cam question

[DogHouse]

Need to tap into some cam knowledge:
I've been running a roller cam with a fairly tight lobe separation of 108 deg. Duration is 256/262 @.050, .714/.714 lift. I understand how duration affects the power curve and idle quality, but am less clear on the lobe separation aspect. I know that a smaller angle increases overlap, thus leading to choppier idle and more potential for water reversion in the exhaust. If I want to change to a larger separation of say 114 deg to reduce reversion, is there a rule of thumb as to how much more duration I should run to maintain the same power curve and idle quality (or lack thereof) that I have now?
Thanks in advance for any suggestions!
-brian

[LakesOnly]

I am running a 108-degree cam, too.
I am by no means a cam expert, but if you ease up on the lobe separation, and then increase duration, aren't you (to some extent) bringing back the very overlap (and reversion) that you tried to eliminate by changing your lobe separation?
Reversion is a problem only at idle...not higher r.p.m.'s. If exhaust reversion (and sucking water in through your exhaust) is why you are considering changing cams, then just run Bassett's stainless steel ball-check T-Valve that kills water flow to the headers at idle and opens up above (maybe) 2000 r.p.m.
If you add too much duration with too wide a lobe separation, you may bend valves with the pistons. This is one reason why valve overlap is also manipulated by way of a narrow lobe separation.
LO
[ March 12, 2003, 02:42 PM: Message edited by: LakesOnly ]

[Blown 472]

My little knowledge on lobe sep, the wider the better idle you will get do to less dilution of the intake charge.

[Craig]

Narrow lobe separation tends to increase midrange torque and results in a faster reving engine. Wide lobe separations result in a broader powerband and more peak power, albeit at the price of a some what lazier initial response.
Some of the rules of thumb I've read are as follows;
*detonation tolerance - wider
*cranking compression - narrower
*max torque - narrower
*max horsepower - combination specific
*broad flat torque curve - wider
*higher rpm - generally wider assuming good
flowing heads
*lower rpm - narrower
*poor heads - narrower
*great heads - wider
*nitrous - wider
*blower - wider
*idle quality - wider
[ March 13, 2003, 07:04 AM: Message edited by: Craig ]

[DogHouse]

Thanks for all the good info. Theoretically I shouldn't have any reversion because I run Lightnings with fully jacketed tailpipes out the transom. Unfortunately I recently found out that they were not sealed properly when installed at the factory, so they've been leaking at the junction between the collector and the tailpipe. Hopefully I'll get them sealed up right this time when I put them back together. Since I also have the motor apart right now, I was thinking of going to a wider cam as insurance in case of more leaks down the road. I'll try some different profiles in Dyno2000 and see what it comes up with.
-brian

[Craig]

One word to fix all that ills you, SUPERCHARGER :D

[Boater Bill]

Another question on this subject. Can someone explain the difference (if any) between lobe separation and lobe centers and their respective effect(s). My cam is 107 degree lobe center intake with 117 degree lobe center exhaust, giving 112 lobe separation? Cams are a black science to me. Thanks.

[DogHouse]

Craig:
One word to fix all that ills you, SUPERCHARGER :D Yeah yeah yeah, keep that up and I'll forward your response along to Amy, and she'll straighten you right out! :D

[Craig]

Cam centerline is a measurement of the cams phasing, or the relative position of the center of the lobe compared to the position of the engines crank. Lobe separation is measured in camshaft degrees while the centerline relates to crankshaft rotation. A cam ground with a 110 degree lobe separtaion will read an installed centerline of 110 if installed "straight up", or with no advance. Since the cam is connected to the crank via the timing chain, the cams centerline can be adjusted forward or backward, changing the installed centerline angle.
This adjusting does not change duration, lift or when the valves open in relation to each other, those things are ground into the cams profile when it's made. Most often, retarding the camshaft (opening the valves later relative to the crank) moves the power up the rpm band and can increase horsepower while decreasing lower end torque. Advancing the cam (opening the valves sooner) has the opposite effect.
Some cam builders offer cams with advance ground into them. When these cams are installed "straight up", they are actually advance.

[Craig]

DogHouse:
Craig:
One word to fix all that ills you, SUPERCHARGER :D Yeah yeah yeah, keep that up and I'll forward your response along to Amy, and she'll straighten you right out! :D Now let's not do anything rash here :D I catch enough hell from my own!

[DogHouse]

Craig:
A cam ground with a 110 degree lobe separtaion will read an installed centerline of 110 if installed "straight up", or with no advance.Did you mean "intake c/l" instead of "lobe separation" here?
Btw, I can't believe that Princess T would ever give you hell for anything!
:D

[DogHouse]

Checked out a profile from Erson vs the Comp that I've been running. With the 114 center and more duration (4 int, 6 exh), I can boost top end by about 20hp and reduce overlap at the same time. Mid range torque does suffer though. If I install it 4 deg retarded it brings the torque back up, still makes a few more ponies on top, and still has the added benefit of reduced overlap. Hmmm, might have to give this a try...