On Wed, 4 Jan 2006, michael lunsford wrote:
> Bob Lang's run down on the circle B head is right on target with what
> I have learned and heard on this list over the past 10 years. One
> thing he failed to mention is that as far as I know the Circle B
> heads, along with some of the other heads he mentioned, were only on
> the later motors (CF motor designation I think).
Ummm. the assertion about Circle B heads only being fitted to late (after
CF1) TR6 is false. I have four early TR6 complete motors with narrow-port
Circle B heads.
> All of the heads on
> the CF motors had better breathing properties than the earlier heads
> because they used the long runner intakes which, according to others
> more knowledgeable than I, provided more torque than the earlier short
> runner intake heads.
this is all true.
> There were other properties of the later heads
> that improved upon the earlier versions but they escape me at the
> moment. The modifications to the heads on the CF motors (this
> includes all later heads, not just the Circle B) supposedly
> compensated for the reduced power which came as a result of a
> reduction in compression (8.5 to 7.75 I think) necessitated by new
> emission rules. I have an early motor (CC designat!
> ion) with
> a circle B head from a CF car. This combined with a 9.5/1
> compression ratio considerably improved the power output of my car.
The late - style head will flow better than the US fed. spec. "Early"
head. The exhaust valve sizes are different between early and late (1.26
for the early cars, 1.19 for the late heads.
One "trick" for the late head is to install the early exhaust valve. This
probably changes the torque peak (moves it up the rev range), but I have
not data to prove that.
FWIW, my .040 over race motor with 11:1 (estimated) compression and a mild
cam has a nice flat torque curve at 144 lb/ft from 2500 past 6000. This
car has an early head that is essentially unmodified. If I put in better
valves (stainless valves with a better "profile") I could get the power
and torque up, but to get to HP numbers above 160, you have to port the
head (a lot). This is where the late-style heads shine... you don't have
to play with the ports nearly as much.
No doubt about it, the late-style heads are better than the early heads,
but with some messaging, you can wring some power from the early heads.
Note: late style heads can also be referred to as "PI" heads. The PI cars
from the get-go had the wide-port design.
> Mike Lunsford, 1970 TR 6
regards,
rml
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