Quadrajet Problem Solving > Dialing in your rebuilt Quadrajet carburetor
Seconary rod suggestion?
Cliff Ruggles:
Cam specifications will usually have "advertised" specs and .050" numbers for comparison purposes.
They will also list lobe lift, and usually lift at the valve with a given rocker arm ratio.
The problem with "advertised" specs is that some companies will use a different open point for those numbers. This can make comparing camshafts a bit confusing as one company may "rate" their cams at .004" tappet lift and another at .006", for example.
The specs look confusing at a glance, but really all camshafts can be compared to each other if you know what specs to look at and what they mean.
For example I'll put up some cam specs below to describe what one may see on a cam card.
Advertised duration: 280/290
.050" duration: 230/240
Lobe lift: .300"
Lift at the valves with 1.5 rocker arms: .450.
Lobe Separation Angle: 112
Intake Centerline (recommended): 108
You will also see intake opening and closing points @ .050" for most camshafts as well.
Most cam companies will grind their camshafts on "tight" LSA's. 110 is very common and many will go even tighter. The Lobe Separation Angle moves the lobes closer together. With any given cam this increases overlap vs moving them further apart. It also typically places the intake advanced further and closes the intake valve sooner in the cycle.
Factory camshafts will typically have much wider LSA's. This reduces overlap and improves vacuum at idle, power right off idle and broad/smooth/flat power curve.
Tighter LSA does pretty much the opposite. It decreases vacuum production at low rpms/idle speed, even to a point where it causes "miss-fire" or "lope". Stinky exhaust is another by-product of tight LSA. Tighter LSA also narrows up the power curve, pulls VE down in the RPM range, and increases cylinder pressure at VE.
It is my opinion that companies grinding camshafts are playing into the "bling" by producing camshafts that put some "attitude" in the idle quality. I certainly can't argue with that, nothing sounds better than your "fresh" engine build "thumping" pretty hard at idle like it's ready to make a hard run down the track.
The problem is that folks associate improved performance with a "rough" idle, and to be perfectly honest nothing could be further from the truth.
Engine power is a product of the CID, compression ratio, head flow and camshaft being used. The intake and exhaust are also players here. So basically the camshaft is simply a small part of a big plan, and not the deciding factory by itself as to how successful the end result will be.
For most street driven engines we use and prefer cams with wider LSA and longer seat timing. I also prefer higher compression even though many folks have bought into the nonsense we read daily on the NET about 9.5 to 1 static compression as pretty much the limit for pump gas.
Higher compression can be your friend with these things as it allows for a larger cam to be used with less negatives at idle and low rpm's.
continued below......
Cliff Ruggles:
I love leading by example with these things. Recently we were asked to help out a customer who just built and dyno'd his first Pontiac 455 engine build. Comp Cams recommended their XR276HR camshaft, basic specs are 224/230 @ .050" on a 110LSA. The big 455 had 250cfm ported heads on it and 9.3 to 1 compression. Despite the "low" compression ratio it pinged on the dyno with the tiny 224/230 cam in it and didn't make very impressive power.
I was asked to help out as EVERYONE involved with the project on their end was blaming the Q-jet. Instead of talking carburetors I had the engine builder install a much larger cam, 236/245 @.050" on a 114LSA. I had him set the ICL at 110 instead of 106 like the previous smaller cam. My cam also had much larger .381" lobes, so bigger everyplace than the tiny XR276HR cam.
They made the swap, more dyno runs and sent me the results. They also noted that the larger cam actually idled much "smoother" than the smaller one and not the first hint of detonation when they cranked some timing to it to make the best dyno numbers.......Cliff
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