Chevy Car Questions & Answers - Performance Q&A

Kevin McClelland Sep 1, 2009 0 Comment(s)

The problem is keeping the car running correctly. Under hard acceleration it pings like there's no tomorrow! Every time I take it back to the shop, they keep it a couple of days, change the plugs, and send me on my way. The engine has less than 300 miles on it. I have taken it to a couple of shops and they told me the carb and manifold should be changed to a single-plane with a 750 carb. Is this true? I've put a lot of money into this car and need some help. Also, what type of plugs do you run in angle-plug heads? Any advice you can give me would be greatly appreciated.
Ken
Via e-mail

A: Well, you've given us just enough information to make us dangerous! First let's talk about the heads. We would love to know if you have aluminum or cast-iron Pro Topline cylinder heads. The famed Racing Head Service is back in the cylinder head business. RHS, part of the Comp Cams family, has been quiet for several years and has had a resurgence with the acquisition of New Zealand's Pro Topline. RHS has renamed the cylinder heads "Pro Action" cylinder heads. This is important when it comes to spark plug selection. If you truly have the Kiwi cylinder heads and they are the cast-iron version, they accept a 14mm, 0.450-inch reach, tapered-seat plug. If the cylinder heads are RHS stateside iron castings, they are machined to accept the 14mm, 0.708-inch reach, tapered-seat plug. If you have the aluminum heads, either from Pro Topline or the new RHS Pro Action, both use a 14mm, 0.750-inch reach, flat gasket-style plug. Since you're having pre-ignition/detonation issues, you'll want to run a relatively cool heat range spark plug. All these plugs have an extended tip electrode.

Let's start with the aluminum head selection at 14mm, 0.750-inch reach, 5/8-inch hex gasketed plug. With NGK plugs, you need a BCPR7ES, Champion RC9YC, and in AC pickup, a FR1LS. As for the Kiwi-built iron heads, which take a 14mm, 0.450-inch reach, 5/8-inch hex tapered seat, use NGK R5674-7, Champion RV9YC, and AC is R41TS. Finally, if you have the newer Pro Action RHS head, you need the 14mm, 0.708-inch reach, 5/8-inch hex tapered seat. Pick up an NGK BPR6EFS, Champion RS9YC, or AC R42CLTS. This should put the proper heat range into your engine for a baseline. You may need to move up a heat range based on the performance, but we're starting to the cold side based on your pinging issues.

Now for your engine rattle. First of all, either you have too hot a plug (hopefully we took care of that above), you're running too much ignition timing for the cylinder pressure, and/or you do not have a high enough octane fuel. Also, the engine could be too lean on your fuel mixture. If your engine is equipped with cast-iron cylinder heads with the 10:1 compression, it's going to be tough to keep this engine out of detonation at peak power spark advance with the current pump super unleaded of today. Yes, this engine may have made 450 hp on an engine dyno under controlled conditions, but bolt that engine in a car, heat sink the engine, and pull around a 3,800-pound Chevelle, and she'll knock.

Is the RPM Air-Gap and your 650 Speed Demon out of place on your engine combination? Absolutely not. We've made over 460 hp and 444 lb-ft of torque with a 350-cid small-block. Yes, this was with a 750 carb, but you're right in the ballpark with the 650 cfm. A Victor Jr. and a 750-cfm carb would run well but lose some midrange torque for the gain of a few horsepower

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