It was a fairly hot day today (over 30 degrees celcius) and I was cruising down the highway with the ac on which I don't normally have on.. and noticed that the car was not boosting as per normal..

I know the TSVS setup will reduce boost levels if it thinks its seeing knock, so with the very hot air im assuming it was pulling a bit of boost for that reason, but having the ac system turned on drops the boost even more (I have no actual numbers to give you as I dont have my boost guage hooked up yet), but for comparison, the car with the ac off will peg the boost needle near the top (on a colder day it will hit the plus sign above the top of the guage) but when i turn the ac on, it only makes it to about bar 4 of 6 on the guage..

TLDR version: any one else with a GT-4 with no boost controller installed notice a reduction in boost with the ac on? or is mine just doing something odd..?

for reference, I checked between ac on and off in 3rd gear every time.

I have the same thing happen.
Car doesn't pull as hard with ac on in comparison
to having it off. Had same thing happen with my 90 gtfour.

pretty sure its like this for all cars

Not too sure, however boost pressure should increase with engine load. Try quick shift to 3rd or 4th with WOT and you should see full boost. Altitude levels may be another factor to consider though.

yup, i get a little lower boost than normal with the a/c on, even with the TVSV disabled.
normally makes ~14psi, with the a/c on i get ~12.

>
Yeah the power reduction is expected in all cars, however I wasn't expecting the drop in boost. It seems I'm not the only one that experiences this, and doing the same experiment today yielded the same results.. so I'm going to chalk this up to "business as usual"

My car does not like ac either. You can hear engine sound different. Cant say I noticed a drastic change in performance but it does sound different with ac on

Ya, I usually turn my A/C off when I pull out into traffic or when I'm passing someone because it makes such a difference....

wow thats interesting that it does that, on the GE it lowers the power drain from the climate control/AC when you hit high rpm (apperently). I think theres a reference of it in the toyota engine manual.

This post has been edited by Rusty: Jul 7, 2010 - 12:54 AM

In most cars it's normal because when you engage your ac the compressor puts extra load on the belt. If you also have a any type of lightened pulleys or flywheel your rpms will drop very quickly between shifts.

sorry I think I worded it wrong, was trying to say, the AC disengages/ doesn't rob the engine of power in higher rpm

Never really noticed that problem in mine besides just hitting less boost due to hot weather.

when you change the throttle position quikly and/or reach high rpms, the stock setup WILL turn a/c off for a moment to allow accel as usual.

from BGB:

"WHEN THE VEHICLE SUDDENLY ACCELERATES FROM LOW ENGINE SPEED, THIS SYSTEM CUTS OFF AIR CONDITIONING (AIR
CONDITIONER) OPERATION FOR A FIXED PERIOD OF TIME IN RESPONSE TO THE VEHICLE SPEED, THROTTLE VALVE OPENING
ANGLE AND INTAKE MANIFOLD PRESSURE IN ORDER TO MAINTAIN ACCELERATION PERFORMANC"

yup

>


hmm.. pretty similar to most cars really.. a lot of manufacturers will turn the comperessor off if the car is at WOT.... mine is not doing this.. which doesnt matter really.. I had just never seen a turbocharged car actually physically lose boost pressure due to the ac being on (after talking to a fellow gt-4 owner in my city, his acts the exact same way as mine).. my car only builds about half its normal boost pressure when the compressor is running.. as does his.. kind of weird but apparently its normal for them..

but hey, when its above 40 degrees celcius, I'd rather have the ac than the horsepower

With any car, the ac functions because of the compressor, which is belt driven off the crank. With the ac turned off, the clutch on it is disengaged and there is no load from it. when the ac is turned on the clutch on the compressor kicks in and you get the rpm drop because of this. This is caused by the load from the size of the pulley and the amount of torque required to turn the compressor so your ac works. If youve ever done an underdrive pulley set up, this principle comes into play. Underdrive pulleys are just smaller pulley on the alternater, powersteering, and even the waterpump in some cases. By having smaller pulleys you have less force to turn them, and you unleash 10-20hp depending on how small you go. The engine isnt gaining any power, its just allowing the engine to put more of it to the ground instead of your alternator and so on.
Anyways, like i was saying all cars have the problem. When boost is in play, the turbo gets its power for the amount of exhaust gases from the engine. You add more resistence to the motor(not motor under load. but more forces for the motor to deal with) you get less exhaust, and thats why you arent able to get full boost. Same thing applys to superchargers aswell.

engine load is not exhaust load. having the AC on makes more load on the engine thus making a little less exhaust.

both wrong. engine output * rpm = exhaust flow. if you produce 100 HP at the crank, at 2000 RPM, exhaust flow will be the same no matter what accessory load there is on the engine. im betting the lower boost numbers has to do with the added heat in the engine bay from the ac condenser, or some kind of stock boost reduction.

DEATH has a/c on his 3s that doesnt reduce boost. BUT its ran on a switch, bypassing the stock setup...

I actually hit boost easier with the A/C on due to increased loads I would assume - same as going up hill.