Holden 304, Camshaft selection

[greenfoam]

I've wanted to write this for quite some time since alot of people make a bad choice when it comes to camshafts.

Firstly an overview of camshafts.

What do the numbers on the camsheet mean?
Lets use this random CROW CAMS grind as an example

Part number 4503

Cam Type: Hydraulic type Profile

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.050 Valve Timing

     Inlet Opens   1          B.T.D.C.      Closes:  45    A.B.D.C .050" Duration:    227

     Exhaust Opens  44      B.B.D.C.      Closes:  -4    A.T.D.C .050" Duration:    220

Advertised Valve Timing

     Inlet Opens   33          B.T.D.C.      Closes:  77    A.B.D.C Advertised Dur:    290

     Exhaust Opens  75      B.B.D.C.      Closes:  27    A.T.D.C Advertised Dur:    282

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The .050 figures are what the cam is doing at .050 thousands of an inch lift, this is one of the two standard figures cam companies use and the best one to see what the cam is all about. Advertised timing or seat timing is _normally_ what it happening at .004 of an inch but different companies messure them at different places so .050 figures are where it's at.

The only really usefull thing about the Advertised duration is that the less difference there is between .050 and Advertised duration is the steeper the ramps and the more aggressive the cam, these cams COME racing called high velocity cams or other peopel call them extreme energy etc etc rar rar. The thing to watch out for with cams like that is that they will be much more likely to valve bounce at low rpm than a cam with a more gentle lobe.

LSA is the Lobe sepertation angle, this is just the amount of degrees inbetween the peak (Centreline) of the intake and exhaust lobes.

See paintCADtm below



The red part in the middle is the where both the Intake and Exhaust lobes are both holding valves open, this is called the Overlap. Now it's quite easy to imagine the closer you move the lobes together then the larger amount of overlap you have and also the lift during the overlap will be greater. So this can be good or bad depending on the rest of the engine.

To be successfull with alot of overlap you need mostly an exhaust with zero backpressure and infact negative pressure to help draw the intake charge into the cylinder, on street exhausts that are unable to do this alot of overlap is usually a bad idea because all you end up with is "reversion" which is exhaust gas running back up your intake.

Apart from that problem the LSA also affects the way the powerband of the cam behaves. A narrow LSA like 104 degrees will be more likely to have a narrow but strong power range. Where a cam with 117 LSA will have less peak cylinder pressure but a wider powerband. In the end somewhere in the middle and something that matches the rest of the combo will be best.

Also the most important thing for street driving cars and LSA is the way it affects your idle vacuum and idle quality. A cam with 108 LSA will have alot of reversion at low rpm (idle) and so will make very little power and struggle to run at those low revs. Infact you will need to increase the idle speed just to make it run. The Factory 304 cam is 117 LSA. Too wide to make a strong powerband but ends up with very little overlap and a very strong and low idle. Good vacuum also makes your brakes work well , which can be handy.

Sleepy again. more later:P

[VQXTC]

LOL........... Ok Greenfoam look forward to it........

cheers

[greenfoam]

Oh yeah everything I want to type is in my head just need to get it out might take a while. I'll make it worth the wait

[kniever]

hey foamy any chance you could get around to finishing this?

[savage1987]

lol. I guess you're going to tell me that the best cam for my stock manifold 5L with no other mods is going to be a 276 (Crow "stage 2"??) as they will make enough torque down low to keep up with the stage 3 in the same car..... am I way off the mark? If the 5L doesn't sell this w/end a cam will be on the cards

[NZVK308]

BUMP!

Waiting for further info with interest..

[vrcalais5ltr]

bump i know you want to finish it

[savage1987]

I know you'll finish this in good time Jason, but the thing I want to know is this: do the same rules apply with the 215i when it comes to cam selection as to the 5L, or does the extra few cubic inches change everything a little bit?

[immortality]

I know you'll finish this in good time Jason, but the thing I want to know is this: do the same rules apply with the 215i when it comes to cam selection as to the 5L, or does the extra few cubic inches change everything a little bit?

extra cubes change things (for the better )

[myutesfarked]

Oh yeah everything I want to type is in my head just need to get it out might take a while. I'll make it worth the wait

Green Foam please contact me. Need some advise on parts selection and I'm willing to pay for it so long as you accept paypal. PM me how much you want and a contact e-mail and I will get on it (^.o)

[savage1987]

^Give up, he's not on the forums any more mate

PM me your email address and I'll see if he will get in touch.