ez2cdave

Guys,

I have a question . . .

Typically, the Torque Band or Peak HP moves about 500 RPM +/-, for each 10 degrees change in Duration in a motor of a given displacement.

QUESTION: How much do the "effective" Cam Specs vary as displacement changes and what specific "adjustments" are required to put the cam back "on-spec" for any given displacement ?

EXAMPLE: A cam for a 350 motor is used in a 427 stroker motor . . .

Obviously, the cam will be a lot "milder" in the larger motor. The object is to be able to "readjust" the Lift, Duration, Lobe Separation, Valve Timing Events, & Overlap to "square up" the Dyno Sheet to produce the same performance curve in the 427 as it would in the 350, but with the numbers "bumped up" on the sheet due to the higher torque produced by the larger cubic inch displacement motor.

THOUGHT & IDEAS ???

This way, specs for custom camshafts could easily be determined by folks without having to deal with "salesmen" at companies or making the wrong choice of cams for their vehicles !

Dave

ez2cdave

bump

NO TAKERS YET ?

Adrenaline_Z

I think I know what you're asking, but the complexity of achieving your
desired goal (albeit hypothetical) is next to impossible.

There's no chance of reproducing a set of profiles between different diplacement
motors because of the vast differences in component specs.

You have to deal with valve sizes, runner sizes, flow variations, compression,
piston speed with respect to RPM, and the list goes on.

Once you start changing stroke lengths and displacement, getting the valve
timing to mimic torque curves on an entirely different engine is out of the question.

Furthermore, a cam with more duration / lift will have ramps and lifter accelerations that differ from the benchmark cam.

Unfortunately, you will (or anyone) need to consult someone who can spec a camshaft
for your engine combination to perform as you would like.

Paint_It_Black

iTrader: (1)

Adrenaline made some good points and is right in his info. However, you are about 5 years too late for an intelligent discussion on this site, but I'll give you the quick version.

The cam in the 346 is designed to fill a 43 cubic inch cylinder. That same cam in a 427 needs to fill a 53 cubic inch cylinder. It will run out of air at higher rpms. The pressure differential in areas of the stroke will change a bit, but you still have to open the valve longer for more fill. How much more is rpm dependent, intake dependent, and cylinder head dependent. This may also (will probably also) change the correct valve events. In other words, you better have a damn good knowledge about how engines work to properly spec a cam. No one who already knows this is going to tell you because it's how they put food on their tables.

Adrenaline_Z

P.i.B. makes a good point about the pressure differentials and the timing
required to fill the cylinder at varying RPM.

An understanding of how the air behaves as it accelerates into the cylinder
and interacts with pressures within as the piston rises and descends will
help you select cams with specific valve opening points, durations, and
overlaps.

There are some excellent resources out there for learning the basics and
even some more advanced "how it works" detail.

Some stuff that I've read that might help:

Scientific Design of Intake and Exhaust Systems
Engine Mechanics & Techonolgy Edition II (by Steckner)
Reher_Morrison Championship Engine Building

Paint_It_Black

iTrader: (1)

Not to mention how different lobes and ramps can/will effect power output even with the exact same cam profile.

Adrenaline_Z

For sure!

Let's add rocker ratio to increase valve opening velocity with respect to
the lifter base. Faster opening valves are always good if the valvetrain
can handle it.

I wonder if Dave will be checking back on this ? If not, I'm sure we'll get
the ball rolling for other readers.

Paint_It_Black

iTrader: (1)

True on the rocker ratio when the head can handle it too.. more mid and high lift duration relative to low lift is great.

But I can think of a few cases where I would opt for a lower ratio!

1) If the cylinder head stalls at lift under where the higher ratio would put the valve (Larry Meaux had a formula for this, I can't find it though).

2) If the high lift wasn't all that impressive, mid lift was solid, and low lift was amazing (which means, amazingly BAD.. low lift head flow sucks!).

3) The cylinder head completely blows the engine away. As in.. it flows gangbusters and has no trouble filling the cylinder at the rpm the engine is being used. Then I might even go with a smaller ratio to lag the air column coming in. It will maintain a pressure differential and continue to pull on the air as the piston starts it's compression stroke. This is assuming the same cam with a different rocker ratio though. At that point it's the cam that's wrong with the combination and the smaller rocker is just a band aid.

ez2cdave

I wonder if Dave will be checking back on this ? If not, I'm sure we'll get
the ball rolling for other readers.[/QUOTE]


************************************************** *******

No need to wonder. . . LOL !

I am trying to get a handle on cam specs for the following motor combination (below). . . 1995 Pontiac Firebird Formula

Originally, I was considering a LINGENFELTER GT-11 but don't think it is aggressive enough in a 427 motor . . . The goal is a minimum 500 RWHP and 500 RWTQ in a smooth idling, mild-mannered daily-driver, at least until the "hammer" is dropped !

( Goal is a BROAD, FLAT Torque Curve with 6000 - 6500 RPM max. / 4L85E transmission (rear gears & converter are TBD ). . . 600 ft/lb +/- flywheel torque / 600 HP +/- @ flywheel . . . SMOOTH to VERY MILD IDLE with less than 40 kpa MAP @ idle of 950 RPM or less )

Basic Engine Components:

LS7 block (4.125" bore) ... CNC LS3 cylinder heads w/68-70cc chambers (2.165 hollow-stem intake valves / 1.59" sodium-filled LS9 exhaust valves )... Callies Dragonslayer 4340 forged crank (4.000" stroke) ... Callies 4340 H-Beam rods (6.125") . . . MAHLE coated pistons (11:1 CR) ... Comp Cams Ultra Pro Magnum Roller Rockers (1.8:1 ratio - Chrome Moly) ... etc, etc, etc.

Head Flow Numbers: (CFM)

LIFT INT EXH
----------------------------
.200 153 118
.300 225 180
.400 280 201
.500 324 217
.600 350 230
.650 355 236

Dave

LS1-NAVY

iTrader: (1)

love threads like theses always love to learn new things

Paint_It_Black

iTrader: (1)

Don't let intake duration fool you on the LS3 heads. It wants something different than the LS1/2/6 wanted.

Adrenaline_Z

I would agree with that statement. I'm not aware of any formula that would
indicate a stall scenerio however? This is something normally measured on
a flow bench, or perhaps by observing the port shape/design. If you can
find this formula, I'd be very interested in checking it out. Of course we could
always contact Larry but I haven't seen him around other forums lately. I know
he's very busy with his business.

Depending on the lobe shape, I can also agree with this statement. The cool
thing about higher ratio rockers is getting the valve to accelerate off the seat
sooner, and also providing more duration at each lift point without changing
the valve open, and valve close positions.

I could see some added benefit of having the valve open for a slightly longer
period of time (except for seat timing) in this case.

This scenario escapes me because I'd want more air mass flowing into the
cylinder to overcome any opposing pressure that might force the mixture
back into the intake runner. This generally speaking is necessary for naturally
aspirated motors to achieve higher VE figures. Maybe I'm missing your point...

Adrenaline_Z

Should be fairly easy with that much displacement. I'm putting down 440
with a well behaved idle, and it's only 346 cubes.

Seat to seat timing of about 285 degrees, or thereabouts...


Something with not much overlap and duration (~ 285 as suggested above).


I'd get the valve to lift somewhere in the 0.650" max. range since the
head is starting to plateau. With a 1.8:1 rocker, you can get the "Area
under the Curve" going with the aggressive lobe shapes available these days.

Your cam will probably end up with specs. such as (seat to seat): 280/285, 0.650"/0.650", 114.

That gets you a fairly mild cam to keep the idle and street manners tame enough, and
enough displacement to achieve the power figures you desire.

Ask your cam guy / engine builder once you finalize the gearing and engine
components. Please update us on what you get. I'd like to know your results!

Paint_It_Black

iTrader: (1)

Adrenaline, the formula from Larry I was talking about what to determine your max lift based on where your head stalls. It doesn't calculate where the head is going to stall. sorry I was vague, it was late when I posted and I was tired.

ez2cdave

bump message

Adrenaline_Z

Ahhh, thanks for clearing that up. I figured he might have some CFD software
with pre-scanned/digitized ports to allow for such a calculation.

ez2cdave

************************************************** *******

The engine components are pretty much "finalized", except for the camshaft specs.

I intend to try to stay with 3.08 gears (3.42 is next ratio in 12-Bolt, with 25.66" diameter tires (315/35/17) for the following reasons:

(1) Retain top end speed (170 mph +/- in 3rd gear w/4L85E) comes in handy, at "certain times" ... LOL !

(2) Acceleration is mainly improved by the torque afforded by the increased displacement (346 to 427 )

(3) Economy and lower cruise RPM ( $$$$ & parts life )

Dave

Adrenaline_Z

I can't agree with the 3.08 gear, or reason #1 but it all depends what
you're trying to achieve.

It's not the best 1/4 mile set-up, but you should still get a reasonable
launch with the amount of power the engine will make.

What's the estimated date for having this project complete?