Short and sweet question: I want a nice ‘thumper’ cam for the stock 283 in my ’30 Ford. Can you help me choose the right one?
T.W.
Your question is definitely to the point, but for better or worse my answers tend to be much longer. The reference you used would lead me to suggest one of COMP Cams’ Thumpr Series camshafts. While I like these cams, I don’t think one would be the right choice for your small displacement engine. I think COMP took the concept of valve overlap too far with these cams to obtain the characteristic idle ‘chop’ that everybody seems to want.
Learn How to Compare Camshafts by Valve Timing Events and Why It Matters!
Let’s investigate the concept of valve overlap and then we’ll get into choosing a cam for your 283. I’ll start with duration, which is the amount of time in crankshaft degrees that the intake or exhaust valve is open. As we increase duration, there is also a short period where the exhaust valve is nearly closed when the intake valve is opening. This is overlap. Assuming the same number of degrees of lobe separation angle, increasing duration on both the intake and exhaust will tend to increase the amount of overlap where both valves are open.
While more overlap creates that characteristic chop or staccato idle that everybody wants, it will kill some low-speed performance. Your 283ci small block Chevy doesn’t make a lot of low-speed torque to begin with, so using a big cam with lots of overlap will really kill what little it has. Having the engine in a lighter hot rod like a Ford Model A can help mitigate some of the worse effects.
You didn’t mention your transmission but I will guess it is probably a TH-350 automatic. Adding a big cam to a small displacement motor while using a stock or near-stock torque converter is a common mistake. The tight stock converter will tend to pull the engine down at idle and often cause the transmission to clunk into gear when shifted from Park to Drive at idle. A bigger cam will require a looser converter to make that transition smoother. I’m going to assume you don’t want to invest in a new torque converter so we’ll look at cams based on that.
For your situation, I would shoot for a cam between 218 and 224 degrees of duration at .050-inch with a lobe separation angle (LSA) of 110 degrees. The LSA can indicate how much overlap the camshaft has, but context matters. For example, a cam with 250 degrees of intake and exhaust duration and a 110-degree LSA will produce a much greater amount of actual valve overlap compared to a camshaft with 218 degrees of duration and the same 110-degree LSA.
We’ve listed several flat-tappet hydraulic camshafts in the Parts List that I think will work well in your combination. What they won’t do is create the big idle chop that you want. One thing I’ve noticed is that these little Chevy engines positively respond to a dual pattern camshaft with four to six degrees of added duration on the exhaust side to help improve higher RPM horsepower. Pay attention to the LSA; the single pattern listed cams below use a tighter LSA of around 106 degrees and the dual pattern cams use a 110-degree LSA.
You didn’t mention what other components are on your 283. The smaller cams will work very well with a dual plane intake like the Edelbrock Performer, a 600 CFM carburetor, and a set of headers. This combination will help low- and mid-range power without sacrificing too much top-end power.
I will assume you are running stock iron cylinder heads like the old factory Power Pack heads. These heads use a small 58cc combustion chamber to maintain compression. Many believe that adding heads with larger valves and ports will help improve power. The problem is heads with large valves also have larger combustion chambers; 64cc is a common size.
That difference may seem insignificant, but with the 283’s short 3.00-inch stroke, you need that very small chamber to create compression. A stock bore 283 with 58cc chamber heads, a 0.015-inch deck height, pistons with 6cc valve reliefs, and a 0.019-inch thick head gasket will make 9.2:1 compression, which is good.
Bolting on 64cc chamber heads with bigger 1.94-/1.50-inch valves will yield a small improvement in airflow, but compression will drop to 8.5:1 and kill power. If your engine already has these larger-chamber heads, I would strongly recommend replacing them with small-chamber heads to keep the compression up. I’d even mill the heads 0.010-inch to improve compression even more.
If you are dead set on a big cam that offers some thump, you might consider something like the Summit Racing™ Muscle Car cam that duplicates the factory cam from the 370 horsepower/350ci LT1 small block. It specs out at 242/252 degrees duration at 0.050 with .459-/.485-inch of valve lift. It really isn’t a good choice for drivability but will give you the chop you want. The cam will also kill some low-end torque but will scream above 3,500 RPM. You’ll need a looser converter and some carburetor tuning to make the cam work at part-throttle. It’s up to you to determine how much you are willing to sacrifice to get that choppy idle sound.
Parts List
Summit Racing™ Muscle Car Cam, 224°/224° @ .050“, .450″/.460″ lift SUM-1122
COMP Cams Xtreme Energy Cam, 224°/230° duration @ .050“, .477″/.480″ lift CCA-12-242-2
Howards Cams Flat Tappet Cam, 223°/223° duration @ .050“, .450″ lift HRS-112431-06
Melling M-Select Cam, 224°/224° duration @ .050“, .436″ lift MEL-22233
