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Speed Secrets: Less Valve Lash = More Horsepower?

Speed Secrets with Richard Holdener has always had an unrelenting focus on the big parts of engine building, like camshafts, cylinder heads, intake manifold, and so on. However, there is more to achieving the right setup than just choosing the correct big ticket items. In an effort to present the affects the little details have on an engine, Richard turns his attention to valve lash adjustments and pushrod length. 

In 1971, GM lowered the static compression ratio from 11.0:1 to 9.0:1 due in part to a change in cylinder heads with a much larger compression chamber, making a 1971 LT1 Small Block Chevy engine a good starting point. Further adding to its appeal, this SBC features a 4 bolt main block, a steel crankshaft, and forged pistons. Comp Cams supplied a number of parts for this engine, including guide plates and rocker studs. Comp also supplied a set of stamped steel long slot rockers for use with the Duntov 30-30 solid lifter camshaft. A camshaft that acquired its name from the recommended valve lash of .030 inch intake lash and .030 inch exhaust lash. 

It is known from experience that the recommended lash setting is a bit on the loose side. With that in mind Richard started this builds off at .024 inch of lash and ran the LT1 on Dyno with a period correct dual plane high rise intake manifold, 750 carburetor, and long tube headers. With a baseline of 361 HP @ 5,900 RPM and 375 Lb-Ft of torque @ 4,400 RPM the question at hand was, what difference could be expected from further tightening the valve lash?

Reducing the lash down to .012 inch resulted in a power gain up top with a penalty of a lose in low speed torque. Although, after adjusting the lash it was apparent that the pushrod length was a little on the short side. So Richard replaced the 7.8 inch pushrods with longer 7.9 inch pushrods. This had a similar result as the lash adjustment. The LT1 engine picked up top end power but lost low end power. Leading to the undeniable conclusion that this is what you can expect to find when you go looking through the little things on the Dyno. 

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