Discussion: advertised duration vs @.050 duration - Chevelle Tech
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post #1 of 50 (permalink) Old Feb 5th, 04, 1:03 PM Thread Starter
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hi guys

is it me or have there been lot of cam posts lately? i thought this post would benefit me and others, so here we are.

i have a pretty good knowledge of camshaft theory and specs but i have always shied away from concentrating on the @.050 duration specs. i notice that some magazines/catalogs just publish the @.050 numbers and lift. i also notice that some people go around quoting @.050 numbers off the top of their head like its second nature to them.

i do know about lobe intensity, for example the way i understand it you could have two cams, both with the same advertised duration. the cam with more @.050 duration has more lobe intensity......correct????

so other than that are there any other benefits/reasons to concentrate on the @.050 spec ?????

Brandon

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post #2 of 50 (permalink) Old Feb 5th, 04, 1:56 PM
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You are correct about the lobe intesity.

The problem with advirtised duration is standardization. Comp rates there cams at .006", GM at .001", Crane at .0045", etc. Everybody is different.
Take my Comp Cams 292 camshaft. It's 292* @ .006, but here it is on everybody else's ratings:
GM: 318*
Crane: 298*

So you can see, there is a big difference between .001 ratings and .006. But GM, Crane, and Comp will all agree that this cam is a 244* @ .050. It just sets a level playing field.

When selecting a cam, I take into account lobe intensity and lift when making my choice, but mainly focus on duration at .050" and LDA.

Matt Jones
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post #3 of 50 (permalink) Old Feb 5th, 04, 1:59 PM
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That's good to know, thank you

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post #4 of 50 (permalink) Old Feb 5th, 04, 2:19 PM Thread Starter
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Matt great info thanks, but it just opens more questions for me.

i use the dcr program alot, thanks Pat! the dcr program asks for the advertised duration to figure the dcr. but it seems to me there could be a problem getting the correct dcr due to the different specs the manufacturers rate their cams at.

for example i have a comp 305h magnum in the 427. comp rates their cams at .006 . but at .001 this cam would be something like 331 advertised ?????

is this a problem? is there a way around this? i mean the difference between 331 advertised and 305 could effect the dcr quite a bit.

Brandon

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post #5 of 50 (permalink) Old Feb 5th, 04, 2:47 PM
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Always thought that seat, .050, and .200 durations would give a better picture. With the three measuremnts, ( even if you threw seat away since they arent standardized), the durations at .050 and .200 would at least allow you to compare the cam's flank, the slope of the cliff the lifter has to rise on.

I realize its only one dimension, and doesn't give you all the facts, but using .200 durations would give us a much better idea of what the cam was about, I mean short of a system of simulatneous equations to describe 4 or 5 planes!

Would allow you to compare .050 and .200 duration to really get a sense of how agressive a cam it is.
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post #6 of 50 (permalink) Old Feb 5th, 04, 3:48 PM Thread Starter
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Gene
when you get time could you explain further?

also when figuring dcr is there anything i/we can do to take in account the different measurements that the manufacturers take their specs at?

Brandon

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post #7 of 50 (permalink) Old Feb 5th, 04, 4:26 PM
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General rule of thumb, on typical street hydraulic cam, subtract 6* from cranes advertised numbers to get a comparable seat timing number as a comp cam. I use comp cams as my standard when figuring dcr's because I have used a lot more of their cams than anybody elses, and have a better idea how they act.

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post #8 of 50 (permalink) Old Feb 5th, 04, 5:04 PM Thread Starter
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thanks for the clarification Gene.

Quote:
I use comp cams as my standard when figuring dcr's because I have used a lot more of their cams than anybody elses, and have a better idea how they act.
travis

i can understand that, but it seems to me the dcr you came up with would be off wouldnt it? since a comp cam is measured at .006 the cam is actually larger than the advertised spec they give. but the engine doesnt care where the cam is measured, it sees the whole cam.

why would a cam manufacturer do this? to make their cams look like they have more lobe intensity than they actually do?

thanks

Brandon

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post #9 of 50 (permalink) Old Feb 5th, 04, 6:04 PM
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SAE call for the cam's seat timing to be measured at .004" tappet lift. This is the allowance for valvetrain compressibility. At .004" the valve should be just starting to come off the seat. Measuring at .006" means the cam is longer by what ever .002" of lift works out to. Chevy's .001" lift is totally unrealistic since the valve is still on the seat.

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post #10 of 50 (permalink) Old Feb 5th, 04, 6:29 PM Thread Starter
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Pat

that explains it, thanks. i believe ive seen you mention this before but i never put two and two together. i also wasnt even thinking about the seat area of the lobe.

thanks to all who replied.

Brandon

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post #11 of 50 (permalink) Old Feb 5th, 04, 10:18 PM
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Your right, the numbers would be slightly lower than cranes with a similar .050 cam. However, I know for a fact that with vortec heads and a xe268, my dcr using comps .006 timing numbers comes out at 7.88. It runs fine on 89 octane, and 87 octane in a pinch. A 7.88dcr reading with a crane cam would not be the same using cranes seat to seat timing numbers. Make sense? Now if I wanted to swap cams, and want to use a crane cam, I can adjust the crane seat timing numbers to get a dcr reading that would be comparable to my comps reading, and would have a very good idea if it would also run on 89 octane or not.

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post #12 of 50 (permalink) Old Feb 5th, 04, 10:38 PM
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Ahhhh---A subject worthy of a chapter in my up-coming book.

Pat,
Thanks for the SAE .004" explanation. IRREGARDLESS of where a cam manufacturer rates his hydraulic cams, the SAE has very good scientific proof that .004" is the correct rating point for hydraulic cams. Don't throw up about Rhoades lifters, they are a special case.

You should always use SEAT timing when working the DCR formulas, or any 'Desktop-Dyno'-style computer simulation programs. When I get through here, I hope no one thinks that the .050" number is the one to use. You can even have cams with the same .050" duration number, but DIFFERENT timings at .050", and an engine will see them as 2 different cams.
I hope everyone will not think that I skip around too much, but here goes.....
We'll start with the intake opening, although the exhaust actually opens first. I suppose you've got to have charge in the cylinder in order to have something to exhaust out....
An intake opening point in itself does not tell what duration or LSA/ICL a cam is on. Cams range from Symmetrical to various degrees of UnSymmetrical-ness. Each of these various cams, all of the same overall seat duration, will be seen by the engine as different cams. I have even said that the most important degree in your cam is the one BEFORE the intake valve opens. If we could get an instantaneous reading of the exhaust gas volume and pressure right before the intake valve opens, we could predict how the cylinder would fill. Identical opening points, but different exhaust gas volume and pressure(REVERSION) will cause different rates of intake filling. When the intake valve starts off the seat, this higher-pressure exhaust gas flows partially into the intake runner. What keeps it from going too far is that the piston is slowing down for TDC, and pushing less and less on the exhaust gases. In fact, we can use where the intake valve opened as a relative guide to the amount of reversion.
Now what about the .050" number?
First off, a good part of the .050" duration is on the WRONG side of TDC. The piston isn't sucking charge in while it is moving upwards toward TDC, it is PUSHING stuff OUT of the cylinder. Airflow into the cylinder does not start in a N/A engine until after TDC, and AFTER the reversion that had entered before TDC has been cleaned out of the runner.
The SOONER airflow starts into the cylinder, the higher the port velocity(the Rate of Filling the Cylinder), and the longer inertia ram will continue to fill the cylinder after BDC.
To keep this post relatively short, I won't go into the effects of high-lift area.
When the intake valve closes, the port velocity governs how the cylinder is still filling, even 60 to 90 degrees ABDC. Low port-velocity, later closing means less cylinder pressure and less torque. High port-velocity will use late intake closing to pack even more charge in, and delivery high cylinder pressures.
I will close this with a small example that I have used many times in the past---my 288R lobe.
Imagine a symmetrical 288R on a 102 ICL---It opens at 42 BTDC and closes at 66 ABDC.
My April 1980 288R on a 102 ICL opens at 39 BTDC and closes at 69 ABDC.
This cam has 3 less reversion in the intake runner, so it starts airflow earlier(a little more than 3, because of the way the back=pressure drops). This gives it a higher velocity in the runner, and a higher rate of cylinder-filling.
Now look at the closing points. The symmetrical cam, with a lower port-velocity and rate of cylinder filling, shuts the intake valve EARLIER, allowing LESS charge in the cylinder.
My 288R Unsymmetrical cam shuts the intake valve 3 later, with a higher port-velocity and a higher rate of cylinder filling. More charge in the cylinder, higher cylinder pressure, more torque.
Engines see the seat opening and closing points, and the .200" duration numbers, as a close guess as to the cam. They actually see the entire valve lift curve as the cam, not a number like .050". This number is for our convience, to have a commonly-understood point to talk about cams. Engines don't know what .050" durations are.
It's worked this way for almost 24 years now.
If you have other questions, just ask. I may have to answer some tomorrow....

UDHarold

UDHarold

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post #13 of 50 (permalink) Old Feb 5th, 04, 11:29 PM Thread Starter
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Harold thanks for the reply, excellent info.

thanks to everyone who replied, great info from all posters.

[img]graemlins/thumbsup.gif[/img]

Brandon

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post #14 of 50 (permalink) Old Feb 6th, 04, 1:18 AM
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Harold, why don't you see if you can get Lunati to spec their adv dur at .004" lift (I don't know what they use right now) and give the seat timings rather than .050". As you have said, .050" valve timing is meaningless.

Pat Kelley
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post #15 of 50 (permalink) Old Feb 6th, 04, 6:07 AM
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First off, a good part of the .050" duration is on the WRONG side of TDC. The piston isn't sucking charge in while it is moving upwards toward TDC, it is PUSHING stuff OUT of the cylinder. Airflow into the cylinder does not start in a N/A engine until after TDC, and AFTER the reversion that had entered before TDC has been cleaned out of the runner.

How much effect does the scavenging pulse have on this effect? Can the cylinder pressure be negative before TDC?

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