What would our budget, factory...
What would our budget, factory smog 360 heads do with a little street porting? Mopar Muscle's Steve Dulcich broke out the grinders and fired up the flowbench to find out.
For those of you just joining us, last month we set out to build a basic budget 360. We weren't aiming to throw together a cheaper-than-dirt hone and rering job, but rather illustrate what can be done with savvy parts selection and reasonably priced machining.
We tapped the PAW catalog for all of the parts that went into the building of a stout street/strip short block. Being the lone Mopar Muscle writer out west, and therefore local to PAW, I also used PAW's machining service to handle the full block prep on a junkyard core '79 360 E-58 High Performance smog motor. The result was a righteous short block for $1,128.
As laid out last month, the plan was to take the basic short block we built, top it off with a low-buck top end and juice cam, spin it on the dyno, and see what we had. Once those steps are completed, we'll go back in and redress our short block with better heads, a hotter cam, and aftermarket valvetrain components and run it again. Thus the moniker, Double Take 360.
Putting together a low-buck top end means using the production head castings. In our case, we were going to reuse the universally despised late seventies smog heads-#4071051 castings-that came with the motor. Now, we know that just bolting on a set of these heads in stock form is not a prescription for power, but here's the facts: All of the 360 smog heads from the '70s through '80s were very similar in port configuration to the high performance design that debuted with the fabled 340 HP "X" heads-casting #2531894.
To get the most from these stock small-block smog heads, we set out to perform some basic porting to see how much flow we could unlock. This issue, we'll work over the intake side and follow up next issue with the exhaust ports.
Since our aim with the stock heads is to put together a budget combo, we weren't going for the full-on Pro Stock porting job, but realistic do-it-yourself street porting. Using the equipment at Specialized Motor Service in Riverside, California, I performed all of the cylinder head machining using a Serdi 100 seat and guide machine, while the subsequent port modifications were done at home using an air die grinder and a selection of carbide bits and grinding stones-a true home porting job. All of the flow tests were done complements of airflow expert David Vizard's Quadrant Scientific flowbench, which we used last year in our series on big block heads ("Go With The Flow," Mopar Muscle, January, February, March 1999). We don't expect everyone to have access to a Serdi, let alone be trained to use one, so we'll include the price of the machining at Specialized Motor Service in the porting costs next issue. As for the porting, we'll spill it all in great detail so our results can be duplicated by anyone with the grit to try it. Jerry at Specialized can perform the mods for anyone who'd rather pay-up than break out the grinder.
Take A Seat
Job one in modifying the production smog heads is to cut a performance seat. The valve seat form is critical to the airflow, and is the only factor of consequence at lower valve lifts.
Think about it. Up to approximately .150-inch lift, the biggest restriction to flow is the amount of area the valve has opened. Virtually any port can be upstream of the valve, from a stock 360 smog port to the latest Winston Cup race port, and there will be plenty of air backed-up to the valve waiting to get through. The form at the valve and seat where the air must get passed at lower lifts will determine how much air will actually squeeze through. The seat form is also important as the lift increases, but other factors such as the bowl and port shape will become the major restrictions at higher lifts and influence how much air a set of heads will flow.
Starting at the bottom of the flow curve (low lifts), we were looking to optimize the cylinder head airflow. The two ways to do this are to provide the best seat possible, for the reasons described above, and to increase the amount of area open to airflow.
How would we know if we were...
How would we know if we were doing any good? We tested a stone stock 340X head, casting #2531894. These are widely regarded as the best production small block Mopar heads, earning their "X" nickname by the large X cast between the exhaust ports (arrow). The stock flow turned in was astounding-reaching a higher peak flow than the stock 440 heads tested last year on the same bench.
The chamber side view of a...
The chamber side view of a stock 360 head. The 360 heads carry a smaller 1.88-inch intake valve than the 340 head's 2.02-inch, but the same size 1.60-inch exhaust valve. The ports are remarkably similar. Stock flow was not too bad compared to stock Chevy heads on file, but not even close to the excellent 340 "X" heads. Notice how the straight side roof channel is dammed by the tail of the guide boss, where the boss hooks over into the flowpath (arrow A).
The seat angle can only be...
The seat angle can only be changed when increasing the valve size, and since we were moving up to a 2.02-inch valve, we had a perfect opportunity to do the job. Stock-type tuliped valves (left) have the valve face flare up to the stem at too steep of an angle to take a 30-degree face. The flat-headed Milodon intake valves we used (right) can be faced with the shallower 30-degree angle.