Cam/Valve Timing...what I learned
02-01-2004, 02:44 PM
#1
Melting Slicks
Thread Starter
Cam/Valve Timing...what I learned
I'm rebuilding my engine...L48 original with 50K miles. I measured .001" wear on all journals. Nothing is out-of-round or damaged so I'm just going to clean it up, paint it, install new bearings and seals then put it back together. But, while it is out I'm doing some measuring...a friend lent me a timing wheel and dial gauge micrometer. Here's a step-by-step of the valve timing:
Assume that at the beginning the cylinder is full of compressed air/fuel mixture:
- At 12º BTDC ignition occurs and air/fuel mixture burns.
- Momentum of the crankshaft continues over TDC to ATDC before combustion is completed.
- Keep in mind that the piston reaches bottom of stroke at 180º ATDC.
- At 107º of crank (there is still 78º of power stroke left) exhaust valve opens .0015"
- Exhaust valve continues to open to .003" at 112º ATDC (.002" at lobe)...this is where advertised duration begins.
- Exhaust valve continues to open to .075" at 143º ATDC (.050" at lobe)...this is where actual duration begins.
- At 180º (bottom of the power stroke) the exhaust valve is open .261" (.174" at lobe).
- Exhaust valve is at maximum lift to .410 at 247º ATDC (.273" at lobe) and starts to close.
- Exhaust valve at 340º (20º BTDC) is down to .01035" (.069" at lobe).
- Intake valve now begins to open and at 340º (20º BTDC) is open .0015" (.001" at lobe).
- At 343º (17º BTDC) Exhaust is at .0915" (.061" at lobe) and Intake is at .003" (.002" at lobe)...this is where Intake advertised duration begins.
- At 347º (13º BTDC) Exhaust is at .075" (.050" at lobe)...this is where Exhaust actual duration ends.
- At 350º (10º BTDC) Exhaust is at .0645" (.043" at lobe) and Intake is at .01125" (.0075" at lobe).
- At 355º (5º BTDC) Exhaust is at .0495" (.033" at lobe) and Intake is at .018" (.012" at lobe).
- At 360º (0º TDC) Exhaust is at .0375" (.025" at lobe) and Intake is at .02925" (.0195" at lobe).
- At 373º (13º ATDC) Exhaust is at .015" (.01" at lobe) and Intake is at .075" (.050" at lobe)....this is where Intake actual duration begins.
- At 395º (35º ATDC) Exhaust is at .003" (.002" at lobe)...this is where Exhaust advertised duration ends.
- At 467º (107º ATDC) Intake is at .3885" (.259" at lobe)...this is max lift for Intake.
- At 565º (205º ATDC) Intake is at .075" (.050" at lobe)...this is where Intake actual duration ends.
- At 581º (221º ATDC) Intake is at .02925" (.0195" at lobe)
- At 608º (248º ATDC) Intake is at .003" (.002" at lobe)...this is where Intake advertised duration ends.
- All valves are closed and Compression stroke begins at 608º (112º BTDC).
From this data I determined the following cam specs for my cam as it is now (worn):
Advertised Duration (I/E) - 265/283
Duration @ .050" (I/E) - 192/204
duration of rising edge @ .002" (I/E) - 124/135
duration of falling edge @ .002" (I/E) - 141/148
Lobe Lift (I/E) - .2585"/.2796"
Valve Lift (I/E) - .38775"/.4194"
Lobe Center @ 107º (I/E) - 107º/113º
Lobe seperation = 220º
Lobe overlap @ .002" = 52º
Both lobes are open about an average .020" at 0º TDC
There are a few things I'd like to point out. The first is that the exhaust valve begins to open a little after half way through the power stroke, and the intake valve is still open during the compression stroke! In fact at 143º (still 38º of power stroke left) the valve is at .075" which is where valve timing specs indicate that the valve is considered open and allowing air flow. The Intake valve is still open .075" at 205º which is 25º into the compression stroke. Of course this is so that the exhaust valve is open enough to release the pressure on the exhaust stroke, cause if the valve started to open at the bottom of the power stroke then it would never open in time to release the pressure and the engine would just stop...or slow down a LOT to give the pressure time to leak out through the barely open exhaust valve. And in the case of the Intake valve it had to be open all the way sometime during the intake stroke and couldn't open too much when the exhaust valve was open and the duration is well over 180º so there has to be some sort of lap. Of course ideally a servo controlled valve which could open completely instantly would open at the exact bottom of the power stroke, but unfortunately a mechanical valve train does not achieve that fantasy.
The cam lobes need to have a nice smooth inclination so that they don't push against the push rods too hard and cause them to bend. Also, the declination must be smooth enough to prevent the lifter from flying off the lobe like a ski jump and hitting against the cam lobe and causeing additional impact forces which are greater than the constant force against the lobe as the lifter presses against the lobe. The most wear on a cam lobe is of course at the top of the front face of the lobe.
Anyway...I thought I would share this info.
Here's some other things I've measured:
The Intake valve is seated .088" above the bottom of the head.
The Exhaust valve is seated .103" above the bottom of the head.
Piston rim (highest point on the piston) reaches TDC .04" below the deck.
Piston top (flat center area) is .108" below the deck at TDC.
The bottom of a piston eyebrow is .208" below the deck at TDC
Not counting the head gasket there is a total clearance of .296" for the intake valve and .311" for the exhaust valve. After looking at all the factors I can safely say that valve springs are the only issue concerning the installation of a high performance hydraulic or solid lifter cam. It will only be an issue for a roller cam since the lobe angle is much steeper, but a properly designed roller cam will open later and close sooner to compensate and preserve the valve to piston clearance.
Any comments? Suggestions? Defamitory remarks? All are welcome (even the defamitories cause hey...a good argument gets the blood flowing) :)
Assume that at the beginning the cylinder is full of compressed air/fuel mixture:
- At 12º BTDC ignition occurs and air/fuel mixture burns.
- Momentum of the crankshaft continues over TDC to ATDC before combustion is completed.
- Keep in mind that the piston reaches bottom of stroke at 180º ATDC.
- At 107º of crank (there is still 78º of power stroke left) exhaust valve opens .0015"
- Exhaust valve continues to open to .003" at 112º ATDC (.002" at lobe)...this is where advertised duration begins.
- Exhaust valve continues to open to .075" at 143º ATDC (.050" at lobe)...this is where actual duration begins.
- At 180º (bottom of the power stroke) the exhaust valve is open .261" (.174" at lobe).
- Exhaust valve is at maximum lift to .410 at 247º ATDC (.273" at lobe) and starts to close.
- Exhaust valve at 340º (20º BTDC) is down to .01035" (.069" at lobe).
- Intake valve now begins to open and at 340º (20º BTDC) is open .0015" (.001" at lobe).
- At 343º (17º BTDC) Exhaust is at .0915" (.061" at lobe) and Intake is at .003" (.002" at lobe)...this is where Intake advertised duration begins.
- At 347º (13º BTDC) Exhaust is at .075" (.050" at lobe)...this is where Exhaust actual duration ends.
- At 350º (10º BTDC) Exhaust is at .0645" (.043" at lobe) and Intake is at .01125" (.0075" at lobe).
- At 355º (5º BTDC) Exhaust is at .0495" (.033" at lobe) and Intake is at .018" (.012" at lobe).
- At 360º (0º TDC) Exhaust is at .0375" (.025" at lobe) and Intake is at .02925" (.0195" at lobe).
- At 373º (13º ATDC) Exhaust is at .015" (.01" at lobe) and Intake is at .075" (.050" at lobe)....this is where Intake actual duration begins.
- At 395º (35º ATDC) Exhaust is at .003" (.002" at lobe)...this is where Exhaust advertised duration ends.
- At 467º (107º ATDC) Intake is at .3885" (.259" at lobe)...this is max lift for Intake.
- At 565º (205º ATDC) Intake is at .075" (.050" at lobe)...this is where Intake actual duration ends.
- At 581º (221º ATDC) Intake is at .02925" (.0195" at lobe)
- At 608º (248º ATDC) Intake is at .003" (.002" at lobe)...this is where Intake advertised duration ends.
- All valves are closed and Compression stroke begins at 608º (112º BTDC).
From this data I determined the following cam specs for my cam as it is now (worn):
Advertised Duration (I/E) - 265/283
Duration @ .050" (I/E) - 192/204
duration of rising edge @ .002" (I/E) - 124/135
duration of falling edge @ .002" (I/E) - 141/148
Lobe Lift (I/E) - .2585"/.2796"
Valve Lift (I/E) - .38775"/.4194"
Lobe Center @ 107º (I/E) - 107º/113º
Lobe seperation = 220º
Lobe overlap @ .002" = 52º
Both lobes are open about an average .020" at 0º TDC
There are a few things I'd like to point out. The first is that the exhaust valve begins to open a little after half way through the power stroke, and the intake valve is still open during the compression stroke! In fact at 143º (still 38º of power stroke left) the valve is at .075" which is where valve timing specs indicate that the valve is considered open and allowing air flow. The Intake valve is still open .075" at 205º which is 25º into the compression stroke. Of course this is so that the exhaust valve is open enough to release the pressure on the exhaust stroke, cause if the valve started to open at the bottom of the power stroke then it would never open in time to release the pressure and the engine would just stop...or slow down a LOT to give the pressure time to leak out through the barely open exhaust valve. And in the case of the Intake valve it had to be open all the way sometime during the intake stroke and couldn't open too much when the exhaust valve was open and the duration is well over 180º so there has to be some sort of lap. Of course ideally a servo controlled valve which could open completely instantly would open at the exact bottom of the power stroke, but unfortunately a mechanical valve train does not achieve that fantasy.
The cam lobes need to have a nice smooth inclination so that they don't push against the push rods too hard and cause them to bend. Also, the declination must be smooth enough to prevent the lifter from flying off the lobe like a ski jump and hitting against the cam lobe and causeing additional impact forces which are greater than the constant force against the lobe as the lifter presses against the lobe. The most wear on a cam lobe is of course at the top of the front face of the lobe.
Anyway...I thought I would share this info.
Here's some other things I've measured:
The Intake valve is seated .088" above the bottom of the head.
The Exhaust valve is seated .103" above the bottom of the head.
Piston rim (highest point on the piston) reaches TDC .04" below the deck.
Piston top (flat center area) is .108" below the deck at TDC.
The bottom of a piston eyebrow is .208" below the deck at TDC
Not counting the head gasket there is a total clearance of .296" for the intake valve and .311" for the exhaust valve. After looking at all the factors I can safely say that valve springs are the only issue concerning the installation of a high performance hydraulic or solid lifter cam. It will only be an issue for a roller cam since the lobe angle is much steeper, but a properly designed roller cam will open later and close sooner to compensate and preserve the valve to piston clearance.
Any comments? Suggestions? Defamitory remarks? All are welcome (even the defamitories cause hey...a good argument gets the blood flowing) :)
02-01-2004, 03:32 PM
#2
Le Mans Master
Member Since: Oct 2002
Location: Las Vegas - Just stop perpetuating myths please.
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Re: Cam/Valve Timing...what I learned (Rockn-Roll)
:D Thanks R&R. I have always wonderd about valve to piston clearance and saved your research here. There is a computor program using a electronic detecter to measure cams called the "cam doctor". Used to verify camshafts for us novice mechanics as the accuracy required is difficult to maintain with hand tools. But it looks like you have a machinists background or maybe millwright?
David Vizard has several good books that would compliment your research here. I have to recommend them as once I started reading them I could not stop until I read them all - and then read some again. They'r sold in paperback but somtimes you can find them at the library. He describes why a flat tappet cam is velocity limited and a roller is acceleration limited. Cam/ lifter intensity. Lifter bounce and spring pressures. Compares all the lifter types and where there performace is optimum.
BTW that lobe separation angle is 110* in crank degrees. :thumbs:
David Vizard has several good books that would compliment your research here. I have to recommend them as once I started reading them I could not stop until I read them all - and then read some again. They'r sold in paperback but somtimes you can find them at the library. He describes why a flat tappet cam is velocity limited and a roller is acceleration limited. Cam/ lifter intensity. Lifter bounce and spring pressures. Compares all the lifter types and where there performace is optimum.
BTW that lobe separation angle is 110* in crank degrees. :thumbs:
02-03-2004, 05:00 PM
#3
Melting Slicks
Re: Cam/Valve Timing...what I learned (Rockn-Roll)
I'm rebuilding my engine...L48 original with 50K miles. I measured .001" wear on all journals. Nothing is out-of-round or damaged so I'm just going to clean it up, paint it, install new bearings and seals then put it back together. But, while it is out I'm doing some measuring...a friend lent me a timing wheel and dial gauge micrometer. Here's a step-by-step of the valve timing:
Assume that at the beginning the cylinder is full of compressed air/fuel mixture:
- At 12º BTDC ignition occurs and air/fuel mixture burns.
- Momentum of the crankshaft continues over TDC to ATDC before combustion is completed.
- Keep in mind that the piston reaches bottom of stroke at 180º ATDC.
- At 107º of crank (there is still 78º of power stroke left) exhaust valve opens .0015"
- Exhaust valve continues to open to .003" at 112º ATDC (.002" at lobe)...this is where advertised duration begins.
- Exhaust valve continues to open to .075" at 143º ATDC (.050" at lobe)...this is where actual duration begins.
- At 180º (bottom of the power stroke) the exhaust valve is open .261" (.174" at lobe).
- Exhaust valve is at maximum lift to .410 at 247º ATDC (.273" at lobe) and starts to close.
- Exhaust valve at 340º (20º BTDC) is down to .01035" (.069" at lobe).
- Intake valve now begins to open and at 340º (20º BTDC) is open .0015" (.001" at lobe).
- At 343º (17º BTDC) Exhaust is at .0915" (.061" at lobe) and Intake is at .003" (.002" at lobe)...this is where Intake advertised duration begins.
- At 347º (13º BTDC) Exhaust is at .075" (.050" at lobe)...this is where Exhaust actual duration ends.
- At 350º (10º BTDC) Exhaust is at .0645" (.043" at lobe) and Intake is at .01125" (.0075" at lobe).
- At 355º (5º BTDC) Exhaust is at .0495" (.033" at lobe) and Intake is at .018" (.012" at lobe).
- At 360º (0º TDC) Exhaust is at .0375" (.025" at lobe) and Intake is at .02925" (.0195" at lobe).
- At 373º (13º ATDC) Exhaust is at .015" (.01" at lobe) and Intake is at .075" (.050" at lobe)....this is where Intake actual duration begins.
- At 395º (35º ATDC) Exhaust is at .003" (.002" at lobe)...this is where Exhaust advertised duration ends.
- At 467º (107º ATDC) Intake is at .3885" (.259" at lobe)...this is max lift for Intake.
- At 565º (205º ATDC) Intake is at .075" (.050" at lobe)...this is where Intake actual duration ends.
- At 581º (221º ATDC) Intake is at .02925" (.0195" at lobe)
- At 608º (248º ATDC) Intake is at .003" (.002" at lobe)...this is where Intake advertised duration ends.
- All valves are closed and Compression stroke begins at 608º (112º BTDC).
From this data I determined the following cam specs for my cam as it is now (worn):
Advertised Duration (I/E) - 265/283
Duration @ .050" (I/E) - 192/204
duration of rising edge @ .002" (I/E) - 124/135
duration of falling edge @ .002" (I/E) - 141/148
Lobe Lift (I/E) - .2585"/.2796"
Valve Lift (I/E) - .38775"/.4194"
Lobe Center @ 107º (I/E) - 107º/113º
Lobe seperation = 220º
Lobe overlap @ .002" = 52º
Both lobes are open about an average .020" at 0º TDC
There are a few things I'd like to point out. The first is that the exhaust valve begins to open a little after half way through the power stroke, and the intake valve is still open during the compression stroke! In fact at 143º (still 38º of power stroke left) the valve is at .075" which is where valve timing specs indicate that the valve is considered open and allowing air flow. The Intake valve is still open .075" at 205º which is 25º into the compression stroke. Of course this is so that the exhaust valve is open enough to release the pressure on the exhaust stroke, cause if the valve started to open at the bottom of the power stroke then it would never open in time to release the pressure and the engine would just stop...or slow down a LOT to give the pressure time to leak out through the barely open exhaust valve. And in the case of the Intake valve it had to be open all the way sometime during the intake stroke and couldn't open too much when the exhaust valve was open and the duration is well over 180º so there has to be some sort of lap. Of course ideally a servo controlled valve which could open completely instantly would open at the exact bottom of the power stroke, but unfortunately a mechanical valve train does not achieve that fantasy.
The cam lobes need to have a nice smooth inclination so that they don't push against the push rods too hard and cause them to bend. Also, the declination must be smooth enough to prevent the lifter from flying off the lobe like a ski jump and hitting against the cam lobe and causeing additional impact forces which are greater than the constant force against the lobe as the lifter presses against the lobe. The most wear on a cam lobe is of course at the top of the front face of the lobe.
Anyway...I thought I would share this info.
Here's some other things I've measured:
The Intake valve is seated .088" above the bottom of the head.
The Exhaust valve is seated .103" above the bottom of the head.
Piston rim (highest point on the piston) reaches TDC .04" below the deck.
Piston top (flat center area) is .108" below the deck at TDC.
The bottom of a piston eyebrow is .208" below the deck at TDC
Not counting the head gasket there is a total clearance of .296" for the intake valve and .311" for the exhaust valve. After looking at all the factors I can safely say that valve springs are the only issue concerning the installation of a high performance hydraulic or solid lifter cam. It will only be an issue for a roller cam since the lobe angle is much steeper, but a properly designed roller cam will open later and close sooner to compensate and preserve the valve to piston clearance.
Any comments? Suggestions? Defamitory remarks? All are welcome (even the defamitories cause hey...a good argument gets the blood flowing) :)
Assume that at the beginning the cylinder is full of compressed air/fuel mixture:
- At 12º BTDC ignition occurs and air/fuel mixture burns.
- Momentum of the crankshaft continues over TDC to ATDC before combustion is completed.
- Keep in mind that the piston reaches bottom of stroke at 180º ATDC.
- At 107º of crank (there is still 78º of power stroke left) exhaust valve opens .0015"
- Exhaust valve continues to open to .003" at 112º ATDC (.002" at lobe)...this is where advertised duration begins.
- Exhaust valve continues to open to .075" at 143º ATDC (.050" at lobe)...this is where actual duration begins.
- At 180º (bottom of the power stroke) the exhaust valve is open .261" (.174" at lobe).
- Exhaust valve is at maximum lift to .410 at 247º ATDC (.273" at lobe) and starts to close.
- Exhaust valve at 340º (20º BTDC) is down to .01035" (.069" at lobe).
- Intake valve now begins to open and at 340º (20º BTDC) is open .0015" (.001" at lobe).
- At 343º (17º BTDC) Exhaust is at .0915" (.061" at lobe) and Intake is at .003" (.002" at lobe)...this is where Intake advertised duration begins.
- At 347º (13º BTDC) Exhaust is at .075" (.050" at lobe)...this is where Exhaust actual duration ends.
- At 350º (10º BTDC) Exhaust is at .0645" (.043" at lobe) and Intake is at .01125" (.0075" at lobe).
- At 355º (5º BTDC) Exhaust is at .0495" (.033" at lobe) and Intake is at .018" (.012" at lobe).
- At 360º (0º TDC) Exhaust is at .0375" (.025" at lobe) and Intake is at .02925" (.0195" at lobe).
- At 373º (13º ATDC) Exhaust is at .015" (.01" at lobe) and Intake is at .075" (.050" at lobe)....this is where Intake actual duration begins.
- At 395º (35º ATDC) Exhaust is at .003" (.002" at lobe)...this is where Exhaust advertised duration ends.
- At 467º (107º ATDC) Intake is at .3885" (.259" at lobe)...this is max lift for Intake.
- At 565º (205º ATDC) Intake is at .075" (.050" at lobe)...this is where Intake actual duration ends.
- At 581º (221º ATDC) Intake is at .02925" (.0195" at lobe)
- At 608º (248º ATDC) Intake is at .003" (.002" at lobe)...this is where Intake advertised duration ends.
- All valves are closed and Compression stroke begins at 608º (112º BTDC).
From this data I determined the following cam specs for my cam as it is now (worn):
Advertised Duration (I/E) - 265/283
Duration @ .050" (I/E) - 192/204
duration of rising edge @ .002" (I/E) - 124/135
duration of falling edge @ .002" (I/E) - 141/148
Lobe Lift (I/E) - .2585"/.2796"
Valve Lift (I/E) - .38775"/.4194"
Lobe Center @ 107º (I/E) - 107º/113º
Lobe seperation = 220º
Lobe overlap @ .002" = 52º
Both lobes are open about an average .020" at 0º TDC
There are a few things I'd like to point out. The first is that the exhaust valve begins to open a little after half way through the power stroke, and the intake valve is still open during the compression stroke! In fact at 143º (still 38º of power stroke left) the valve is at .075" which is where valve timing specs indicate that the valve is considered open and allowing air flow. The Intake valve is still open .075" at 205º which is 25º into the compression stroke. Of course this is so that the exhaust valve is open enough to release the pressure on the exhaust stroke, cause if the valve started to open at the bottom of the power stroke then it would never open in time to release the pressure and the engine would just stop...or slow down a LOT to give the pressure time to leak out through the barely open exhaust valve. And in the case of the Intake valve it had to be open all the way sometime during the intake stroke and couldn't open too much when the exhaust valve was open and the duration is well over 180º so there has to be some sort of lap. Of course ideally a servo controlled valve which could open completely instantly would open at the exact bottom of the power stroke, but unfortunately a mechanical valve train does not achieve that fantasy.
The cam lobes need to have a nice smooth inclination so that they don't push against the push rods too hard and cause them to bend. Also, the declination must be smooth enough to prevent the lifter from flying off the lobe like a ski jump and hitting against the cam lobe and causeing additional impact forces which are greater than the constant force against the lobe as the lifter presses against the lobe. The most wear on a cam lobe is of course at the top of the front face of the lobe.
Anyway...I thought I would share this info.
Here's some other things I've measured:
The Intake valve is seated .088" above the bottom of the head.
The Exhaust valve is seated .103" above the bottom of the head.
Piston rim (highest point on the piston) reaches TDC .04" below the deck.
Piston top (flat center area) is .108" below the deck at TDC.
The bottom of a piston eyebrow is .208" below the deck at TDC
Not counting the head gasket there is a total clearance of .296" for the intake valve and .311" for the exhaust valve. After looking at all the factors I can safely say that valve springs are the only issue concerning the installation of a high performance hydraulic or solid lifter cam. It will only be an issue for a roller cam since the lobe angle is much steeper, but a properly designed roller cam will open later and close sooner to compensate and preserve the valve to piston clearance.
Any comments? Suggestions? Defamitory remarks? All are welcome (even the defamitories cause hey...a good argument gets the blood flowing) :)