December 2012 Chevy High Performance Q&A

Totally Broke(n)

Kevin McClelland Oct 22, 2012 0 Comment(s)

Flat or Dished

Q. I see some cylinder heads are available with different-size combustion chambers, and I’m wondering what would be the better way to go, larger chambers with flat-top pistons or smaller chambers with reverse dome pistons? Assuming all else is equal on a pump-gas Chevy small-block. Thanks!

Adam Hubley
Via email

A. Yes, cylinder head manufacturers have come to the rescue and offered multiple chamber volumes to allow engine builders like you to tailor their compression to the application. Common combustion chamber sizes for small-block Chevys are 58, 64, and 76 cc from the factory. There were many others, but you can see that there was a big gap between 64 cc and 76 cc. The aftermarket has adopted 72 cc as another common volume.

As for flat versus dished, we prefer the flat-top piston. With a dished piston you have less quench area to create mixture motion in the combustion space. Also, it’s tougher to completely evacuate the combustion space of spent fuel (exhaust) with a dished piston. That said, that’s volume in your fresh charge that cannot burn again, possibly reducing output. With a flat-top piston you can run a larger combustion chamber and do a better job of unshrouding the valves, which increases flow. You can also transition from the inlet port, across the back of the chamber, and into the cylinder better, which keeps the fuel in suspension with the incoming air. This reduction in separation also increases output by having consistent mixture throughout the combustion space.

We’re assuming you’re referring to small-block Chevy builds. With big-blocks it’s tough to build compression because the standard open-chamber cylinder head has a chamber volume of 119 cc. Even with 454 ci you must run a slight dome to make 9:1 compression.

Look at all the options on the market for cylinder heads and their chamber sizes, and then you can hone in on a piston head design

Officially Jealous!

Q. Keep up the great work with your articles; they are the first thing I flip to when I get my newest issue. I’ve been a Chevy guy my whole life and have spent years restoring many Bow Ties. I have a numbers-matching, all-original ’67 Camaro SS 350. Did this car come from the factory with multileaf or monoleaf springs? All the research I found told me that ’67 Camaros came with monoleaf springs. Mine has multi on it, and I would like to know which spring it came with from factory before I start my restoration.

Dave Bruley
Arlington, OH

A. Yes, all ’67 Camaros were equipped with monoleaf rear springs. The multileaf springs found their way onto the Camaro in 1968, when they also had the stagger shock option to help reduce wheelhop, but it did very little to control the spring wrap that causes wheelhop. In another attempt to control wheelhop, GM added the right rear axle radius rod option on limited L-48 (295 hp/350), and all L-78 (375 hp/396) engine packages. The 350 cars were equipped with a round radius rod, and the 396 cars had a stronger square tubing bar. The two radius rod upgrades were very similar, but on the big-block–equipped vehicles GM added a stop bracket, which mounted below the radius rod on the rear axle. This stop would come in contact with the radius rod when the spring would begin wrapping up and create a nice little traction bar. Unfortunately, GM only felt it needed this on the right side of the vehicle. It would have been very cool on both sides.

If you have multileaf springs, you may have a ’68 or ’69 12-bolt rear axle, as the spring perch brackets on the rear axle are different between the mono and multileaf springs. The depth of the perch accommodates the either four- or five-leaf multileaf spring, depending on the spring option and original build. The monospring perch is very shallow. The bottom line is with the rubber isolators installed between the spring and rearend, and the spring retaining plate, which incorporated the rear shock mount, will pull up flush to the rear axle spring perch.

Enjoy your resto. Send us in-progress photos—we’re still jealous.


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