I have a 1971 Chevrolet 307 that I’ve decided to hot rod. In my search to find the best setup, I’ve had many different answers, mostly from helpful suggestions to buy a 350 and just toss the motor in a ditch. What is the best setup for a 307 to get the most power I can out of it without blowing it up? — J.V.
Jeff Smith: Let’s start with this engine’s configuration. While universally panned, the 307’s layout is really nothing less than a “stroker” 283.
A 283 uses a 3.875-inch bore and a 3.00-inch stroke crankshaft. While the 283 was among the best-performing engines of its day and was certainly durable, it was also small.
So Chevy added the 327’s 3.25-inch stroke crankshaft to the 283 to produce a 307 c.i.d. Essentially, this was the opposite approach of the much-vaunted Z/28 302 that paired the 283’s short 3.00-inch stroke with the 327’s 4.00-inch bore.
While it is true that a larger 350 cubic-inch small block will make more power given its much larger displacement, it is certainly possible to make decent power.
Let’s use a standard of 1.15 foot-pound of torque per cubic inch as a measuring stick.
Any LS engine running on all eight cylinders can make more than this, but we’re limited by a small bore and weak (by current standards) cylinder heads.
But with 1.15 ft.-lb. per cubic inch, a 350 c.i.d. small-block will make 402 ft.-lbs. of torque. Let’s round that off to 400 ft.-lbs.
That means we could probably make 400 horsepower with that combo.
Using that same 1.15 figure, a much smaller 307 c.i.d. engine will only make 353 ft.-lbs. of torque. That’s 50 ft.-lbs. less torque and probably around 355 to 360 hp. That’s the price we pay to use a smaller displacement engine.
Of course, there are ways to make more horsepower by revving this smaller engine higher and by using big-port heads, but that does not fit within our goal of a mild, affordable street engine.
With the physics established, the 307 will respond to all the same techniques that work on any engine to improve performance.
The only real limitation to making power is the small bore and this isn’t really that big of a deal. But this does prevent the use of “big valve” heads because 2.02-inch intake valves physically will hit the side of the bore. The smaller 1.94-inch valves will work however.
My good friend, the late John Lingenfelter, once said that the key to any powerful engine design is a great cylinder head.
He told me you could put in a junk cam and the engine will still make power. So if you want to make power, we need to concentrate on a cylinder head that will flow a decent amount of air.
It just so happens that several companies are now casting small-bore small-block Chevy heads that use an intake valve that is less than 2.02-inches so they will fit the small bore.
If that’s still too much money, a set of stock Chevrolet iron Vortec heads can be located for a reasonable price, but these heads are limited to only about 0.4250–inch valve lift unless you modify the valve guides to allow valve lift to reach 0.500-inch.
Another option is an iron Vortec casting with 1.95/1.50-inch valves and 67cc combustion chambers. These heads have been modified to accept up to a 0.520-inch valve lift. The valve springs are somewhat low on the spring pressure in order to work with a mild performance flat tappet camshaft.
If you wanted to run a hydraulic roller cam, the springs would need to be changed. One major advantage to this head is that a pair of these complete castings can be obtained for just under $800.
Vortec heads also require a unique intake manifold mounting pattern which means these heads must be paired with an intake manifold that uses a Vortec bolt pattern. As an example, Holley makes a Speed Warrior, Edelbrock has a Performer, and Summit offers an intake that will bolt up to this Vortec pattern. These are all dual plane intake manifolds.
We’d also recommend a mild performance camshaft that would work with these components. In all of our dyno testing on small-block Chevys we continually returned to a very nice flat tappet camshaft offered by Comp that we felt worked very well.
This is the COMP XE268 cam with 224/230 degrees of duration at 0.050-inch tappet lift with 0.477/0.480-inch valve lift for the intake and exhaust. We never tried this cam with a smaller displacement engine like a 307, so this may be one step too big. This only means you could downsize by one step and go to the XE262 which offers 218/224 degrees of duration at 0.050 with 0.462/0.469-inch valve lift.
This smaller cam would still offer decent power gains but would also idle relatively smoothly and still deliver extra torque and horsepower over the stock cam which is probably roughly 190 degrees of duration and much less lift. Either cam will offer a major boost to power.
Keep in mind that Vortec heads also are designed to work with what are called guided rocker arms. The Vortec heads do not have a hole drilled in the head that “guides” or keeps the pushrod in place. Often these holes are not sufficient so engine builders went to guide plates.
What Chevrolet did on the Vortec head is extend the length of the valve tip above the valve spring retainer and then used what is called a guided rocker arm. These stamped steel rockers have two guides that retain the rocker arm over the valve tip.
We’ve included a Chevrolet Performance part number for an affordable set of these rocker arms. Of course, you can also go to full roller rocker arms but these will still need to be guided versions unless you modify the heads for guide plates.
Vortec heads also use a center bolt valve cover as opposed to the traditional perimeter bolt design. It’s another expense you will need to consider.
One last note is that Vortec heads require a specific style spark plug with a slightly longer threaded portion (called the reach) which is 0.708-inch instead of the shorter 0.480-inch reach spark plug used on must iron small-block Chevy heads. Be sure to use the longer reach spark plug or the engine will run really poorly because the operating end of the plug will be half-buried inside the threaded portion of the spark plug hole. This really kills combustion efficiency so make sure to use the right plug. We’ve included a Champion spark plug number, but any quality plug like AC Delco, Autolite, Bosch, or NGK will work just as well.
We also did a quick compression ratio evaluation if you were to use these heads. The stock iron 307 heads use a 70cc chamber while the Chevy Vortec heads are 64cc and the Summit Vortecs are 67cc. With a stock piston, we’ll assume the piston is 0.020-inch down from the deck, which is a common situation with ‘70s engines. With a steel shim head gasket and assuming 4cc in the piston valve reliefs, the compression for a 64cc head is 9.52:1 while the 67cc head is slightly lower at 9.2:1. These are decent compression ratios and should help performance that will work with 91 octane pump gas.
It won’t make as much as a 350 engine just because of its smaller displacement, but you can still build a decent street engine out of this 307. Have fun!