Ken:
The later head has much better profiles on the ports, and so
flows much better than the earlier head. Also, despite the fact that the
earlier head has a larger exhaust valve, I believe the later head flows
better on the exhaust port than the earlier head. The reason is valve
shrouding. The larger exhaust valve moves the valve too close to the
wall of the combustion chamber. If you are looking for a performance pop
on the later head, install some nice stainless valves with waisted stems
and swirl polished faces. Don't forget a 3 angle valve job too.
The downside to the later head is the much lower compression
ratio - this will cost you a lot of power unless you mill the head to
bring the compression up to something respectable.
The later head was used to restore some of the power lost to the
lower compression ratio, which was necessitated by the tightening
emission standards. The later cam was better than the earlier cam as
well, once again to regain lost performance. HP was virtually the same
even though torque had fallen off.
Just my $0.00002
Cheers,
Vance
-----Original Message-----
From: owner-6pack@Autox.Team.Net [mailto:owner-6pack@Autox.Team.Net] On
Behalf Of owenskenbrenda@bellsouth.net
Sent: November 09, 2007 10:28 AM
To: 6pack@autox.team.net
Subject: [6pack] Early vs Late Head
I plan to install Richard Good's triple carb setup on my 71 TR6. The
head (early model) was overhauled less than 2 years ago and is working
very well. I do have an extra head that is a 1974, but, it is the
"rough" and will need to be overhauled in order to use it. My question
is: other than the intake ports on the later year head being spread
wider on the head, are there any other differences between early and
later model heads? Also, since the exhaust valves are smaller, would
this not be a reason to use the early model head (without machining the
later head to accept the larger valves). I do realize the valve springs
are different. I am wondering if the perforance gains between early and
late model lies in the head or really is in the longer intake manifold.
Looking for all the performance gains I can get, so, if there is a real
reason to overhaul the extra head I will do so.
Looking forward to everyone input!
Thanks,
Ken Owens
71 TR6 SM2 Autocrosser
Fortson, GA
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