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Yes, I notice this tiger/lamb performance on a daily basis. Engine and input temperatures are the cause. If you have a set of temp guages and sensors collecting info on the engine, outside air, IC in and out temps, you'll be able to observe how and when performance is effected. The dividing line for my C900T seems to be when the IC output air temp goes above 120F degrees. That also corresponds to IC input temps being above 150 and the engine temp above 180. And even if the cooling system is able to keep your engine temp around 210 during a hard drive, the increased turbo temperature can raise the IC input well above the engine temp. Even if driving at a good clip under those circumstances, but the outside air is a balmy 85 or more, the IC output remains high.
Just yesterday I had to drive during rush hour traffic in the late afternoon, beginning in one of the hottest parts of the S.F. Valley. Outside temps were 95 or more. Sitting just waiting to get on the freeway, I watched the engine and IC temps rise and rise. Then during one stretch with a line of cars waiting bumper to bumper to get on the entrance ramp, sitting in the sun, I watched the outside air temps rise to 115! With cooling fan blazing, the engine temp still shot up to 220. I half expected something horrible to happen any second, as that was the first time I'd driven this car in such extreme a setting.
As far as a performance measure goes - under that circumstance, the last thing I want to do is contribute to an already over-heated situation. I knew all I'd get is a dirty look from my engine anyway. Once i began to move, even at a slow to moderate pace in that heat, the engine and IC temps went back down to their normal "hot" temps. Heavy traffic wouldn't have allowed a boost test, but I know from experience it would have been less than average. Best was the drive home at night in 65 F moist temps. That's when the IC works best and full boost remains constant.
Solutions? I hear good things about water injection. I'm also looking at an IC fan. Plus installing a water/alcohol spray system to wet the outer IC surface. Then there's avoiding the S.F. Valley in the afternoon during the summer.
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