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GT40 Iron Head Flow Tech by Peter L. Turek
11/03/95 ported GT-40 heads Flow Summary and 08/21/96 porting and valve swap summary
CFM flow chart
---------+--------+--------+--------+--------+--------+--------+--------
Valve | | | | | | |
Lift | .100" | .200" | .300" | .400" | .500" | .600" |
---------+--------+--------+--------+--------+--------+--------+--------
stock | | | | | | |
intake | | 111 | 159 | 183 | 185 | 190 |
---------+--------+--------+--------+--------+--------+--------+--------
#1 prting| | | | | | |
intake | | 115 | 166 | 193 | 208 | 210 |
---------+--------+--------+--------+--------+--------+--------+--------
1.94 prtd| | | | | | |
intake | 98 | 156 | 207 | 225 | 237 | 242 |
---------+--------+--------+--------+--------+--------+--------+--------
Stock Vs. 1.94" valve & porting on intake
CFM Inc. 45 48 42 52 52
% Inc. 40.54% 30.19% 22.95% 28.11% 27.37%
#1 porting Vs. 1.94" valve & porting on intake
CFM Inc. 41 41 32 29 32
% Inc. 35.65% 24.70% 16.58% 13.94% 15.24%
CFM flow chart
---------+--------+--------+--------+--------+--------+--------+--------
Valve | | | | | | |
Lift | .100" | .200" | .300" | .400" | .500" | .600" |
---------+--------+--------+--------+--------+--------+--------+--------
Stock | | | | | | |
exhaust | | 28.5 | 125 | 129 | 129 | |
---------+--------+--------+--------+--------+--------+--------+--------
#1 Prting| | | | | | |
Exhaust | | 101 | 142 | 172 | 174 | 173 |
---------+--------+--------+--------+--------+--------+--------+--------
1.6 prtd | | | | | | |
Exhaust | 127 | 155 | 178 | 195 | 203 | 203 |
---------+--------+--------+--------+--------+--------+--------+--------
Stock Vs. 1.60 & porting on exhaust
CFM Inc. (yeah ) 126.5 53 66 74
% Inc. (right!) 443.86% 42.40% 51.16% 57.36%
#1 porting Vs. 1.60 & porting on exhaust
CFM Inc. 54 36 23 29 30
% Inc. 53.47% 25.35% 13.37% 16.67% 17.34%
Intake-to-Exhaust ratio
.100" .200" .300" .400" .500" .600"
------------------------------------------------------------------------
Stk int
stk ex 26% 79% 70% 70%
------------------------------------------------------------------------
stk int
prtd ex 89% 94% 94% 91%
------------------------------------------------------------------------
prted int
stk ex 25% 75% 67% 62%
------------------------------------------------------------------------
prtd int
prtd ex 86% 89% 84% 82%
------------------------------------------------------------------------
1.94 int
1.60 ex 130% 99% 86% 87% 86% 84%
Notes:
* stock exhaust not flowed at low lifts- it wasn't worth measuring!
* Also note, stock GT-40 exhaust "fell off a cliff" after .500" lift.
GT-40 exhaust only appears to flow nice in the mid-range, I attribute
this to excessively small valve seat throat diameter and large intrusi
left by the bowl/throat cutter. Also, the exhaust benefitted a good
deal by laying back the exhaust short side, widening the bowl a good
deal, and especially raising it's roof a large amount.
* The intake side received mostly minor work. Mild flattening of what
little push rod bump existed, taking off the rough edges around the
intake guide and around the bowl just below the valvejob. The bowl
was widened and reworked only on the shorter side turn wall of the
port. This resulted in roughly 10 CFM at .500" and at .600" lift
with smaller gains at lower lifts. I theorize more widening on this
same side would produce more gains as it wasn't widened so much, but
rather recontoured. The opposite side might benefit as well, but as it
curves under the valvejob, it might be more worthwhile to leave it
alone and concentrate on the shorter side wall as it will have
most of the flow and taking some more material out strategically
should have the same effect as cutting down the poushrod bump. And
the widening will almost certainly release more CFM.
(The comments below reflect the mods made with the 1.94/1.60" valve install)
* The final porting of the GT40 iron head in which 1.94/1.60 Manley
Race flow valves were used resulted in truly great flow numbers.
The large Manley valves appeared to aid both the intake and exhaust
significantly, with the exhaust gaining the most as predicted
due to an inferior small 1.54 valve for the port size and bowl
area after porting the 2nd time around. The Short-side radius
turn on the exhaust also was laid back perfectly this time
with a slightly decreasing radius from the valve job to the exit.
The exhaust port floor was untouched and the roof extensively
modified esp around the critical guide and thermactor hole locations.
The combustion chamber was lightly worked around it's outer edge to
eliminate any rough edges and lips that might hurt low lift flow.
* The Intake port on the 2nd porting with the 1.94" valve installed
was simply a bowl widening on the short turn port "wall" (not the
floor) and smoothing of the radius. After that operation, the bowl
was opened up and blended with a bottom cut for the larger valve.
The bottom cut left a rough edge and also made the narrowest point
in the intake port valve throat area. The 242cfm # was with this
rough spot still on the casting. It was later removed and the
bowl hand-blended (opened another .030" in radius), but it was
never reflowed. Thus it is surmised that the GT40 iron will go to
over 242cfm (maybe 250-260cfm).
* No special cuts were used on the intake valve which was a bone
stock Manley Race flow valve.
* Note the extremely high low lift exhaust flow #s. This is excellent
or ideal for a forced induction motor and even the killer low lift
intake flow is outpaced by the incredible exhaust flow from
.100" to .300". Also note the high intake to exhaust ratios throughout
the entire lift range.
* The extremely high low lift flow #s on both the intake and exhaust
and the high intake to exhaust ratios may require a revised camshaft
profile to take into account that the overlap period may have
substantially higher scavenging or reversion effects given the
increased low-lift flow over the unported heads.
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