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Camshaft math and explanation for idiots.

General engine tech -- Drag Racing to Circle Track

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Bill Chase
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Camshaft math and explanation for idiots.

Post by Bill Chase » Tue May 19, 2020 4:19 am

What are the formulas for figuring out valve timing when armed only with @ 0.050, lsa and icl?

Figure you guys will have the simplest way for a math challenged idiot like me??

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Re: Camshaft math and explanation for idiots.

Post by 77cruiser » Tue May 19, 2020 8:39 am

Jim

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Stan Weiss
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Re: Camshaft math and explanation for idiots.

Post by Stan Weiss » Tue May 19, 2020 8:49 am

ECL = LSA + (LSA - ICL)

IVC = (Intake duration / 2) - ICL

EVC = (Exhaust duration / 2) - ECL

IVC = Duration - ( IVC + 180))

EVO = Duration - (EVC + 180)

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Re: Camshaft math and explanation for idiots.

Post by piston guy » Tue May 19, 2020 1:38 pm

Bill as you already know cam timing is just a math game. Here are some simple examples to show you what to do I'm using simple @ .050 numbers and a simple 110 LSA. This is essentially the same math as Stan has posted.
My example information Duration @ .050 260 , LSA 110 ICL 106 . Intake opening is determined by taking 1/2 of the duration @ .050 and subtracting either the LSA , OR the ICL "if" it is different than the LSA ( VERY often) So 130-110 = I.O. of 20 OR 130-106=24@ 4*s advanced. The E.C is the "remainder" of the IO and 180. So 260 total duration minus 20 (IO) minus 180 =60 (E.C.) Because 20-180-60=260. Calculating the numbers for an advanced cam is a little more tricky. So 130-106=24. 260-24-180=56(EC) Again 24+180+56=260 . BUT The exhaust numbers need to be calculated differently. When the cam is "advanced" , the exhaust timing goes in the opposite direction. To find the ECL , you double the actual LSA , in this case 220 and subtract the ICL (106) leaving 114 . So 106+114+ 220 and 1/2 is the original LSA of 110. Back to "my " method we calculate the EC first. So 130-114=16 ( EC) Now the EO is 260-16-180=64 EO 16+180+64=260.
A cam card would read dur@ .050 260, LSA110 , ICL106 , IO 24, IC 56 , EO 64, EC16 A dual pattern cam is done the same way. Smaller cams and wider LSAs go to negative numbers but are also less critical on being degreed in.
I hope this helps despite being exactly what Stan posted.

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Re: Camshaft math and explanation for idiots.

Post by Bill Chase » Tue May 19, 2020 2:02 pm

Thanks guys, sorry for asking so many novice questions, just trying to expand my knowledge base.

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Re: Camshaft math and explanation for idiots.

Post by David Redszus » Tue May 19, 2020 3:03 pm

Next question.

How do valve events change when viewed at: seat to seat (.000"), 0.006", and actual lash settings?

How valid is a .050" number?

How much difference is there in valve events from cylinder to cylinder?

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Re: Camshaft math and explanation for idiots.

Post by piston guy » Tue May 19, 2020 4:11 pm

Three great questions.
Valve events @ .000, .006, and actual lash are all over the place because cam grinding companies have different ideas on "take up" ramps . Plus rollers and flat tappet lobes differ in intensity ( yes at low lift too). This is why ( MANY ) years ago the industry adopted the @ .050 numbers. This WAS before concepts like inverted flank , asymetrical lobes.

A .050 number Is a point past those "ramp up" areas. Is it perfect with some of the latest .500+ lobe lifts? Maybe not .

Accuracy of valve events from cylinder to cylinder is an interesting question. As far as how the cam itself is ground, it depends on how the cam is indexed ( manually for each lobe) or programmed on CNC grinding machines. As far as "when running" there are several influences. Cam Core material and diameter, spring load , oil pump load ( or lack of) , gear drive , chain , belt , etc. I know one grinder that would advance the rear lobes when the oil pump drive was on the opposite end of the timing chain. He did it to "allow" for torsional twist he envisioned. "I" never saw a measurable difference on an ET slip at the drag strip. Others may have and have different opinions than mine above and I welcome responses to my opinions.

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Re: Camshaft math and explanation for idiots.

Post by 67racer » Sun May 24, 2020 6:58 pm

David asked,
"How do valve events change when viewed at: seat to seat (.000"), 0.006", and actual lash settings?"
They change

"How valid is a .050" number?"
in my case, not very. All lobes were pretty consistent measured from OEM 1mm. lift height. Wasn't until I started measuring at .001" event open and close that differences were found.

"How much difference is there in valve events from cylinder to cylinder?"
2.4 liter Ecotec, OEM cams, Engler mechanical IR injection on methanol:
I'm using .004 to .006" lash on intakes, .005 to .007" on exhausts
Many hours spent gutting OEM hydraulic lifters and then trial and error shimming to above specs.
So long as I can keep them within above tolerances, negligible difference in event timing. Outside tolerances, it starts making a difference.

I have struggled with reversion on the rear cylinder since acquiring this engine. Corona provided some time to work on it. With stacks removed and a sudden throttle blip a faint, soft cloud at each of front three cylinders is produced. Rear cylinder SPITS fuel droplets. Rear cylinder header flange very hot compared to front cylinders.

Decided to measure all cam lobes. 7 of 8 intake lobes measure pretty close using the .001" event open and close. Front intake on rear cylinder has a 5* opening head start on all other lobes. Exhaust lobes on rear cylinder measure the same. I have no idea what OEM production tolerances are but think that 5* likely contributes to the problem. Also, rear cylinder header primary is approx. 6" shorter than the front three. Attempted new cam purchase...national back order. What to do in the meantime? Maybe on track higher RPMs overcome it somewhat?

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Re: Camshaft math and explanation for idiots.

Post by frnkeore » Sun May 24, 2020 9:14 pm

This is the way I learned cam timing LSA, using the timing @ .050 (any other opening will work, too but, LSA, may not be exactly the same as .050,because of ramp differences).

Open degree (we'll call it 30 on intake) plus close (call it 70), plus 180, will = 280, for total duration @ .050.

1/2, 280 = 140 minus intake open = 110, that's the lobe center for the intake.

If the exhaust has the same profile (i.e. 70/30), it will also have a 110 CL. You subtract the lower number, from the 1/2 figure, if it's IN or EX.

What ever you advance the intake (i.e. 6° or 104°) you have to add to the EX (i.e. 114°), the LSA stays the same, you've just rotated the cam lobes, is all.

If the EX has different timing, such as 80/30, you add those numbers, the same before. 80 + 30 + 180 = 290°, 1/2, 290 = 145, minus 30 = 115°. That's the new CL for the exhaust and different from the IN CL so, you have to add the 110 and the 115 = 225 ÷ 2, gives you the new LSA of 112.5°. If you advance that cam 6°, you get 106.5 for ICL and 118.5 for the ECL. The numbers will always total LSA of the cam. You always subtract the lower number of the timing, from the 1/2 the duration number.

If you run those numbers on different advertised cam specs, you get comfortable with doing it.

Regarding telling cam lobes apart, with the same @ .050 timing, look for the difference between the .050 duration and the .200 duration. Most cam grinders give those numbers. You can further distinguish cam lobes if, they give .300 lift duration but, not many do.

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