Exhaust diameter

[Conex]

Curious what the correct exhaust diameter should be. Mine is 2" at the header, but after the muffler it is 1.5". I cant imagine any need for backpressure and wondering if it would be any benefit to change it to 2" after the muffler. Also, any advantage to a more of a performance type muffler? Because you know that the stock "B" is so out of control with power . Just looking for any kind of gain.

 

[The Dude]

I will argue on a low RPM deisel that you will see NO performance gains by changing your exhaust.

You will see better (lower) EGTs, that's it. I have dyno'd my trucks before and after with no HP or torque gains.

Zero back pressure is what you want. Go as big (up to 3") as you can and keep the temps low.

 

[Greg_B]

I will argue on a low RPM deisel that you will see NO performance gains by changing your exhaust.

Hi Brad

I'm still trying to get my mind around that one. I do believe the lower temp thing...having seen it, but have a hard time understanding how there would be no gain when turfing a restrictive dangly stock exhaust with a free flowing larger version. Anything that helps it breath has to translate...no? Why bother porting and polishing then...

Maybe I'll wander down to BD when I have time and talk to the folks I know there. See what their experience is, though they typically do a lot of mods all at once.

gb

 

[The Dude]

Porting and polishing on a low RPM engine is a waste of time. (well the % gains are very small) When you start revving 9000-11000 rpms, it makes a difference.

It is the same for the exhaust.

Our (land cruiser) deisel engines are very efficient as it is and they turn at a relitivly low RPM. Maybe there is a 1% gain, but on a 85hp or 140hp engine, who cares???

Even on my Duramax I had, it was pushing 500hp and over 1200ft-lbs. On the dyno we cut the exhaust off and no hp or torque gains but I did see a drop of 150* on the EGTs

 

[crushers]

this is backed by the reports from Alamo here in the city.
the volumn of air and fuel is not changed so why would the performance increase?

 

[Conex]

Another reason I ask is.....when I installed the lift and 33's, my highway temp climbed higher than I was comfortable (it used to run below the first mark on the gauge, after- it was halfway between the first mark and the middle). I removed the thermostat and it brought the temp back down to where it was before the tires (on the highway), but I lose all temp when just cruising around. I would like to put a thermostat back in, but need to control the highway temps somehow. Do you all think a larger diameter exhaust would solve or help? BTW I have been looking for a decent pyro with no luck.

 

[roscoFJ73]

this is backed by the reports from Alamo here in the city.
the volumn of air and fuel is not changed so why would the performance increase?

My diesel shop swears by a 6hp or kw increase on a 1HZ with a straight through exhaust when tested on the dyno in conjunction with a small adjustment in fuel.
Theyve tested it on the dyno
I know someone is going to say its the fuel adjustment that does it but the shop says its a combination of a free flowing exhaust and boosting the fuel.

Im guessing its the fuel being turned up with the bigger diameter exhaust keeping it cool and allowing the extra exhaust to escape,so one wont work without the other.

 

[crushers]

easy enough to test, turn up the fuel...

 

[Greg_B]

the volumn of air and fuel is not changed so why would the performance increase?

My thinking was there would be an increase in efficiency using the existing settings...

gb

 

[crushers]

don't get me wrong Greg, i don't know why, just assumptions.
the larger exhaust will purge quicker but the intake side is the same...

 

[Greg_B]

don't get me wrong Greg, i don't know why, just assumptions.
the larger exhaust will purge quicker but the intake side is the same...

If all things are equal, yes.

However questions: If you have a restriction on either side one would think it would impact efficiency (power?). If you are unrestricted (for the most part) on the intake stroke, and restricted on the exhaust stroke (can not get a full breath out) it will not matter that the intake is unrestricted. The reverse is true too. If you are unrestricted on the exhaust side (for the most part), and have a plugged filter or intake restrictions your mileage will suffer, you will have more smoke (reduced air volume), as the engine can’t get a full breath in. Whatever impedes movement is a drag. A turbo restricts an engine on the exhaust stroke. It's saving grace is the positive pressure over static air pressure assisting on the intake stroke, and increased air volume, allowing more fuel and thus a bigger bang. Yes...no?

Anyway, all this is moot if the real world analysis at the rear wheels shows minimal or little gain with no other changes then a larger exhaust. Reduced EGT's, and faster turbo response is a good enough reason in my mind to go bigger.

Brad, your Duramax was already tuned and had a larger exhaust, and you were taking it off to go bigger yet...or was it tuned up and still on the stock exhaust?

rosco's point is well taken in that one could not up the fuel and/or boost and make the most of those possible gains without ensuring the air pathways in and out are the least restrictive as possible. They kinda go together I guess.

Engine design engineers must have a blast building engines and checking theories against real world tests eh?

Uggg...I gotta drop this and get packing...

gb

 

[Fantom]

I got a free flow exhaust 2.5" and ran it from the motor, below the pass doors and it stops before the back wheel. Now it's 1/2 the lenght, 3/4 less turns and larger. Honestly I did not notice any significant change, but combined with a K&N filter I did notice a change in milage and a bit in horse power. Granted I did not put it on a dyno. Worst case scenario the truck sound meaner now and pisses people off on the highway that have their windows down.

 

[Fantom]

Another reason I ask is.....when I installed the lift and 33's, my highway temp climbed higher than I was comfortable (it used to run below the first mark on the gauge, after- it was halfway between the first mark and the middle). I removed the thermostat and it brought the temp back down to where it was before the tires (on the highway), but I lose all temp when just cruising around. I would like to put a thermostat back in, but need to control the highway temps somehow. Do you all think a larger diameter exhaust would solve or help? BTW I have been looking for a decent pyro with no luck.

Did you change the gears?
What were you running before (tire size)?
Mine is not turboed but it sits around 1/2 to just below 1/2 normally and it does not complain. If yours always sat at a 1/4 then you might have another issue there. Have you tried changing the thermostat for a new one.
If you are running stock gears and bigger tires then the motor is working harder to push them then before, this will result in a rise in temps. You go faster but put more stain on it.

 

[crushers]

If all things are equal, yes.

However questions: If you have a restriction on either side one would think it would impact efficiency (power?). If you are unrestricted (for the most part) on the intake stroke, and restricted on the exhaust stroke (can not get a full breath out) it will not matter that the intake is unrestricted. The reverse is true too. If you are unrestricted on the exhaust side (for the most part), and have a plugged filter or intake restrictions your mileage will suffer, you will have more smoke (reduced air volume), as the engine can’t get a full breath in. Whatever impedes movement is a drag. A turbo restricts an engine on the exhaust stroke. It's saving grace is the positive pressure over static air pressure assisting on the intake stroke, and increased air volume, allowing more fuel and thus a bigger bang. Yes...no?

Anyway, all this is moot if the real world analysis at the rear wheels shows minimal or little gain with no other changes then a larger exhaust. Reduced EGT's, and faster turbo response is a good enough reason in my mind to go bigger.

Brad, your Duramax was already tuned and had a larger exhaust, and you were taking it off to go bigger yet...or was it tuned up and still on the stock exhaust?

rosco's point is well taken in that one could not up the fuel and/or boost and make the most of those possible gains without ensuring the air pathways in and out are the least restrictive as possible. They kinda go together I guess.

Engine design engineers must have a blast building engines and checking theories against real world tests eh?

Uggg...I gotta drop this and get packing...

gb

my take is simple, no matter what happens to the exhaust side it can not affect the intake side. but even turning up the fuel will make a difference on the intake side...
now if someone wants to figure out the volume of air being moved though the combustion chamber and out the exhaust feel free, if the exhaust is not big enough to move the exhaust gasses then a restriction will occure. as long as the exhaust can move the volumn of air coming out then no matter how big the exhaust is, there is no real restriction to the flow, now that being said a larger exhaust will allow hot gasses to move quicker and thus cooler.
i feel it comes down to volumn of air flow.
the intake side of things is simular, the compression can only take so much volumn, too much fuel and the smoke is black too little there is a lack of power. if you can run just the right amount of fuel for the amount of boost and then the engine is running at its true potenial with out harming it.
me, i like playing with stuff so a test here and there is fun. an engineer probably loves to work things out on paper first but they must get pretty frustrated when real life doesn't back up their findings...
a larger exhaust (with in reason) is probably one of the best things you can do to your diesel in terms of longevity. allows it to run cooler, the trubo to spool up quicker as you said Greg.
one side note though, if you go to big and like to do deep water crossings the larger exhaust also reduces the amount of pressure the engine can push out the exhaust and thus the water pressure from the outside CAN create too much back pressure and kill the engine. BTDT. if you like deep water then stick with a smaller exhaust diamater...
this is fun, eh Greg?

 

[Conex]

I was running 31's and I know it is the larger tires causing the heat as I noticed it immediately. For some reason nobody ever has a t-stat when I go look for one (closest parts house is 1 hour away). I will check again today, for the 5th time.

Just in theory and with the amount of pressure I have coming out of the tail pipe I am assuming that I DO HAVE back pressure. The previous opinions seem very valid as this is not a piston ported 2 stroke and back pressure is not needed and probably counterproductive as even though there are valves, if the exhaust is restrictive it would be fighting tail pipe/manifold pressure to escape the chamber. If I were to go with a larger diameter exhaust would it be neccessary to change the fuel rate? Would lower EGT's equate to an overall lower engine operating temp or only lower EGT's? Am I missing something?

 

[crushers]

EGT and water temp are totally unrelated.
the larger exhaust will allow the engine to run cooler EGTs but the cooling will be only affected by the cooling system. try a good rad flush first. then the thermostate (or both at once) you could also try water wetter.
you really shouldn't have any heating problems with 31s, those are pretty small tires...

 

[Greg_B]

EGT and water temp are totally unrelated.

EGT and water temp are totally related, in that they both come from the same source (containment of the combustion process).

They may react different at different times, however they are related. Run too high EGT's for too long and it will reflect in stressing the coolant system (amoung other things).

For some reading on EGT's, impact and other thoughts go here:

http://www.bankspower.com/Tech_whyegt.cfm

http://www.steves.co.za/Engine_Protection_Units_1.html

hth's

gb

 

[Conex]

Will see if I can locate a t-stat. I went from 31x9.5 to 33x12.5 and that is when my temp came up (somewhat expected), just trying to get the temp back to what it was before and not fluctuate so much.

 

[Greg_B]

I will argue on a low RPM deisel that you will see NO performance gains by changing your exhaust.

Lower EGT's can translate into increased efficiency, which is a performance gain of sorts? I think I see what you are saying though, in that you experienced no increase in total output measures (rear wheel HP and Torque). Reducing intake or exhaust restrictions (if present) will allow the engine to become more efficient though...

There are so many variables that would effect all of it. I'll have to see about accessing a dyno for before and after comparisions on a few stock trim JDM's. I am very curious to see the rear wheel results too.

gb

 

[tmarx]

This thread is almost tailormaid for me.Please help a lurker.
After many searches on this forum and lots of deliberations I have booked my truck to be turboed and intercooled in 3 weeks time. It involves a Mitshibishi TD04 15T water cooled unit and an Aqua intercooler.As I am mechanically challenged the work will be done buy "specialists" who are 1600kms away.Their standard settings are 7 PSi and Egts at 700 deg C.The latter I am not happy about and have indicated that I would like a pyro to be fitted simultaneously and Egts to be 650 max.The OEM exhaust was always going to be chucked for an oversize freeflow unit.I am having a hard time deciding whether to upgrade the exhaust now or to wait untill after the turbo etc.Also, should it be 2.5" or 3"?
Any comments or last minute advice please .