The lobe separation angle has nothing to do with dynamic compression except in an inferred manner. The event that controls the dynamic compression is the intake valve closing point, and cams with a narrow LSA tend to be installed more advanced than those with a with a wider LSA. Advancing the cam closes the intake valve sooner and raises the DCR.

 

Most of the cams nowadays are made with built-in advance so maybe you can install it "straight up" so that it is retarded to drop your DCR a schosh.

Say a CC cam at 110/106, installed using a 4 degree bushing at 110/110..

You move the intake closing point later by 4 degrees.

pdq67

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With a narrower LSA the cam has more overlap. With more overlap both the exhuast and intake valves are open at the same time for a longer period. On the low end of the rpm range more air and gas coming into the engine will go right out and your CR will go down technically because there will be less air/gas to compress. Fuel economy, idle quality, and low end torque will also go down. When you rev it high, however, the air and gas have less time to escape so less makes its way out of the exhaust, compression will go up because of it as well as power. When calculating CR we dont take into account the cam overlap so most people aren't going to say LSA/ overlap effects the CR.

 

The DCR equation is affected mainly by the intake valve closing point---ABDC. Cylinder pressure is affected by the actual compression ratio and by the actual volume of air/gas in the cylinder when the intake valve closes. This is why blower engines make so much power, even with late intake valve closings----There is more air and gas in the cylinder when the intake valve actually closes.
Overlap affects cylinder pressure only in how it causes the cylinder to fill with clean air/gas mixtures---If overlap delays cylinder filling, low cylinder pressures, if overlap does not delay cylinder filling, then high cylinder pressures.
At least this is the way I've seen it, since about 1977.....

UDHarold

 

With a narrower LSA the cam has more overlap. With more overlap both the exhuast and intake valves are open at the same time for a longer period. On the low end of the rpm range more air and gas coming into the engine will go right out and your CR will go down technically because there will be less air/gas to compress. Fuel economy, idle quality, and low end torque will also go down. When you rev it high, however, the air and gas have less time to escape so less makes its way out of the exhaust, compression will go up because of it as well as power. When calculating CR we dont take into account the cam overlap so most people aren't going to say LSA/ overlap effects the CR.

=
????
Please explain to me how more overlap or less overlap has anything to do with cylinder filling at low speeds??
There is no charge coming into the engine at lower speeds on overlap.

You are at the end of the exhaust cycle & the beginning of the intake cycle so how is there an incoming mixture to exit the exhaust?
In fact what will happen is you will have exhaust gases still in the cylinder that will end up exiting into the intake tract hindering the beginning of the incoming charge
Cylinder filling has not even started at this point at low rpm's,,

The cylinder only sees what is trapped when the intake valve closes which is completely on the other side of the cam so at low speeds this has nothing to do with overlap except for slightly delaying the beginning of cylinder filling.

Overlap can increase cylinder filling at higher speeds due to the inertia of the outgoing charge so the cylinder will start filling sooner due to what should be better scavenging which is also dependant on the exhaust system but at low speeds overlap is not your friend.

To the original question, all else being equal,, widening the LSA will lower effective cylinder pressures at low rpm.
What you are doing is advancing the ex valve & retarding the intake valve so the intake valve is going to close later.
This may delay or eliminate the onset of detonation somewhat at lower engine speeds.

As was stated, retarding the cam you have will give the same general effect regarding effective cylinder pressures at low speeds.

 

More overlap = more time exhuast valve and intake valve are open at the same time. As the engine begins to draw air and gas into the cylinder on the intake stroke the exhuast valve is still open. Air escapes right out of the exhaust valve as its coming into the engine. Less air and gas are in the engine. Is less air and gas is in the engine, less air and gas is compressed. If you compress 1 cubic foot of air and gas in a 12 to 1 compression engine it then becomes 1/12th of its original size and it now 1 cubic inch. If we start off with only 10 cubic inches of air and gas because 2 cubic inches went out of the exhuast valve and its in a volume of 1 cubic foot then it is under less pressure, instead of being at, well say 1 KPa (although that is wrong, im just using it to simplify things) as it started off when 12 cubic inches of air and gas occupied 12 cubic inches of volume, it is now at .83333 KPa. Now if we compress it 12 times its at 10 KPa whereas the cam with 0 overlap (although not possible really if you want any power) has compressed the air to 12 KPa. Youre not changing the CR of the engine, which you might have thought I was saying, but the pressure of the air and gas is decreased and I beleive that blue66s goal was to lower the pressure of the air and gas by using a larger cam, which still would not work at high engine speeds because once you rev it high air and gas dont have much time to escape. This is why you need a certain CR with big cams because they have a narrow LCA and therefore alot of overlap and with a low CR you would loose even more low end power. This is also why you loose vacuum with a big cam, the air is less dense and less pressurized and its velocity decreases and so does vacuum.

 

More overlap = more time exhuast valve and intake valve are open at the same time. As the engine begins to draw air and gas into the cylinder on the intake stroke the exhuast valve is still open. Air escapes right out of the exhaust valve as its coming into the engine. Less air and gas are in the engine. Is less air and gas is in the engine, less air and gas is compressed. If you compress 1 cubic foot of air and gas in a 12 to 1 compression engine it then becomes 1/12th of its original size and it now 1 cubic inch. If we start off with only 10 cubic inches of air and gas because 2 cubic inches went out of the exhuast valve and its in a volume of 1 cubic foot then it is under less pressure, instead of being at, well say 1 KPa (although that is wrong, im just using it to simplify things) as it started off when 12 cubic inches of air and gas occupied 12 cubic inches of volume, it is now at .83333 KPa. Now if we compress it 12 times its at 10 KPa whereas the cam with 0 overlap (although not possible really if you want any power) has compressed the air to 12 KPa. Youre not changing the CR of the engine, which you might have thought I was saying, but the pressure of the air and gas is decreased and I beleive that blue66s goal was to lower the pressure of the air and gas by using a larger cam, which still would not work at high engine speeds because once you rev it high air and gas dont have much time to escape. This is why you need a certain CR with big cams because they have a narrow LCA and therefore alot of overlap and with a low CR you would loose even more low end power. This is also why you loose vacuum with a big cam, the air is less dense and less pressurized and its velocity decreases and so does vacuum.

First, the engine does not draw air into the cylinder, it is pushed

At low engine speeds the cylinder is not starting to fill when both valves are open, it may be just the opposite.
So, again at lower engine speeds, there is no way part of the incoming charge is escaping out the exhaust, there is no incoming charge until the cylinder becomes a lower pressure area than atmospheric (unless you have a blower on it) & this does not happen during overlap at lower engine speeds.
As I stated, at higher speeds things change,, you now have a column of exhaust gas that has inertia & may be causing the cylinder to become lower in pressure assuming you have an efficient exhaust system.

The piston is on it's way up on the exhaust stroke & as soon as the intake valve opens part of whatever is left in the cylinder it headed out both valves again, at lower engine speeds.
You cannot equate what is forced into the cylinder to what is expanded on the power stoke in simple terms to say X amount of pressure is left in the cylinder.
The pressure left is going to be higher at the end of the exhaust stroke than what it would have been if we were just talking about a piston going up & down in a cylinder. You have the end pressures from combustion.

Bottom line, the overlap cycle has little to nothing to do with cylinder filling at low speed. except for contaminating the intake charge,,

 

Then what exactly does overlap effect? And what do you mean air and gas is pushed in? And you seem to be completely ignoring the fact that the exhaust valve is open on the intake stroke.

 

Then what exactly does overlap effect? And what do you mean air and gas is pushed in?

=
I should let Harold answer this but here's my simplistic take,,,

The "good" part of overlap effects how an engine breathes at higher rpm.
In other words, at higher rpm you do not have time to fill the cylinder so you need some help.
The inertia of the exhaust gasses help to draw the cylinder down to a lower pressure than is possible with the time you have during the intake cycle so,, in effect, overlap rears it's ugly head but in a good way, it helps to give you a lower pressure area sooner than would be possible than if you just slammed the ex valve shut before TDC & starts the column of nice fresh new charge into the engine sooner,,,
Sort of mother natures idea of blower
Also why you can get in excess of 100% VE normally aspirated

Second question,,,
Mother nature is very picky,,, she hates low pressure areas, wants everything to be equal so,,,
air or gas or whatever always flows from a high pressure area to a low pressure area, it is not drawn there.

 

Oh ok. Thanks, that makes sense now. I see what you mean about mother nature being picky, I was overanalyzing it before. So I just have one more question, why does an increase in overlap make lower engine speed power decrease? Also, why do you need more compression for bigger cams?

 

Oh ok. Thanks, that makes sense now. I see what you mean about mother nature being picky, I was overanalyzing it before. So I just have one more question, why does an increase in overlap make lower engine speed power decrease?

=
My fingers are getting tired
For the reasons part of which you touched on, both valve are open,, biggest one is you are contaminating the intake charge by pumping a bunch of crap into the intake port to mix with the fresh intake charge & delay cylinder filling
Poor mans EGR

 

Mike, Don't stop now.

Some people think of overlap as affecting compression; it doesn't. But it does effect WHAT you are compressing. With long overlap, you are simply compressing intake mixture diluted with exhaust. That's why it doesn't idle well. This is one of those things I always expound on: Exhaust diluting the mixture is a GIANT power killer. That is why it is so important to have leak free intake valves: To keep the combustion pressure out of the intake tract, so as to avoid intake mixture dilution.

Don't believe everything you read in magazines. There are a lot of misprints.

 

For all the youngins out there,can we say "REVERSION" which is when the incomming intake charge (in the intake manifold)is somewhat contaminated with the ex gasses from the prior firing stroke/s which is common with long duration hotter/higher perf cams as is mentioned above here by Mike.

Scott

 

Oh ok. Now it makes sense. I guess my veiw was a bit cloudy and messed up. Thanks guys.:beers:

 

Mike, Don't stop now.

Some people think of overlap as affecting compression; it doesn't. But it does effect WHAT you are compressing. With long overlap, you are simply compressing intake mixture diluted with exhaust. That's why it doesn't idle well. This is one of those things I always expound on: Exhaust diluting the mixture is a GIANT power killer. That is why it is so important to have leak free intake valves: To keep the combustion pressure out of the intake tract, so as to avoid intake mixture dilution.

Don't believe everything you read in magazines. There are a lot of misprints.

Now that is an understatment,,,
But you can always believe the HP numbers,,
=
I'm back,,, had to go play in the shop & visit with a TC member that came by to visit & get the "okie tour" of our shade tree operation

Oh ok. Now it makes sense. I guess my veiw was a bit cloudy and messed up. Thanks guys.:beers:

Join the crowd,,, I am almost always in a "cloud",,, the further I go the behinder I get,,
What was the question :clonk:

 

I think, for some reason, that I was under the influence that overlap keeping the intake and exhuast valves open at the same time does the same thing as duration increase and that air and gas went out the exhuaust like increased duration sends air and gas back into the intake and decreases cylinder volume, causing a need for higher CRs. Why I thought that dont ask me. I never quite understood why an increase in overlap increased power so again, I thank you for clearing that up. Its hard to ask people too because alot of people are really, really confused about cams lol.:clonk:

 

You might be thinking of the specific case when a blower is involved. Then, yes fuel-air gets pushed out the exhaust during overlap.

 

LOL I think I was just confused but thanks for being nice:beers:

 

Mike,

While we are on this subject, can you explain why torque plate valve jobs have seem to gone out of favor? They were an absolute necessity when I was working on 348's. Those heads bent all over the place for me, but maybe that was just me. Years ago, I also checked 461 heads that moved after being bolted on. Is head bending alleviated with the use of studs? I know my AFR's didn't bend.

 

Mike,

While we are on this subject, can you explain why torque plate valve jobs have seem to gone out of favor? They were an absolute necessity when I was working on 348's. Those heads bent all over the place for me, but maybe that was just me. Years ago, I also checked 461 heads that moved after being bolted on. Is head bending alleviated with the use of studs? I know my AFR's didn't bend.

=
Hi Ken,
Mainly because it has been found that in most cases there ain't no power here.
The seats & valves run so hot compared to sitting on a bench that at least in my opinion you are pretty much amusing yourself with the plate for doing a valve job with most heads.
Been tested a bunch by different folks who basically found nothing there.
There are some heads that really should be done with a plate and there are some that the spark plug will actually move the seat more than bolting a plate to it.
I haven't used one in years either but I am sure there are heads I have done that would benefit from one.
I just do not have the time to check in most cases.

Another part & a big one is economics,,, very very few people are willing to pay for our time to do some of this stuff
If you break down the hours spent doing a performance engine to what the end customer pays for it most people of sound mind would wonder what the hell we do this for :clonk:

I think it was Carl, (CNC Blocks ) that made the analogy to a plumber that charges $90+ an hour & has a pipe wrench & a snake

I am a very small shop & still have easily $800k+ in equipment which I also get to maintain & people expect to get a block bored & honed & cleaned which to do right takes a few hours of time, not to mention electricity, stones, cutters,,,,,, for $150 & do it correctly.
What is wrong with this picture,,,,
BTW, not real numbers,, if you want your block done for $150 you can take my number off your speed-dial

Sorry, kind of went off on a tangent here,,
=
Here is a little interesting reading for you from some of the best,,,

http://www.speedtalk.com/forum/viewtopic.php?t=3114&highlight=torque+plate

http://www.speedtalk.com/forum/viewtopic.php?t=1686&postdays=0&postorder=asc&highlight=torque+plate&start=15

 

=
Hi Ken,
Mainly because it has been found that in most cases there ain't no power here.
The seats & valves run so hot compared to sitting on a bench that at least in my opinion you are pretty much amusing yourself with the plate for doing a valve job with most heads.
Been tested a bunch by different folks who basically found nothing there.
There are some heads that really should be done with a plate and there are some that the spark plug will actually move the seat more than bolting a plate to it.
I haven't used one in years either but I am sure there are heads I have done that would benefit from one.
I just do not have the time to check in most cases.

Another part & a big one is economics,,, very very few people are willing to pay for our time to do some of this stuff
If you break down the hours spent doing a performance engine to what the end customer pays for it most people of sound mind would wonder what the hell we do this for :clonk:

I think it was Carl, (CNC Blocks ) that made the analogy to a plumber that charges $90+ an hour & has a pipe wrench & a snake

I am a very small shop & still have easily $800k+ in equipment which I also get to maintain & people expect to get a block bored & honed & cleaned which to do right takes a few hours of time, not to mention electricity, stones, cutters,,,,,, for $150 & do it correctly.
What is wrong with this picture,,,,
BTW, not real numbers,, if you want your block done for $150 you can take my number off your speed-dial

Sorry, kind of went off on a tangent here,,
=
Here is a little interesting reading for you from some of the best,,,

http://www.speedtalk.com/forum/viewtopic.php?t=3114&highlight=torque+plate

http://www.speedtalk.com/forum/viewtopic.php?t=1686&postdays=0&postorder=asc&highlight=torque+plate&start=15

Mike,what gets me the most is people can't understand after they have bought a new Dart block that it will need the same work done to it as an o.e.m. block will.They think you just pull them off the pallet wash them up a little bit and start installing all the parts.

 

It's a shame that only 282 people have read this post with all the information it contains. Should have put "Hooters" in the title.

 

Hey its off topic but I see that you mentioned a 348. Does anyone make aftermarket heads for the 348/409?

 

Are the "HOOTERS" on the LSA or DCR team. More importantly which team will be "SKINS"

 

Hey its off topic but I see that you mentioned a 348. Does anyone make aftermarket heads for the 348/409?

=
Good question... I have heard Lamar Waldon (spelling??) is making an aluminum head but don't know anything about it.
Also heard they are in the $5000 range bare
This is all second hand though, don't have any real information.

I have 2 sets of 409 & a set of 348 heads in the shop along with a couple of blocks I need to get to some day,,,
One of the 409's is going to be a 450" stroker in it's new life,,

Some of us will do anything for money