Good point. I wouldn't exactly say that the fuel mass is trivial - at least not at anything much above idle - particularly in terms of spinning the turbo. BUT you are correct in your statement that the volume of (hot) exhaust gas is significantly larger (due to heat expansion) than the volume of (cool) air pulled in through the intake. Of course taking advantage of that heat & volume diferential is the main reason the exhaust impeller has the power to spin the intake compressor wheel fast enough to end up producing a net increase in engine power output.

That point also supports my arguments that the 3" exhaust will reduce back pressure compared to the stock 2.5" pipe, and result in more available power...

As far as ability of an exhaust system to flow enough air for a given amount of horsepower without causing significant back pressure, this is from one of David Vizard's books:

1. For avoiding significant restriction from back pressure, the pipe should flow at least 2.2 CFM per horsepower produced.

2. A straight pipe will flow ~115CFM per square inch of area (using inside diameter of the pipe)

For a 3L motor, a 2.5" exhaust is more than you'll ever need. Modified Grand Nationals with 4+ L and massive turbo's get by with a 3" downpipe.

DUDE! David Vissard is a gasser guy! He may know all there is to know about building HP in gassers, but beyond the basic mechanical similarities, you can throw everything you (or he) know about gassers out the window when you start working on diesels.

First and foremost, gassers need some backpressure to make peak HP and torque (especially torque). That doesn't apply to diesels - they produce their best HP and torque with NO backpressure at all.

That 2.2 CFM per horsepower formula equates to 264 CFM on a 120 HP 3 liter turbodiesel and applying your 115 CFM per square inch formula to that gives you a 1.75" ID pipe. That may be big enough for a 120 HP gasser that needs some backpressure for optimum HP & torque, but it isn't sufficient for a diesel that will produce its best power with the exhaust venting to free air (zero backpressure).

Extensive research on the 6.9 liter and 7.3 liter Ford V8 turbo diesels has shown that there is steady performance gains as you increase the exhaust incrementally from 3" (7+ square inches cross section) on up to 4" (12-1/2+ square inches cross section) and then the improvement starts to plateau to the point that there is no more real significant improvement beyond about 4-1/2".

Based on those figures the same principles should hold true for the 3 liter in our W123 Mercedes going from 2-1/2" exhaust (4.9 square inches cross section) to 3" (7+ square inches cross section).

The only problem is that its pretty easy to crank a lot of fuel out of a Powerstroke where it's a feat of engineering on a 61x/60x. So given the fuel we've got, what good can actually come of increasing exhaust size? I would suspect lower EGTs in some driving conditions as well as a possible advancement of the boost curve (builds boost at lower rpms). Having driven around for a day with only a 2' section of 2.5" downpipe (less resistance than a 3" full system) I doubt exhaust restriction affects power appreciably. My exhaust is getting rough, and I do plan on building a turbo back 3" system when it fails, if only for the better sound, so maybe I'll have the good fortune to eat my words.

Actually the 6.9 and 7.3 Fords that I am referring to are the PRE-Powersmoke IDI diesels - VERY similar in design and operation to our OM61x/OM60x engines. Both engines have glowplugs, mechanical injectors, mechanical (piston) injection pumps, steel injector lines, precombustion chambers, timing set by rotating the IP, 22:1 compression ratio, etc., etc., etc.

From what I've read around here there are more adjustable parameters and they have more adjustment range on the Mercedes pumps than there are on the Stanadyne rotary pumps that come on the old Fords.

The two engines actually have more things in common than they do that are different. Probably the biggest differences are V8 vs. I-5 and hydraulic lifters with pushrods vs. overhead cam with no lifters.

As for your experience with the 2' downpipe, doesn't that just go to show that, like I said, the real benefits of the 3" pipe can only be had if it is 3" all the way up to the turbo? If I could find a good quality piece of 3" accordion-style flexpipe to replace the piece of 2.5" flex right after the turbo flange in the stock setup, I'd get a couple of 3" mandrel-bent elbows and fab a complete 3" down pipe assembly. Then I'd have some 3" bent up to replace everything all the way from turbo to exhaust tip.

Betcha' that would give some noticeable improvement in spool up and every other aspect of performance...

so is the exhaust outlet on the turbo 3 inches? do u have to bolt on a piece of that flex pipe or can you just attach a regular piece of 3 inch pipe and then use some mendral bends to form a downpipe?

The turbo outlet and flange is around 2.75" inside diameter, so you'd have to build an adapter. Due to the tendancy of the 5 cylinder to shake at idle (I suspect it has something to do with the fact that 2 pistons fire every other rev, and 3 pistons fire every other rev), and the amount of "rocking" the engne does in its mounts, you need that flex pipe. I wouldn't try bolting up a piece of solid pipe without the ability for it to flex for fear of putting too much stress on the outlet of the turbo housing.

Here is a picture one of the members posted of where he mounted his thermocouple that shows the downpipe mounting flange where it bolts to the turbo outlet, and the section of flexpipe below the flange/elbow

http://www.peachparts.com/shopforum/...5&d=1240385673

The fact that the exhaust has to go through the smaller outlet of the turbo before getting to the larger diameter pipe shouldn't negate the benefit of the larger pipe. Pushing the expanding gasses through a smaller "oriface" doesn't create as much restriction if they can more freely expand on the other side - at least not as much as it would if their expansion were to continue to be constricted by entering a pipe that is even smaller than the "oriface" they just passed through (which is the case with the stock setup - as you can see above)

No. Doesn't show a damn thing. Try this: find a really skinny straw, like the kind you might stir coffee with, and cut a 1" section of it. Then find a big straw, like one you might drink a milkshake with, and leave it full lenth. Which one is harder to blow through, the 1" section of small straw, or the long section of big straw? If you cut a short enough section of little straw, it will be easier to blow through than the longer section of big straw. My point is we haven't shown anything without numbers on a flow bench.

I would be willing to bet, however, that the gains available are minimal at best, because the extremely short section of 2.5" exhaust was not noticeably different than the full 2.5" exhaust system complete with muffler, resonator, bends, etc. Shortening the straw has just about the same effect as making it's diameter bigger, see?

EDIT: However, a 3" exhaust might sound better, might net better mileage and lower EGTs.

The pic of the downpipe shows all kinds of restrictions. Non-mandrel bent as I stated earlier.

Good illustration of what I was saying in my last post - the flange (super short section of skinny straw) won't have much effect if it is dumping into the 3" pipe...

I agree that nothing is PROVEN without flow bench numbers, however, one can logically infer a lot with a fair degree of accuracy.

That is one way of looking at it. On the other hand it can just as easily be argued that the reason it wasn't noticeably different is that 2' of 2.5" pipe with two 90 degree bends in it provides enough restriction that whether the rest of the system is connected or not makes no appreciable difference.

Whether it sounds better or worse would be totally a subjective matter of opinion - but It will certainly sound different. EGTs and turbo lag will both almost certainly be decreased, and MPG may be increased - but only if the larger pipe actually does provide a power/efficiency boost.

I am starting to speak of this idea as "it will" as opposed to "it would" because I am going to do this before the summer is over. I have a spare downpipe to pirate the flange from, and two pieces of 3" pipe that each have two 90-degree bends in them, and I have a complete 3" ATS turbo exhaust system for a Ford IDI that a guy gave me - lots of pipe with mandrel bends of many different angles.

I have a wire-feed welder, and an angle grinder, and today I found an 8" section of 3" flex pipe on Ebay for under 30 bucks - including shipping. So all I have to buy is that piece of flex and maybe a 2.5"-3" adapter and I'll have everything I need to fabricate a 3" system from the turbo flange a-l-l the way to the rear bumper - for next to nothing.

I've got a couple of more urgent projects to finish and then I'm going to do it - just to see what difference it makes and what it sounds like.

Sorry man, but you are drawing an incorrect conclusion from a picture that doesn't give you enough of a view of the bends in the downpipe to support your statement.

I just went and pulled my spare downpipe out of my pile of spare parts and looked it over. You are technically correct that the bends aren't quite mandrel-quality bends, but they are about the closest thing you can get to a 90 degree mandrel bend without actually using a mandrel. The bends have a "constricted" section where the diameter is "squeezed down" less than the thickness of the pipe wall - on the inside radius of the bend only.

If the straight pipe is 2.5" ID then I'd wager that the bends are probably 2.4" ID - virtually no constriction. The flex section appears to be the same ID as the pipe as well. Its ribbed construction might create some turbulance in the laminar flow of exhaust gasses right at the inner surface, but it doesn't really represent any restriction.

I also compared the flange where it bolts to the turbo to a piece of my 3" pipe and the outer rim of the flange is pretty much exactly the same diameter as the pipe. That means that when I join the flange to a piece of 3" pipe, I'm going to have to use a 2.5" to 3" adapter and I'll end up with about a 1" section where the pipe is 2.5" (for the clamping ring) before it flares out to 3".

Is that a short enough "skinny straw" for you Greasy Beast? ;)

If you have the stock fuel injection pump going to a 3 inch exhaust from a mufflerless stock exhaust will yield only little gains. If you are changing to a 3 inch do some tests now and record the results and the conditions. Repeat those tests with the new larger exhaust and post them both.

I'll be going from a completely stock exhaust (resonator, muffler, the works) to a 3" straight pipe all the way from turbo to tailpipe. That way I can simply unbolt my stock system, set it aside (use it as a template to copy when building my new one) and replace it with the 3"...

I'll record some before & after data (RPMs @ start of boost, max boost, 0-60 times, etc.), but the one thing I won't be able to get is EGT before & after since I plan on installing the EGT probe and doing the exhaust all at the same time. It only makes sense to do both at once since I'll have the turbo disconnected and off anyway, right?

Sounds great- we all look forward to seeing the results.:)

well i got some money now and im going to do this too... so the flex pipe that comes right after the turbo looks like it would be ok to start the 3 inch piping right after that all the way to the back of the car

btw i blew in a short coffee mixer straw one that is perfectly round and tiny in diameter and a regular size big straw and the big straw was still easier to blow thru. i think this will work, plus my exhaust is crap right now anyway so i need somthing new and clean anyway, plus when i upgrade to a bigger turbo whether it be now or later this will be a good thing to have anyway