CAM help please ?

[Blown_FC]

After reading a few replies to my earlier post on Cams/Converters, I had a reply from CROW CAMS on my same query to them.....they aren't very talkative !

Their whole reply to my half page enquiry was " I WOULD RUN A 35604 CAM AND A 2200-2500 CONVERTER"

They did attach the spec sheet, which just confuses me, so I'm hoping someone here can explain in more detail than CROW CAMS....

Inlet Open - 43 btdc / Close - 81 abdc /  Duration-304
Exhaust Open - 83 bbdc / Close - 41 atdc / Duration-304

They called these "ADVERTISED VALVE TIMING"...the "Vlave Timing" figures were different...

Inlet Open - 4 btdc / Close - 42 abdc / Duration-226
Exhaust Open - 44 bbdc / Closes 2 atdc / Duration-226

What does all this mean ( I know btdc is before top dead centre ) and all that, but where does a 30/70 or a 28/73 cam come into it

Cheers for any help

Mark

[Dr_Terry]

Hi Mark.

Yes, cam timing figures are confusing aren't they ?

How the figures work is, using the 30/70 as an example, is the inlet opens at 30 degrees BTDC (before top dead centre) and closes 70 degrees ABDC (after bottom dead centre) & the exhaust is opposite, it opens 70 degrees BBDC (before bottom dead centre) & 30 degrees ATDC (after top dead centre).

If you add the figures 30 & 70 together & then add 180 degrees this gives you the total duration (= 280 degrees) of opening time.

That isn't the hard bit. When they say it starts opening, how is that measured, & why do you see so many different figures. Just about every cam grinder uses a different standard. Some consider 5 thou (0.005") to be open, some use 10 thou, some use 8, some use 20. These 1st few degrees are often called 'Ramps'. But if you measure the same cam using all of these methods the duration looks different.

Several years ago in the USA, they came up with a universal standard, that is used by almost everybody, where they measure the duration at 50 thou. So now if you use the 50 thou number you cam directly compare any 2 cams with no confusion.

Problem is, a 300 degree duration cam (@ 5 thou) measures only 225 (@ 50 thou) & in advertising, bigger sounds better doesn't it. And everybody knows a big cam is heaps better than a small cam, yeah right !!

So a cam that is really 225 duration can be advertised as a 300 degree cam or even more, depending on how you measured it. (The advertising dept just love it)

The other thing too, is it also depends on where you install the cam. If you take the 30/70 cam and install as is, it will open & close as listed above. If you use an offset key or some other method to advance it say, 2 degrees, then the same cam will now open & close at 32/68 & (72/28 for the exhaust). This is called assymetric timing & is preferred by modern engine builders & car manufacturers.

Hope this partially answers the question for you.

Dr Terry.

[Blown_FC]

Thanks Dr Terry, you seem to really know your stuff !

But now I'm a little confused as to which cam to go for, because both you and another member on the forum said to go for the 28/73 cam, but Crow Cams are saying 43/81 ?    They sound miles apart ?

What characteristics would each of these cams provide...in other words...which one is the easy driving type cam and which is more lumpya nd get up and go ?

Given that CROWS have advised a 43/81....what do you think about it ?

Thanks again and sorry for all the questions, but you are rally helping me here, and I appreciate it heaps !

Cheers

Mark

[Glenn 'Stinky' Stankevicius]

Just to add to the confusion, I would suggest trying Waggot Engineering (VIC I think), apparently these people did a heap of dev work on Holden Sixes and should be able to give you a good cam if you give them the correct info. My motor is running a Waggot 238 (see post for other details http://www.fefcholden.org.au/cgi-bin/yabb/YaBB.cgi?board=howto;action=display;num=1057555170;start=3)
I have the exact specs at home if you are interested.

Another thing you may like to try, I have a trial copy of a DOS based Dyno program that has always been dead-accurate for me. It is geared more towards drag-racing and quarter mile times, but if you are shooting for a specific HP then it is good too. It takes into account almost every engine variable. Send me a PM or email if you are interested, it is fairy small so it can be emailed.

If you are still fitting a CRS blower kit, it might pay to ask Rod what type of cam to use. He may well have done some development on this. AFAIK, blown motors are a different kettle of fish when it comes to cams.

[Dr_Terry]

Hi Guys.

That 43/81 cam seems a bit big to be used in a daily driver.

If it is the same cam that you have listed above, that has 225 degrees @ 50 thou & 304 degrees advertised duration, I would suggest going a bit smaller.  Around 210 degrees @ 50 thou is more suitable for the street.

It is however, a personal choice. Some people like a lumpy idle, no low down torque & power band that starts around 3500 rpm.

With cams, power doesn't come for free, there is always a compromise. The whole engine has to be designed as a package, heads, manifold, cams & carbies etc have to work as a team otherwise its a pile of parts with less power than a factory stocker.

Regards.

Dr Terry.

[Leon (FCCOOL)]

As far as I know blower grinds are usually pretty mild, infact I almost put a blower on my own FC, I even had it bolted on. I was told that a gemini convertor would do fine on with the blower, these convertors are supposed to have a higher stall than the 6 cyl trimatics and are cheap, as for a cam I was going to replace it with a standard one or get a blower grind. next time you see someone with a blower cruising around like they have a lumpy cam check to see if they are taping on the accelerator pedal.