Re: 505 Turbo- why do the heads crack?

Brian Holm, Peugeotholm (peugeots@plainfield.bypass.com)
Mon, 04 Jan 1999 19:49:00

My opinion on head cracking is that it comes from repeated thermal stress
in the area between the exhaust valve seat and the water jacket, where the
cracks occur across the headgasket face. I believe that the stress is
greatest when the thermal gradient is steepest, when the coolant is cold
and the valve seat hot, namely when someone has their foot in a cold
engine. I think that people who hotrod the car when it is cold, or just
move into fast traffic or have to climb a hill with a cold engine are most
at risk for early head cracking. Shutting a hot, hard-working engine down
without a cooldown period may well also cause the same areas to develop hot
spots as the heat transfer is impeded into the stagnant coolant.

One other factor I have discovered in talking with owners of high-mileage
turbos with their original heads is that the owners nearly always changed
the oil at 2-3M miles regularly. This may simply be an indicator of a
general level of care and attention to the vehicle rather than directly
causal, as I have a hard time linking clean oil to heads not cracking
directly.

My advice is to take it easy on the loud pedal until the temp gauge is up
at least a little, and especially in cold weather. If you live or work at
the foot of a hill or must get right into fast traffic, you may be doomed
if you aren't prepared to dirve around a little first.

Brian Holm

At 11:47 AM 1/3/99 -0500, Jim Lill wrote:
>On Sun, 3 Jan 1999, checker767 wrote:
>
>> What can one do to preserve the heads on 505 Turbos?
>>
>> I'd greatly appreciate any insight regarding this problem!
>
>There's two types of things that can nuture head cracks. Power-related,
>such as overboost and detonation, and thermal-related. The latter is a
>function of the dissimilar thermal expansion rates of the aluminum head
>and the iron block. The aluminum head is used in a turbo motor to minimize
>hot spots in the combustion chamber and curb detonation. The iron block is
>a necessity for strength. The N9 series block is capable of 400+HP.
>
>Power-related failures should be seldom seen in a car with properly
>working boost, fuel and ignition controls. If you've never seen the LED
>flash, that suggests that the ignition system is probably OK. If you did a
>"plug read", you can get an idea of the fuel mixture, but even an
>occasionnal leaness should not toast a head.
>
>My suspicion on your car, given you suggest no overheating and it being an
>N9TEA with high boost capability, is that your boost is spiking at times.
>The N9T and N9TE use simple wastegate control and the boost is limited to
>about 9 psi. The N9TEA uses a fancy electrovalve scheme and the boost can
>run to almost 12 psi. I'll guess it can spike even higher. You need to
>examine the whole wastegate hose and electroical scheme. If you want to be
>safe rather than speedy, you could route the line direct and eliminate the
>electrovalve and get something like 6 psi max. I'd spend $35 and get a
>"real boost gauge" like a VDO in any event to monitor the boost.
>
>As a general caution, to minimize thremal stress, I'd never run it hard or
>even hot without a 15 second minimum cooldown. This not only helps the
>turbo's bearing etc. live longer but can help the head cool down. The use
>of Reline "Waterwetter" can be a help.
>
>-Jim
>
>
>
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Brian Holm, Peugeotholm
Supplying parts for Peugeots only, since 1969
Plainfield, VT 802.454.7132 Fax 454.1310
Alternate E-mail: peugeots@together.net