Hey all you cam nerds! Who here is taking the time to put pencil to graph paper when making camshaft selections!? I recently inquired about having a ‘regrind’ not knowing enough about the dynamics once I got the specs back I started diving into the theory. It’s pretty neat what kind of numbers you can derive out of timing specs alone. The more I learn the more questions I have. Rod length affects cam timing?
Rod length effects cam timing, port flow effects cam timing, static compression ratio effects cam timing, the weight of the car effects cam timing, the gear ratio effects cam timing, the transmission type (auto or manual) effects cam timing, carburation has an effect on cam timing. And if you get into the Professional level, the track that you are racing on as well as the weather has an effect cam timing. Sorry, didn't mean to throw a proverbial monkey wrench into cam timing plotting...but if you want to plot some of the details, might as well see the whole picture. I've seen first hand about 90% of the above have a direct effect on cam choice. Mike
Lotsa folks feel "the long rod theory" is who-ee! In other words.....'the rod needs to be long enough to connect the piston to the crank throw!!' Suppose to "dwell" longer at TDC...therefore the cam timing can be longer. I like the long rod theory because you have a very small/light-weight piston on the end of the connecting rod! I think it really pays dividends when getting the last 1/2 HP out of an all out engine. (Think Pro Stock....NASCAR) A lot of Shivel-lay fellas like to put a 6" rod in SBC stuff to help relieve side load but not so much about cam selection.Week end 'play-purty' spend the $$ on something else. What I think is fastenating is how Lobe Separation Angle on a cam can change THE WHOLE operation of the cam!! Yep!!!..I'm a cam junkie/nerd! 6sally6
At one point I thought that I understood cams and cam timing. I had been running a de-stroked 400 sbc with a Racer Brown 42R cam and was very happy with the results. I was going to run some USAC shows and they were limited to 355 cu in. I had an LT1 short block, but it was only 8.5:1 compression. I had run it previously and it ran good. However, I had been running an Isky 505-T. The engine had no bottom end torque with the Racer Brown cam. I think the Isky was about 270 degree duration and the Racer Brown was a bit over 300 degrees. Went back to the Isky and got the bottom end torque back. At that point I realized that I really didn't understand cam timing/duration. The best bet is to talk to the cam supplier and tell them what you are running and what you want to accomplish.
The selection of the best cam for any engine of choice is a crap shoot at best. While you can generally pick the traits you are looking for, and a little research and measurement is necessary..........you won't know for sure how any cam will perform in a specific engine until you pay your money and try it out. There are too many variables that affect the end result.
You didn't say the purpose of your quest - just trying to learn some theory, or actually pick a cam for a specific application. If you are actually trying to pick a cam to use, I have two suggestions for you: 1. Find an engine combo online or in a book that shows dyno results and copy it exactly. 2. Actually the best solution, call a cam company of you choice and talk with their tech department. You will need to have all of your engine specifications worked out before you call.
There is a great book out there for SBC cam selection. The name of the book is Max Performance Chevy Small Blocks by David Vizard. Fantastic cam information in there that can be applied to other engines. Also, if you are building a flathead, there is a book out there by one of the guys at H&H Flatheads.
Not really seeking any particular advice on my build per say but after studying an application my head is spinning. I think @jmaountainjr hit the nail on the head. The general builder will have to rely on others experience. There are so many variables that require individual attention that without the right tooling (I.e flow bench etc.) it would be impossible to find the perfect timing for a unique build.
Like I said above.......I'm a real camshaft junkie! jaracer........did the Racer Brown cam have a long LSA? (ex. 112-113*LSA) Isky grinds MOST of their stuff on a 106-108*LSA. Gives a LOT of torque down low but.........is finished by 5500-6000 RPM. Lotsa guys refer to it as "a Light Switch" cam. ....BOOM its making power and then BOOM it's done. Other camshafts with longer LSA (like in a stock application) may be 114-116* LSA. A looong Gradual power curve. Duration (how long the valve hangs open) is important for power but ....only to a point. (intake has to close eventually so the exhaust can open AND STILL make some compression in between!) I always thought it was weird how a tight LSA 106-108* gives gobs of low end power BUT.......idles so snotty at low RPM's. Weird,huh?! Anyhow........I bet the Isky cam had a tighter LSA than the Racer Brown! 6sally6
Plus, now they offer different cam timing for varying intake manifold runner length, all on the same shaft. (Center ports being shorter, outer ports being longer.) Yes, I'm a cam nerd...
Got my new Dec 2020 issue of Hot Rod yesterday. Marlan Davis/Billy Godbolt do an outstanding job stepping thru how each variation of an engines major components affects camshaft choice. Very interesting reading. Might also be on [email protected] (See, if you hadn't let your subscription lapse you would already have this )
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