I'm trying to figure out this cam card. As i understand it to figure overlap from the intake opening and exhaust closing events is to add them together.
Looking at this card the ex valve closes at -2.5 ATDC (which means 2.5 BTDC?) The in valve opens at 5.5 BTDC. If I add those together I get 3 degrees of overlap. Whaaat?
Can someone explain this to me. I'm feeling a little dense over this. I must be missing something.

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Ok I think I see what is going on with this cam card info. I still don't know how to convert it into something I can understand. It looks like all the timing events are maybe at .050 lift? Or maybe they are at lash and the numbers I'm looking for are at .050. Knowing that does not help me figure out how to get what I need. Is there a way to convert this or do I have to get it from Lunati?
The numbers I've seen on other cam cards have much later Intake valve closing events and the over lap can be easily figured. If I plug this intake valve closing number into a DCR calculator I get crazy high DCR's.

 

I was trying how to explain this. You don't have to add or out think yourself. What is says is what it is!

Maybe draw it out on paper. So think of it this way the spark plug just fired and it drives the piston down. At 41.5 degrees before "Bottom Dead Center" the exhaust is opening. As the piston stops at the bottom and comes back up the bore to push the exhaust out. The next valve event is the Intake opening at 5.5 degrees before "Top Dead Center" The exhaust valve is closing after the intake so this is "Overlap" of 8 degrees later at 2.5 degrees after "Top Dead Center" the exhaust closes.



The intake stays open to 33.5 After BDC and come up on the compression stoke to fire again.


The cams advertised numbers are 270/270 which means nothing for comparisons. The .050 duration numbers give a some what truer definition. If you really want to know what cam lobe is compare the .200 of an inch duration numbers

 

I understand about the events and how they interact with each other. It's just that to enter the valve closing event into a DCR calculator it needs something like 60* not 33.5* to be some kind of realistic number. And I'm pretty sure this cam does not have 3* of overlap that would be crazy narrow and cause really high cylinder pressures unless it was a really short cam. It should be somewhere around 54* of overlap.

 

DCR calcs are iffy at best, but maybe this would help you from the cam card.

See you have 219/219 @.050 and 270/270 @ probably .006 inch. Just do a little math and you get 270 -219 = 51 place 1/2 of that 25.5 degrees to each side of the .050 numbers. So total over lap is @ 58 degrees. but only at very low lift and minimal flow rates

Yes this cam would only work on a low compression motor. What is so iffy about DCR calcs is a higher lift cam with roller lobes will fill a cylinder a higher percentage increasing DCR. Then another iffy addition is a true tuned port intake system. It has the ability to efficiently "Ram" in adition cylinder filling also raising DCR over it's "tuned" rpm range even creating higher DCR.

 

FWIW, I agree that duration @.050" and @.200" are better for evaluating a given cam's performance characteristics than is advertised duration. However, I also appreciate that knowing tDCR* can be a useful tool (tho it isn't the holy grail) for many a pump-gas build. Pat Kelley's calculator appears to be the best you'll find online. If the card is accurate, just enter the advertised durations, LSA and ICA, and proceed...


* The "t" I put ahead of DCR above stands for theoretical. In a working engine, actual DCR = CR x VE% at a given RPM. Given that an appropriately cammed, high SCR engine will tend to produce significantly higher max cylinder pressures and temps than will a lower SCR build with the same tDCR, as SCR's increase tDCR becomes less able to predict the practicality (relative to octane) of a combination. Hope that's worth $.02

 

I see a note that says "Timing figures at .050" lift". So u really need the full duration numbers. Ok here goes.
Intake center line is 104 BTDC (whether full duration or 0.050" lift) with 270 full duration gives 270/2=135, Int CL 104ATDC + 135 = 31*BTDC and 104ATDC-135=59*ABDC.
Next LSA=108 which = 216 at crank, so 104ATDC + 216* = 112BTDC = Exh CL.
Now again 270/2=135 and 112BTDC + 135* = 23*ATDC and 112BTDC - 135* = 67*BBDC.

All this makes your timing card look like this 31-59/23-67 where Int opens = 31*BTDC, closes = 59*ABDC (Int CL = 104*) and Exh opens 23*ATDC, closes = 67*BBDC (Exh CL = 112* BTDC). Of course your overlap is still 270 - (108 * 2 = 216) = 54*.

Whew, i swear Lunati cards have reverse rotation.

Good luck Reel',
cardo0

 

You need intake closing point for DCR calculations? Here you go.
http://forums.corvetteforum.com/c3-t...lculation.html

 

I see how I could use that for a ballpark figure. One would have to assume a symmetrical profile. I know the profile on this cam is asymmetrical closing the valve at a slower rate that it opens it.

Great info! I suspected that a roller cam with greater lift poetential at the same relative cam position as a flat tappet would provide more efficient filling of the cylinder and thereby provide more cylinder pressure. That combined with 1.6 ratios and I could run into trouble. I just couldn't find it written anywhere so you are the first one to confirm my suspicion.
My concern now it too much DCR If I get 7cc pistons. Before it was too little DCR when I was looking at using stock 17cc pistons. From what I have read 7.5 to 8.5 DCR is acceptable with 91 octane or better, aluminum heads and a decent quench like .040".
I'm using this claculator which allows altitude input as well.

http://www.jeepstrokers.com/calculator/

I'm at 3000 feet minimum, but up to 5000ft and much higher density altitudes as it gets hotter, so I'm using 3000 for my calculations.
Deck is at 9.025 and using .015 head gasket with 61cc heads and stock con rods, bore and stroke on a 350 CI engine.

using this info I get 8.04 DCR if I input advertised duration. Since the additional cylinder filling is an unknown I'm not completely sure if my margin is good enough with pump gas on a hot day at high rpm (well 5500rpm or so) or under a heavy load like WOT and a shift to 3rd with a 3.08 rear and a TH350.
I'd like to use a flat top piston like the 7cc one to maximize the effect of good quench.


Yes this is good supporting info. I expect to get reasonably good (relative to stock) VE% with side pipes and STS baffles which claim to assist in cylinder scavenging at higher RPMS.
Haven't read a lot about VE yet, so not a 100% clear on the factors that affect it, but logically I can see how good scavenging should increase effeincey.
As I understand it VE increases as RPM's increase due to intake charge momentum, making it harder for it to go out the exhaust valve during overlap. like I said though haven't done a lot of reading on that yet so I may have it wrong.

 

That cam is to small for your 61 cc heads.......... I didn't figure it out, but your static is near 10.7 or something. It will just ping cruising down the road at light throttle

 

Static comp is 10.38 at sea level, dynamic 8.64 using a 7cc piston and the .015 gasket. These numbers are too high computed at sea level.
Static comp is a volume/volume computation, so a fixed comparison, however if you factor in altitude and loss of air density this would reduce effective static compression right? That in turn would reduce cylinder pressure and move the engine further away from detonation?
So if I plug in my numbers here.

http://www.jeepstrokers.com/calculator/

and use 3000 ft I get; static compression 9.78 and dynamic 8.04. This is using advertised duration. Am I all wrong on this altitude compensation thing?
My original post was trying to get advertised intake valve closing to get a better idea of where I might be at since using what looks to be a .050" intake valve closing does not compute properly.

 

I compute everything at "sea level" never heard of any adjustments to the formula. I always figure a safety margin. If everything is right and tight quench, aluminum heads 8.3. Iron heads 7.8. Never failed me. HP sdacrifice is minimal. Getting it together and having detonation issues sucks. I gave you a formula for computing intake closing point. A 10.4 to 1 static needs about a 282 minimum advertised intake duration to work with premium. Great heads may increase this. A high gear diff and big drop between gears will increase this. Running hotter intake charge will increase this, a hot day will increase this, a little to fast of timing curve will increase this, a little lean on carb A/F ratio will increase this, a little carbon buildup over time weill increase this, a headwind will increase this, get it. Play it safe, none of this tDCR is carved in stone, better a little less than pust the envelope.
.

 

I some how missed that initially this morning, thanks for the formula. Using that formula it figures out to a 59* intake closing.

I agree and yes I get it.
What I don't think were factoring in here is air density. My daily driver requires minimum 87 octane fuel according to the owners manual. At this altitude I can get and use all the time 85 octane. How can I get away with this without detonation? Cylinder pressure has been reduced due to elevation. I use it in the summer, winter, under heavy loads and with the air on in a head wind on the hottest days under heavy loads. It does have a ping sensor, of course, and will retard timing in an attempt to reduce ping if it is sensed but that also throws a check engine light if it occurs.
Travel to higher altitude destinations and you'll notice 85 octane is available at the pump, 91 octane is the highest octane available without ethanol additive. Travel to lower altitude destinations and 87 octane is the minimum and 93 is the highest (pure gas no ethanol)
what if I lived in Vale CO. The average elevation there is 8,150 ft. Using this cam,7cc pistons and .015 gasket now DCR is down to 7.01:1. The engine does not know what elevation it is at only what the cylinder pressure is right? So now I could even use 85 octane fuel. Since the drop in air density as you increase altitude is pretty linear then why would you not factor in lack of air density even at 2000, 3000, or 5000 feet?
Each cam has an operating pressure that it needs to be most effecient and produce acceptable power levels at specific rpms as near as I can figure. Too low of an operating cylinder pressure and the engine is a dog until pressures can be brought up via rpm or cylinder pressure or whatever.To high and it detonates. The common denominator here seems to be cylinder pressure and the numerous factors that can affect it.
Given that and the constraints of pump fuel limitations, cylinder pressures and the way to control that ie overlap, head flow, intake flow,exhaust flow, valve lift, cam duration etc. why shouldn't altitude and associated air density be added to the list?
I know this is over the top for a street built engine but we spend a lot of time picking out just the right cam, intake, heads, exhaust all in an attempt to get the most for our efforts so I can't see dropping what looks to be an important player.
Car manufacturers have to consider operations of the engine at all altitudes from the continental divide at 11,000 feet to Death valley CA, I don't. So when I build my engine, not a costumer's who I don't know where he's going to be driving it, shouldn't I factor in where I'm going to be operating it?
I'm not trying to be a smarta** or someone who is trying to reinvent the wheel just trying get the most for my efforts and understand why certain things are done or not done on these engines.
With all that said I tend to agree that 7cc pistons is pushing the edge too hard and backing it off to 10cc's or so may be a better way to go. Having some problems getting a 10cc piston on a rotating assembly at a reasonable price with a 1.56 CH. Still playing with various combinations which may include still using my stock rotating assembly.

 

I believe that you need to run higher static compression at higher altitude. I did a C/R for your listed setup and it was 10.38 which should work with aluminum heads.

It has been my observation that roller cams run very well with 8 - 10 degrees more duration than their h-flat lifter counter parts. Roller cams run smoother and have higher vacuum.

 

Thank you gkull. I was beginning to think I was alone on this line of thinking here.

Could one also express this as; a roller cam can be run with greater overlap ("X" number of degrees) than the flat tappet counter part, and still be true?
I chose a greater overlap than I would have in a flat tappet but with a shorter duration and tighter LSA.

 

FWIW, with ~10.4:1 and aluminum heads I'd put a 276* to 282* (adv) roller in it. You can always have it ground with wider LSA (~112*) to improve manners a bit and broaden out the TQ curve with minimal loss of peak HP.

 

I am pro wider LSA 112 -114 is 385 or less ci. The trade off is less peak TQ, but a higher overall average across the rpms. Better fuel mileage.

Then go up 8-10 dur. @ .050

 

That's interesting. That duration ground on a 112 would be 55* of overlap, almost exactly the same as 270/270 on a 108. The big difference would proabably be the intake valve closing point. If I ballpark the intake centerline at 108* then closing would be at 66*. That would drop DCR to 7.61 at 3000 ft from 8.04 with the 7cc pistons. Static CR of course remains unchanged.

 

Ok I got a new question. If I install this cam with the 7cc pistons and find that detonation is a problem for some reason, could a fix be to reduce the rocker ratio?
This would limit the valve opening somewhat and then the engine would have less air to compress reducing cylinder pressure. Nearly the same effect as a later valve closing angle. This is a low rpm motor not more than 5500 rpms.
Would this depend on what rpm the detonation is occuring? As rpms rise the VE increases and so does cylinder pressure so reducing valve opening would create more of an obstruction to air movement at higher rpms, than low rpms, right? Lol, why would someone intentionally restrict air movement.
I know about all the conventional fixes for detonation like enrichening fuel mixture and pulling timing,colder plugs, but I don't hear rocker ratio mentioned.
Another band-aid fix, if it's close, could be retarding the cam timing a couple degrees.
Try finding 10cc pistons with a 1.56 compression height. Not really available. Could move up to 12cc's. Personally I'm still liking the 7cc pistons just still have that nagging doubt that I may be way off.

 

if it pings on 85, try 89, or 91, that should do it.
If it pings at low rpm, DOWNSHIFT.
Learn to tune/drive.
Set timing where it won't ping. Sounds easy, but guys get stuck on a number and won't even try less.
Run cold air intake in summer.
Cooler thermostat can help.
Higher rocker ratio has the effect of a larger cam with more overlap.
An open carb base gasket will allow exhaust to dilute the mixture, and can be tried to reduce pinging. I'd do that LAST as it kills the low end, but would be better than pinging. Guys do this all the time and don't even know it, by using a single plane or air gap.

At the drags, put in 1-2 gallons of race gas.

What do u want on your tombstone? "he played it safe"
or "he took a chance now and then"

 

Lol! Always interested in what you have to say Matt. Don't always agree but I like your "outside the box" style.
I figure that this will be strictly a 91 octane engine given the CR''s it'll be running so I probably won't be trying anything lower.
I was doing some reading yesterday and came across some info that basically stated once rpms are high enough that VE reaches 100% or nearly so and this is the point at which max torque is achieved.
So I take this to mean that the overlap is no longer bleeding off pressure, or very little pressure and is not much of a player any more.
It is at this point where I believe I may run into trouble with high DCR numbers and experience detonation if I run it up too high. That got me to thinking about reducing rocker ratio to reduce cylinder pressures.
That was my line of thinking anyhow, not sure if it has any merit though.
Hadn't thought about the open carb gasket. I have both the open style and the four hole style carb gasket. Thought I would experiment with the four hole style based on the results you achieved using one. My old stock manifold has four holes instead of one big hole like in the eldelbrock performer that I'll be using. Putting the four hole gasket will make it more like the stock design. Could be restrictive, but not sure as I haven't experimented with it yet.