.484 purple shaft grind info.

[T67POWER]

Cam company's have listed for each cam they sell on what it needs as far as ,compression,rpm,stall speed,gear .vehicle weight and carb size.
This is a really good way to understand how cams of different durations work.

Cam dynamics can be very hard to understand and what it really boils down to is learning the basics.
I still consider myself a basic mechanic.
I only can draw from the limited experience that i have.
I have a good understanding of what works for cams for basic engines.
Talking with each other and giving advice is a great way to learn but sometimes we get off track and or over are heads.
Strokerscamp has some valid points that are being taken from a good source.(mopar performance publication)
This is also in my small block engine book.

I have David Vizard's book on how to build horse power and i also have the small block mopar book written by Larry Shepard.

They both talk about cams and how to select them.
Larry Sheppard talks about duration as a key to cam selection.
David vizards talks about overlap and the LSA as a key to cam selection.

They both talk about every other part of cam dynamics also, but each has his theory on cam selection and his theory on the subject.

If i could sit down with ether one of these guys for a cup of coffee i would buy all the coffee they could drink and then some.

All in all being different is just part of the fun of being a gear head.
Hell,bench racing is second nature to us.
I think this might even turn into a cam race.LOL

 

[krazykuda]

Let me use what your saying in this comparison of cams.

MOPAR PURPLE SHAFT>>.850 x 284 ADVERTISED DURATION =241.4 @ 50

COMP CAM XE284H>>.750 X 284 ADVERTISED DURATION=213.0 @ 50

The XE284H is not 213.0 @ 50 (WHY?) It's being measured the same way.

/QUOTE]

OK, let me see if I can explain this clearly. Your information/calculation above may be correct. The difference is what they call "ramp speed". In the example above the MP cam is advertised duration of 284 and 241 at .050. This means that the MP cam goes "from 0 to 50" in 43/2 degrees or 21.5° from open to .050. The other 21.5° gets it from 50 to 0 on the closing ramp.

The comp cam goes from 0 to .050 in (284 - 213) = 71°/2 = 35.5°.

So the MP cam takes 21.5° to open from 0 to .050 where the comp takes 35.5° to go from 0 to .050. This means that the MP cam has a faster "ramp speed" than the comp cams. Or the MP cam is opening the valve faster than the comp cam. Or the comp cam is opening the valve slower than the MP cam.

Some schools of thought say that the faster ramp speed is more important. It has its benefits and drawbacks. Some say that the valve doesn't flow very much air below .050 and only want to look at cams and compare the specs at .050. I like to do what you did and look at advertised duration and .050 to see how fast the valves are opening. You have to keep in mind that the faster the cam opens the valve, the faster it closes the valve. The faster the valve closes, the HARDER it closes. the HARDER it closes the more WEAR it will do to the valve seat. Or the valve seat will get "beat up" quicker on a faster opening/closing valve than a slower one because it is closing harder (faster with more force) than the slower one. Therefore if you want durability (long lasting) seats, the slower valve speed might be more desireable than the faster one. However the faster valve speed will have more time "open above .050", so the racers (expecially 1/4 mile) would like the faster valve opening for maximum valve opening above .050 which will flow more air/fuel hence make more power. They are not as concerned about valve seat durability.

This is why I like to look at BOTH advertised duration and duration at 50 when picking a cam. They can have the same duration at .050 and different advertised duration or vice versa. Also keep in mind that some manufacturers use different "lifts" for their "advertised duration". One might use .001" "lift where another might use .002" or .005". When I degree a cam I like to use .001". This means that just when the valve opens at .001" lift is when I start my "advertised duration" and then when the valve is back down at .001" lift to record the valve closing angle for the total advertised duration. You will understand this if you have ever degreed a cam as it is hard to say exactly at 0.000 opening and closing the valve just opens and closes. It is easier to see .001" on the dial indicator. It is too difficult to measure "0" opening and closing at a specific angle as it is to pinpoint it at .001 and be able to repeat the reading. A measurement that cannot be repeated more than once/twice (repeatable) is not accurate or effective.

The only way to accurately compare two cams is to degree them yourself so you have the same opening/closing reference points, as the different manufacturers use different points of reference themselves. However, you can use their advertised and .050 specs to evaluate ramp speed and make your selection on which feature you feel is important.

So, hopefully this gives you more insight on the whole issue of cam selection. Hopefully you all can understand what I have explained without getting confused.

 

[T67POWER]

Thanks for the info.
I multiplied the comp cams advertised duration by .750 to show that it came out wrong.
The comp cam is really 240 @ 50 per comp cams info.

So then i used .850 as the number to use and it came out right at 240@50.

This was using information that is provided in some of the mopar engine books.

Your so right about ramp speed.
This is one thing that when shopping for a cam i never bought into.
What i mean by buying into is---- ALL THE EXTRA PARTS NEEDED TO RUN THE FASTER RATE CAMS.
When you have to go purchase.
1)new valve springs.
2)cut the valve seat area in the head
3)cut the valve guide boss down
4)install new positive valve seals
5)new rocker arms (roller)
6)new push rods
I think it's a waste of money to buy all these parts for just a cam change on an engine -----WITH STOCK HEADS.

Notice i said a stock headed engine.
If the money was put into a new short block with the new cam, plus you did some work on the stock cylinder head ports you would have a better performing engine ready for aftermarket heads in the future.