Looking for feedback on how I'm degreeing my cams - Modular Fords

  • Cam Selection & Phasing the How's and Why's

    Here are a few hopefully provocative thoughts on the how's and why's of cam selection and phasing.

    Blower motors produce much higher cylinder pressures than their n/a cousins which is part and parcel to how they generate more torque and therefore horsepower. These higher cylinder pressures provide both benefits and challenges we do not have in n/a engine design equivalents. Those distinctions can be used to advantage as we approach cam phasing decisions for our engines. What I am going to share may seem at first, counter intuitive but it is as real as the supercharger on top of your engine.

    Lets use Comp 106450 cams and phase them to the crank like this;



    This is a fairly typical phasing for these cams. I am going to suggest (along with some reasoning) that you might want to phase them somewhat differently than they are illustrated above. The magenta circles in the image above represent opportunities to optimize the cam phasing. The short rules for a supercharged application like ours look something like this;

    ◉ Whenever possible perform cam measurements at 0.050" valve lift. Some cam specs are at 0.050" lobe lift,

    ◉ Choose cams with the smoothest, fastest opening ramps you can obtain. When you plot velocity, acceleration and jerk you want a plot that looks like this;



    With all metrics smooth and flatlining as the probe (follower) arrives at max lift as in the chart above. Unbelievably this is not the case for all cams. The chart above is the opening ramp for one of four cams. The two intake cams and the two exhaust cams need to have their opening and closing ramps each cut differently to produce the same motion at the valve head. The reason is the difference in geometry between right and left side heads, the cam placement and the relative cam rotation.

    ◉ Always select and install the cams to produce between 5˚ and 10˚ of crankshaft rotation at or near TDC overlap where both intake and exhaust valves are seated. If you don't you will be supercharging the atmosphere by blowing boost out your exhaust, giving up low speed torque, power, and increasing IAT2 temps.

    ◉ On 4V engines try to install your Intake to open at TDC. Our 4V heads have stunning low lift air flow. They do not need the "encouragement" of overlap that 2V designs require to get the airflow in motion. To achieve your 10˚ dwell time at or near TDC overlap where both valves are closed you will need to close your exhaust valve at, you guessed it, 10˚ BTDC.


    This next one is going to be tough,

    ◉ I am going to suggest that you want to open your exhaust valve in close proximity to 80˚ BBDC! That will bring a gasp from most folks who have come from a n/a environment and also more than a few who have come from a supercharged environment.

    Here is the back story

    Most folks pick cam timing and in particular phasing based on things like LSA and centerline. That will get you in the ball park but it can and will, also mislead you. The cam durations for the intake and exhaust profiles should be chosen to optimize the particular cycle of the four cycle engine they are designed to service. When you have best optimized the intake it is whatever it is. When you have best optimized the exhaust, same thing, it is whatever it is.

    Once you are done, the angular displacement between the two lobes for the two different events will simply fall out and again, be whatever it is. Attempting to "force" either to a particular figure obtained somewhere will probably not provide the best overall performance for you.

    It can not be said too often, our cylinder heads have excellent low lift flow. The flow is so good Ford waits until the piston is already 22˚ down from TDC before they even start to open the intake valve! Then they close it only 27˚ ABDC! Ford starts late and close early! The engine's Volumetric efficiency (Ve) is crippled by these choices.

    Now consider a camshaft that Ford build's for a factory race event, the Daytona Prototype competition at Daytona Beach where they do not compete against little guys like you and I but rather against other factory teams like GM and their LS engined similar race cars. You simply run what you brung, within the rules (usually) and hope you brung enough. One of Ford's now famous Daytona Prototype cars used an intake cam measured at 0.050" valve lift that had an intake opening point of 1˚ BTDC and a closing point of 49˚ ABDC. When you do the math it is a 230˚ intake event? 1 + 180 + 49 + 230 intake degrees.

    Remember when you were a kid and raced other kids on your bike? You would stand up on the pedals, grip the handle bars and push as hard as you could on the pedals. There was an interesting phenomena we would all run into. By the time the pedal got to about 3 O'clock no matter how hard we pushed our ability to further accelerate the bike dramatically diminished. The reason for that is the same as the reason the bike accelerated harder, the harder we pushed on that same pedal from 12 O'clock to 3 O'Clock.

    If you look at the sprocket from the side and break the pedal location down into a vertical Y component and horizontal X component, the horizontal X component would increase in length until 3 O'clock and then begin to decrease in length as you move towards 6 O'clock.

    That horizontal line is the length of the lever arm you have available to you to exert a twisting force on that front chain sprocket. From 12 O'clock to 3 O'clock it increased in length, leverage and so did the acceleration you could realize from your push on that pedal. At 3 O'clock everything changes and the lever begins to shorten as rapidly as it had increased moments earlier. See the pic below;



    Replace the words TDC and 90˚ BBDC with 12 O'clock and 3 O'clock and you have the front sprocket on the bicycle you rode as a young boy. Now go back and think about racing your bike and also think about producing power in your engine. At 90˚ ATDC or if you will 90˚ BBDC the leverage you (on your bike) or your crank (in your engine) has, begins to deteriorate as the pedal (the piston) reaches it's maximum rate of speed at 3 O'clock (90˚ BBDC). For your engine it is even worse than that! On your bike you could push as hard at 4 or 5 O'clock as you did at 12 O'clock you just didn't have much leverage. Your engine can not push as hard!

    As the piston moves downward in the cylinder, the combustion gases expand into the now rapidly expanding cylinder volume. As that happens the in cylinder pressure, which pushes on the piston, drops dramatically. Not surprisingly when the piston is at mid stroke (90˚ ATDC) that cylinder volume is expanding fastest, the in cylinder pressure is falling fastest and the horizontal component of the crank pin's circular motion is decreasing fastest. The net, net bottom line is there is little to no work being done or torque being produced from 90˚ ATDC to BDC!

    So what to do? Well lets take advantage of what cylinder pressure we have left for exhaust scavenging! If we open the exhaust valve around 90˚ ATDC, we still have some cylinder pressure left that we can use to send a strong exhaust pulse down the header tube for that cylinder. When that pulse hits the collector it reflects a negative pulse of near equal proportions back up the exhaust pipe. If you guessed that pulse can be used to scavenge the exhaust gases from the cylinder, you are correct! The only thing we need to do now is make sure the exhaust valve is still open somewhat when that pulse and its sister pulses reach our exhaust port. If those pulses arrive near TDC overlap, with the intake valve closed they scavenge the remaining exhaust gasses from the chamber and you have a clean, empty chamber waiting for the next intake charge!

    There is one more important piece to this puzzle. An 8 cylinder engine fires every 90˚and max cylinder pressure occurs between 8˚ and 12˚ATDC. Lets just pick 10˚for a working number. If the cylinder we are watching is 90˚ down the hole then the next cylinder to fire is already at TDC with a burning mixture inside of it. Turn the crank another 10˚ to the burn's peak pressure point and the previous cylinder we were just looking at is 100˚ down it's own cylinder or, if you will, reading from BDC it is 80˚ BBDC! That is exactly where I suggest you open the exhaust valve! You are blowing down the last cylinder in the firing order at the time the current cylinder in the firing order is exerting its maximum push against the crown of the piston. You are loosing no power whatsoever.

    In fact there is a very good argument to be made that says you actually produce more torque using this method. The early opening and subsequent exhaust scavenging reduces the pumping losses associated with pumping out the now much lower pressure exhaust gas, languishing in the cylinder. The usual approach to pumping the exhaust gases out the exhaust port is to use the upward motion of the piston to literally pump it out. The early opening exhaust uses existing exhaust pressure and reflective sound waves to scavenge the cylinder reducing those pumping losses.

    Now lets close that exhaust valve just in time so we don't blow any fresh charge out the exhaust port and start supercharging the atmosphere as the intake begins to open. That means ~10˚ of crank rotation between exhaust closing and intake opening. If the intake opens at TDC then the exhaust must close no later than 10˚ BTDC. Now lets add up the exhaust pieces that are laying around. From 80˚ to BDC is, well 80˚. From BDC to TDC is 180˚ but we are stopping 10˚ short of TDC so that means 170˚, 80 + 170 = 250˚ viola! Our exhaust profile wants to be 250˚

    This is what those cams look like on Mark's cam chart illustration software;



    The phasing works out to a 125˚ LSA with both the intake and exhaust cams advanced 10˚. Lets do the same thing with the Crowers. This is what Mark's illustration s/w will look like;



    The numbers come out as a 124/125 LSA with both cams advanced about 5.5˚

    The very late closing intake is a function of the relatively long duration (238˚) intake event. For a race only application at high boost it would decidedly play to your advantage at high rpm. For a daily driver it would kill off some low speed compression effectively lowering your dynamic compression and, the good news, making the engine more tolerant of lower quality gas you might accidentally get at a station. A 220˚ Intake event, closing 40˚ ABDC would produce a much better mannered street machine.

    It is possible to kill off a little of the late closing of that intake by adding 5˚ to both the intake and exhaust cams phasing by advancing them. Mark's cam chart s/w would illustrate it like this;



    My preference for a street driven vehicle however would be to reduce the intake timing to close the Intake valve at 40˚ ABDC. The 40˚ ABDC number is coincidentally the exact same spot the '00R Cobras and the latest GT500's close their intake valves although they do not open them as early. At this point we have optimized the opportunities that I highlighted in the first Cam Chart at the beginning of this article.

    BTW a fair question is does anybody besides Ed do this . The answer is yes. Here is a very popular cam used in blown alcohol engines for about the last 4 decades. Today with cutting edge custom cams we have gone to much higher lifts in the 1 inch+ area (but similar cam phasing), however this particular cam is still used in the vast majority of blown alcohol engines today. Here it is;




    and here is what it looks like using Mark's cam chart s/w




    Remember this is a 2V engine and need's to "encourage" the low lift flow and scavenging effect at TDC overlap that 4V engines do not have to do. Look at the overlap that is required to do this. The Cam Chart illustration is prepared with a 3˚ advance which is a common way to run the cam. Look at where the exhaust valve opens ...


    Ed
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    This article was originally published in forum thread: Looking for feedback on how I'm degreeing my cams started by MalcolmV8 View original post
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