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Isky cam's interesting comments about split duration cams

53K views 39 replies 23 participants last post by  427L88 
#1 ·
Isky says longer exhaust duration cams simply kill low-mid range torque and no more...and that single pattern cams alway make more power...

"...Why is it that so many people (racers & cam grinders alike) insist on running a cam with longer exhaust duration regardless of what equipment is employed? The answer is "habit". Most of them have been somewhat successful in doing it their way and will probably never change unless virtually forced by circumstances to do so.

Before we go any further however let's review what it actually is we are trying to do with an engine when we attempt to make more power. Our best result comes when we are cognizant of the fact that an engine is basically an air pump. We pump it in and out (although in a different form) and we have problems when one side or the other is restricted. Balance or the equilibrium or flow should be our objective, unless of course we are not trying to make more horsepower!

Example #1 (Oval track racing) Here, I have often observed that the most experienced drivers are those who are most likely to run a single pattern (equal on intake and exhaust duration) cam. Why? Because such cams always, I repeat always make more torque! These veterans have a more educated foot and greater experience in feathering the throttle in the corners. They can therefore, utilize the benefit of added torque, in the lower to mid RPM range, to their advantage.

Their counterparts, the younger drivers on the circuit, generally are not as experienced and may at times actually get "crossed up" in the corners especially with a lighter car or when they are learning the ropes. In their case, a longer exhaust duration is often the more appropriate choice. It will often help them to drive better, more "flat footed" if you will, without consequence. But please for the sake of accuracy, let us be truthful. The benefit comes from an actual bleeding off of low to mid range torque, which is always what happens when Exh. Duration is lengthened, not from any improvement. The improvement, (if any) would come because of an improvement in scavenging at the extreme upper end of the power curve and would usually be marginal at best. Yet the so-called "extra power" potential of a longer Exh. Duration cam is most often why they are touted - power most people are backing away from at the end of the strait away!

Example #2 (Drag Racing) At the drag strip it's a little different and I feel more honest. Here, racers have long enjoyed longer exhaust and longer durations across the board (If I may add specifically for the purpose of "killing" low-end torque) to keep the tires from too easily breaking lose. This has been successful and sometimes actually results in a slight increase in top end power - something you can actually use in drag racing since it is a full throttle endeavor through the lights. Keep in mind here though, it's quite possible that a longer duration cam overall would have done just as well or better. In other words if you needed that longer exhaust for top end, perhaps the intake could have benefited from such a lengthening as well. ..."
 
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#2 ·
we'll see what UDHarold or dacaman has to say about this!

to me, this sounds very old school....maybe they need to get into the 21st century:D

I'm stickin with my Lunati voodoo 268:thumbsup:
 
#3 ·
My understanding one of the reasons that split duration cams were developed to make up for cylinder head flow differences between intake/exhaust ports .....so not sure a blanket statement can be made on which type makes most power cylinder head design needs to considered in the combination ???
 
#5 ·
No cam expert here, but my voodoo 262 in my 327 has alot more torque and top end power with the 1.6's over the 1.5's. I guess it's a matter of the combination of parts.
 
#8 ·
Like GRN69CHV, I feel strongly both ways. You have to get it out before you can get it in. Street cars with typically restrictive exhaust systems should benefit from a little more time to get the exhaust out. That said, if the head flow has the proper bias between intake and exhaust, there should be no reason for unnecessary blowdown.

In my opinion, the most important part of any cam design, with any timing, is to clean the exhaust gas out of the combustion chamber. Left over exhaust gas is the greatest killer of power at any RPM. When an engine "comes on the pipe", it runs better because residual exhaust gases have been removed from the chamber.

To settle this argument, one only needs to perform standard lash loops on the dyno to determine what a particular head/piston/manifold/carb/compession setup wants. I'm confident that most competent tuners are this sharp, and I would think it would be mentioned in the Isky article. Maybe the Isky article should have been about lash loops and teaching how to determine what cam YOU NEED for YOUR engine and the way YOU drive.
 
#9 ·
Our best result comes when we are cognizant of the fact that an engine is basically an air pump. We pump it in and out (although in a different form) and we have problems when one side or the other is restricted. Balance or the equilibrium or flow should be our objective, unless of course we are not trying to make more horsepower!
I'm glad Isky says this so I ask. . .How can you calculate, figure, select the cam for a given engine without taking the flow numbers in account? We are talking about the item that controls the air potential in and air potential out....
 
#11 ·
Of course single pattern cams make more torque. That's why Pro-Stocks use 20* bigger exhaust cams at .050, to kill off ALL the torque in the engine.........
There are all kind of reasons to run single-pattern cams.
There are just as many, or even more, reasons to run dual-pattern cams.
There are even reasons to run reverse-pattern cams.
It all boils down to two things, What the cylinder head flow needs, and what the customer wants the engine to do.
I have made lots of hydraulic cams that are single-pattern. I have made hydraulic cams that are smaller, and bigger, that are dual-pattern cams. Both types worked fine for what they were supposed to do. I even tried to talk Holley into 1 or 2 single-pattern VooDoo cams, but then, Comp didn't make any single-pattern Xtreme Energy cams.
At UltraDyne I had a single-pattern flat tappet finish 2nd in the Winston Amatuer Championship.
There are reasons for everything. The trick is knowing the right reason for any application.
I sold 2 reverse-pattern roller cams to a customer today. About 90% of what he buys, and he buys them 2 to 6 at a time, are reverse-pattern. He also helped me develop the 283/289F6 flat tappet, and he's bought a lot of those.
My most popular flat tappets are 6 to 8 degrees larger on the exhaust than on the intake.
The most popular rollers are mostly 8 degrees larger, some only 4, some 16 to 20.
A cam should always be made for what the customer wants the power band to be like.
All of the above are the correct answers.

UDHarold
 
#14 ·
How is Rocker Arm Ratio going to increase duration? Lift sure but duration is ground in right? You lost me on that one. :confused:

Troy
 
#16 · (Edited)
Many people incorrectly assume that a change in rocker arm ratio has no effect on the duration the valves are open. In actuality there is some change to the duration at the valve depending on the lobe profile of the camshaft. Comp Cams explains that by lifting the valve an additional 7-8%, but beginning and ending the lift at the same point, the opening and closing rates become steeper. This increases duration, measured at the valve at 0.050" lift, by 1-2 degrees. You can measure the actual duration change by using a degree wheel and dial indicator set on the valve retainer. Coincidentally the principle of a steeper rate of opening and closing is what Comp Cams has engineered into their Extreme energy camshafts to enable more valve duration for a given lobe duration.
.
 
#17 ·
Many people incorrectly assume that a change in rocker arm ratio has no effect on the duration the valves are open. In actuality there is some change to the duration at the valve depending on the lobe profile of the camshaft. Comp Cams explains that by lifting the valve an additional 7-8%, but beginning and ending the lift at the same point, the opening and closing rates become steeper. This increases duration, measured at the valve at 0.050" lift, by 1-2 degrees. You can measure the actual duration change by using a degree wheel and dial indicator set on the valve retainer. Coincidentally the principle of a steeper rate of opening and closing is what Comp Cams has engineered into their Extreme energy camshafts to enable more valve duration for a given lobe duration.
M.Manner is right! There will be an increase in duration with high lift rockers depending on the ratio and the steepness of the opening and closing rates. I've seen 2-4 degrees on degree wheels.

If your standard ratio is 1.5, and you get a small base circle cam, you can get back the duration you loose with the small base circle by going to 1.65 rockers, with the added bonus of some extra lift. (I believe UDHarold says this no longer occurs with the CNC Landis grinder, which is why he will be grinding my next cam.) To me, buying a standard lift roller rocker for an intake valve is a waste of money. A higher ratio arm gives you more to tune with by changing lash.
 
#18 ·
Yup! Im in harmony with the others.....it does change duration. Its not the lobe itself obviously that changes, but whats going on at the valve changes(thats what really matters).

So for example, you got a camshaft that has 230 degrees at .050(that meanst .050 lobe lift). This is the degrees of rotation between .050 on the opening side of the lobe to .050 on the closing side of the lobe. With 1.5 rockers, the valve is at .075 lift when lobe lift is .050. So while we measure duration at .050 lobe lift.....the engine is "measuring" duration at .075 valve lift I guess you could say. Duration at .075 valve lift in the engine would obviously be 230 degrees as well(230 at .050 lobe lift is the same as .075 at the valve.)

With 1.6 rockers.....things change. At .050 lobe lift, the valve is at .080(larger than .075). The engine is seeing something different with the same .050 lobe numbers.

But to bring it back on track to duration.....remember .050 lobe lift is .075 valve lift with 1.5's. What matters is what the valve is doin......so think about this and pretend we are measuring valve lift duration instead of lobe lift. With 1.6 rockers, at .075 VALVE lift, you have roughly .046 lobe lift. The duration at .046 lobe lift is greater than .050 lobe lift. So if we were measuring at the valve......duration at .075 valve lift with a 1.6 rocker is greater than duration at .075 valve lift with a 1.5 rocker. We dont measure duration at the valve. Duration specs are based on the cam lobe. But all the engine cares about is what is happening at the valve......and things do change at the valve.
 
#20 ·
All these facts, or theories, change with RPM. What is true at low RPM is not true at high RPM. All cams are a compromise, until we go to digitally-generated valve lift curves.
The longer you can keep an exhaust valve closed, the longer power stroke you have, and the more torque gets put into the crank. Single-pattern cams keep the exhaust valve shut longer, so more bottom-end torque.
At higher RPMs, past peak torque, the most important thing is getting the exhaust out of the cylnder so a fresh charge can get in. If you can't get the exhaust all out, you can't fill the cylinder as full, because old exhaust is still in it.
You notice this most often when you hit peak power. The Torque/Horsepower curve takes a bigger drop, caused by an inefficient exhaust system or cam.
Single-pattern cams make good bottom-end, but limit the top-end horsepower.
Dual-pattern cams make good top-end, but give up a little bottom-end torque.
Reverse-pattern unblown cams are a special case of type one and type two.
They aren't usable except in very limited carburetion applications, where the carburetor limits the maximum amount of horsepower.

UDHarold
 
#21 ·
That was an excellent and clear explanation Harold.. I learned something.

mike
 
#22 ·
Right on the money with the exuast opeing point. I had the Comp NX279HR in the 403 motor for a short while. Specs were 279/294 adv, 230/242 @ .050, 113LSA/108ICL. First thing I noticed was how pronounced the exhaust became. By design, this cam was intended for N2O, but I used it to mate the 2.19/1.72 valve combo in my heads. I restropect, the fact that the exhaust got so loud was due to blowing down the cylinders at/near peak pressure, probably a poor move with a 3.76" stroke. Could definitely see the benefit of added exhaust duration with a stroker though. Comparing a 4.25 stroke to a 3.75, that's a half inch of added stroke to play with. Once you get past 90* ATDC, that piston gets down in the hole very fast compared to the shorter stroke.
 
G
#24 ·
I gotta say that the "Cam-Father" said, "IN THE RIGHT COMBINATION, a single pattern cam always made more power"!!

I figure that Ed is right as is Harold too!!

IN THE RIGHT COMBINATION!!

But, like Harold said more about, the right combination is HARD to HIT w/o cubic MONEY to find it!!

Right Harold?????????

And Harold, I also gotta say how much we look forward to your lectures on this stuff b/c I figure that you are about the only CAM EXPERT that has offered to educate us gearheads on the Boards AND I for one really appreciate it!!

pdq67

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#25 ·
And Harold, I also gotta say how much we look forward to your lectures on this stuff b/c I figure that you are about the only CAM EXPERT that has offered to educate us gearheads on the Boards AND I for one really appreciate it!!

pdq67[/QUOTE]

I also appreciate Harold taking time to provide the facts .............:yes:
 
#26 · (Edited)
Paul, the combo is akin to yours, 4.25" stroke, GM iron heads, ovals with valve angles laid back to enhance low/mid lift flow. Old L88 block should go .100 over. Qjet on top/ M20/3.31. Shifting at mid 5000. Guess it could get $$$ with a roller cam setup, but it doenst have to have one. I Do think a highly modern and intense roller cam would make the motor. And the valve trick.

Mr. Brookshire gave me a grind a while back, which I beleive he designed but never ran any of. 248@.050 square, I forget the seat dur/lift. Specifially for a 496, hot street.

But I need one to idle like a puppy. NO LOPE.Another reason I want a "square" cam. "Anti-thumper", which gets on quick and lasts for a bit. Willing to pay for a roller grind to get the great accelerations/lift off pretty small dur numbers. Would like in excess of 500 ft lbs at 2000 rpm to 5300 rpm. Quick Tq peak and some range to it. Wide lsa, I'd reckon. 112-115 LSA, 264/235/152 dur .600-.650". ??

Yup, thank you Mr. Brookshire for the facts and for the tons of wisdom alongside them!

Now, can you design my "Contra-Thumper" cam for me? ;)
 
#29 · (Edited)
I use an Ultradyne single pattern HR in my Biscayne. Oval port 781`s with their somewhat weak exhaust port. Not a real good candidate according to most for a single pattern cam. It`s 233 @ .050 with .625 lift on a 109 sep.

I have never dead hooked, I use BFG drag radials and have to drive it out carefully then mat it. With that in mind I`m sure there`s a couple tenths left in it. Shifts are usually around 5600-5800 rpm 1st-2nd and 6000 rpm 2nd-3rd.

534" in our Camaro ran pretty decent IMO using another single pattern HR Ultradyne.
Dart 325 heads this time. Same shift points as above btw.
With humidity and heat, a greasy track and spinning off the line with 1.47 60 FT it ran 9.97 ET @ 136 MPH

X-pipes on both too, maybe they are helping with scavenging the spent gases. Some believe an X pipe OVER scavenges, while that may or may not be true, I`d venture to say the cams used were probably your typical dual pattern designs.
 
#33 ·
Didn't no one ever tell you: "You'll burn the valves unless you got some back pressure?"

Since I've switched to Desmonic Freen Valves, all burning has been eliminated.
 
#31 ·
If Ed is the Camfather then what is UD...Sir Camelot?
 
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