As can be seen from the torque graph Fig. 2 on page 72, the 180cc ports (black curve) produced the best output up to 3,400 rpm, peaking at a stout 482 lbs-ft. The 200cc port (red curve), though was not lagging by much, and from 3,400 rpm up it ran up with or close to the bigger ports.
If we look at the torque curves and also consider the hp curves in Fig. 3, we can see that for our 383 incher, with the cam it had and the rpm range it would operate in, the 200cc ports delivered the best curve. The 215cc (green curves) heads made the highest hp by pumping out 478 horses, as opposed to 457 for the 180cc runners, 472 for the 200, and 475 for the 230s. The price the 215s pay over the 200 to achieve this seven hp advantage is that they give away up to 10 lbs-ft of torque in the rpm range from 2,300 to 3,200.
The 230cc port runner heads failed to deliver any worthwhile superiority anywhere in the rpm range on our engine. Indeed the smaller 215cc port heads beat the 230s everywhere! This, in case it was needed, is living proof that an engine is not a simple air pump, nor for that matter is bigger better. Had we targeted an engine capable of more rpm or one with bigger displacement, then the bigger port heads would have paid off. Experience with ports in the 230-245 range show that every bit of the port size is needed if you are building a 440-cube small-block. If we look at a comparison on a pro-rata basis, a 235cc port on a 440-inch small-block Chevy is only equivalent to a 186cc port on a 350.
So how do you decide what port volume your small-block Chevy should have for best results? As good a rule of thumb as any is to base the port volume, and we are only talking traditional 23 degree heads here, on the projected power output. A point to note here is that if you rate your engine's final output too optimistically, you will end up with a port that is too big and the target output will not be reached. In other words, you will have shot yourself in the foot.
Check out the chart (Fig. 4). This will give you a good starting point for port volume selection. With Dart's selection of port volumes, they pretty much have the range from a relatively mild 350 (180cc ports) to a rampant 440-incher (230cc ports) covered. One more point worth noting for those higher hp engines here is that these Dart heads are really easy to port. Doing so can get the port volume right where it needs to be along with more flow.
Just remember that a port a little too small will be a far better deal to drive than one that is a little too large. Just 20cc too big can easily cost 20 lbs-ft at a point in the rpm band that is most often used for a true street driver.
One final point, just in case you are wondering. With a bigger cam and 10.5:1 CR, our 200cc Platinum Darts, on this engine, allowed it to crank out 500 lbs-ft and a tad over 502 hp.
| FIG. 4 |
|PROJECTED HP ||PORT VOL. |
| MIN - MAX ||MIN - MAX |
|350 - 400 ||140 -160 |
|400 - 450 ||160 - 180 |
|450 - 500 ||180 - 200 |
|500 - 550 ||200 - 220 |
|550 - 600 ||220 - 230 |
|600 - 700 ||230 + |
This chart gives a good working guideline as to the intake port volumes to target for a small-block Chevy. Remember, a little too small is much better than a little too big. If the port volume of the heads you have seems a little too big, go for all the compression ratio the fuel intended will stand, as this will, to some extent, compensate.
T & L Engine Development Inc.
Dart Machinery Inc.
353 Oliver St.