There's a saying that when it comes to engine building, “the power is in the ports.” Airflow is the key to turning up the power output of any engine, and with choices in performance cylinder heads, there is plenty of flow to be had. However, when money is tight, budget constraints often dictate cylinder head selection. We see plenty of Mopar street machines running around with OE cylinder heads. Sure, a swap to aftermarket heads might be in the cards for later when the funds are available, but is that a reason to give up substantial horsepower right now? Port work can improve the situation substantially, but grinding on that old iron has its practical limits. Professional porting cost bucks, and unless the engine is being built for a class that demands stock iron, it's generally impractical. That said, is it worth pulling out the tools and giving the heads a little rework? If you can handle a grinder, the answer is almost always “yes.”
Even if you are paying for professional porting services, it might be worthwhile to consider the options. The key is to keep the scope of the work at a level that makes sense in terms of time, money, and effort. We have seen guys go nuts on factory iron, and produce impressive results, but that is not what we were after here. Our goal is finding the maximum flow gains for the minimal amount of work. To get there, we looked at practical steps that produce a big improvement with the minimum level of modification.
A good example is the set of OE Mopar small-block iron castings presented here. These heads are a part of a low-buck, mild 360 street rebuild project, and were fitted with OE 1.88/1.60-inch valves. A valve size upgrade for OEM small-valve heads is usually worthy of consideration, especially if new valves are going to be a part of the reconditioning process anyway. In the case of the Mopar small-block, an increase in intake-valve diameter to the early 340 engine's 2.02-inch specification is a low-cost and common upgrade.
The relative benefit of installing the larger valves will vary with how well the seat work is done on the head casting. We have seen installations where the valve seat was simply cut with a larger 45-degree seat angle, and sized with a 60-degree bottom cut. This kind of installation will hardly change the port throat diameter, and yield little if anything in increased flow, except at very low lifts. On the other hand, a professionally cut seat, sized to the valve diameter, including a throat cut at a minimum of 75 degrees will dramatically open the port's bowl. The point is that the results from installing a larger valve can vary substantially depending upon how the work is performed. For our heads, the valves were installed with a competition seat job, featuring a five-angle (30-45-60-75-82-degree) cut that makes the most of the larger valve's diameter. After machining, the large increase in the port bowl's throat diameter was readily apparent by the ledge left at the point where the newly machined area met the as-cast portion of the bowl.
After the seat work, the large protruding ledge left by the seat machining must be blended into the bowl. The stepped mismatch left at this point will represent a massive flow disruption, negating any potential benefit of the machining operation. To illustrate this effect, we flow tested the cylinder heads before and after machining. As seen in the accompanying flow chart, the high-lift flow in this form was clearly impeded. Since the porting carbide cutters need to be pulled out and put to work to blend the seat cuts, we wondered just how much additional flow can be gained by a few very basic porting steps.
1. Our subject heads here were a set of Mopar small-block 587 castings from the mid 1970s. Featuring 1.88-inch intake valves, these and other similar heads were very common on the majority of LA 360 engines. The idea for our rework was to gain the maximum performance for the least effort.
2. To quantify the results of the modifications, we used the SuperFlow 600 flow bench at Dr. J’s Performance. The same basic modifications and results can apply to many other Mopar cylinder heads.
3. Step one when installing larger valves is to set up the seat cutter to take advantage of the increase in valve size. Dr. J’s set the fixture to .005 inch under the major valve diameter. The actual cutter is a proprietary 5-angle tool custom made for Dr. J’s to their specifications.
4. Seat cutting equipment is the only way to go for an accurate valve job, especially when removing large amounts of material. Old fashioned stones have no place in this type of work.
5. The cutter completes all five angles at once for a consistent form. It’s easy to see how much larger the throat diameter has been made by the large step remaining in the casting below the machine work.
6. The outside diameter of the seat measures 2.0115 inches, taking advantage of virtually the valve’s entire diameter, while leaving a small tolerance. Beware that some machinists will cut the seats significantly undersized.
The porting work considered here was just basic. We were looking for the biggest gains for the least time-consuming level of rework. Rather than go for the classic sandpapered shine, we sought to just leave the modifications in a smooth carbide-cut finish, and quantify the results of this level of modification with additional flow testing. The first step was to simply blend the machining cuts into the port bowl, a level of work usually referred to as a “bowl blend.” While in the vicinity, a mild reworking of the port short turn was also performed, and the head went right from the porting stand to the flow bench, producing the results seen in the third columns of the flow charts. As can be seen, this level of modification really unlocked a healthy amount of flow, with strong gains everywhere on the flow curve. Testing showed an increase of 35 cfm at peak intake flow, with the exhaust showing a 22 cfm gain.
At this point we could say that the performance character of the heads was radically changed, with flow numbers that would produce a very respectable street performance engine. In keeping with our objectives of maximum improvement for a minimal level of rework, we looked to one more popular modification on the intake side of the port: the port match. The intake port is opened at the manifold side to match the desired gasket size, in this case a standard Fel-Pro 2013 gasket for the small-block Mopar. Potential flow gains from gasket matching will depend upon the architecture of the cylinder heads, and in the case of these particular castings, the narrow and restricted pushrod pinch area is a definite source of restriction. Again, leaving the work in a smooth carbide-finished state, the port entrance was gasket matched, working into the runner to enlarge the pushrod restriction to the practical safe maximum allowed by the existing iron. Back on the flow bench, the port match/pushrod pinch work showed a gain of 14 cfm, leaving no doubt that this area represented a restriction.
So what did we get for our modest cylinder head modification efforts? Looking at the flow curve, the lower lift and midrange numbers really equal some of the better aftermarket cylinder heads in terms of flow. The competition-style valve job is responsible for that result. Since a valve job is going to be a part of any cylinder head rebuild it is worth the effort to find a shop that will do a performance cut. Looking at the peak numbers, the heads deliver flow on par with a good set of entry-level aftermarket heads, and actually exceed the flow of some of the offerings on the market. With an improvement of 50 cfm from just three steps, we consider this to be a very practical level of modification in terms of cost/time versus the improvements. That's enough extra flow to support up to 100 extra horsepower in an all-out race-style engine, and 50 or more in a typical street piece.
Cylinder Head Flow
SuperFlow 600 Flow Bench
Tested at 28-inch depression
Tested at Dr. J's Performance
Stock: factory Mopar 587 small-block casting
V-Job: Dr. J's competition 5-angle valve job*
Bowl: blend machining step and short turn
Match: port match and enlarge pushrod restriction**
* machining terminates with large mismatched step into bowl
**port match intake only
|Competition valve job
7. An additional machining step to the valve job is to sweep the chamber with a cutter that is designed to provide a finished top-cut, and move the chamber walls outward to further de-shroud the chambers to the vicinity of the head gasket line.
8. Here is the completed machine work on the intake side. Note the smooth profile running from the chamber, right into the bowl to the end of the machine work. The large stepped-mismatch where the machining ends needs to be blended by hand.
9. The exhaust ports on these heads retain the stock 1.60-inch valve size, but the radius seat-cutter used by Dr. J’s enlarges the throat area below the valve.
10. To improve flow, especially at low lift, the intake valves are back-cut with a 30-degree angle, improving the transition at the back side of the valve.
11. A basic bowl blend will smooth the transition from the bottom cut into the port. We went a step further to gain more flow, lightly blending the short turn on the intake and exhaust ports. The port work was left at a smooth carbide-cut finish, rather than pursue any additional polishing with sanding rolls. Flow was up considerably.
12. Looking for another incremental improvement, the intake side was gasket port matched, and the pushrod restriction was enlarged to give the port more area to breath. This modification is very effective at improving flow with these particular castings.