3" Exhausts and low RPMs

[ToyTruck]

I have a 3" mandrel bent exhaust on and it's got great power at mid to high RPMs but i noticed some loss of power at the lower RPMs. Have any of you guys with 3" systems noticed this? If I change the downpipe to 2.5" and leave the rest of the 3" on might the low range improve? I was thinking maybe the thing might not have enough backpressure off boost.

 

[Dion]

I was thinking maybe the thing might not have enough backpressure off boost.

Forget 'backpressure' being necessary/relevant on a turbo diesel, it's not.

 

[ONLOCK]

Get yourself a chip and tune it. This will help a bit. Back pressure doesnt really affect diesel's or turbos really

 

[ST-X DEON]

That shouldnt effect low RPM i got 3" but put DP chip on at same time and it hauls ass!!

 

[the_bluester]

Actually I have to post an enhancement to what was posted.

Backpressure helps NO engine in any configuration, turbo or otherwise. People are great at looking at an effect and calling it a cause (My car with a small exhaust had more torque at low RPM than it does now with a big one, the small exhaust makes more back pressure so the backpressure is what makes the torque)

Cart before the horse IMO. A small exhaust on a naturally aspirated engine will (If it is designed right) produce more low and mid range torque due to the potential for better cylinder scavenging, which dissapears at low RPM with a bigger exhaust system. At low RPM even a small exhaust will create insignificant back pressure. It is the scavenging that improves low end torque not the (non existant) back pressure.

Perception is a wonderful and varied thing, I would nearly bet you that if you dynoed it before and after the 3" exhaust you will find no significant torque difference unless some characteristic of the bigger pipe send the electronics a bit wobbly for some reason, which is doubtful. As was posted before, the bigger pipe makes little difference at low RPM on a turbo, the turbine does a pretty effective job of slicing up the exhaust pulses making them effectively useless for scavenging. I reckon the difference is more likely to be the perception of a soggy low RPM delivery created by how much more lively it is coming on to boost and on boost than it was before.

 

[Old.Tony]

I gotta say that was a brilliant explanation.

I reckon the difference is more likely to be the perception of a soggy low RPM delivery created by how much more lively it is coming on to boost and on boost than it was before.

This sounds a LOT like the experience you get when you use your mate's faster computer. You're happy with yours, everything works like it should. You go to a mate's place, his zips along like the blazes, and suddenly when you get back home yours is intolerably slow.

Why? Your computer didn't change at all. Your perception changed.

The trouble with perception is that it's real and has a powerful effect on how you look at a thing.

I can't agree more - many of our improvements (EGR mod, 3" exhaust, DPF Delete, pod air filters etc) would need to be dyno'd to see the real improvement - if any.

 

[the_bluester]

Perception is everything!

My track car feels like a complete slug off boost. Could not pull the skin off a rice pudding territory. The thing is that the work I have done on it over the years has actually slightly improved its off boost performance (The compression ratio is slightly higher for one thing) but it is also producing far, far more power on boost than it used to and the off boost performance feels even more awful in comparison than it did to begin with.

You only THINK an off boost YD25 feels flat!

 

[Leprechaun]

It's amazing how during a day of racing how different the car feels from the start to the end of the day, you would swear the handbrake was stuck on but in reality you just get used to the performance and wish there was more.

 

[Bosshog]

Get yourself a chip and tune it. This will help a bit. Back pressure doesnt really affect diesel's or turbos really

That shouldnt effect low RPM i got 3" but put DP chip on at same time and it hauls ass!!

I too got a custom made 3 inch from turbo to tail and a straight thru pipe. With the wick on the chip turned up it absoutely 'F'ing flies.

 

[the_bluester]

It's amazing how during a day of racing how different the car feels from the start to the end of the day, you would swear the handbrake was stuck on but in reality you just get used to the performance and wish there was more.

Try when you have had the car set up for a couple of years without any major changes. I am itching for more power at the start of the day, let alone by the end of it!

 

[ToyTruck]

Thanks for the replies guys. I have the DP Chip on but it is set to the factory exhaust coz I got it before the 3" exhaust. Haven't had the time to retune it yet.

That's probably it, maybe I just forgot how the stock unit performed. Gotten used to the power so it doesn't feel like much anymore. The accelerator pedal actually feels like a hair trigger, much more responsive in any speed except from standstill.

I still think the yd25 is pretty dead at idle to a bit over 1000 RPM though, the thing keeps inching backwards when you let go of the brake to launch it from standstill on an incline. Never got that from any 3.0L diesels.

 

[DieselTuner2]

...

 

[Leprechaun]

I have tried it on quite a number of D40's over the years.
The 2.5" does make more power for the first 2/3rd's of the rev range and the 3" slightly out does it towards the top end.
Hence the reason we sell 2.5" not 3" systems for D40's.

Everybody is quick to sling around 3" is necessary BS. Nobody has done any testing or they would see that the 3" system doesnt reduce the backpressure any more than 2% over a 2.5" system.

2.5" System is good for about 360hp of air flow before it becomes a restriction.
3" is about 500 max

There are heaps of test results showing a 3" significantly lowers the turbo destroying piston melting high EGT's. That's why it is favored over the smaller diameter, nothing to do with power or back pressure. 565C (post turbo) is regarded as a safe reliable operating temp, once you chip up a diesel those high temps are reached very easily and quickly.

From my research the guy's who push the smaller diameters usually don't have the facilities to bend 3" pipe so they peddle what they can make money on.

 

[MAD D40]

Actually I have to post an enhancement to what was posted.

Backpressure helps NO engine in any configuration, turbo or otherwise. People are great at looking at an effect and calling it a cause (My car with a small exhaust had more torque at low RPM than it does now with a big one, the small exhaust makes more back pressure so the backpressure is what makes the torque)

Cart before the horse IMO. A small exhaust on a naturally aspirated engine will (If it is designed right) produce more low and mid range torque due to the potential for better cylinder scavenging, which dissapears at low RPM with a bigger exhaust system. At low RPM even a small exhaust will create insignificant back pressure. It is the scavenging that improves low end torque not the (non existant) back pressure.

Perception is a wonderful and varied thing, I would nearly bet you that if you dynoed it before and after the 3" exhaust you will find no significant torque difference unless some characteristic of the bigger pipe send the electronics a bit wobbly for some reason, which is doubtful. As was posted before, the bigger pipe makes little difference at low RPM on a turbo, the turbine does a pretty effective job of slicing up the exhaust pulses making them effectively useless for scavenging. I reckon the difference is more likely to be the perception of a soggy low RPM delivery created by how much more lively it is coming on to boost and on boost than it was before.

Another reason for the reduced performance is correct me if im wrong here
at the end of the exhaust stroke you have a certain degree of overlap between the inlet and exhaust valve I assume its on diesels as well.
If you have back pressure from a small exhaust the overlap allows the exhaust gas to remain or be sucked back into the combustion chamber resulting in less fresh air giving a set fresh air to fuel ratio.
If you install a larger exhaust the exhaust gasses can escape freely(er) thus leaving less exhaust gasses to be sucked into the combustion chamber viz the cams overlap and then gives a higher fresh air to fuel ratio and causes some form of leaning out.
Whether the sensors are able to adapt to this new ratio or not im unsure but the reason for having a retune after fitting an exhaust is so you can configur fuel to air ratios.

In saying this Im not sure if the turbo restricts the back pressure to X amount regardless of exhaust size but it obviously reduces exhaust temps.

 

[Old.Tony]

Curiously it appears in the D40's manual (valve clearance measurement, page 210 of EM.PDF) it does seem that both inlet and exhaust valves may be opened at the same time.

If you look at the diagram in point 5, you can check the clearance in the valves marked with an 'X' but the others would be mid-stroke. This is measured at TDC of the compression stroke for cylinder #1, so naturally the front cylinder can have all 4 valves measured.

Cylinder 4 in this operation cannot be measured on either inlet or exhaust, which would indicate that they are indeed part-way through a movement.

That's a different way of doing things, I just learned something new.

 

[Bosshog]

There are heaps of test results showing a 3" significantly lowers the turbo destroying piston melting high EGT's. That's why it is favored over the smaller diameter, nothing to do with power or back pressure. 565C (post turbo) is regarded as a safe reliable operating temp, once you chip up a diesel those high temps are reached very easily and quickly.

From my research the guy's who push the smaller diameters usually don't have the facilities to bend 3" pipe so they peddle what they can make money on.

I agree completely with you Leprechaun. However D40's run extremely hot straight out of the factory (refer to the last para in this article). I have a pyro gauge fitted and here's some general temps recorded:

• Idling 225-250C
• Round town at 60KPH 350-400C
• Cat converter conditioning at idle 280-300C
• DPF regen burn at idle 350-400C
• Normal highway driving 450-550C
• Underload pulling up a hill 650-700C (read canning it)
• Normal highway driving with DPF regen under way 550-600C

So I have concluded that my larger exhaust has aided in lowering temps even after the chip has been installed. Oh and my Chip is wired into the pryo and will be auto switched off should temps exceed 750C.

 

[Leprechaun]

750 post turbo? I've read that a 200C drop across the turbo is average. Scary stuff considering Aluminium melts @ 650C.

 

[tweak'e]

Another reason for the reduced performance is correct me if im wrong here
at the end of the exhaust stroke you have a certain degree of overlap between the inlet and exhaust valve I assume its on diesels as well.
If you have back pressure from a small exhaust the overlap allows the exhaust gas to remain or be sucked back into the combustion chamber resulting in less fresh air giving a set fresh air to fuel ratio.
If you install a larger exhaust the exhaust gasses can escape freely(er) thus leaving less exhaust gasses to be sucked into the combustion chamber viz the cams overlap and then gives a higher fresh air to fuel ratio and causes some form of leaning out.
Whether the sensors are able to adapt to this new ratio or not im unsure but the reason for having a retune after fitting an exhaust is so you can configur fuel to air ratios.

In saying this Im not sure if the turbo restricts the back pressure to X amount regardless of exhaust size but it obviously reduces exhaust temps.

sort of right.

if turbo is making 15psi then exhaust backpressure on the turbo is more than 15psi. now that means there is also 15+psi exhaust pressure inside the cylinder (assuming perfect flowing ports) so even with good cam timing it will have 15+psi of exhaust gas trapped in there which dilutes the incoming air. this is also known as internal EGR (some engines deliberately leave exhaust valves open a bit to long to increase internal egr for emission purposes).
so lowing the exhaust pressure by increasing exhaust flow reduces the amount of internal egr. also it can reduce the amount of external egr. so its certainly a win-win.
by reduce internal egr it reduces the heat from the exhaust gasses and allows more air in which also cools it.

fuel/air ratios are not that important on a diesel. theres no need to retune just because you changed the exhaust.

one problem you can get is with variable turbo's. they are ECU controlled and may operate incorrectly with a big exhaust which may cause the low rpm loss of power or other faults like overboosting.

 

[brendan]

750 post turbo? I've read that a 200C drop across the turbo is average. Scary stuff considering Aluminium melts @ 650C.

thats correct but you would think it would be an alloy of some type and raise the melting point. but even then still not going to be safe.
in my opinion after the turbo is a waste as the turbo uses heat to drive it as well, the difference in temps can varie all the time. befor the turbo is the only true way your going to know the exact EGT most people are worrie the prob will break off, this is crap and un herd off, just make sure your not going to use an ebay special EGT gauge and you will be fine.. i did alot of research and could not find one story of the prob breaking off.

 

[DieselTuner2]

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