Chevy Small Block Evolution - Performance Q&A

Kevin McClelland Feb 1, 2010 0 Comment(s)

Double-Hump Dinosaurs
Q: I'm old-school (just turned 60) and am building a 327 that has been bored 0.040 over. The block casting number is 3903352, with stock rods; pistons are Speed-Pro flat-top with two valve reliefs on each; a PN H660CP hypereutectic, 350hp cam with 0.447-inch lift and 222 degrees duration; an Edlebrock Performer intake; and a Holley vacuum-sec 600-cfm carb. I'm on a tight budget, and I can get a free set of 462 double-hump heads that are ready to bolt on, but I'm afraid they are 58cc and that will push the compression ratio too high to run pump gas. I also have a set of 461 heads, but they will have to be completely rebuilt. I also have a friend with a set of Vortec heads-he says they're just as good as the double-hump heads, but if I go with them I'll have to change intakes. I would like to use the 462 heads if possible because it will be by far the least expensive. If I could use these, what gasket would I need? Could you tell me the approximate power output of this combination? Thank you very much for your advice.
Larry Johnson
Via email

A: Old-school? Haven't you heard, 60 is the new 40! The 461s and 462s are almost as old as you are. Wouldn't you want to take advantage of some of the new technology out there?

First of all, the cylinder heads you listed (even the Vortecs) have combustion chambers between 62 and 64 cc. Making some assumptions on the deck clearance, valve relief volumes, and head gaskets, your compression ratio checks out at 9.1 to 9.3:1, depending on the 62- or 64cc heads. These compression numbers were calculated with a standard Fel-Pro PN 1003 performance head gasket that displaces 9.1 cc. This is a completely safe compression number for pump gas on your little 327. Except for the cost of the manifold, it's a no-brainer to go with the Vortec heads. They will pick up an easy 30 hp over either of the early double-hump heads.

If you wish to raise money for the Vortecs and the accompanying manifold you'll need, why don't you try selling your double-humps? They are going for good money these days on CraigsList ( and eBay ( from people who are restoring early iron. In good condition and with the proper date codes, they can bring a pretty penny.

Search around for the best price on a Performer RPM Vortec PN 7516. This manifold with the Vortec cylinder heads should produce somewhere around 350-360 hp on your combination, and around 350 lb-ft of torque. The original 350hp 327s may have been rated at 350 hp, but they were usually far from it. You will have one potent little Mouse with this package. Good luck.


Popular Shortbed
Q: I'm about to begin the buildup of a 468 BBC for my '69 Chevy C10 shortbed Stepside truck. I'll be building toward a Pro Touring style theme, and the truck currently has a stock peanut-port 454 out of a '78 3/4-ton truck and a Tremec TKO five-speed (3.27 First) with 3.73:1 gears in the 12-bolt rearend. I'm shooting for a reliable package with a nice spread of torque and good top end, since this build is for a 3,800-pound street/strip truck. Here is what I have acquired so far from top to bottom: a Holley 870 Street Avenger, an Edelbrock 7561 Air Gap RPM-O intake and 61459 RPM-O Heads (2.25/1.89 intake/exhaust valves, 110cc chambers and 290cc runners) with Crower 68340-16 springs and 10-degree locks/retainers, a Comp Cams X4278H cam, a Cloyes Hex-a-Just timing set, Crane full roller rockers, and Comp Magnum pushrods. I will try to get 10:1 compression with SRP -23cc dome pistons, swinging on stock truck rods, and a steel crank with ARP bolts throughout the bottom end of the two-bolt block. Headers are Hedman Elite 1.75-inch long-tubes and will dump into a 3-inch dual exhaust. Does this sound like a strong combination for my goal? What should I expect for peak power and torque? I have considered running a bigger cam to take advantage of the flow the heads offer, maybe a Comp XR274 solid roller, but I don't know if this will give me much more power or create a headache on the street, since I'll also be running power brakes and A/C. Thank you in advance. I love the magazine; keep up the good work!
Adam Reed
Little Rock, AR

A: You have chosen a very popular truck of late. These are bringing very high dollars on the open market, and their styling is timeless. Stick shifts in light little trucks can be a blast, especially with a big-block. Let's look over your combination.

When you run your truck over a set of scales you'll be very surprised what it weighs, probably at least 4,500 pounds! We think it's going to come in closer to 5,000, though, especially with a big-block on the nose.

Some quick math with your components gives you a compression ratio of 10.3:1. This is assuming your -23cc dome piston is 0.020 inch down in the hole, and you would use a Fel-Pro PN 1037 0.039-inch-thick head gasket displacing 9.7 cc. This is right at the max for compression ratio on the great fuel we are served these days. Small-blocks can handle this amount of squeeze better than a big-block. A manual transmission will give you the control over the load on the engine at slow engine speeds-this is where the engine would want to knock.

The foundation of your engine combination is very sound. We're a little concerned about the XR274 camshaft. You may want to run a larger 'shaft, but we'd stick with a little smaller one. You're going to move your truck with torque, not horsepower. Your current combination should make around 500 hp, with the torque coming a little higher. The 274 camshaft specs out with 230/236 degrees duration at 0.050 inch tappet lift. This cam is going to have idle vacuum in the low teens, and with power brakes and A/C it may struggle to idle well when the A/C is on. Getting the truck stopped is a whole other problem. The XE268 comes in with duration numbers of 224/230 at 0.050 inch. This slight change will help the idle stability and give you more area under the curve with a broader torque curve. The few extra ponies the 274 will give you will be at an engine speed you rarely use. Put the beef in the torque curve, where you need it.

Good luck with your build-up. Send us photos and an update when you get it done. The Readers Rides section needs a nice little street-brawler pickup.



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