Q&A

Ask Away! with Jeff Smith: Picking a Cam & Cylinder Heads for a Mild Street Small Block Chevy

I am building a mild street engine and researching my existing heads for air flow. The casting number on my cylinder heads reads 10147898: It appears these are 1995 through 1998 350ci crate engines, 78cc chambers, with 1.84/1.50-inch valves.

Are these heads okay to use? I’m also not having any luck making a decision to choose the proper cam.

Thanks

D.S.

Let’s also take a look at your cylinder heads. The 10147898 heads were used in late 1980s and early 1990s Chevy light duty trucks. They have a large 76 to 78cc combustion chamber and small valves. These are not considered a good flowing cylinder head that would make decent power. There are a number of far better choices for heads in both iron and aluminum.

My good friend the late John Lingenfelter once told me that you can make great power with good heads and a less-than-ideal cam, but you have no chance to make good power with a great cam and junk heads. The main reason the entire LS family of engines make outstanding power is because the factory LS heads offer excellent airflow.

Upgraded Cylinder Heads for a Small Block Chevy

The best way to improve power on your small block Chevy would be to not attempt to save those existing heads. Assuming your 350 small block Chevy is using flat top pistons with valve reliefs, a cylinder head with a smaller 64cc chamber will immediately help by adding compression. If your budget is tight, I’d suggest a set of affordable iron Vortec heads. These heads flow much better than almost any production Chevy head. These have become so popular that now several companies offer a new casting for an affordable price.

Summit Racing offers an iron Vortec head (part number and link at the bottom of this post) that is a brand new Vortec casting. The Vortec heads were originally used in late 1980s trucks and feature a 64cc chamber and 1.94/1.50 inch valves. These heads do demand their own unique Vortec style intake manifold bolt pattern. That adds to the cost of the conversion if you already have a small block Chevy performance intake manifold since a traditional small block intake will not fit the Vortec heads for a number of reasons.

The Vortec heads also employ what are called center bolt valve covers using four mounting bolts in the center of the valve cover as opposed to the perimeter style valve covers used on traditional small blocks. Your current heads may already be center bolt and if so, their valve covers will work. The Vortec heads also use what are called guided rocker arms. These use two small nubs or guides stamped into the rocker arm tip to maintain the rocker arm on the valve since these heads do not use guide plates.

Summit Racing offers a Vortec style dual plane intake manifold that will work with the Vortec heads and we’ve included a part number in the Parts List at the end of this answer. Of course, Edelbrock, Holley and a few others also make manifolds if you choose to go that route. A dual plane style intake is the right choice for nearly all mild street applications as opposed to a large single plane manifold.

We mentioned that these smaller chamber heads will also improve the compression ratio and this is one reason why the Vortec heads are a good choice. With a 76cc chamber head on a 350 Chevy, your compression ratio will be around 8.5:1 assuming the use of a 0.041 inch thick head gasket and the piston is between 0.015 to 0.020 inch below the deck surface. Just by changing to smaller 64cc heads, the compression ratio improves from 8.5:1 to 9.7:1 with no other changes.

A full compression ratio increase is worth between three and four percent of power. So if your engine was making 350 hp and you increase the compression ratio by one full point—this will result in adding roughly 15 to 365 horsepower. Plus, fuel mileage will also likely improve, as will throttle response due to improved combustion efficiency.

While compression helps, the Vortecs also improve airflow which the above example does not address. The Vortec heads also flow dramatically better than your existing heads. So this might add another 10 to 15 hp with the Vortec heads and now that 350 hp baseline is now closer to 375 to 380 hp—or roughly a gain of 25 to 30 hp!

Selecting a Camshaft to Match the Cylinder Heads

Now, if you want some input for a camshaft to go along with a set of Vortec heads, I’d choose a camshaft with duration numbers at 0.050 between 214 and 224 degrees. This is a mild duration that will offer good low-speed torque while improving top-end power. Our recommendation would be to go with a dual pattern cam which means the exhaust duration would increase 6 to 10 degrees over the intake duration. This will help peak power without sacrificing mid-range power.

Along those lines, Summit Racing (as one example) offers a hydraulic flat tappet cam with these specs:

  • 214/224 at 0.050
  • 288 / 298 advertised duration
  • 0.444 / 0.466 inch lift intake and exhaust. 
  • 112 Degrees Lobe Separation Angle

The lobe separation angle (LSA) is 112 degrees, which will improve idle vacuum to make this cam idle happily with perhaps around 14 to 15 inches of manifold vacuum. This should be enough to allow the power brake booster to operate properly.

With this cam, a set of Vortec heads, a tall dual plane intake manifold, a 650 to 750 cfm Holley carburetor, and headers, this engine could pull down around 400 to 425 hp at around 6,000 rpm. We’ve built several small block Chevys with similar combinations and that’s nearly always the power they produce.

The nice thing about an engine like this is it offers decent overall power, excellent throttle response, and great street manners while still making over 400 hp. This would make an excellent street engine and you don’t have a ton of money invested in it. Of course, this assumes the short block is in good shape and piston ring seal is good.

Cam, Intake & Cylinder Heads Parts List

  • Summit Racing Iron Vortec Heads (x2), SUM-151124
  • Summit Racing Vortec Dual Plane Intake, SUM-226018
  • Summit Racing Flat Tappet Hydraulic Cam, SUM-1103
The Vortec heads offer an affordable way to make good power with a mild street small block. List price for a pair of these heads is very affordable and they are ready to bolt on. As we mentioned in the answer, these heads will require a specific Vortec intake manifold. (Image/Summit Racing)
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One Comment

  1. Hi Jeff, Interesting article. How would this answer change if the engine is equipped with EFI? Or, is it better to change the EFI over to a carburetor?

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