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Discussion Starter · #1 ·
Hey dont make fun of me. Yeah i did a swap, yeah i have driven it 6000mi this year, yeah its my daily driver, yeah i dont know most of what goes on inside the Diesel engine.


So here is my question.

Does exhuast restriction increase EGTs? and if so then wouldnt it follow that a turbo would increase EGT and therefore a smaller exhuast housing increase EGT?

I know that a turbo will force air into the engine bringing down EGTs but it seems that there is a balance point. For instance, if i took my stock 4bt and got a smaller exhaust housing would my EGT go up because of restriction or down because of the extra air?
 

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yes the restriction would go up cause increased egt's. unless you went with something unreasonably small your egts would not get overly hot. that being said you probably also won't get to hot of egt's without more fuel because obviously more fuel burnt means more temp.

so i'm guessing your asking this because you are trying to decide what turbo to go with. it really depends on your purpose of use for an optimum towing setup you will want a free flowing exhaust with no turbo smaller than an hy35 cause your temps will go up considerably after that, the load you are towing will help spool whatever turbo you tow with unless its overly large.
if its your dd then i would stick to a smaller turbo so you can spool, it is likely you won't have egt issues unless you drive the crap out of it.
best deffense is propper gauges and monitoring.
also i live up north doesn't get to hot up here things might change a little if you have really warm summers, an intercooler would also help
 

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Discussion Starter · #3 ·
Well, thats what got me thinking about it but Im really just interested in whats going on it there.

Im trying to figure out how EGTs get higher with exhuast restrictions. the same amount of fuel is injected regardless of restriction. I guess its because increased backpressure will not allow all of the exhuast air to escape, and then the next intake stroke wont have as much cool, fresh air.
 

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Well, thats what got me thinking about it but Im really just interested in whats going on it there.

Im trying to figure out how EGTs get higher with exhuast restrictions. the same amount of fuel is injected regardless of restriction. I guess its because increased backpressure will not allow all of the exhuast air to escape, and then the next intake stroke wont have as much cool, fresh air.
ok i see so your looking for a better understanding of the theoretical reasoning behind it. well as far as i understand it when you compress a gas it creates heat so the increased backpreasure itself directly causes an increase in heat, and i think what you said is a large part of it high back pressure means not all the exhaust gasses will be evactated meaning warmer cylinder temps, with more gas taking up space less fresh air will also be drawn in, this could also cause a less than complete burn or the fuel burning as it leaves the cylinder causeing an increase of temps. might be something i missed
 

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The increase in EGT due to restriction is really minor, it follows the ideal gas laws.
Basically you need a huge difference in pressure to raise EGT's by 50 deg C.

For this reason on a non wastegated turbo, smaller exhaust housings will drop EGT's up to around the point where your valve springs can't close the exhaust valves properly.
 

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Discussion Starter · #6 ·
OK so wich one is correct? Will going with a smaller housing Drop my EGT or Raise it? The Ideal gas law would say that a restriction to expansion would retain Temp. and a free flowing exhuast would allow expansion and a drop in Temp. Im leaning towards Dieseldakota's answer of an increase in Temp. But i may have misinterpreted Dougals answer. Or maybe correollis has something to do with it.
 

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OK so wich one is correct? Will going with a smaller housing Drop my EGT or Raise it? The Ideal gas law would say that a restriction to expansion would retain Temp. and a free flowing exhuast would allow expansion and a drop in Temp. Im leaning towards Dieseldakota's answer of an increase in Temp. But i may have misinterpreted Dougals answer. Or maybe correollis has something to do with it.
Housing size is used to tune the boost delivery. EGT is a consequence of that, not the cause.
The extreme case of a larger housing is no boost, which takes you back to NA power levels.
The extreme case of a smaller housing is too much boost and EGT"s that are too cold to provide much power.

It's about finding the right size for your application, I don't think you'll find the smaller housing to be too small.
 

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Discussion Starter · #8 · (Edited)
Dougal,

I think that is only partly true. Because with a big housing and no boost the IP will only deliver the "no boost" fuel load. BUt i think if i took my EGT readings at highway speeds with my big housing and then went a smaller one, with no other changes i bet the smaller one would be a higher EGT. whats your take?

Im trying to logically figure out if a non wastegated turbo is better or worse than a wastegated one. My thinking is a wastegated one will reduce my egts at peak RPM, and will provide less exhaust backpressure at highway speeds.


Housing size is used to tune the boost delivery. EGT is a consequence of that, not the cause.
The extreme case of a larger housing is no boost, which takes you back to NA power levels.
The extreme case of a smaller housing is too much boost and EGT"s that are too cold to provide much power.

It's about finding the right size for your application, I don't think you'll find the smaller housing to be too small.
 

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Dougal,

I think that is only partly true. Because with a big housing and no boost the IP will only deliver the "no boost" fuel load. BUt i think if i took my EGT readings at highway speeds with my big housing and then went a smaller one, with no other changes i bet the smaller one would be a higher EGT. whats your take?

Im trying to logically figure out if a non wastegated turbo is better or worse than a wastegated one. My thinking is a wastegated one will reduce my egts at peak RPM, and will provide less exhaust backpressure at highway speeds.
The smaller housing will give you lower EGT's at highway cruise. Guaranteed.
I've just gone from a T25 with a 0.49 A/R turbine housing to a T25/T28 hybrid with a 0.64 A/R housing. My boost at 100km/h cruise dropped from 8-9psi to about 6psi and EGT's increased.

A wastegated housing is normally used to limit boost which can cut pumping losses at higher revs. It all depends on your setup whether it'll help or not. Generally 4BT's have a short enough rev range that it's a close call either way.
 

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Discussion Starter · #10 · (Edited)
Dougal,
I think im following you now. What your saying is that the restriction caused by the turbo will cause a small theoretical increase in EGT (not being allowed to expand following the ideal gas laws) but will be overwhelmed by the Increse in Airflow netting a Lower EGT at operations that the turbo is providing boost. But as a side note Boost will also be higher at a given power setting (representing the leaner/cooler mixture in the cylinder)

And all the wastegated turbos will only bypass the turbine if max boost is reached right? so if you cant spin it to max it will never bypass making a waste gate a waste of space, and Conversely if you can spin it to Max boost you would still benefit from the extra air provided by disabling the wastegate (assuming you dont overspin the compressor)?

Is that right?

The smaller housing will give you lower EGT's at highway cruise. Guaranteed.
I've just gone from a T25 with a 0.49 A/R turbine housing to a T25/T28 hybrid with a 0.64 A/R housing. My boost at 100km/h cruise dropped from 8-9psi to about 6psi and EGT's increased.

A wastegated housing is normally used to limit boost which can cut pumping losses at higher revs. It all depends on your setup whether it'll help or not. Generally 4BT's have a short enough rev range that it's a close call either way.
 

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cut... So here is my question. Does exhuast restriction increase EGTs? and if so then wouldnt it follow that a turbo would increase EGT and therefore a smaller exhuast housing increase EGT? I know that a turbo will force air into the engine bringing down EGTs but it seems that there is a balance point. For instance said:
Here's my example,
to my stock 4BT I added a 52mm intake inducer (46mm stock). It's an improvement, but still smokes too much. EGT go lower
Replaced the 16cm exhaust with a 14cm, better.
Exhaust is 3" with many bends.
Turned up the fuel by 1/2 turn. Lots of fun to drive. only smokes if I go to to WOT, if i roll the throttle on, no smoke. EGT's much better.
 

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Dougal,
I think im following you now. What your saying is that the restriction caused by the turbo will cause a small theoretical increase in EGT (not being allowed to expand following the ideal gas laws) but will be overwhelmed by the Increse in Airflow netting a Lower EGT at operations that the turbo is providing boost. But as a side note Boost will also be higher at a given power setting (representing the leaner/cooler mixture in the cylinder)
Yes, that's exactly it.

And all the wastegated turbos will only bypass the turbine if max boost is reached right? so if you cant spin it to max it will never bypass making a waste gate a waste of space, and Conversely if you can spin it to Max boost you would still benefit from the extra air provided by disabling the wastegate (assuming you dont overspin the compressor)?

Is that right?
Essentially yes. Engine manufacturers produce engines with a lower state of tune where the extra boost isn't needed. A wastegate is used to limit boost to a lower level and also reduces pumping losses at higher rpm.
But people who're tuning diesels usually are after the most boost they can get and set fuelling to suit. So wastegates aren't as important.
 

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Discussion Starter · #13 ·
Thanks man! I appreciate you sticking with the thread long enough to help me work through it.

Yes, that's exactly it.



Essentially yes. Engine manufacturers produce engines with a lower state of tune where the extra boost isn't needed. A wastegate is used to limit boost to a lower level and also reduces pumping losses at higher rpm.
But people who're tuning diesels usually are after the most boost they can get and set fuelling to suit. So wastegates aren't as important.
 

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Discussion Starter · #14 ·
Hey Dougal,

I was rereading one of my other posts, and i ran across a posting that said stay away from a small (9cm) exhuast housing if you do any towing or mountains because EGTs will be too high. Whats the deal with that? How could ex housing be that big of a deal when it should be flowing more air?
 

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Hey Dougal,

I was rereading one of my other posts, and i ran across a posting that said stay away from a small (9cm) exhuast housing if you do any towing or mountains because EGTs will be too high. Whats the deal with that? How could ex housing be that big of a deal when it should be flowing more air?
I think someone has their wires crossed. Smaller turbos are far better for towing than big turbos. But high altitude makes turbos work harder and it's possible to run a turbo into overspeed at altitude which would be perfectly fine at sea-level.
 

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Discussion Starter · #16 ·
Dougal,
One more question, Over on my Idle thread, 47ford said that exhuast backpressure will increase egts i.e. mufflers, exhuast brake. I would assume they do this by not allowing intake air to flow in against the increase base cylinder pressure that exists when back pressure is increased. So why does a small turbo housing not cause this? Is it because the turbo will increase intake air pressure by a comparable amount?
 

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There are two ways the exhaust brake increases EGT.
The first is simply through increasing the load on the engine, the pumping loss makes the engine work harder, the governor injects more fuel to keep the rpm's constant.
The second is heating due to increased pressure (ideal gas law etc).

I've measured exhaust backpressure at up to 2x boost in the worst conditions (40psi, 20psi) and under boost in the best conditions (18-19psi, 20psi). When cruising it's about 1.5 times boost on my engine (12-13psi backpressure, 8-9psi boost).

Turbos do increase egt's in the same way exhaust brakes do, but the cooling effect they provide through more boost is far greater, so the result is lower EGT.
 

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Discussion Starter · #18 ·
Great Info man keep it coming
 
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