Selecting for robustness and reliability
If a valve spring for a race engine is to survive a service life of 1400 race miles or 1600 absolute miles then it needs to be robust and reliable. In the Indy Racing League's IZOD IndyCar Series, where the Honda Indy V8 is the sole motive power, Honda Performance Development (HPD) has gone with a steel, flat-top, double-valve spring without insert, to achieve the balance needed on road courses, street circuits, short ovals, speedways and superspeedways.
HPD has tried different designs to get the spring right, looking at the right length of the spring and getting its diameter right, so there has been a lot of development going into the choice of its geometry, shape and materials.
Incredibly, the engine builders have been using the same specification of spring since 2006, even as the Indy cars have gone from 3.5 litre engines to a 3-litre mill, and traded methanol fuel for ethanol. The cam profile hasn't changed since 2006 so lift has been consistent. As with all of its suppliers - who remain unnamed for confidentiality reasons - HPD relies on their expertise during design and ongoing development.
"It's a two-way process with our suppliers," says Roger Griffiths, manager of HPD's development division. "You have to respect the fact that your supplier makes so many of them (valve springs). And they'll quite often have an idea. So we have those kinds of conversations all the time concerning recommendations. Sometimes their solution might be on the expensive side, however, so we ask if they have something else."
When HPD does make changes, it normally tests in-house before applying to any race engine. "On valve springs, even though we have the same design and the same supplier, whenever we get a new batch we test some to make sure they are durable enough to go the distance," Griffiths says. "We never put a new component or batch in an engine without signing off on it, using our durability processes."
The steel valve spring used by HPD for the Honda Indy V8 has some coating on it, he says, "And we replace valve springs with each engine rebuild. It's a progressive curve with valve springs.
"Obviously, the revs haven't changed, so that is a plus. When we did begin the Honda Button Push to Pass that gives a couple hundred extra revs (last year), we were concerned that the springs would be okay at 10,500 rpm as opposed to 10,300 (maximum revs)."
In order to verify there would be no issues with the 200-rpm change, where drivers can have the extra power for as much as 20 seconds at a time, HPD took what Griffiths describes as "a whole bunch" of used springs and tested them at higher engine speeds. "We didn't have any problems; it wasn't an issue, but it was something we wanted to make sure, in introducing the Honda Button, that with the higher revs we weren't going to compromise the reliability of the valve spring - or the entire engine. So we did the testing on the valve springs and we were happy with the result," he says.
The value in having more than one supplier - which HPD has for its valve springs - is the ability to find the right vendor for the right application. Also, in some cases, HPD might be concerned by the vulnerability of a supplier, so it would not be sourcing springs from a single source.
"We have found with both our suppliers - we have used US-based and foreign vendors - that they can make the components we need and are happy to do so," Griffiths says. "We've had no issues with our suppliers, and this is what it comes back to - we pay a lot of attention to selection and how we develop components with those suppliers."
Fig. 1 - Close-up of generic double valve spring
Written by Anne Proffit