Discussion: advertised duration vs @.050 duration [Archive] - Chevelle Tech

: Discussion: advertised duration vs @.050 duration

Feb 5th, 04, 2:03 PM
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 ?????

Feb 5th, 04, 2:56 PM
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.

Feb 5th, 04, 2:59 PM
That's good to know, thank you ;)

Feb 5th, 04, 3:19 PM
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.

Feb 5th, 04, 3:47 PM
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.

Feb 5th, 04, 4:48 PM
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?

travis g
Feb 5th, 04, 5:26 PM
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.

Feb 5th, 04, 6:04 PM
thanks for the clarification Gene.

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?


Pat Kelley
Feb 5th, 04, 7:04 PM
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.

Feb 5th, 04, 7:29 PM

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.

travis g
Feb 5th, 04, 11:18 PM
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.

Feb 5th, 04, 11:38 PM
Ahhhh---A subject worthy of a chapter in my up-coming book.

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....


Feb 6th, 04, 12:29 AM
Harold thanks for the reply, excellent info.

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


Pat Kelley
Feb 6th, 04, 2:18 AM
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.

Feb 6th, 04, 7:07 AM
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?

Feb 6th, 04, 7:58 AM
For comparison, here are some #'s I got when checking out my xe274.
Add up the duration at each lift,.005 IO and .005 IC,and add 180 for total or adv. dur.
open Close
.003 33 .003 74= 287*
.004 31 .004 70= 281*
.005 30.5 .005 66.5= 277
.006 30 .006 65= 275
.050 10 .050 39= 229
I edited it 3 times to get it to look close to right.

Note: my cheesey 9" degree wheel was probably the reason for the numbers being slightly off.
I couldn't borrow the neighbors 18" wheel at the time.

Feb 6th, 04, 8:33 AM

The 'scavenging pulse' is much talked about, but very little understood. This is not about you asking, just about those writers who mix theories together without understanding any of them.
Pulsing is in both the intake and the exhaust, and is at a rate entirely different from that of port velocities. The pulses are referred to as 'Sonic' pulses, because they travel at the speed of sound, about 1100 fps in the intake, and around 1500 to 1800 fps in the exhaust, depending upon the exhaust gas temperature.
These pulses work just like organ pipes do, they travel from one end of the pipe to the other, reversing polarity(going from positive to negative and vice versa) when they reach an open end.
So, when the exhaust valve opens, a positive exhaust pulse is formed and starts moving through the out-flowing gas, towards the end of the header tube. When this positive pulse hits an expansion, such as the end of the header and into the collector, a negative pulse is formed and heads back up the tube. Measuring in the tube shows a slight increase in pressure as the pulse heads out, and a slight decrease in pressure as it travels back up the tube.
The trouble is, this pulse is travelling so fast that it can make several trips back and forth, and these are RPM dependent. The length of your header determines how many trips the exhaust pulse makes. A properly-designed header will allow 3 round-trips for the torque peak, and 2 round-trips for the HP peak.
Here are the problems----Everyone does not like reversion, so we have anti-reversion measures in place, such as step-headers, anti-reversion plates between headers and head, etc, etc, etc. These WORK! And in doing so, they minimize the effects of that sonic pulse in the exhaust. There is a bigger pay-off minimizing reversion than there is maximizing the exhaust sonic pulse. If you do want to maximize it, there is one small problem. Remember the 2 peaks, max torque and max HP? Between those 2, an extremely strong exhaust sonic pulse arrives during overlap, and blows charge right out the intake runner. Sprint cars with individual intake runner FI and 'weedburner' straight exhaust pipes sometimes show fuel standoff above the ram tubes--this is the positive pulse getting all the way to the end of the intake runners. Racing motocycles use this system all the time, it really works. But the usable power band is so narrow they have to go to 8-speed transmissions.
No one has yet invented a one-way valve that will allow negative pulses back to the cylinder, and keep positive pulses out.
The way I open my exhaust valves gives an extremely intense exhaust sonic pulse, and this is what gives my cams that "Sound".


Feb 6th, 04, 8:49 AM
And it's a beautiful thing; 'rings' those pipes like a symphony! :cool:

Wonder how that X pipe effects the waves then? ( it sure is alot more complex than simple scavenging as I understood it).

Feb 6th, 04, 11:26 AM
There are those who belive that the "scavenging" effect that is so often times talked about really doesnt exist... That despite these waves goeing back and forth there is still an above atmospheric condition present the vast majority of the time in the exhaust system...Jere Stahl has basically said that in most N/A cases the power increases one sees with exhaust system modifications are the result of better management of the exhaust back pressures.

In an slightly old book (1982) I have a pic of an occiciliscope (sp) showing a time / pressure trace of an intake port during a steady state rpm/load dyno run..... it basically is showing a sine wave and then the valve opening and closeing events....
in this pic it clearly shows a small reversion spike when the intake opens and the trace sort of follows as the cylinder fills and then as the intake closes......what is really intresting is what happens after the intake closes... the scope trace line jumps nearly twice as high as the intake reversion spike....... If you were to consider say an intake manifold like a single plane where all 8 runners are drawing from a common plenum you actually have all 8 runners blasting pressure back into the plenum ( reversion ) and different times, either from intake opening or the piston pushing it back into the intake tract with a late closeing ( long cams )...with the carb trying to feed A/F ...lets say you have any given cyl intake opening, you would have the closest carb barrell to this cyl feeding the majority of the air with the other 3 feeding lesser amounts but still all 4 trying to supply air.But other cyl pulsations are blasting up and back in the intake...so as you are trying to feed one cyl a reverse pulse in another blasts A/F back into the plenum and it may then see the air coloum trying to fill that cylinder and participate in supplying air.... or a pulse could hinder this also....So these pulsations are making the intake like a pin ball machine with stuff bouncing around all over the place.... it's not just a nice even flow from the carb to cylinder and the a nice even flow to another cylinder....

This was just from a steady state scope and I would think that an accelerating one would be just as chaotic or worse .... yet somehow scavenging can somehow take place where a powerfull pull from the exh would overcome all this and help pull A/F into the cyl ? I supose it could happen yet it would seem to have to be in a narrow RPM range and somehow reach a condition of near equal pressures in both intake and exh ?... if it happens at all ?

Fact is I sure dont know ....

Feb 6th, 04, 2:06 PM
Is there something wrong with my math?
Let's say my horsepower peak is at 6000 rpm.
6000 rpm = 100 rps. Since the exhaust valve opens once every 720, it will open every .020 seconds. At 1600 fps the sonic exhaust pulse will travel 32 feet in this time. Two round trips down an 8' pipe would get it back in .020 sec. Do I need 96" long primaries to get the pulse back at the right time?

The Comparative Aircraft Flight Efficiency Foundation did a study on light aircraft exhaust systems and produced some interesting graphs.


The EPG and Aircraft Exhaust Systems (http://www.cafefoundation.org/aprs/epg.pdf)

Aircraft Exhaust Systems IV (http://www.cafefoundation.org/aprs/EPG%20PART%20IV.pdf)

Feb 6th, 04, 2:28 PM
Excellent links. Now I just need to absorb it all.

Feb 6th, 04, 4:18 PM
Originally posted by UDHarold:
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. another question, what about solid and roller cams? are they rated at a different spec than hydros?

also, i know with a solid you have valve lash and takeup ramps (correct term??) built in the lobe for the lash. would you have to take this in account when figuring dcr?

Feb 6th, 04, 5:54 PM
I seem to be reading that duration at .050" is different between a solid cam and a hydraulic cam even when it's the same number??

Am I missing something or just confused??

I can see where the opening points of each cam will be different due to a hy-lifter always riding, although not pumped up solid yet on the base circle whereas a solid lifter will have the lash looseness to contend with before it can kick the lifter up.

Please straighten me out here so I can figure out just what is going on..


Best Car Insurance (http://autoinsurances.wearecares.net) | Auto Protection Today (autoprotection.wearecares.net) | FREE Trade-In Quote (trade-in.wearecares.net)

Feb 6th, 04, 6:37 PM
Im definately no pro here but I dont see how you could say .050 is meaningless. If there are 2 solid cams with 250@ .050 lift and one of them is a 300 advetised duration and the other is a 288 advertised. There is going to be a big difference in how the cam acts. Again lets say there are 2 cams with the same 290 advertised duration. one of wich is 235@ .050, and the other is 255@ .050 they will act differently also. So i guess i dont know why .050 would be irrrelivant. Please explain

Pat Kelley
Feb 6th, 04, 8:39 PM
Solid cams should be measured using this formula: Lash/rocker ratio + .004". The .004" is for valvetrain compressibility. I don't think any mfgs rate each of their cams like this. Usually they pick a number, say .020" and use that for all their wide lash cams. They use a different lift for tight lash cams. The only lash .020" is correct for with 1.5 rockers is .024" lash. With any other lash the adv duration will be different by varing amounts.

PDQ, solid cams need to be about 8 larger to be about the same size at the valve as a hydraulic cam. This is because of lash. The lifter has to take up the lash before it pick up the valve. So with .024" lash and 1.5 rockers, the lifter will have risen .020" before the valve starts moving. This time, in degrees, needs to be accounted for. 8 is the number usually used however it isn't totally accurate for all cams but generally close enough.

Nickel333, reread what I said. I said that the .050" valve timings (IO/IC, EO/EC) are useless to both man and machine. They tell you nothing of value. They are useless for any calculations and the engine doesn't give a hoot about them either.

.050" duration is useful for comparing cams and getting an idea of the ramp speed.

Feb 6th, 04, 8:46 PM
ahhh ok i read it wrong, i see what your talking about now. graemlins/thumbsup.gif

Feb 6th, 04, 10:11 PM

Thanks for jumping in here b/c I still don't get it??

Granted, a solid cam has to take up lash coming off the base circle up through the ramps.

AND a solid has to compress the loose valvetrain a schosh, but doesn't a hy-lifter compress the lube a schosh too as it comes off the base circle and ramps just like a solid cam does??

ANd regardless, .050" up the side of the lobe is .050" regardless of which cam we are talking about isn't it??

Now if a solid was lashed at .055", then the supposed .050" duration numbers would be shorter b/c they would now be at .055" b/c the solid lifter wouldn't even be lifting anything yet!! Right??

Bear with me here b/c other than hy- vs solid, I have always been under the impression that given identically ground lobes, the hy- cam would be rated wider b/c of being way down on the ramp whereas the solid cam has to be rated shorter or at least up to the point of taking up all it's lash..

What am I missing here b/c I really want to be able to talk about this better then I can now..

Thanks for bearing with me on this..


Best Car Insurance (http://autoinsurances.wearecares.net) | Auto Protection Today (autoprotection.wearecares.net) | FREE Trade-In Quote (trade-in.wearecares.net)

Feb 6th, 04, 10:22 PM

If I may bring up some ancient history here....

I dug up an old late 1970's Engle cam catalog.Looking at thier lobe spec chart they said this :

" Hydraulic grinds are checked at .040 to give an approximate relation to the solid lifter cams which are checked at .050.The difference in ramp length between hydraulic and solid lifter cams is useually between .008 and .012 "

That said even Engle has abandoned that measurement and certinally by the 80's had changed to measure thier hyd cams at .050.In my quest for old catalogs and info the only cam mfg I have ever run across useing .040 on hyd only was Engle...Not saying there were others but they are the only one I have run across..... BTW even in the late 70's Crane had some of thier hyd cams were rated at .002 and .003 ...Before basically standardizing on .004.

It seems to me that .050 is .050 wether hyd or solid but the seat timing and intensity would be different between hyd and solid.

Feb 6th, 04, 11:05 PM
In a perfect performance world I guess there would be a standard for all cams to be measured or compared from, but even then it wouldn't take into consideration the design/shape of the lobe which also must greatly affect cylinder filling and exhaust scavenging etc. Heck, some cams are actually launching the lifters now smile.gif I will continue to look at the numbers provided and make my best guess as to what cam I need and also ask opinions of knowlegable others but as I see it, it will always be the cam grinder's magic and alot of trial and error that will find the best power. How many cams does a professional race engine builder stab in before the perfect grind is found? Alot I bet.

Feb 7th, 04, 1:18 AM
Just a few general comments on items posted since my last post-----
1st--On exhaust pulsing and the length of exhaust pipes: Using the given 6000 RPM and the 1600 fps sonic velocity, we see the engine takes 20 ms per revolution of the cam(6000 rpm/60 spm=100 rps, 2 revolutions/1 cam revolution and 100 rps=50 cam rps, 1000 ms/50 rps=20 ms per 720 crank duration). Assuming a 300 cam, 300/720=.41666 the time the exhaust valve is open is .41666X20 ms, or 8.333 ms. Now 8.333 ms X 1.6ft/ms--1600 fps/1000 ms/sec--equals 13.333 ft total for the exhaust pulse to travel while the valve is open. I have earlier said that most exhaust systems tune for the 2nd reflection for peak power. We have 13.333 divided by 4--from seat to end, back again,from seat to end again, and back again, positive, negative, positive, negative--equals 3.333 feet, or 40". This is from the exhaust valve seat to the end of the primary tube. This is a typical primary tube runner length, once the port length is taken out. It's funny how these things work out.......
2nd--About valve lash and hydraulic cams:
We have already established(Pat Kelly and me) that the SAE considers .004" ramp height to be the correct one for hydraulic cams. This ramp height times 1.5 rocker ratio is .006", the amount of 'normal' deflection in a stock valve train. We have already discussed hydraulic cams, so this is just about solids. What I call my 'normal' ramps, of .026" valve lash, are designed at .01734" ramp height. Multiplying .01734" times 1.5 equals .02601", minus the .006" for valve train deflection, and we have .020", the cam height number I use to rate my cams' durations. Over the years, I started keeping that as a standard measuring number, although I'm starting to abandon it at Lunati. Get ready for each series of cams to have its' own measuring height---Advertising is at play.....
Ramp heights are chosen for many different reasons. However, valve lash-.006 should equal checking height. As the lash gets smaller, this doesn't work, and I sometimes feel we are trying too hard. After all, the valve lash should be adjusted until the car runs its' best.
3rd--What really is .050"? Let us take 4 cams, all with 254 at .050, and all on 102 ICL. These cams range from symmetrical to various degrees of unsymmetrical-ness. Their timings at .050, all on 102 ICL, are 26 BTDC-48 ABDC, 25 BTDC-49 ABDC, 24 BTDC-50 ABDC, 23 BTDC-51 ABDC. This is obvious that an engine would see these as FOUR different cams, but they are all 254 at .050, 102 ICL. And you have to remember, all those degrees BTDC are HURTING the engine, by pumping high pressure exhaust gas into the intake BTDC. The actual flow doesn't start until after 10 or more ATDC, when the piston has cleaned the reversion out of the intake runner. This is why I have stressed the .200 numbers so much, they give a much better picture of the cam's high-lift area.
I quit at this point, answer some more tomorrow night.


Feb 7th, 04, 1:40 AM
Check my post above again after I made it easier to read.
It doesn't matter what kind of lifter is used.
If you change the rated dur.lift spec, the total advertised dur will change. The intensity # is BS made up by magazines to make them sound like they know what they are talking about. If you use the rated dur. to compare anything w/o knowing what the lift spec was, the number is useless. Even then you have to guess what the difference between .004 to .006 is.

For a better comparision, use .050 and .200. for an intensity # Can't fudge there for advertising.
Or pick the cam with the highest lift with the same .050 #'s.

On the header length thing, check out a catalog some time. The only ones near 40" are for sprint cars. SC's for a chevelle bbc are 35", comps are 30".

Now the lower the rpm, the longer the exh needs to be.
And the smaller OD too. The 1600fps stays the same but the engine turns slower so the exh gas needs to travel farther to keep the timing the same.
At least according to Headers by ED and a few other things I've read which quoted engineering equations and such. Also, the rpm used in the equations is for where you want to increase torque.
Which basically means that theory wise, you can't fit long enough ones under your car.

Feb 7th, 04, 2:10 AM

As a point of intrest I'm pretty sure Harvey Crane was who coined the various " Intensity " terms.He actually went a bit farther with it with his " Span " concept....

If you took say a mech lifter cam rated at say 290 deg at .020 and 250 deg at .050 you would have a major intensity of 40 degrees. 290-250=40... he then would clasify his " span " numbers depending on the cam being symetrical or asymetrical.... if symetrical you would take the 40 deg intensity and divide by 2 and end up with a span of 20-20..20 deg for opening and 20 deg for closeing. if asymtrical (you would have to plot this ) it "could" be say 18 - 22 or 16-24... ect ect..ie opening side 18 plus closing side of 22 = the intensity level of 40 deg... or perhaps
16 deg for the opening side and 24 deg for the closeing....reflecting the asymetrical's shorter opening side and longer closeing side.... but always the two span number equal the intensity number...

How usefull this info is is totally up to debate but it is a different way of looking at something.

Feb 7th, 04, 2:37 AM

I use 'Off-Set' for what Harvey calls 'Span'. Here is my usual example, the 288R lobe:
opening/closing points at .020---39 BTDC 69 ABDC
opening/closing points at .050---24 BTDC 51 ABDC As you can see, "Off-Set' equals-15 open 18 close At different 'intensity' levels, the off-set will be another set of numbers. My 304R, same lift as the 288R, is 268 at .050. Its' off-set numbers are 16 open/20 close--36 intensity, vrs 33 for the 288R.
Most cam grinders consider anything shorter than 30 intensity to be 'inverted-flank' cams. My MSP III series, which have been extremely successful, are 30 intensity.

Regarding the 40" headers.... Please re-read my post. The 40" includes the exhaust port length, from the flange to the exhaust valve seat. This length must be subtracted to get primary pipe length.
BTW, a properly designed header should have the 3rd harmonic at peak torque, the 2nd harmonic at peak power. Calculations are done as in my example......


Feb 7th, 04, 2:56 AM
Thanks for the lesson Harold !

Pat Kelley
Feb 7th, 04, 3:01 AM
Hydraulic Intensity is Harvey Crane's baby. It's not exact and only give an idea of the steepness of the ramps and the rate of lift. Cam lift and duration are good indicators, too. And .200" tappet lift is even better. Given 2 cams with the same .050" and seat to seat duration, the cam with greater .200" duration is more "intense". The engine will see them as different cams, because they are. This is why there are so many cams on the market that look the same in the catalog but perform differently. I agree, .050 and .200 are good numbers to use for intensity.
The difference between .004" lift and .006" can be measured with a degree wheel, as you did. Without measuring, it is a guessing game.

The duration @ .050" is tappet lift. As a solid starts up the ramp, it has to take up the lash +.004" before the valve starts lifting. A hydraulic cam starts lifting the valve at .004" of tappet lift. Once a hydraulic lifter is filled with oil and under pressure it is, for all practical purposes, solid since a liquid cannot be compressed. As a solid lifter starts up the ramps and takes up the lash, it has risen some part of the .050" tappet lift before the valve starts to open. As a hydraulic lifter rises, it starts lifting the valve right away (at .004" tappet lift to be exact). If you measured two cams, one solid and one hydraulic, with identical .050" duration and lift, the hydraulic lifter cam will move the valve off the seat sooner than the solid cam would. The rule of thumb is 8 @ .050". But like all ROTs, there are exceptions and measuring is the only way I know of to be sure. I guess what I'm saying is while the lifter has risen .050" on both cams, the solid cam is seen as smaller by the engine since the intake valve opens later and closes earlier due to the lash in the system. Same with the exhaust

Feb 7th, 04, 10:54 AM
Thanks Pat,


Best Car Insurance (http://autoinsurances.wearecares.net) | Auto Protection Today (autoprotection.wearecares.net) | FREE Trade-In Quote (trade-in.wearecares.net)

Feb 7th, 04, 11:33 AM

This is why solid cams make so much more HP than hydraulic cams. Youu gain more in reversion-reduction and in increased port velocity than you lose in the .050 differences. If you increase the .050 by 8 so that the .050" off the seat(.050" VALVE LIFT) is identical, the actual off the seat duration at .001" valve lift is still much shorter for solids than for hydraulics.
Shorter duration=less reversion, more torque and horsepower.
On the other hand, you don't have to set lash with a hydraulic cam.......


Feb 7th, 04, 11:39 AM
Originally posted by UDHarold:
BTW, a properly designed header should have the 3rd harmonic at peak torque, the 2nd harmonic at peak power. Calculations are done as in my example......

UDHarold I'm a little confused about the terms describing the waves. A harmonic is an overtone of the wave, a multiple of the basic frequency. Are we talking about multiple reflections of the same frequency or multiple frequencies, as in second and third harmonic.
Does the peak control the wave or the wave drive the peak? It seems that you are tied physically to a torque peak that's at 2/3 the rpm of the power peak. Does the same basic ratio work on the intake runners, too?
How does the collector fit into this equation? What's the difference in the reflected wave from a zoomie header as opposed to one with a collector?

Feb 7th, 04, 12:09 PM

First of all, I am not an exhaust system designer, but a cam designer. My knowledge of exhaust/intake systems is governed by their interaction with cam design.
With that being said, remember, we are dealing with physical systems, and physical reactions to those systems. No matter what length primary tube length you have, this sonic pulse will travel in it, and reflect back and forth. As it does, the amplitude(height, intensity) of the pulse will decrease, and the duration of the pulse will increase. What this means in real life is that the more trips the pulse makes, the weaker and less effective it is. As was earlier noted, if you tune for just the 1st trip, you have EXTREMELY long tubes, both intake and exhaust, to the point of un-usability.
Because the actual velocity of sound in your exhaust system may be higher or lower than our 1600 fps example, the same length in two different engines may give different results.
Also such things as step headers are designed to PREVENT the reflection of sonic pulses back to the exhaust valve seat. Getting the exhaust OUT is more effective than trying to maximze the exhaust pulses. If you succeed in maximizing the pulses, you have a HUGE positive pulse coming into the combustion chamber, about halfway between peak torque and peak horsepower. It causes a major loss of HP, right where you don't need it. An engine with a highly-tuned intake/exhaust system needs a large number of transmission gears to keep the engine ABOVE this loss of HP.
Exhaust systems seem to be best when they get the exhaust gas out, and keep it out......


Pat Kelley
Feb 7th, 04, 2:44 PM
So, Harold, you don't go along with Isky's 5th cycle theory, the overlap period. During which the exhaust gases pull the intake charge into the cylinder. I think this is what the exhaust system questions are getting at. I think understand what you are saying about this. That a system that can utilize exhaust pulses to pull the charge is very limited in the rpm range that this will happen. Requiring, as you said, a whole bunch of closely spaced gears to keep the engine in this narrow rpm range. Outside of this narrow rpm range reversion is rampant and so detrimental that the small gains in scavaging are more than offset by the losses to reversion.

Feb 7th, 04, 7:48 PM
Is this what makes a small, say 210 duration at .050" cam, have a smooth always building torque curve unitl it peaks and starts dropping whereas a bigger, say 240 duration cam has generally a bad dip in the torque curve that generally shows up around 3000, then build's really high before backing down???


Best Car Insurance (http://autoinsurances.wearecares.net) | Auto Protection Today (autoprotection.wearecares.net) | FREE Trade-In Quote (trade-in.wearecares.net)

Feb 7th, 04, 11:47 PM
Great info here - a good read.


Feb 8th, 04, 1:40 AM
To go a little further into exhaust sonic pulses, and their harmonics:
I have already posted that the relected sonic wave's travel time is dependent upon the length of the total(primary tube length + length of exhaust port up to exhaust valve seat) length, the duration of the exhaust cam, and the RPM.
The duration tells you how long the exhaust is open, and therefore how much percentage of the RPM is available for reflections. Because this sonic wave will have a certain velocity(1500 to 1800 FPS), when we look the overall equations, we can tell what sort of pulse (positive or negative, weak or strong) is going to show up at overlap to either help or hinder intake filling. If the pulse is positive, it will HINDER intake filling, if negative--What you guys call 'scavenging'--it will help cylinder filling. The more reflections the pulse makes--the lower the RPM--the weaker and wider the pulses will be.
A question was 'what if we tuned for the 1st reflection?'. It would work, but the primary runner would have to be 80" long......This works in the intake also, only the speed of sound in the intake is 1100 fps, the speed of sound in normal air. The exhaust's speed of sound is much higher, because of the high exhaust gas temperature.
Such things as step-headers are specifically designed to PREVENT the return of sonic waves in the exhaust system.......


Feb 8th, 04, 2:01 PM
i just wanted to thank everyone again for responding. lots of great info here, i think it would be nice if this post was archived for future reference.


travis g
Feb 8th, 04, 3:48 PM
Wow...definately some good reading here.

From what I am gathering here, it would seem that if you have an engine with a bit too much compression, that a hydraulic cam would give you more cylinder pressure bleed-off versus a similar sized solid, and might allow for pump gas useage where a solid would have more cylinder pressure. Yet, this same hydraulic cam that could be used for bleeding off some cylinder pressure is also going to create even more intake reversion than a simlar sized solid, and thus considerably less low end and midrange power, up to a point. Then, up to the point that that a hydraulic lifter starts pumping up, a similar sized hydraulic cam could actually make MORE top end power than a solid. I have seen dyno tests that showed this before, with a 236*.050 hydraulic vs. solid, and the soid made considerably more torque under 3K rpms, but the hydraulic had a slight power advantage over 5500 rpms. And all this is why a comparable sized soild cam with pull more idle vacuum than a hydraulic. Does all this sound right? It seems to make sense to me.

travis g
Feb 8th, 04, 3:55 PM
On a similar note, there was s dyno test in a mag here a while back comparing a HE268 to an XE268 (not exactly apples to apples I know), and the XE made more low end power and the HE made more top end. The HE, with its "less intense" lobes, would have more intake reversion at lower speeds, thus the lower low end output, but that same design causes an increase in top end breathing when there is less time for any bad effects from reversion...effectively acting like a longer duration cam than what is it. Am I way off base here?

From all the previous info, it would seem that if I was building a max grunt/midrange truck motor, then a moderate sized solid flat cam would be the way to go.

Good, good stuff.

Feb 8th, 04, 6:19 PM
I designed the HE268 in 1977.......


BTW, the HE268 is 218 at .050, the XE268 is 224/230 at .050, plus it has more valve lift...

travis g
Feb 9th, 04, 1:47 AM
Harold, I take it by the short response you might think I'm ripping on the HE cams. That is definately not the case. I have used the HE cams for many years and absolutely love them. Please re-read my last 2 posts in this thread...just trying to gain more info is all.

Feb 9th, 04, 1:56 AM

YOU misunderstood me. I wasn't picking on you at all. I too read that article and almost hurt myself laughing so hard at the authors trying to explain how a cam of 218 at .050, .454 valve lift, could out-power a cam of 224/230 at .050, .477/.480 valve lift, ON THE TOP-END.
I was merely stating that I designed the HE268 when I was CC's original designer. No slam at you at all.
I don't think the HE268 has more reversion than the XE268, but probably less. It was designed as an unsymmetrical cam. CC's website has timing numbers for the XE line, and they sure look symmetrical to me.


Feb 9th, 04, 2:37 AM
Hi Harold,
Were the "Magnum's" just an extension of your HE's or are they a different family designed by someone else