Help with Cam selection (again)

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AMXrated

AMC Nut

Joined: Jul/26/2015
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Posted: Oct/13/2018 at 12:50am

Also, noticed Crower had this cam on clearance on their website.

Too small for my application, but it's a good deal for $98.

https://www.crower.com/clearance/amc-290-401-power-compu-pro-hydraulic-camshaft.html

You can have my Hurst shifter when you pry it from my cold dead hand.

jpnjim

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Posted: Oct/13/2018 at 6:27am

AMXrated wrote:

@jpnjim: Thanks for the thoughts/analysis. I am absolutely blown away at how much you guys know about this stuff. The last time I worried about cam specs, we called them 3/4 Racing cams...probably dating myself.

I've heard really good things about Howard's, but never got a reply from the guy after I asked him to reconsider his spec's targeting a 1600-5500 RPM range. I even sent him a follow-up email this morning.

Your welcome,

I like to try to wrap my head around the numbers sometimes,

so these threads can get pretty interesting

I'll give you my understanding, right or wrong here it is.....

Cams are tough to figure because different manufacturers can use different points in the cam to advertise their duration at,

I'm just going to type about Hydaulic Cams here because Solid Cams add even more issues to the numbers to confuse things (me!) even more.

There are crazy aggressive 300 degree duration cams and very mild 300 degree duration cams,

the STOCK 401 cam is very mild, but has an advertised duration of 296 intake/304 exhaust!

(The stock 304/360 cam is 264/264 duration,

401 stock lift is .4576, 304/360 lift is .4256).

So you can look at the advertised duration and lift and take a good guess at the cams intensity.

Then you read their descriptions, which are all over the place too.

Some advertise their cams in the range they best work,

others list the entire range that the engine would work.

So the same came could be listed as working from 2800-5500 by one company, and 2200-6000 by the other cams advertising department.

Then they started using net duration.

That's the .050" duration in the lists I copied earlier.

.050" duration is great because its a standard point that all the manufacturers seem to be using (hydro cams),

and comparing one .050" number to another .050" number is apples to apples, for the most part.

If you want to figure intensity you look at the advertised duration (usually .006", but not always).

A big .050" duration with a not big .006" number = a fast acting/intense cam.

The .200" number would most likely back this up.

How long the cam stays open past .200" says a lot about the cam's profile,

the bigger the .200" number is compared to the .050" number,

the more intense the cam will be.

So you can have a lazy 225 degree (at .050") cam that acts like its smaller than its middle of the road numbers wpuld suggest,

or you can have a pretty intense 225 degree (at .050") cam that is much faster acting

^the fast acting cam might have the same .200" duration as a slower acting 230 degree, or bigger cam so the intense cam could move almost as much, or maybe even more air during its shorter event timing.

Lift tells you a lot too, but if you have the .006", .050" and .200" numbers you can start to compare them all to each other and see how it all fits togther.

^this seems to work for me,

but I'm the guy who will keep reading the same thing 20 times over until I think I understand it or my brain starts to hurt.

Lobe seperation is the next big thing to consider but I'll leave that for next time. LOL

71 P-code 4spd Javelin/AMX
some Jeeps and some Fords

jpnjim

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Posted: Oct/13/2018 at 6:55am

heres an intense 224 degree .552 lobe compared to a less intense (but not mild) 231 degree .512 lobe:

lobe name .006" .020" .050" .200" 1.6:1 lift

HM2243450A 271 248 224 144 .5520

1hf231320 285 258 231 143 .5120

The first cam would be considered at the mild end of things with 224 net duration,

but its open past .200" for 144 cam degrees of rotation,

longer than the 143 degrees that the much bigger 231 cam is open.

That tells you that the 224 cam is going to breath a lot better than a typical 224.

Now look at the .006" numbers,

the intense 224 has just 271 degrees advertised duration,

that tells you it's opening quickly, closing quickly and getting its work done fast without a lot of gross duration

^that can be good for cylinder pressure (all things being equal, lobe seperation and cam advance plays a big part in this too).

The more serious (but less intense) 231 cam has 285 degrees advertised/gross duration,

so it would have much more overlap with it's other lobe (all things being equal), most likely lower DCR (not because of the overlap, and again, all things being equal),

and it would be easier on valve components compared to the intense 224.

If you didn't have all the other numbers,

you could look at the 224 degree .552 lift and guess that it would be more intense than a 231 .512 lift cam.

If you found a 231 degree .480 lift cam (at the same 1.6 ratio) it would be a good guess that the smaller lift cam was even less intense.

Last thing,

lifter size can allow for greater intensity,

AMC's and Chryslers have the widest lifters of all regular production V8's,

many cams were originally designed for narrow Chevy lifters,

and it shows in their mild intensity and lower lifts (Small Block Chevy's use 1.5:1 rockers so that makes their valve lift lower too)

Edited by jpnjim - Oct/13/2018 at 7:00am

71 P-code 4spd Javelin/AMX
some Jeeps and some Fords