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High Performance Valve Jobs
By Dave Emanuel
Valve jobs are like a number of other automotive machining operations – there’s a right way and a wrong way to do them, but what’s right for one application may be wrong for another. Although a standard "cut and buff" approach is entirely satisfactory for most car and light truck engines, performance enthusiasts invariably look for something better.
That ‘something’ is usually a multi-angle valve job. And for good reason; on a typical V8 engine, it’s worth about 10 horsepower. A good multi-angle valve job (three angles are most commonly used) includes a 70-degree bottom cut, which is the same angle found in many stock heads. But as opposed to the 70-degree cut meeting the 45-degree cut of the valve seat, a three-angle valve job incorporates a 60-degree cut between the two cuts to smooth the transition from the valve seat as well as to narrow the valve seat.
The seat itself is cut with a 45-degree cutter, while a 30-degree cutter machines the transition from the top of the valve seat to the combustion chamber surface. The accuracy of the valve job exerts a significant influence over performance because it affects airflow, as well as the quality of the seal between the valve face and seat.
Although some engine builders vary intake valve seat width according to application, most performance engine builders rely on a .060" wide intake valve seat for both street and race engines. However, the general preference is for exhaust seats .080" wide for street applications and .060" wide for racing.
The reasoning for this is that the narrower seat provides a slight flow improvement and the heads on a race engine are removed for service frequently enough that the seats will be recut before they get beat up too badly. With a street engine, the slightly wider seat is required for longevity.
While these dimensions have been used successfully in thousands of high performance street and race engines, they are by no means written in stone. It’s important to note that some cylinder head specialists may have other ideas or have developed other combinations that they prefer.
As experienced performance engine rebuilders are likely to explain, in many cases the trade-off with a valve job is between flow and heat dissipation. A wider seat provides more area for the valve to transfer heat into the head. That’s why you can’t use the same valve seat dimensions on heads built for the street or oval track that you can on drag race heads.
Every engine builder has his own technique, but there are still certain things that are either right or wrong for a particular application.
You can get by with narrow seats that are out near the edge of the seat surface in a drag engine because it’s going to come apart before any damage is done. But on street, oval track or bracket engines, where you’re trying to get long-term durability, you have to give up a little flow to get reasonable valve life.
Regardless of the application, the valves should be lapped. You can cut the prettiest-looking seats, with every angle just what it’s supposed to be, and there’s still no assurance that you’ll get the best possible seal. When you lap the valves, you’ll know for sure.
Another aspect of the valve job to be leery of is the positioning of the valve head with respect to the combustion chamber roof. This is especially true with used heads. Each time a valve job is done, some material is removed from the valve seat. If a head has been the victim of numerous valve jobs, or was used for on-the-job training, the seating surface may have been ground so much that the valves are sunk.
Sinking the valves should be avoided at all cost because it hurts airflow and, consequently, power output. Sinking the valves shortens the distance between the seat and the short-turn radius which is exactly opposite of the desired condition.
If a pair of heads has a lot of redeeming features, but has seats that have been sunk like the Titanic, either new valve seats should be installed or larger diameter valves substituted for the original ones. One of the reasons that small valve (1.94"/1.50") Chevrolet small block castings respond so well when 2.02"/1.60" valves are installed, is because the valve seats are raised. This increases the distance from the seat to the short turn. Remember, larger valves improve airflow two ways – with increased diameter and better seat positioning.
Ever since leaded fuel all but vanished from the pumps, valve seat life has been a big question – especially in high performance engines. Numerous tests have shown that without lead to provide a cushion between the valves and their seats, rapid seat erosion results.
Consequently, most factory-installed heads manufactured after 1971 have induction-hardened exhaust seats. While they’re certainly advantageous, they’re not always necessary. Valve seat erosion is typically only a problem when an engine is run continuously under a heavy load.
While valve seat wear is unquestionably greater with unleaded fuels, it won’t be severe enough to cause problems under normal operating conditions. But when you have to stand behind your work, cylinder heads without hardened seats present a significant potential liability.
If you’re facing the prospect of doing a high performance valve job on a set of heads without hardened seats, it’s advisable to cover your posterior. If you suspect that the heads will be used on an engine that’s run hard, advise the customer IN WRITING that valve seat degradation may be extremely rapid due to the use of unleaded fuels.
One alternative is for the customer to purchase a set of heads with hardened valve seats, but that may not be a well-received suggestion. Many people own extensively modified heads that don’t have hardened seats. After having spent hundreds or thousands of dollars on head modifications, they won’t be too anxious to scrap them just because they need a valve job. On the other hand, you don’t want them coming back and complaining because the valve job didn’t last.
Simply stated, high performance engines deserve high performance valves. Aftermarket swirl polished stainless steel valves are used by virtually all high performance and racing cylinder head specialists because they exert just as much influence over airflow as the machining done to the cylinder head castings.
Some companies offer valves that are longer-than-stock, and many offer valves with undercut stems for increased flow. Note that nearly all factory replacement type valves have a .001" taper from one end of the valve stem to the other. The taper doesn’t mean the valves are poorly machined; the stem diameters are smaller near the head to accommodate heat induced expansion which would otherwise cause galling.
Titanium is the material of choice for valves used in "damn-the-expense, let-the-sponsor-pay-for-it" classes of drag, oval track and road racing. Owing to their light weight and high strength, titanium valves allow for more radical cam profiles and higher power potential.
However, at seven to eight times the cost of a top quality stainless steel valve, the expense can’t be justified in anything less than a professional class of competition. Special machining techniques are required with titanium, and it is entirely possible that a set of heads with titanium valves will appear in your shop if you promote high performance valve jobs.
Irrespective of the material, most high performance valves incorporate a back-cut to improve air flow. This extra cut is analogous to the multiple angles around the valve seat — it makes for a smoother transition and increases airflow.
Strictly speaking, assembly of the cylinder heads isn’t part of the valve job, but many customers want it to be done so the heads are ready for installation when they pick them up. In many instances, the supplied springs, retainers and locks can be reused, but their condition should always be carefully checked before reassembly. It’s also advisable to verify lift specifications to assure that the valve locks don’t go crashing into the valve guides at maximum lift.
Like most other high performance machining operations, multi-angle valve jobs represent excellent profit potential. But they can just as easily become a thorn in your side, sucking money out of the bottom line with the speed of a nitro burning Funny Car. As such, good communication with the customer and precise machine work is essential.
(Illustrations from "Small Block Chevy Performance" by Dave Emanuel, published by HP Books, a division of Berkley Publishing).