PT-200 Valve Spring Tester Articles and Tutorials 

Some Notes on NHRA Pro Stock Valve Springs

Valve springs are definitely a critical component in 10,000+ RPM NHRA Pro Stock engines. Our PT-200 Valve Spring Tester has been  use over the past 15 years by most of the top Pro Stock engine builders (WJ, Cagnazzi, Jenkins, Maskin, Schmidt) and its been pretty interesting to watch the development of the valve springs used by these engine builders.

When we first introduced the tester back in '93, our 1000 lb max test pressure was fine for the springs in use at that time. But as the engine RPM's kept going up, the builders needed stiffer springs to keep the valves and valve train under control and we kept having to redesign the tester to increase the maximum test pressure, in addition to accommodating the taller springs in use.

We had to go to 1200, then 1300 and now are at maximum of 1500 lbs of test pressure. A typical Pro Stock spring will now have around 450 lbs of seat pressure and a staggering 1400 lbs when the valve is fully open. Since the springs are taller than previous generation springs, they require a pretty large installed height of around 2.3".

The amazingly high rate of these springs means you could put one of the springs under each tire of a full size pickup truck and the springs wouldn't even be compressed all the way.

One issue that comes up though from making stiffer springs is that the spring weight itself becomes an increasingly significant part of the total weight of the valve train. The current springs are made taller to increase the spring rate, but this requires extra weight, and also requires a longer and thus heavier valve stem. This weight increase limits how far you can go with steel alloy valve springs.

To reduce this weight, titanium valve springs have been used in Pro Stock engines. Since they are so much lighter than steel springs, the weight reduction allows less spring force, so a typical titanium PS spring will have around 350 lbs of seat pressure and 1100 lbs of open pressure. The negatives on titanium springs are the high cost and the failure mode, when they fail they don't just lose pressure, they tend to fracture and come apart.

While mulling over that 1400 lb open pressure figure, keep in mind that at 10,500 RPM the valve and its spring is going through a complete open and close cycle of over 1" of lift almost 90 times per second. So in the time it takes to say "per second" thats around 100 complete cycles of this 1400 lb spring.

Its pretty amazing and impressive that working with engines and valve trains pushed this far to the limit that Pro Stock racing is so extremely competive, with the top teams often being within a few hundredths of each other on the track.

If you haven't seen one of the high speed videos of a valve spring at race RPM, take a look at:

PAC Racing - Valve Spring Video

The wave motion of the spring coils up and down are what are called "spring harmonics" and have to be carefully controlled to reduce undesired valve motion.

Paul Titchener, Power Technology

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