Pro-Flo XT Cam Selection

[cturney]

As we are waiting on our XT for a big block (3557) I am curious about cam selection guidelines. I have read in the forums that the cam choice must take into consideration the use of efi.

Are there any limiting factors on the cam used with the Pro Flo XT systems?

Could we find out the specs on the cams that were used in the test setups to determine base the base maps?

Any suggestions or words of wisdom on what should be done to get a good cam match for these systems would be greatly appreciated.

(we have been drooling over this system since we first read about it and can't wait to get our hands on one to try out!)

[cturney]

This is somewhat of an answer to my question. but very little guidelines defined other than the need for sufficient lobe separation. I've read this 3 or 4 times now. Would this note as a guideline be enough information to work with a cam specialist to get a suitable grind?

Camshaft Profile
Cam profile theory (in total) is beyond the scope of this guide but a few key concepts are important to understand
when tuning a fuel injected engine. Valve Overlap – Overlap is the period when both intake and exhaust valves
are opened together; when the exhaust valve is closing at the end of the exhaust stroke and the intake valve is
opening prior to the intake stroke occurring. In racing engines, increased overlap helps scavenge the spent
exhaust gases out of the chamber and pull the fresh intake charge into the chamber. The effectiveness of this
phenomenon though decreases significantly at low engine speeds. In fact, increased overlap tends to dilute the
incoming fresh charge with exhaust gases at low speeds causing decreased idle / cruise vacuum, rough idle, poor
fuel economy and increased exhaust emissions. Factors that affect valve overlap are duration and lobe centerline
angle or LCA. For a street cam, which must by necessity strike many compromises, the use of wider LCAs
produces more acceptable results than tight ones. Wider LCAs allow the use of longer duration cams to improve
top end power without penalizing low RPM drivability. Edelbrock recommends a minimum of 112° LCA for fuel
injected applications. It is possible to tune an engine with tighter LCAs but the task is much more difficult due to
low and/or fluctuating manifold pressure at idle. Remember from above that the main fuel map is used to
command the required fuel flow and that manifold pressure is used as an indication of engine load at a given
RPM. At idle and cruise conditions, large overlap cams will cause a decrease in engine vacuum. To the engine
controller, this is seen as an increase in manifold absolute pressure. Normally what happens when manifold
pressure increases? Fuel flow must increase as well. More load right? The problem is that the increased
exhaust dilution necessitates a leaner mixture for proper running not richer.

taken from (http://www.edelbrock.com/automotive_new/mc/efi/dl/calibration_selection_matrix.pdf)

[S. Armish]

That section wasn't really meant to be a guideline for cam selection; rather; its intention was to make customer/tuners aware of the influences of cam profile on tune-ability. The Pro Flo XT system will allow you to tune pretty much any combination (cam profile included) because it allows you the choice of MAP-N vs Alpha-N. Beyond that, proper cam selection depends on everything else...obviously (heads, manifold, etc).

One thing to keep in mind with the XT manifold design in particular is that it's benefits are greater the higher you rev the engine. In other words you'll see the most gains over a comparable 4 barrel style EFI intake above 4500 RPM or so.

Intake duration at .050" and LCAs are included with each calibration description in that document.