Future magazine article
09-19-2002, 11:48 AM
#1
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Future magazine article
This is a future article for FAMILY AND PERFORMANCE BOATING MAGAZINE, please read it and give me some feedback before I turn it in.
Dennis Moore
THE HIGH PERFORMANCE V8 EXHAUST SYSTEM
By Dennis Moore
Improving the exhaust system on an engine is one of the best ways to increase engine performance. Most people assume that an increase in performance through the exhaust will always come with an increase in the noise level of the engine. This may be true in some instances but not always. By understanding the nature of the exhaust system a boat owner can increase the performance without increasing the noise level. An exhaust system on a V8 engine is different than the exhaust system on a four or six cylinder engine. A four cylinder engine will have an exhaust pulse discharging into the exhaust pipe every 180 degrees of crankshaft rotation. A six cylinder engine will have an exhaust pulse discharging into the exhaust pipe every 120 degrees of crankshaft rotation and an eight cylinder engine will have an exhaust pulse discharging into the exhaust pipe every 90 degrees of rotation. At 4,500 rpm the four cylinders exhaust system has only 9,000 exhaust pulses per minute. A six cylinders exhaust system has 13,500 exhaust pulses per minute and an eight cylinder has 18,000 exhaust pulses per minute. With 180 degrees of engine crankshaft rotation between exhaust pulses (on a four cylinder engine) the exhaust pulse will have traveled down the exhaust pipe a considerable distance before the next exhaust pulse enters the pipe. With an exhaust pulse entering into the exhaust pipe every 90 degrees of crankshaft rotation (on an eight cylinder engine) the exhaust pulse will have barely started its travel down the pipe when another exhaust pulse is released right behind it. If there is any restriction in the pipe (that slows down the speed of the exhaust pulse, such as a small exhaust pipe or muffler) the exhaust gases can back-up inside the exhaust pipe (and the engines cylinders). This is known as back pressure and can prevent the engine from making maximum horsepower. A simple solution (it would appear) for reducing back pressure is to divide a V8 engine into two separate exhaust systems, one for each bank of four cylinders. Some less informed people (not readers of FAMILY AND PERFORMANCE BOATING) think that this solves our problems of exhaust pulses stacking up in the exhaust pipe. They think that instead of having an exhaust pulse dumping into one exhaust pipe every 90 degrees we now have two four cylinder engines with each engine having an exhaust pulse dumping into its own exhaust pipe every 180 degrees. If the exhaust pulses dumped into each exhaust pipe alternately, an exhaust pulse on the left side of the engine and then an exhaust pulse on the right side of the engine (back and forth), we would have an exhaust pulse entering each exhaust pipe every 180 degrees of crankshaft rotation. Unfortunately this is not the case because a V8 engine can not simply be divided into a left side four cylinder engine and a right side four cylinder engine. In any V8 engine we have two cylinders on each side that discharge an exhaust pulse into the exhaust pipe in succession (only 90 degrees of crankshaft rotation separates the pulses). On the left side (port) we have cylinders 5 and 7 discharging exhaust pulses within 90 degrees (of crankshaft rotation) of each other and on the right side (starboard) we have cylinders 8 and 4 discharging exhaust pulses within 90 degrees (of crankshaft rotation) of each other. When the two cylinders on each side fire in succession they will temporarily overwhelm each exhaust pipe with exhaust gases creating a restriction to exhaust flow. This sharp spike in increased exhaust pressure (inside the exhaust system) may not only create a restriction to exhaust flow but may cause some exhaust gases to reenter another exhaust port/cylinder through a (still open) exhaust valve. For example let’s look at the left side of the engine; when the #5 cylinder has expelled all of the exhaust gas from the #5 exhaust port/cylinder, the #7 exhaust valve will just be opening with high pressure exhaust gas shooting out from the #7 exhaust port/cylinder. Because the exhaust pipe is overwhelmed with exhaust gas (because the cylinders fired in succession) some of the gas from #7 may enter into the #5 exhaust port/cylinder. The exhaust gases will contaminate the fresh air/fuel mixture and hurt the production of horsepower in the #5 cylinder. As mentioned earlier both sides of the engine have this problem with cylinders #8 and #4 on the starboard side (the #4 cylinder will contaminate the #8 cylinder with exhaust gases).
An easy solution to this problem would be to install an exhaust pipe that is so large that there is no restriction to exhaust flow. This solution is also a very noisy solution; a larger exhaust pipe will definitely be much louder. A quieter solution would be to isolate the cylinders that will pass exhaust gases between themselves. Keeping the #5 and #7 cylinders exhaust pulses separated (as much as possible) on the port side and #8 and #4 cylinders exhaust pulses separated on the starboard side will help power production by reducing exhaust gas contamination. An exhaust manifold that is manufactured with separate passages, in an attempt to separate the cylinders that fire in succession, will increase performance without increasing the noise level.
Dennis Moore
THE HIGH PERFORMANCE V8 EXHAUST SYSTEM
By Dennis Moore
Improving the exhaust system on an engine is one of the best ways to increase engine performance. Most people assume that an increase in performance through the exhaust will always come with an increase in the noise level of the engine. This may be true in some instances but not always. By understanding the nature of the exhaust system a boat owner can increase the performance without increasing the noise level. An exhaust system on a V8 engine is different than the exhaust system on a four or six cylinder engine. A four cylinder engine will have an exhaust pulse discharging into the exhaust pipe every 180 degrees of crankshaft rotation. A six cylinder engine will have an exhaust pulse discharging into the exhaust pipe every 120 degrees of crankshaft rotation and an eight cylinder engine will have an exhaust pulse discharging into the exhaust pipe every 90 degrees of rotation. At 4,500 rpm the four cylinders exhaust system has only 9,000 exhaust pulses per minute. A six cylinders exhaust system has 13,500 exhaust pulses per minute and an eight cylinder has 18,000 exhaust pulses per minute. With 180 degrees of engine crankshaft rotation between exhaust pulses (on a four cylinder engine) the exhaust pulse will have traveled down the exhaust pipe a considerable distance before the next exhaust pulse enters the pipe. With an exhaust pulse entering into the exhaust pipe every 90 degrees of crankshaft rotation (on an eight cylinder engine) the exhaust pulse will have barely started its travel down the pipe when another exhaust pulse is released right behind it. If there is any restriction in the pipe (that slows down the speed of the exhaust pulse, such as a small exhaust pipe or muffler) the exhaust gases can back-up inside the exhaust pipe (and the engines cylinders). This is known as back pressure and can prevent the engine from making maximum horsepower. A simple solution (it would appear) for reducing back pressure is to divide a V8 engine into two separate exhaust systems, one for each bank of four cylinders. Some less informed people (not readers of FAMILY AND PERFORMANCE BOATING) think that this solves our problems of exhaust pulses stacking up in the exhaust pipe. They think that instead of having an exhaust pulse dumping into one exhaust pipe every 90 degrees we now have two four cylinder engines with each engine having an exhaust pulse dumping into its own exhaust pipe every 180 degrees. If the exhaust pulses dumped into each exhaust pipe alternately, an exhaust pulse on the left side of the engine and then an exhaust pulse on the right side of the engine (back and forth), we would have an exhaust pulse entering each exhaust pipe every 180 degrees of crankshaft rotation. Unfortunately this is not the case because a V8 engine can not simply be divided into a left side four cylinder engine and a right side four cylinder engine. In any V8 engine we have two cylinders on each side that discharge an exhaust pulse into the exhaust pipe in succession (only 90 degrees of crankshaft rotation separates the pulses). On the left side (port) we have cylinders 5 and 7 discharging exhaust pulses within 90 degrees (of crankshaft rotation) of each other and on the right side (starboard) we have cylinders 8 and 4 discharging exhaust pulses within 90 degrees (of crankshaft rotation) of each other. When the two cylinders on each side fire in succession they will temporarily overwhelm each exhaust pipe with exhaust gases creating a restriction to exhaust flow. This sharp spike in increased exhaust pressure (inside the exhaust system) may not only create a restriction to exhaust flow but may cause some exhaust gases to reenter another exhaust port/cylinder through a (still open) exhaust valve. For example let’s look at the left side of the engine; when the #5 cylinder has expelled all of the exhaust gas from the #5 exhaust port/cylinder, the #7 exhaust valve will just be opening with high pressure exhaust gas shooting out from the #7 exhaust port/cylinder. Because the exhaust pipe is overwhelmed with exhaust gas (because the cylinders fired in succession) some of the gas from #7 may enter into the #5 exhaust port/cylinder. The exhaust gases will contaminate the fresh air/fuel mixture and hurt the production of horsepower in the #5 cylinder. As mentioned earlier both sides of the engine have this problem with cylinders #8 and #4 on the starboard side (the #4 cylinder will contaminate the #8 cylinder with exhaust gases).
An easy solution to this problem would be to install an exhaust pipe that is so large that there is no restriction to exhaust flow. This solution is also a very noisy solution; a larger exhaust pipe will definitely be much louder. A quieter solution would be to isolate the cylinders that will pass exhaust gases between themselves. Keeping the #5 and #7 cylinders exhaust pulses separated (as much as possible) on the port side and #8 and #4 cylinders exhaust pulses separated on the starboard side will help power production by reducing exhaust gas contamination. An exhaust manifold that is manufactured with separate passages, in an attempt to separate the cylinders that fire in succession, will increase performance without increasing the noise level.
09-19-2002, 12:10 PM
#3
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Dennis, it`s a very informative article and you do a great job of explaining the theory. As a reader, the first thing that popped into my mind was: "where do I get this manifold?" And "Who makes a manifold designed to do this?" You may want to touch on that information as well.
09-19-2002, 12:44 PM
#6
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Posts: n/a
Sounds Great Dennis,, I am very curious about this subject as I posted previously noise restrictions are coming/here in many places. Mainly what are the benefits power wise of going to seperated exhaust pasages in a header or manifold like gils or imco's for thru prop (quiet) or a well muffled exhaust. How much power/torque gain can you expect without going to stright thru hulls as well ? The #5 cylinder will no doubt run cooler at a minimum. The #4 and 8 don't seem to interfere as much with each other perhaps sine they are further apart in the log, at least on my engine 502 mag i can see the dif on the #5 plug versus the rest, its a lighter shade of tan
09-19-2002, 01:18 PM
#8
Gold Member
Gold Member
Good question Cobra Marty. I've often wondered that myself.
There has to be way to rig up marine manifolds when the engine is on the dyno so we can get a realistic look at how the engine will perform as a package. Those long tube dyno headers can make a difference.
There has to be way to rig up marine manifolds when the engine is on the dyno so we can get a realistic look at how the engine will perform as a package. Those long tube dyno headers can make a difference.
09-19-2002, 02:37 PM
#9
Gold Member
Gold Member
I read you proposed article and enjoyed it.
It reads very close to parts of the chapter on exhaust maifolds in your SBC Marine Performance book.
You didn't mention any scavenging effect the pulses create, if any, of an idividual runner type manifold vs an open chamber type production manifold.
You talked about how the exhaust pulses can stack up and possibly dilute the incoming fuel/air charge but not how a properly functioning system can help draw the fresh charge into a cylinder.
In the part about how high pressure exhaust gasses flowing from one cylinder to another could happen and that "an easy solution would be to install an exhaust pipe so large that there is no restriction", but you didn't mention how a pipe (collector) that large would actually hurt performance by reducing exhaust gas velocity and increasing the likelyhood of the pulses stacking then diluting the incoming fuel/air charge.
Very good though.
It reads very close to parts of the chapter on exhaust maifolds in your SBC Marine Performance book.
You didn't mention any scavenging effect the pulses create, if any, of an idividual runner type manifold vs an open chamber type production manifold.
You talked about how the exhaust pulses can stack up and possibly dilute the incoming fuel/air charge but not how a properly functioning system can help draw the fresh charge into a cylinder.
In the part about how high pressure exhaust gasses flowing from one cylinder to another could happen and that "an easy solution would be to install an exhaust pipe so large that there is no restriction", but you didn't mention how a pipe (collector) that large would actually hurt performance by reducing exhaust gas velocity and increasing the likelyhood of the pulses stacking then diluting the incoming fuel/air charge.
Very good though.
09-19-2002, 03:09 PM
#10
Geronimo36
Gold Member
Some headders are quieter
Dennis,
Good points made and I agree. For instance the open style of a Gill Offshore manifold does not take into effect the separation of those cyclinders (#5, #7). On the other hand the high end "Stainless Marine" system does. It divides #1 and #3 together, and number #5 and #7 into separate collecters then discharges them later down the line when the pulses equalize. I remeber learning about this scavenging effect on Turbo Charged Engines. The reason why the Grand National was such a good turbo Car. A motor with multiple of 3 cyl has matched exhaust pulses and can actually make more power, where a V8 does not.
Also, I agree that a headder system may actually be quiter than a manifold type of system. I believe it is due to resinace inside the manifold? Last winter a buddy of mine installed CMI elbo tops in place of his gill offshores. The motors were quiter at idle and at cruise.
Good points made and I agree. For instance the open style of a Gill Offshore manifold does not take into effect the separation of those cyclinders (#5, #7). On the other hand the high end "Stainless Marine" system does. It divides #1 and #3 together, and number #5 and #7 into separate collecters then discharges them later down the line when the pulses equalize. I remeber learning about this scavenging effect on Turbo Charged Engines. The reason why the Grand National was such a good turbo Car. A motor with multiple of 3 cyl has matched exhaust pulses and can actually make more power, where a V8 does not.
Also, I agree that a headder system may actually be quiter than a manifold type of system. I believe it is due to resinace inside the manifold? Last winter a buddy of mine installed CMI elbo tops in place of his gill offshores. The motors were quiter at idle and at cruise.
