I’m looking at building a 307 small block Chevy. With the cam I want Summit Racing says I have to get the compression up some. I’m currently looking at a set of 327 heads with 64cc chambers to get at least 9:1 compression using 1.94 inch intake valves.

Will these head work on my 307?

J.S.

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The short answer to your question is that yes, those heads will work. But as usual, there’s more to the story.

Let’s start with some basic information first. The 307 is really just a stroked 283. The stock bore size is 3.875 inches with a stroke from a 327 at 3.25 inches. So what we have is a small bore, long stroke small-block as opposed to the 302 Chevy engine which used the 327’s 4.00 inch bore but was de-stroked with a short 283 3.00-inch crankshaft

While the 307’s small bore tends to shroud breathing slightly, the longer stroke is helpful for making compression. We will assume here that this engine will retain its stock flat top pistons. With a flat top piston with four valve reliefs, a 64cc cylinder head, a stamped steel gasket, and a piston deck height of 0.015 inch below the deck the compression ratio ends up right round 9.2:1.

The above combination assumes 64cc style 327 cylinder heads with 1.95/1.50 inch valves. A larger intake valve won’t fit inside that small bore so don’t be tempted to enlarge the intakes. Those older 327 heads like the 462 heads are certainly okay but you might consider using a set of later, Vortec style heads instead. The Vortec heads were first used on small blocks in the early 1990s and retained the smaller 64cc chamber and the same 1.94/1.50 inch valve sizes.

The difference is the intake flow characteristics.

The Vortec heads flow significantly better on the intake side and if you want to do some minor port work on the area right below the exhaust valve seat by removing the sharp edge on the short turn radius side of the pocket, these heads can be worth additional power. These Vortecs flow much better than those older small block heads.  

pistons inside an engine block
A stamped steel head gasket will generally be 0.015 inch thick which will help tighten up the piston-to-head clearance which improves compression. These gaskets are fine with cast iron heads but do not seal as well with aluminum heads. These gaskets are especially helpful when the piston at TDC is more than 0.020 inch below the deck. (Image/Jeff Smith)

Upgrading to OE-Style Vortec Heads

While you might be able to find a used set of Vortec heads, be careful as these heads often had cracking problems. But even if you find a decent set that are not cracked, a valve job with new guides, valves, springs, and seals will still set you back $800 or more depending upon how much work is put into the heads and the quality of the parts. For a bit more money, you can have a brand new set of Vortec heads from Summit Racing ready to bolt on.

All Vortec heads use a completely different intake manifold bolt pattern that requires its own specific intake manifold since it only requires four bolts per side as opposed to six for the traditional small-block. We’d recommend a dual plane intake manifold for your combination. Edelbrock offers several but they can be pricey. Summit Racing offers a very affordable dual plane intake specifically for the Vortec head that will also do the job.

The next thing is that Vortec heads also demand a center bolt valve cover compared to the traditional perimeter bolt valve covers. Plus the Vortec also uses a guided rocker arm that uses small guides built into the rocker arm tip that straddle either side of the valve tip to locate the rocker over the valve tip as opposed to guide plates. We mention these things because these will be required parts to use with the Vortec heads. But we feel that the investment is very much worth it for the additional power the engine will make.

You may appreciate this article: Are Vortec Truck Cylinder Heads Really Better Than Stock SBC Heads?

Understanding Your Camshaft

You didn’t mention which camshaft you intend to run but because Summit Racing has suggested increasing the compression, we will assume that the camshaft will be aggressive. We’ll assume you are building a daily driven street engine but unless your car is equipped with a manual trans like a four or five-speed and has gears deeper than 3.08:1. We suggest keeping the duration on the camshaft in question to no more than about 220 degrees at 0.050 for the intake and roughly 226 degrees on the exhaust.

Even this is pushing it for engine that is only 307 cubic inches and barely 9.2:1 compression. If you want more cam to sound good on the street, it’s best to push the compression up to around 9.75 to 10.0:1. Longer duration camshafts tend to bleed off cylinder pressure due to the overlap between the intake and exhaust where both valves are open simultaneously. The longer 3.25-inch stroke will help slightly, but with an engine only displacing 307 cubic inches, it would be best to shoot for a camshaft closer to around 215 to 218 degrees on the intake at 0.050 inch tappet lift.

A shorter duration camshaft will improve low-speed torque and throttle response at engine speeds below 3,000 rpm. A shorter cam will not sound as aggressive as a longer version, but overall it will perform better and you will likely be happier with the engine’s overall street performance. Keep in mind that you are building a small-block that is barely 300 cubic inches. It will not feel nearly as strong as a 350 purely because of its lack of displacement. But as long as you are content with its less than powerful status, then this combination of a set of Vortec heads at 9.2:1 and a cam of around 218 degrees of duration at 0.050 with a dual plane intake will certainly run very well.

In terms of power, you might see 340 horsepower with the above combination but not much more. To put all this in perspective, a bone stock 4.8L LS truck engine (293 ci) with a carbureted intake will make more horsepower with the same camshaft but not quite as much torque because its 16 cubic inches smaller yet.

The difference is in the cylinder heads as the LS heads are that much better!

Vortec Head Conversion Parts List

  • Summit Racing Vortec cylinder heads (x2) – SUM-151124
  • Edelbrock Vortec Performer RPM dual plane intake – EDL-7116
  • Summit Racing Vortec dual plane intake – SUM-226062
  • Summit Racing Stage 1 Vortec dual plane intake – SUM-226018
  • Blueprint Engines dual plane intake (w/gaskets) – MLL-BPP350VINT
  • SCE Vortec intake manifold gaskets – SCE-111127
Keywords
307 , Ask Away , small block Chevrolet , vortec cylinder heads
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Author: Jeff Smith

Jeff Smith has had a passion for cars since he began working at his grandfather's gas station at the age 10. After graduating from Iowa State University with a journalism degree in 1978, he combined his two passions: cars and writing. Smith began writing for Car Craft magazine in 1979 and became editor in 1984. In 1987, he assumed the role of editor for Hot Rod magazine before returning to his first love of writing technical stories. Since 2003, Jeff has held various positions at Car Craft (including editor), has written books on small block Chevy performance, and even cultivated an impressive collection of 1965 and 1966 Chevelles. Now he serves as a regular contributor to OnAllCylinders.