Dual Exhaust Question

[Jim Shepard]

I'll be putting Fenton-replica cast iron headers with dual exhaust on my B2B. I've seen duals with a cross-over pipe. What does a cross-over pipe do? My pipes will be side-by-side and dumping out under the right running board in front of the right rear wheel. They won't be more than 4-5 inches apart. Is a cross-over pipe necessary?

[moparmonkey]

Cross-over pipes are supposed to improve scavenging by making use of the exhaust pulses coming down through each tube. For separate dual pipes, you'll have 3 pulses per cycle in each exhaust pipe. But, if you use a crossover, you'll add those three pulses from each pipe together (back to the original 6 pulses you had with a single pipe) improving scavenging. That's why a lot of the performance pipes for v8's now use "X" pipes instead of a straight crossover, higher velocities and even better scavenging (in theory).

But it does depend on timing and firing order. Having the pipes close together wouldn't negate using a crossover, in fact, the shorter the crossover pipe the more likely it is to actually work. Placement of the pipe also makes a difference, as does design (ie, "X" vs "H"). There's actually a lot of theory that goes into it if you want to be technical.

Regardless, they aren't absolutely necessary. They do tend to make the exhaust note a little more mellow, and depending on who you listen to they may improve power/efficiency a little.

Edited September 13, 2011 by moparmonkey

[Dave72dt]

It's not as important on a I6 as it would be on a V6 or V8. All your exhaust comes out the same side and you can pick which cyinders to group together to get an even pulse. Firing order on all I6's as far as I know is 153624. Because it takes 2 revolutions of the motor to fire all six cylinders, you group every other cyl into one group and the others into the second. Happens to work out to 123 in one group and 456 in the other. Balances the load on the crankshaft also. On a V engine, you group one side together, regardless of firing order, and can get uneven pulses.

Anyone by chance know the firing order of a straight 8?

[olddodgeguy]

I'm not sure they all were the same but generally the order was 1-6-2-5-8-3-7-4. Mike

[Dodgeb4ya]

I'm not sure they all were the same but generally the order was 1-6-2-5-8-3-7-4. Mike

All these different firing orders are for the V8 engines!

1-8-4-3-6-5-7-2

1-8-7-2-6-5-4-3

1-3-7-2-6-5-4-8

1-5-4-8-7-2-6-3

1-6-2-5-8-3-7-4

1-8-7-3-6-5-4-2

1-5-4-2-6-3-7-8

1-5-6-3-4-2-7-8

1-5-3-7-4-8-2-6

1-2-7-8-4-5-6-3

1-2-7-3-4-5-6-8

The I-6 engines are just one-best I can find.

The Chrysler straight 8 is 16258374.

Bob

[Young Ed]

Some of the reason for the difference in v8s is that different companies counted the cylinders different. Some are 1 2 3 4 down one side and 5 6 7 8 down the other while some are 1 3 5 7 one side and 2 4 6 8 on the other.

[Powerhouse]

Would the crossover pipe have a similar effect as adding a second exhaust flange to a stock exhaust manifold... under where the heat choke is? I have seen this done before but didn't know if they blocked off the passage to the rear 3 exhaust ports in the manifold.

[Don Coatney]

Would the crossover pipe have a similar effect as adding a second exhaust flange to a stock exhaust manifold... under where the heat choke is? I have seen this done before but didn't know if they blocked off the passage to the rear 3 exhaust ports in the manifold.

I have a modified stock manifold on my engine and the heat riser baffle is welded making this manifold a true split.

To answer the original question. I dont believe anyone has successfully built a Mopar flathead 6 that will make 200 horse power. The prime reason for splitting the exhaust is the sound that is sweet music to my ears. Adding a crossover pipe does little to increase the sound but does add to the cost of the conversion.

Edited September 20, 2011 by Don Coatney

[48Dodger]

Back pressure, scavenging, etc....all for HP racing stuff in my opinion. For the daily driver its tone. Makes a great engine sound even greater.

48D:D

[Powerhouse]

So if you put all 6 cylinders through 2 exhaust ports wouldn't it be the same as 3 through 1 port and the other 3 through 1 port. What would the sealing off do really? Wouldn't 6 through 2 be more breathing room?

I'm obviously confused...any light would be appreciated. Thanks.

[Powerhouse]

I just read a little something on the web...I don't know if it's credible or not...

http://www.x-pipe.com/Scavenge.html

Here's Some of it:

Urban Legend #1:

Headers produce scavenge, thereby increasing exhaust airflow, allowing the engine to burn more air/fuel, and make more power.

The Truth of the matter:

Most headers designed to create scavenge actually slow down exhaust airflow, creating back pressure, which is why longer duration exhaust cams can improve the performance of a header equipped engine. There is only one way to make more horsepower; increase dynamic cylinder pressure.

FACT:

Only dynamic cylinder pressure can create horsepower. Scavenge is defined as a suction or a less than atmospheric pressure.

REASON:

There is no way that a suction can create an increase in dynamic cylinder pressure. Maybe headers do work, in some applications, but scavenge alone can not be credited with creating performance improvements.

Urban Legend #2:

Scavenge removes burned exhaust gases from combustion chambers, to allow more fresh air fuel to be burned, making more horse power. More scavenge is better. In fact, mankind has gone to the moon, but mankind has never, ever, created too much scavenge.

The Truth of the matter:

Everything has diminishing returns. More scavenge may not improve power. It seems illogical to state that more vacuum creates more dynamic cylinder pressure.

FACTS:

Scavenge, created and maintained by a resonant 4into1 header, does not remove burned gases from the combustion chamber of an internal combustion engine. As the combustion mixture is burned during the power stroke, the mixture begins its burn from the sparkplug and continues burning toward the descending piston. When the exhaust valve opens in the latter portions of the piston's descent, the gases nearest the valve, and sparkplug, are the first to leave the chamber, under hundreds or even thousands of pounds per square inch pressure. As the light, hot, energetic combustion product gases pass out of the chamber through the exhaust port, chamber pressure drops as potential energy (cylinder pressure) is converted into kinetic energy (exhaust gas velocity). This high velocity exhaust gas column then tends to stay in motion, even after all of the hot, burned, exhaust gases have left the chamber and the chamber pressure drops to zero. Scavenge is then created as the still rapidly exiting exhaust gas column evacuates the chamber by sucking any unburned air/fuel, remaining at the bottom of the chamber, out the exhaust port. No dynamic cylinder pressure increase here.

About 60 degrees before the end of the exhaust stroke, the intake valve opens, exposing the intake tract to the scavenge created vacuum now in the combustion chamber. If the scavenge vacuum is greater than manifold vacuum at that instant (say the throttle is wide open), air/fuel mixture will be sucked out of the intake manifold, through the combustion chamber, past the still open exhaust valve, and out the tail pipe. No dynamic cylinder pressure increase here. In fact, cylinder pressure will be reduced in the very next compression cycle because a portion of the available air/fuel has been lost, never to be burned. Opps, you have just lost dynamic cylinder pressure, the one thing needed to make power.

Urban Legend #3:

Fuel mixture must be made richer in fuel when a scavenge header is installed. The reason is that the header's powerful scavenge allows the engine to breath better, burning more air/fuel, making more horsepower.

The Truth of the matter:

Fuel must be added because the scavenge header is sucking the same amount of fuel, the amount that must now be added, out the tailpipe; the phenomenon is known as over-scavenge.

FACTS:

All intake systems compensate for airflow changes by proportionately adding fuel to air passing through the intake. Carburetors use a simple Ventura that generates a stronger vacuum signal with greater airflow, pulling more fuel through jet circuits, spraying it into the intake air stream. Fuel injection systems use various airflow sensors and air/fuel maps to precisely meter fuel to match air flow. So, even if intake air flow were to increase because of the installation of a scavenge header, the correct amount of fuel would always be added; there would be no need to rejet or remap air/fuel mixtures.

Typically, the more highly tuned the header, and the less the back pressure after the header (i.e. mufflers or catalytic converter restriction), the greater the over scavenge generated and the greater the amount of fuel that must be added to the fuel delivery curve. The additional fuel has to be added because an equal amount of the originally jetted or mapped fuel is lost. How? By over scavenge of the initial air/fuel charge, the richest portion of all air/fuel delivered to the combustion chamber. How do we know this? Because 1) fuel economy (BHPHR/#fuel) does not improve, 2) the mixture leans out only at the scavenge power peak (resonance) rpm, it becomes richer (needs less supplemental fuel) below and above that rpm, & 3) HC (unburned hydrocarbon) emissions go up within such tuned rpm range.

REASON:

Scavenge quickly becomes over scavenge, sucking the richest portion of delivered air/fuel out of the intake manifold and through the combustion chamber during the 60 or so degrees of valve overlap, when both intake and exhaust valves are open at the same time. Ironically, 3, 4 and 5 valve engines are the most likely engines to suffer over scavenge due to the much greater valve curtain from which air/fuel is allowed to escape under minimal vacuum conditions. High flow or ported 2 valve heads suffer the same fate due to increased air flow propensity. So, the better the head, the more likely that scavenge will become over scavenge, and dynamic cylinder pressure will drop.

[48Dodger]

Again, imo, you're talking about HP engines. When you get into dynamic compression, air ramming, volmetric efficiency, etc...every moment of the engine is suspect to causing a loss of horse power. If we're talking about about a 230 with 8:1 comp and VE of .8, we're talking about tone, not performance per se.

Maybe someone can post the stock performance numbers of a 230 ci. I believe Jim got his from a later year truck.

48D

[Jim Shepard]

It came out of a '55 1-ton...

[48Dodger]

http://dodgepilothouseclub.org/know/Australian_Truck_manual/australian.htm

DPETCA Knowlegde section in the Aussie Truck manual puts a 230ci engine at 6.25 to 1 copmpression. Jim are you changing anything other than the carb set-up (single to dual) ? Or is there milling of the heads etc.....

48D

[Don Coatney]

If you look at a top fuel dragster engine there is an exhaust pipe for each cylinder with no crossover connections. These engines make massive horsepower and sound good from a distance. However these engines are fully rebuilt after turning a thousand reveloutions or so. At 8000 RPM's for a 4.5 second run the engine only spins 600 times, and each individual cylinder only fires 300 times.

Edited September 24, 2011 by Don Coatney

[Jim Shepard]

http://dodgepilothouseclub.org/know/Australian_Truck_manual/australian.htm

DPETCA Knowlegde section in the Aussie Truck manual puts a 230ci engine at 6.25 to 1 copmpression. Jim are you changing anything other than the carb set-up (single to dual) ? Or is there milling of the heads etc.....

48D

The motor is bored .060 over and it has a "mild" RV cam. The carb is a Stromberg WW (2bb).