DinSum

iTrader: (7)

I've noticed recently that my rx8 isn't performing nearly as good as it was during the winter, its very low on power in comparison.

So lately I been paying a lot of attention to how the car feels based on the temperature. when its like 3 degrees celsius it feels like an absolute rocket! barley touch the gas pedal the it just wants to go! If its 12 degrees celsius or less it feels really responsive and fast, if its between 12-18 degrees it starts feeling like its losing power, as soon as the temperature rises to 19 celsius or above it really starts feeling like a honda civic, it still drives extremely smooth and the mpg is the same as cold weather if not a little better but its so slow!

Is this normal for a rx8 to have this kind of performance difference based on outside temperature? I do understand that the colder weather gives a bump in performance with pretty much all cars but this is a huge performance difference which i've never experienced with any of my previous cars. I purchased my rx8 last year right when winter started so I don't really know if this is normal for the rotary engine.

my 0-60 time went from 6 sec flat to 7 seconds.... maybe even longer

I've always run 87 octane, maybe 87 is good for winter but bad for summer?

some info:

-hollow cat
- 1 CEL for rear o2 sensor
-bhr ignition
-148,000km
-compression is slightly above minimum spec and very even on both rotors

Any suggestions are greatly appreciated

ShellDude

iTrader: (12)

A lower ambient temperature results in denser air. The increase in measured air results in more gas being injected.

More air + more gas == more power (basically)

I'm assuming 87 octane in Canada is different than 87 in the US? If it's the same scale then try 91. I run 93 religiously... 87 causes my car to knock due to premature combustion (detonation).

RIWWP

iTrader: (2)

1) colder air is denser, denser air can make more power. This is true regardless of tune.

2) if your tune is naturally lean, the extra air will make you even leaner, dropping power, so winter wob't be a dramatic improvement over summer

3) if your tune is naturally rich, then the extra air in winter will lean it out, making a significantly obvious improvement as temp drops.

ShellDude

iTrader: (12)

You really think it's leaner, RIWWP? I would think that the MAF would account for the additional air and/or there's a multiplier (trim) of some kind applied to IAT (although I've never seen an OE table for it).

I completely agree though... more power could certainly arise from a leaner run.

I've also wondered what impact the throttle body heater has on the broad hypothesis regarding CAI. The air was already measured, gets stuff into the throttle body where it's heated and expands. Since NA engines suck in air what impact does this have on the manifold pressure? Does it create a mini boost of sorts or does the air just back up out thru the throttle body.

RIWWP

iTrader: (2)

Yes, it does account for the denser air, though I remember a technical discussion with MM of why it can still shift the afrs slightly.


I personally think that while the throttle body coolant passage does heat the air, it's not more than the hot engine bay heats the air in the UIM, or the significantly heat soaked LIM.

ShellDude

iTrader: (12)

Maybe it happens faster than the ECU can account for it.

DinSum

iTrader: (7)

Good to know, so it looks like im running rich

Is there anything I can do? Do I just live with it or is it possible to get a cobb AP and get it tuned a little differently to get more power in summer at the cost of some mpg's? I guess lean things out a little

RIWWP

iTrader: (2)

The cobb ap is a flashing tool, so if you have a differently tuned flash file, you can use the ap to flash that onto the ecu. Getting that flash file is something you will either have to learn to do with downloadable software, or pay someone else to do.

Generally yes, our cars run rich, but don't just dive into leaning it out. Learn all about it first. There are very good reasons to run rich in terms of engine health. EGTs is a big one.

zoom44

i can tell the outside temp by the sound of my engine. there's a certain really nice resonant growl it makes that when i hear its always within a degree of, if not exactly, 45 degrees.(f here) there's also the way it takes off and sound in the 50s thats different and into the 60s.

lower than 45 the whole car is stiffer and sounds like it. doesnt accelerate like id like it to until its well warmed up. over 65 into 75ish the intake sound is different yet and the car feels looser even from dead "cold".

after that its hard to tell except when the AC stops keeping up then i know its over 95ish

DinSum

iTrader: (7)

Yeah I don't know anything about tuning yet, I would pay the fee and have MM tune it for me :/ I just wonder if it will make any difference for me because it would run me like $600 to get the cobb and then the tune

ShellDude

iTrader: (12)

Also be sure to account for the two months of your life you're going to have to set aside to get a good tune. Not a knock against MM, just the reality of remote tuning.

In my experience our car (as well as many others) will always run better in cooler temperatures. There's not much a tune can do to help that. For example, I have a $1,200+ ECU solution, have spent countless hours tweaking tables and trims, and I still notice the difference between a cold and warm day.

I found myself just a week ago thinking the exact same thing... it tends to be an annual experience.

Try running a higher level octane fuel in the Summer. I suspect you'll be pleasantly surprised with the results.

j9fd3s

well its more like there are more oxygen molecules per unit of air. AFR is a ratio, so air AND fuel can change. if fuel is the same, but air is denser, the AFR should be leaner.

POWER should go up, as there are more oxygens in the combustion chamber when its colder

i don't know how the Rx8 works but it does have an IAT, so it does, or should see the temp drop and adjust.


i think at high revs the air probably can pass through the throttle body without heating up noticeably. how many BTU's of heating can the TB possibly have? and how many CFM/Gs/airflows go through it?

999miki

Difference between 3°C and 19°C can account for 6% increase in power purely by higher density. Then you have effects of more effective cooling ie overall better thermal managment of whole engine so timing and fueling is more optimal than at higher teperatures.

MPG may be better at higher temperatures due to slightly more opened throttle body for same mass airflow - same power for cruising and overall, higher air temperate aids in engines efficiency.

Loki

iTrader: (1)

How long ago was this diagnosed?
" -compression is slightly above minimum spec and very even on both rotors?"

Because, I mean.. that would do it.

DinSum

iTrader: (7)

maybe 3 weeks ago, 104 psi on one rotor and 101 on the other

04Green

iTrader: (1)

This is the benefit of a CAI. Also why they do not show an impovement on a dyno, open hood and fan defeat the benefits.

Racing beat duct is a good way to do it as well.

Loki

iTrader: (1)

Ok, i'll bite. How was this measured? You should have 3 numbers per rotor @ a given rpm. If this was done with a regular comp tester, the result won't be meaningful.

RIWWP

iTrader: (2)

Assuming you pull the valve, video tape the compression tester face, and do all the math correctly, keep in mind that nearly every analog compression tester has a volume of air between the actual threaded part that you insert into the plug hole and the sensor, in the tube.




That volume of air is compressing/decompressing, and is effectively changing the compression ratio of the engine. It WILL result in lower compression readings than if the sensor is at the plug hole.

DinSum

iTrader: (7)

I used my standard analog compression tester, made sure to see three even bounces when holding the release valve, then let go of the release valve and see what number all three faces climbs to.

Then I use this calculator:

Foxed.ca - Rotary Compression Calculator

I get my rpm from Torque app

I also am 280 meters above sea level

ive read a few posts stating that even the compression testers that mazda use throw wrong numbers and they need to be calibrated all the time. The way I see it you will never get the EXACT compression numbers but you try to do everything right and be as close as you can be