Posted By: emarine01
Cam duration vs cubic inches - 09/26/11 12:31 AM
Ok you guys all know the old saying... cubic inches eats cam duration... so with that being said.... About how much rpm will cubic inches consume with the same cam < Example > 360 cid vs 408... same 12 to 1 compression solid roller with
265@.050... lets assume the heads are up to the task on both engines.... The main reason for the question is that most cam grinders give rpm ranges for stock cid engines... so when you throw more stroke into the equation the rpm is somewhat of a guess
Posted By: 340B5
Re: Cam duration vs cubic inches - 09/26/11 01:14 AM
Man you ask tough questions....makes my head hurt. But I'll be looking forward to the answers.
Posted By: 440Jim
Re: Cam duration vs cubic inches - 09/26/11 01:19 AM
Yea, that is a tough question, and I don't have the answer.
But if you change the question to the same heads, intake, headers, cam and only change the CID, you might get some guesses.
I do agree that cubic inches like more duration, but that is based on the same heads/flow. And typically head restricted combos with the extra cubic inches.
You don't need a lot of duration with small CID and good head flow. But you do need the valve train to handle the extra rpm.
Posted By: Guitar Jones
Re: Cam duration vs cubic inches - 09/26/11 02:01 AM
I don't have any answers for you but I'm hoping it isn't much.
I'm basically going from a 363 to a 408 with the same heads but a bigger valve, 20 cfm more flow, a solid roller from a flat tappet but the duration at .050 numbers are within 1 degree of each other but about .030 more lift. Since I can only buy 1 ratio of pro gear for my 8.75 it's much harder for me to change the RPM range I can run the car in. However since most racing down here these days is 1/8 mile it's a lot more forgiving.
Posted By: emarine01
Re: Cam duration vs cubic inches - 09/26/11 02:18 AM
Maybe 440Jim hit it on the heads
The need for more duration for the cubes assumes a restricted head flow on the larger cid combo
Posted By: 340B5
Re: Cam duration vs cubic inches - 09/26/11 02:38 AM
Ok you got me thinking about it
360/1728 X (RPM/2) = CFM
CFM /(408/1728) = (RPM/2)
The CFM answer from the 1st formula for the 360 is used in the 2nd formula to get the the RPM at which the 408 draws the same air.
Posted By: dannysbee
Re: Cam duration vs cubic inches - 09/26/11 02:45 AM
The same cam and head with only a cubic inch change will make aprox the same hp. The larger engine will just make it at a lower rpm. For the 408 to to make its power peak at the same rpm as a 360 the camshaft will have to have more duration and heads need a larger port that moves more air.
Posted By: viperblue72
Re: Cam duration vs cubic inches - 09/26/11 03:20 AM
1 degree more duration per 10 cubic inches will keep you in the same power band everything else considered. This is just a general rule and hopefully this is correct cuz that's what I remember. So if you add 60 cubes add 6
degrees@050 would be best.
Posted By: emarine01
Re: Cam duration vs cubic inches - 09/26/11 03:29 AM
Anyone got a calc that can take a crack @ the math inputting 7500 rpm for the 360? I seem to be too retarded to get any recognizable numbers on my phone calc
Posted By: dannysbee
Re: Cam duration vs cubic inches - 09/26/11 04:07 AM
Aprox 6700 for the 408 with the same heads and cam as the 360 with a 7500 power peak.
Posted By: Cab_Burge
Re: Cam duration vs cubic inches - 09/26/11 04:24 AM
I don't know the math but I did switch heads, intake and carbs. this year on my pump gas 518 C.I. street motor keeping the same solid roller camshaft similar to yours, 260 @ .050 intake and 266 @ .060 exhaust side. The motor had a set of CNC ported Eddy RPM heads (intakes flowed 310 CFM at .700 lift at 28 inches of water on a Superflow 600 bench)with a six pak intake and carbs, it made peak power at 5800 RPM the last time it was on a engine dyno and peak torque at 4600 RPM. I switch the heads to a set of Indy SR max wedge ports that flowed 350 CFM at .700 lift on the same flow bench as the Eddy RPM, the peak HP is now at 7000 RPM and peak torque was at 4800 RPM
The car use to cross the finish line in the 1/4 mile at 6200 RPM with the old heads and intake system and now crosses at 7000 RPM with the Indy 400-3 intake and a single Holley 1050 CFM Dominator, no other changes
I say use the cam you got to start with and see if you like the results
Posted By: emarine01
Re: Cam duration vs cubic inches - 09/26/11 04:30 AM
Wow, that's 300 more rpm drop than my best guess
So If we apply the 1* per 10 cube rule 360 to 408 lets say 5* more duration, about how much will rpm rise?
Posted By: dannysbee
Re: Cam duration vs cubic inches - 09/26/11 05:24 AM
We know the 360 has to turn 7500 to produce the peak hp. In 7500 revolutions the 360 ci engine moves 2,700,000 cubic inches of air. The 408 will move the same amount of air in 6618 revolutions. May not be exact but I bet it is close. A friend did this with a truck. He built a new 408 short block and used all the parts off his 360. Truck was about a tenth faster and that was it. He had to lower his shift points.
Posted By: emarine01
Re: Cam duration vs cubic inches - 09/26/11 05:42 AM
Thanks for running the math, the results make sense , very close to some dyno sheets from builds listed on here, That is a good math formula to keep, Thanks to all.... I gota hit the sack, early day tomorrow
Posted By: RemCharger
Re: Cam duration vs cubic inches - 09/26/11 08:50 AM
You have to visualize your engine packing in air ,and not letting it escape.. (oxygen molecules).
Posted By: PorkyPig
Re: Cam duration vs cubic inches - 09/26/11 03:54 PM
Going by rapidly fading memory here, Grumpy Jenkins book had some cam selection stuff that talked about reducing the duration 5 degrees for each 25 ci drop to keep the same basic operating range. Of course, as soon as you also start changing to heads with different runner volumes, that kind of gets shot to he11.
Posted By: emarine01
Re: Cam duration vs cubic inches - 09/26/11 09:58 PM
Mike, the 260/262 roller is it a 296R-6? And have you thought about how your gona raise the plenum floor on that larrrrge port W5 intake
Posted By: Moparmal
Re: Cam duration vs cubic inches - 09/27/11 09:48 AM
FWIW, a couple of "learned" engine builders on your side of the Pacific discussed this on another site a while back -
They both ended up agreeing that a rule of thumb would be to add appx 8 degrees @ .50 when selcting a 408 camshaft over a 360.
Not sure how they arrived at that figure - experience probably
- but it seems to concur with whats been said here.....
Posted By: dannysbee
Re: Cam duration vs cubic inches - 09/27/11 01:52 PM
If you put the 440 avs on the 383 and put a good set of valve springs on it so it will turn the rpm needed to match the 440 air flow. Put a 3.91 gear in the 383 and a 3.23 in the 440 to match the power curves. You will be surprised how close they are.
There are exceptions to every rule, sure the over cammed 360 will run better as a 408. You have a hydraulic cam and can't turn enough rpm to meet you heads limit. In both those cases a 408 will run better than the 360.
The point I am trying to make is don't think you are going to take the heads, cam, induction, and headers off of your well matched 360 put them on a 408 and make 13-14 percent more Hp. If you do there was a problem with your 360.
Posted By: emarine01
Re: Cam duration vs cubic inches - 09/27/11 10:47 PM
From the 4 inch throw small block posts on here over the years It seems that the smaller cid 408 to 416 with good heads 300cfm to 330cfm make peek power around 6500 to 6800 with cams in the 260 to 270 @.050 range.... There really are too many variables for a rule approach
Posted By: polyspheric
Re: Cam duration vs cubic inches - 03/25/12 07:59 PM
Doesn't work, it's not linear.
If the heads are already small, the 408 will run out of steam even earlier.
If the heads are too big the 408 will show better at all speeds.
Posted By: POS Dakota
Re: Cam duration vs cubic inches - 03/26/12 11:33 AM
I would say that when picking a cam DURATION, the focus needs to be based more on the stroke than the overall cubic inches.
You can have a 5 inch bore, or a 4 inch bore. They both travel the same distance down the bore and spend the same time doing so in degrees of crank rotation, (no?) so I would say that generally they would both need roughly the same duration as one another and that the larger bore for example would need not necessarily more duration, but more of an opening to breathe through (larger cylinder head port and maybe higher valve lift) than the smaller bore, but not necessarily duration. A larger cylinder bore (larger cubic inches) isnt going to benefit from hanging the valve open any longer than the smaller bore.
Now take (again as an example) a 4 inch stroke and a 5 inch stroke.
The 5 inch stroke is going to have a piston travelling down the cylinder on the intake stroke (for example) for a longer length of time empasized in degrees of crank rotation than the 4 inch stroke. So by that virtue, you are going to want to open the valve for a longer time in crank degrees of rotation on the 5 inch stroke than the 4 inch stroke as the 5 inch stroke is spending more time creating a depression in the intake port, or breathing in.
So I would say that when picking a duration for a cam. It is really more reliant for the most part on what the stroke is than the actual cubic inches.
I briefly skimmed through the thread and correct me if I am wrong, but I didnt see any mention of stroke...but this is why an engine that has a longer stroke (408 over a 360) needs more duration, and also explains why the 360 with too much duration becomes a dog.
Think about that for a second....the 360 with 270 @.050 lobe isnt able to make the vac that the 408 would because the valves are open too long and a lot of that fuel isnt being retained in the cylinder to be burned, or even exhausted.
And this is why the 360 will have the higher powerband, and the same cam in the 408 will have a few hundred less rpm. Because of the stroke. Not really because of the cubic inches.
If I had a 360 and 408, (one larger than the other only because of bore size and not stroke for example) I would only want a larger port feeding the larger bore, and maybe more valve lift to help ensure that since the larger bore will indeed breathe in more air than the 360, but it's still spending the same amount of time doing so as the 360.
With a longer stroke, there is more time spent in degrees of crank rotation breathing in and that is where duration comes into play.
Posted By: polyspheric
Re: Cam duration vs cubic inches - 03/27/12 12:04 AM
More mistakes than content = too much work.
Posted By: emarine01
Re: Cam duration vs cubic inches - 03/27/12 01:09 AM
There are surly some mistakes but...If you read into the content, I think some of it is just the wording is wrong.... just simple stuff like time & speed. Even though its not technically correct, if you think about some of the opinions they have merit or express another way to look at things, most of the greatest achievements on this planet stemmed from mistakes. I understand why you don't post the long version anymore.... but bother to comment....
Vic, this was not to you.
So if the 265 solid roller has a power band of say 5000 to 7500 in 340 cid... whats the power band @ 408? & to take it one step further if the cam is 108 in @ 104 in the 340... would it make sense to install it straight up in the 408?
) or theoryist 
I can call Comp Cams race dept. tomorrow and get their engineers involved, if you want, 
Also I did mention I was bored