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  #1  
Old 06-15-2009, 06:49 PM
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Performance exhaust

my exhaust looks like crap.. i need somthing new. i was wondering if i put on a 3 inch pipe from the downpipe back if i would gain any power? or at least not loose any power. or better yet the turbo back, maybe a reducer from the turbo to a 3 inch from the turbo back, and then to a flowmaster diesel muffler or whatever anyone could recomend for a good muffler. I have no muffler or resonator right now just the stock exhaust with that stuff off, it is starting to sag and whoever i bought it from has a downpipe on there that hangs down way too far causing the whole exhaust to be too low. i dont want to loose any power i have from the straight pipe but i would like a performance muffler if i would not suffer any power loss. otherwise id like to do 3 inch all the way back.

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Last edited by TurboSDL; 06-15-2009 at 06:57 PM.
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  #2  
Old 06-15-2009, 10:44 PM
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A larger exhaust will help conduct more heat way from the engine quicker and make it possible to run higher fuel levels. I'm not sure, on it's own that it will contribute much power but I'm pretty sure that it won't cost power.

Eventually, I'm going to be running a larger exhaust system on my car with an Aeroturbine diesel rated muffler.
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  #3  
Old 06-20-2009, 09:18 AM
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I would say a 2.5" pipe with no cats, resonators, etc and a high quality muffler would support whatever power levels you could want. You only need to expel 3L of exhaust per revolution.
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Old 06-20-2009, 05:17 PM
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I went with a glasspack where the two center mufflers were,then no muffler at rear.Quicker spool and a little time off 0 to 60.
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  #5  
Old 06-20-2009, 10:31 PM
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Until you do more with the delivery of fuel and cooling the intake air nothing more than the stock exhaust "straight pipe" will really do much good. In theory you want a vacuum on the exhaust of a turbodiesel, but the curve of gains by size increase gets flat quickly. The modified stock IP with up to 16psi stock turbo and close to that with a VNT needs little if any more than stock "SP". Going from stock SP to a 3 inch SP on the few that reported it showed little improvement unless you had a finnish IP or other fuel mods.
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Old 06-21-2009, 01:02 AM
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Quote:
Originally Posted by Goatman View Post
I would say a 2.5" pipe with no cats, resonators, etc and a high quality muffler would support whatever power levels you could want. You only need to expel 3L of exhaust per revolution.
Actually wouldn't that be 1.5 L per rev since each cylinder only fires/exhausts every other rev?

However, at 3000 RPMs that's still 4500 LPM, or roughly 159 dubic feet per minute. That's still a fair amount of volume to have to push through a pipe with a cross section of just under 5 square inches (pi x r squared = 3.14159 x 1.25 x 1.25 = 4.91). Whereas with a 3" pipe you're pushing that same 159 cubic feet per minute through a pipe with a cross section of just over 7 square inches (3.14159 x 1.5 x 1.5 - 7.07). While 2.16 square inches difference may not sound like much, it is roughly a 44% increase (2.16 / 4.91 = .4399).

Diesels run best with NO back-pressure at all (as MTUPower said - a vacuum on the exhaust would be better still) so 44% increase in flow area = a 44% decrease in back pressure. Gotta' be some benefit to that - though you'd need to run the 3" all the way to the outlet of the turbo to get the most benefit out of it...
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  #7  
Old 06-21-2009, 03:44 AM
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Stock is just about 2.5". Go for the 3", it can't hurt anything and should help EGTs and let your turbo spool a bit faster.
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  #8  
Old 06-21-2009, 08:47 AM
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Quote:
Originally Posted by rcounts View Post
Actually wouldn't that be 1.5 L per rev since each cylinder only fires/exhausts every other rev?

However, at 3000 RPMs that's still 4500 LPM, or roughly 159 dubic feet per minute. That's still a fair amount of volume to have to push through a pipe with a cross section of just under 5 square inches (pi x r squared = 3.14159 x 1.25 x 1.25 = 4.91). Whereas with a 3" pipe you're pushing that same 159 cubic feet per minute through a pipe with a cross section of just over 7 square inches (3.14159 x 1.5 x 1.5 - 7.07). While 2.16 square inches difference may not sound like much, it is roughly a 44% increase (2.16 / 4.91 = .4399).

Diesels run best with NO back-pressure at all (as MTUPower said - a vacuum on the exhaust would be better still) so 44% increase in flow area = a 44% decrease in back pressure. Gotta' be some benefit to that - though you'd need to run the 3" all the way to the outlet of the turbo to get the most benefit out of it...
Don't forget that the turbo, putting out 1 bar of boost, will double all your numbers here since it creates "artificial" displacement.

I don't have the spreadsheet on this computer but it calculates the lb/min. of air moved when a given amount of HP is generated.
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  #9  
Old 06-21-2009, 11:54 AM
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When an engine is rated as 3L, it should, under perfect conditions, and 100% Volumetric Efficiency, displace 3L of air every revolution. With NA engines, that almost never happens, untill they are highly modified, and then usually only at WOT with some sort of efficient ram air induction.

Turbo motors can provide 100% VE numbers, and much more so when the air is cooled (more O2 per CI) with an efficient intercooler. That is the only reason I stated the engine expells 3L of exhaust.

A 2.5" exhaust will be more than an enough for that tiny motor. The gains to be had are in replacing the bends in the stock system (not madrell bent) and removing the restrictions (cats, resonators, mufflers).

Once you get those fixed, you can still add an intercooler and be OK, pipe diameter-wise.
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  #10  
Old 06-21-2009, 04:58 PM
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Quote:
Originally Posted by Goatman View Post
stock system (not madrell bent)
Incorrect.
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  #11  
Old 06-21-2009, 10:02 PM
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Quote:
Originally Posted by rcounts View Post
Actually wouldn't that be 1.5 L per rev since each cylinder only fires/exhausts every other rev?
Yes.....
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  #12  
Old 06-21-2009, 10:50 PM
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Quote:
Originally Posted by Goatman View Post
When an engine is rated as 3L, it should, under perfect conditions, and 100% Volumetric Efficiency, displace 3L of air every revolution. With NA engines, that almost never happens, untill they are highly modified, and then usually only at WOT with some sort of efficient ram air induction.
NO, the total displacement of the engine is 3 liter BUT each cylinder fires and exhausts only once every two revolutions (it is a 4-stroke), so it only exhausts 1.5 liters MAX if it is NA and has 100% volumetric efficiency. This is not possible with an NA engine, so the actual exhaust gas volume is LESS than 1.5 liters per rev.

Quote:
Originally Posted by Goatman View Post
Turbo motors can provide 100% VE numbers, and much more so when the air is cooled (more O2 per CI) with an efficient intercooler. That is the only reason I stated the engine expells 3L of exhaust.
A turbo producing 1 bar would effectively double the volume, but since the VE is less than 100% to start with, the actual exhaust flow will still be less than a full 3 liters. Even with 1 bar pressure in the intake manifold, all the other factors that prevent the NA from reaching 100% VE still exist and still reduce the flow.

So although the volume of air in (and exhaust out) will nearly double (the turbo's backpressure will also reduce the VE very slightly) it still isn't going to flow the full 3 liters volume. That would take more than 1 bar pressure. For example if the NA has say 85% VE, then to flow the full 3 liters per rev, the intake pressure would need to be roughly 1.18 bar.

1 bar pressure doubles the flow from 1.275 liters per rev (1.5 L x 85%) to 2.55 liters per rev. 1.18 bar would be required to make it flow roughly 3 liters per rev.

Quote:
Originally Posted by Goatman View Post
A 2.5" exhaust will be more than an enough for that tiny motor. The gains to be had are in replacing the bends in the stock system (not madrell bent) and removing the restrictions (cats, resonators, mufflers).

Once you get those fixed, you can still add an intercooler and be OK, pipe diameter-wise.
Agreed removing bends is going to improve flow, but I still say that the 44% increase in flow capacity of the pipe is going to help as well - IF you can do it ALL the way to the turbo. Of course there's the rub - taking the 3" pipe all the way to the turbo isn't so easy to do...
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1984 300 Coupe TurboDiesel
Silver blue paint over navy blue interior
2nd owner & 2nd engine in an otherwise
99% original unmolested car
~210k miles on the clock

1986 Ford F250 4x4 Supercab
Charcoal & blue two tone paint over burgundy interior
Banks turbo, DRW, ZF-5 & SMF conversion
152k on the clock - actual mileage unknown
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  #13  
Old 06-21-2009, 11:28 PM
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Stock "modified" IP = stock exhaust minus muffler or straight pipe = very close to max output.

Myna pump or other fuel mods = larger than stock exhaust diameter may be needed.

It's not complex.
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  #14  
Old 06-22-2009, 11:14 AM
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Aren't all of you forgetting that it would INTAKE 1.5 to 3.0L of air, but the whole diesel combustion process produces extra exhaust gas? For all practical purposes the mass of the exhaust gas is the same as the intake (amount of fuel mass is trivial I think, maybe I'm wrong though?), but the VOLUME is much much higher.

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  #15  
Old 06-22-2009, 08:22 PM
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what is the exhaust outlet size on the turbo?

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