<blockquote>quote:</font><hr>... on the exhaust side, the benefit I see to a tuned exhaust is cylinder scavenging. You create a negative pressure on the cylinder, thus it sucks out more exhaust and pulls more air/fuel in, thus bigger bang, thus more power.
Phil, this is true with TUNED headers that are run into each other through various means, some have "x" pipe between the two header collectors, some have "H" pipe in the same place, some ,like the '66-68 or 69 ford gt40s had a 360 degree system where the odd firing cylender on each side was run to the other bank and helped scavenge even more. This actually gave an exhaust sound more like a 6 cylendar since it was a smoother sounding. Also regular 4into 1 and sometimes tri-ys can be tuned to their respective bank(head) and you get the shock wave of one exhaust pulse arriving at the collector just when the next exhaust pulse enters the runner leading to the collector, thus "pulling" with it the next pulse, this is only effective within certain rpms and therefore, you must tune runner length as well as diameter to the rpm /power band that the engine will be running in. confused yet? [img]smile.gif[/img]
But isnt' the actual torque of the motor generated when the bang happens and the fuel/air mixture ignites? (This is only part of it, the middle part. This is the belief that too many people fall into, where they think "If I put a huge duration cam in my engine and leave everything else alone, I'll get radical power!" its a scale, all must be balanced- such is life)
Thus the motor is closed to the negative(or positive as the case maybe in smogged vehicles) (or ambient in Matt's ideal) pressure on the exhaust side, right? (sometimes this is true, it does depend on alot of variables. valve timing and the like as to when the exhaust valve closes...to be continued...)
Or is valve overlap such that at low RPM a high velocity exhaust will actually suck fuel/air out of the cylinder (or maybe reduce compression?) reducing torque?
This is an interesting question, as it can be true also, and false- if you have a long duration cam, you will, at low rpm (idle-2000 rpm lets say) have your intake valve opening and exhaust valve closing, causing not only the 2 gasses to intermingle fresh a/f mixture with spent exhaust gasses in the cylender, but also in the intake manifold and exhaust header, this is why most drag cars with lumpy cams sound really cool/sick when they idle, the plugs cannot ignite exhaust gas a second time so the engine sounds like its going to die. this problem alleviates itself as rpm climbs and the engine then makes a hellacious effort at turning rubber into toxic smoke! if you have a short duration cam, you will more likely not allow the exhaust system to "see" the intake system through the cylender. short duration cams also have a very smooth idle and run very well untill they hit high rpms, say 5000 rpm in a typical v8 where they really have no power, as opposed to the drag car that has rough idle and makes most of it's power at and above 5000rpm. Now, saying that, I'll say this, the exhaust pipe diameter, flow characteristics make almost no difference at idle or low rpm, idle to 2000 rpm. what it does do though, with headers, is allow for some extra scavenging throughout the entire rpm range of the engine, most noticeable at higher rpm, 2000 to 5000 rpm in the typical v8.
open pipes, larger diameter, just allow the pumping of the piston in the exhaust stroke to work that much less to free the cylender of spent gasses, the exhaust pulse from the burning(exploding more accurately) gasses doesn't exit itself into the cylender under its own force, but is pushed out by the piston which is of course tied to the crank making its work the result of a different piston's power stroke. now where the argument seems to be, apparently, (I don't really understand the thinking too well) is that larger pipe allows the exhaust gas to slow down and presumably it gets tired and finds a starbucks some where and waits to find a muffler to hang around in, while it drinks a mocha latte', then all the rest of his gas buddies do the same until you've got a traffic jam around the coffee bar and no one leaves.
The idea that exhaust gasses moving fast like water down a waterfall is completely rediculous, it can be compressed (unlike fluid) and will back up or slow if restricted.) However, since even though the pipe is larger, and the gasses may not be moving as fast at a given rpm, there is still the same amount of them, just moving slower. this is good since it will allow more gas to be packed into a pipe (ristricted somewhat just by the nature of being a confined tube for containment or conveyance)without it being over packed and back pressuring the system as quickly, which would cause the piston's exhaust stroke to sap more power from the power available to spin the crank and make motion in the old drivetrain thus motivating the vehicle. In the smaller pipe, the gasses have to move faster and are more dense because they are RESTRICTED by pipe diameter and this will be more easily packed by the normal running of the engine, the longer the pipe the more it wil restrict also, although to a somewhat less extent.
Whereas more restrictive exhaust (read mid velocity) will scavenge the cylinder at lower RPMs without depleting the fuel/air mixture, right? (once again, depleting f/a mix is really only a concern with long duration cams)But a more restrictive exhaust will be just that (restrictive) at higher rpms causing exhaust gas to remain in the cylinders thus reducing power at higher RPMs? (this is acttually more likely, but like I said more likely if using a long duration cam, the exhaust valve is usually closed sooner not allowing this in most street driven v8s with a mild performance cam.
Thus why VVT (I don't mean to seem dumb now, but whats vvt? valve to valve timing?) flattens torque curves. You can have the right amount of scavenging for a larger RPM band right?
To answer this and hopefully wrap all this together, I'll say this: larger rpm band meaning broader power band (torque at low rpm and hp at high rpm instead of both at only high or only low rpm)yes, if you allow a larger exhaust pipe, you allow more flow, this translates into not only more axhaust gas, but the efficient handling(quicker exitting) of more gas, but it works on lower rpm as well as (designed for) higher rpm. Losing the exhaust gas and freeing the space it occupied for more exhaust gas is what big pipes are all about. this is only the exit side of the engines power circuit though, intake manifold, carb and intake port/valve in combustion chamber, is the beginning. spark, compression ratio/pistons, exhaust port/valves/combustion chamber, headers/pipes/ mufflers/cat, are the end. the cam is the brain behind the intake valve to the exhaust valve, everything else thats bolted on must be matched to it all and also to the vehicle and purpose.
help me out guys
Hope that helps phil. -Matt
[ March 31, 2002: Message edited by: Dive 30 ][/QB]<hr></blockquote>
[ April 01, 2002: Message edited by: Matt W ]</p>