Quote:
Originally Posted by grant
The 135i will make very little torque at all at 1.4k when it's not under load (torque curves on turbo motors are very dependent on load, unlike n/a motors which only depend on revs). Turbo motors only build boost when under load - when they're doing work (not when revving motor in neutral).
They actually launch much better with conventional torque converter automatics where you can brake torque (put car in drive and press throttle while holding car still with left foot braking). When launching a turbo with manual gearbox, you need to rev it up pretty high. Even then launches are not very impressive. Launching a turbo works MUCH better from a rolling start...
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I do a very harsh thing with my turbo A4 for a better launches. I guess what you said about needing load for turbo engines to build power really explains why it works. I've never really thought through the engine dynamics behind it, and needing to have load to build power.
I heel/toe the gas and brake at the same time while slipping the clutch right before launch. It works like this: I level the gas to get the RPM to around 3500 RPM with the clutch all the way to the floor and the brake firmly applied. Then I start letting out the clutch until the RPM's start to drop to around 2800 RPM while easing on a bit more gas. When I hit about 2800 RPM, I swivel off the brake and floor the gas while letting the clutch all the way out at the same time.
This is all done in one quick motion over about 1-2 seconds. I don't hold the clutch at 2800 RPM for any time at all, I just go straight to launch the moment the engine starts really pushing hard against the brakes.
I'm going to guess that I will burn the tire off if I try that in a RWD turbo 135i, so I'm feeling damian's pain. I know how to launch a turbo engine with AWD, and I know how to launch a torquey NA 6-cyl 535i with RWD. (Just start feeding it gas and then play out the clutch to keep it at a fairly constant launch RPM and let the engine do all the work. No planning ahead is needed, it's all just the feel of the moment.) But I'm not sure how to launch a turbo engine with RWD.
I'm going to have to figure out how to build load before launch and then transfer to acceleration without the harsh clutch release -- and without too much clutch slip. It would be easier if I didn't need to load the turbos to make power before launch. It doesn't sound like higher RPM's is going to be a good substitute for generating load. I don't think just reving up the RPM's is going to get the turbos to full boost.
I'm not sure about asking the 335i guys on the 335i boards. Everything I've read there seems to indicate that manual 335i owners really aren't getting good launches. The Autos seem to be doing much better, like grant said. Especially the 335i's with mods like Shiv's Proceed. They have been putting down fairly poor runs at the strip compared to what you would expect from the power boost of the Proceed. I'm going to guess that part of the problem is a lack of boost/load at launch.