Picking a Predator 212 Cam and Springs

The cam is where a 212 stops being a 9-horsepower engine, and it is also where most of the money gets wasted, because the cam catalogs sell rpm and the engine cannot use rpm it is not built for. A cam card has four numbers that matter. Learn to read them and the choice mostly makes itself.

The four numbers

Lift. How far the valve opens. The stock cam is a low-lift grind; the aftermarket runs from about .265" (GPS HOT265, FIRE265) through .308"–.310" (GPS .308, DynoCams 308/310) to .335"–.356" (DynoCams 335, 356, GPS .356). On a small head with small valves, lift is the number that matters most below about .300", because the port is not the restriction — the valve opening is.

Duration at .050". How long the valve is open, measured where it counts. DynoCams publishes it: the 255 is 236/236, the 275 is 240/239, the 308 is 244/245, the 310 is 255/260, the 335 is 260/265. More duration moves the powerband up and costs bottom end. On a 212 that is geared for a go-kart or a mini bike, bottom end is what gets you off the corner.

Spring pressure. The cam maker tells you what springs the cam needs. DynoCams' cards say 18–22 lb for the 255, 26 lb for the 275, 32 lb for the 308, 34–36 lb for the 310 and 315, 36 lb for the 335 and 356. Run a cam on lighter springs than it calls for and the valves float below the cam's peak — the dyno curve just stops climbing, and you paid for rpm you never see. Run heavier springs than it calls for and you wear the cam and lose power to friction. The spring is part of the cam, not an accessory.

RPM ceiling. Also on the card: 7,000–7,200 for the 255 and 275, 8,200–8,400 for the 308, 8,500–9,000 for the 310 and up. This number is a promise about the cam, not the engine. The stock rod does not care what the cam card says; the rod guide covers that.

Matching the cam to the rest of the engine

Governor in, stock rod: the stock cam. The dyno sheets show the stock cam done at 5,000 rpm and the governed engine done at 4,700; there is nothing here for an aftermarket cam to do, and a cam that needs springs and rpm will make the engine worse in the range it can actually reach.

Billet rod, governor out, stock head, Stage 1 carb and pipe: a mild grind — GPS HOT265 or FIRE265, DynoCams 255 or 275 — with the springs the card names. These make the engine pull to 6,500–7,000 and keep the low end the stock carb can feed. The cam-swap page and the stock-vs-HOT265 page run the number on your build.

Rod, 22mm or VM22 carb, ported or milled head: the .308"–.310" class. This is the cam that wants the carb and the head, and it wants an ignition coil with a rev limit set under the rod's rating. The DynoCams 255 vs 310 page shows how the peak moves up when the engine can breathe.

Ported head, 24–26mm carb, race pipe, race fuel: the 335/356 class and the GPS .356. These are race cams with race maintenance and 36-lb springs, and they belong on an engine with a rod, a flywheel, and a coil that can all live at 8,500. The 335-vs-400 page is the top of the ladder on this site.

A note on what is and is not anchored

The stock cam and the Stage 1 kit are anchored to Red Beard's Garage's sheets. The aftermarket cams on this site are modeled from their published cards — lift, duration, spring, rpm — and from the anchored engines around them. No one has yet put the DynoCams or GPS grinds on a dyno with a stated correction factor and the sheet on screen; when they do, those combinations get checked and the pages get corrected. Until then, the cam pages are estimates and say so. What is not an estimate is the direction: every cam listed above makes more power than stock once the rod, springs, and carb can support it, and none of them does anything useful on a governed engine.

Do not skip

Springs matched to the card. A rev-limiting coil once the engine can reach the cam's ceiling. Valve lash set after the springs go in. And on the .308"-and-up cams, check piston-to-valve clearance if you have milled the head or run a thin gasket — a small engine has small margins.

Build the engine in the calculator and change only the cam. Then change only the springs. If the number moves on the springs and not the cam, you found the restriction.

Sources: DynoCams and GPS published cam specifications as listed in the simulator; Red Beard's Garage — Road to Horsepower Ep. 1 (tQjitfEMDxM) and Ep. 2 (vaL4-d5Fz70) for the stock-cam anchors. No published, correction-stated dyno pull yet exists for the aftermarket grinds; the cam pages on this site are labeled as estimates for that reason.

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