NHRA Chassis Certification - Performance Q & A

Kevin McClelland Feb 15, 2010 0 Comment(s)

As for the maximum engine displacement of an LS engine design, the GM Performance Parts block comes in the winner at a max bore of 4.250 inches and a stroke length of 4.50 inches. With this combination you will have a displacement of 511 ci! Now that's one nasty small-block.

With a production Gen IV engine block of 4.060 inches and a 4.125-inch stroke crank you achieve a displacement of 427 inches. If you step up with an LS7 block, with its 4.125-inch bore, or sleeve a production LS1 with the 4.250-inch stroke, you will see 454 inches of nastiness. Hope this has whetted your appetite.
Sources: dartonsleeves.com, gmperformanceparts.com, racingheadservice.com, scatcrankshafts.com, worldcastings.com

Aussie Runner
If you can help me with this one-off special I'd be very grateful. I'm not trying to reinvent the wheel, but here goes. To satisfy the rules of the Lawmen we are restricted to a 308-cid engine, so we want to use a few legal and irregular options based on a 350 block and a 3.1-inch-stroke crank which should fit nicely (I hope). Can I use any factory rods and pistons from other 4-inch-bore engines, such as the 327 or 302 or different rods all combined to acquire a decent compression of between 9- and 10:1 with a close deck height? The cylinder heads would be 461s and the cam would be 222/222 from the 350/290hp series flat-tappet, which is plenty for my needs. The carb would be a Barry Grant 575 Demon and I would use the standard GM HEI. Hope you can help, and all the best from Noel in Oz. Noel Foreshew Sydney, Australia

Where did you come up with a 3.10-inch-stroke crankshaft? Here in the States GM only used that in Gen I engines in 1975 through very early '77 in Chevy Monzas and Novas in their 262ci small-blocks. This engine was very short lived because of its weak 110hp rating. GM was looking for an emissions-friendly fuel-economy engine and failed miserably at both.

Down under, Holden may have used this combination longer for you to have your hands on one of these cranks. Or do you have a specially ground crankshaft that was used in some type of racing engine? Either way, you could use factory 302 pistons for your 308 buildup. The only issue is that the pistons are going to come up out of the deck by about 0.025-inch. You can have a set of factory 5.7-inch small-block rods offset-bushed 0.025-inch down and shorten the rods to 5.675-inches. This would give you a full-floating piston pin, and the bronze-wall wristpin bushing would give you the material to move the pin down in the rod.

Going with a slightly shorter connecting rod will allow you to use factory 302 small-block pistons. They had nice forged TRW slugs from the factory with an approximately 0.125-inch dome, which gave you 11:1 compression with 64cc combustion chambers. Speed-Pro now only offers one piston with a 12:1 dome. For a really nice piston check out Wiseco Pro-Tru forged pistons, PN PT042H, offered in 0.030-, 0.040-, or 0.060-inch overbore. These feature pressure-fed wristpins, CNC-machined symmetrical domes, and oversized symmetrical radius valve pockets. They come in at 11.1:1 compression with a 64cc combustion chamber. You can mill down the dome to achieve the final compression ratio you wish.

Hope this keeps the Lawmen happy. Good luck with your little Mouse.
Source: wiseco.com

Hot Big-Block
As an avid reader, I really appreciate the effort you put into helping people with their problems. I was hoping you could help me with mine. My '68 Camaro has a 325hp 396 with 5,000 miles on it. On hot days, say 90 degrees F and up, if I get stuck in traffic the temperature climbs from 180 until it overheats at 230.