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Small Block Chevy Angle Plug Combustion Chambers - Ask Marlan

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Straight Talk on Angle Plugs
What is the difference between a small-block Chevy angle-plug and straight-plug head as far as performance goes? Is it worth spending the extra money on expensive headers just to use angle-plug heads (especially if you already have two sets of straights)?
Mike Cushing
Chicago, IL

On small-block Chevy angle-plug combustion chambers, engineers moved the plug tips closer to the roofs of the chambers, supposedly to improve combustion efficiency and gain power. The first heads to feature the angle-plug configuration were a special parts-counter-only version of the conventional production straight-plug, 64cc chamber, "492" casting. Other than the angled plug location, the casting was identical and carried the same "492" casting number; hence, it offers a useful yardstick for comparing just the effects of altering spark plug location. The consensus was that original angle-plug heads were worth 10-12 hp on high-compression engines using high-dome pistons because they reposition the plug higher in the chamber so the flame front is no longer blocked by the piston dome. In fact, angle plugs made practical the use of even higher domes (with correspondingly higher compression ratios); previously, really big domes on small-blocks often proved counterproductive because of the flame-front blockage problem.

Because the angled plug's firing tip also ends up more centered in the chamber and oriented slightly toward the exhaust valve, the hottest part of the mixture is ignited first. With the plug located closer to the chamber's quench area, maximum turbulence is directed toward the plug, thereby improving combustion speed and pressure rise. In theory, this improves combustion efficiency even on engines without domed pistons, but on a street engine any power gain that results from changing spark plug location in an otherwise identical head is so slight it's not worth the hassle of conversion.

The angle-plug version of the 492 heads is no longer available, having long ago been superseded by modern GM and aftermarket angle-plug, off-road heads with redesigned intake and exhaust runners and higher-efficiency chamber shapes. In the modern era, Chevy also installed angle-plug heads on production L98 and LT1 aluminum-head engines. Whether off-road or production, these heads feature many improvements besides a revised spark plug location, so comparing today's modern high-efficiency angle-plug castings with old-style straight-plug castings is like comparing apples and oranges. Although, as we've seen, spark plug location alterations aren't that important for a flat-top piston engine, the modern-tech heads' other refinements can definitely offer a significant performance enhancement on a serious performance engine.

Clearance Sale
I transplanted a TH350 in place of my original Powerglide trans. My problem is that the original solid linkage interferes with the speedo cable. The new trans is correctly geared for my 3.42:1 rearend, so I just need a 1.0:1 speedo box to offset the speedo cable input. I've been to several local trans shops and no one sells one. If you could point me in the right direction I'd really appreciate it.
James Lee Garrison
Mesa, AZ

A miniature 90-degree offset 1.0:1 speedo adapter box requiring only 1 1/2 inches of clearance is available from Lance Martin Automotive. The 90-degree leg can be reclocked in relation to the other leg as needed. One drawback is that miniaturization makes the unit more costly; it'll set you back about $80. Alternatively, you could always install an aftermarket floor shifter with cable linkage.

• Source:
Lance Martin Automotive:

Dept. CC:
815 Hilbert Rd:
Fallbrook, CA 92028-1608:

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