Hey guys. My question pertains to the intake centerline of a certain BBC cam. This spec is not listed on the cam card, so I have taken it upon myself to find the intake centerline through mathematics.
The cam we are dealing with:
Single Profile Cam -
4* Advanced
Advertised Specs -
280 Duration
.323 Cam Lift
.565 Valve Lift (1.75 rocker ratio)
Intake Opens 30* Before TDC
Intake Closes 70* After BDC
Exhaust Opens 70* Before BDC
Exhaust Closes 30* After TDC
60* Overlap
Specs @ .050"
232 Duration
6* Overlap
Intake Opens 6* Before TDC
Intake Closes 46* After BDC
Exhaust Opens 46* Before BDC
Exhaust Closes 6* After TDC
The way I'm figuring is that the centerline of the lobe should be at exactly half the duration of the lobe:
280*/2=140* OR 232*/2=116*
To find the exact center from TDC, I then subtracted the amount of degrees before TDC that the intake opens:
140* - 30* Before TDC=110* After TDC OR 116* - 6* Before TDC=110* After TDC
Now, add in the 4* advance that is ground into the cam:
110* - 4* Advance=106* Intake Centerline
By the above mathematics, I get a 106* intake centerline. Did I do this correctly?
I got confused there, but its pretty simple.
A cam with a 110 LSA installed straight up (I don't mean dot-to-dot since cams may be ground off center, I mean like centered on actual cam events) will have an intake centerline of 110* Advancing it 4* will make that ICL be 106. Advancing the cam takes the same number of degrees off the ICL that you advance. Make sense?
I got confused there, but its pretty simple.
A cam with a 110 LSA installed straight up (I don't mean dot-to-dot since cams may be ground off center, I mean like centered on actual cam events) will have an intake centerline of 110* Advancing it 4* will make that ICL be 106. Advancing the cam takes the same number of degrees off the ICL that you advance. Make sense?
http://www.***boat.com/forums/attach...1&d=1171276975
Uh...no...
The intake centerline of a cam is the relation of the lobe center line (the split angle between the lobes, the LCA). If the cam is installed "centered" with all valve events timed equally on both sided of piston TDC, the ICL will be the same as the LCA. If it is advanced or retarded in relation to the crankshaft TDC, then the ICL will be larger or smaller than the LCA.
The ICL is not a part of the cam grind, it is a relation to the crank TDC position. You determine the ICL by your cam instalation.
I wasn't very clear on it till recently with the help of a book on engine blueprinting I bought at B&N a ferw weeks ago. Cleared upo a lot on some of the fine points of engine build and specing.
Hey guys. My question pertains to the intake centerline of a certain BBC cam. This spec is not listed on the cam card, so I have taken it upon myself to find the intake centerline through mathematics.
The cam we are dealing with:
Single Profile Cam -
Specs @ .050"
232 Duration
6* Overlap
Intake Opens 6* Before TDC
Intake Closes 46* After BDC
Exhaust Opens 46* Before BDC
Exhaust Closes 6* After TDC
The way I'm figuring is that the centerline of the lobe should be at exactly half the duration of the lobe:
280*/2=140* OR 232*/2=116*By the above mathematics, I get a 106* intake centerline. Did I do this correctly?
No, first off where did you get the 6* overlap from?
start with your 6* BTDC + 180* (between TDC & BDC) + 46* ABDC = 232*
now 232*/2 = 116* or 116*-46* ABDC = 70* before BDC is your int lobe center.
46* BBDC + 180* (between BDC & TDC) + 6* ATDC = 232*
232*/2 = 116* or 116*-46* BBDC = 70* after BDC is you exh lobe center.
Now the distance between these two (70* ABDC to 70* BBDC) 220*/2=110* this is you lobe seperation and as was stated earlier this cam is straight up no adv or ret per you specs.
If you were advanced 4* you would have a 106* int c/l your specs would be:
int open 10* BTDC, close 42* ABDC
exh open 50* BBDC, close 2* ATDC
and your overlap is 12* @ .050" and it is will not change with adv or ret the cam.
No, first off where did you get the 6* overlap from?
start with your 6* BTDC + 180* (between TDC & BDC) + 46* ABDC = 232*
now 232*/2 = 116* or 116*-46* ABDC = 70* before BDC is your int lobe center.
And 70* before BDC is 110*.
46* BBDC + 180* (between BDC & TDC) + 6* ATDC = 232*
232*/2 = 116* or 116*-46* BBDC = 70* after BDC is you exh lobe center.
Which would be 250*.
Now the distance between these two (70* ABDC to 70* BBDC) 220*/2=110* this is you lobe seperation and as was stated earlier this cam is straight up no adv or ret per you specs.
OK this is where I get confused. Where is the 220* coming from? 70* ABDC would be 180*+ 70* which would equal 250*. 70* BBDC would be 180*-70*, which would be 110*. Between 250* and 110* would be 140* wouldn't it?
If you were advanced 4* you would have a 106* int c/l your specs would be:
int open 10* BTDC, close 42* ABDC
exh open 50* BBDC, close 2* ATDC
and your overlap is 12* @ .050" and it is will not change with adv or ret the cam.
Oops...6*...I did mean 12* overlap...that was a mistype on my part.
Hey guys. My question pertains to the intake centerline of a certain BBC cam. This spec is not listed on the cam card, so I have taken it upon myself to find the intake centerline through mathematics.
The cam we are dealing with:
Specs @ .050"
232 Duration
6* Overlap
Intake Opens 6* Before TDC
Intake Closes 46* After BDC
Exhaust Opens 46* Before BDC
Exhaust Closes 6* After TDC
I'm getting 12° overlap and with cam installed with the above figures:
110° Intake Centerline
110° Exhaust centerline
110° Lobe Seperation Angle
Ok I'll try it this way since the computer or sight is acting up for me.........
Quote:
Originally Posted by Carnivalride
No, first off where did you get the 6* overlap from?
start with your 6* BTDC + 180* (between TDC & BDC) + 46* ABDC = 232*
now 232*/2 = 116* or 116*-46* ABDC = 70* before BDC is your int lobe center.
And 70* before BDC is 110*.
Is 110* after TDC or 0*
Quote:
Originally Posted by Carnivalride
46* BBDC + 180* (between BDC & TDC) + 6* ATDC = 232*
232*/2 = 116* or 116*-46* BBDC = 70* after BDC is you exh lobe center.
Which would be 250*.
Is 250* after TDC or 110* from TDC (360*)
Quote:
Originally Posted by Carnivalride
Now the distance between these two (70* ABDC to 70* BBDC) 220*/2=110* this is you lobe seperation and as was stated earlier this cam is straight up no adv or ret per you specs.
OK this is where I get confused. Where is the 220* coming from? 70* ABDC would be 180*+ 70* which would equal 250*. 70* BBDC would be 180*-70*, which would be 110*. Between 250* and 110* would be 140* wouldn't it?
You are measuring from the center line of the exh lobe to the center line of the int lobe so your 250* to (TDC) 360* = 110* + (TDC) 0* to your 110* = 220* and you divide that by 2 so you get 110* for lobe separation. Remember the cam and the crank dont turn at the same ratio.
OK so I read through one of my engine books...
Carnivalride...this whole thing makes sense now. What was messing me up on understanding your explainations was due to something I never knew about "lobe centerline angle" (aka 'lobe centers') and LSA.
LSA is spec'd in camshaft degrees while lobe centerline angle is spec'd in crankshaft degrees. After I read that, your statement where you said "Remember the cam turns at half the speed of the crank" made perfect sense. I mean, I've always known that the gear ratio between the crank and cam was 2:1, but I couldn't figure out what that would have to do with these specs since I thought LSA and lobe centers were both spec'd in crank degrees.
Thanks for all your help guys.
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