Coffee Marten

I just bought an RDX. The K20C4 is paired as I understand it with an MHI TD04 Turbocharger. Looking at it's performance ratio it's good for a compression ratio of 2.5 even at it's maximum airflow (2.9 at lower airflow). This suggests it's rated for up to 37.75 psi of boost at atmospheric.

My question is why - other than a bit of extra turbo lag - would there be any decrease in power at altitude? I'm assuming the ECU would just work of of absolute manifold pressure and while I fully get that if the turbo is near it's performance limit it would limit power output it seems like the TD04 has the headroom to easily compensate for any altitude performance including pressure drop across inlet components (e.g., 85% sea level pressure: 14.7 psi * 0.85 * 2.5 pressure ratio = 31 psi boost pressure >> the max allowable boost of 20.8 psi).

What am I missing? Does the ECU not command the turbo based on absolute pressure and does indeed limit boost to 20.8 psi above whatever atmospheric is? That seems unnecessarily complicated. Am I miss-understanding the TD04 capabilities? Again - I get the 0-60 times aren't going to be the same because the turbo is going to have to spool to higher rpm to achieve the same manifold pressure but I'm just not getting why the power output after spool would be different at any conceivable operating altitude.

Thanks!

ttribe

What is the year of your RDX?

Coffee Marten

2024!

SamDoe1

I'm confused. Are you saying that you are somehow seeing the ECU limit boost pressure to 20.8psi? You're right that the ECU will use absolute manifold pressure to gauge wastegate position and control the turbine speed.

There's also a ton of other variables that the system will use too, fuel type, coolant temp, oil temp, transmission temp, what gear you're in, what drive mode you're in, etc...

Coffee Marten

No - as far as I know the boost gauge doesn’t show a real boost number - it just sort of fills up - I’m not tapping into the OBD and I’m not pushing the car to it’s max anyway (and certainly won’t be for the first 5000 k!). I guess that’s a good question - is the max on that gauge 20.8 psi of boost or is it really giving a manifold pressure?

I’m just interested purely in theory how
much the turbo can compensate for altitude.
I’m also curious because I drove the MDX and the RDX was a lot more fun - but there are lots of differences other than the engine - however it might be where I live the turbo 4 really has more output than the NA V6.

Coffee Marten

No - as far as I know the boost gauge doesn’t show a real boost number - it just sort of fills up - I’m not tapping into the OBD and I’m not pushing the car to it’s max anyway (and certainly won’t be for the first 5000 k!). I guess that’s a good question - is the max on that gauge 20.8 psi of boost or is it really giving a manifold pressure?

I’m just interested purely in theory how
much the turbo can compensate for altitude.
I’m also curious because I drove the MDX and the RDX was a lot more fun - but there are lots of differences other than the engine - however it might be where I live the turbo 4 really has more output than the NA V6.

My question would be answered if anyone Dyno’d their stock RDX in Denver but I don’t know if anyone bothered… not sure the RDX is a car worth modifying which is when folks would do that…

SamDoe1

If the boost gauge "just fills up" then there's no way to know what it means. Without a scale, it seems like it's more for show than anything else. The ECU will use the manifold pressure to control boost, I don't believe cars even have ambient pressure sensors. The turbo can compensate for inlet pressure up to its max flow rate ability which should be well within the altitudes you're talking about.

It's an RDX...not a performance car. Don't worry about boost pressures and turbine speeds, just enjoy driving it.

ttribe

MODERATOR NOTE: Thread moved to model specific forum.

Twism86

As you go up in altitude oxygen density decreases. So the higher up you go, they less oxygen molecules are available in a given volume of air, and your engine will make less HP. Having a turbo just jam more air into the engine at higher pressure will not compensate for this.

subiefanIL83

I think its inaccurate to say it doesn't compensate for lower oxygen density at altitude. All engines lose power at altitudes, however the turbo allows that loss to be less than if you just had a N/A engine. Instead of loosing the 15% of an N/A engine, a turbo may reduce that to 10% (just pulling numbers from the air for an example). I'm sure there is data out there regarding this.

John from PA

Good explanation of the Acura turbo system at Diagnosing Acura RDX Turbo System Problems