As a side note, remember that 6000 RPMs has each valve opening and closing 50 times per second!!!!!! Thats almost like a vibration instead of a controlled movement. This is why valve train componants take such a beating, and must be of the best quality available.........MP
Now lets draw on our prop people. Most prop people will tell you to prop for a 1000 rpm over max torque. This avoids torque loading the drive and such.
So lets see what we have:
(502/2) * 4500 / 1728 =653.64
653.64/8= 81.7
Let's use a duration around 224 and see what we get;
224/720 = .31
81.7/.31 = 263.54 cfm
Hmmm, our Canfield 310 is 266CFM at .400 lift. We now have arrived at our "infield ballpark" duration and lift. . . .
I will continue. . .in at bit. . .
Chris
So in a jet, how do you know what your peak rpm's will be with a given cam? How do you know at what rpm the engine will make peak power at?
Do you want to spin the engine to its peak power rpm point?
I guess what im trying to ask is if you want your engine to spin an A impeller to 5500 rpm's, where do you need your motor to make peak hp and tq?
So in a jet, how do you know what your peak rpm's will be with a given cam? How do you know at what rpm the engine will make peak power at?
Do you want to spin the engine to its peak power rpm point?
Taylorman,
Use the above formula to calculate how much rpm you can turn with the heads you have.
Again the head can only flow so much air. The cam can be fudged for more power by adding duration but what that effectively does is raise and narrow your powerband. Fine for a Lenco car, not good for a boat that has to reach plane.
You want the engine to spin to the max rpm range you have cammed it for.
Again, I am giving you all a way to narrow down the selection of the right cam for your engine using some basic match calculations. Is it exact, no. Will it get you pretty close...yes.
Chris
I like it!
Now we just have to get chris out of one of those LOW rpm out drive deals (5,500 give me a break) and into a real prop boat. Yes chris, you need a nice flatbottom that wants to run in the 8,800 + range. Just think of all the fun you would have building engines for it.
This thread should be pinned at the top as it's taking some of the voo doo out of cam selection for some of us.
For my runner bottom (prop boat) I would like a torque peak of 5000 rpm and to spin to 6800+, so the following would apply? The only thing Im not clear on is why .400 lift was used in your example and how does compression effect these calculations?
Thanks for sharing this info Chris, this is really cool stuff.
(440/2) * 6000 / 1728 = 763.8888
763.8888 / 8 = 95.48
Cam:
250 / 720 = .347 (275.) 95.48 / .347 = 275, etc.
255 / 720 = .354 (269.)
260 / 720 = .361 (264.)
265 / 720 = .368 (259.)
270 / 720 = .375 (254.)
My cylinder heads are similar to the list below (GM iron rect.), the 265 @ .050 figures line up well with my lift at .500, this seems to be in the right neighborhood to me. If I had to match to my .400 lift using the above formulas, it would dictate a 290 @ .050 duration
.300 lift intake = 188
.400 lift intake = 233
.500 lift intake = 259
.600 lift intake = 291
Ok, so im trying these numbers. BBC .030 over = 460ci. I would like to be able to turn 5500 rpms. That gives me 732 required air flow and 91 cfm per cyl. Then i try a 250 cam and that gives me 263 required air flow. The Canfield heads do more than that at .500.
So does this mean that my cam does not need to be larger than .500 lift? How do you figure out what kind of hp this combo will make? I assume desk top dyno?
Whoops, my bad, I read prop for 1000 rpm over torque peak, so for this exercise I would end up with 440/2 * 6800/1728 = 865.74/8 = 108.22, then 108.22/368 = 294 (265/720 = 368), so 265 duration at .050 is still the answer only now I need .600 lift insteed of .500. This will make a cool Excell spreadsheet when we have all the data.
Taylorman, I'm no expert but I think the 310 Canfeilds are a bit big for a 460 cube @ 5500 max rpm, I'd chase down some oval port numbers and run the numbers again with a cam around 245 @ .050 and .600 lift, should offer better port velocity. Just my 2 cents
Im with you on the excel. Ive got one im making as he gives more calculations.
I didn't get what he meant by propping 1000 rpm's over peak tq. Does he mean that the engine will spin 1000 higher than the engines peak tq rpm. How is that? How can rpms go up when peak torque is not?
You list ".400". " lift (in your tests) and then quote most people (who ask for advise) over ".700" " lift for normally asperated motors. I don't need "exercises" in performance, I need to learn 'new' ideas in making power. Are you using "computer data, or doing ACTUAL DYNO TESTS?" I find that many of the 'data based camshafts' are too big for the actual use. Just my 'old guy opinion'. What are you looking at when you post cam specs? I'm ready to learn, after building 18 boats that have won 33 world championships in IHBA. I didn't use any computers, but I'm still willing to learn! Please tell us how this all works!!!!
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