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Comment by msandford | Hacker News Reader

How does it handle a car that's iffy on starting? It's probably not hard to get something that'll crank a starter for a few seconds. But what happens if it needs to crank for 60 seconds over a 90 second window?

I've got a 2006 Forester with a manual. I'll occasionally stall it backing out of a parking spot, so I'll have to start it again. For some reason it's much more difficult to start in between cold and warmed up, so it'll take several seconds of cranking to fire back up. If I were to stall it again trying to get going, would I then be in trouble?

It doesn't happen often, perhaps only once a month. But if I'm stuck there for 30 seconds while the LiFe batteries recharge the supercap I'm not going to be happy.

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It's hard for me to say exactly how many times you can start it before having to wait for the caps to charge back up, since we've measured a surprisingly high variation of required starting current over a bunch of different cars. One of our testers drives a Datsun 240z (it's really sweet) and sometimes needs a few cranks to get it going. He hasn't had to change his behavior from his old battery, so he's getting at least a few attempts over a short period without needing to wait for recharging. One other cool trick we can do is charge the capacitors slightly higher to deliver more current on a second starting attempt, which will hopefully reduce the number of cranking attempts you need.

Subaru ECU cold start logic is, well, questionable at best. As a side note, if your car is stalling that frequently, I would suggest adjusting the clutch engagement position. Sounds like its either very low or way too high.

It's definitely driver error and not the car's fault. I try not to rev too hard and slip too long, and it occasionally results in a stall.

I'm spoiled from having driven a number of very powerful manuals. Big engines mean a lot of rotating mass which means that no revving is needed, just pop the clutch and go. That extra rotating mass also means that the ECU has more time to notice the drop in RPM and give it a little extra gas to bring idle back up.

Not so in the Subaru. It's a very light engine and seemingly a pretty light flywheel too.

Just out of curiosity- Do you recall the approximate weight of flywheels in referenced "big powerful manuals"?

IIRC the Subaru flywheels for the Impreza weigh in at about 20 lbs - and this is likely similar or identical to what is used in the Foresters.

No idea, but one of the cars that I learned to drive stick in was a friend's 2003 Mustang Cobra. All cast iron engine and something like 400WHP. I don't know that the weight of the flywheel was everything, the crankshaft weighs a lot and I think even the moving mass of the pistons and rods translates into rotational inertia. Finally having 8 cylinders versus 4 means that your engine response time is theoretically twice as fast.

When the clutch engagement time is say 1 second and your engine is idling at 900 rpm you're only doing 1.5 rev/sec. In a four stroke, four cylinder engine that means you're getting on average 1.5 combustion events per second. In the 1 second engagement, you get 1 or 2 combustion events. In the 8 cylinder you get at least 2 and probably 3.

I thought about doing the math for how much energy is in a cylinder firing versus stored in the rotational inertia of the moving parts, but honestly I don't know that I really care. I know anecdotally that big engines make it easier to get cars moving for some reason, and that smaller engines make it a little more difficult. I just speculated as to why I thought that might be.

I'd gamble it has more to do with the fact you have to overcome the resistance of 2 extra axles and mechanical differentials of the AWD forester that is making it appear to be a bit more difficult than the lazy v8 2WD Cobra.

With that said, check out where the engagement of your clutch is. I've had people complain about grabby clutches(stalling) only to find out the engagement point is almost at the top of the throw (Former Subaru tech)

You could just start the car in neutral, then the clutch engagement zone doesn't matter...

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