Free Parts Cost - Performance Q&A

Kevin McClelland Mar 1, 2010 0 Comment(s)

Nothing Is Free
I know I've written this before, but it hit me again: The most expensive parts you will ever get are the ones you get for free. Many times your buddy will help you out with some very cool parts-but they end up being completely wrong for your application and you spend a ton of money to use them. This time, the heads, manifold, headers, and wheels were free so I thought I was safe on this one, but I found a new way to break a lot of parts.

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As you have read in the past, I've had nothing but trouble getting the rings to seal in our Stock Eliminator engine for the wagon. I'm on my fourth build to seal this thing up and I've tried different brands of rings, as well as various cylinder wall treatments to match the "trick of the week" rings. After beating my head against the wall, I've gone back to standard back-cut, low-tension 5/64-inch-wide rings on my pistons. When I was reassembling the engine I was very confident that this was the answer. As I had hoped, the engine sealed right up and all eight spark plugs were dry as a bone. Success ... or so I thought.

We were at the track for our first test day and I noticed an excessive amount of blow-by through the breathers on the valve covers. We made a few passes down the quarter-mile, and it was time to start tuning the engine to find its best performance. With the engine safe on our AEM wideband air/fuel ratio gauge, I decided to advance the timing 4 degrees. The engine has a whopping 9.8:1 compression and is running on Rockett Brand 111-octane racing gas required by NHRA. Past experiences with these engine combinations have shown to make peak power at 42 degrees. So after advancing the timing to 42 degrees we made two runs-with no performance gain. After these runs the blow-by was worse and the engine began to crank through some cylinders. All I could think of is, what have I done now?

On teardown we found that it had broken the top rings on four of the eight pistons! Now, these were standard rings like I had mentioned above. How did the rings break? Sure enough, it came down to one of those free parts. A good friend gave me one of his off-shore SFI-approved lightweight dampers. What I found was that the degreed damper was incorrect. When I build my engines, I set the TDC with a dial indicator on the piston and make a pointer to ensure TDC. However, when I set the timing by the degreed markings on the damper to 38 degrees, it was really 42, which meant I'd advanced the timing from 42 to 46 degrees! All I can figure is that the engine rattled with the 46 degrees of lead and the extreme cylinder pressure broke the back-cut rings.

I thought I'd figured out just about every way to break something, but this was a new one. This is something to think about when you're using off-brand components to save money. If you have to go this route, be sure to always check them over with a fine-tooth comb!

Go Flat When You Can
Q: I have a '70 El Camino with a 350 engine. I had to replace a head gasket and saw it has dished pistons and 187 casting (76cc) heads. I want a bigger cam, but I need more compression. With the compression around 8 to 8.5:1, I was thinking of using ProComp 58cc heads to raise the compression. Which is the better combination: dished pistons with small chamber heads or flat-top pistons with larger chamber heads?

Dennis Stabler
West Hills, CA

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A: We will always go for the flat-top piston and the properly sized combustion chamber for the compression ratio needed for the application. The flat piston crown will give you the most mixture motion when they almost touch the deck surface of the heads at top dead center. The quench area forces the air and fuel into the combustion chamber, mixing the fuel into the air, which gives you a more uniform burn across the chamber.




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