlap is is a thing and we need it for Scavenging but I would caution you especially on the the newer engines that have really good flowing heads you don't need a Gob of overlap this is an old school thinking I have a I have a buddy named Larry and he is the most old school guy you've ever heard of he hates Electronics he hates fuel injection he thinks the new Motors are terrible and he's he still likes the old camel hump Chevys and the 327s nothing against them they're great Motors I'll probably never stop building the old classic small blocks or and Big Blocks but the fact of the matter is is that's really old technology it it just the the heads are just absolute garbage they are the only thing that those old heads are good for anymore is a classic car restoration they are they just don't flow compared to what we have now and compared to aftermarket heads so I want to caution you on overlap you don't need a ton of overlap even though it sounds cool and it idles rough you really got to watch that because you're losing cylinder pressure at low RPM and it kind of makes the engine run like a dog I've I've I had a guy bring me a a big block Chevelle like last year and it had an LS swap in it had a Corvette LS motor in it and they had a big giant monster cam in it that was Island rough and and you know that car ran terrible it just did not run good we ended up doing a cam Swap and we backed the cam way off and I think we probably gained 80 to 100 horsepower another issue that we have is exhaust valve opening exhaust valve opening okay so exhaust valve opening is a little bit different now we have something we have something called cylinder blow down now the definition of cylinder blow down is this in a race engine let's say you got like a pro stock or something it's a naturally aspirated race engine the goal with the engine builders especially at high RPM is to so I've got these events intake compression boom power so now I'm on my power stroke my piston is on the Power Stroke I'm on my way down if I have a well-tuned exhaust system that scavenges the cylinder very very well I can open my exhaust valve a little bit earlier when I'm about two-thirds of the way down the bore let's say my top dead center point is here my bottom dead center is here I'm about two-thirds of the way down listen the fire's out the Piston is simply traveling down from the inertia of the pressure that was exerted on it by the expanding gases when they ignite it so when I'm about two-thirds of the way down the bore I want to open my exhaust valve then because remember the exhaust valve has an incredible draw now we can increase the draw that the exhaust has by putting a better exhaust on headers do a very good job of increasing the draw on that cylinder here's the thing I have burnt exhaust in here now I want to evacuate as much of that burnt exhaust out of that cylinder as I can before the exhaust stroke starts right so I've got this exhaust pulse and I've got all of these header pipes that are Scavenging and pulling on these exhaust ports when I open that valve those exhaust Mount molecules are going to start to blast out that header tube even while I'm still on my way down on the power stroke now stock cams aren't very good at this they usually open that valve too late or later than it should and there's reasons for that for emissions and and all there's all kinds of reasons for it the manufacturers the engineers I don't want to get into all that because honestly a lot of that stuff is like over my head um this really is rocket science here but we uh we do know that cylinder blow down so talk about the Pro Stock for a second Pro stocks will have a hundred percent cylinder blow down what that means is the intake and exhaust systems are designed so well that from the point where the exhaust valve opens and honestly exhaust valve probably opens a lot sooner than it does with it with a street motor from the point where the exhaust valve opens right here to bottom dead center they will almost completely Evacuate the entire cylinder using Scavenging from the exhaust system now this does a couple of things number one it makes the engine produce a lot more power because remember these exhaust molecules in here they are solid pieces of matter they are a gas right but if you look at them at the molecular molecular level say that 10 times fast they are actually physical pieces of matter or molecule the more that is left in that cylinder after I start up on my exhaust stroke the more pumping work the engine has to do to physically push that exhaust out hopefully that makes sense so if I get a camshaft that opens this valve sooner rather than way down here like a stock cam I'm going to start to scavenge that cylinder and I'm going to start to evacuate a lot more of that exhaust and that frees up horsepower because I don't have to physically Pump It Out now this is highly dependent on a well-designed exhaust system the length and diameter of the header pipes and header tubes is critical because the Scavenging effect actually uses negative pressure waves okay so there's a lot going on here so when it comes to here's your header pipe so we're just going to look at one header pipe I know this is really short the header pipes are usually longer but here's the thing every time there's an exhaust pulse in here right and remember this is going really fast this is happening I mean if you're doing 6000 RPMs you got 50 50 exhaust Strokes in one second on every cylinder so that that's that's a this thing is moving that means the Pistons going through the four-stroke cycle 50 times per second so you got all these exhaust pulses on this so what happens is you get a shock wave an exhaust pulse is a sound wave it's a compression wave and it's basically moving at the speed of sound so you got these pulses every time that comes up there's a pulse that comes down here each one of these is an exhaust pulse now you get down here when these exhaust pulse get to The Collector where all where where they expand what happens is they hit the collector and they expand rapidly well when they expand for every action there's an equal and opposite reaction we understand that the expansion of that wave actually sends a negative pressure wave back up the header tube to the exhaust valve so this thing expands here boom and now we'll draw it down here I have a negative pressure wave and there's there's a bunch of waves and they're traveling back and forth they're reverberating from The Collector to the valve a well-tuned exhaust system is going to put this pressure wave when this pressure wave comes in your exhaust valve is closed here what we want is we want that negative pressure wave to come back hit the exhaust valve boom and when it hits it it bounces off and it goes back toward the collector so this pressure wave is bouncing between the collector and the valve boom boom boom boom boom boom and that's actually what you hear when the engine is running when you got headers or whatever you got a loud exhaust what you hear is you hear all these pulses and the exhaust is going actually what you're hearing is you're hearing shock waves reverberating back and forth in that header that's why it sounds that way so what I want is I want my shock wave to come back from The Collector hit the valve bounce off and turn around and head back again once that shock waves hits the exhaust valve and bounces off this shock wave has a lot of energy here if I can tune my exhaust right and if I have a longer head or tube that means that it's going to take longer time for that shock wave to get from The Collector back here and that generally puts my shock wave back at the valve at a lower RPM so it scavenges the cylinder better and this is why long tube headers work better at lower RPMs they make more power because what I'm doing is I am using that shock wave so let's say my shock wave came up boom it hits the valve turns around it's got all this energy now that's headed deflected back to the collector and just as I do that guess what my exhaust valve does my exhaust valve opens this is my exhaust valve opening event at the same time down here my piston is on its compression stroke and I have all this exhaust in here that listen that compression wave has a lot of energy and that compression wave actually pulls very hard on that cylinder and it will do a very good job of Scavenging that cylinder so this is why the exhaust system the header length and the design of the headers is important we call it a tuned exhaust system the definition of a tuned exhaust system is the exhaust system is going to put the shock wave at the valve just before the valve opening turn around and scavenge the cylinder in the RPM range where the cam is making its optimal power right it's only going to hit the valve and bounce off in a certain RPM rate it's either going to be really high if I have big short headers or it's going to be at a low RPM if I have smaller long tube headers getting these two events now finding out exactly when that happens in a particular engine is difficult and what RPM range it is but as a general rule if you want your engine to run really good and you have a mild cam shaft like in a Tahoe or a Suburban or something you want a long header you want a smaller tube long head or small tube for more velocity restriction to flow creates velocity if I had a garden hose here and I turn around and you across the room and I went haha you know the water's running on the floor here you wouldn't be worried about it but if I take and I restrict the garden hose with my thumb what happens in that hose is restriction to flow creates pressure pressure is going to build up in that hose and now I got velocity and I'm going to spray the guy across the room the same thing happens in here a smaller tube header is a restriction to flow now we don't want it too small because then it's going to choke everything off but we don't want this big giant header tube here because the problem is we don't have enough restriction to flow and the exhaust gases slow down so we want the header to diameter sized right as a general rule you got a big heavy vehicle you want small tube headers you want small long tubes because long tubes are going to make the compression wave scavenge the cylinder or Evacuate the cylinder better at lower RPMs if you got a high RPM Cam and you're running seven eight thousand RPMs you want a really short giant header on that thing like a race car for most street cars small tubes small long tube headers are going to be the best choice because that's what that does is that puts my compression wave at the valve opening event pulling on the cylinder between about 2500 and 35 to 4 000 RPMs which is where 95 of the street cars are going to run there's a lot going on here you guys and opening that into valve earlier is going to really be beneficial especially if I have a tuned exhaust system so those are the two events you really want to think about that's intake valve closing and exhaust valve opening

Channel: Myvintageiron7512

Views: 7,196

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Id: -M2BKX5-fCc

Length: 16min 37sec (997 seconds)

Published: Fri May 19 2023