Cam required???
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Topic: Cam required???
Posted By: Lunker
Subject: Cam required???
Date Posted: 04 Jan 2014 at 12:46pm
As I have posted before, I am putting together a WD45 Block with a bore of 4.5" stroked to 5.1" with a CR of 10:1. I am porting the head and intake manifold some with larger 1.8" intake valves and bigger Carb. I expect to run at about 2000 RPM. What do I need to do to the Cam???
When folks talk about degreeing the Cam, Do they change the duration and timing of the lobes etc.
Thanks, Lunker
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Replies:
Posted By: wi50
Date Posted: 05 Jan 2014 at 11:33am
first off "degreeing the cam" refers to the measureing process of checking the intake centerline. It's a process in which you take a dial indicator and measure TDC, take and find TDC and checka sat distance on each side and split the difference for true TDC. I'll install a degree wheel on the crank nose and clamp a pointer on the block, I'll estimate TDC on number 1 cylinder and set the wheel and pointer to TDC. Then I'll rotate the engine backwards some and start to rotate the crankshaft clockwise to say .100" before the piston hits TDC, then I'll look at the degree wheel and maybe it's at 16 degrees BTDC. Then I'll rotate past TDC and stop when the piston is down in the bore that same .100" and take a reading on the degree wheel, say it's 13 degrees ATDC for example. 16 and 13 degrees on each side are not correct, I need to rotate my degree wheel back 1.5 degrees and lock it down, check again.
Now that you get a true TDC measured out it's time to check the intake lobe of the camshaft. Same process, measure it a set distance on each side of the peak lift, say .050" or .100" and average the numbers un your degree wheel. What you may find is that your camshft has an intake centerline (peak point of lobe lift) at something like 113 degrees ATDC.
That's to late. Most of the stock cams I've measured have an intake duration (number of degrees of engine rotation that the intake valve is open) of about 182-186 degrees at .050" lift. I'm just going to say that it's 184 for all practical purposes, years of wear and manufacturing variations come into play here. What this means is that your intake valve is at half of the 184 degrees, or 92 degrees left in it's cycle when the piston is 113 degrees ATDC.
So with 92 degrees left plus the 113 ATDC, you get to 205 degrees ATDC or 25 degrees ABDC before the intake valve closes, or quits flowing air for all practical purposes.
If you rotate the camshaft back to say 104 deg intake centerline, you close the intake valve sooner at say 16 degrees ABDC.
The lobe seperation angle is ground into the cam and is set in stone, you can't change it without regrinding the cam. So by changeing where the intake centerline is, you also change whrer the exhaust centerline is. The "re phasing" or re timeing the camshaft differently to the crankshaft changes when these events happen in relationship to the piston position in the bore.
Think about it this way, by retarding the camshaft from 113 int cent to 104, you increased your compression stroke a few degrees. You also change where the valves overlap in relationship to the piston being at TDC and starting a new intake cycle.
A stock cam will work better by just changeing the intake centerline. You either slot holes and rotate it on the gear and lock it down or cut new keyways in a keyed gear.....trial and error.
Now when I'm getting a cam ground, I take into consideration the other engine factors. For the smaller engines with limeted crbs and cylinder heads I'll use a stock cam or one with say 195 deg duration @ .050 lift in some of the pulling engines. I keep the lobe seperation at 105 to 106 degrees.
For the larger engines I'll get one around 200 degrees @ .050" with a bit wider lobe seperation, say 106 to 107. For the longer stroke engines I'll go out further on the numbers 210 degrees and 108 LSA.
It all varies a little, engines with better breathing ability will take a little larger cam. Faster running engines will tolerate more cam. Air flowing through the carb, manifold and into the cylinder has inertia, sometimes there's enough inertia (with higher engine speed or better flowing induction package) to keep filling the cylinder longer as the piston comes up past BDC on compression stroke so I'll re time the camshaft a bit differently or rub a little longer duration.
These engines share intake ports and with the fireing order it's easy to put to big of a camshaft in them and loose power also.
What you are doing though get a cam in the 194 to 200 degrees of duration @ .050" lift on the intake lobe, 5 or 8 degrees less on the exhaust if the cam grinder is willing to do so and keep the lobe seperation 106 or 107 degrees.....it'll work great. If you're cheap and confused a stock cam will work good....kust make sure you get it timed to the crankshaft correctly. Set the intake centerline somewhere in the 104 to 106 ATDC range.
I can get into more detail on lobe seperations, cam ramps, etc. This post is long enough and most reading it are going to be lost with even the basic information given. Draw things out and put some thought into the process and what's happening in the engine. If you can understand it you'll be further ahead than 99% of the competition.....the other 1% will agree and have no right answers either.
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"see what happens when you have no practical experience doing something...... you end up playing with calculators and looking stupid on the internet"
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Posted By: wi50
Date Posted: 05 Jan 2014 at 1:20pm
It's typical to see 2 numbers listed for cam duration. One is refered to as advertised duration, or .050" duration. The other is seat duration. Advertised is typically 30 to 40 degrees less than seat duration, or the time that the lifter is .010" or .012" off the base circle.
The start and end of movement is quite gentle to keep the valves from slamming against the seats when closing and to acclerate them slow enough to makr parts last. To slow though and the engine will burn valves.
Just because the cam has XXX for duration and another is very similar or identical doesn't mean that they are. Different grinders have different master lobes, or even the same grinder can use different master lobes to achieve a set spec. There's endless possiabilities for lobe geometry.
Everyone wants more lift. Lift isn't that inportant on these particular engines, they quit flowing air well before the valve makes it to it's peak lift. Lobe shape is more important here. I'll give up lift for a different lobe shape. The time or duration and when the events happen is important, but the net valve lift just isn't with these poor flowing heads.
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"see what happens when you have no practical experience doing something...... you end up playing with calculators and looking stupid on the internet"
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Posted By: Dipstick In
Date Posted: 05 Jan 2014 at 1:29pm
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wi50, I started losing it in the first sentence, and by the time you got to TDC, I think that means "the dumb cam", I was totally confused! LOL
A couple of years past, I called Barry Cams, and I think I talked to a Scott. I told him what I was wanting to build an engine for,(plowing) and he gave me a series of data for the grind, based on a 175 cam with some improvements. It's amazing what just a small change in numbers does in effect!
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You don't really have to be smart if you know who is!
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Posted By: mgburchard
Date Posted: 05 Jan 2014 at 3:21pm
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The higher the velocity is in the port the wider the lsa needs to be to widen the power curve.
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Posted By: WF owner
Date Posted: 05 Jan 2014 at 7:03pm
Thank you Marty!!!! Every time I read one of your posts, I learn something. You should make online videos !
Thanks again for your input in this forum. There are a few of you guys that have (and continue to) helped me immensely!!!!!
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Posted By: Lunker
Date Posted: 07 Jan 2014 at 12:09am
wi50 (Marty)!
Thanks for the lesson and info on cams. I think I follow most of it, but will print it out and reread it a dozen time, then maybe I can figure it all out!!! Thanks again!
Lunker
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