Imo Apita

Had a look at the Crane website.
They have a SR cam which is close (within a degree or two) of the Comp XR 268R. Just shopping around. Noticed something that I was already curious about before; Crane mentions "cruise RPM" on their website in the selection of a cam.
With the above mentioned cam they recommend I guess a cruise RPM of 3000.

I have not seen a cruise RPM mentioned on a cam website before.
Since I'm contemplating a 700R4 swap also this has tickled my curiosity.
Ran some numbers and with the 3.08 rear end my cruise RPM at like 70mph would be 1800-1900 rpm with the 700R4.
This would be way out of the Crane recommendation.

The question I was struggling with earlier is the following:

• 2000-5500 intake
• 2000-6000 Cam

Highway easy cruise RPM of 1800-1900. Would it be an issue if that falls both outside the RPM band of the intake and the cam?
Does it even matter? Question of the automatic downshift to get up in the power band? Am I wasting precious brain power struggling with this?

Or would I eventually be better off by changing the 3.08 to a 3.77 to bring the 70 mph cruise RPM within the RPM band of both the cam and the intake?

Curious minds would like to know.
If I ever get this engine done it will be an aluminum head 383.

MotorHead

XR286R is 3000 to 7000RPM I putt around at less than 3000RPM all day and cruise at 80mph @ 2000RPM with a 3.08 rear gear so I wouldn't worry about it if you are making some power.

Here's mine at 7000RPM

SteveG75

Solid rollers also act "shorter" than a hyd roller with the same advertised duration. Has to do with the lash and ramp profiles.

Lt1er

3.08 is a horrible choice for a tranny with a .70 OD.

I've used various Crane cams in different vehicles of mine over the years. My take of their listed cruise RPM is really just a sweet spot RPM where the engine is working at max efficiency.

Like MotorHead suggested, Depending on the amount of TQ your motor produces it might run pretty well at sub 2000 rpm even though the listed cruise RPM has a mid point of 3000 rpm. But at some point a motor will lug when it is at to low of rpm to produce sufficient power to maintain a constant speed

midyearvette

cruise rpm is an important number if you are running an auto with overdrive.....if you miss the number , you will be behind the torque curve of the mill which would make a horrible driver
ask crane for their recommendation.....you may want to change the rear gear for a different rpm...jmo....

Imo Apita

hence the plan to swap for 3.77 or more when the money tree recovers.....

Now I'm no cam expert so I've been playing with the Comp Cams software.
After putting in my variables the XR268R comes up as the #2 best choice.
Max torque 400+ at 4000RPM and max HP 440+ at 5000 RPM.
Sounds like a reasonable cam to me.

gkull

when you play with roller cams the real deciding factor is the static c/r ratio With 10.5 aluminum heads is 232 @.050 intake min. as compression goes down less duration works.

Dynamic compression ratio is the key. So many variables, the online calc programs only give a rough ratio. Like restrictive exhaust or like great intake manifold ram effect

v2racing

MotorHead,

I've been meaning to ask you how you get .640 lift out of a 286R? What rocker ratio do you have?