Model A camshaft
- Thread starter glrose67
- Start date
Synchro909
Well-known member
Why not ring Clive Cams and ask what grind number 566 is?
Synchro909
Well-known member
So what grinds is it?
This might help:
https://www.fordgarage.com/pages/camshaftspecs.htm
https://www.fordgarage.com/pages/camshaftspecs.htm
Jim Brierley
Well-known member
Interesting that he says the B has only one profile. The one shown is for the latest B design. I grind cams, mostly A/B's, and that one is the most rare of 3 different profiles. Most B cams have .302" of lobe lift, some have .315" and the latest is shown in the Ford Garage article.
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Synchro909
Well-known member
So, it seems that cam shaft is a B 6250 grind?
Given the EXCELLENT condition of the distributor/oil pump drive, it is an excellent cam shaft. Good pickup, Greg.
Given the EXCELLENT condition of the distributor/oil pump drive, it is an excellent cam shaft. Good pickup, Greg.
Bob Bidonde
Well-known member
What is the significance of intake & exhaust valve overlap? Functionally, they are separated by the compression stroke, so I see no interaction of the overlap. However, I suspect the overlap means something to setup cam grinding.
The Master Cylinder
Well-known member
I always thought "overlap" was the number of degrees the exhaust and intake valve are open at the same time at the end of the exhaust stroke / beginning of the intake stroke.
You're looking at it in the end of the intake stroke / beginning of the compression stroke.
Jim Brierley
Well-known member
You are correct. Performance cams have greater overlap, so actually have less power at low speeds but gain power as RPM increases, due to inertia of the air/fuel. Sounds weird, but it works.
Bob Bidonde
Well-known member
I can imagine at very high engine RPM, the intake & exhaust valves may overlap open because of the time it takes the valves to open and close. However, does that matter in a very low RPM motor such as the Model A's?
Jim Brierley
Well-known member
During the overlap period intake air is mixed with exhaust, causing the rough idle common in performance engines. At high RPM's inertia of the gases force intake air into the exhaust, and packs more air into the cylinder. More air in the cylinder, almost like lightly supercharging, equals more power. Valves open and close slowly, in comparison to when the lifter is further up the lobes ramps , so the valves are only open a small amount, but enough to cause what is termed a ramming effect. Air has weight, so the extra filling of the cylinder is effective.
Another thing that seems odd is that the exhaust valve starts to open long before BDC of the piston. This is to allow the pressure in the cylinder to bleed down before the piston starts back upward, which would take horsepower away.
That's enough to make a guys head hurt!
Another thing that seems odd is that the exhaust valve starts to open long before BDC of the piston. This is to allow the pressure in the cylinder to bleed down before the piston starts back upward, which would take horsepower away.
That's enough to make a guys head hurt!
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8EL
Senior Member
Bob, This is a complex association, many variables to power and drive ability are determined by these valve timing events. It goes much deeper than what I see in previous postings, to touch on a few, this much I know...
As rpm increase, air demand is greater. To supply the additional air and fuel, designers open the intake valve sooner, which allows more time for the intake charge to fill the cylinder. With an early-opening intake valve, at high rpm the exiting exhaust gas also helps draw the intake charge through the combustion chamber and out the exhaust-that's good for purging the cylinder of residual gas, but it also increases fuel consumption by allowing part of the intake charge to escape before combustion and can make for a rough idle. In a perfect world, the optimum intake closing point would occur just as the air stops flowing into the chamber.
An early-opening exhaust valve may benefit scavenging on high-rpm engines because most useful cylinder pressure is used up anyway by the time the piston hits 90-degrees before BDC on the power stroke. Later exhaust valve opening helps low rpm performance by keeping pressure on the piston longer.
Lift, the magnitude of the valve peak opening (don't forget to factor rocker arm ratios on push rod engines), duration, the value in degrees that the valve has left it seated position. Lobe displacement angle (some refer to this as lobe separation angle), the displacement angle (LDA) between the center lines ground into the cam, are all engineered for the engines intended purpose.
Duration, lift, and LDA combine to produce an "overlap triangle." The greater the duration and lift the more overlap area, LDAs remaining equal. Given the same duration, LDA and overlap are inversely proportional: Increased LDA decreases overlap (and vice versa). More overlap decreases low-rpm vacuum and response (why race cams usually employ vacuum pumps for accessory function, and also produce that lopey thumping exhaust note we associate with race engines), but in the midrange overlap improves the signal provided by the fast-moving exhaust to the incoming intake charge. This increased signal typically provides a noticeable engine acceleration improvement.
Less overlap increases efficiency by reducing the amount of raw fuel that escapes through the exhaust, while improving low-end response due to less reversion of the exhaust gases back up the intake port; the result is better idle, a stronger vacuum signal, and improved fuel economy.
Due to the differences in cylinder head, intake, and exhaust configuration, different engine combos are extremely sensitive to the camshaft's overlap region. Not only is the duration and area of the overlap triangle important but also its overall shape. Much recent progress in cam design has been due to careful tailoring of the shape of the overlap triangle. The most critical engine factors for optimizing overlap include intake system efficiency, exhaust system efficiency, and how well the heads flow from the intake toward the exhaust with both valves slightly open.
Like I said very complex issue, but the take away from this would be anyone serious about making power, time spent to understand this would be well spent. No component has more effect on the power an engine makes than its induction system. The cam is the blueprint for this combination....Hope this helps.