Thread: Cam overlap

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Help me out here...if you have two solid lifter cams with nearly identical profiles (lift & duration) but one is 108 degrees of overlap and the other is say 112, what differences will I find in performance?????

Originally posted by techinspector1
..... lobe separation angle (LSA).

That's why ews da man!

.....in relationship to the crank centerline.

ummm, opps, I spoke too soon! That should read "cam centerline in relationship to TDC"

TyphoonZR; HE! HE! Glad to see you finally got your name back.


~ Vegas ~

Richard,
Thanks.....That's EXACTLY what I was looking for! And yes, I had the terminology wrong, thanks for knowing what I was trying to spit out!
Bob

Thanks for the link, tech. That explains it very well.

Bob, L.S.A. refers to the number of degrees that separate the intake and exhaust lobes. So if you have the exact same cam specs on two cams except for the LSA, then the higher number would indicate more torque whereas the lower number would give you more hp.

Having said all that, allow me to give you a for instance.

I am of the opinion that engine buffs speak in a very confusing language. The reason I say this is because they always address the intake valve prior to the exhaust valve when in fact in the four cycle engine, it is the exhaust valve that does it’s job prior to that of the intake valve, in my opinion.

Comp Cams machines their cams so that most of their cams have 110 degrees of lobe separation. In addition to this, they have discovered that by advancing the installed position of the cam, the signal to the intake occurs a little sooner because the exhaust valve opens and closes a little sooner thus giving better throttle response by taking advantage of the scavenging effect of the exhaust sucking the intake charge into the chamber! All (most) of Comp Cam cams are machined with 4 degrees of advance in them, you can not change this value once it is machined without re-machining it.

So, if a cam has 110 degrees of lobe separation, this means that the exhaust lobe is machined so that the lobe peak is separated by 220 degrees from the intake lobe peak, therefore the centerline of those two factors is 110 degrees.

However if you install the cam with 4 degrees advance, then the exhaust opens earlier and after it has passed TDC, it will be at it’s highest peak 114 degrees after TDC of the piston. The intake lobe at this point will be at it’s highest peak and this will occur 106 degrees BTDC because you are narrowing the distance of the intake cam lobe peak to that of piston TDC. It all has to do with the position of the piston.

It gets a little crazy and most guys do not understand the mathematics. What might help is if you come to terms with the fact that the two cam lobes (exhaust and intake lobes of any single piston) are in fact separated by 220 degrees of..........CRANK SHAFT ROTATION....in the above case, or 110 degrees of cam rotation due to the fact that the cam turns at half speed.

So, when looking at a cam in hand, nothing makes sense!

So, L.S.A. (lobe separation angle) can never be changed once a cam is ground, however L.C.A. (lobe centerline angle) can be changed by installing a crank shaft cam gear which has different key-way markings machined into them. Crane offers a timing set which offers as many as 9 different settings.

Again, in this case, a cam which is machined with 110 degrees of separation, can only have the centerline of the lobes moved in the advance or the retard direction, which is the relationship of the lobes position to that of the piston. Whatever amount you advance the exhaust valve by,...meaning that you achieve a higher number of degrees, ....the intake lobe is minus the same number of degrees in each case, in relation to the position of the piston TDC.

Likewise, if you retard the cam so that the exhaust valve opens later, so will the intake valve. If the cam is ground with 110 degrees L.S.A., the total number of degrees of lobe separation will always remain at 220 from the centerline of the two lobes of the cam, however not necessarily form the centerline of TDC of the piston.

So, if you advance a cam 4 degrees, the intake valve will be at it’s highest point 106 degrees prior to TDC of the piston therefore opening sooner on the down stroke of the piston, therefore the exhaust gases which are on their way out will not only scavenge (suck) unspent fumes into the exhaust, but also will assist in filling the combustion chamber more adequately.

Now, imagine the same piston continuing on it’s journey downward. After it hits bottom and is on it’s return back up to make compression for the combustion stroke, because you advanced the cam, the intake lobe will finish it’s cycle 4 degrees of crank rotation sooner, thus making more compression and thereby yielding more torque.

Hmm, it makes sense to me! Hahaha!

First of all I did not read any responses prior to making my previous post, so now I will address those whom I need to.

Yes Richard, I could consider TDC the centerline of the crank, however we are actually discussing L.C.A. = cam lobe centerline. I think it would make it too confusing to call TDC,... centerline. Hey, to each his own. I am the odd man out.

For instance, the entire industry speaks of intake lobe first and then exhaust. Now that just doesn’t compute in my little brain. Am I going to change the entire industry, hahaha! I guess each one tries their best to communicate a notion to the other person in the way they see it. In the end, they may still fail , however another person comes along and says the same thing a little differently, and wohlaa, he gets all the praise and is the hero! Hahaha, oh well! Why did it work for the second guy, cause the two of them were on the same wave length!

Vara, me too! I really didn’t like that other nick, it didn’t suit me!

Apparently Bob’s query has been satisfied while I was working on a reply, oh well, maybe it will speak to someone else!

TyphoonZR.....You've made it much clearer and I appreciate that. I got fast on the keyboard and didn't see yours until a few minutes ago. Thanks!