Diesel exhaust downpipe and turbo exhaust theory

My 7.3L powerstroke van has some exhaust issues.... and by that I mean its stock and boring.

I found a few aftermarket systems, some of which have 3.5" downpipes and some have 4" downpipes. The stock downpipe is a very crimped 3"

I'm getting some feedback that suggests a 4" downpipe will sacrifice low end torque and I should stick to a 3.5". I thought that was just a naturally-aspirated issue and turbos (especially diesels with zero valve overlap) benefited from "bigger is better" thinking.

Your expert thoughts on downpipe sizing for a diesel turbo?

Bigger is better.

mguar wrote: In reply to curtis73: So let me see if I understand you.. You want to spend money for something you don't need and won't be able to use?? OK we all do that at times.. So when you get the new pipes installed do you plan on racing the Van? (Just trying to get application information right).. How much additional fuel are you willing to buy? I mean you could go to 6 inches if you wanted to and were willing to accept the fuel costs.. (assuming new exhaust manifolds or adapters)

Apologies: I don't mean to sound terse, but do you know much about diesels? Diesels are fueled by the injector pump, not a throttle. Increasing diesel engines' power is often accompanied by an increase in MPG. Read this article. It outlines taking a diesel from 170hp/17mpg up to 972hp/27mpg.... and this article could be duplicated ad infinitum.

The question is not about application or MPG, the question is about turbo exhausts and low RPM torque production.

EvanB wrote: Bigger is better.

That's what I thought. I'm looking to get a few more ponies while reducing EGT, but many "respected" diesel guys have told me not to do it for fear of losing low end. I wonder if they're just still on the "backpressure" bandwagon?

I assume they are still on the backpressure bandwagon. Everything I have read indicates that with a turbo you just want as little restriction as possible. I don't have any firsthand experience or dyno graphs to prove it but that is my understanding.

The turbo itself is a much larger restriction than any piping behind it (unless you weld in a crazy straw.) Anything after the turbo is strictly to point the hot gases away from the cabin, and maybe muffle the sound a bit.

At some point, you'll stop gaining meaningful flow; that may be at 4", or maybe 5"... but you won't be doing any harm, beyond carrying around a bigger, heavier pipe than you really need.

Turbo = bigger is better, no exceptions. Your low end is provided by said turbo. Bigger exhaust = faster spool = more power = more low end.

they are on the backpressure bandwagon. With a turbo, the name of the game is getting gases OUT of the spinning wheels. The faster you can get them out of the turbo, the faster the exhaust can go through.. which builds boost faster (and lower in the RPM range) and would negate any "low end loss"

I personally have always wondered if the "need backpressure" myth came about for the same reasons the vw 16v vs 8v low end torque myth came about.. the engines with less backpress "feel" so much stronger at the top end than the low that people automatically assumed that the low end power was being sacrificed

Go with the biggest exhaust that fits, period. I never never met a diesel that doesn't want bigger exhaust then factory.

As to the referenced article, they cheated on the HP number by spraying a bit of laughing gas. But on fuel only, they made what 550? 600? Plus Cummins are awesome MPG makers since they don't have 2 extra cylinders worth of pistons, rods, rings, and weight being dragged around the crankshaft....

+1 on the backpressure lot.

I would expect that a larger less restrictive exhaust would help the turbo spin up lower in the rev range, which should theoretically help with the torque, not adversely influence it.

Its a 7.3 L diesel, how much low end will you be losing by going bigger? And will you be missing your lost TQs?

I can tell you my 97 F250 2wd 7.3, same exact setup as the later vans, is supposed to make 535 ft/lbs torque@1800... But it feels like 200. Of course it is completely bone stock.

When your turbo spools off idle, any claim of backpressure affecting torques is nullified. As large as it can be (THAT's WHAT SHE SAID).

mad_machine wrote: I personally have always wondered if the "need backpressure" myth came about for the same reasons the vw 16v vs 8v low end torque myth came about.. the engines with less backpress "feel" so much stronger at the top end than the low that people automatically assumed that the low end power was being sacrificed

Nah, it's from people who figure that a 3" pipe must have less backpressure than a 2" pipe (on a 200hp nonturbo car) and then when they go to a 3" pipe and low-end suffers, they think it's because the engine "needs backpressure".

The reality is, the 3" pipe has MORE backpressure, because it doesn't have enough velocity at low engine speeds, so the engine has to push the exhaust out rather than the exhaust's own inertia showing itself out the door.

Turbos are a different story - all the effective inertia is used up by the turbo, so there's little if any self-scavenging effect. Get the pinwheel spinning and everything else is secondary!

Ok... that's what I thought. I asked the "expert" for clarification on the other forum and he hasn't written back yet, but I have come to respect his tech on other subjects. Maybe he knows something about the 7.3L fueling curve or something that I'm not thinking of.

Bigger is better with everything turbos.

mguar wrote: It doesn't seem funny to you that the best minds in the industry who are trying to squeek out fractions of a Mile per gallon on over the road semi's etc.. missed such an obvious thing? (972 horsepower and 27 miles per gallon?) Heck why hasn't Dodge gone that route? If they could advertize 972 horsepower and 27 miles per gallon they'd absolutely dominate the pickup truck industry. Some how the laws of physics don't apply.

You're missing a LOT of things. The OEMs have to worry about emissions down to the hundredths of a gram, about per-vehicle costs, about trying to assemble the thing in as rapid a time as possible, about NVH concerns that an enthusiast couldn't care less about, about potential heat issues if some bozo drives full throttle up a mountain in the desert while towing an overloaded trailer and the A/C on full blast, so on and so forth... and then they have to warranty it.

My counter argument: Who is to say that the OEMs have anywhere near the same priority list as Joe Carguy?

Before you spend the money on an exhaust system, look into a simple reflash.

I use a Diablosport Predator tuner with my 7.3L. They give 3 or 4 different performance levels. Economy, towing, performance, etc.

I use an economy tune, which adds about 65 hp. No exhaust change. I get a kick in the pants, can spin the wheels at will, tow anything I want, and still get 22 mpg.

The bigger performance tunes will move enough air that they will require bigger exhaust, but you can get quite a bit out of that engine without any exhaust changes.

mguar wrote: Some how the laws of physics don't apply. More power for less energy input.. Hmmm remarkable!! Is it possible that it's like Formula one engines? 850 horsepower from extremely tiny engines that are good for about 1000 miles and that's it? Or top fuel dragster engines that make 6-7000 horsepower for 1/4 quarter of a mile?

No laws were broken, the cycle is more the same it is just more efficient. Don't forget MPG is calculated at a little to no load state, where the 972 was at WOT. If you look at the mods they did, I can see why. Take normal service parts mark up. (double it) then the distributor (double it again), then the manufacturer (double) now take the $14K they spent on the parts and divide by 6.

puts you at an extra $2333 in cost on a truck that was already the most powerful (at the time) and the cheapest. Not to mention will now pass emissions? Will they have a reliable enough drivetrain to keep warrenty costs inline with targets. Lots to go into the detune, also think about longevity.

How long with that engine last? If you did the reliability calculations alone on the bearings it will probably die in under 100K so that isn't good for a diesel.

There are reasons my friend.

mguar wrote:

curtis73 wrote:

mguar wrote: In reply to curtis73: So let me see if I understand you.. You want to spend money for something you don't need and won't be able to use?? OK we all do that at times.. So when you get the new pipes installed do you plan on racing the Van? (Just trying to get application information right).. How much additional fuel are you willing to buy? I mean you could go to 6 inches if you wanted to and were willing to accept the fuel costs.. (assuming new exhaust manifolds or adapters)

Apologies: I don't mean to sound terse, but do you know much about diesels? Diesels are fueled by the injector pump, not a throttle. Increasing diesel engines' power is often accompanied by an increase in MPG. Read this article. It outlines taking a diesel from 170hp/17mpg up to 972hp/27mpg.... and this article could be duplicated ad infinitum. The question is not about application or MPG, the question is about turbo exhausts and low RPM torque production.

OK thank you for that information; It doesn't seem funny to you that the best minds in the industry who are trying to squeek out fractions of a Mile per gallon on over the road semi's etc.. missed such an obvious thing? (972 horsepower and 27 miles per gallon?) Heck why hasn't Dodge gone that route? If they could advertize 972 horsepower and 27 miles per gallon they'd absolutely dominate the pickup truck industry. Some how the laws of physics don't apply.

it's a Jewish conspiracy to make us buy more oil from the Arabs. Bobby Fisher was right...

Mguar; Its no different than a gas engine. Chevy put out some 454 cubic inch engines that only made 215 hp and only got 10 mpg. It has to do with emissions, warranty, market desire, etc.

The huge difference between gas and diesel is how they are fueled. Gas engines use throttles, cam timing, intake size, head port size, and a few million other things to control where it makes power in the RPM band. Diesels don't have throttles (well, some new ones do, but it has nothing to do with RPM, its an emissions thing). They have no valve overlap, and they are entirely controlled by how much fuel is injected and how much air gets pushed in by the turbo.

Adding power to a diesel simply means adding more fuel and more air. You aren't changing the daily-driver characteristics. In stock configuration, let's say my powerstroke injects fuel from 1 unit at idle up to 10 units at full accelerator. When you modify a diesel, you are basically adding more fuel potential at the top end so that it still injects 1 unit at idle and 20 at full accelerator. Since diesels typically don't have boost controllers (no chance for detonation) they respond to the extra exhaust mass as increased boost. Of course there reaches a point at which the turbo is pushed beyond its efficiency and turbo upgrades are necessary to keep up, but its a remarkably simple loop.

There are daily-driver diesel trucks putting down 1200 hp and 2100 lb-ft of torque. I've seen trucks pull into a dyno day with a travel trailer, unhitch, turn a knob on the dash (literally) and lay down 1000+ hp on the dyno at the rear wheels. Then they turn the knob back down, hitch up, and drive home.

The main MPG benefit comes from changing the injector timing. I may have these numbers wrong, but I'm pretty sure my powerstroke has a base injection timing of 13 BTDC. Many of the more aggressive tunes put that number closer to 27 BTDC. The benefit is that the fuel has more time to exert its pressure on the piston, and there is more time to inject fuel. The reason OEMs don't do it is because it makes a lot more vibes and noise, and it also spikes NOx emissions way beyond what the EPA would allow.

Many diesels can make 200 additional HP with simply a cold air intake, a modest computer tune, and a free-flowing exhaust.

I'm shooting for 450 hp at the wheels eventually. I just want to make sure the exhaust I put on it will keep up with the future plans.

BTDC timing? Heresy!

Diesels are a strange breed. Problem is, thanks to that article I'm starting to want a old diesel Dodge. I don't even like Dodges

Am I the only person who is happy with "just" 570 pound-feet of torque?

mguar wrote: That's fine.. It's a choice and who am I to say what choices he should or shouldn't make.. My choice was to spend money and time on things that would make a whole lot of difference

And you sir (as well as myself) are why there is an automotive aftermarket.

DaveEstey wrote: Am I the only person who is happy with "just" 570 pound-feet of torque?

I am too, when 571 isn't available