28 June 2013

Well that wasn’t much fun…

Drove the car to work this morning, and driving in was uneventful. Driving home however was much more stimulating due to very heavy traffic (apparently everyone’s leaving town for the upcoming short week.) That meant driving along at 5-20 miles per hour for more than an hour. Just to keep things interesting, in first gear, engine speed was a little high, and in second, a little too low. And finally, it was about 100 deg and promises to be hotter this weekend. Stuck in the situation, it was a chance see how coolant and oil temperatures reacted to the heat:

1. The radiator fan apparently moves plenty of air, because once engine rpm rose a bit, coolant temperature dropped about 6-10 deg immediately.

2. However, while stationary or at low engine rpm, the coolant temperature would not drop.

3. Oil temperature got as high as 110 deg C, too high in my opinion, especially considering how lightly-loaded the engine was. This was not surprising since without a cooling medium, no oil cooler can work well. On the other hand, the engine was loafing and wouldn’t be expected to be dumping much energy into the oil… apparently the engine thought otherwise. It’s very likely that I’ve vastly underestimated the amount of heat that a turbocharger adds into the system…

The above points mean that though the fan was doing its job, coolant flow is insufficient at low rpm. It was recently recommended that I have an electric water pump and yes, it would keep coolant moving regardless of engine rpm. That should keep the coolant temperature from not dropping while at low speed. Obviously for track use this isn’t a big deal, but moving the coolant faster will help cooling regardless of speed. And since the car will spend a lot more time on public streets than on-track, it seems like a good idea (though I wonder how I would know if it quit… high coolant temp I guess!)

Preface the following with: transmission gears 1-4 are dog-engagement, and this turns into a long-winded thing…

Playing with the clutch in traffic and having plenty of time to think about it, I realized that I’ve been using the clutch wrong. Using the clutch or even double-clutching wasn’t working as well as expected, and I think I now know why.

There are three different rpms to juggle: engine/clutch, the transmission input-side gear cluster, and the output-side gear cluster (with the latter always being some proportion of road speed.) Unless all three are matched, grinding will result when shifting. Say the car’s speeding up and it’s time to shift. Immediately after moving from one gear to neutral, the engine/clutch, input and output-side gear clusters are all initially spinning at the same speed, then start diverging at different rates. The engine and input-side gear clusters start coasting down, while the output cluster will spin at a fixed percentage of road speed. Because the clutch is pushed in, the engine is now disconnected from the transmission, engine speed and the input gear cluster speed will also begin diverging.

I understand that the secret to shifting a dog-box is to double-clutch, where when passing through neutral, the clutch is briefly let out to match engine speed to the input cluster speed. Next, the clutch is pressed again, the shifter moved into the next gear and the clutch let out… and this is where my doubt starts. That is, even though the above is done, all it accomplishes is to make the engine and input cluster spin at the same speed, while the output cluster continues to spin at some proportion of road speed, but doesn’t have anything to do with input shaft speed. Unless all three speeds perfectly match, the gears will grind. I think that when shifting from first to second gear, there’s no way around waiting for the engine to spin down until it matches the output cluster speed. You just have to wait, and nothing can speed up the process.

Yes, a dog-box can be forced into gear at any time depending how rude you’re willing to be, but consider what’s happening. In an instant, the engine is  forced to change speed and match the output gear cluster. Lets say that instead of waiting for the engine to spin down from 6000 rpm to 4000 rpm so that second gear can be smoothly engaged, the driver just “makes it happen.” In about 0.01 seconds, the engine speed is forced to drop 2000 rpm, which briefly has the effect like throwing a wrench onto the spinning front pulley bolt. It’s going to go “BANG”, and if nothing breaks, the tires will likely chirp. After seeing people do it on-line I tried it exactly once and won’t ever do it again. Yeah I’m sure it saves X seconds per lap, but it’s my car and I’m the one who has to pay for fixing things… Also,  breaking the rear tires loose has got to be a bad idea during a race. So, I can’t see any way to smoothly shift from 1->2, 2->3, or 3->4 without just waiting it out. That said, the longest wait will always be the 1->2 shift because the gear ratios have the largest percentage difference. The other shifts take less and less time the closer together the ratios are. After realizing that I had to wait just the right about of time, I could shift with no ugly noises at all, even without using double-clutching.

Now on the other hand, I can see how double-clutching works when going from 2nd to 1st. In this case, engine speed must be increased to match the gear speed and blipping the throttle can quickly match them up. However, when going from 2nd to 3rd, we just have to wait, or at least, I will be.