cam math. question

[j.d.roost]

I was looking over my web cam card and it says that intake opens at -7 before tdc. That means that the cam is opening 7 ° After tdc right? So I will need to subtract that from my 33° closing and add 180 for duration @.50?

My exhaust closes -7 after tdc. So is that 7° before tdc??( I have never seen a cam card like this)
If so do I reverse the math. Opening (33) - 7 +180(206). Then devide that by 2 and add the 7 back to get centerline?
I read somewhere that you need to subtract opening and closing when cams have low or 0 overlap. Am I on the right track?
Thanks for any help

 

[AWD_ENVY]

Never heard of any special math when cams are at low overlap.


Your on the right Track, but make sure you WebCam card is for Duration @ .50 VALVE lift.... if it says CAM LIFT you need to still factor in the rocker ration of 1.7

So if it's in Cam lift you take the .50" X 1.7 or the rocker ration and you get .85" valve lift mesurment.

Here's a good tech artical on cam math if you dont know alot yet.
http://www.pipercams.co.uk/TechnicalAdvice.html

The duration is the addition of the open(-7)+close(33) of the cam, plus 180,which equals 206. Then divide by 2 resulting in 103. Deduct the number of degrees before tdc(-7) that the valve started to open.
The result 110 Lobe Center.

Although something doesn't sound quite "right" with your numbers.... can you post the car ?

 

[NOSNUSE]

that means that the valve is at full lift at 7 btdc, for the intake the minus sign is just an indication of where the timing is in relation to top dead center.as far as the exhaust that doesn't sound right can you show a picture of the cam card.

 

[j.d.roost]

cant scan tonight so here is the specs on the card
int. Ex
v.lift .400 .385
Duration 272° 256°
[email protected] 206 206
lobe center 110° 110°

int opens -7° btdc ex opens 33° bbdc
int closes 33° after ex closes -7° after
bdc tdc

valve timng is with 0 lash @.50 inches of lift

If you have a low overlap cam, the intake opening may be AFTER TDC, if so, you will have to SUBTRACT that figure from the closing number and add 180. This will be the duration at .050 inches of lift. (39 - 10 + 180 = 209) Divide the number by two, and add back in the opening number. (209 / 2) + 10 = 114.5 This will equal lobe center. The same thing may happen on the exhaust side with your closing number, it could close Before TDC. If you need help, please call us direct. Thank you.
I have never seen a cam card like this either. I also have never seen an exhaust cam close before tdc.
If it were -7 before tdc ( prob a standard card they print out, then fill inthe blanks) I think they ment 7° AFTER tdc?? If you look at the statement from the web page this is the only way their math works out.
On our engines the is no rocker ratio right? If there is and I take my measurements off the valve retainer ( I have a solid lifter) what is the rocker ratio and where do I use it.

Thanks for everyone's time
jd

 

[AWD_ENVY]

Originally posted by NOSNUSE
that means that the valve is at full lift at 7 btdc, for the intake the minus sign is just an indication of where the timing is in relation to top dead center.as far as the exhaust that doesn't sound right can you show a picture of the cam card.

Full valve lift is NOT at 7 BTDC.... Max lift valve lift occures @ Lobe Center

(-7) is where the cam is in relation to the cam/valve/engine timing.....

 

[j.d.roost]

@.50inches of valve lift sorry
quote is from web cam page
www.webcamshafts.com

 

[j.d.roost]

that means that the valve is at full lift at 7 btdc, for the intake the minus sign is just an indication of where the timing is in relation to top dead center.as far as the exhaust that doesn't sound right can you show a picture of the cam card.

think it means that @.50 of lift the degree wheel should read 7 after tdc (aka - 7)
?
jd

 

[AWD_ENVY]

Ok sorry... yes you need to change your math... IF the opening point is BTDC add to 180, if ATDC subtract from 180.

My math above stating the 110 degree center is correct.

You don't need to worry about the rocker ration as your values you wrote are "v.lift" which is mesurments at the retainers. On some cams such as Crowers & Some Web's, they give you the numbers in CamLift. This number will be off by the rockerratio(DSM ratio = 1.7).

You will need to make a solid lifter that you can adjust to 0 lash. Stock hydro ones will smush(if that make sense.. ) and give bad readings.

If you are degreeing the cams In the car, you can use a degree wheel NO BIGGER than 7". FYI......

 

[AWD_ENVY]

ok.... the .50" is NOTHING MORE than a INDUSTY STANDARD point to read values.

When you put a Dial indicator on the retainer,.... Centering to "0" on the Bottom Circle Diameter, you will move the timingbelt/cams/ect... untill you get a reading of .50" AT THAT TIME(if properly degreed) your degree wheel down on the crank should read the manufacture's Valve "Open".

This Occours WAY before Full lift

 

[j.d.roost]

I]When you put a Dial indicator on the retainer,.... Centering to "0" on the Bottom Circle Diameter, you will move the timingbelt/cams/ect... untill you get a reading of .50" AT THAT TIME(if properly degreed) your degree wheel down on the crank should read the manufacture's Valve "Open". [/I]
Right , thats what I thought. I do have a solid adjustable lifter.I guess the problem is that I am not sure -7btdc means 7° after tdc. (on the degree wheel)
Same with the ex close -7 after tdc (7° btdc??)

ps. I have an old 1/4 midget(go cart type car)d.wheel 7". I can pull the belts and pully and bolt it right up.
thanks again