Reply to "4V Heads"

A lot of guys who "stuff" the intake ports end up reducing air flow, or worse yet they make their car accelerate more slowly. This usually means they were too heavy handed with their "stuffing". There is a proper way (or amount) to stuff the intake port, but even stuffed properly it has never been demonstrated (as far as I know) that it will actually result in a faster accelerating street engine. So at best stuffing the intake port will result in no improvement. The time spent stuffing the intake port was time that could have been spent better doing something else. Hence my advice to leave the 4V intake port unmodified because you can't improve it.

A better way to spend your time (and to extract additional power from the 4V heads) is to install 2.12 inch intake valves in the D3ZE heads (if those are the castings you're working with) and then to "detail" the areas above and below the valves seats. Work on the valve pockets without increasing their size. Sharp edges in the valve pockets can be removed. The valve pockets can be smoothed to establish smooth, well blended transitions from the valve pocket to the throat, "rounded" throats, smooth, well blended transitions from the throats to the valve seats. The large cast-in valve guides can be removed from the valve pockets entirely and replaced with press-in bronze valve guides which are 0.500 inch OD. The valve seats are normally recessed a bit, and therefore sharp edges or “lips” surround the valve seats. These “lips” can be knocked down, so that the transitions from the valve seats into the combustion chambers are smooth, having no sharp edges, no steps, and no lips. The valves in quench combustion chamber heads can be unshrouded where there is insufficient clearance between the valves and the combustion chamber walls.

This type of detailing is accomplished with modern heads via a CNC machine, but with these old iron castings it has to be accomplished by hand.

Finally the width, diameter, and angle of the valve seats can be sized and “profiled” with progressive cut multi-angle valve seat cutting. The first cut should be 70° (as per Ford), the valve seat angle should be cut at 45°, and the undercut should be 30°. The seat width of the intake valves should be no less than 0.060 inch, the exhaust seat width should be no less than 0.080 inch.

The reward for this detailing is usually a 55 cfm improvement in intake port air flow at 0.600 inch lift (a 20% improvement) and a 40 cfm improvement in exhaust port air flow at 0.600 inch lift (a 22% improvement); plus an improvement in horsepower and acceleration.

To answer the other questions:

I would agree that too much hoopla is centered around gas velocity in street applications. Not that gas velocity isn't a good thing, but how many cylinder heads are you aware of that are that far off? Gas velocity really matters at wide open throttle. How often is your street engine operated at wide open throttle? Related to gas velocity however is port cross-section and port volume, these things impact an engine's power band; it is the power band tuning of the intake port which is far more important to a street engine.

There’s an automotive urban legend that when an engine having relatively large intake ports lacks low speed torque that its the port size which is responsible for the poor low rpm performance. In most cases it is the combination of a camshaft with more intake duration than what's ideal for the application, excessive overlap (narrow LSA), insufficient compression (8.0:1), a single plane intake manifold, and/or the exhaust system (i.e. headers that are too large in OD, too short in length, or too restrictive of a tailpipe/muffler system). Its NOT the intake port size.

Tuning for higher output

While intake port cross section will affect the port velocity and the power band, in most cases if you're tuning a performance street car's engine for higher output you're better off going slightly larger on port size and slightly conservative on the cam duration. A good dual plane intake and long tube headers with a low restriction exhaust sure helps. The concept is to tune the power band via the size of the induction system; this makes it possible to utilize a camshaft with parameters more appropriate for street operation. The 351C with 4V heads is a perfect example of this. It can be a very powerful street engine that accelerates like none other without sacrificing drivability. I've proven this to owners over and over again (as long as 400 to 450 BHP is considered powerful). In actuality I like to advise owners to not place too much emphasis on dyno numbers, but rather the engine's power band and acceleration. From that perspective the 351C with 4V heads never disappoints.

Camshafts with wider (114°) LSA smooth out the idle, widen the power band, and improve low rpm performance, but they reduce overlap. Many engines need an increase in the amount of overlap to allow their cylinders to fill better at higher rpm; this is not true for the 351C 4V however. A 351C 4V “street engine” does not need an increase in overlap because the shallow Cleveland combustion chambers make more high rpm power at 50° overlap than most engines make at 65° to 70° overlap.

The 351C 4V has good high rpm capability built into it; it doesn’t need the same help in this area which other engines do. As the 351C with 4V heads is “uncorked” with a 750 cfm carburetor, a high lift camshaft, and a better exhaust system you’ll find it is tuned for a power band extending from idle to 7000 rpm, with peak horsepower occurring at 6500 rpm. This is achieved without taking steps to improve high rpm power. With that state of tune it is capable of good drivability and good low rpm power. Taking this approach low rpm power does not have to be sacrificed. However tuning the 351C 4V the same way you would tune the induction of a SBC or SBF (long intake duration camshaft, narrow lobe centerlines, increased overlap, single plane intake manifold) shall take a power band that was already high enough and push it higher ... too high and out of range for what is useful for a street car.

Lowering the power band

There are really only 2 good choices for lowering the power band of a 351C equipped with 4V heads. (1) The first is to install cylinder heads with smaller intake ports, i.e. 2V cylinder heads or perhaps the CHI heads with small ports. Not for the increase in port velocity but because the induction system shall be tuned for a lower rpm power band. (2) The second is to install a long stroke crankshaft. With no other changes the stroker crank will raise piston speed and thus lower the engine's power band tuning. A 3.75 inch crank with a 6 inch long connecting rod is the only stroker whose engineering I can recommend, at that stroke the peak horsepower of the 4V intake port will be lowered to approximately 6000 to 6100 rpm.

Filling the 4V intake port just can't lower the power band enough to accomplish this.

There is more to the subject of gas velocity than simply more is better. If that were so, then why do so many enthusiasts purchase aftermarket heads with bigger intake ports? There is such a thing as too much velocity just as well as too little. The 4V intake port was a professionally designed port, nothing about the port should lead anyone to believe it would be simple to improve. The designers did indeed pay attention to gas velocity, and the sales department summed it up by writing "the ports were carefully sized". That's quite an understatement; it's like describing a bottle of 18 year old Macallan's as “good whiskey”.

At this point I could go on and describe the 4V intake port in detail, but I've already taken quite a bit of time writing this. I know I can be long-winded. So I'll close by explaining that only the inlet of the 4V intake port is large, by the push rod bulge (only 3/4 of an inch into the port) the port cross-section has stabilized at a cross-section that is over 1 square inch smaller than the inlet (27% smaller).

-G