Advertised duration: 0.020" vs 0.015"?

[plovett]

Anybody have a ballpark figure on how much duration changes when measured at 0.015" and 0.020"?

I'm looking at Compcams solid flat tappet profiles. Some are measured at 0.015" and some at 0.020". Just trying to find a way to compare them.

I know the difference will vary depending on the specific profile. I'm looking for a very general idea. Maybe 4 degrees?

thanks,

paulie

[af2]

Anybody have a ballpark figure on how much duration changes when measured at 0.015" and 0.020"?

I'm looking at Compcams solid flat tappet profiles. Some are measured at 0.015" and some at 0.020". Just trying to find a way to compare them.

I know the difference will vary depending on the specific profile. I'm looking for a very general idea. Maybe 4 degrees?

thanks,

paulie

No way to tell and why the hell they do that is beyond my pay scale!!

[hoffman900]

Echoing others here in the past, I don’t understand why manufacturers don’t advertise the events at hot lash. That will tell you wayyy more as it’s what the engine actually “sees”.

[pdq67]

I am pretty sure Camking does use hot lash as the advertised numbers.

pdq67

[modok]

General purpose of such a specification would be to compare cams minus the lash ramp....so, why not...do that?

[Scotthatch]

Or just get used to numbers at .050

[F-BIRD'88]

Roughly 8 degrees for the comp solids, roughly.

[hoffman900]

Or just get used to numbers at .050

Not really. Unless you’re using the same lobe family and have a general understanding of that lobe family’s performance characteristics, then .050” tells you nothing as far as cam comparison.

Go through several cam lobe catalogs and look how many lobes have say 270* @ .050”. They all will not perform the same - not even close.

Pdq, yes, Mike does. Logically it makes the most sense.

Here is a good thread: https://www.speed-talk.com/forum/viewtopic.php?f=15&t=22828

[BradH]

Roughly 8 degrees for the comp solids, roughly.

Based on various lobes I've mapped, that's a pretty good estimate.

[B Original]

Or just get used to numbers at .050

.050 And .200 lift should weigh heavy into your analyzation also. I've seen .050 numbers vary 4 degrees or more with the same advertised duration. These numbers tell you more about the aggressiveness of the
Profile. I use advertise numbers as an indicator but I don't put much weight into the different manufacturers standards of measurement

[Scotthatch]

I use .050 duration in a lot of my math but technically what is needed is duration at lift that is10 % of valve diameter but that's a pain to figure and the duration at .050 gets me close enough to work .. I agree that comparing advertised and .050 is a quick way to decide on lobe intensity a lot of times I will hunt down the lobe used in the master lobe list as there is a little more information

[groberts101]

Or just get used to numbers at .050

.050 And .200 lift should weigh heavy into your analyzation also. I've seen .050 numbers vary 4 degrees or more with the same advertised duration. These numbers tell you more about the aggressiveness of the
Profile. I use advertise numbers as an indicator but I don't put much weight into the different manufacturers standards of measurement

Right on. A good custom spec'd aggressive lobe design can be well more than 10°difference to other shelf style cams @.050 but having identical seat timing, even when said cams are supposedly "much more aggressive" than some other shelf grinds. Assymetrical lobes can be tricky to scale their seat to seat #'s in comparison to other symmetrical designs too.

The other thing I see being done quite often when people pick their own cams from the catalog is making the assumption than comparing seat timing numbers is what controls the bulk of the advertised and expected powerband.. and even idling characteristics. Put two similar seat spec'd cams against one another and see if the one with much more aggressive ramp rates doesn't affect said characteristics. You will quickly learn that even miniscule amounts of lift at .001 or .even .010 doesn't have near the same impact than slightly higher lifts gained with faster lobe acceleration rates do during the overlap periods. Takes much better designed(sized/lengthed) induction and exhaust systems to tame the much more aggressive lobes back down to get back to where the shelf cam is acting at very low piston speeds.

It is possible to have a bigger cake and get to eat it more of it too.. just takes more design work and cash to get there, is all.

[CamKing]

Or just get used to numbers at .050

.050 And .200 lift should weigh heavy into your analyzation also. I've seen .050 numbers vary 4 degrees or more with the same advertised duration. These numbers tell you more about the aggressiveness of the
Profile. I use advertise numbers as an indicator but I don't put much weight into the different manufacturers standards of measurement

If the cams have different lash settings, you can't compare them at advertised, .050" or .200" duration. Just changing the height of the lash ramp, will make a cam look more aggressive then it is.

[B Original]

You will quickly learn that even miniscule amounts of lift at .001 or .even .010 doesn't have near the same impact than slightly higher lifts gained with faster lobe acceleration rates do during the overlap periods. Takes much better designed(sized/lengthed) induction and exhaust systems to tame the much more aggressive lobes back down to get back to where the shelf cam is acting at very low piston speeds.

Additionally something that I've seen controversial between some of the posters on here lots of time has been tied to air flow is low and mid lift. Aggressive ramp ads to the average Lyft during a valve opening cycle the peak lift has little importance to the performance of a camshaft rather the higher average lift. For instance with a mild ramp and a .550 lift camshaft the average opening for that camshaft during it's 270 degrees seat timing Maybe.220 but another camshaft of the same seat timing with.497 lift could have .250 average Lyft so the more aggressive ramp profile can potentially allow higher volume of charge by the valve opening

Although a lot of times it is better to choose higher lift with a less aggressive ramp as a compromise for an engine that's being used Street driven to the effect of being able to use a milder spring for a longer span life cycle from the spring

[groberts101]

You will quickly learn that even miniscule amounts of lift at .001 or .even .010 doesn't have near the same impact than slightly higher lifts gained with faster lobe acceleration rates do during the overlap periods. Takes much better designed(sized/lengthed) induction and exhaust systems to tame the much more aggressive lobes back down to get back to where the shelf cam is acting at very low piston speeds.

Additionally something that I've seen controversial between some of the posters on here lots of time has been tied to air flow is low and mid lift. Aggressive ramp ads to the average Lyft during a valve opening cycle the peak lift has little importance to the performance of a camshaft rather the higher average lift. For instance with a mild ramp and a .550 lift camshaft the average opening for that camshaft during it's 270 degrees seat timing Maybe.220 but another camshaft of the same seat timing with.497 lift could have .250 average Lyft so the more aggressive ramp profile can potentially allow higher volume of charge by the valve opening

Although a lot of times it is better to choose higher lift with a less aggressive ramp as a compromise for an engine that's being used Street driven to the effect of being able to use a milder spring for a longer span life cycle from the spring

Much more complicated than that and all boils down to the available time the engine has to actually make use of its airflow window. Which is of course why most serious engine builds having much higher rpm potential prefer to use steeper seats and larger lifts. They could give a rats ass about lift anywhere much under .200".

And if you don't believe me.. just ask them. Or just patiently wait for one of them to show up and pounce on that blanket statement. Say it.. and they will come..