[TRobinson6]

This will be my first winter with this car and now that Florida has had a cool spell PJ feels like a different car as it sure likes the cool dense air. The motor seems so much more responsive and the power can be felt in the seat of my pants. When it's hot out it's such a wimp, almost like a couple of the ponies are cooling off in the shade. But now I have to put up with more squeaks and rattles that I don't have when it's warm/hot.

 

[jzchen]

What octane fuel are you using?

 

[DreamCarOwner]

You are correct in your assumption Mr.Robinson. I note that when the ambient temperature is 80 degrees or above Max is sluggish and has a little less power. It usually manifests itself as a flat spot. You are driving along and a hill comes up. You shove the accelerator half way down and the car picks up a little bit of speed but not alot, you shove it on down almost to the floor, but not enough to cause a backshift, nothing happens. The engine tends to labor more but sometimes it will actually lose speed. Then you let up some on the accelerator and the engine accelerates the car. It has a "flat spot" or just feels like it is running way lean. Then when the temperature is in the 70's or cooler the engine is alot more responsive and feels as though there is more power. I'm sure that little engine bakes back there and alot is ask of it, especially in hot weather with the A/C on and a passenger. The engine is breathing in air through the side vent so it should be getting cooler air than what is in the engine bay. I think in hotter weather, the 10 to 1 compression and gasoline even at 93 octane causes the engine to spark knock and the computer is retarding the timing to eliminate it, just cutting power and throttle response in hotter weather. In cooler ambient temperature the engine doesn't spark knock as much so timing can run more advanced which would enhance throttle response.DCO

 

[InjuredAgain]

I think in hotter weather, the 10 to 1 compression and gasoline even at 93 octane causes the engine to spark knock and the computer is retarding the timing to eliminate it, just cutting power and throttle response in hotter weather. In cooler ambient temperature the engine doesn't spark knock as much so timing can run more advanced which would enhance throttle response.DCO

This is the most probably explanation. Gasoline engines tend towards pre-ignition during high throttle opening, low-RPM operation and exacerbated by higher temps. Most modern engine management systems will start retard timing once the intake air temps reach in the 90-100 degree F. range, which will be practically all the time once outside air temps getting up around 80 degrees F.

The other thing is actual pre-ignition during operation. Our Rav4 is specified to run on 87 octane gas, but that same engine with probably very close to identical programming, is specified to make more power on 91 octane gasoline when used in Lexus models. We have a very heavily speed-patrolled section of slightly uphill freeway that we drive very often. When running on 87 octane gas and with the cruise control set at 65 MPH/1800 RPMs, the engine always comes out of torque converter lockup mode climbing this grade. With 91 octane gas, it almost never comes out of lockup mode. And that's mostly temperature dependent. On the warmest days, even on 91 octane, it will sometimes unlock climbing that grade, probably because we're also running the A/C but also because the higher intake temps are causing timing to be retarded.

 

[jzchen]

So....

Do you two think a less restrictive air filter like a K&N would make an improvement? (Seems like the engine is being starved of air)? Hot air being less dense than cold air, principle of hot air balloon...

 

[jetfuel]

...in the art of making an engine perform better is not one thing or another that will achieve the desired goal....but a combination of changes to the original design that will give us that extra edge over the competition...
...air filter..
...air filter and intake runner
...air filter..intake runner...exhaust
...and so on and so forth...

Jetfuel...hair dryer>

 

[jzchen]

Hair dryer, aka turbo, being best of all, no?

 

[jetfuel]

...yep...
...one of the first mods I made was removing the long intake tube to the throttle body..everybody went for the long tube thy called the "cold air intake"....
...I went a different way and caught hell for it...we called the "stubby"

...it performed better than expected over stock with time slip to prove it...

Jetfuel...those were the days

 

[DreamCarOwner]

Isn't it important to keep whatever air intake you are using connected so that it pulls in outside air? I know there is some air circulating when you are driving but in stop and go traffic I'm sure it retains some serious heat back there. You don't get the radiator fan moving air around, so lay your lunch on there and go for a drive and let it cook! I wonder why Mercedes never installed a small heat activated fan that would blow air around in the engine compartment? DCO

 

[Smart Bob]

The 453 has an engine compartment cooling fan.

 

[InjuredAgain]

Air intakes are compromised in many ways. Having a direct route for outside air to get to the filter raises the risk of water, leaves, bugs, and dirt getting in.

Pre-ignition is most likely to occur at low RPMs and high throttle openings. To minimize heating of the intake airstream, you want the flow to be fast so there's minimal conduction from the walls of the intake tract. However, fast flow at low RPMs means restriction at high RPMs.

In the first generation Miata, the best performing air intake actually utilized the high pressure region at the base of the windshield, rather than trying to duct through the bumper grill or an opening in either the corner parking light or flip up headlight cover. Any intake along the side of the vehicle like I think it is in the 451 is operating in a low pressure region unless the body in front of the intake is shaped to create a pressure wave there, or the intake juts out into the airflow.

 

[DreamCarOwner]

The 453 has an engine compartment cooling fan.

Does the fan exist there to cool an intercooler for the turbo? I have never got to see any of the 450 or 451 with a turbo. I also have not got to investigate the 453. Just too isolated where I live. I have noticed a few more Smart 451's in the area. Have been asked to work on a couple and I have always gladly obliged. It is just so sad the the Smart story is ending so abruptly for the ICE. DCO

 

[1stiski]

The 453 has an engine compartment cooling fan.

Does the fan exist there to cool an intercooler for the turbo? I have never got to see any of the 450 or 451 with a turbo. I also have not got to investigate the 453. Just too isolated where I live. I have noticed a few more Smart 451's in the area. Have been asked to work on a couple and I have always gladly obliged. It is just so sad the the Smart story is ending so abruptly for the ICE. DCO

453: fan there to keep motor compartment cooled when it gets a bit stuffy , I?m not sure on when the fan switches on and at what temp , but it doesn?t run all the time .