thinking it thru vs building what your buddies say

[grumpyvette]

Im posting this copy of a thread here to get some of you guys to stop and think thru your combos, while theres guys here that can more than likely build better engines than I can, the vast majority are guys that need all the help you can get, so I hope this helps

 

 

 

ok CONFESS! did you think it thru, or did you just build it the way the magazine did or your buddies said you should?

 

now each of us has to do what he feels is correct, but Im constantly amazed at the guys that would rather impress thier buddies with a long list of radical race only parts like cams and heads, intakes, ETC. than think thru a project, research the potential carefully and build it to do exactly what THEY want and can use VS what thier (BUDDIES) say they should build. now IM no EXPERT, but I have build well over 80 bbc engine (I lost count) and I do know what will and what won,t work in most applications...both thru extensive software testing and EXPERIANCE building the real things

example

I recently suggested a solid roller cam for a bbc application where street driven depenability was a significant factor, useing a 4.25" stroker,crank,(496 displacement) I suggested a cam with approximately a .620 lift and 247 durration which perfectly matched the application,rpm range and expected power range, keeping in mind that the spring pressures on those roller lifters ran between 360 seat lbs and 480 open lbs which is on the low end of the roller lifter range, and the ramp design was designed for excellent valve control with long term durrabillity, because , I don,t see swapping out $300-$800 worth of components every few thousand miles as a realistic option just to gain a few hp that a wilder cam would produce, now Ill be the first to admit that the larger cam would potentially produce noticably more PEAK HP,and this is important, AT AN RPM LEVEL youll rarely reach, BUT not AVERAGE HP within the normal opperational rpm band and theres a differance!

WAS I wrong with my suggestion? hell, Ive been married 37 years, Im used to being told IM wrong, I decided to run all the specs and find out, nothing but the cam specs was changed, I WAS SHOCKED TO SEE THE RESULTS, SOFTWARE PREDICTED, EXPERIANCE and SEVERAL PREVIOUS ENGINE BUILDS MADE ME THINK I WAS CORRECT, BUT LOOK AT THE RESULTS

the wilder cam with its aproximately .700 lift requires a spring rate of aproximately 360 seat lbs to 800 open lbs

and the ramp accelleration rates are on the control edge, life expectancy is in the low HUNDREDs of HOURS vs years for the milder cam, designed for limited street use VS the wilder RACE ONLY DESIGN

now lets compare what the probable results will be..heres what the milder cams results predicted thru my software dyno,looks like this

 

rpm, torque, horsepower

2000--510......195

3000--575......329

4000--626......477

5000--641......610

6000--585......668

 

the wilder cam

 

2000....277....105

3000....381....218

4000....461....351

5000....545....519

6000....555....633

7000....510....679

7500....500....710

 

yeah those 700 lift and 267 duration numbers look impressive on the spec card.....but do they fit your application??? BTW the piston speed limmited you to about 6300rpm before youll start breaking parts

[EZ-E]

To me it is obvious which the better cam is and Im not what you would call the one that can tear down any engine and build it.

 

I would think for the durablitiy the power and the torque of the milder cam is there really any choice at all?

[Michael]

What was the duration on the 0.700”-lift cam? It’s remarkable that just a cam change - with what on first blush appears to be a minor change in specs - will cause low-end torque to almost fall in half!

 

Now here is a follow-up question: fully-assembled cylinder head combos spec’d for mechanical roller cams tend to have very stiff springs! So for mild mechanical roller cams, often the combo becomes over-sprung. My Brodix Race-Rite heads have springs with 580 lbs advertised at 0.650” lift. But the cam manufacturer recommends 400-450 lbs at 0.650”. Correcting this mismatch was not practical in the short-term, so I let it be. How risky is this? I have the Isky Red Zone lifters and 3/8” chromoly pushrods.

[dr_hunt]

.700 lift is not a wild cam by any means in a BBC. But what may be happening is that the heads don't flow well enough to make use of that cam would be my guess. So, I think that epitomizes grumpy's point of matching parts.

 

I went to the drags last night and a buddy of mine had his chevelle out. BBC 468, 4 bolt, steel crank,, roller cam, Dominator on alky, eagle rods, 13:1's. He had iron oval port heads, was running 12.10's to 12.30's in the quarter. He purchased some Brodix BB3 heads, slapped them on and his best time of the night was a 12.00, mostly 12.10's, so he didn't gain all that much, if any, with the rectangular port brodix heads. So a quick check with brodix and the BB3 cylinder head has a 370cc intake port and is designed for 555 to 632 cubic inch engines!!!!

 

Is it any WONDER that it didn't pick up any power??? No, probably not, his TQ went in the toilet and his HP did too!!! But being able to say you have BB3 heads is, well, worth what when you are bench racing? Not much in my book.

[Lason]

I was told by all my buddies to go with a mild 350 in my Z, instead I went with a LS1/powerglide combo with a straight axle/4-link conversion so I guess Im on the flip side of your original scenario

[grumpyvette]

its NOT the lift so much, as the valve opening and closeing points,(durration and LCA) matched too the displacement and compression ratio, that effects the tq curve

keep in mind the piston compresses ZIP /NADA untill both valves close,spread the LCA and add durration and that huge 400-500-600, or larger inch displacement, will be almost useless at low rpms simply because a great deal of potential cylinder pressure and fuel/air mix is getting wasted, in reversion, or pumped out the exhaust, un-used

youll make your best power when your most effectively filling and burning the highest percentage of the cylinders volume, and that occures only at the point where both the number of power strokes per minute and the volumetric efficency filling the cylinders are near maximum, stroke, cpr, port cross setion,valve size,exhaust scavaging, effect, the results but its very easy to screw up the parts sellection where one component restricts all the other parts effiency. in the case above all but the cam stayed the same and all parts matched at 5500rpm with the milderr cam, the wilder cam was designed for 7400rpm, and bleed off excessive cylinder pressure under 5000rpm, yeah its an extreme in mis-matched parts but I see it frequently

read thru this carefully ITS an excellent source of info

 

http://www.newcovenant.com/speedcrafter/tech/camshaft/1.htm

 

 

http://www.popularhotrodding.com/tech/0607phr_camshaft_basics/

 

theres nothing new here, but most guys still screw it up because they don,t match ALL the components flow and rpm bands,or they over cam, or use retrictive heads or not enought compression