Exhaust and Turbo Spooling - Effect on Part-Throttle Performance???

[AdmiralYoda]

I've always wondered what the effects are on exhaust and turbo spooling, especially at low speeds and part throttle. Conventional wisdom with turbo's says that a larger diameter downpipe and catback exhaust improves turbo spool speed and improves performance by improving efficiency. At full throttle this makes sense. The turbo works less pushing the air out becoming more efficient and makes more power.

Take for example the GM 2.7L turbo 4clinder L3B engine. It has a dual volute turbo to spool up quick and provide peak torque around 2k RPM's depending on the application. AFE makes a larger mandrel bent downpipe and catback exhaust for this engine in the Silverado/Sierra configuration (off-road only). Video's on the Youtube show the turbo spooling up MUCH quicker even at part throttle. The trucks sound like performance diesel engines right off idle.

So more than likely this improves performance at wide open throttle...but I've always been curious what would happen under normal part-throttle conditions. You'd be into boost quicker and off-idle torque would improve...but what about part throttle fuel efficiency, throttle response, overall drivability, etc?

Is this a free lunch (off road only)? Or does getting into boost sooner kill fuel mileage? Thanks!

 

[Cujet]

I have a good bit of race car turbo experience, starting from the late 70's and getting serious in the '80's.

Turbochargers and exhaust restriction don't go well together. We always considered a 7 degree divergent to be a minimum turbine outlet setup. In other words, the turbine exducer diameter should smoothly expand to a large pipe diameter.

 

[Pew]

All the cars I've seen with aftermarket performance downpipes also had tunes (me included) so if that's the case with the YT video you watched, I'm willing to bet the lower boost threshold was 99% the tune. I don't know the science of it but, on any turbo car the less restrictions after the turbo the better (which is why you see turbo-dumps straight out the hood or front of the car.) Also, it seems like some cars nowadays combine the catalytic converter with the downpipe so if that's the case with the silverado/sierra, you get double the benefits of removing the restrictive catalytic converter and improving the downpipe size/shape ("off-road" use only.)

At partial throttle, assuming you're running a downpipe with a stock tune, you won't notice a different when cruising around normally. Yes going into boost earlier will eat into your fuel economy but if you're putting a downpipe on, chances are you've already got some other modifications and fuel economy isn't your first goal.

The shape of the downpipe seemed like it was just as important as well as long as the outlet size were the same according to popular tuners like AMS and ETS.

 

[AdmiralYoda]

@Pew Normally I'd 100% agree with you, but to my knowledge tuners are just starting to break into this ECM, which I think dates to 2019. A few of the larger companies are working on it but have admitted the new ECM is tough to work with.

My hunch is the good'ol'boys on YT aren't breaking into the ECM and doing a better job than the larger tuning companies but I could be wrong.

I don't have much experience with turbos (yet)...but is it safe to assume that the vehicle manufacturers want you to cruise at RPM's that do not experience much boost to save fuel economy?

On the other hand a vehicle only needs XX horsepower/torque to maintain a certain speed/load. Would the factory ECM just dial down the ignition advance and fuel curve if boost is present but a load is not?

 

[Pew]

@Pew
I don't have much experience with turbos (yet)...but is it safe to assume that the vehicle manufacturers want you to cruise at RPM's that do not experience much boost to save fuel economy?

Would you be able to link the video? I'm not sure what goes into the overall selection of RPM/gear/boost but I will agree with you that they'll want you to cruise around at the lowest possible RPM for emissions and MPG (makes sense in my head in order to pass EPA stuff.)

On the other hand a vehicle only needs XX horsepower/torque to maintain a certain speed/load. Would the factory ECM just dial down the ignition advance and fuel curve if boost is present but a load is not?

From my experiences load and boost are always mutually inclusive with one another but boost will always follow load. Once the load drops, the turbo bypass valve/blow-off valve will open to dump the excess air in the intake plumbing into the atmosphere or back near the beginning of the air intake. The only time boost should be present when load is not is during anti-lag process where ignition gets retarded so much that raw fuel gets dumped into the turbo/exhaust (ie: rally car launches.) This does cause big temp spikes and destroys your catalytic converter if you have one, or fireballs out the exhaust.

 

[Greasymechtech]

More responsive at part throttle

My 1.4T likes to breathe. Intake/exhaust helps it respond better. I tend to shift exhaust, I was in the the 2200-2300 shift point where engine is a gas guzzler and drops to the upper 40's(46-48mpg).

Exhaust and intake restrictions asphyxiate the turbo. There is always room for improvement.

MPG is based on your RPM caused by go pedal position, and not the mods.

The worst mod is the tune, giving me 4+ psi extra boost, which can be addicting and the RPM gain excessive after 2200rpm meaning I can't short shift quick enough and zip around enjoying the 3k-4k shift points and the pathetic 38mpg.

I'd wager the GM 2.7 has much more available with common mods, good fuel, and a mild tune.

 

[AdmiralYoda]

@Greasymechtech That is pretty much the idea, I'm looking to have my cake and eat it too. Not looking for a full blown hot rod but would like some extra pep when I want it and extra efficiency when I don't. Looking at the 2023 GMC Canyon's. 430 lb-ft of torque at 3000 RPM but 90% of that is available at 2000 RPM.

Not looking to go too nuts but perhaps a GM performance parts intake and a free-er flowing exhaust. Maybe the larger/smoother downpipe with cat when it becomes available. Last thing will be the tune, which may not be available for a few years. If I do go with the tune I'd like some way to return it to stock (eco mode) on the fly.

In factory trim 65mph cruising is 1500RPM and 75mph is about 1700RPM. Some extra grunt at lower RPM makes me think it will spend less time up-shifting or lugging which makes me think it should be more efficient.

 

[dumkid]

on diesels, you chuck the factory exhaust and tune in the bin and you get better mileage from less pumping loss/restriction, better mileage from better tune (earlier injection) and more power from higher max fuel/air flow. off road only of course, don't drive on public roads.