Hoopics

Took my wife's RDX for a longer drive last night, including a stretch on the interstate for the first time. I noticed that when cruising at 55 to 70 mph, the RPM seems nice and low, between 1800 and 2100 or so, and it holds speed with no problems at all.

I also noticed that any acceleration whatsoever at highway speed starts to kick in the turbo. Never having had a car with a turbo before, I have no idea if this is normal, but it seems really inefficient. Based on where the engine is in terms of RPM it plainly seems to have power to spare for at least gentle acceleration, so why trigger the turbo so quickly? I have to wonder if the miserable gas mileage (and my wife is averaging 14 mpg or LESS) is in part due to the heavy (unnecessary?) dependence on the turbo.

I'd love to hear from anyone who has had turbos in other cars for comparison, or whose RDX behaves differently on the highway.

flar

My last car had twin turbos, but no boost gauge. All I can say is that the mileage did not tank nearly as badly as my RDX when I needed to accelerate - even if I accelerated briskly. I think the difference was that that car used the turbos to flatten the torque curve (it literally gave the same torque from about 2k to 6k RPMs) whereas the RDX uses the turbo to make a 4 feel like a 6.

Fishbulb

Its a 2.3L engine trying to move almost 4000lbs. Turbo use is generally required to make any sort of passing maneouver on the freeway.

rdxfirstmd

You're right. I have no problem with the RDX turbo and i average 20 mpg city/hway combined. Not very bad, but not great though

Hoopics

I'm not talking about a passing maneuver. I'm talking about any variation in speed above 50 mph, regardless of how easy on the accelerator. I find that any depression of the gas pedal at highway speed beyond the amount necessary to hold constant speed causes the turbo needle to jump up, generally to about the 1/3 mark on the turbo gauge. It is possible to accelerate gently to keep the turbo from going too high, but I was unable to keep it from engaging at all.

Can anyone else gently accelerate on a flat highway from, lets say, 60 to 65, without the turbo kicking in at least a little?

F.Rizzo

You are correct IMHO Hoopics. There is almost no way to modulate this turbo with the throttle. All my other turbo cars had a feel with the throttle that you could keep the car out of boost, the RDX does not do this and the boost comes on very early and very smooth. Like you - most of the time I did not realize that car was under boost.

After 3 mos of ownership, I say the car should have had the V6 in it.

FWIW: my MPG goes up by 1 when I manually turn off the A/C.
.

PhillyGM

Anyone know if there is a programmer or anything available that could maybe alter the engines use of the turbo?

The reason the turbo probably kicks on all the time is that modification to the intake side of the turbo for low power increases. It seems they never made an option to have the turbo shut down for highway cruising and only kick on when the pedal is depressed in a certain manor(ie passing).

It seems fairly simple to me to do so though I'm not an expert on turbos. I dont know if there is a way to kind of "free-wheel" the turbo so it doesn't provide additional boost.

The programming would be simple. The programming would basically engage in a certain gear or speed and when the pedal is held at a certain throttle position range. It is drive-by-wire so that would be fairly easy to do.

Just something to think about.

neo1738

well its most likely due to the double wastegate, this is how the car gets power at lower rpms, on the down side, your getting boost more often. i dont think it would be possible to electronically change that, its a physical feature of the car. you might b able to remove the dual wastegate but then you'll be throwing check engine lights. As to the 6cyl this was addressed in a previous post and the 4cyl was most likely not just a "fuel effiecency" thing but also a weight ratio thing. It helps keep the car mroe balanced and not front end heavy thus reducing understeer more.

happasaiyan

the [relatively] poor gas mileage is no doubt due to the aggressive tuning of the ECU. in the ECU, you will find a TPS (throttle position) vs boost map...which allows for positive boost probably from as low as 10% TPS.

i have a modded STi that i have tuned myself, and i have set it up so that i can only get positive boost from 35% and higher TPS. doing this, however, makes sure you can tell you are driving a turbo'd car from the non-linearity of boost vs TPS. acura was trying to make the RDX drive as seamlessly as possible. you have people who complain about "boost lag" and then want it so that the car doesnt spool up so soon. you cant have your cake and eat it too.

however, if i were to get my hands on a K-Pro, i would definately set the entrance into positive boost somewhere around the 35% mark, as yes, i experience the same things you guys do...but like any car, you have to learn how to drive it- i have found that you either have to accellerate at about 5% TPS (to not get into boost) or to accellerate rather quickly up to the speed you want and then cruise to get the best mileage. if you accellerate slowly up to speed but are somewhere in the 30% range, you will find that you a) dont accellerate quickly enough but b) also eat up gas...which is a bad combo for mileage.

so far i am averaging 17.9mpg with a 24mph (i think) average. i can get up to 25mpg on the highway.

happasaiyan

a) the special things to the RDX turbo were done to the hot-side of the turbo, not the cold (intake) side.
b) you cannot "free-wheel" a turbo
c) for the rest, see my prev post

neo1738

yea but even changing the TPS, wont the dual wastegate still give you boost at lower rpm even if you dont want it?

happasaiyan

from my knowledge of the setup on the RDX (its hard to find explicit, tech documents on it), you should be able to use the wastegate to regulate boost no matter which hot-side stage you are in (the smaller, quick spool stage / the larger slow spool, higher flow stage)...

...which in case, the TPS vs boost map will regulate boost using the wastegate.

regardless, you will be able to get some boost even with the wastegate completely open, as most internal wastegates still allow boost...but it should lessen how fast is spools up...and hopefully net some additional mpg.

flar

The RDX seems almost too jumpy off the line for my tastes. It almost seems like it has a chip on its shoulder (pun not intended) about its 4 cylinder and wants to dispel that inferiority complex immediately with a touchy throttle.

35% might make a good threshold for having maximum flexibility for deciding if you really want turbo, but even backing off on the "jumpiness" would be a nice gesture. Otherwise it's downright tiring trying to accelerate mildly without kicking in the turbo.

happasaiyan

i agree.

on my STi, i have it so that it doesnt start to build until about 35%...then from there, it is linear up to about 75% then full boost from 75% and up...

...on the RDX, if i am flooring it, even when i lift my foot up to start slowing down, it doesnt slow down because the map is still trying to push almost full boost...its a little funky for my tastes. i still havent gotten used to it. it feels like the map is something like this:

0-10% - 0 boost
10-30% - 1/3 boost
30-50% - 2/3 boost
50-100% - max boost

even though its not that simple, thats roughly what it feels like to me.

PhillyGM

Happasaiyan, yes, that's the side I was referring too. My terminology was incorrect, I'm more familiar with blowers than turbos.

Well, it sounds like with a little bit of tuning, the problem can certainly be diminished. I think Acura was trying to do what you mentioned before, basically get the car up to speed quickly and coast from there. The problem is this doesn't take into consideration traffic circumstances like bumper-to-bumper. Push the pedal a little too far, car jumps, hit the brakes, then do it all over again, watch the mpg plummet. A little tweak like this would certainly help that.

I wonder if anyone is working on a way to allow a turbo to spin without one side providing boost. It would be impressive technology but you could have the vanes on the intake(cool) side turn like adjustable vanes in a turbofan to the point they dont move any air and when you reach a certain point on the tps they start turning to engage more boost. Has this ever been tried or even considered?

Blowers just disengage the pulley to free-wheel while cruising for mpg purposes, maybe Acura should have gone for that instead and avoided this issue altogether.

PhillyGM

Just an add on to the turbo mod part. What if you added a third valve and some pipe on the hot side that allowed no exhaust to hit the turbo until a certain tps point and it just flowed around it and out like a regular car?

F.Rizzo

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mav238

Anytime you start to put load on the engine, the turbo will kick it, especially the variable flow technology in the RDX.
My SAAB 9-3 sports sedan turbo also does the same thing... during acceleration manuevers on the highway, the turbo will spool up.
Following a remap of the ECU by SAAB (a TSB), the turbo is always "spinning", albeit low speed, apparently to get around the issue with the car stalling and also to decrease the effect of "lag".

PhillyGM

Mechanically it may not do that exactly, but the boost does come off at cruise. I was drawing from something I read a while back, memory failed me. It basically acts like a blowoff valve, not disengaging the pulley, but it seems to stop boost and comes back only when you dip into the throttle enough. The turbo doesnt seem to do that or at least do that as well.

"Supercharger By-Pass Valves

A supercharger by-pass valve allows pressure to escape while at cruising speeds. Since a supercharger pushes out air based on RPM, the supercharger is still pumping out air while just cruising around. The air will run through the tubing, then hit the throttle plates, which are only partially open. The air then wants to run backwards out of the supercharger, which is hard on the internals of the supercharger. The by-pass valve allows that air to escape when in cruise situations. When you're not driving hard, the manifold is in vacuum, since the engine is sucking more air than the throttle plates are allowing to pass. The by-pass valve stays open during manifold vacuum, but as soon as you give it enough throttle to open the plates enough to allow the manifold to see boost, the by-pass closes and all your supercharger boost is shoved into the engine."