I have a glass bonnet on my V8, the headers exit an inch below the fender
mounting surface and out, literally, over the tires. In 25k miles of street
and auto-x use I have seen no ill effects on the bonnet.
As for other considerations, I took some measurements back in the planning
stage and found the surface of a black car in the Colorado summer sun would
reach 165F.
I installed some DEI heat shield, the woven fiberglass stuff with one
aluminized side, onto the undersides of the fenders with 3M heavy duty
adhesive. I then ran the hell out of the car and idled it, the tops of the
fenders never went over 120F. Again, in 25k miles no problems. I can
still, to this day, rest my hand on the fenders or bonnet at any time, with
no burning sensation.
For most of the miles, the headers were only painted, things have only
gotten better since I ceramic coated them. Also, the bonnet has never had
any insulation installed onto it, just regular old Krylon black. It is
still shiny and intact, even though it lives only two inches from the
headers.
I have always been of the opinion that the bonnet insulation was for sound.
I can say that the difference in engine noise between a stock B with and
without insulation is huge. The insulation damps out most of the high
frequency noise, especially tappet noise.
Just IMO, nothing more.
James Nazarian
71 MGBGT V8
71 MGB Tourer
87 BMW 325es
-----Original Message-----
From: owner-mgs@autox.team.net [mailto:owner-mgs@autox.team.net] On Behalf
Of Max Heim
Sent: Sunday, February 20, 2005 6:10 PM
To: MG List
Subject: Re: Under bonnet temp
I wouldn't jump to any conclusions... Some cars have fiberglass bonnets,
yes, but are they as close to the engine and manifolds as the MGB bonnet?
And are their engine compartments as restricted ventilation-wise?
I can get a fiberglass hood for my 1967 Barracuda, but there must be a foot
of air space between the cylinder heads and the underside of the hood, and
it has a full-width insulation pad. Not to mention a relatively enormous
amount of room for air circulation.
I think you would have to assume at least 200 degree temps for the underside
of the bonnet. But if it's really a concern, an empirical test is in order.
--
Max Heim
'66 MGB GHN3L76149
If you're near Mountain View, CA,
it's the primer red one with chrome wires
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