CAMSHAFT guys, explain lobe centers to me

[Roothawg]

I have never been real sure about lobe centers and how that affects cam performance.
Please explain.

[JOECOOL]

I'm not be berry good splainer sumtimes so hear goes. The lobe center is just a way of measuring the cam actions. The base of the cam is harder to find as some manufacturers use the measurement at the bottom and some at + .050. If you go thru the cam specs and the cycles of a 4 cycle engine you will notice that there are times when both valves are partially open . One will be closing and another will be opening. The further those are apart the more torque you will have as it doesn't allow them to be open at the same time for as long ( more Compression). Like one cam is 112 degree and the next is 108 the 108 cam will allow less "leakage"? from the intake to the exhaust and vise-versa.Thats why a performance cam will usually have a rougher idle than a stock one. As the RPM's go up this is not as critical as the higher velocity of the air/fuel moving thru keeps it from getting contaminated.
An RV cam is just a cam with small duration and lift, it increases torque at low rpm the same raeson as the hi-po cam decreases low end torque. Now an RV cam also is able to trick your engine into thinking it has more compression . Because the intake valve is not completely closed at bottom dead center, the piston starts up on compression as the valve finally closes. The Rv cam closes earlier so that the piston has more movement with the valve closed so it thinks it's got more compression. Wow I told you I wasn't Berry good at this ,hope it helps.

[Blownolds]

Simply put, for the SAME lobe profiles on each lobe, if you have them at a smaller lobe separation angle, then there will be more overlap and if yuo have them at a wider lobe separation angle, then there will be less overlap.

Supercharger profiles use wider lobe angles, typically 114 but occassionally even 116. This is because while both valves are open at the same time during overlap, the supercharger can push the intake charge right on through the exhaust and so a wider lobe center helps to minimize this.

Perhaps someone can explain why some race cams for naturally aspirated race engines use 106 or 108 lobe centers instead of 110 or 112.

[hog mtn dave]

I'm no expert and apparently a slow learner. About once a year I go to this part of comp cams website.

http://www.compcams.com/Technical/TimingTutorial/

My understanding is that the right cam ground on 106 with the proper valve adjustment can make a car lots of fun. I'd post a picture but haven't figured out how to do that yet.

[Roothawg]

See that's what I was unsure of. I know the street cams have a lot higher lobe center(number wise) i.e 114 vs. 106.
I was looking at one with 106 degree center.

[hog mtn dave]

106 will give you a bunch of bottom end end, wheelstanding power, at least in a 406. Aren't you running a small motor? I know the cam that I am using is considered "small" for my motor, but would be huge in a 265. Would it work? I leave the answers to those questions to smarter folks than me.

I would love to see your car on the back bumper though.

[Tudor]

I know one effect of the lobe centers and overlapping is to leave the exhaust valve open at the end of the cycle or close it real slow while the intake is startgin to open. Then the exhaust leaving the cylinder head will create a vacuum and help accelerate the flow into the head from the intake.

[C9]

Another reason for wide lobe centers is to calm down the idle.
I ran a KB cam with 118 degrees separation and once the engine was warm it idled like an old Ford tractor.
This particular cam advertised by KB (KB = Kenne-Bell, the Buick guys) as the biggest 'cheater' cam you could get.
More than likely true if you ran quiet muffs.
(Adv duration was 288 & 294 int & exh if I remember right.)

Later on I went to a Crower cam with the same adv duration and the lobe centers on that were 112 degrees.
The Crower had a 'badder' more hot rod lopy idle.

A totally subjective view of performance gaines from the cams was that the Crower had a slight edge.

Both cams worked ok with the 9/1 CR, but 10/1 would be better.

Right now I'm running a very mild Crower with 112 degrees separation and 260 & 266 adv duration.
The effective compression ratio is higher with this cam and the power band comes on farther down the rpm scale.
I seldom run this cam over 4000 rpm - due mainly to the cars light weight and big engine.

With the ignition timing set where it runs best power-wise, the engine runs great on 87 octane.
No pings, rattles or other scary noises - summer or winter.


Reason for the lopy idle is the overlap of the cam is kicking part of the intake mixture back up into the intake manifold.
The incoming charge mixes with that and you have an overly rich mixture.

Overly rich mixtures are easily duplicated by judicious use of a manual choke when rolling through the drive-in.
The old hands will probably catch you at it though.
Specially when you try to do the Don Garlits, blower special, seriously lumpy idle.
The black smoke from the exhaust tells the tale.
Too much is a dead giveaway.

Not that I ever did that.... http://www.jalopyjournal.com/ubbthreads/images/graemlins/grin.gif

[Blownolds]

Better check with the cam grinders on that 106, because street cams are typically 110-112 for naturally aspirated, with pump gas.

[hog mtn dave]

My experience with a cam ground on 106 is not on a street motor, but it can be done. It will not be a docile piece. It won't be a daily driver. You won't be able to sneak up on anybody. It will be a tire shredder. It will be fun.

[buffaloracer]

Two things cause overlap. Duration and ground lobe center angle. More overlap, more Rump-Rump. With much duration, 106 degrees of lobe center will not be much fun to drive around town. It is true that you can run more compression that way. When the cams are advertised with good low end they are talking about from 4000 to 6000 rpm.

[Doug Evans]

Roothawg Hey man I could explain it too you in depth and how it effects the engines out put on different lobe centers. But it will take much time, I also have a tune up program on my computer that will give you a detailed tune up for your injectors, but I need all the specs on your engine. Also a #0 hilborn pump DOSE flow 1.57 gallons per min.@ 1500 rpm, ( that's pump rpm, not engine rpm ). If you got a flow sheet for your pump send me the numbers along with the specs of your engine and I will send you the spread sheet with the tune up mumbers. And you are still welcome to borrow my pump, if you want. I always look at the flow mumbers at 6000 rpm that's why the numbers sounded so low to me.

[ray]

this is one area the ricers really got it made! we are stuck with whatever the cam company thinks is right for our motors, imports, with DOHC etc. adjustments of cam timing, exhaust and intake independently, is just a few twists of the allen wrench away. dammit anyway.