Small Turbo vs Large Naturally Aspirated Engine

[nthach]

So, having driven a small TGDI I4(2018 Cruze 1.4T and 2018 CR-V 1.5T), “big” TGDI I4s(2019 Subaru Ascent and @The Critic let me drive his family’s Lexus TX500h F Sport Handling), hybrids with a “regular” sized I4(2025 Camry and 2023/24 Prius in both “regular” and PHEV variants), I much prefer the “bigger” TGDI - the torque is a plus but it feels like a 3.5+L V6 to me. The smaller I4 feels like it needs to be revved up - not as much as a 1990s Honda but the CVT/stepped AT programming/gear ratios are working against it. The Honda 1.5T in the CR-V feels kinda gutless compared to the 2.5L A25A-FKS/2AR-FE in the non-hybrid RAV4 but it feels like it wants to pull harder when you get it in the “sweet spot”, while the RAV4 feels like it’s out of steam once you get higher in the revs. Toyota did a counterintuitive thing with a “bigger” engine in their hybrids - the first two generations of Prius were dogs to drive, the next two generations with the 1.8L I4 while it wasn’t much of a HP bump helped them feel a bit more confident, but the newest one pulls above its weight class. ~100hp once the hybrid batteries are depleted feels better than 76hp in a 2500lb car.

That TX500h though, it feels fluid, there’s not that much turbo lag. I don’t miss the V6 at all in my parent’s Camry. The 2.5L A25A-FXS combined with the 5th gen THS-II system from the current Prius does a fine job moving it around. I think the gas engine is good for 170-180hp with the hybrid system adding in 40-50hp for 230ish hp/300ish ft-lbs. Same deal with the current Prius - it’s smooth and quick unlike the ones of generations past.

 

[grasscutter]

I took a ride in a Honda Accord one day. I asked my neighbor for a ride to the auto parts store.

That thing was a rocket ship. It had the 1.5 liter turbo 4-cylinder. I was amazed by the acceleration of that car.

 

[RDY4WAR]

NA simply because turbos have oil and coolant lines to prolong the life of the turbo and the associated piping for the intercooler. If the intercooler is air-to-water then you have even more coolant lines. Turbos also have wastegates which will eventually fail but they're going away.

I'd be willing to bet there's been less problems with wastegates and coolant/oil/air plumbing than there have been hemi lifter failures.

 

[Hohn]

I'd be willing to bet there's been less problems with wastegates and coolant/oil/air plumbing than there have been hemi lifter failures.

As would I. Properly designed and assembled engine plumbing is stone-axe reliable because it has no moving parts. The only way it goes bad is degradation (cheaping out on lesser grade stuff) and rubbing (improper design or assembly.)

 

[Hohn]

I took a ride in a Honda Accord one day. I asked my neighbor for a ride to the auto parts store.

That thing was a rocket ship. It had the 1.5 liter turbo 4-cylinder. I was amazed by the acceleration of that car.

You should experience my 2.0T. It has faster torque rise off idle than a hemi does.

 

[Hohn]

Small turbos with early boost are inherently harder on engines (high load at low rpm). This probably only matters if you're like me and keep cars/engines into the 250k plus mile range. Then down the road there's the potential for additional expenses like turbo coolant and oil line failures (pretty common and very expensive due to labor cost). Turbos also don't last forever, especially small ones working hard and hot (also not a concern if your a 5 year/100k mile get a new car guy). The advantages are the obvious: gas mileage when you aren't beating on it, weight and packaging if its the common 2.0L turbo that just about every manufacturer has. I've had 3 turbo cars and 2 of the 3 got bigger aftermarket turbos for reason one. The only one that didn't was an BMW N54 because I couldn't afford to.

The high torque at low rpm is a big reason I believe these small-displacement turbo engines benefit from much thicker oils than the cap recommends. I’m two grades higher in my accord and while I’m pending a used oil analysis the somewhat prove it, the engine is very, very happy.

 

[Snagglefoot]

I have a soft spot for turbos. Our new F150 has the 3.5L ecoboost. We test drove the 5.0L which had good power but doesn't quite plant you in the seat as well.

2025 F150

5.0L V8
- 400 HP @ 6000 rpm
- 410 TQ @ 4250 rpm
- Fuel economy
-- 17 mpg city
-- 22 mpg hwy

3.5L TT V6
- 400 HP @ 6000 rpm
- 500 TQ @ 3100 rpm
- Fuel economy
-- 18 mpg city
-- 23 mpg hwy

What's new to me is how quickly and easily these turbos spool up. I'm used to my aftermarket turbo'd cars being fully spooled at 3500+ rpm. Going up a hill today, the F150 made 7-8 psi at 1400 rpm. It never even dropped out of high gear and climbed the hill seemingly effortlessly.

Good info. Additionally a torque curve can show potential buyers just how much more torque is available over the RPM range with a turbo vs reporting the peak torque in a NA engine.

 

[BMWTurboDzl]

I'd be willing to bet there's been less problems with wastegates and coolant/oil/air plumbing than there have been hemi lifter failures.

Hahaha.

 

[BMWTurboDzl]

The high torque at low rpm is a big reason I believe these small-displacement turbo engines benefit from much thicker oils than the cap recommends. I’m two grades higher in my accord and while I’m pending a used oil analysis the somewhat prove it, the engine is very, very happy.

A lot depends on whether oil pressure is map based. If the the targets are too low I don't think you'll gain as much bang for the buck by going higher. This is why some owners of the BMW N55 would spin their bearings. The target pressure is set to low at the low rpm range and the engine makes a lot of torque early.

 

[altaylar]

Even though this is a little bit engine spesific question, as a general rule of thumg, there is no subtitution to displacement.
For short city driving, I suspect turbo engines work better, but on a highway driving you might find NA engine more efficient.

 

[BMWTurboDzl]

Even though this is a little bit engine spesific question, as a general rule of thumg, there is no subtitution to displacement.
For short city driving, I suspect turbo engines work better, but on a highway driving you might find NA engine more efficient.

It's actually the reverse because turbo engines typically run off boost on the high and have less rotational mass compared to the NA engine with similar HP.

 

[RDY4WAR]

When you can ram into the engine with turbos, you can use smaller valves, smaller cam lobe profiles (particularly the exhaust side), and longer intake runners, all of which also benefit low end torque and fuel efficiency when not in boost.

 

[altaylar]

It's actually the reverse because turbo engines typically run off boost on the high and have less rotational mass compared to the NA engine with similar HP.

Interesting, so far my experience have been those downsized engines need to run st much RPM on highway (gearing) and end up burning much more. Whereas on city driving where there is acceleration and deceleration, the mass.of bigger engines make them lowerr efficiency

 

[Hohn]

A lot depends on whether oil pressure is map based. If the the targets are too low I don't think you'll gain as much bang for the buck by going higher. This is why some owners of the BMW N55 would spin their bearings. The target pressure is set to low at the low rpm range and the engine makes a lot of torque early.

In other words, how the oil pump capacity and pressure relief are set compared the engine’s torque curve?

 

[fdcg27]

In an era when GM has made a four cylinder turbo engine the default choice for its bread and butter half ton pickups, can we really doubt that a smaller turbocharged engine effectively replaces a large NA one?
After all, GM lives or dies on the sales of these units, so I'd doubt they'd take any risks with durability in their engine offerings.

 

[stanlee]

I'd be willing to bet there's been less problems with wastegates and coolant/oil/air plumbing than there have been hemi lifter failures.

I'd bet you'd be wrong. A well maintained Hemi lifters fail like clock work around 200k miles (which most these days consider the lifespan of a vehicle), 100k ish for the 3.6 Pentastar and Chevy 5.3/6.2 any given day. The inability of 2 or 3 companies to make a reliable valvetrain does not change the fact that nearly EVERY BRAND of modern turbo engine have hard to access oil line failures, coolant line failures and loose worn wategate failures over time just as predictable as hemi lifter failures. The valvetrain issue is a manufacturer issue (along with their insistence of using cylinder deactivation no different than Nissan insistent on CVTs despite the known failure rate) as every Japanese and German manufacturer manage not to have wide spread lifter failures. The turbo issues are inherent to gas turbos and the heat associated with them regardless of manufacturer. None of that matters to your typical 5 year/100k mile buy a new car person but the 2nd and 3rd hand buyers need to do their research of likely pending expenses regardless.

 

[barryh]

I'm totally sold on forced induction because in every day driving I don't use the high revs where a naturally aspirated engine develops peak torque and power. Small naturally aspirated engines have of late tended to have peak torque at something like 4000 rpm which makes them feel very weak at lower revs. They'll move but you have to work them hard to do it My last two cars have been a Mercedes C class 1.6 with supercharger and a VW Golf with a 1.5 with a turbo, both being around 150 HP and with peak torque at 1500 -3000 rpm Of the two, the supercharger was brilliant at producing instant torque from idle onwards but the turbo is vastly more economical on fuel. There's not a chance that a naturally aspirated engine could return the fuel economy that I'm getting not least because they couldn't pull the gearing of 30mph/1000rpm without the engine being a much bigger displacement and using more fuel.

 

[bighead2586]

You should experience my 2.0T. It has faster torque rise off idle than a hemi does

A bummer Honda makes neither the 2.0T nor the 3.5 V6 for the Accord anymore. Both those engines were very strong. The 2.0T is ridiculously quick off the line but the 3.5 sounds much better to my ear.