DaveMcLain wrote: ↑Sat Jan 13, 2018 1:26 pm
Back when I made all my designs symmetrical, I checked centerline off of max lift. I couldn't understand why Harold's cams always wanted to be a couple more degrees advanced then my similar sized cams wanted to be. Harold's were asymmetrical. It turned out, advancing his more put his opening and closing points closer to where mine were.
From everything I've seen, where max lift occurs, isn't as important as where the valve opens and closes.
I think that's very true because once the valve is closed it is closed and no more flow will happen in either direction. But really if the max lift point was moved around a few degrees how would it make a heck of a lot of difference to performance? The opening and closing events are more absolute when it comes to the engine.
Because it would effect the entire shape leading up to and down from that point. Billy's comments should extend to not just overlap, but the entire lift curve. It's not so much the area as it is the shape. Remember, you're governed by physical constraints of the system. That may be velocity, acceleration, or jerk. Different types of valvetrains have different thresholds (like a OHC bucket follower vs say a OHV pushrod engine with very long pushrods).
Mike Jones designs his cams this way. He figures out the valve lift curve, then works backwards through the valvetrain geometry to produce a lobe. What he ends up with is entirely contingent on the shape he calculates as well as the valvetrain geometry. That might mean an assymetrical valve lift curve that results in a symmetrical cam lobe or any other combination of the two.
Billy has shared this (example he gave was a sliding follower cam that needed to be ground at 105 LSA to produce 115 LSA at the valves), and Harold also did some design work on OHC Pintos at one point. You also have to make sure the thing is dynamically stable (acceleration shape / values , jerk shape / values, etc. Hertzian stress, can it even be machined?, etc.)
Harold used a design philosophy that any lift before TDC is bad and that the engines does most of its heavy lifting in the first 75* of crank rotation. Harold's cams were very aggressive on the opening side and spent more time coming back down.
Billy Godbold shared that in NASCAR in the late '90s into the 2000s, they spent a lot of time focusing on lift @ BDC. Paraphrasing; for a given duration / centerline, the more lift you can have at BDC the more power you're going to make - almost every time.
As Mike and Harold have both shared here. You can open the valve too fast on the intake side. As Billy has shared, and Mike has said, you sometimes want to slow the valve down to increase velocity. The example Billy gave was on the exhaust side. Makes you wonder about how much would square valve lift profiles really get you?
An engine works off of pressure differentials, if you think in terms of crank rotation and time, it only knows what happened the degree before where it is now, so the lift at every degree (in as small of increments as you want to make them) matter. Not just seat-to-seat and max lift.
For example from Mike Jones:
A restrictor plate cup engine that turns 8,000 rpm runs a 304/312 seat duration cam. A non-restricted cup engine turning 8,000 would require a 304/312 seat duration cam. The non-restricted engine would be pulling in a lot more air at 8,000, yet it would require the same amount of overlap duration. That flies in the face of your theory.
Mike Sloe (spl):
Wouldn't it depend on your definition of "most important"?
Harold defines "most important" as "has the greatest effect on power output". According to this definition, intake close can, in no way, be considered the most important event.
If intake closing was "numero uno", changing it would have the greatest effect on power output. Closing the intake valve, say, 8° late would have a substantial effect. The loss of compression and the addition of reverse flow would murder low end torque.
These thing DO NOT happen. Our torquiest cam is one of Harold's that closes the intake valve 8° later than the the very best Comp grind. It outpowers that same Comp grind throughout the entire power curve, especially on the bottom.
This gain proves that the opening events are MORE IMPORTANT than either closing event. If nothing else, it shows how much effect the opening events have on intake closing.
Intake closing can't possibly be the "most important" if it is affected by the opening events so much, can it?
You CANNOT outpower Harold's truck cam by dicking with the intake closing event. We've tried!!!
This is why intake closing is trumped by BOTH opening events events.
However, the closing ramps do have the greatest effect on dymanic stability, though. The Comp cam would sack a set of springs in one race.
P.S. The Asmus parameters NEVER make the most power, in our engines, at least.
Disclaimer:
These are only the results. I'll leave it to the engineers to explain(or explain away) my observations.
If the theory doesn't match the results, change your theory.
UDHarold wrote: ↑Tue Jan 29, 2008 9:02 pm
Let us examine 3 cams:
25 BTDC-75 ABDC 280*, 115 LSA/ICL
35 BTDC-75 ABDC 290*, 110 LSA/ICL
45 BTDC-75 ABDC 300*, 105 LSA/ICL
All three close at the identical intake closing point.
Does your common sense tell you they will make the same power, or different power curves, each desireable for different situations?
I have seen unsymmetrical cams, with delayed closing points, make better power than similar duration symmetrical cams, even from my early days at Competition Cams, cira 1977.
My current-designed NASCAR Truck cam shuts the intake valve 8*, right, EIGHT degrees, later than a similar duration cam from a major cam manufacturer.
According to the dyno, my design makes better torque and horsepower, and revs a little higher.
I must being doing something wrong.
UDHarold
UDHarold wrote:
thought some of you might like to read part of page 3 from my UltraDyne 1982 cam catalog.
"UltraDyne's unsymmetrical cams are designed as two entirely separate profiles. The opening side is short, to minimize reversion, which is the entry of burned exhaust gases into the intake port. The actual point of opening is critical here. Yet, we still need a large cam after TDC. Therefore we open it fast. If we opened it late, but at a slow or normal rate, the cam would be either too small or be retarded too far, or else have an extremely high acceleration rate, all bad features. By using the highest possible opening rates, we are able to catch cams 6 to 10 degrees larger in duration before they get to TDC. From TDC on throughout the profile an UltraDyne cam acts as if it was 6 to 10 degrees larger than its seat duration.
Isn't this bad? Isn't the cam too big? No! Up to the limits set by port flow and engine geometry, engines love big cam lift and duration. What kills most big cams in engines is the early intake valve opening. The sooner an intake valve opens, the more and higher pressured exhaust gases will enter the intake port and the longer they will delay cylinder filling with a clean air/gas mixture.
Instantaneous flow in the intake port is directly related to the instantaneous flow occurring monentarily before. A port that starts flow late can never catch up. The sooner we start flow into the cylinder, the higher the port velocity the flow will have at any larger degree of crank rotation....."
I had explained to Dave Vizard back in the mid-80's that the part that does all the heavy work is the first 75 degrees ATDC of crank rotation. This is where the piston is accelerating up to maximum velocity. After this point, the piston is slowing down, although the inertia in the air/gas mixture keeps increasing port velocity as the piston nears BDC. As the piston is slowing down, the inertia increases at a ever-decreasing rate. But if everything is done right, the air/gas mixture will still be ramming in at valve closure, ABDC.
Just some food for thought,
UDHarold
PS---You may read this for yourself, but not in a 1982 UltraDyne catalog, at "wayback machine" on Google. Just enter
www.ultradyne.com in their search box. Page 5 also repeats the information on inertia ram.
I have delayed answering your question because the answer is very long, and there are many different interpetations of it. Eacfh Cam designer has his own theory, and often they contradict each other. Here are some of my thoughts and practices on cam lobes vs airflow.
First--AFAIK, there is NO airflow into the engine BTDC, except for various supercharged/turbocharged engines. BTDC in an unblown engine the piston is pushing things, ie--exhaust gases, out of the cylinder/combustion chamber. These gases have positive pressure(measurable back-pressure) and block the intake port as the intake valve opens. We call this REVERSION. I believe no intake airflow can start into the cylinder until this reversion is cleaned out of the intake port. This is why I design all my cams to minimize reversion, by delayed intake valve openings. I have done it this way since 1977, and I have seen that it works.
Second--I believe the part of the intake stroke that does all the 'heavy lifting' is the first 75* ATDC. Notice that max valve/lobe lift occurs 100* tp 114* ATDC--The piston has been slowning down for 25* to 39* before max valve lift; This is why max valve lift isn't the most important thing. What most people see with very high valve lifts is the actual valve movement from TDC to 75* ATDC, as mentioned before.
Third--The potential airflow(the airflow curve at various valve lifts) and the valve movement controlled by the opening side of the cam lobe generate the potential airflow into the cylinder---The rod/stroke ratio comes into play also, as the downward piston movement, and the rate-of-change of the piston, let the atmospheric pressure push the air into the cylinder.
So, mid-lift flow is very important, if not most important, in filling the cylinder.
The only caveat is when the port volume is very large(over 33%) of the cylinder volume, and the rod/stroke ratio is very long, 1.9:1-2.25:1. Then very fast ramped cams can move the valve faster than the air can follow the valve into the cylinder and the engine always acts over-cammed. I see this in NASCAR engines with flat tappet cams, it is not restricted to rollers.
I hope this has been informative and given you something to ponder. Even if port airflow hits a plateau and levels off, it is OK. Extra valve lift above that point is for dynamic control. High valve lift never hurts port flow, unless boundary air conditions start decreasing airflow above some valve lift. Then the port must be fixed, or the net valve lift stay below that point.
.
Here are some thoughts I've had for a very long time......
Engines work off of pressure differentials--ie, differential equations......
In unblown engines we have a positive pressure(back pressure) in the exhaust pipe, and a negative (vacuum) in the intake port BTDC. The piston is moving upwards, pushing the exhaust gases out the exhasut port. The numbers aren't far apart, say 1 1/2 lbs backpressure, a few inches of vacuum, but it does not allow flow into the combustion chamber. Then TDC, and the piston starts down. As it moves downward, it is creating a volume where no volume had existed---and atmospheric air pushes in to fill that volume. This is even why cars run better on cold days that in the heat of summer, the cold air weighs more, and fills the cylinder faster because of inertia.
I'll be a little scattered myself, because of the lateness of the hour, but two points of interest.
First---The engine never has long-term memory. It only knows what is happening NOW, and what just happened. Here is an example:
Intake opening 40* BTDC--- Is this a
288* cam on 104 ICL?
292* cam on 106 ICL?
296* cam on 108 ICL?
300* cam on 110 ICL?
304* cam on 112 ICL? or a
308* cam on 114 ICL?
With symmetrical cams, all those intake durations and ICLs open at 40* BTDC.
Or, if it is an unsymmetrical cam, it might be a 288* cam on 101 ICL, like in my UD 288/296R6.
At .200", the 288 is as fat as many 300* cams, but the upper part of the cam lobe is way advanced compared to them. The engine sees the overall shape of the cam lobe, not some number we use to measure it with. The higher the port velocity, the better the inertia ram.
Second---Most engine builders, probably you too, have tried the same cam lobes ground on different LSAs, say like 106 and 108. If you dyno them in the same engine, and on the same Intake CL, both cams obviously have the same intake cam on the same ICL. Oddly enough, they have different power curves, and everyone says "Sure, because of the different LSAs!". Remember, they both had the same intake lobe on the same intake CL. The power curves differed because the effects of the exhaust back pressure and the resulting reversion were different for each test, and each cam filled differently because of their reversions. Once you start thinking on this, it leads all sorts of interesting directions......
Enough for me tonight, see you all tomorrow.....
UDHarold
