BTDC or ABDC?

 

BTDC on the compression stroke

 

The intake closes at "X" number of degrees After Bottom Dead Center, not Before Top Dead Center.
After the plug fires, maximum cylinder pressure will occur when the piston is 12 to 14 degrees After Top Dead Center.

Maybe you can keep track of the valve events better if you think about it this way. There are 360 degrees in a circle. From ZERO to 90 degrees is AFTER TOP dead center. From 90 degrees to 180 degrees is BEFORE BOTTOM dead center. From 180 to 270 is AFTER BOTTOM dead center. From 270 back to zero is BEFORE TOP dead center.

But remember, in a 4-cycle motor, there are 720 degrees to a complete cycle. Let's say the intake valve begins opening at 20 degrees BTDC. The piston comes up to TDC and reverses direction, heading down the bore with the intake valve open. It gets to the bottom of its stroke and reverses direction, heading back up the bore. At some point after BDC, the intake valve will close. This is the most critical event in the valvetrain. If you close it too early, you may not fill the cylinder completely. If you close it too late, the piston, which is coming up the bore, will push some of the mixture that was just pushed into the cylinder by atmospheric pressure, back up the intake track and out of the motor. You can see this at night by shining a strong light across the throttle body or carburetor. It looks like a white fog. So the cylinder does not begin to pressurize until the intake valve closes, which will be at some point after BDC. The piston will continue to the top of the bore and the mixture will go BBBAAAANNNG, sending the piston down the bore and turning the crankshaft. At some point after TDC with the plug firing, the piston will arrive at a point between 90 degrees and 180 degrees from the top of the bore and the exhaust valve will begin opening, blowing the burnt gases out the exhaust port and into the exhaust system. The piston will go to the bottom of the bore and then reverse direction, heading to the top of the bore while pushing out all the burnt gases. At some point with the piston coming up the bore, the intake valve will also open. Now, we have both the intake and the exhaust valves open. This is called the overlap period. As the piston comes to zero at the top of the bore and reverses direction, headed down the bore, the exhaust valve closes and the intake valve remains open, again allowing atmospheric pressure to fill the cylinder with an explosive mixture of air and fuel. And there you have it.
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The intake closing at 74* and all the other valve events are cam timing and fixed in how the cam was ground and installed in the engine. To look at this simply you have 180* between BDC and TDC and the valve closes at 74* AFTER the piston passes BDC. This leave 106* to build compression before TDC.


The spark @36* is ignition timing. Which 36* would be a total number in most situations controlled by an advance curve starting at 8*,10*,12* depending. To put it simply fuel burns at a controlled rate so the faster the engine spins the earlier the spark need to occur to get complete combustion.


For comparison a stocker cam might have an IVC of 42-45 ABDC and build 150psi as tested by a compression guage. At 74 ABDC you might be lucky to get 115psi and bottom end power will be down. I'm not sitting here doing math and there are dynamics at play when it comes to how these events affect powerbands but it gives an idea of the effects and measured compression ratio calculations are important if trying to select a cam.

 

For comparison a stocker cam might have an IVC of 42-45 ABDC and build 150psi as tested by a compression guage. At 74 ABDC you might be lucky to get 115psi and bottom end power will be down. I'm not sitting here doing math and there are dynamics at play when it comes to how these events affect powerbands but it gives an idea of the effects and measured compression ratio calculations are important if trying to select a cam.

Hipster g this is what I was really trying to understand was the amount of psi in the cylinder thanks the way you and tech explained it makes perfect sense now

 

Ok thanks tech I see it a lot clearer now I got It. I was confusing myself by forgetting about 280° of crank rotation before bottom dead center and after bottom dead center which is when the intake valve would close and the cylinder would start to build pressure right?

 

Let's say that we have a cam that has 220 degrees of intake duration @0.050" intake valve lift. The timing on such a cam might be intake valve opens at 5 degrees before TDC and closes at 35 degrees after bottom dead center. So, we would add 5 plus 180 plus 35 and come up with 220 degrees of crankshaft rotation. Back in the days of the "camshaft wars", different cam grinders used different opening and closing points to make their cams look bigger than the competition. This finally got to the point of ridiculous and so the cam grinders got together and set a standard for hydraulic cams of 0.050" (duration measured with the valve off its seat by fifty thousandths of an inch) for the opening and closing events on hydraulic camshafts. Solid lifter camshafts were standardized at 0.020" (duration measured with the valve twenty thousandths of an inch off its seat). The "advertised duration" figures that you see are measured with the valve 0.004" or 0.006" off the seat.

I might just as well continue your education with this......
"Hydraulic Intensity" is a term that was coined by the late, great Harvey Crane, the brains behind Crane Cams. It is a way to determine the rate of opening and closing of a valve and to determine how much work the cam lobe is doing. If you will subtract 0.050" duration from advertised duration, you will come up with a number somewhere between about 25 and 80. A cam that has a H.I. of 65 to 80 is a very "slow rising lobe" camshaft. It is very easy on the valvetrain and might be chosen by a machine shop rebuilder who wants the motor to last a long time (preventing comebacks) and be very quiet in its operation (to please the little old lady who drives the vehicle to church and back). An H.I. of 50 to 65 might be chosen by a person who wants a little more from his or her motor, while still being reasonable from a wear and tear and noise standpoint. An H.I. of 25 to 50 might be chosen by a hot rodder who is not so concerned with longevity of the motor, but wants maximum performance. Such a cam will be harder on the valvetrain than a cam with a higher H.I. and valvetrain noise might be objectionable to some....... And by the way, when Harvey was alive, he ground all his cams on a H.I. of 62.
.

 

As to how Tech described this and others have made contributions this I'll add to the knowlwdge dump you got.

There is a thing called Dynamic, or in some circles Compensated, Compression Ratio. The name is to differentiate his from the Static Compression Ratio which is the common calculation of all cylinder inclusive volumes divided by those volumes found above in including the piston crown to the combustion chamber roof with the piston at TDC.


The Dynamic or Compensated Compression Ratio is a trigonometry calculation of the stroke used up to the point of intake valve closure. The movement in position and velocity is not linear but is related to crankshaft degrees as the rotation of the crank through the connecting rod opens, closes, and changes types of triangles that appear between crank pin, rod, and piston pin.

In general the later the intake valve is closed the more stroke length is consumed and thus the engine's swept stroke thus displacement is smaller than calculated by the SCR, yet the key to a high rpm engine is in the late closing intake valve. But their are no free lunches which I'll get to.


Early 4 cycle engines used a vacuum intake valve that used a spring loaded disk that used the pressure differential between the outside atmosphere and the inclosed cylinder and piston. So us the piston fell from TDC the difference in pressures forced the valve open and controlled the duration of that event. So basically the valve was late to open and early to close. This prevented sufficient breathing to allow more than a few hundred rpm at best. Once engineers figured out that the key to a high rpm engine lie in the use of induction inertia to force feed the cylinder the cams of the day sprouted intake lobes along side the existing exhaust lobe and of course the costly exhaust valve mechanism was duplicated for the intake as well, so you can see the effect of finance on engineering as the vacuum intake valve was thought to be a manufacturing cost savings more important than other factors, but time and necessity proved this wrong, but in all things we keep marching down this road.

So mixture has speed and mass so it also kinetic energy that can be put to the use of filling the cylinder(s) being fed. So to the point where events are so fast there isn't enough ime to fill the cylinder.

Time out for wifey.

Bogie

 

Alright back from wifey's need to tell me about the neighbor lady's problems which in my opinion stem from too much money and time combined with too little brain.

I was getting into mixture has speed and mass so it also kinetic energy that can be put to the use of filling the cylinder(s) being fed at least up to the point where events are so fast there isn't enough time to fill the cylinder. This leads into why there is no free lunch with long duration cams that really pick up the topend power. But the cost is a sizeable loss on the lower rpm's because at lower rpm's comes less velocity of the mixture and also less amount of mixture so the sum total of kenitic energy in the flow is considerable less. Therefore, at these lower rpm's the rising piston develops enough reverse thrust that a good portion of the previously inducted is pushed back out. This creates a host of problems but we'll stick with the effect on compression pressure which is not the same as compression ratio. Roughly you can get away with thinking or picturing the problem that below the torque peak, mixture is pushed by the rising piston past the still open intake back into the induction track if not all the way into the aircleaner if that is being used. At and above the torque peak the induction system with the combination of passage velocity and increased mixture mass is developing enough countervailing force that the mixture is being rammed into the cylinder against the forces created in the previously inducted mixture taken in before BDC by the rising piston. The cost (remember no free lunch) is the lost torque under the torque peak and the movement of that peak higher in the rpm band the later the intake valve is closed. Also, going on is while both the torque and horsepower peaks go higher in the rpm band and in amount, they also get closer together which becomes a converter stall and gearing issue, but I digress.

Now one thing we know from decades of test data is that engine efficiency falls off remarkably when the compression ratio adjusted for stroke lost up to the intake closing point falls under 8:1. So we use the concept of Dynamic or Compensated Compression Ratio to determine what SCR is needed for any particular cam's intake closing point that keeps the DCR/CCR (sounds like one of my favorite rock groups) at or reasonably above eight to one. This preserves compression pressure which is what is shouldering the load of making power, and improving extration of energy from the fuel used; compression ratios are only the means of getting to this end.

Well it turns out that we also know that this correction factor for static compression ratio also builds back much of the torque lost from the lower density trapped mixtures at lower rpm's as one of the reactions of liquid fuels is that the harder they are squeezed the more energy they release. There are other constraints and limits imposed on this mostly by finding cost effective structural materials but this does lead into theoretical study Ford engineering released back in the 1960's which asserted that a compression ratio of 100,000 to 1 would provide unlimited power and infinite mileage. But this probably, also, makes unlimited NOx emissions as well. But of course with the materials we have even 20 to 1 in a diesel is hard to contain so 100,000 to 1 is just nonsense.


The other place the DCR/CCR calculations help is up on the top end where the mixture again looses in cylinder density as the time to take a breath gets shorter. At this end getting these compression values right helps slow the power decay past the peak to where the engine is still making good usable power, this can be helpful at shift points by finding that some over rev at the shift allows the engine to fall into a better part of the power curve for acceleration.

All right enough already, Bogie

 

Adding to Tech's hydraulic intensity description/discussion - some more info to keep in mind is that the last little bit of valve lift, .005-.015", above the seat doesn't flow anywhere near the amount of air when it's at .300-600" off the seat. But this number 'just off' the seat is what we use for calculating DCR and and SCR. If you think about it, then it seems silly to consider the duration without considering the hydraulic intensity.

Here's an example - look up a CS1062R cam (easily found on Summit's site). .050" duration of 220/231 int/exh, but at .006" 287/304. WOW - does that calculate to an absolutely AWFUL DCR unless you have 11:1+ compression! I ran that cam in a 10:1 327 and it worked great - idled pretty well, mild rumble, but would run to 7000 rpms in a blink of an eye. With a hydraulic intensity 67/73 it was a 'big' cam with little cam manners and worked great. But if I had followed the DCR 'acceptable' numbers, then I would've been in the 7th circle of SCR/DCR hell lol.

DCR is a guide, use it as one is my best advice. SBC's are the most hot rodded motor in existence in the US and whatever you're wanting to do has been done hundreds of thousands of times - a Comp XE268 or XE274 in a 350 at 9.5:1 will run strong with a set of Vortec heads in a normal-ish street car of 3000-3500lbs - adjust from there.

 

Wow thanks for all this info 👍 this is awesome. Bogiesannex1 I'm a little confused how the dynamic compression is calculated. I understand the first part of this paragraph but not this part
"The movement in position and velocity is not linear but is related to crankshaft degrees as the rotation of the crank through the connecting rod opens, closes, and changes types of triangles that appear between crank pin, rod, and piston pin"
Is the anyway you could explain this to me again I might mixing up some of the words? I've been reading through your guys posts trying to wrap my head around all this info it's a lot.

Rick

 

it's a lot.

I agree Rick, it is a lot, but just take it slowly and get your head wrapped around it a little at a time. In just a short period of time, you should be able to correct all those "dime store cowboys" down at the Sonic Drive In. Stick with us and just keep asking questions. All of us enjoy teaching those who really want to learn this hobby.

 

If you visualize an X-ray view from the front of the block where you can view the changing relationships of the crankpin (center of the rod journal of the crankshaft), the straight line that would represent the connecting rod and the centerline of the piston pin (also called wrist pin) as the crankshaft turns in the block.
https://www.google.com/search?hl=en....c_e99jU88H8&ved=0ahUKEwjBtdKAvLDkAhXSm-AKHQUwDmAQ4dUDCAU#imgrc=G5o_CaPkeqf80M:

I can't be certain that is what Bogie is referring to, but I think it is. He's sharper than I am and sometimes his stuff goes over my head. :thumbup:

 

It’s a lot easier to visualize if you look at like this.
The upward moving piston can’t trap the A/F til the intake closes so the faster you open the valves, the longer you can hold them open, and the faster you can close them will net the power.
At least up to the point where the parts break because of the related slamming open/closing takes it toll.

My 412ci FT engine breaks something ever 25 shows so it gets new lifters, cam, springs every 20 nights. That’s only around 20 miles of street driving at 8k rpm

 

LOL :thumbup:

 

To Raven57; here is a link to one of many compression calculators out there. This tracks with my Excel model calculator so I know it's good. Plus a new name of Effective Compression Ratio, haven't looked at this site in awhile and went "cool"!


https://uempistons.com/rt-4-calculators.html


You need to run both the Static and Effective in that order, the model needs intake closing point in crank degrees, they say to add 15 degrees to the .050 value which works well with modern short ramp cams like the Comp XE and Lunati Voodoo. When you get into copies of OEM and older design aftermarket cams which use a lot more ramp you should use 20 to 25 degrees more for those ramps.


This is an iterative process to drive out the SCR needed to get the (CCR/DCR/ECR) my general rules for a street driver are for the (CCR/DCR/ECR):


- For PreSMOG iron heads and iron heads like the post first gen SMOG heads like the L98 or LO5 8 to 8.5 : 1.


- SMOG iron heads 7.8 to 8.0 :1


- L31 Vortec iron and aluminum L98 can go 8.2 to 8.7:1


- Aftermarket and GM Fast Burn aluminum with L31 type chambers can do 8.5 to 9.0 0r higher if you have EFI, a lightweight vehicle, or stiff gearing.


- If you live in Denver you can add .5 across the board to get more bang out of the lower atmospheric pressure.








Bogie

 

I think the last time the Trans Am vintage cars raced on the circuit was at Watkins Glen in 2016, that would have been something to see.



I don't remember exactly but I think the displacement limit was 305 or 307 inches. Most everybody piled on in some form or other. The Ford 302 owes its existence to it, the Chevy 302 is still a legion.Even AMC had a good time.



The current Gen one SBC at 305 inches has nothing to do with it, that was developed to meet SMOG and mileage standards not for racing.


Bogie

 

Early 4 cycle engines used a vacuum intake valve that used a spring loaded disk that used the pressure differential between the outside atmosphere and the inclosed cylinder and piston. So us the piston fell from TDC the difference in pressures forced the valve open and controlled the duration of that event. So basically the valve was late to open and early to close. This prevented sufficient breathing to allow more than a few hundred rpm at best. Once engineers figured out that the key to a high rpm engine lie in the use of induction inertia to force feed the cylinder the cams of the day sprouted intake lobes along side the existing exhaust lobe and of course the costly exhaust valve mechanism was duplicated for the intake as well, so you can see the effect of finance on engineering as the vacuum intake valve was thought to be a manufacturing cost savings more important than other factors, but time and necessity proved this wrong, but in all things we keep marching down this road.

Has anyone tried to use vacuum to open and close a valve also determine the duration of a valve again but with different or new technologies?

 

New technologies are into solenoid activated valves eliminating the cam completly.
Really only racing motorcycles and a handful number of cars at this point have tried it with varying success. But it is being done.

ICE will become less important here soon once batteries get better. You will have an electric for your torque then a small high reving ICE engine for your high way speeds. As you dont need much horsepower to maintain highway speeds the ICE will be mainly used as a generatior.


People believe that high torque ICE will be around forever. But the simple fact is that if you have a hybrid that allows you to lay down 70% of that packs charge into that electric motor at once your going to make even a solenoid activated engine look slow.

One major thing electrics offer that ICE can not help to compete with is open the door for direct drive. This eliminates the transmission, lowers the cog by moving the motors down, even allows for a more aerodynamic shape.

A few cars and trucks will be coming out in the next 2 years with direct drive. It should become mainstream in 10 years. Battery tech keeps becoming better. But that is still the main hurdle at this point.

 

Am i understanding this right if I have 64cc chamber heads to get a dcr of 8.5 there would need to be 544cc of fuel in the cylinder when the intake valve closes?