396ci Small-Block - Tech

This popular big-block displacement comes in a new flavor!

David Vizard Jan 10, 2011 0 Comment(s)
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Chevy stroker motors have been with us since almost day one but are hardly what you would call common. The advent, back in the ’80s, of cheap 400 cranks with reduced main bearings to fit a 350 changed the stroker scene forever. The 400’s 3.75-inch stroke gave a 0.030-over 350 some 383 cubes and, when done right, delivered satisfying results. So popular was this 350 stroker conversion that the supply of 400 donor cranks soon became an issue but Scat stepped in and saved the day by introducing a superior cast-steel crank at a cost less than a stock 400 crank. Building a stroker 350 was now so cost effective that the 383 became a major part of the Chevy small-block culture.

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IHRA Pro Stock racer Terry Walters of Terry Walters Precision Engines (TWPE) in Roanoke, Virginia, was one of those caught up in the 383 popularity wave but, unlike most, he got to build and dyno a lot of 383s.

As a result of his engine building skills, Walters’ Pro Stocker was one of the first into the 200-mph club. Such results on the asphalt dyno tell us in no uncertain terms that TWPE must be somewhere close to the top of the field when it comes to building power. In simple terms, winning performances in such a competitive arena are achieved by continually pushing the envelope. That same mind-set applies to the engines that Walters sells to his customers. For those who could afford the additional cost of a forged crank even more inches from a 350 were on the table, still at a cost-effective price. A 4.00-inch stroke in a 350 means that 408 inches are possible but Walters is not a fan of this combo. Here’s why and it’s proof that little in the way of optimal engine component combinations is simple.

Bigger Is Not Always Better

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If a 408 is possible, then why go for anything smaller? First, stuffing a 4.00-inch crank into a 350 block is way more problematic than say a 3.75-inch stroke as per a 383. Roughly speaking, four out of every 10 blocks fail the water jacket pressure test when the bottom of the bores and the pan rails are cut to clear a 4.00-inch stroke crank/rod combination. For a 383 only about one out of 10 fails the pressure test. This means anyone contemplating a 408 has to pay part of the cost of a 40 percent block scrap rate. This considerably ups the price for block prep of a 408.

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The next factors to consider are those of geometry. Cubes gained from a stroke increase have a small but, at the limit, significant negative in the way of piston friction. With a 4.00-inch stroke a 6-inch rod is about as long as is practical. It delivers a rod-to-stroke ratio of 1.5:1 and that is getting to be a little short. The short rod-to-stroke ratio means more piston side thrust and this, added to the fact that the crank is going to drag that piston farther up and down the bore, simply compounds the friction situation. The result is that although a 408 can deliver, getting the best from a 4.00-inch stroke is far more costly and the gains are disproportionately low to a combo with a little less stroke.

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