Seconary rod suggestion?

[55 Tony]

Pretty high pressure readings for 454 CID with a 230 @ .050" cam on a 110LSA.  I'd suspect the true static compression is higher than you have calculated or the cam is advanced further than you think it is, or a little of both.......Cliff

Yes, I believe the head was milled a little or lot more than I accounted for.  I really wish I had known about cc'ing them.  It was my first build ever.  I have replaced timing chains before and I'm pretty darn sure it's 2* advanced and no more.  In your opinion, if I have all the low end torque that I can use, would you suggest setting the cam straight up?  It's not an easy job with a BBC in a 55 chevy.  Although the cam says 1500 to 5500 and I redline it at 5500rpm anyway.  Comp told me the 2* advance would lower the power band about 500rpm at each end, but I know the guys on the phone are hit and miss.  Oh, and by the way, the motor has almost 8000 tough miles on it with no problems so I guess all in all I did a few things right?

[Cliff Ruggles]

Advancing camshafts to improve low end power is a highly inaccurate assumption.

It gets regurgitated over and over on the Boards, but I doubt if very many folks have actually done any testing in that area to see what really happens with moving cams around in terms of engine and vehicle performance.

It is true that advancing the cam or lowering ICL closes the intake sooner, but it also moves the exhaust to cylinder scavenging is effected, often adversely.

Several years ago I decided to test that sort of thing and moved the cam I was using in my 455 4 times to see what would really happen.

It started out installed at 109ICL per the cam card.   I had many hundreds of runs on the car so accurate comparisons could be made.  I also logged many thousands of street miles so would also compare street performance as well.

First move was to 111ICL.  It really didn't effect it much anyplace, just a very slight reduction in vacuum at idle and maybe just a tiny bit "lazy" at very light throttle openings.  At the track it ran almost exactly the same at every point as it did at 109ICL.

Next moved it to 113ICL, and that really took some "snot" out of it.  Idle vacuum dropped a solid 2" at 750rpm's, noticeably "lazy" right off idle, and it lost nearly almost 2 tenths and 2 MPH at the track.

Moved it to 107ICL next.  The vacuum at idle returned, and it "felt" little better on the street, with slightly improved throttle response right off idle.  It didn't really "feel" any better past about 2000rpm's, but when you are working with a big 455 engine that makes TONS of torque anyhow, "seat of the pants" evaluations are a bit difficult right to start with.  Anyhow, race day came and at the track it KILLED it everyplace, slower in 60' and ET and MPH at every point. 

Lesson learned, advancing a cam doesn't necessarily improve low end power, engine power across the loaded rpm range, or vehicle performance at any level......FWIW......Cliff

[55 Tony]

So ... read the card, set it and forget it?

Something else I *think* I realized.  Bare with me, I don't know cam shaft lingo.  But some of the numbers on roller cam specs make it look a bit wild when comparing it to a flat tappet cam.  But since the flat tappet is, well, flat, the cam lobe hits it sooner and holds it longer than with a roller on the same cam.  Is that right?

[Cliff Ruggles]

There are limitations with flat cams not present with roller designs. 

The roller cams can get the valves up to full lift quicker/sooner and hold them there longer.

Flat cams would chew up the lobes trying to do the same thing........Cliff

[55 Tony]

Let me ask a different way.  When stating specs for cams, do they give the actual lift of the cam lobes or the lift of the lifter and rod as it travels over the cam lobes?

[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