LS exhaust suggestions

[glassman]

the problem was in the crossover of the old system, when revved up, it sort of created a pressure in the tube of the crossover.
Ordered the Dynatech s/s kit, just came in the mail today and will install tomorrow....

Mike

[Ron Sutton]

Originally Posted by glassman

the problem was in the crossover of the old system, when revved up, it sort of created a pressure in the tube of the crossover.
Ordered the Dynatech s/s kit, just came in the mail today and will install tomorrow....

Mike

Was the cross over tube placed at the wrong distance for the engine's pulses? ... or ???

[glassman]

Good Question Ron, not really sure. It was the system on my sbc installed last year, but clearly not designed for the LS...i'd like to know though...

[Ron Sutton]

Originally Posted by glassman

Good Question Ron, not really sure. It was the system on my sbc installed last year, but clearly not designed for the LS...i'd like to know though...

It's not quite common knowledge, but H-pipes & X-over tubes ... and mufflers for that matter too ... need to be placed in a precise location for optimum operation. If they're out of place significantly ... it throws off the timing of the exhaust pulses & they actually run into each other ... and literally "back up" or "clog" flow ... similar to what happens in stop-n-go highway traffic.

Maybe the pulse timing was "OK" for the previous engine & this engine was different enough it caused a problem.

For all our serious cars, we design the system based on these timing pulses.

Best wishes !

.

[Vince@Meanstreets]

Originally Posted by Ron Sutton

It's not quite common knowledge, but H-pipes & X-over tubes ... and mufflers for that matter too ... need to be placed in a precise location for optimum operation. If they're out of place significantly ... it throws off the timing of the exhaust pulses & they actually run into each other ... and literally "back up" or "clog" flow ... similar to what happens in stop-n-go highway traffic.

Maybe the pulse timing was "OK" for the previous engine & this engine was different enough it caused a problem.

For all our serious cars, we design the system based on these timing pulses.

Best wishes !

.

Makes sence with the firing order change. But that much back pressure? He was getting 7 on the blowitoffmeter.

[glassman]

Y"eah, it does make sense, great analogy with the traffic issue. Sort of a volume vs flow equation. Makes sense. Meanwhile we think "its just exhaust", but a chain is only as strong as its weakist link....working on it tonite, got the old system out, and need to make two cuts, yep, four dead dewalt batteries and no "corded" sawzall at home....til tomorrow.....

[glassman]

Originally Posted by Vince@MSperfab

Makes sence with the firing order change. But that much back pressure? He was getting 7 on the blowitoffmeter.

Man, i wish i could use the "blowitoffmeter" at my yob, it would of come in handy today

[Ron Sutton]

Originally Posted by Vince@MSperfab

Makes sence with the firing order change. But that much back pressure? He was getting 7 on the blowitoffmeter.

Hi Vince,

I don't know the exhaust pulse back up was all coming from his X-over or H-pipe ... because obviously I don't know anything about his system other than what he posted. It could have been just from that if it was placed in the exact opposite of optimum, or from a combination of errors. The firing order doesn't effect it, because regardless of which cylinder is firing, there is a pulse every 90 degrees of crankshaft rotation.

I like using the analogy of freeway traffic. If all the cars are in sync, everyone can cruise 65mph. If one yahoo gets on the brakes, it causes a chain reaction as the other cars slow up. If all those cars were to brake at the same point every time ... like when there's a wreck on the side of the road ... the back up would be ongoing ... consistently at that point.

Conventional dual exhaust systems can suffer minor pulse back up if the mufflers are in the wrong spot as the exhaust pulses expand. But again, it is minor.

Cross over pipes & H-pipes can help "clean up the freeway" when there is a high volume of traffic ... if they're placed correctly. It's like opening up another lane ... as long as the cars, I mean exhaust pulses "merge" well. The difference between the exhaust pulses merging well ... or running into each other (creating back pressure instead of relieving it) ... is a timing issue.

The exhaust gases continue to expand while exiting. If we can relieve some of the pressure in one side ... through a X-over and/or H-pipe ... without that pressure running into the exhaust pulses in the other side ... we can optimize exhaust flow.

If on the other hand, we place the X-over and/or H-pipe in the incorrect spots ... the timing is off ... and the pressure from one side runs into an exhaust pulse on other side ... causing it to "stall up."

Like with freeway traffic ... if we open a lane for a car to get over ... to avoid slowing as traffic congests ahead of him ... and the car pulls into a clear opening ... all is well. But if he tries to merge at the same time a car ... already traveling in that lane ... gets there, one of them needs to use their brain & slow up. Exhaust pulses don't have brains, so they just run into each other, creating back pressure.

I learned a little about exhaust expansion, pulses & timing the expansion pulses when we raced 2-stroke karts. On 2-strokes you could really move the power range around with small changes in the pipe length before the expansion chamber. 1/8" was noticeable & 1/4" was significant. If you were significantly too short or long ... it killed the power.

The parameters are different, because these cars sure aren't 2-strokes. But the concepts that exhaust gases expand as they exit, and understanding we can affect the timing of those pulses with exhaust design ... are similar.

The optimal power output & torque curve of an engine is only affected by the header primaries (diameter & length) & the header collector (diameter & length) ... if it is an open header race engine.

If it has an exhaust system past the collector, that assembly, its components & their exact placement will affect the power output & torque curve also.

There are really 3 things to keep in mind:

A. If the tubing size is too small, it will act as a restriction & reduce flow at higher engine rpms ... affecting (reducing) upper rpm power the most. How much depends on how small is the tubing compared to what is optimum. If optimum is 3" ... 2.5" tubing will cause a small loss in upper rpm power ... while 1.5" tubing would cause a substantial power loss at upper rpms, even limiting the rpm potential of the engine.

B. If the tubing size is too big, it will act as an expansion chamber, slowing the exhaust air speed (but not the volume). This MAY reduce top end power A LITTLE or A LOT depending on what it does to the harmonics. It WILL reduce low rpm engine response & low end torque. How much loss depends on how large is the tubing compared to what is optimum. If optimum is 2.5" .... 3" tubing will cause a small loss in low rpm engine response & low end torque ... while 4" tubing would be more substantial.

C. Exhaust flow harmonics is a big deal. Since we're all PT here, I'll keep it to V-8's. You have 8 cylinders pressurizing the exhaust system at different times. This creates exhaust pulses. The camshaft duration, centerline & overlap ... along with over key engine criteria such as how long the pistons "rock" at TDC, rpm, etc ... define the pulse pattern. I cant' say this strongly enough ... for optimum power you need the exhaust pulses working in harmony.

As long as the pulses are NOT running into each other ... the exhaust flows out smoothly & at a high rate of cfm. If the pulses run into each other ... the exhaust gases slow & "stall up." It's like comparing a smooth flowing group of traffic on the freeway ... to ugly, stop-n-go traffic. When exhaust pulses run into each other, the exhaust flow speed slows & exhaust cfm is reduced ... wasting power.

Engines are basically air pumps. To build power, you need to get the air (and fuel) into & out of the engine ... smoothly ... and in high volumes.

Several things in the exhaust affect the exhaust gas pulses, including:
1. Primary tube diameter ... or diameters if it is a "step-header" ... and of course length.
2. Collector style ... be it the traditional 4-1 ... or more developed 4-2-1.
3. Collector diameter and length.
4. If merge collectors are used ... and if so ... exactly where & how small.
5. If Vortex cones are used ... and if so ... exactly where & how restrictive.
6. Diameter of the exhaust tubing needs to be the same diameter as the collector, assuming it is correct
7. If a H-pipe is used ... and where it is placed, exactly.
8. If a X-over is used ... and where it is placed, exactly.
9. Where the mufflers are placed, exactly.
10. How long the exhaust pipe is after the mufflers.

Getting the placement of these exhaust components correct ... to create a harmonious freeway for the exhaust gas pulses & flow ... is key to getting optimum power out of a performance or race engine. I've seen street cars with 450hp pick up 40 hp when the exhaust system was corrected. Meaning they were losing 40 hp before.

Going back to “B” ... if the exhaust tubing is too big ... it changes the speed of the exhaust gases ... affecting the pulse timing. If it is off a little, and your engine made crazy big HP, you may not notice. If it is off more, the exhaust gases run into each other, slow & "stall up" ... and exhaust flow cfm is reduced ... hurting power output.

If you're looking to get the optimum power through out the rpm range, getting the exhaust system designed correctly is just as important as selecting the right carb, intake, heads or cam. It's all part of the big "package" picture.

I feel optimizing the exhaust is very important & often overlooked. We invest the time, effort, expense ... and design every piece of our exhaust system ... with the same care as building the engine ... to optimize total power output & torque curve.

The pieces of the puzzle that define the exhaust gas pulses from the engine are:
Engine power output (flow volume)
Bore/Stroke ratio
Rod/Stroke ratio
Compression ratio
Number of cylinders
RPM range
Valve area
Both sides of cam lift & duration
Degreed centerline, lobe center & resulting overlap
Exhaust port length

-------------------------------------------------------------------------------------

I feel this is way too involved for the average hot rod or PT car.

But if the placement of a H-pipe or X-over is in the opposite of optimum, it will have effects like we have discussed here.

[glassman]

Thanx for the explanation Ron, everything we do with our "sport" is getting more and more complex, however, more and more efficient too.

With knowledge comes power and power is nothing without control...

[IMPALA MAN]

Originally Posted by Ron Sutton

Hi Vince,

I don't know the exhaust pulse back up was all coming from his X-over or H-pipe ... because obviously I don't know anything about his system other than what he posted. It could have been just from that if it was placed in the exact opposite of optimum, or from a combination of errors. The firing order doesn't effect it, because regardless of which cylinder is firing, there is a pulse every 90 degrees of crankshaft rotation.

I like using the analogy of freeway traffic. If all the cars are in sync, everyone can cruise 65mph. If one yahoo gets on the brakes, it causes a chain reaction as the other cars slow up. If all those cars were to brake at the same point every time ... like when there's a wreck on the side of the road ... the back up would be ongoing ... consistently at that point.

Conventional dual exhaust systems can suffer minor pulse back up if the mufflers are in the wrong spot as the exhaust pulses expand. But again, it is minor.

Cross over pipes & H-pipes can help "clean up the freeway" when there is a high volume of traffic ... if they're placed correctly. It's like opening up another lane ... as long as the cars, I mean exhaust pulses "merge" well. The difference between the exhaust pulses merging well ... or running into each other (creating back pressure instead of relieving it) ... is a timing issue.

The exhaust gases continue to expand while exiting. If we can relieve some of the pressure in one side ... through a X-over and/or H-pipe ... without that pressure running into the exhaust pulses in the other side ... we can optimize exhaust flow.

If on the other hand, we place the X-over and/or H-pipe in the incorrect spots ... the timing is off ... and the pressure from one side runs into an exhaust pulse on other side ... causing it to "stall up."

Like with freeway traffic ... if we open a lane for a car to get over ... to avoid slowing as traffic congests ahead of him ... and the car pulls into a clear opening ... all is well. But if he tries to merge at the same time a car ... already traveling in that lane ... gets there, one of them needs to use their brain & slow up. Exhaust pulses don't have brains, so they just run into each other, creating back pressure.

I learned a little about exhaust expansion, pulses & timing the expansion pulses when we raced 2-stroke karts. On 2-strokes you could really move the power range around with small changes in the pipe length before the expansion chamber. 1/8" was noticeable & 1/4" was significant. If you were significantly too short or long ... it killed the power.

The parameters are different, because these cars sure aren't 2-strokes. But the concepts that exhaust gases expand as they exit, and understanding we can affect the timing of those pulses with exhaust design ... are similar.

The optimal power output & torque curve of an engine is only affected by the header primaries (diameter & length) & the header collector (diameter & length) ... if it is an open header race engine.

If it has an exhaust system past the collector, that assembly, its components & their exact placement will affect the power output & torque curve also.

There are really 3 things to keep in mind:

A. If the tubing size is too small, it will act as a restriction & reduce flow at higher engine rpms ... affecting (reducing) upper rpm power the most. How much depends on how small is the tubing compared to what is optimum. If optimum is 3" ... 2.5" tubing will cause a small loss in upper rpm power ... while 1.5" tubing would cause a substantial power loss at upper rpms, even limiting the rpm potential of the engine.

B. If the tubing size is too big, it will act as an expansion chamber, slowing the exhaust air speed (but not the volume). This MAY reduce top end power A LITTLE or A LOT depending on what it does to the harmonics. It WILL reduce low rpm engine response & low end torque. How much loss depends on how large is the tubing compared to what is optimum. If optimum is 2.5" .... 3" tubing will cause a small loss in low rpm engine response & low end torque ... while 4" tubing would be more substantial.

C. Exhaust flow harmonics is a big deal. Since we're all PT here, I'll keep it to V-8's. You have 8 cylinders pressurizing the exhaust system at different times. This creates exhaust pulses. The camshaft duration, centerline & overlap ... along with over key engine criteria such as how long the pistons "rock" at TDC, rpm, etc ... define the pulse pattern. I cant' say this strongly enough ... for optimum power you need the exhaust pulses working in harmony.

As long as the pulses are NOT running into each other ... the exhaust flows out smoothly & at a high rate of cfm. If the pulses run into each other ... the exhaust gases slow & "stall up." It's like comparing a smooth flowing group of traffic on the freeway ... to ugly, stop-n-go traffic. When exhaust pulses run into each other, the exhaust flow speed slows & exhaust cfm is reduced ... wasting power.

Engines are basically air pumps. To build power, you need to get the air (and fuel) into & out of the engine ... smoothly ... and in high volumes.

Several things in the exhaust affect the exhaust gas pulses, including:
1. Primary tube diameter ... or diameters if it is a "step-header" ... and of course length.
2. Collector style ... be it the traditional 4-1 ... or more developed 4-2-1.
3. Collector diameter and length.
4. If merge collectors are used ... and if so ... exactly where & how small.
5. If Vortex cones are used ... and if so ... exactly where & how restrictive.
6. Diameter of the exhaust tubing needs to be the same diameter as the collector, assuming it is correct
7. If a H-pipe is used ... and where it is placed, exactly.
8. If a X-over is used ... and where it is placed, exactly.
9. Where the mufflers are placed, exactly.
10. How long the exhaust pipe is after the mufflers.

Getting the placement of these exhaust components correct ... to create a harmonious freeway for the exhaust gas pulses & flow ... is key to getting optimum power out of a performance or race engine. I've seen street cars with 450hp pick up 40 hp when the exhaust system was corrected. Meaning they were losing 40 hp before.

Going back to “B” ... if the exhaust tubing is too big ... it changes the speed of the exhaust gases ... affecting the pulse timing. If it is off a little, and your engine made crazy big HP, you may not notice. If it is off more, the exhaust gases run into each other, slow & "stall up" ... and exhaust flow cfm is reduced ... hurting power output.

If you're looking to get the optimum power through out the rpm range, getting the exhaust system designed correctly is just as important as selecting the right carb, intake, heads or cam. It's all part of the big "package" picture.

I feel optimizing the exhaust is very important & often overlooked. We invest the time, effort, expense ... and design every piece of our exhaust system ... with the same care as building the engine ... to optimize total power output & torque curve.

The pieces of the puzzle that define the exhaust gas pulses from the engine are:
Engine power output (flow volume)
Bore/Stroke ratio
Rod/Stroke ratio
Compression ratio
Number of cylinders
RPM range
Valve area
Both sides of cam lift & duration
Degreed centerline, lobe center & resulting overlap
Exhaust port length

-------------------------------------------------------------------------------------

I feel this is way too involved for the average hot rod or PT car.

But if the placement of a H-pipe or X-over is in the opposite of optimum, it will have effects like we have discussed here.

Love the detail Ron. The number one mistake I see every day is people wanting to go too large on the tube size (primary and collector). I think the best thing you said in the entire post was how it is a complete package the must be matched together. It's no different than buying a huge cam and putting it an engine with stock heads and a two barrel carb. The best advice I can add to Ron's post is this: When you are ready for exhaust (or any product for that matter) call the pros. If you have a shock question, there is a pretty good chance a shock manufacturer will know what you will need. The same with any product. I know if I have a trans question, I can try to figure it out on my own or just save a bunch of time and headaches by just calling someone who lives and breathes it every day.