4" Diesel Exhaust On A 6.0 Gasser

redneck tom · July 6, 2009

Hey guys! I came across a great deal on a 4" inch exhaust, Will this hurt anything on my truck? will my mileage be affected? how loud will it be in the cab?

redvett · July 6, 2009

Diesels require a fairly unrestricted exhaust and have high torque ratings, a gasser has much lower torque rating and require a more restricted exhaust to keep the engine in a lower power band. Even a 3" exhaust is too large and will raise the torque band and lower fuel economy. A 4" exhaust on your 6.0L truck would kill bottom end power and be quite noisy. Remember most Diesels are turbocharged and that in it self quiets the exhaust sounds.

asilverblazer · July 6, 2009

I'd bet it runs worse. There would be no exhaust gas velocity to carry it out of the engine.

alwcurlz · July 6, 2009

Diesels require a fairly unrestricted exhaust and have high torque ratings, a gasser has much lower torque rating and require a more restricted exhaust to keep the engine in a lower power band. Even a 3" exhaust is too large and will raise the torque band and lower fuel economy. A 4" exhaust on your 6.0L truck would kill bottom end power and be quite noisy. Remember most Diesels are turbocharged and that in it self quiets the exhaust sounds.

1+!

BuckFeverZ · July 6, 2009

Diesels require a fairly unrestricted exhaust and have high torque ratings, a gasser has much lower torque rating and require a more restricted exhaust to keep the engine in a lower power band. Even a 3" exhaust is too large and will raise the torque band and lower fuel economy. A 4" exhaust on your 6.0L truck would kill bottom end power and be quite noisy. Remember most Diesels are turbocharged and that in it self quiets the exhaust sounds.

IMO a single 3" exhaust would be fine. On the other hand, 4" is way to big for a gasser.

Z71kris · July 6, 2009

no back pressure=no good!

Draenor · July 6, 2009

Diesels require a fairly unrestricted exhaust and have high torque ratings, a gasser has much lower torque rating and require a more restricted exhaust to keep the engine in a lower power band. Even a 3" exhaust is too large and will raise the torque band and lower fuel economy. A 4" exhaust on your 6.0L truck would kill bottom end power and be quite noisy. Remember most Diesels are turbocharged and that in it self quiets the exhaust sounds.

1+!

+2

no back pressure=no good!

+1, You'll probably burn out your valves too.

alwcurlz · July 6, 2009

Some other info I found that might be useful I'll post here.

exhaust theory: shiny

Tell me if there's something wrong with this reasoning...

There's often some amount of intake and exhaust valve overlap. That said, the speed of the exhaust gas out of the exhaust ports is higher than the of speed of the intake charge through the intake ports because the pressure differential across the exhaust valve versus the exhaust gas is higher than the pressure differential across the intake valve versus the intake charge(note: the heads are designed such that exhaust gas is moved away from the intake valves by the exhaust valve's pressure differential creating an area of lower pressure near that intake valve to allow the pressure differential across the intake valve to result in the intake charge moving into the cylinder.)

If the exhaust gas is evacuated too quickly, some of the intake charge can be drawn out of the exhaust valve, resulting in lower fuel economy and possibly lower performance. The tendency for this to happen will be directly proportional to the ratio of the exhaust valve pressure differential to the intake valve pressure differential. As a function of engine speed this ratio will decrease, thus at low engine speeds the tendency to lose the intake charge to the exhaust will increase. To combat this, an exhaust system is designed such that more backpressure will exist to decrease the exhaust valve pressure differential. This will in turn decrease that ratio inhibiting the loss of intake charge at low engine speeds.

Of course, with very flexible variable valve timing you could combat this very easily by making the amount of valve overlap proporational to engine speed. And also... that backpressure does waste energy. A force is applied indirectly through the gas as a result of that backpressure, so the piston performs work against that backpressure over the distance it travels -- so backpressure is used as a necessary evil to allow low engine speed usability and economy(which for street going daily drivers is one of those necessary things), but robs some amount of power at high engine speeds.

Also, of course, you're right about the cost aspect. That's a major factor in all aspects of design.

Regarding your idea of a larger pipe increasing backpressure(or perhaps I read you wrong?) -- I feel that you're not quite correct on that. I think that due to that pulsed nature there's probably a good medium somewhere where the effective backpressure is the least. Also, it should be possible to take advantage of resonances in the exhaust system to help scavenge exhaust at some frequences. Maybe when I get more time I'll work it out or throw it in a CFD package... For now, all I can give is an analogy: Does it take more force to push and pull an air plunger pumping air through a small pipe or a large pipe(with the same plunger volume in each case)? the small pipe, so the "backpressure"(force over unit crossectional area at some point in the exhaust path) is higher.

All in all, I think the benefits of the exhaust system really depend on the specifics of the application. In some engines that have a lot of valve overlap at low engine speeds, the hit from having reduced backpressure on low rpm torque may not be worth the extra torque at high RPMs. If a variable valve timing configuration is setup such that the backpressure is unnecessary, it's likely that the exhaust system was already setup to take advantage of it by the manufacturer. Of course, on some applications, the exhaust system may have been designed in a different country where emissions & noise regulations were different... and instead of redesigning the entire thing different(possibly more restrictive) cat and mufflers may have been attached for that country.

Ok.. that was fun.

Spurshot · July 11, 2009

Well, the factory has seen fit to put a 3" single exhaust on a 367 hp engine already. If you were making or trying to make more than 400hp it may be of benefit to go to that 4" or if you have a small exhaust diameter now. All that dissertation on cam timing overlap kinda touched on, but not directly stated that exhaust restriction (or lack of restriction) can have a similar effect as slight changes in cam timing overlap. Given the mild cams of these engines, a reduction in backpressure will have a better scavenge and the effect of a increase in overlap at WOT.

An engine is an air pump. The more air you can pump thru it in a given time, the more horsepower you can make. You want to make horsepower, torque is a static measurement of force and but one component of horsepower. Horsepower is a measurement of work in a given amount of time.

alwcurlz · July 11, 2009

An engine is an air pump. The more air you can pump thru it in a given time, the more horsepower you can make. You want to make horsepower, torque is a static measurement of force and but one component of horsepower. Horsepower is a measurement of work in a given amount of time.

What about these ricer dudes who state they get 300 or up to 400 hp from there little 4 or 6 cylinder engines? I've come to understand that that HP rating is mis-calculated. The duration of work performed you mentioned makes sense. I've often wondered about torque and haven't researched it much but, it seems pretty important and more important than HP, or am I wrong on that?

66nova · July 11, 2009

An engine is an air pump. The more air you can pump thru it in a given time, the more horsepower you can make. You want to make horsepower, torque is a static measurement of force and but one component of horsepower. Horsepower is a measurement of work in a given amount of time.

This is very true. BUT a bigger exhaust system is not necessarily the most efficient way to get the exhaust out. I also do not believe that very many engines NEED back pressure. A properly designed and properly sized exhaust will actually evacuate more exhaust from the motor then a big wide open exhaust. I have seen a lot of cars make more hp and torque with exhaust then without and it is not because of back pressure. It's due to scavenging. If the exhaust is too big you lose the signal from the sound wave that is helping "suck" the exhaust out. You will not gain this signal back until way up in the rpm range. This is the reason engines "lose torque". They do not necessarily lose it but it is moved up the rpm range. The torque curve is altered.

What about these ricer dudes who state they get 300 or up to 400 hp from there little 4 or 6 cylinder engines? I've come to understand that that HP rating is mis-calculated. The duration of work performed you mentioned makes sense. I've often wondered about torque and haven't researched it much but, it seems pretty important and more important than HP, or am I wrong on that?

This depends on who you ask. This has been an argument for years. But in my opinion it truly depends on what you are doing and trying to accomplish.

Most racers will take a high hp car over a low end torque monster any day. If you take two cars that are properly set up to their motors(reread that...PROPERLY SET UP...ie gears, converter, shift points etc) that other wise are the same the high hp car will win everytime.

Now if you are talking about 10,000lbs from a dead stop up a 30 degree grade, give me the torque monster.

Spurshot · July 11, 2009

I believe a well designed exhaust system that has significantly lower restriction will make more horsepower than a equally well designed exhaust system with higher restriction, providing that each is appropriately tuned (fuel and ignition) optimally. Scavenging can occur very well at low restrictions, in fact, it thrives on that.

There's a lot of misunderstanding of what defines the term torque. It's simply a static force. Also, a transmission, rear axle, or any other gear reduction device is a torque multiplier. An engine that makes 400 hp at 10,000 rpm is capable of doing the same work as the engine that makes 400 hp at 5,000 rpm...assuming you use both engines at their rpm where the 400 hp is made. On a race car, the 10,000 rpm 400 hp engine may be an advantage since it's likely much smaller and lighter. Both engines, while turning their respective rpms at their 400 hp ratings, with gear reduction to turn a ...say 33" dia tire a given rpm while on identical trucks up the same grade, will put out identical torque at that tire and pull that grade equally.In a truck towing 12,000 lbs of horse trailer, the engine making the very same amount of horsepower at half the rpm will be easier to drive and will have less wear from rotating parts.

Again, torque is one of the components of horsepower. I'm not minimizing it. But without movement (rpm) it doesn't have the capacity to do work. Horsepower IS the measure of work. Work is what you are doing by moving a truck down the road, accelerating it, going up hill, etc..

Engines with "broad torque bands" also have broad horsepower bands. It makes sense once you grasp the concept that torque is just a static force, just like when you measure how tight a bolt is with a torque wrench. The high rpm example engine above will need a lot of help from gearing (transmission gears) to keep it within it's best horsepower band. The high rpm engine can be as useful, it just may require more gear changes and closer ratios to do it. The Jap trucks are doing it to some degree.

66nova · July 12, 2009

I believe a well designed exhaust system that has significantly lower restriction will make more horsepower than a equally well designed exhaust system with higher restriction, providing that each is appropriately tuned (fuel and ignition) optimally. Scavenging can occur very well at low restrictions, in fact, it thrives on that.

Well for the most part we are going to have to agree to disagree. I agree that you do not want a lot of restriction. Thats where the properly sized exhaust comes in. A properly size exhaust will not be restrictive but it also will not be too big to not allow for a good scavenging. We are talking about relatively small engines here. Even the 6.2 is really not that big.

I also don't think there is any amount of fuel or timing you can add to one of these trucks to make of for the change in torque curve (especially while staying within a usable octane range). In fact very few fast cars, that do run open exhaust (which would be you least restrictive system) will run with out a extension bolted on to the collector. In fact at one point we ran open headers when ever we ran. It did not matter where the time was set or how adjusted the carb we would always run 1-2 tenths faster with the extension bolted on. Now we never run without exhaust and we run as fast and faster then with the open with extension. Thats with 427 SBC motor with NOS (as lot of pressure) running in the 9's

If you or driller want to give it a try, I say go for and let us know what the dyno says. But I ask you to explain to me why none of the exhaust manufacturers are making a 4 in system for a gas motor. If they coulc even claim a single hp more then the competition I have a feeling they would market it.

Sorry but a properly sized exhuast (for a given cubic inch and rpm range) is always going to make more power then an oversized exhaust. This is not just me and my experience saying this.

Spurshot · July 12, 2009

There's some good reading on exhaust systems of internal combustion engines out there.

I'm not without some experience with race engines myself. I worked for a leading race cylinder head company, the owner exposed me to a lot of diverse engine types including lots of Hemis, Bignotti's Offy's, and I personally did the exhaust mod of a set of 351 Cleveland heads for Bob Glidden. And I did singlehandedly design and build the equal length 4 into 1 exhaust system for that competition aerobatic plane in my avatar.

I believe scavenging and backpressure are mutually exclusive of each other. I believe you don't need one to have the other.

A backpressure "signal" is not the "signal" needed for efficient scavenging on a multi-cylinder 4 cycle engine, but rather the sudden pressure drop in the exhaust caused by another cylinder exhaust pulse in an adjacent tube in a collector, produces the scavenge.

A 2 cycle engine can indeed benefit from a backpressure signal following a low pressure signal. The low pressure signal over scavenges some of the intake charge into the exhaust system, then the sonic reversion pulse pushes it back into the exhaust port after the intake port has been covered, thus providing a bit of supercharging effect. It can be pretty dramatic in a 2 cycle. Since the speed of sound rises dramatically with temperature, in exhaust temps, the speed of sound is almost twice what it is at sea level ambient temps. A 4 cycle doesn't have this particular opportunity since the timing is completely different (intake is open after the exhaust closes).

66nova · July 12, 2009

I believe scavenging and backpressure are mutually exclusive of each other. I believe you don't need one to have the other.

A backpressure "signal" is not the "signal" needed for efficient scavenging on a multi-cylinder 4 cycle engine, but rather the sudden pressure drop in the exhaust caused by another cylinder exhaust pulse in an adjacent tube in a collector, produces the scavenge.

Ok, let me restate this. I do not believe and engine needs backpressure. I fully understand that you want the exhaust out as quick and as efficently as possible. But, the way to accomplish this is not by just opening the exhaust up as big as you can. This is old school way of thinking and is what was done 10+ years ago.

The pressure drop you talk about above actually emphasizes my point. How great this pressure drop you have is determined by the exhaust diameter. The greater the diameter the less this drop will be noticed. Now a too small of system is going to have too much backpressure and the exhaust will not be drawn out effectively.

This is the reason a huge 4+ inch single exhaust is not used on a gas motor, but if more flow is needed a dual exhaust system is ran with 2 smaller pipes and a cross over pipe. This pressure drop is can be actually utilized this way and still get the volume of exhaust out thats needed. Again I will say, this is the reason none of the exhaust companies or automans offer a 4+ inch single system. They already have the tube and bends for the diesels so it's not going to cost them much to make it.

So a properly sized system will actually get the exhaust out more quickly by utilizing the pressure drop within the rpm range the engine operates in. I think I have used this comparison before, but its a good comparison. This is the same reason we don't run a 1050 dominator on a relatively mild 350 engine. Due to the large venturi size it would be a slug driving it around on the street. Now if you took that motor to the dyno you could most certainly make more hp with the dominator then say a 750 that you may be using for good driveability assuming you wind it up to 7000+ rpm.

I agree that there is a lot of good reading out there on exhaust but there is also a lot of bunk.