Of "high performance engines" and jump-starting cars

My batter died in a grocery store parking lot Sunday morning. The owner of the car next to me (a BMW X5, this becomes important) came out while I was lamenting my situation. “Problem solved,” I thought, “I have jumper cables; his car most likely has a 12v battery.” So I say, “Hey buddy, would you mind giving me a boost? I have cables.” His reply, “Sorry, but since this is a high performance engine, I’m not supposed to do that.” Huh? One, this wasn’t a Ferrari, this was an X5, a mini van on a 5 series frame. Two, what does your engine have to do with your battery? I just want a few seconds of juice. I didn’t say anything, because honestly I don’t know if he’s right or just full of it, but believe me, I wanted to say, “Hope you never have any problems with your ‘high performance’ car, cause Karma’s a mother.” So folks, I put it to you. Is there anything to this dude’s claim? I was in an '01 Vovlo S-40 if that matters.

In looking for info on the BMW X5 the owner reviews are amazingly negative re relability. It’s like it’s $ 50,000 Yugo. If the car is that flaky it’s possible that it has some weird electrical issues as well.

Wouldn’t matter if it was a Ferrari. Doesn’t matter what engine. All that matters is it’s a 12-volt system. He was full of it, and full of himself, in a way that only a pretentious twit with a special toy that makes him special can be.

The only warning I’ve heard about modern cars with dead batteries was not to push start them, it ruins the catalytic convertor if too much unburnt fuel passes over the innards.

And with that you’ve validated my initial rage. He was all that you would expect from this sort of chump. I honestly wanted to say, “Listen bro, just say’no,’ don’t make up some lie that makes us both that much more stupid.” But, I wasn’t sure and didn’t wan’t to be mean.

It’s an automatic, so I don’t think push starting was an option. Don’t you have to “pop the clutch,” to do that?


Actually, urban legends notwithstanding, I have push-started automatic transmissions. You need to push them with a car because they need to get up to 20-30 mph before the push car drops back and the automatic car drops it into drive. I imagine this is pretty hard on the car.

I assume this only works on some cars. I used to freak out my friends by shutting off the car while driving on the freeway (only if I was the driver, not something I would do to someone else) and then drop it into neutral and re-start the engine. I found that on some cars if you left it in drive and turned the key back to ‘run’ it would start back up, but on some cars it wouldn’t work.

Only if the tranny has a tail-shaft-driven fluid pump. Otherwise there won’t be sufficient pressure in the torque converter to couple to the engine. GM Powerglide transmissions are one such type. I’d imagine this would be hardest on the torque converter than anything else, but that’s not my area of expertise. GaryT?

I’m not an automatic transmission specialist, so I don’t know the details. I doubt it would stress the torque converter, because I can’t think of how there could be damage in a fluid drive coupling. I did start my '64 Galaxie with a C4 automatic once by rolling it down a hill.

Well, there might be something to it, although the fault doesn’t seem to be so much “high performance” as that the BMW electrical system seems unable to cope with potential high voltages.

From here :Tip #3: Due to the varying nature of car electrical systems, never jump start your BMW using another car that is running. Use the other vehicle’s battery power alone to start it because a 14.5 volt running system (i.e., GM) can seriously damage a 12.6 volt system (i.e., BMW) due to the overvoltage.
To me, that sounds like bad engineering, as most cars will be called upon to perform a jump start at some point in their lives.

I’m thinking stress on the vanes. Is it possible for these to break or deform under sufficient fluid stress?

Actually, it doesn’t say anything about not using your car to jump start another, or about not jump starting your car from somebody else’s. Just hook up the cables with both cars not running, then try and start the BMW if it’s dead, without starting the car with the good battery.

It doesn’t say whether or not people should, after attaching cables, start the BMW if it’s the one with power, though.

this is jumping a BMW off of another car (which it says should be off), not using a running BMW to jump another.

The answer is:

You could have received a jump from the X5.

You didn’t, because the owner was either pompous, uninformed, or misinterpreted what he read.

to be fair, it was probably all three. Still, he was a snide little jerk about it – "I have a ‘high performance’ engine . . . " Who says something like that?

Eh, so what happens when you get the other car started and it has a higher voltage system? It’s gonna take a while to yank the cables off. Even when off, how isolated is the battery from the rest of the system? I’m not saying that it’s guaranteed that you’ll do damage to the BMW, but according to my cite, it’s at least plausible that bad things will happen.

That doesn’t make sense. Every “12-volt” battery has an actual normal rest voltage of 12.6. That site also says: Most alternators now use internal voltage regulators to maintain the proper system voltage, from 12.6-14.5 volts. You should check your BMW’s repair manual or with your local dealer to obtain the exact proper voltage for your BMW. I see some problems with this statement as well.

Now, not being a BMW expert, I’ll concede that perhaps they have a system that acts a little differently from every other car in the world, but frankly I have serious doubts about that. What they call “running” voltage (= charging voltage) does vary from car to car to some degree, but mainly varies with the electrical load on the system at the time and the ambient temperature. Talking about “exact proper voltage” suggests there’s a precise specific voltage that applies to the car, and that’s just not how it works on any car I’ve ever seen.

12-volt systems are considered to have a nominal charging voltage of 14.2, and variances of plus or minus 0.5 volt are considered normal. If BMW regulates voltage so that it can be as low as 12.6 while actually charging, that’s a new one on me. Even if that were the case, their systems still have to charge at a higher voltage (like they say, up to 14.5) at times, otherwise the battery could never be recharged from the drain of starting. I don’t see how getting a jump from a car with that voltage could damage anything, since the system itself has that voltage at least some of the time.

Voltage spikes of, say, 15 or 16 volts can certainly do some harm – to any car. But I’m very skeptical that any properly working car electrical system can damage a BMW system during a jump start.

I can’t swear it’s impossible, but I haven’t heard of it. Seems to me that a push start couldn’t exceed the stress of foot-to-the-floor acceleration.