Any improvements with larger exhaust on 617 diesels?

[mholbrook]

I'm wondering if anyone has changed out the exhaust fromt the downpipe back for a performance improvement?

It would seem that a 3" system all the way back plus a K&N in the stock box would make it easier to pump the air. Maybe it is just not cost effective.

I wouldn't mind having a deeper "big diesel" sound. Jake Brake anyone?

Any comments appreciated.

Souping up diesel engines for more power is a COMPLETELY DIFFERENT ball game than doing the same with a gas engine.

With a gas engine, anything you can do to improve breathing will add power, more fuel/air in results in more power out.

In the case of a diesel, however, improving breathing by itself does NOT result in more power. Remember, the power is regulated by varying the amount of fuel added to the combustion chambers. Without other modifications, the only thing that an opened up exhaust would do would reduce pumping losses. The effect of this gain would probably be immeasurable using even the most sensitive of dynamometers.

The factory added power with the turbo and other breathing improvements and THEN providing more fuel in the combustion chamber. The added breathing allowed additional fuel to be added with no ill effects.

Even though this is all scientific fact, there has been much debate in this forum regarding this. You can debate laws of physics all day long and they will not change. I suppose that some believe that if you debate it long enough, you can make the Sun rise in the West.

Have a great day,

[wagonboy]

mholbrook, if you do decide to go for it, please post your findings.
Wanna know how a diesel sounds with a flowmaster as a muffler.

[rwthomas1]

While I completely respect LarryBibles opinion I beg to differ on this point. It has been proven time and again, by reputable sources and tests that a free-flowing exhaust system can increase hp/torque in diesels. I am not an engineer so I have no idea if this is due to a reduction of "pumping losses" or what but in some situations it DOES improve things. When I first started towing with my truck there was a hill I couldn't get over at more than 45mph floored in 3rd gear EGTs at 1100*. I removed the factory garbage 2.5" downpipe/exhaust and installed a nice 3" downpipe/4"exhaust system. The difference was drastic. Same hill 55-56mph in 4th gear, EGT's at 900-950* There is one important point to consider here. The factory exhaust system was a VERY poor design that caused excessive backpressure. (the intake side was similarly restricted but that is another issue) I agree that the laws of physics don't change but I think that anyone can see that any significant restriction in either the intake or exhaust system of ANY engine will cause pumping losses and inefficiencies that will prevent the engine from making its full rated power at a given fuel setting. It has been proven ad nauseum that the US diesel truck manufacturers don't produce the most free-flowing exhaust systems and the aftermarket has responded with better options. I would certainly hope that MB did their homework and the exhaust systems on our vehicles are as efficient as possible which would negate any need to make modifications. With respect to that a simple test could be performed. A pressure gauge plumbed into the exhaust pipe 2-3 feet from the turbo would be a great way to assess the efficiency of the MB system in real-world conditions. I would expect between 0.5-1.5psi and this would not warrant any changes. However if it was 3-4psi under any conditions a change to a free-flowing system would likely make a noticeable difference. Just my 2cents.... RT

[The Warden]

One other thing...what's the diameter on the stock pipe? I presume that my exhaust system is stock, but the pipe's already pretty bloody big. I think it's either 2 3/4" or 3" as it sits...certainly bigger than the 2 1/2" pipe on my truck (with an engine that's double the size of a 617).

I do believe that the more restriction-free you can get an exhaust system, the better (if nothing else, to allow the engine to work less to get the exhaust out, thereby lowering EGT's), but short of straight-piping I'm not sure what kinda gain you would get out of modifying the exhaust...seems to me that Daimler-Benz already did a lot of that.

OTOH, I think that steps could be taken to make the intake more restriction-free...

[rwthomas1]

You are correct Warden. The stock MB pipes are pretty darn large to start with which leads me to believe that MB did their homework on this one. I do wonder about the muffler design and why there is two.... Turbos quiet the exhaust pulses in the process of doing their job and muffler technology has progressed quite far since the mid-80's. I think that you could likely get away with one muffler such as a Flowmaster but I would hate to make my MB any louder particularly from the drivers seat. I would like to experiement with this and maybe I will get around to testing the exhaust system for backpressure and changing mufflers to see if I can improve it any. Quite likely I will find that it is already fine, as I would expect from MB. RT

[BoostnBenz]

Originally posted by The Warden
OTOH, I think that steps could be taken to make the intake more restriction-free...

Agreed there, I've been thinking of buying another air snorkel and such, then have dual intakes on my air cleaner housing.

As for the exhaust, you always lose torque when upgrading exhaust pipe sizing. Why is this? You are decreasing the backpressure, meaning that there is less pressure in the motor when the compression cycle begins. This reduction in backpressure means that the turbo lag time will benefit from this (time for the turbo to spool). I doubt there is much to gain in horsepower by making the exhaust larger. Odds are, also with a straight through muffler like MB used there isn't much to gain there either.

Jeff M.

[psfred]

I doubt adding a larger diameter pipe will do much of anything, MB diesels don't suffer from low horsepower output or excessive back pressure. All you would get is more noise, not something I personally desire.

A NA diesel moves the max volume of air at about 1500 rpm. For a turbo, airflow is only limited by compressor speed and intake crossectional area, the intake manifold can pretty much be ignored (and MB makes very high flow efficient manifolds anyway).

Peter

[rwthomas1]

As for the exhaust, you always lose torque when upgrading exhaust pipe sizing.

Not true and not applicable to turbodiesels. You could run the exhaust straight out of the turbo housing and have no decrease in torque. Turbos on diesels and gassers benefit from the lowest backpressure possible. This allows the turbo to spool up as quickly as possible. I am going to test my MB out of pure curiousity. I will drill/tap the exhaust pipe a few feet back from the turbo and put a pressure gauge on it and take it for a ride. I'll just stick a pipe plug in the hole when I am done. I'll let y'all know how it goes. BTW headers and big 3" duals work GREAT on NA diesels like GM 6.2's and IH 6.9/7.3's, increase in torque! RT

RT,

I am not trying to be combative here. I will take your word for the increase on your truck, but this is only telling me that your truck had and EXTREMELY poorly designed exhaust system.

The ONLY gain that could be derived without somehow injecting more fuel would be a drastic decrease in pumping losses.

As psfred indicates, the MB system is not too bad, any power increase would be totally insignificant without fuel system modifications.

REMEMBER, on a diesel engine you regulate power by regulating the amount of fuel injected into the cylinder. Fuel air ratios are acceptable on a VERY broad range.

In a GAS engine, the fuel is mixed with the air before being drawn into the cylinder. This means ANYTHING you do to make the engine breath better increases power within certain acceptable parameters. A diesel in contrast, draws ONLY air into the cylinder, the fuel quantity is determined by the fuel system alone.

Have a great day,

I own a golf TDI so sometimes I check out the TDI forum. A lot of the guys add a chip, larger injectors and completely eliminate their mufflers! Supposedly the car is not much louder, but produces a more throaty sound. Some of the links provide sound clips of their street driven diesels running no mufflers. At the very least, it's interesting.

[BoostnBenz]

Originally posted by rwthomas1
Not true and not applicable to turbodiesels. You could run the exhaust straight out of the turbo housing and have no decrease in torque.

Why is this? Your saying the backpressure before the turbo remains constant? Hmm, don't know a whole lot about diesels, but it just seems like since the air can't get out that more air will be in the cylinder on the next fire. Granted not clean/fresh air, but more air creating more torque. Or are you saying since there is no TB plate that it reachs this equilibrium of pressure in the motor no matter what the backpressure is (well other than when the turbo kicks in) while the motor is filling with fresh air.

Yea the TDIs sound pretty cool. As for the lack of turbo, that is where the term "turbo muffler" was coined. People are misleaded into thinking it is a performance muffler, but really they make no claim of that as any or no muffler is fine on turbos (owner willing). Actually it seems that most 'turbo' mufflers are some of the worst performing mufflers out there. I put some on my old Grand Prix, a couple years later to find they were the worst flowing that DynoMax sold. Ah well, not my car or my problem anymore.

Jeff M.

[wagonboy]

Hi, dunno about right off the turbo housing but when I upgraded to a big 16g on my VR4, I broke her in with just the 3" DP, boy was that a lot of fun. When she got on boost, car was starting to point to the stars, then fuel cut . Hopefully someone would try to install a free flowin muffler on our turbo diesels and post here. I need to accelerate the destruction of my tranny so I can get a rebuild and get to mess around with the Alda and boost pressure.

[BoostnBenz]

Yea I remember the first time I turned my 14b upto 15-16psi when it was 40 degrees outside, WOW!!! So I can only imagine how fun a Big16 would be, time for injectors to help fight that nasty fuel cut. I havne't hit it yet, but injectors (720s) are high on my list, somewhere lower on that list, towards the bottom is a BR50 turbo (capable of ~500hp).

Jeff M.

[psfred]

Diesel engines don't make that much noise at any power output short of full -- they sound pretty much the same without the muffler as with at idle. Not at all like a gas engine. You will only get more sound at full power.

And the mufflers are flowthroug, anyway... even on my 72 280 SE.

Peter