This question is based on a sentence from the book "Hot to build Max Perf Chevy Rat Motors".
Quoted from that book:
"Manual Trans vechicles or larger Cubic motor are more fogiving of Cam duration increases than an Auto Trannie or a smaller cubic inch motor."

Based on that statement, and knowing that cams are stated to make power up to certain RPM's.
Let's say for an example, a cam will make power between 2000 - 6800 rpm (stated by the Manufactuer). Could one make the assumption that as the Cubic inches increase, the RPM drops, if the Carb, Intake, and Valve size remains the same? If this is not true, what is the relationship between RPM, Cubes, and a Cam?

Thanks,
Fred.

Could one make the assumption that as the Cubic inches increase, the RPM drops, if the Carb, Intake, and Valve size remains the same? If everything is the same except the bore and stroke then the larger engine will top out at a lower rpm than the smaller engine because of airflow requirements,

Your heads actually dictate what the rpm capabilities the engine will have, you could build a peanut port headed 502 with a monster cam but it will still be dead at 4000 rpms. And rpm range listed on cam descriptions are general- a 396 will respond differently than a 427, 454, 502. I would choose what rpm range and match the heads, pistons, and intake and exhaust,to that, then ask a cam expert like UDharold what cam to run. JMO

Stroke has more to do with how an engine acts with a particular cam than does bore. Increased bore size makes more power due to less valve shrouding plus increased piston surface area for the expanding charge to act on. Increased stroke makes more power by virtue of increased leverage (since the lever is longer), plus the utilized length of each cylinder bore allows more air-fuel mixture to be drawn into the cylinder during the intake stroke. Bore size doesn't care about cam timing since valve size, etc. aren't changed at all. Stroke is much more sensitive to cam changes since valve timing plus stroke length determine how much of the mixture can be pulled/rammed into the cylinder.
At least that's how it was explained and demonstrated to me.

I had the same cam in a 396 and a 454 and the 454 made power from 2500-7000 while the 396 had power from 3500-7700.

along the lines of what harold is saying.........i know when i thumb threw the cam books for bbc....most are stated for a 454 bbc so i know if the cam is rated 2000 to 6000 rpms.....then i know my 396 bbc is going to run at least 2500 to 6500 rpms......with that cam.........i believe most rated cams for sbc they are using 350 engines for the ranges so smaller cubic inch motors will rev higher in the rpm range as stated...and the biiger then 350 inch engines will rev less......

Same goes for a cam for an old 265 SB motor vs a 400 SB motor..

If the rod length is held constant, a longer stroked motor will have the piston just a schosh farther down in the hole when the intake valve closes then a shorter stroked motor, therefore it can use a slightly bigger cam.

A rule of thumb I have read about was something like 5 degrees bigger for every 25 inches of motor increase or something like that??

Anybody remember this too?????

pdq67

Think I had read 10 deg @ .050 for every 50CI.

Utilizing a "rule of thumb" such as this is grossly oversimplifying the process of cam selection. There is a lot of information necessary in order to select a cam that will work right in any particular application. Thus the reason why cam manufacturers (or anyone else) will request so much information when assisting with cam selection. Such info as vehicle weight, gear ratios, transmission type, manifold design, valve sizes, intended vehicle useage, etc., etc.

Generally a larger engine can effectively run a cam with more duration and/or lift (all else being equal) soley because of volume differences. For an overly-simplified analogy consider the difference between emptying a bucket of water into the bathroom lavatory and emptying the same bucket into the bathtub. The lavatory may be filled to capacity and even overflow, but there will be little or no affect on the bathtub.

Thanks guys,

This is good data, it will without question help. I understand Overlap, Lift, duration, advance, etc, but wondered why all BBC where grouped together with no spearation in the cam listings. Knowing that Cubes/heads had a effect (didn't think about stroke), I expected that they based the RPM, etc on one of the engines. I didn't know if it was the smaller enging, picked a middle of the road engine, or the 454. I did ask UDHarold what cam he suggested, but wanted a better idea of exactly how it would react in my cubed motor. Thanks for your help.

Fred.

The "rule of thumb" is just that, not some magic formula; but it mostly works out, in most typical setups, especially street type usage. It's true that stroke changes and cam duration changes go more hand-in-hand, whereas bore changes and lift changes go together (assuming that head flow limitations don't enter the picture).

Look at the cam section in the Crower catalog sometime. http://www.crower.com/pdf/2-52.pdf

I have spent hours and hours playing with D2K to see what kind of "trends" develop while holding a motor combination constant except for size increases and cam size increases starting from a 240 and increasing it by 5 degrees up to 300 degrees!!

Run a family of cams on one motor and then up the motor and run the same cams through the bigger one. I've looked at 265, 283, 302, 305, 307, 327, 350, 383 and finally 400 motors!!

One problem that happens that I did adjust for was the need for the motor to have a higher compression ratio with cams bigger then about 270 on SB's and 280 on BB's!! (AND, yes, I ran both motor families over time)..

I'm not saying to take a D2K study like this to the bank, but generally, it will help you a lot in figuring out what happens here. As will taking one motor and one cam and changing the LCA/ICA angles over a range from say 100 degrees ICA to 120 degrees!! Here you can see how a wide LCA cam will increase power above 5252 rpm just like the mag's say's it will in comparison to one with a narrow LCA, (114 to 106) sorta deal!!!!

This is how I picked up on the little solid cam spec'ed as follows; 250/250, 104/108, .440"/.440" gross lift for my about 9.75 to 1 CR, 283 motor when I was trying to max it's grunt!!


It theoretically will make almost a flat torque curve up to 4500 rpm and will do the same thing if bumped up to a 256/256 cam in a 350 motor too...

Also you will see that lift if varied and everything else is held constant isn't all that big a power producer player when compared to duration and LCA/ICA changes...

Interesting to say the least...

But not very funny!!

pdq67

RB69SS396Conv,

Thanks for pointing me at the Crower site, their breakdown is exactly what I was looking for. They give rpm for each size engine, which makes perfect sense. All the others that I have looked at just give 396-454, and one RPM range. Which is what confused me on other cam sites, and prompted the question.

pdq67,

I've been in that mode over the last several weeks, playing with different setups on D2K.
I have been picking out different cams and then using the IVC angle to computer the chamber volume. Using the Chamber volume and Sweep volume, I calculate the static compression my engine would need for this cam. If they match, I then plug it into DYK and compare the results. This combined with the rpm range I want to run, and the max lift I can have, is how I narrowed down the list.
The thing that started to confuse me, was that the rpm stated by the Manf, was never what D2K gave me. The above data helps out a great deal in explaining that.

Thanks,
Fred.

Cam Motion Cam Company lists their cams by motor size and tranny type , or at least an old cat. of theirs that I have is that way if you want to look into them??

Hope this info is up-to-date????

http://www.maxracesoftware.com/race_car_pictures.htm

Cam Motion
2092 Dallas Drive
Baton Rouge, LA 70806
Phone: (225)-926-6110
Fax: (225)-923-2101
Sales: sales@cammotion.com

http://www.cammotion.com/

pdq67

pdq67,

Thanks, no wonder you have such a solid handle on this stuff.

Thanks,
Fred.

Not at all SS,

Thank Pat Kelley and his great Dynamic Compression Ratio Calculating site that he now call's Effective Compression Ratio and Dema Elgin that I read about in Ed Staffel's great book.

It all started to jell after I realized after reading Ed's book that you can't trap any charge until after the intake valve closes AND that is dictated by advertised duration numbers, LCA and ICA AND stroke length if rod length is held constant.

This stuff is way COOL to me as is the theory behind 87 Octane/11+ to 1 motor combinations!!!

Try this link to it if it will work...

http://members.uia.net/pkelley2/DynamicCR.html

pdq67

PDQ. I run a true 11.1:1 compression 350 motor on 89 at the track and everywhere. 87 could be done but why do it when 89 ethanol is cheaper!

There you go!! Low octane AND high compression and good motor combustion eff.!!

That's really what it is all about, imho....

pdq67

pdq67,

Thanks, I've been on Pat Kelley's web page a number of times, and use his program quite often. He did a great job with his site and program, it helped me alot. I think I've read his article about six or seven times now. I see that he has added items since the last time I was there. I need to keep this link active.

Thanks again,
Fred.