Effect of displacement on Camshaft selection

[fe66comet]

Eh I guess I will have to call Barry then I thought he had that covered?

[jayb]

If those are the springs that Barry recommended then I wouldn't be too concerned; he has done a whole bunch of hydraulic roller motors, and he certainly knows more than I do on that topic.  Did he also give you the cam grind?

[fe66comet]

Talked to Barry whoops me dummy, he used these and explained the characteristics of the spring and said you cannot bind a beehive more or less.

http://m.summitracing.com/parts/cca-26055-16

[afret]

Yeah, that one makes more sense.

[plovett]

Per the original question.   A respected engine builder once said:

For a given engine combo if I have a known good cam profile I add 8 degrees of duration @ .050" lift for each additional 50 cubic inches. I also like to tighten the LSA 1 to 2 degrees for each additional 50 inches.
Both combos will HP peak in the same RPM window and have similar idle characteristics...

He was talking about building a similar but larger engine from the same engine family with the same or similar parts.  Just a displacement increase.

I assumed he tightened the LSA because the larger displacement motor can tolerate more overlap (in addition to the increased overlap from more duation) and still have similar idle and low rpm characteristics.  And also because the larger displacement motor needs even more overlap to help breathing at high rpm, given that it has the same or similar induction system trying to feed more cubes.   I'm not sure, but that's what I assumed.

The 8 degrees per 50 cubes would yield 14.4 more degrees for a 90 cube increase which seems to be in line with the other replies.

The big variable not addressed by this, which Ross mentioned, is the rest of the induction system.   I would think if the heads, intake, etc. were marginal on the small cube motor, then the large cube motor would need even more cam duration added to keep a similar rpm range, and the results would not be very good.   If the heads, intake, etc. were replaced with higher flowing pieces on the bigger motor then you might need less cam duration increase than the formula calls for.  The rpm range is determined by displacement and the flow characteristics of the entire induction system.  The cam is just one part of that.   That's the thing with rules of thumb.   They by intention oversimplify the situation.   They can still be useful though.

JMO,

paulie

[fe66comet]

It would seem relevant that if comparing say a 352 two barrel engine to a 428 CJ the parts on the induction side would be different anyhow. The two barrel head would be useless from the get go.

[plovett]

Absolutely.  I was just trying to make the point that the cam is only one part of the equation.  Also when people put a stroker kit in their motor, they often reuse the induction system they had on the motor before adding the stroker kit.  It may or may not be a good choice for the increased displacement.  It's something that should be considered.

JMO,

paulie

[jayb]

Thanks for that info Paulie, I've never seen that particular rule of thumb before.  Makes sense though, and is in line with my own experience...

[plovett]

de nada, enchilada! (ya sure ya betcha!)   

paulie

[hotrodfeguy]

Sorry I was out of town (was at Blair Patrick's FE house of wonder) But in Daves defense he does have a nice set of CJ valved and worked D2 heads with a good CJ intake. And is going in a truck to pull his race car so it may not be a total induction package for racing but should provide a nice package to tow and snap for going down the track. I think it will catch a few chevy guys off guard thinking it's a garden 390 in a truck till it drives by at the digs on the big end  And that would be worth getting on you tube