There is very little difference in performance for the 2 exhaust port sizes.
The disadvantage of
the larger is a greater potential for cracking b/n inlet and exhaust valve.
I have a Longman GT6 head, originally on my 1380 but now on the shelf.
Here are a few interesting bits of info.
The valve springs supplied were not of the best quality - I had them
replaced soon after
purchasing with competition valve springs.
After only 1500 miles (< 2000km) after purchase, multiple cracks were found
in this head,
some of them quite deep. It was the opinion of the engine builder that these
cracks were
probably there at the time of porting and machining the head. Apparently it
is not
uncommon for heads with 'small' cracks to have the seats machined out and an
unleaded
seat fitted. It is one way of salvaging a cracked head. Unfortunately, it is
quite likely that
the crack will reappear. This head was sourced from Mini Mania back in '95
I had the cracks welded up by a specialist welder. This worked OK.
However, since the performance of the motor was below expectations, I had
the head
flow benched. At 0.5" lift it flowed 115cfm (corrected). This is OK but not
all that flash.
I had a new head made up that flows 128cfm using the same valve sizes. This
head is a really
good performer for the valve sizes. It is interesting to note that the
exhaust efficiency was not a
problem even with the higher inlet flows - hence demonstration that you
don't need a bigger
exhaust valve.
So, the GT6 is a pretty average 'Hot Road' head. Not the worst by any means,
but well
off the best. I wouldn't be buying one since for about A$1000 (US$650) I can
get a better
head made by a local experienced specialist. I also would *know* the
condition of the
head the job was started with.
Mike
.
----- Original Message -----
From "Mark Endicott" <endicott at nashville.com>
To: "Spridgets" <spridgets@autox.team.net>
Sent: Thursday, January 27, 2000 3:29 PM
Subject: Longman Heads
> I have been looking at the Mini Mania page since it has been to cold to go
> outside for the last few days. I have found a couple of Longman heads
for
> a late 1275 that are similar except for the size of the exhaust valves.
> Both the GT6 and the GT7 spec. 1.4" intake valves but the GT6 has 1.15"
> exhaust and the GT7 1.21". Both are ported and have hardened seats.
Would
> there be an advantage to bigger or smaller exhaust valves on an otherwise
> stock 1275, and how about with a Judson.
>
> Thinking that I am ready for a valve job and new seats anyway, and these
> heads are not all that much more money than having all of the machine work
> done to my head.
>
>
> Mark
> 70 Midget
> Nashville, TN
>
> http://www.NashvilleTN.org
> http://members.home.net/mendicott
>
>
>
>
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