cam grinds for SC

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In a message dated 4/29/99 7:13:04 AM Pacific Daylight Time, jsg at donet.com 
writes:

<< Subj:	 cam grinds for SC
 Date:	4/29/99 7:13:04 AM Pacific Daylight Time
 From:	jsg at donet.com
 Sender:	owner-diy_efi at esl.eng.ohio-state.edu
 Reply-to:	diy_efi at efi332.eng.ohio-state.edu
 To:	diy_efi at efi332.eng.ohio-state.edu
 
 
 I'm about to install a Vortech supercharger on my SBC 305 TPI, and
 I'm thinking about changing cams too. The vortech FAQ has 
 this to say about cams:
 
 4) What is the correct cam to run with my
    supercharger?
 
        Cam selections are best discussed with the various
        cam manufacturers. Many people select cams which
        are not the correct choice for centrifugal
        superchargers. When discussing the cam with a
        manufacturer make sure they understand a Turbo cam
        or a cam for a Roots style supercharger is different
        than a centrifugal cam. Generally speaking the
        supercharger and the cam do the same thing (increase
        volumetric efficiency), be cautious of choosing a cam
        that has large amounts of lift and duration. Ideally for
        a street application retaining decent idle, emissions
        and drivability are important, for this reason we
        suggest a "moderate" camshaft. Given the choice
        between two cams we suggest the milder of the two for
        street applications.
 
 Of course that is of little help, and I would like some idea of what's
 going on before contacting manufactures. So, what does make for a good
 cam for applications like this? How do these cams differ from turbo or
 roots specific cams?
 
 john
 
  >>
Hi John,
	Man, where do I start........this thread could be extremely long but 
I'll try to keep it to the point.....Turbo cams diff from supercharged cams 
for two major reasons.....backpressure between the exhaust valve and turbo 
and spool up times,and truely a bunch more incidental ones....but that's a 
whole different discussion......
  	As for the supercharged stuff here goes......There is probably a very 
good cam for your app available off the shelf or if not can be custom ground 
at a reasonable cost,,,but I will need more info from you to recommend 
one......As for some experience on the subject I would like to share with the 
group this regarding lobe separation and overlap cycles.......overlap cycles 
affect octane requirements and fuel efficiency...... example....540 bbc 7.5:1 
compression 100 octane fuel 775 hp 5500 rpm 11 lbs boost.  engine would not 
detonate on the dyno with 92 octane but if loaded for long periods of 
time(boat) would begin detonating within 7-8 minutes unless we used 100 
octane....cam was 222-226  @.050 on 116LC......Customer wants to run 92 
octane,  when decreased timing far enough to eliminate detonation    EGT's 
would go too high for extended use....advise of decrease in fuel mileage and 
install cam 246-254 @.050 on 116LC.  engine makes the same 775hp at 5500 but 
now is only 8lbs of boost.   Intake air temps within 10 degrees of each other 
but requires almost 20% more fuel(using Horiba airfuel meter and allowing an 
offset for the overlap to scew the sensor readings)  Egt's are now 1375-1400 
and stable on 92 octane for unlimited periods of time........The overlap 
cycle helps cool the surface temp of the combustion chamber and piston and 
exhaust valve...but results in a waste of charge out the exhaust.......same 
customer later wants 1000 hp, turn boost up to 11 lbs  and engine makes 980hp 
at 5600 but jumped from 11lbs boost at 5600 to 14lbs  of boost at 5800.....VE 
of engine fell off at 5600 and supercharger did not.....Install new cam with 
exact same overlap cycle but intake valve closing event 20 degrees 
later.....engine makes exact same power to 5600 but makes 1184 hp at 6100 rpm 
at same 11lbs of boost......This told me my heads were flowing as much air as 
the supercharger could push(pressure had equalized in the cylinder in 
comparison to the manifold) until there wasn't enough time(RPM vs valve 
event) to continue to fill.....again required 100 octane fuel to make the 
higher horsepower but was very stable and hey how long does it take to change 
pulleys to run 92octane?????   As far as cams go in reference to centrifugal 
vs. roots....with a roots type the only thing holding the air back is the 
throttle blade,,,but with a centrifugal it is also rpm dependant and the 
wrong camshaft will produce less torque at low speeds than you probably had 
stock.......and in reference to the cylinder head stuff, if I had poorer 
heads on this app I would have gained hp by adding intake closing event 
time....another way to help a poor environment such as a stock engine......I 
have ended up with as high as 121LC cams on some supercharged street engines 
just to fill the cylinder more without giving up the effiency.....but you  
might think about an EGT guage if your gonna tow.........I know I have just 
touched the surface of this conversation but will attempt to answer any 
questions I can.....I don't know everything there is to know about every app 
regarding supercharger cams but am more than willing to help with the 
experiences I have had.
	John, I would like to know any mods done to your engine,,,,static 
compression ratio,,,,rpm band and how much hp you're trying to achieve.....is 
it going to be a flat tappet hydraulic or hydraulic roller?????
 	Anyway, I hope that helps and I'll try to respond as quickly as 
possible,,,,,just doesn't seem to be enough hours in the day....it's 4:30 am 
here and gotta be up soon to tune some stuff at the track....ttyl
-Carl Summers