Actually SGS Thompson that specifically makes protectors for
automobiles and load dump (do what 'too small' transzorb would
also do) acknowledge in their app notes that:
" ... there are several existing products products able to
clamp this overvoltage at the board level, for example the
LDP24 or RBO series. The protection at the alternator level
is a quite new concept and all the technical problems do not
seem to be completely solved."

This would explain why an 'up to 270 volt' transient is not
routinely observed. Still, automotive electronics typically
should withstand about 50 volts ballpark without damage.
Transzorb could be but one part of that protection system. Or
a DC to DC converter that specifically has such protection
could be obtained. A weakness of Transzorbs are their low
power abilities. The advantage - those avalanche diodes can
handle so much more power than conventional zeners. My first
design that used Transzorb was maybe 20 years ago. GS was
selling Transzorbs even long before then.

Load dump does not occur frequently. But that one time can
be so destructive. Best to consider load dump when attaching
any electronics to automotive power because even trivial
protection does so much.

Rich Grise wrote:
This is what transzorbs
http://www.vishay.com/docs/88301/15ke.pdf
and hash chokes are for.

I'm not affiliated with Vishay, formerly General Semiconductor,
just a satisfied customer. I've used them to protect circuits
against the transients caused by a 5 KV arc in an ion gun. :-)