Vizard's secret '128' cam spec # revealed

[turbo2256b]

I might of missed it is there any tweaking needed to the formula for a single plane VS a dual plane intake?
Also ave to wounder the affect of a ported dual plane that flows more air than a VrJr with the just draft removed?

[hoffman900]

Guys,

I think you are missing David's point.

It's a number used to get within a good ball park for engines for a certain engine type with a narrow modifications list.

If you're engine is a typical 10:1 street / strip engine like you can find at just about every cruise night, this isn't likely for you.

[bigfoot584]

Guys,

I think you are missing David's point.

X2

Several posters have done a good job of twisting it.

[hoffman900]

Guys,

I think you are missing David's point.

It's a number used to get within a good ball park for engines for a certain engine type with a narrow modifications list.

If you're engine isn't a typical 10:1 street / strip engine like you can find at just about every cruise night, this isn't likely for you.

Edited

[falcongeorge]

Question for David, pretty sure I know what its going to be, does using an IR intake affect the optimum LSA for a typical 2 valve wedge combination?

[David Vizard]

Question for David, pretty sure I know what its going to be, does using an IR intake affect the optimum LSA for a typical 2 valve wedge combination?

Yes but not as much as I had expected. Tried different cam LCA in a Pontiac 455 and a 383 SBC with 48 Dellorto's on. The Pontiac was no different to a single or two plane. The Chevy wanted one degree tighter and the difference in power was only about 7-8 hp. Would like to have done more with IR but ran out of time. However I suspect I may have an opportunity to do a lot more with an IR system on a 289 Ford later this year so we will see what transpires

DV

[falcongeorge]

Question for David, pretty sure I know what its going to be, does using an IR intake affect the optimum LSA for a typical 2 valve wedge combination?

Yes but not as much as I had expected. Tried different cam LCA in a Pontiac 455 and a 383 SBC with 48 Dellorto's on. The Pontiac was no different to a single or two plane. The Chevy wanted one degree tighter and the difference in power was only about 7-8 hp. Would like to have done more with IR but ran out of time. However I suspect I may have an opportunity to do a lot more with an IR system on a 289 Ford later this year so we will see what transpires

DV

Thanks David. I was guessing it didnt make any difference.

[turbo2256b]

sO DAVID USING YOUR FORMULA DOSE NOT MAKE ALL THAT MUCH DIFFERENCE BETWEEN A DUAL PLANE AND A SINGLE PLANE. nOT yelling my fingers are fried

[groberts101]

Question for David, pretty sure I know what its going to be, does using an IR intake affect the optimum LSA for a typical 2 valve wedge combination?

Yes but not as much as I had expected. Tried different cam LCA in a Pontiac 455 and a 383 SBC with 48 Dellorto's on. The Pontiac was no different to a single or two plane. The Chevy wanted one degree tighter and the difference in power was only about 7-8 hp. Would like to have done more with IR but ran out of time. However I suspect I may have an opportunity to do a lot more with an IR system on a 289 Ford later this year so we will see what transpires

DV

Thanks David. I was guessing it didnt make any difference.

I've learned much from David and many others around here and do not wish to dilute this thread anymore than it already has been to this point.. but since the question has already been asked, I am finding it very hard to wrap my head around the physics involved here.

With a split plenum designs outright ability to tame an overly large camshaft at the carbs booster.. I simply can't imagine separating cylinders from one another would not impact a camshafts optimum overlap number for a given application and the Powerband related to it.

Now, I don't claim to be at or near the level of understanding/accomplishment as many on this site but was just hoping for more detail on the matter of an induction designs impact on the camshafts design and how it relates to this formula.

[David Vizard]

sO DAVID USING YOUR FORMULA DOSE NOT MAKE ALL THAT MUCH DIFFERENCE BETWEEN A DUAL PLANE AND A SINGLE PLANE. nOT yelling my fingers are fried

The 128 formula predicts the same whether or not a single plane or dual plane intake is used. In practice manifolds cam make a difference but there are some many different intakes out there that it is impractical for me to test them all - it would cost a fortunes when done. I am sure there will be exceptions but for the most part on Chevy's ans small block Fords there is usually only 1 to maybe 2 degrees different between a dual p[lane and a single plane. Usually the dual plane seems to be OK with a slightly wider LCA of 1 or 2 degrees but - (big but here) it is less sensitive to being a degree too tight so short of actually dyno testing and putting about 3 cams through the motor to see what's what I would say stick to the formula if you have nothing else to go by.


DV
PS put your fingers in a jug of ice water.

[David Vizard]

Guys,

I think you are missing David's point.

It's a number used to get within a good ball park for engines for a certain engine type with a narrow modifications list.

If you're engine is a typical 10:1 street / strip engine like you can find at just about every cruise night, this isn't likely for you.

Sorry, got to pull you up on this one. This was all looking good until the last line. If you are building a street motor and are on a limited budget this formula will be worth about 40 lbs-ft and as much as 50 hp on a typical 383 SBC build as you will get the right cam not one of those incorrectly speced wide LCA catalog cams.

Of course if it is totally a cruiser as you mentioned then no one really cares whether it has 300 hp or 400!

DV

[na svt]

I apologize if I missed it in the previous pages, but is there a formula like "128" for coming up with the optimum LCA where max power from 6500-8500 is desired?

[Walter R. Malik]

I always liked Harold Brookshire's school of thought on selecting cam events:

Eacfh Cam designer has his own theory, and often they contradict each other.
I used to say the most important degree in your camshaft is the degree BEFORE the intake valve opened. I still do. If a measurement was made in your cylinder of the volume and pressure of burned gases existing there before the intake valve opened, that would give you a figure of merit for what the engine would flow in clean air and fuel, for whatever intake lobe is on the engine. We see this in dyno-testing the same intake and exhaust lobes, but with different LSAs, when we install the intake lobe on the same ICL for both tests.
In both tests the intake lobe is on the identical LSA, the only difference being the position of the exhaust lobe. The differing positions of the exhaust lobe produce different amounts of exhaust gas pressure and volume just before the intake valve opens, and starts airflow earlier or later, producing different torque and BHP curves in this engine.

Here are some thoughts I've had for a very long time......
Engines work off of pressure differentials--ie, differential equations......
In unblown engines we have a positive pressure(back pressure) in the exhaust pipe, and a negative (vacuum) in the intake port BTDC. The piston is moving upwards, pushing the exhaust gases out the exhasut port. The numbers aren't far apart, say 1 1/2 lbs backpressure, a few inches of vacuum, but it does not allow flow into the combustion chamber. Then TDC, and the piston starts down. As it moves downward, it is creating a volume where no volume had existed---and atmospheric air pushes in to fill that volume. This is even why cars run better on cold days that in the heat of summer, the cold air weighs more, and fills the cylinder faster because of inertia.

Most engine builders, probably you too, have tried the same cam lobes ground on different LSAs, say like 106 and 108. If you dyno them in the same engine, and on the same Intake CL, both cams obviously have the same intake cam on the same ICL. Oddly enough, they have different power curves, and everyone says "Sure, because of the different LSAs!". Remember, they both had the same intake lobe on the same intake CL. The power curves differed because the effects of the exhaust back pressure and the resulting reversion were different for each test, and each cam filled differently because of their reversions. Once you start thinking on this, it leads all sorts of interesting directions......

I have had a conundrum about this very kind of thing, which happened in some recently past dyno testing, consuming my thoughts for the past half year or so.

This is the best explanation about what was happening which I have received from anywhere.

[na svt]

Take a look at the graph. Study it for a while and you will see that a 108 Lobe Centerline Angle (also called a Lobe Separation Angle) for this particular engine (a 355 SBC with 186 heads and a 10/1 CR) gave the widest and best output on a 108 LCA. Spread that angle and watch the torque and HP drop.

Were these test conducted with the cam installed straight up?

[na svt]

dv-01.jpeg

Are these seat to seat durations?