1987 Mazda Rx-7 Turbo II
no power steering
50w20 oil
outside temps 85 to 105
operating temps seem normal

Alright this one has been baffling me lately. It has gotten hot in Gerogia in the past month or so and I've started using my A/C which I used to not do because it would blow hot sometimes. Lately it has been working fine. I notice if I turn it off, my car idles lower, and when it's on my car seems to have better throttle response, and accelerates faster than I've ever felt it before. So I am wondering. What would make my 7 seem more powerful when running the A/C? I heard it's supposed to rob power. Did Mazda put an "A/C compensation" mode or something on it?

 

when you turn on the a/c it adds load to the motor which raises the idle slightly.

other then that i think its just in your head, and the horsepower robbed by a/c is like barely measurable.

kevin.

 

but shouldn't the load lower idle? It rises from about 800 to 1100. I know if I press in the clutch the idle needle moves a millimeter up from the lost rotational mass, but lowers back when I let go of the pedal. So I am pretty sure the A/C's resistance should be lowering my idle.

 

i'd use 10w30 oil

 

wierd, my 90 NA feels horrible with the AC on. horrible low end torque, lots of bogging and the revs drop a lot faster, I'll probably end up taking it out soon. It blows cold but it takes a while to actually cool the car.

 

The load lowers the idle, but the BAC raises it to compensate for the added load.
Also, don't use 10w30, especially in hot environments. It doesn't provide enough lubrication in high load situations, especially for an older engine.

 

All cars do this.... They always have... My 78 cutlass supreme did this when the compressor kicked on. What is the debate? Yes it robs power, no it is not that noticeable any thing driven off the rotation of the engine "robs" power.

 

yes, please don't use 10w30 oil. Its too thin for older, high temp motors like rotaries. 10w30 will just burn off and you'll soon wonder why the motor is oil starved. Use 20w50.

 

a\c power rob is crap. dont you watch mythbusters? haha i think they did mileage, but its the same thing.

and i didnt know aobut the oil thing... ive been using castrol on my rebuild.... but hell, next time i change my oil i guess it wouldnt hurt to switch.. haha

 

and 87 turbo II - your lucky your first car kicks ***. mine was a 80 SA. but my new car makes up for the years of low(er) powered 12a's. i think all of us 17yr olds should drive TII's.. haha. that was my friends first car too, cept his was a grand's worth of ****..

 

My 7 Has almost 200,000 miles on it, 137,348 when I got it. I have used 10/30 Castrol since aquisition and has done great for me. Only loose about 1qt in 2500miles. Anything much thicker, has trouble making it through OMP lines until full temp. Run 100 to 1 mixture of 2 stroke in the fuel and 10/30 castrol or valvoline. Change every 2,500 miles.... forget about it. The apex seals need as much help as possible and premix secures great oiling, no smoke, 1oz per gallon and who knows how long you can make one last. OH and A/C robs power PERIOD!, Remove it, loose the smog pump too... Power to weight.

 

A/C > No A/C .


Notice the period.







BTW, my car idles better with the A/C on too. That's because the BAC is giving the car extra air at idle.

My idle doesn't hunt or bounce, but it's incredibly inconsistent and finicky. And it tends to idle a little too low with the A/C off.

 

I did watch the mythbusters episode you refer to - the myth they busted however, was that driving with the A/C on saves fuel over driving with the windows down instead for "4-60" air-conditioning. That said, they drove their test vehicles - a pair of Ford Explorers, round and round an oval track at 45 mph until each ran out of fuel, one with all the windows down, the other with the a/c on. It would have been more meaningful if they had done the test more than once, reversing the vehicle using a/c and windows down, since it could have just been manufacturing variance that caused the higher fuel use on the a/c vehicle anyway. And, had the test been at freeway/highway speeds, it's likely a/c would get better mileage over windows down, as aerodynamic effects only really become significant above 55-60mph.

Back to the OP's question, with a/c on, my na does idle faster due to the compensation of the BAC for the load on the engine (any a/c car does this, especially ones with low torque, to prevent the a/c from stalling the engine or making it rev too slow to maintain oil pressure and electrical charging, As other posters have noted, the car really requires a heavy foot and generally higher revs to drive with a/c on, as the a/c puts a fairly heavy load on a relatively torqueless engine - I really feel it when revs are anywhere below 3500, like the car has a lot less power, or is carrying bags of cement in the back! If the OP's car feels more responsive, perhaps it is already idling too slow, and the idle boost is putting it back to where the engine actually generates some torque. Or perhaps the idle boost is enough on his turbo car to actually spool the turbine up a bit and reduce lag? Either way, the a/c is certainly using engine power that could otherwise go to the wheels.

With the a/c off, there should be no noticable impact on power, but removing it would save some significant weight, which is just like adding power

 

Most Americans could make up for the A/C just by losing their guts.

 

im in Canada theres no need for air conditioning, ahahaha.
But before I took it out, I had lower torque and it gave me more space under the hood.

not much of a power difference, so if you in the hot states id leave it in. LOL

 

Thanks everyone, I guess it makes sense. I didn't see an explination for the better throttle response or faster acceleration but maybe starting from an 1,100 RPM idle just seems faster with my driving style.

 

The extra air probably give you a bit more torque do to leaning out the fuel air ratio, which is rich on stock NAs.

 

I can feel my 240 drop from 6hp to -12hp when I turn on the A/C. But A/C is the only reason it is my DD.

 

You said it, not me! lol
Too damn true - a lot of the time. We had SNOW on the ground on May 24, if you can believe that - and this past weekend hit 30C (about 86F) - 8 days apart. I find the heat thrown by the exhaust makes the interior of my 7 heat up even with dark window tint and using a sunshade in the front window when it's parked in the sun, so even though it's main use is as a track car, not daily driver, I've kept the a/c.

 

That is a false statement. The BAC pulls metered air, meaning that it is air that the AFM sees so it has no effect on the a/f ratio. Throttle response might be better just because the idle is higher and in a range where the engine has better response. The BAC adds so little air that I doubt it would have any noticable difference at WOT.
Also, I really don't like that mythbusters episode. It was done with very un-scientific methods. They used two DIFFERENT vehicles. This breaks every rule of scientific testing as there could be very large differences between the two vehicles. They should have at least switched which vehicle had the windows open and averaged the two runs or something. Also, even assuming they had done a fair test, you can't assume that all cars get better mileage with a/c than open windows just because some Explorers or whatever did it. There are huge variables with this: Engine power, efficiency of the compressor and power required to drive it, aerodynamic drag caused by the opening of the windows, just to name a few. I can guarantee that my MX-3 daily driver gets better mileage with the windows down. When I have the a/c on it feels like I am running 1 cylinder down. When I am accelerating in low gears and turn of the a/c it almost feels like a small supercharger kicks in. However in my wife's 6s, you can't really even notice a difference with the a/c on so it probably gets better mileage with the a/c on.

/rant

 

My idle actually drops with the A/C on... sometimes low enough that the car will turn off if i don't give it a little bit of gas. It is worse when it is really hot outside also. What gives?

 

See this is what I expected.

 

So wait

A/C > Windows Down when traveling about 55 MPH? Fuel consumption wise.

 

AC is easier on gas than the windows down above about 100km/h/65mph.


A/c off, windows up and sunroof cracked/blower on is even better on fuel though :P

 

Sounds like your BAC isn't working properly to increase the bypass air flow to step up the idle to compensate for the increased load.