I know that a lot goes into correctly choosing the correct cam. However, I am hoping that some basic engine building knowledge will help me understand the principles in "general" terms to what would be a better LSA for a sbc400 for street use (under 5000rpms).
A tighter seperation angle of 108 or a wider 112.
Also, please tell me why so I can try to follow the logic.
Right now everything on the car is stock, except for a q-jet and edelbrock dual intake sitting on a shelf in the garage. The heads are stock camel hump with press in studs (what is the max lift for stock press in stud heads) with 1.94/1.5 valves. It has a stock powerglide, stock converter, etc.
Basically, the entire car (63 impala ss) is completely stock.
Originally, I was thinking a Crower cam in the range of 214/218 duration, .456/458 lift, maybe a 110 or so lsa. I know this cam is maybe on the smallish side, but I was hoping to use the original heads and still build a solid "mild" engine (think weekend/summer driver) on a tight shoe-string budget.
Thanks!
because your basic design goal is to maximize the 1500rpm-5500rpm power band on pump gas, as a basic guide a 400 or 383 with its reasonably long 3.75" stroke should use a fairly tight LSA like a 106-108 with the minimal duration required that will allow you to maintain about an 8:1 dynamic compression ratio.
this combo of limited duration and a tight lsa traps the most displacement as the valves seat earlier in crank degrees compared to a wider lsa so youll compress and burn more fuel/air mix and produce more low rpm torque, than the same duration on a wider lsa, thus balancing your static compression and breathing potential, which for the street should fall in the 9:1-9.3:1 range
moving to a wider lsa like a 112 will bleed off a bit more cylinder pressure and smooth the idle, and use a bit less duration, like a 215-220 duration cam, but at the cost of a bit of mid rpm torque.
building a combo for max mid rpm torque will generally mean a cam in the 220-225 duration range like this, but your going to want a 3.36-3.42:1 REAR GEAR RATIO
this will idle with a slight lope but pull hard from about 1600rpm-up to 5500rpm, because you don,t intend to spin over 5500rpm a hydraulic cam and some fairly small efficient ports in the 180cc range will work well.
http://www.summitracing.com/parts/BRO-1021004/
matched to a good dual plane intake like this
http://www.summitracing.com/parts/WND-8501/
and a fairly small carb like a vacuum secondary 650cfm
http://www.summitracing.com/parts/QFT-650PROVAC/
but your port size and valve size effect results
viewtopic.php?f=52&t=1070
viewtopic.php?f=52&t=727
heres the cam I suggested its got a 225 intake duration and the intake closed at 37 deg (.050 dur)
heres a very similar cam with 226 dur on a 112 lsa
notice the .050 intake closes at 41 degrees
heres a chart make your own comparison
http://www.crower.com/valve-timing-chart
below is a piston location chart in crank degrees
http://www.iskycams.com/ART/techinfo/ncrank1.pdf
a bit of math shows you compress and burn a total of about 10 cubic inches more fuel/air mix with the tighter lsa, if all 8 cylinder fire
youve got two potential routes to chose from.
your options will control where in that 1500rpm-5500rpm power band you concentrate the peak power.
if you want to maximize the engines potential power and don,t mind a lope at idle you can select a bit more duration and a tighter lsa.
if you want a smoother idle and don,t mind sacrificing a bit of power to gain a bit more drive-ability, then if you go the common route with a cam like this.
heres the cam I suggested, notice the intake valves opens earlier,with the larger cam but closes at a similar time
35 vs 37 degrees ABDC
the exhaust valves open at similar times also
47 vs 51 degrees yet the tight LSA allows a good deal more breathing time for the ports to flow,which is important to power in the mid and upper rpm range with the larger duration and tighter lsa,especially on a 400 with slightly restrictive heads, now theres no question that the crane will idle a bit smoother and work reasonably well,and if the cars basic transportation thats important but if your willing to put up with a bit rougher idle the crower cam will pull harder in the mid and upper rpm range,
If I had a manual transmission in my car theres little choice, Id select the crower, with an auto trans its not as clear a choice, because although the crower will produce more average power, its crane that will run a bit smoother and have more just off idle power, why not call both cam companies and get their input, Im reasonably sure youll find that a cam similar to the crane design is suggested if you express your concern mostly for durable street driving, and a cam similar to the crower design is suggested if you tell them you want to maximize your cars power curve but still keep it drivable for every day transportation.
if your much more concerned with the car as basic transportation then Id go the CRANE ROUTE
IF you want the car to have a bit more get-up-@ go, especially on the upper end of your power curve
the crower will produce that at the cost of a bit rougher idle
both routes will work
http://www.cranecams.com/
http://www.crower.com/index.php/contacts
