Does size matter?
How much valve spring does an engine tuner need to propel 8000 hp down a 1000 ft dragstrip? Is it best to use titanium, or is steel sufficient? And are two springs best, or should there be three?
For Bob Vandergriff Jr's C&J Energy Services National Hot Rod Association Top Fuel dragster, this year competing full-time in the Full Throttle Drag Racing Series of 22 events, the choices are predicated on a change to the set-up configuration for the 2011 season.
Working to gain reliability on the dragstrip and to be competitive with teams that have had full-season set-ups far longer than they have, Vandergriff's team is spending the first few races of the year trying to find a happy combination that will work best for their engine.
The objective is to be aggressive enough to qualify and to make the field of 16 for Sunday eliminations, but also to keep parts in the engine from ending up as a molten mess after a run. For these reasons, according to crewmember Rob Hauser, the team has chosen to use double-spring PAC valve springs in the current combination as they find the sweet spot.
"The objective is not to beat up the valve springs but to take them out before they've lost their longevity," he told me. "We test them every run while the head's being serviced, check each one on our spring checker and, as long as the spring maintains the correct level of spring pressure, we'll continue to use them."
The steel double spring the team has chosen - for both intake and exhaust valves - has 895 lb/in pressure at production. Installed pressure is 550 lb and, when open, the spring generates about 1150 lb, he said. "Once it falls below that level, we'll take the spring out and set it aside. We may then sell it to an Alcohol team or a smaller-budget team to offset some of the costs.
"We use a double spring instead of a triple - it's two separate springs, instead of having a damper in between like the titanium springs," Hauser explained. "We like this steel spring - it seems to be 'living' quite nicely - and they last longer than the titanium springs, maintain the pressure and cost a heck of a lot less." Weight is a bit higher, at 5 oz (141.75 g), but the smaller diameter offsets this additional weight of steel versus titanium.
The valve springs in use at Vandergriff Motorsports are a standard spring and are straight-cut parts, not a conical or beehive valve spring. "In the older spring we used, in order to get the spring pressure we needed, it had to be a triple spring, but now the spring is actually physically taller than we used to run. The wire diameter is smaller now so we can still get a decent coil bind to allow us to have the camshaft the size we wish to run without having problems with coil bind."
When coil bind occurs in a Top Fuel engine, the valve fails to open and can break pushrods and/or rocker arms. "When one coil touches the other and they're all squished together, we get breakage on the opposite side," Hauser said.
"Since this valve spring is taller, with about half again more rotation than what we used before with a triple spring, we get higher spring rates so that when we do squeeze it down, it goes right back to its 550 lb operating pressure. In the past, the larger spring would only give up about 425 lb of spring pressure, but the two of these together work out to 550 - this is enough to get the valvetrain right where we need it.
"We're using this valve spring as part of our new tune-up for the 2011 season, and we're very happy with the service life we're getting. We had instances in the past with premature failures and that would cause catastrophic failures, which are mega-expensive," Hauser said. "It costs enough just to maintain our engines without catastrophic failure - that adds a whole new dimension to the situation."
Fig. 1 - Vandergriff Racing is using a double steel valve spring this year to complement its new tune-up configuration (Photo: Anne Proffit)
Written by Anne Proffit