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Old 01-14-2004, 01:01 PM   #1
grumpyvette
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will a bigger cam make my vette faster?

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.
yours

http://dab7.cranecams.com/SpecCard/D...1=Display+Card

mine
http://www.compcams.com/Technical/Se...umber=12-433-8
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

io 36
ic 76
eo 84
ec 36

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
.050
INTAKE
8.0 BTDC
42.0 ABDC
exhaust
56.0 BBDC
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 rampadded 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]
http://www.69mustang.com/hp_torque.htm

http://www.ubermensch.org/Cars/Technical/hp-tq/

http://vette.ohioracing.com/hp.html

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.
example


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]
http://www.bracketracer.net/carmath/dynochart.asp

http://www.miata.net/garage/tirecalc.html

http://www.wallaceracing.com/reargear.htm

http://users.erols.com/srweiss/calcmph.htm

http://users.erols.com/srweiss/calcrpm.htm

http://users.erols.com/srweiss/calcrgr.htm

http://www.prestage.com/Car+Math/Ge...io/default.aspx

http://www.geocities.com/z28esser/speed.html

http://server3003.freeyellow.com/gparts/speedo.htm

http://www.pontiacracing.net/trannyratios.htm

http://www.tciauto.com/tech_info/gear_ratios.htm


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
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Old 01-14-2004, 05:04 PM   #2
Patrick
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Great post. I hope to start putting together my combo later this month, and cam selection is something I'm very confused about!
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Old 01-14-2004, 05:06 PM   #3
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How do you know so much Grumpy. I hope i could be so engine savy like you. Great post as usual.
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Adam "Buy the ticket, take the ride."
Old 01-14-2004, 10:26 PM   #4
-Jordan-
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hey grumpy what are you doing with your 85' vette right now. I remember that you said it was getting worked on ? is it stock.?
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substitutes for the word "rice"
"SLICE" (stupid looking idiotic car econobox.)
ASANG" (all show and no go)
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Old 01-15-2004, 06:30 AM   #5
grumpyvette
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stock????

theres not a single part in my engine(INCLUDING THE BLOCK) thats orriginal,in fact my current 383 has not a single chevy produced part, its a full roller 11:1 cpr all forged engine
was it worth the price? no not really, I could build it today for slightly less and I could builld a BETTER engine for the same money today, its been a learning process, thats sure! but then I built the basic engine over 12 years ago!
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" IF YOU CAN,T SMOKE THE TIRES AT WILL,FROM A 60 MPH ROLLING START YOUR ENGINE NEEDS MORE WORK!!"
Old 01-15-2004, 06:28 PM   #6
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hahah sweet .. do you have any pics ? how is the interior ?
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substitutes for the word "rice"
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