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Author
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Topic: Overlap question
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Mpcoluv
Gearhead Posts: 1341
From: Charlotte NC usa
Registered: Apr 2001
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posted 02-11-2002 11:12 AM
   
Hi,
Is there a rule of thumb for calculating overlap?
A friend built a motor almost identical to mine but used a 244 dur @.050 114 LC cam.
The engine idles MUCH smoother that my .224/232 dur @ .050 108 LC cam.
Am I correct in assuming that my smaller cam with the lighter lobe centers has more overlap?Thanks ------------------
Jim Sams
'69 mustang Coupe
372C
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steve'66
Gearhead Posts: 9533
From: Sonoma,CA,USA
Registered: Mar 2000
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posted 02-11-2002 08:54 PM
Jim,Yes you are correct, I think. Although I'm all confused now.  LOL SteveW
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Mpcoluv
Gearhead Posts: 1341
From: Charlotte NC usa
Registered: Apr 2001
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posted 02-11-2002 09:11 PM
Just talked to David Vizard (lives nearby) and he says that overlap is the sole determining factor in idle quality.
So either I have bogus overlap numbers of the two cams or my motor has poor ring sealing.... :mad
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Rustang
Gearhead Posts: 821
From: Clarion PA
Registered: Nov 2000
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posted 02-11-2002 10:16 PM
The narrower the lobe center the more radical the idle.------------------
'68 mustang 351 clevor- 10.92@124
'67 Stang, 351W -11.18@118
'69 351C Torino-14.90@100
'78 Pickup 351W-15.56@88
'79 Pickup 460 ET=??
http://www.rustang-racing.cityslide.com/contents/contents.cfm/712231
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Mpcoluv
Gearhead Posts: 1341
From: Charlotte NC usa
Registered: Apr 2001
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posted 02-12-2002 07:57 AM
Ok, after some research here is what I think is the method for calculating overlap.
If an exhaust valves closes at 11 deg after TDC, and the intake opens 16 deg before TDC (both at .050 tappet lift), then the overlap is 27 degrees (16before + 11 after).
Is this right?
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clevelandstyle
Gearhead Posts: 1309
From: central Indiana
Registered: Jul 2001
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posted 02-12-2002 07:22 PM
I would think that using the seat timing events would be more accurate for this calculation instead of .050" off the seat.  ------------------
Ben
Grabber Green '70 Mach I 351C 4V
Robbin Egg Blue '79 Fairmont 351C 4V
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bluestreek
Gearhead Posts: 1724
From: Athens,GA
Registered: Jul 2001
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posted 02-12-2002 10:08 PM
quote:
Originally posted by Mpcoluv:
Hi,
Is there a rule of thumb for calculating overlap?
A friend built a motor almost identical to mine but used a 244 dur @.050 114 LC cam.
The engine idles MUCH smoother that my .224/232 dur @ .050 108 LC cam.
Am I correct in assuming that my smaller cam with the lighter lobe centers has more overlap?Thanks
Your friends cam looks like a performance S/C or nitrous grind. The 114 degrees LSA would help keep compression up on larger port engines at lower rpms but has enough duration to flow good if needed.
I find it hard to beleive that a 244 @.050 duration cam can have a "smooth" idle, unless it is 11:1 compression, or maybe a roller cam with a special mild lobe. Your mild 224/234 cam would have been a better street grind if it had 110-112 LSA.
The centerline that a cam is installed can effect the idle quality. Things that can cause rough idle: 1. low compression
2. low ignition timing at idle
3. large intake ports
4. rich idle mixture  BS
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Mpcoluv
Gearhead Posts: 1341
From: Charlotte NC usa
Registered: Apr 2001
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posted 02-13-2002 08:05 AM
The engine with the 114 lc cam (N2O grind) has aussie 2V Cleveland heads with a Weiand Xcellerator..
My engine has 4V heads with a Strip Dominator.
The smaller intake volume on my friends engine may be the wild card factor.
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Daniel Jones
Gearhead Posts: 898
From: St. Louis, MO
Registered: Aug 99
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posted 02-13-2002 11:30 AM
Another thing that may factor in is Dave was running the thing on a test stand. Did he have mufflers installed or was he running with open headers? Remember when his brother ran zoomies with a big cam? He said it idled fine but when they switch to full exhaust the idle became very lumpy. Dan Jones
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Mpcoluv
Gearhead Posts: 1341
From: Charlotte NC usa
Registered: Apr 2001
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posted 02-13-2002 12:41 PM
quote:
Originally posted by Daniel Jones:
Another thing that may factor in is Dave was running the thing on a test stand. Did he have mufflers installed or was he running with open headers? Remember when his brother ran zoomies with a big cam? He said it idled fine but when they switch to full exhaust the idle became very lumpy. Dan Jones
I think he has the PAntera exhaust hooked up.
The 4-glasspack version I think.
I'll ask him.
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SundanceKid
Gearhead Posts: 1274
From: UT
Registered: Feb 2001
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posted 02-13-2002 01:29 PM
Mpcoluv, are you confuseing LC (lobe Center) for LSA (lobe seperation angle?)
Your lobe center spec is for the intake measured in camshaft degrees not crank degrees. You use this to measure the cam degrees in relation to the crank. What lobe center is the exhaust ground on?
If you were to measure the degrees between the intake centerline and the exhaust centerline this would be your LSA. You need to know what the exhaust CL or the LSA to determine the powerband and or idle quality. The higher the LSA number the lower in the powerband the cam will make power and the better the idle quality. Also the higher the number the wider the powerband will be. Most stock cams have a LSA of 112. To figure the LSA from the Intake and Exhaust center lines you take the intake CL degrees plus the exhaust CL degrees and divide them by 2. When you reduce the LSA angle you increase valve overlap if you had a LSA of 180 degrees you would have zero overlap.
Make sence?
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Mpcoluv
Gearhead Posts: 1341
From: Charlotte NC usa
Registered: Apr 2001
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posted 02-13-2002 01:57 PM
I'm talking about lobe seperation angle.
Not installed lobe center.
Sorry for the confusing terms
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SundanceKid
Gearhead Posts: 1274
From: UT
Registered: Feb 2001
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posted 02-13-2002 04:57 PM
You have David Vizard as a resorce and you ask us...LOL How great would that be?
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Mpcoluv
Gearhead Posts: 1341
From: Charlotte NC usa
Registered: Apr 2001
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posted 02-13-2002 09:33 PM
quote:
Originally posted by SundanceKid:
You have David Vizard as a resorce and you ask us...LOL How great would that be?
I try not to bug David all the time with silly questions. He would probably avoid me if I did.
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Mike_R
Gearhead Posts: 178
From: Indianapolis, IN 46237
Registered: Feb 2001
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posted 02-15-2002 12:38 PM
Here's an easy way to figure overlap. First off, you have to use the advertised duration rather than .050". Overlap, by definition is the amount of time the exhaust and intake valves are open at the same time from seat to seat.Here's how you figure it. Take advertised intake duration and add that to the advertised exhaust duration. Now divide by 2. Now take the LSA x 2 and subtract it from the first number. That's your overlap. For example on my cam it is: 298 + 308 = 606 606 /2 = 303 111 x 2 = 222 303 - 222 = 81 Sorry I forgot how to write forumlas, I think it might look like this: (advertised IN dur + advertised EX dur)/2 - (2 x LSA) = overlap
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'69Stang
Gearhead Posts: 205
From: Detroit, MI USA
Registered: Jan 2002
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posted 02-18-2002 10:23 AM
The most accurate way to determine overlap is to get the actual valve timing events from the cam card and then figure out what the overlap is. The intake and exhaust numbers for TDC and BDC will let you know what it is exactly! 
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Ron
Gearhead Posts: 137
From: Newcastle, Wa.
Registered: Feb 2002
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posted 02-18-2002 08:47 PM
quote:
Originally posted by Mike_R:
Here's an easy way to figure overlap. First off, you have to use the advertised duration rather than .050". Overlap, by definition is the amount of time the exhaust and intake valves are open at the same time from seat to seat.Here's how you figure it. Take advertised intake duration and add that to the advertised exhaust duration. Now divide by 2. Now take the LSA x 2 and subtract it from the first number. That's your overlap. For example on my cam it is: 298 + 308 = 606 606 /2 = 303 111 x 2 = 222 303 - 222 = 81 Sorry I forgot how to write forumlas, I think it might look like this: (advertised IN dur + advertised EX dur)/2 - (2 x LSA) = overlap
You're computations are correct but using advertised is gravely incorrect.
You must use the timing figures at .050 this is the industry standard
a way easier way of doing this however is simply adding your intake opening degree and you exhaust closing degree together
if it opens like mine at 26 btdc and the exhaust closes at 20 atdc then my overlap is 46
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Mpcoluv
Gearhead Posts: 1341
From: Charlotte NC usa
Registered: Apr 2001
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posted 02-18-2002 10:12 PM
Well, I finally got a cam card for the 114 LSA cam. IT has 18 degrees overlap, while my cam has 10.5 degrees overlap both at .050
But the 114 cam has 2/3 of its overlap before TDC while my cam has 2/3 of its overlap After TDC.
Since the 114 cam is "advanced" compared to my grind, will this affect idle quality much (any)?
I gotta get a cam theory book.....
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Mike_R
Gearhead Posts: 178
From: Indianapolis, IN 46237
Registered: Feb 2001
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posted 02-19-2002 08:52 AM
I agree completely about the duration at .050" being an industry standard used for judging cams, that's for sure. Cam overlap however is (by definition) the number of degrees from the time the intake valve opens until the time the exhaust valve closes. So you can't figure overlap using duration numbers at .050" lift because the .050" duration numbers don't give you any indication of when the valve will actually open or close. Cam lobes can have much different levels of aggressiveness. You could have two cams that have 250 duration @.050" lift, but one has 280 advertised duration and the other has 310 advertised duration which would have a huge effect on the overlap. It's true that not all companies measure advertised duration with exactly the same method, but the number they give is the theretical calculation of what total duration will be, and that's about all you have to go by.A side note to all of this. While the .050" lift number tells you alot about how much horsepower you can build, the advertised duration numbers tell you more about the idle characteristic of a cam. For example the two cams I mentioned before. The one with 280 advertised and 250 @.050" will have a much smoother idle than the one with 310 advertised, and 250 @ .050". This is because of the additional overlap.
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kid vishus
Gearhead Posts: 6590
From: middle of NC
Registered: Oct 2000
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posted 02-19-2002 10:41 AM
The problem with using advertised numbers for anything is each manufacturer could use a different point to get their advertised numbers from. One may get their adv numbers .002 off the seat while another may take them .005 off the seat. Different companies take them from different places to "inflate" their duration numbers. Thats why the only real way to tell anything about a cam is from its duration at .050 and its LSA and centerline. Going by just advertised numbers is a crap shoot at best.
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Mike_R
Gearhead Posts: 178
From: Indianapolis, IN 46237
Registered: Feb 2001
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posted 02-19-2002 12:25 PM
I agree with what you're saying kid. You have to make sure your comparing apples to apples and the .050" duration numbers are the easiest way to do that. If a person is strictly going to race the engine and they don't care too much about the idle characteristics there is really not much of need to look at anything beyond the .050" lift numbers. You can look at the .050" numbers and basically determine your power band, horsepower level, etc. However, If you're trying to work with a street car where you want a smoother idle, more vacuum etc, but still get the power you have to come up with a way to fool the engine. In this case you really need to consider the seat to seat timing also. The seat to seat overlap has a major effect on idle and vacuum. An aggressive lobe is the key to having both power and a smoother idle on a street car. If you only look at .050" number there is now way to tell how aggressive the lobe is. I like to compare lobe specifications at .020" lift and at .050" lift and that gives a pretty good indication of how aggressive the lobe is. Also MPCOLUV, if the cam is advanced it will also tend to make it idle smoother. Alot of times cams have a certain amount of advance already ground into them the cam manufacturer never tells us. So when we install the cam "straight up" it's really not "straight up".
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Ron
Gearhead Posts: 137
From: Newcastle, Wa.
Registered: Feb 2002
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posted 02-19-2002 01:42 PM
quote:
Originally posted by Mike_R:
I agree with what you're saying kid. You have to make sure your comparing apples to apples and the .050" duration numbers are the easiest way to do that. If a person is strictly going to race the engine and they don't care too much about the idle characteristics there is really not much of need to look at anything beyond the .050" lift numbers. You can look at the .050" numbers and basically determine your power band, horsepower level, etc. However, If you're trying to work with a street car where you want a smoother idle, more vacuum etc, but still get the power you have to come up with a way to fool the engine. In this case you really need to consider the seat to seat timing also. The seat to seat overlap has a major effect on idle and vacuum. An aggressive lobe is the key to having both power and a smoother idle on a street car. If you only look at .050" number there is now way to tell how aggressive the lobe is. I like to compare lobe specifications at .020" lift and at .050" lift and that gives a pretty good indication of how aggressive the lobe is. Also MPCOLUV, if the cam is advanced it will also tend to make it idle smoother. Alot of times cams have a certain amount of advance already ground into them the cam manufacturer never tells us. So when we install the cam "straight up" it's really not "straight up".
I notice that too! crower list on their cam card if you install this cam on it intake cl it will be 4 degrees advanced, but crane does not tell you this on their card. Problem I have with the advertised versus numbers taken at .050 is, if you take your reading at .050 and you determine you have 50 degrees of overlap, if you make the same calculations on the advertised it comes out to 80 or so degrees of overlap
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Mpcoluv
Gearhead Posts: 1341
From: Charlotte NC usa
Registered: Apr 2001
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posted 02-19-2002 04:09 PM
quote:
Originally posted by Mike_R:
Also MPCOLUV, if the cam is advanced it will also tend to make it idle smoother. Alot of times cams have a certain amount of advance already ground into them the cam manufacturer never tells us. So when we install the cam "straight up" it's really not "straight up".
I knew that advancinc the cam would beef up low end, but I never knew it would affect idle.
How much vacuum can be gained when you go from "straight up" to 4 degrees advanced (assuming no advance is ground in the cam)?
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