3.5" exhaust

[Honky Kong jr]

Too big? I have 2 10 series flows and 20 feet of 3.5" pipe. It's EMT. I have used this stuff for my last 2 projects. I love the sound it produces. It's heavy wall and galvanized. Engine is a 10.5:1 roller cammed 306 due. 590 lift 407 BBC with a port matched 2.0 intake and 750hp Holley with a 1" open spacer. Exhaust is about 6' long. Looking for opinions here. I've seen the Engine Masters episode when they tested different size exhaust.

 

[74 Shortbed]

I'd say it's too big, unless all the power is made up top.. IMHO..

 

[Green79Scottsdale]

Single or dual? Single... too big but usable. Dual... waaay to big. Others opinions may vary.

 

[Adam G]

Too big? I have 2 10 series flows and 20 feet of 3.5" pipe. It's EMT. I have used this stuff for my last 2 projects. I love the sound it produces. It's heavy wall and galvanized. Engine is a 10.5:1 roller cammed 306 due. 590 lift 407 BBC with a port matched 2.0 intake and 750hp Holley with a 1" open spacer. Exhaust is about 6' long. Looking for opinions here. I've seen the Engine Masters episode when they tested different size exhaust.

I would go with 2.5" exhaust pipe plus galvanized pipe is toxic if you weld on it or heat it to a certain temp I believe.

 

[Rusty Nail]

You couldn't pay me enough to begin to consider that.

 

[1987 GMC Jimmy]

That would murder your low end.

 

[Honky Kong jr]

I'd say it's too big, unless all the power is made up top.. IMHO..

Yes with the cam that's in it the power comes in up top. It is primarily for the mud pit,with light street use. Very light. I have 3" coming so I will decide when I get it.

 

[Honky Kong jr]

I would go with 2.5" exhaust pipe plus galvanized pipe is toxic if you weld on it or heat it to a certain temp I believe.

I know about galvanized pipe. I have used it on a few projects. It all gets ground at the weld seams and heat cycled out side then painted with VHT.

 

[Adam G]

Just trying to keep everyone safe

 

[74 Shortbed]

Yes with the cam that's in it the power comes in up top. It is primarily for the mud pit,with light street use. Very light. I have 3" coming so I will decide when I get it.

Well, you still need torque, just my opinion but if it were me if you do mandrel bends I'd go 2 1/2", if you have to do crush bends then I'd go 3", still think 3 1/2" is a tad big..

 

[Honky Kong jr]

Well, you still need torque, just my opinion but if it were me if you do mandrel bends I'd go 2 1/2", if you have to do crush bends then I'd go 3", still think 3 1/2" is a tad big..

I was thinking the same thing. Just needed to be pushed over that edge. The normal stuff I use is actually 2.750 ID. Thanks for your insight.

 

[bucket]

There have been many articles in recent years that prove back pressure is not needed to make torque. Needing back pressure is basically an old wife's tale it seems.

Going off of that, 3.5 inch exhaust is unnecessary, but it won't hurt power.

 

[Green79Scottsdale]

There have been many articles in recent years that prove back pressure is not needed to make torque. Needing back pressure is basically an old wife's tale it seems.

Going off of that, 3.5 inch exhaust is unnecessary, but it won't hurt power.

I completely agree. I think people use the term back pressure when they should use scavenging. When people think smaller exhaust diameter they think they are adding back pressure. In reality, what they are doing is slowing down the exhaust pulses, and consecutive exhaust pulses begin to pull the next pulse along. The pulses slow down because the cross-sectional area is reduced. With a huge diameter you are giving the exhaust the freedom to escape. The problem becomes it escapes to quickly and does not pull the next pulse with it. When you get to higher rpm's, exhaust pulses are coming more rapidly and thus the scavenging effect happens at a higher rpm range. With smaller diameter exhaust the scavenging effect is optimal at a lower rpm. At least that is my "theory".

 

[bucket]

I completely agree. I think people use the term back pressure when they should use scavenging. When people think smaller exhaust diameter they think they are adding back pressure. In reality, what they are doing is slowing down the exhaust pulses, and consecutive exhaust pulses begin to pull the next pulse along. The pulses slow down because the cross-sectional area is reduced. With a huge diameter you are giving the exhaust the freedom to escape. The problem becomes it escapes to quickly and does not pull the next pulse with it. When you get to higher rpm's, exhaust pulses are coming more rapidly and thus the scavenging effect happens at a higher rpm range. With smaller diameter exhaust the scavenging effect is optimal at a lower rpm. At least that is my "theory".

I also think the header design has a MUCH bigger impact on scavenging than the exhaust itself. But I'm no scientist either.

 

[Green79Scottsdale]

I absolutely agree. I think the first four or five feet of the system is the most important, and the majority of that is headers, assuming long tube. In theory as exhaust gas cools it compresses, so as it travels down the system a smaller diameter exhaust is technically needed.

In a perfect world one would measure the exhaust gas volume, temperature, and exit speed of an engine after it is built. From that they could tune the primary diameter and lengths and merge points in relation to firing order to perfect scavenging.