Derrik Holmes Asks... How Does a Mopar Big-Block 505 Stroker's Larger Size and Better TFS Heads Affect the RPM Range and Overall Behavior of a Cam Originally Designed for a Standard 440 with Iron Heads?
Hot Rod|December 2020
In Mopar world, 505ci big-block stroker engines are among the most popular builds today, with seemingly boundless mild to wild combos available. You can build a 505 Chrysler for not much more than a 440, so it’s almost a no-brainer. Our late sister mag Mopar Muscle’s “Project 505” made 683 hp back in 2009 on this fairly high-end build.
Marlan Davis

Q: I’m planning a Mopar 440 to 500ci stroker build. One thing I have had trouble picking out is the camshaft. The engine will be going in my 1970 Challenger that I want to be streetable but with a healthy lope. I’m looking at a hydraulic roller so I can have the largest usable rpm range. How does the larger displacement affect the behavior of the cam? If the cam is listed to have an rpm range of 2,200 to 6,000 rpm for a 440, will that change for a stroker? Should I be shooting for more duration or lift to accommodate the extra displacement? When do you pick higher ratio rockers versus just getting a cam with more lift? For background, I plan to use Trick Flow heads and an Edelbrock Performer RPM intake manifold with a Holley Sniper EFI system. This will be my first engine build, and I will be buying the stroker short-block from an engine builder.

HOLMES’ 505CI CHRYSLER ADDITIONAL INFORMATION

Direct conversation with Mr. Holmes returned this additional information.

Short-Block

Muscle Motors Racing short-block

Displacement

505.3ci

Bore x Stroke

4.35” bore x 4.25” stroke

Connecting Rod

7.100” center-to-center

Rod/Stroke Ratio

1.67:1

Cylinder Heads

Trick Flow Specialties PowerPort 240

Combustion Chamber

78cc

Valve Sizes

2.190” intake/1.760” exhaust

Peak Flow @ 0.700 Lift

334 cfm intake/262 cfm exhaust

Compression Ratio

10:1 to 10.5:1

Fuel Metering

Holley Sniper TBI, up to 650 hp normally aspirated

Transmission

Close-ratio Chrysler A883 four-speed manual

Rear Gears

3.23:1

Operating Range

2,200 to 6,000 rpm

Intended Use

“I want a hot street car. It will be used for summer cruising, occasional weekend drags, a couple of highway cruises, including someday Power Tour.”

Desired Cam Characteristics

I want people to know there’s a cam in the car. A cam determines a car’s personality! But not totally like a race car that’s trying to die.”

Vacuum-Dependent Accessories

“I have no power-anything that needs vacuum. If I ever added power brakes, I’ll install a vacuum pump if necessary.” (But EFI is vacuum sensitive.)

A: Derrik, you’ve asked some great questions. Conceptually, what we’ll be discussing here applies across the board, to every engine type (but we’ll specifically include recommendations for your 505 big-block Mopar stroker cam). As I’ve said before, catalog cam recommendations, including the published rpm range, are based on “average” engines of the most popular displacement in each engine family and their everyday cylinder heads. For instance, for the ubiquitous small-block Chevy, it’s a 350 with traditional iron production heads. For a big-block Chrysler, it’s a 440 theoretically equipped with decent factory iron heads (such as a generic No. 906). But there are so many variables possible today even with “normal” displacements for a given engine family that you can’t necessarily rely on published rpm ranges, especially when comparing cams across different manufacturers. The interaction between engine displacement (not just total cubic inches, but actual bore and stroke changes), cylinder-head port flow, compression ratio, and even connecting rod length is very complex, as Comp Cams valvetrain engineering group manager Billy Godbold points out. Hopefully without writing an entire textbook, with Godbold’s help, let’s wade through your questions, and some related questions that arise from your questions, to see how cam experts home in on a cam profile recommendation. There’s gotta be a better way than just throwing darts, blindfolded, right?

How Does Engine Size Affect Cam Behavior?

Hoary tribal knowledge handed down through generations of hot rodders maintains, “A bigger engine makes a cam act milder.” This is usually true, but way oversimplified. According to Godbold, “If you dig down deep enough, it appears to be more directly related to intake port flow in cfm and/or piston speed than strictly displacement.” In other words, if two different cubic-inch engines had similar static compression ratios, intake manifold configuration and runner length, R/S (rod/stroke) ratio, L/D ratio (cam lift/valve diameter), cylinder head intake port flow in cfm, and piston speed, the cam specs required to peak at the same rpm (6,000 rpm for Holmes’ 505 Chrysler) would be almost identical. But in the real world, that rarely if ever happens. Bigger engines invariably want (and nowadays can easily obtain) bigger heads. The larger engine’s longer stroke will change the R/S ratio. In other words, multiple things change at the same time. Let’s get out a big shovel and dig down into this big pile of . . . stuff.

What Has More Effect on Cam Behavior: Cylinder Bore Size or Crankshaft Stroke?

In terms of cylinder bore size, Godbold says, “There can be rare times that you go up on bore and actually unshroud the valve to such an extent that the improved breathing makes the cam act bigger if the engine could fill and empty more quickly.” But for most engines that everyday hot rodders deal with, “bore increases usually only have a marginal change on operating range.”

On the other hand, stroke changes change cam behavior big time. “Stroke increases always mean you have more lever arm (that helps a bunch downstairs) and more piston velocity (that creates more signal to the port). Think of increasing stroke like grabbing a longer wrench or using a cheater bar.” Apply the same amount of muscle pressure, and thanks to the cheater bar, you get more force at the seized wheel nut (hopefully without breaking the wheel stud clean off; don’t ask!). “But that can be the tip of the iceberg as the R/S ratio probably has to change (usually to a lower number) to keep everything inside the block—which in turn changes the piston configuration and compression ratio, as well. At the same time, the rotating assembly may be getting heavier, resulting in more parasitic drag losses. If misfire (aka cam lope) and low-end throttle response were two main drivers that keep you conservative on cam timing, the combination of all these changes may well throw any of these concerns out the window. Now your main concern [with a lot of cubic inches] is not choking the airflow above 5,000 rpm. What ends up being a very mild camshaft on a big engine would be huge on something with half an inch less stroke pretty much any way it could possibly be configured.”

How Does Rod/Stroke Ratio Influence Cam Choice?

Optimum R/S ratio itself is still a huge can of worms. The debate never ends. Thinking continues to evolve, and there are proponents of both “short rods” and “long rods.” There is even no universal agreement about what constitutes a “long” or “short” rod, and that definition may vary for different engine families and configurations. At the risk of overgeneralizing, anything under R/S 1.5:1 is to be avoided on a V8 if possible, and—at least in the realm of street/strip domestic OHV V8s—you’ll rarely see anything over 1.8:1. No doubt a higher R/S ratio is better for minimizing cylinder bore wear, and arguably, with lower side-loading and reduced piston speed, results in less power-robbing friction. A long rod has a longer dwell time in the vicinity of TDC, so it may help a poor chamber burn more efficiently, as there’s more time for a complete burn. However, because the piston accelerates more slowly from TDC, there is less and later “pull” on the port from swept volume. Also, because of lengthened TDC dwell time, everything else being equal, a vacuum gauge will show less idle vacuum at the same rpm with a long rod (high R/S ratio). For street cars, the long rod may make reversion back up the intake port worse while the piston camps out at TDC—so the idle, depending on your point of view, can be worse (greater misfire and instability) or better (if you actually want to hear that raspy cam “lope”).

Shorter rods allow for a lower deck height (in high-end racing where you can actually order blocks with custom decks, or—on the Chrysler big-block family— build a 440 or larger engine within the confines of the low-deck 383/400 block). Shorter decks usually mandate shorter rods (a lower R/S ratio, everything else staying the same). With a shorter deck, in a V-type engine, intake runner length also gets shorter. Aerodynamics in an open-wheel Indy-style car will improve. But short rods themselves may have certain mechanical advantages, including effectively lengthening and strengthening the four-cycle engine’s intake stroke by the greater leverage and pull exerted by a short-rod rotating assembly.

What we can say for sure is that the popular 505ci Chrysler big-block combo, with its 4.25-inch stroke and 7.100-inch center-to-center rod, has a higher R/S ratio (1.67:1 R/S) compared to a similarly sized 502ci big-block Chevy with its 4-inch stroke and typical production-based 6.135-inch center-to-center rod (1.53:1 R/S). Again, depending on your point of view, that may be good . . . or not. Looking at it from the cam profiler’s point of view, valve opening and closing points may require adjustment to fine-tune them to the piston’s position in the bore based on R/S ratio as well as its effects on piston speed. Due to greater effective overlap, reduced idle vacuum, and decreased piston-to-valve clearance with a long rod, cam profilers may choose to widen the lobe separation angle (LSA), or, conversely, narrow the LSA with a short rod.

How Does a Bigger Cylinder Head Influence Cam Choice?

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