Firing Order Spacing

[Echo]

Got a question for you guys, since it seems like you know a bit about 180* headers and other exhaust systems. What's so magical about the 180* firing order?

With a 8 -> 2 -> atmosphere merge collector, I can understand organizing the pulses for the LRLRLRLR order, but what about say an 8 -> 4 -> atmosphere (quad exhaust)? Would you still want 180* crankshaft rotation separation, or would 360* be preferred?

Would the 360* (two into one merge collector) exhaust provide even higher levels of scavenging over the 180* exhaust, or worse?

Trying to understand all of this exhaust stuff . Thanks!

 

[Jim Cowden]

360deg separation I would imagine the neg wave used in the scavenge process would have faded to such an extent it would harm scavenge ?.

You need back pressure to generate a negative pressure.
Low back pressure low neg pressure.
High back pressure high neg pressure.
Then its finding the balance for your application.

Read Scientific intake and exhaust manifolds.
By P Smith and J Morrison.

Jim

 

[Echo]

360deg separation I would imagine the neg wave used in the scavenge process would have faded to such an extent it would harm scavenge ?.

You need back pressure to generate a negative pressure.
Low back pressure low neg pressure.
High back pressure high neg pressure.
Then its finding the balance for your application.

Read Scientific intake and exhaust manifolds.
By P Smith and J Morrison.

Jim

My confusion is coming partly from open header cars (dragsters, for example) where the "scavenging" is accomplished by the reflected wave from the pulse hitting atmosphere (if I am not mistaken). These cars effectively have 720 degree headers, no? (or would it be considered 90 degree headers as the atmosphere is acting as the collector) They have very little "backpressure". The momentum of the exhaust gasses traveling out of the pipe is what causes the negative pressure.

I'll take a look at that book. Thanks .

 

[BruceA]

Well, what Ive learnt, is that if you make your exaust system 'too' good, it will 'over scavenge' the intake charge and you will get less power from your engine!! If only I could come up with a variable length exaust system.

 

[Jim Cowden]

My confusion is coming partly from open header cars (dragsters, for example) where the "scavenging" is accomplished by the reflected wave from the pulse hitting atmosphere (if I am not mistaken). These cars effectively have 720 degree headers, no? (or would it be considered 90 degree headers as the atmosphere is acting as the collector) They have very little "backpressure". The momentum of the exhaust gasses traveling out of the pipe is what causes the negative pressure.

I'll take a look at that book. Thanks .

I have had nothing to do with drag engines.
But logic tells me that a blown engine will use the intake pressure to push the gas out and to a point alcahol engines with massive cylinder pressures.
The short pipes they run is probably all it needs to do the job I would say?.
Higher RPM requires a shorter pipe length as a general rule.

You wont be running at full sniff like they are so you will want to get good torque through a wider range and that comes from good cylinder filling,hence getting the old stuff out.
Note. their is a direct relationship between the inlet and exhaust lenths.

I figure the 180deg headers are that way because if you send a pulse down the collector every 90deg it may choke it and have to much back pressure and kill the top end.

You never know maybe what you are thinking about will work ,sometimes you need to do things to find out.

A book by an OZY guy called Alexander Graham Bell (Performance tuning) I think has some good rules of thumb.

Their are long formulas for all this stuff but the short version I use.
99000 divide by max rpm = intake runner length from valve seat.
Exhaust length is X 2 of intake.
You can go in multipuls on the exhaust ,say you have 12" intake and 24" ex you can take the ex 24-36-48 ect.
Next size pipe up on port size for reversion lip.
Larger pipe for higher rpm torque.
Smaler pipe for lower rpm torque.
Longer pipe for lower rpm torque.
Shorter pipe for higher rpm torque.
Merge collectors neck down at rear to primary or just larger than primary size.

This is a rule of thumb and others may have experienced different outcomes as engines are different but it has worked for me.

RAMBO
When you come up with them pipes I would like to see them.
Jim

 

[Jac Mac]

Got a question for you guys, since it seems like you know a bit about 180* headers and other exhaust systems. What's so magical about the 180* firing order?

With a 8 -> 2 -> atmosphere merge collector, I can understand organizing the pulses for the LRLRLRLR order, but what about say an 8 -> 4 -> atmosphere (quad exhaust)? Would you still want 180* crankshaft rotation separation, or would 360* be preferred?

Would the 360* (two into one merge collector) exhaust provide even higher levels of scavenging over the 180* exhaust, or worse?

Trying to understand all of this exhaust stuff . Thanks!

Your LRLR......... sequence is incorrect.
Firing order 15426378 is RLRRLRLL while 13726548 is RRLRLLRL

Merge collectors on pairs of primaries build mid range torque.

In todays world of extended RPM up around 8k plus the 180° GT40 type exhaust cannot be configured with suitable primary lengths simply because the calculated primary length @ those RPM works out TOO short. If the exh ports were in center of the Vee this wouldnt be a problem.
That said most motor combos seldom really hurt for power by using a longer primary, whereas too short can really kill power.

 

[Echo]

Well, what Ive learnt, is that if you make your exaust system 'too' good, it will 'over scavenge' the intake charge and you will get less power from your engine!! If only I could come up with a variable length exaust system.

Although this is not exactly what you are looking for..

If you create an unequal length 180* exhaust system and create an appropriately lengthed (matched) 180* intake system, you can tune the power band to bring your overall performance up, rather than bringing up a small area like with equal length headers.

You'll lose a bit of that 'sound' though because the pulses are arriving in the collector at different times..

 

[Echo]

Your LRLR......... sequence is incorrect.
Firing order 15426378 is RLRRLRLL while 13726548 is RRLRLLRL

Sorry if I wasn't clear in my first post. When I said "I can understand organizing the pulses for the LRLRLRLR order", I meant using 180 degree headers. Cross plane V8s most definitely do not have an even firing order, stock.

Merge collectors on pairs of primaries build mid range torque.

Are you referring to 8 -> 4 -> atmo?

In todays world of extended RPM up around 8k plus the 180° GT40 type exhaust cannot be configured with suitable primary lengths simply because the calculated primary length @ those RPM works out TOO short. If the exh ports were in center of the Vee this wouldnt be a problem.
That said most motor combos seldom really hurt for power by using a longer primary, whereas too short can really kill power.

Interesting, good to know. Thanks!

 

[Echo]

I have had nothing to do with drag engines.
But logic tells me that a blown engine will use the intake pressure to push the gas out and to a point alcahol engines with massive cylinder pressures.
The short pipes they run is probably all it needs to do the job I would say?.
Higher RPM requires a shorter pipe length as a general rule.

Oh, you're probably right. I completely forgot about the intake manifold pressure.

You wont be running at full sniff like they are so you will want to get good torque through a wider range and that comes from good cylinder filling,hence getting the old stuff out.

Only occasionally :happy:.

Note. their is a direct relationship between the inlet and exhaust lenths.

I figure the 180deg headers are that way because if you send a pulse down the collector every 90deg it may choke it and have to much back pressure and kill the top end.

Although I'm still a little fuzzy on this gas dynamics stuff, I figure if you have a tuned 180* system that works exceedingly well at 6000 rpm, wouldn't a 90* system tuned the same way as the previous 180* system work just as exceedingly well at 3000 rpm (half the RPMS, same spacing between exhaust pulses)?

You never know maybe what you are thinking about will work ,sometimes you need to do things to find out.

Yeah.. I just prefer to work it out on paper if possible before jumping into building things and spending money.

A book by an OZY guy called Alexander Graham Bell (Performance tuning) I think has some good rules of thumb.

Thanks, I'll check it out.

Their are long formulas for all this stuff but the short version I use.
99000 divide by max rpm = intake runner length from valve seat.
Exhaust length is X 2 of intake.
You can go in multipuls on the exhaust ,say you have 12" intake and 24" ex you can take the ex 24-36-48 ect.
Next size pipe up on port size for reversion lip.
Larger pipe for higher rpm torque.
Smaler pipe for lower rpm torque.
Longer pipe for lower rpm torque.
Shorter pipe for higher rpm torque.
Merge collectors neck down at rear to primary or just larger than primary size.

This is a rule of thumb and others may have experienced different outcomes as engines are different but it has worked for me.

Wow, that's great. Thanks!

RAMBO
When you come up with them pipes I would like to see them.
Jim

I'm a long way off from building anything. For now I'm just working out all of the math. When I finally build something though, I'll be sure to post back.

 

[Jim Cowden]

Quote from eco
Although I'm still a little fuzzy on this gas dynamics stuff, I figure if you have a tuned 180* system that works exceedingly well at 6000 rpm, wouldn't a 90* system tuned the same way as the previous 180* system work just as exceedingly well at 3000 rpm (half the RPMS, same spacing between exhaust pulses)?



An engine will fill the cylinder fairly well (BMEP) at 2500-3000 rpm on its own anyway ,because of the time the valve is off the seat.
I worked it out once and its surpising how fast things are happening.
6000rpm div 60(sec)=100 RP Second
Div ex cam duration into 1/100 sec
Things are moving fast.
I wouldnt get to worried about the 3000rpm band it has twice as long.

Jim

 

[Echo]

So I got to thinking again, and using 180 degree pairing on an 8 -> 4 -> atmosphere exhaust system would lead to, in effect, an uneven firing pattern among the exhaust pulses (similar to a harley?).

You would have one cylinder fire, and the one it's paired up with fire 180 degrees later, but then it would be another 540 degrees of crankshaft rotation before another cylinder fired into that collector.. Wouldn't that destroy the cross-scavenging effect that merge collectors offer for the second cylinder?

 

[Dave Wood]

So I got to thinking again, and using 180 degree pairing on an 8 -> 4 -> atmosphere exhaust system would lead to, in effect, an uneven firing pattern among the exhaust pulses (similar to a harley?).

I think you are trying to determine exhaust configuration with no regard for engine configuration. Most V8 engines are 90* and a Harley is 45, that is part of the "odd fire" sound. As far as drag engines go, most operate in a very small power window and higher class cars with blowers are not affected by "pairing", as much as by length of tube. Most of the benefits gained from header design have to do with making the most power over the widest range, that's why drag cars are a poor comparison for header configuration...they are designed to make the most power in a narrow rpm area and the gearing,shift points,etc. are set up around that. There have been 180* V8s in a few indy cars. It makes them basicly 2-4 cylinders on a single crank. Header designs were probably not 180 for many reasons that are dictated by crank layout. Much the same with V6 or V12 engines, they are optimum at 60* for smoothness so once again pairing would more likely be different from a 90* V8, with the exception of a few V6 engines that are odd-fire and pretty much a lopped off V8. If I recall correctly, both the odd-fire V6 and the 180(flat) crank V8s make more power, but the vibrations( or buzz) is unacceptable for most production vehicles. Most Harleys run separate pipes too, so there is no pairing...there was one ,I think Sporty, that used a set of pipes that were joined just below the engine ( not a lot of choice for pairings LOL) and it had a broader power band as well...but not the same sound that 2 separate ones have.

 

[Echo]

For the record (and hopefully to clear up any confusion), I am talking about a standard cross-plane, 90 degree V8 with a firing order of 1-3-7-2-6-5-4-8.

I think you are trying to determine exhaust configuration with no regard for engine configuration. Most V8 engines are 90* and a Harley is 45, that is part of the "odd fire" sound.

I just took a closer look at the harley firing order and I realize I was mistaken. I thought the second piston fired 45 degrees after the first and then there was a 675 degree lag until the first fired again. My mistake (and it nullifies any argument I had about the firing )..

Anyway, the point to my harley comment was that when pairing a 8 -> 4 -> atmosphere collector 180 degrees apart, you would end up with one firing, then the next firing 180 degrees, and then a 540 degree gap until the first fired again, negating the scavenging effects of the 180 degree headers. Which is why I was asking about 360 degree headers..

As far as drag engines go, most operate in a very small power window and higher class cars with blowers are not affected by "pairing", as much as by length of tube. Most of the benefits gained from header design have to do with making the most power over the widest range, that's why drag cars are a poor comparison for header configuration...they are designed to make the most power in a narrow rpm area and the gearing,shift points,etc. are set up around that. There have been 180* V8s in a few indy cars. It makes them basicly 2-4 cylinders on a single crank. Header designs were probably not 180 for many reasons that are dictated by crank layout.

Yeah, the dragster comment wasn't the greatest, but it was the only engine I could think of that routinely runs open headers without collectors.

Much the same with V6 or V12 engines, they are optimum at 60* for smoothness so once again pairing would more likely be different from a 90* V8, with the exception of a few V6 engines that are odd-fire and pretty much a lopped off V8.

Actually, V6's and V12's are very very different engines. A V12 can be run at any bank angle the engineer wishes because it is a perfectly balanced engine. It is actually two inline-sixes sharing a cranskshaft (I-6's are perfectly balanced in both the primary and secondary orders).

A 60 degree V6 on the other hand is quite the oddball. Most nowadays use a split pin crankshaft, where the pin is not shared between cylinders and are actually offset from each other 15 degrees (might be 30, but I don't have any schematics in front of me and can't recall off the top of my head).

If I recall correctly, both the odd-fire V6 and the 180(flat) crank V8s make more power, but the vibrations( or buzz) is unacceptable for most production vehicles. Most Harleys run separate pipes too, so there is no pairing...there was one ,I think Sporty, that used a set of pipes that were joined just below the engine ( not a lot of choice for pairings LOL) and it had a broader power band as well...but not the same sound that 2 separate ones have.

Right. My experience with harleys is extremely limited, but I do recall the "potato-potato" sound they have . Thanks for putting up with my questions .

 

ehy jac mac, how goes the alloy motor build ? regards steve.