How 180's work
Attachments
Images (1)
Original Post
Replies sorted oldest to newest
Here is the text for the above article, (easier to read)
A 180-degree header ensures the firing pulse seen by the collector occurs every 180 degrees of crank rotation. On a V8, this requires crossing over two tubes from each bank to the opposite bank’s collector. With a 351C Ford’s 1-3-7-2-6-5-4-8 firing order (where 1 through 4 are on the passenger side and 5 through 8 on the driver side), Nos. 2, 3, 6, and 7 are crossed.
To achieve optimum exhaust scavenging on a four-stroke V8 engine, each cylinder should fire every 180 degrees of engine rotation; on a typical V8 with a cross-plane crankshaft and conventional headers, two cylinders on each bank end up firing consecutively, only 90 degrees apart. The way around this is crossing two primary tubes from each bank over to the opposite bank. For example, on an engine with a 1-8-4-3-6-5-7-2 firing order (with the odd-numbered cylinders on the driver side and even-numbered ones the passenger side), cross tubes from cylinder Nos. 3 and 5, and 4 and 6.
Often termed a 180-degree header, this configuration makes the firing cycle seen by each collector occur 180 degrees apart, improving scavenging as well as low- and midrange torque by permitting a smaller collector outlet diameter than normal for a given combination.
The problem is packaging this bundle of snakes. Four tubes must cross either under the oil pan or around the front or rear of the engine. On a high-rpm race mill, it may be difficult to keep primary-tube length down to 32 inches or less.
If that’s not enough, try to arrange the tubes into each collector so they fire in a rotational firing pattern, as seen on Ford’s Indy engines and Le Mans race cars back in the 1960s. Such a rotational pattern further enhances scavenging: The exhaust gas exiting one tube, passing across the opening of the tube directly beside it, creates more signal on that tube than it would on a tube on the collector’s opposite side.
In theory, the ultimate solution to the V8 firing-order conundrum is the flat-plane crank (aka, a “180-degree crank”). Essentially, this makes the V8 act like two four-cylinder engines, allowing the optimized collector arrangement of a four-cylinder engine without the need to cross tubes. But flat-plane cranks cause such severe vibration they can only be used on a pure race car.
A 180-degree header ensures the firing pulse seen by the collector occurs every 180 degrees of crank rotation. On a V8, this requires crossing over two tubes from each bank to the opposite bank’s collector. With a 351C Ford’s 1-3-7-2-6-5-4-8 firing order (where 1 through 4 are on the passenger side and 5 through 8 on the driver side), Nos. 2, 3, 6, and 7 are crossed.
To achieve optimum exhaust scavenging on a four-stroke V8 engine, each cylinder should fire every 180 degrees of engine rotation; on a typical V8 with a cross-plane crankshaft and conventional headers, two cylinders on each bank end up firing consecutively, only 90 degrees apart. The way around this is crossing two primary tubes from each bank over to the opposite bank. For example, on an engine with a 1-8-4-3-6-5-7-2 firing order (with the odd-numbered cylinders on the driver side and even-numbered ones the passenger side), cross tubes from cylinder Nos. 3 and 5, and 4 and 6.
Often termed a 180-degree header, this configuration makes the firing cycle seen by each collector occur 180 degrees apart, improving scavenging as well as low- and midrange torque by permitting a smaller collector outlet diameter than normal for a given combination.
The problem is packaging this bundle of snakes. Four tubes must cross either under the oil pan or around the front or rear of the engine. On a high-rpm race mill, it may be difficult to keep primary-tube length down to 32 inches or less.
If that’s not enough, try to arrange the tubes into each collector so they fire in a rotational firing pattern, as seen on Ford’s Indy engines and Le Mans race cars back in the 1960s. Such a rotational pattern further enhances scavenging: The exhaust gas exiting one tube, passing across the opening of the tube directly beside it, creates more signal on that tube than it would on a tube on the collector’s opposite side.
In theory, the ultimate solution to the V8 firing-order conundrum is the flat-plane crank (aka, a “180-degree crank”). Essentially, this makes the V8 act like two four-cylinder engines, allowing the optimized collector arrangement of a four-cylinder engine without the need to cross tubes. But flat-plane cranks cause such severe vibration they can only be used on a pure race car.
Attachments
Images (1)
It's a nice article. Very entertaining. Better then porn.
Thanks for posting.
Thanks for posting.
A 180-degree header ensures the firing pulse seen by the collector occurs every 180 degrees of crank rotation. On a V8, this requires crossing over two tubes from each bank to the opposite bank’s collector. With a 351C Ford’s 1-3-7-2-6-5-4-8 firing order (where 1 through 4 are on the passenger side and 5 through 8 on the driver side), Nos. 2, 3, 6, and 7 are crossed.
Attachments
Images (1)
It mentions that a smaller than conventional collector outlet is needed. What size out let do you recommend? 2.5”,3”? I am leaning towards a 3” collector outlet but maybe it would be too large?
I'm pretty sure mine are 3-1/2" 3".
I think this is really a subject of tuning. Everything in the system is "adjustable" to put the most power in a range where it can be used the best.
What is best for YOUR ENGINE, or mine is not the same as someone elses. Not everything is equal and not everything can be generalized.
There are other things that can be used in the exhaust such as step tube primaries for one thing. They are just as effective in changing power curve as anything else is.
Another factor is aftermarket "high port" heads AND some use 1.60" exhaust valves and some use the 1.71" exhaust valves. That is involved in deciding what diameter primary tubes get used.
Why worry unless you are going to run at Lemans?
The "engineers" here among us just want the challenge.
I think this is really a subject of tuning. Everything in the system is "adjustable" to put the most power in a range where it can be used the best.
What is best for YOUR ENGINE, or mine is not the same as someone elses. Not everything is equal and not everything can be generalized.
There are other things that can be used in the exhaust such as step tube primaries for one thing. They are just as effective in changing power curve as anything else is.
Another factor is aftermarket "high port" heads AND some use 1.60" exhaust valves and some use the 1.71" exhaust valves. That is involved in deciding what diameter primary tubes get used.
Why worry unless you are going to run at Lemans?
The "engineers" here among us just want the challenge.
Attachments
Images (1)
It likely depends on many variables like engine size, RPM range, cam...etc. On my 2.7 Lt 4 cylinder race engine that had a usable range to about 8,000 RPM we ended up with a 2.25” merge collector with no power loss at high RPM. BTW, merge collectors work really well. When we went from a 3” regular collector to the 2.25” merge collector we gained significant midrange power. Dynos are great tools but the real important thing is that I could really feel the difference on the track. I doubt that I will ever use headers again that don’t have merge collectors. For our 351CI to 400CI street motors my guess is closer to 2.5” to 2.75” collectors. Burns Stainless could answer the question.
quote:Originally posted by Racecar Mike:...........for our 351CI to 400CI street motors my guess is closer to 2.5” to 2.75” collectors. Burns Stainless could answer the question.
2.375" merge to 3" cone.
http://pantera.infopop.cc/eve/...081005826#3081005826
Best,
Kelly