I got this QUESTION ASKED, "So grumpy my cams got more Duration at .050 236/242 .565/571 with 1.5's I used 1.6's than yours at 230/238 .559/.558 with 1.5's you used 1.6's too.
is it better?"
this is a question I get frequently in several forms, the basic question is always (if I install a larger cam ,won,t my car be faster?) well the answer is.... (sometimes, yes,...BUT NOT ALWAYS
because at about 230 @.050 lift durration in a 350-383 displacement youll find that your starting to loose low rpm cylinder pressure that allows you to make torque at those lower rpms which tends to drop off just as fast as you gain upper rpm breathing ability,in the upper rpms, which tends to improve, with some duration and overlap increases at least to a point. so what your effectively doing is moving the efficient volumetric cylinder filling rpm band higher at the cost of low rpm tq. ( and yeah the rest of the combo has a huge effect on exactly when and where in the rpm range the efficiency starts and stops . )
makeing horsepower is only part of the game.... makeing it peak and haveing almost all of the torque curve fall within your usable gearings rpm band is the trick that allows your car to be fast
keep in mind that while these are the .050 numbers below, with 1.5 ratio rockers that the seat to seat numbers
are a better picture of the cams true effect on the valves and I use 1.6 roller rockers so your not getting exactly the same ramp speed ratios that youll get on 1.5 ratio rockers
heres the measured
2.0 ATDC 117
crane used non-matching opening/closeing ramp profiles with the closeing ramp setting the valve down slower than it opens, ramps have about 28 degrees on the fast open side but about 34 degrees on the slower closeing ramp
added to get the seat to seat figures between the 1.6 ratio rockers and the differant ramp rates the .050 lift durration figures are basically flawed for useful data imput purposes
horsepower is a mathmatical formula for the RATE at which TORQUE can be applied, and youll want to apply that torque at the highest rate you can, which will require you to keep the engine in its most efficient range thru matched gearing, the formula for hp is (tq x rpm/5252=hp
exampleREMEMBER, youll need to keep the engine in the rpm range so the engine stays approximately between the torque peak and just over the horsepower peaks without getting into the engines redline
450 ft lbs of torque at 3000rpm=257hp
450 ft lbs of torque at 6000rpm=514hp
because the torque at the higher rpm useing gearing can be applied faster, here read this[/b]
where most guys go wrong is in not correctly matching the cars stall speed and gearing to the cars tq curve, if you mod the engine for increased high rpm performance but fail to also match the stall speed and gearing to that higher rpm tq curve much of the potential improvement is wasted.
in the close to stock engine above, the engine should be geared to stay in the 3500rpm-5000rpm range for max acceleration (lower in the rpm range if mileage is a big factor)
in the moded engine above the rpm range moved to 4000rpm-6500rpm requireing differant rear gears and slightly higher stall speeds to gain max acceleration in the same car,
you should readily see that a trans that shifts at 5000rpm will work in the first example but would waste most of the power curve in the second example,where shifting at 6500rpm under full power acelleration would make more sence.
a 3.08 rear gear and 700r4 trans matches the first example
but it would take a swap to a 3.73-4.11 gear to allow the engine in the second example to keep its most effective power band matching that second power curve well.
links youll need to figure out correct rear gear ratios
[b]REMEMBER, youll need to keep the engine in the rpm range so the engine stays approximately between the torque peak and just over the horsepower peaks without getting into the engines redline [b]
LOOKING AT THIS GRAPH, IT WOULD SEEM THAT THE CORRECT GEARING FOR MAX ACCELERATION WOULD USE A 4200RPM STALL AND A SHIFT POINT JUST OVER ABOUT 6700RPM heres the graph with your choice in the exact same engine combo
but remember if you change the other parts in the combo the results could easily swap with the comp. cams grind coming out better in that new application