NHRA Chassis Certification - Performance Q & A

Kevin McClelland Feb 15, 2010 0 Comment(s)

Kevin McClelland

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All Cracked Up!
When was the last time you checked all the welds on your race car? Probably never. NHRA requires that the chassis of a race car that goes 9.99 seconds or quicker, or 135 mph or faster, to be certified every three years. This certification is an inspection of the thickness of the chassis tubing with a sonic tester. There are a multitude of specs for the different diameter tubes and their location in relationship to the driver.

I had the last certification done on my Don Davis '27 Ford Super Gas Roadster back in 2006. This year it came up for recertification, and NHRA has changed the SFI specifications for this type of side-steer Roadster, now requiring helmet bars in the rollcage, rollcage gussets, and several new bars in the driver area. The car was originally built back in 1995 and I'm the third owner. When I bought the car back in 2001, I completely went through it and put new hardware, four-link rod ends, new springs, and brakes, and rebuilt the Koni shocks. I've done regular service to the rest of the car over the years.

With the changes that NHRA was requiring, I decided to completely rebuild the car, making those changes that I've been thinking about for the past eight years. The car is completely down to the bare tubing. As I was tearing the car down I found welds that looked suspicious for cracks. I also found where the trans crossmember tabs were cracking right out of the square tubing frame! The chassis is currently at the sandblasters having all the paint and primer stripped off. This will give me complete access to all the welds to inspect for irregularities.

I'm finding this on a standard mild-steel chassis. Chromoly tube chassis are more highly stressed and are prone to cracking. Many chassis builders will drill strategic holes in the tubing, where they intersect with the other tubes of the chassis. These holes will create an air path throughout the entire chassis. If you plan out the build of the chassis you can install a 1/8-inch pipe bung in the tube so you can pressurize the chassis to check for cracks. This is a very simple and effective way to inspect and repair your race car.

Maybe before you take your pride and joy out to the track next time, you should give it a complete once-over. Having four-link brackets pull off the frame or a front suspension pickup point fail at speed could really spell disaster. Take the time to make sure that you and your toy are safe.

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Loaded Question
The driving force behind this is the article in the Dec. '09 issue, "LS Motors Decoded." So in the interest of putting some amazing 21st century technology into my '68 Corvette while keeping the original factory 427ci she was built to contain, I have a question that I'm sure has been at the back of everyone's mind: What are the maximum bore/stroke combinations that these Gen III and IV LS engines can take? Thank you in advance. Jason Bellevue, NE

This is truly a loaded question. There is a displacement max for factory production engines. Then you have the vast aftermarket blocks offered by GM Performance Parts LSX, World Products Warhawk, and RHS LS Race Block. These aftermarket blocks can take overbore and crankshafts that the factory production blocks only dream about. Putting a 4-inch stroke crankshaft into a production block is old hat. We've heard of windage problems with stroke lengths over 4 inches, but there are many buildups out there using factory blocks with 4.250-inch strokes. The factory sleeves in the LS engines will only take minimal overbore, but you can replace the factory sleeves with aftermarket sleeves. You can even go to the extreme of adding deck plates to increase the cylinder wall length for more stroke capacity.