Why does it feel like the engine loses 100 HP when I turn on the AC?
’98 with 38,000 miles on it.

 

It does actually lose about 50 hp. As I recall, it takes about 14 hp to run the compressor, and the rest comes from the ECM changing A/F and timing to deal with the added heat load.

 

Wow really that much - Well I didn’t notice a performance issue with my ‘94 (LT1), or my ‘08 (LS3) - this is my second C5 and I wasn’t impressed with the first one as far as engine response goes - don’t get me wrong I love the car but I think the LS1 sucks - lol. Actually I have to say with the AC off it runs really good.
thanks

 

I'm surprised you didn't notice it on your '94. I used to have a 96 impala and the impala club would have drag days where lots of rookies would run their cars down the drag strip. Anyone who forgot to turn off their AC would lose nearly a second in the quarter.

 

I can’t explain that except maybe how the Corvette was geared compared to the Impala.

 

I ran a ported stock heads/cam package on my 04 from Advanced Induction. Even with the stock intake and throttle body, (did have headers), it made 456 rwhp and I think 426 rwtq. Only complaint I had was because they had milled the heads to bump compression, I got a lot of knock retard in the summer. Was planning on putting a meth injection kit on it when I wrecked the car (some idiot changed lanes into me and literally pushed me under a semi trailer ala smokey and the bandit).

 

I doubt the C5 uses even 30 HP to travel at highway speed - I would be willing to bet it is much lower. Also, unless I am mistaken, the AC compressor is disengaged at full throttle so it should not be an issue for WOT performance. I would be very curious to learn how it loses 50 HP with the AC on. I would bet it is 2 - 4 hp in reality lost to the compressor.

 

Rule of thumb estimate: On a hot humid summer day with your blower on high and outside Fresh air setting and a couple thousand rpm you are in the 8 horsepower range.

 

Maybe 50 hp for all accessories on an older, large vehicle. 50hp.... that's a 37 kW AC unit.

Stock Z06 tune (base probably same), AC compressor kicks on/off around 4700rpm. Can also be based on TPS, but it not set up that way in the tune.

 

So all given said would agree that 15-20 hp would be a fair estimate?

 

The majority of the horsepower loss doesn't come from running the accessories. The majority of the horsepower loss comes from the ECM pulling timing from the engine. Running the AC puts a heat load on the engine's cooling system, and the ECM compensates by pulling timing. And it will continue to pull timing even when the compressor cycles off.

All you have to do to calculate your horsepower loss from running your AC is make a couple of runs at the drag strip. Run once with the AC on and once with the AC off. Then use one of the many online calculators to estimate the car's horsepower during each run.

 

I think you're mixing 2 separate things. High IATs do cause big power losses and some ignition timing pulling. That scenario of the drag strip and running the AC in the staging lanes etc., stuff is going to get really heat soaked, which of course leads to big power losses. Going to get heat soaked bad without AC...but definitely will be hotter with AC running of course. IAT of 80*F vs 140*F is a 10% reduction in air density--at 400hp level that's at least 40hp loss.

I can only guess that's what you're alluding to in your talk of losing big power with the AC--heat soak. The pcm is not pulling timing because AC is running, but it will because of high ECT or IATs.

So yeah, in a lot of stop and go traffic, the power loss of running the AC constantly is probably more pronounced than running at highway speeds etc. But that is an indirect effect of high IATs.

 

No, IAT is a separate parameter. The ECM has programming that says in effect, "When the AC is running, retard ignition timing by X degrees." This is to help minimize pinging. It has an unintended side effect of causing the engine to produce less horsepower.

You can easily test it. Run your AC from the staging lanes all the way through to the finish line.
Now run your AC in the staging lanes all the way up to the burnout box. Turn your AC off, do your burnout and make your run. Note the difference in ET and mph.

 

I'm honestly curious to see how much it loses with AC on. I'll datalog a set of 3rd gear pulls, and import them into virtual dyno. I'll also run my dragy for a 60-100 run, that will roughly correlate to that 3rd gear pull.

 

On a relatively stock LT1 impala ss, it would turn a 14 second car into a 15 second car.

 

I don’t feel the car looses power under air conditioner load ...

 

I don't really either. Feels a little more sluggish at really low rpm, which makes sense as the engine isn't making much power down there anyways.

Either way I'm going to get the logs/runs to put this one to rest. Either tomorrow or sometime this weekend.

 

Time to do a reality check. The same compressor is used on the 2.2 liter 4 cylinder S10 pickup truck. Do you really think you could take 30 horsepower to run the a/c and still be able to run the truck down the road?

i used to be able to calculate the horsepower required to provide about 2 tons of refrigeration which is a ball park number for an automotive a/c system but that was years ago when I actually remembered my engineering thermodynamics. Like I posted earlier, rough ball park maximum horsepower to run the compressor at maximum a/c loading is around 8 horse power. Back when we were developing automotive a/c with an electrically driven compressor, we were able to prototype an ultra efficient system that drew about 2 horsepower at high blower 100 degrees by 40% humidity ambient conditions.

keep in mind that you have a variable displacement compressor. So when you are cruising down the road on a hot day with your blower on medium speed, it will automatically decrease in displacement under lower load a/c conditions which will draw significantly lower horse power.

On the other hand, if you are sitting at idle, it is stinking hot out, the air temp out of the ducts is kind of warm, the compressor is pumping as hard as it can at idle and then you step on it.......there will be a short duration of higher horsepower because it will take some time to pull the suction pressure down and subsequently reduce compressor displacement.

 

What year s-10 ? Interchangeable as in tear off and hook up ?!

 

Oy. Again, it's not parasitic horsepower loss. The computer pulls timing when the AC is on and it's the retarded timing, NOT the compressor load, that causes the majority of the horsepower loss.