Doug Patton, Pro Line Racing Engines
Tellingly, when engine builder Chuck Lawrence received the order to bestow a 520cu in big-block Ford with the sound of a Pro Stock engine, he replaced the normal 112LSA hydraulic roller cam with one of 108LSA. “The result sounded wonderful,” said Mr. Lawrence, “but it didn’t rev as enthusiastically and it made 30hp less than normal!”“If you changed the lobe separation of a street engine from 112 degrees to 106 and didn’t do anything else,” says Jon Kaase, “the engine would idle a lot rougher and generate worse exhaust emissions largely because of unburned fuel.”
In conclusion, lobe separation angles change the amount of valve overlap, which affects many performance factors particularly idle quality, peak torque that can be moved from a lower rev range to a higher range and power bands that can be narrowed or broadened.
- Erson Cams,
- Moore Good Ink
I’m no expert or anything near one, but the way I see it, separation isn’t so much a setting itself as something that comes out from all the other settings. So, you decide how much peak lift for intake and exhaust, and you decide duration for intake and exhaust, and you decide what’s the timing for peak lift for intake and for exhaust, and that determines overlap, so after you work that out, then separation falls out of the numbers.
I suppose if you have general ideas of what some of the numbers are, then looking at separation could be a shortcut to figuring out approximately what the other numbers are. Like if everything else was the same, an engine with more separation would have less overlap than one with less separation and would probably run lousy at low RPM. It might not run better at high RPM if everything else wasn’t optimal, but that’s what the designer would be going for. But if the lifts and durations were different between the two engines, then all bets are off as to what the separation means.
I wonder what the lob separation angle is on my 1977 Buick 350. Peak torque is at only 1,800 rpm.
Looking at LSA on an island is an unproductive way to look at camshaft technology. LSA is a result of cam events which are the real important variables. If you reduce LSA, overlap increases, but it will also increase if you leave LSA the same and increase intake/exhaust duration. And what about profile advance/retard? Wouldn’t it favor higher RPMs if the narrow LSA cam is installed with less advance or retarded?
FYI, stock GM LS cams LSAs in everything truck to Corvettes are in the 114 – 119.5 LSA range, and the ones rated past 5900 RPM use VCT. On the SBF roller cams (pushrods), they are from 107.5 LSA for trucks 114-118 for the rest, yet none was rated for anything past 5500 RPM until VCT was added.
Now let’s take the 5.0L Coyote with stock duration @.050″ of 211° int/exh. It has a park/max intake centerline of 139 / 89 and 123 / 73 for the exhaust. These values make for a minimum overlap of -25.5° and a maximum of +24.5° under different advance/retard conditions. The question is, wouldn’t a higher overlap favor scavenging at higher RPM to increase intake flow and velocity, thus high RPM HP? And the other key questions are, under which overlap conditions? Intake or exhaust biased? And how biased each way and under what RPM/load conditions? Keep in mind that the Coyote is not limited to VCT (LSA is fixed) since it uses Ti-VCT (LSA is also adjustable/programmable).
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Watch the David Vizard Powertec 10 vlog on How to get Pro Stock Torque. According to David, LSA is determined primarily through cylinder displacement divided by effective intake valve diameter. Overlap is then a consequence of duration. He states you can get more torque and power using the narrower LSA and less duration.
Great information, good to know when picking my cam
Can someone tell me definitively which of the two differing comments on the effect of lobe seperation (overlap) on engine torque and idle is absolutely correct.
Dear John,
Thank you for your patience.
Simply put, tighter lobe separation angles (LSA) create greater low end torque, rougher idle and less engine vacuum.
Wrench Safe
Mark
Classic Car Restoration Club Video membership
That doesn’t make sense. The mother of all “lumpy” cams is the Duntov 30 – 30 that was in 64 & 65 365 and 375 HP engines. The LSA for that cam is 132 degrees – it produces very little low end torque but it is also notorious for virtually zero vacuum below 900 rpm and a very rough idle. It appears that LSA is not the only design factor that influences torque curve, vacuum and smooth idle.
Great informative article..
I’m so confused! Cam tech has always been a foreign language for me and between the article and the first comment and then the corrected one, I’m worse off now than before. I gather that a narrow LSA produces a rougher idle; now, does it produce more or less torque in the lower to mid range RPM scale? Also, the article mentioned cam degrees, I can’t get my head around that term as the shaft makes a complete turn of 360 degrees therefore the lobe must also travel 360 degrees. I realize a lifter riding the lobe travels a greater distance but how does that affect cam degrees, and wouldn’t that change with the design of the lobe? As I said, “I’m so confused!”
Hi Doug. Sorry for all of the confusion, A narrower LSA will produce more torque at lower/mid RPMs, As it relates to this article, Cam degrees are the number of degrees of lobe separation between the intake and exhaust maximum opening. On cams with narrow LSA there are times when both the intake and exhaust valves are open. The lobes on the cam are cut to determine: how quickly a valve opens, how long it remains open, how far open the valve becomes and how quickly it closes. The size of the cam lobes does not affect the degrees of cam lobe separation though, although it can factor into the amount of overlap. And you are correct there are 360 degrees of total rotation, that is a constant.
Thanks
Mark-Classic Car Restoration Club
I was just discussing with my son the reason why Pontiac used a VERY conservative cam in the 350 it put in the 74 GTO option. The engine as I recall had 7.6 to 1 mechanical compression. Why? I suspect because it raised the working compression and with its four barrel carburetor gave the car 200+ hp. That doesn’t sound like much but with a manual transmission the actually ran pretty decent. My point was GM had a lot of red tape to work around to get emissions and mpgs demands. Of course, when Chevy cam out with the LS1 it was clear this kind of thinking led to a stock cam with an LSA of 122+ which seemed just as strange to me as a duration (?) of 169 on a sport model. But I think they were fighting the same battle but Pontiac, who new hiw to build engines with horsepower, just didn’t have the technology in 1974.
I’ve got an Oldsmobile motor installed for marine use with a roller cam and in this application the headers are water cooled by spraying water into the headers a couple inches from the exhaust port. The problem is the engine sucks water in from the exhaust into the cylinders and ends up with water in the oil. I’ve done a lot of experimentation to prove this is what’s happening. I was told a wider lobe separation angle would resolve this problem since the exhaust won’t “see” as much of the intake vacuum. Is this true? Please advise. Thanks
Paul
Hello. Sorry I am not a marine expert, but there is no engine vacuum on the exhaust side of the of the engine.
The vacuum is created on the intake side only. I don’t believe your issue is related to cam lobe separation.
Thanks
Mark-Classic Car Restoration Club
Had that happen on my marine old 455 and it was the logs allowing water to “slosh” into the exhaust when the engine was off to the point of hydraulicly locking the engine when trying to start it. Flaps on the thru transom exhaust helped but due to the low profile of a jet boat and the low rise logs water still easily entered and water entering the oil occurred. I’m running headers that rise above the intake and then go back thru the transom now and no problem since switching style. Only other times oil in water occurred was when too much water entered the bilge and got above the dipstick and when I had blown head gaskets. Get good quality layered ones to make them last. Just my 2 cents!
61lincoln
So if I hear this correctly a cam with a larger degree of sepertion has more lope or sounds like a old Harley at slow idle? So does this effect lift on camshaft?
Hello. Lobe separation does not affect the amount of lift that is ground into the camshaft profile.
Thanks
Mark-Classic Car Restoration Club
“Changing the lobe separation angle,” says Doug Patton of Pro Line Race Engines, “changes the amount of overlap that exists during the time the intake and exhaust valves are both open. On a naturally aspirated engine, the lobe separation angle has an effect on whether the engine reaches peak torque a little earlier or later in the rpm range. Typically, narrower lobe separation develops peak torque at lower rpm and widening the separation tends to build peak torque higher in the rpm range. Nitrous engines, which make plenty of power and torque, often run wide lobe separation angles to moderate cylinder pressures and temperatures.” This paragraph is incorrect.To correct my first post: Narrow lobe separation doesn’t make for more low end torque at a lower engine speed. It is the exact opposite .I am sure that the writer or/and the speaker realized that. Either a misspeak or a typo. (And I did it on my first post! )
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I agree with you the original print is incorrect
i agree wider lsa reduces overlap smoother idle. narrower lsa equals more overlap rougher inconsistent idle. article seems flawed. lower lsa builds cylinder pressure later in the rpm range.
“Changing the lobe separation angle,” says Doug Patton of Pro Line Race Engines, “changes the amount of overlap that exists during the time the intake and exhaust valves are both open. On a naturally aspirated engine, the lobe separation angle has an effect on whether the engine reaches peak torque a little earlier or later in the rpm range. Typically, narrower lobe separation develops peak torque at lower rpm and widening the separation tends to build peak torque higher in the rpm range. Nitrous engines, which make plenty of power and torque, often run wide lobe separation angles to moderate cylinder pressures and temperatures.” This paragraph is incorrect. Narrow lobe separation makes for more low end torque at a lower engine speed. I am sure that the writer or/and the speaker realized that. Either a misspeak or a typo.
Very insightful article–thanks, Butch
It can change just about every aspcet of the motors running. It can give you a lopey idle, allow the motor to rev up quicker, and change the amount of air used, changing the exhaust note, among other things. Since the exhaust note is the sound of combustion happening inside the chamber, any change to the state of that combustion will produce an audible result. Generally a rough, loping idle is considered tough sounding, this type of idle is normally atributed to a high duration camshaft.