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Jeff Smith Technical Questions and Answers - Ask Anything

Timing is Everything

Dale Caissie; Kamloops, BC, Canada: I'm having trouble setting up initial timing on my SBC 350. It's a 0.030-over '84 block, balanced Scat rotating assembly, flat-top hypereutectic pistons, Pro Products damper and flexplate, ported Edelbrock Performer RPM heads, Comp XE300HR hydraulic roller cam with matching valvesprings, Crane 1.5:1 roller rockers, Pro Products single-plane intake, with a Holley 750 street HP on top. The ignition is a Mallory Pro Billet PN 8361m with a HyFire digital 6 CD box and matching coil. Here's my problem: Upon trying to dial in initial timing, my advance is way up at 30-plus degrees. This is with an idle speed of 900 to 1,000 rpm with the vacuum advance disconnected. Is my mechanical advance coming in already? How do I compensate for setting that initial advance? I questioned myself on the initial build of a milder combo a few years back that maybe the balancer was for the timing tab to be in the center of the timing cover, not off to the side as most would be, so I double- and triple-checked the location of TDC to the timing pointer, and it's correct. As for tuning, I have out-of-the-box jetting, power-valve and accelerator pump jet. Before I dive into carb tweaking, I've always been told to nail down ignition timing before anything else. Any suggestions?

Jeff Smith: It sounds like the timing is advanced, because that's where the engine seems to run properly. I'm assuming the problem is that if you try to pull the timing back to somewhere around 14 to 16 degrees initial, the engine won't run. While your friends are correct that you need to set the timing correctly to dial in the carb, it sounds like we have multiple issues. Let's start with the basics. You said you checked the timing marks to make sure they are accurate. The best way to do that is to pull all the spark plugs to make it easier to turn the engine by hand. Install a spark plug–style piston stop and carefully rotate the engine clockwise by hand until the piston contacts the stop. Mark the zero (0 or TDC) timing position location on the balancer, then carefully rotate the engine counter-clockwise until the piston hits the stop and mark the balancer again at the zero timing mark. Never use the starter motor to turn the engine with a piston stop in place. This procedure will produce two locations on the balancer. If the balancer is degreed, then simply record the number of degrees on either side of zero and compare the numbers—they should be the same. If your balancer is not degreed, try this simple trick: Place a piece of tape to straddle both sides of the zero mark on the balancer and mark at zero and at both stop points. Then remove the tape and use a steel ruler to measure both distances from your zero mark. If the distance is the same on both sides of the zero mark on the balancer, then TDC is correct. If the mark is not centered, you will need to move either the indicator or remark the balancer. Frankly, it's easier to create a new indicator. Then double-check your work to make sure you went the right way. With this effort, you now have a true zero mark from which to determine both initial and total timing.

Back to your problem: If TDC is correct, let's look at your combination. With a Comp XR300HR hydraulic roller, the specs are 248/254 degrees at 0.050-inch tappet lift with a 110-degree lobe-separation angle. That, my friend, is literally a bottom-of-the-page camshaft that suffers from minimal idle manifold vacuum. So I'm going to guess that you had to crank open the throttle blades to establish any kind of idle speed and that the engine wants all that timing just to stay running. What you really have to do is richen the idle mixture screws to put more fuel to the engine. By doing so, you will be able to pull the timing back slightly. But more than likely, you will need 18 to perhaps 20 degrees of initial timing to keep the engine running at low speeds. This is especially true if you only have 9:1 (or less) static compression. Let's touch on why more initial timing is necessary.

Overlap is the amount of time in camshaft degrees between the exhaust closing and intake opening points. A long-duration camshaft like yours creates that lumpy idle that everyone wants because of the increased amount of time that the intake and exhaust valves are simultaneously open. But this greater overlap also kills manifold vacuum. We calculated your cam's overlap by adding the exhaust closing point at 0.006-inch tappet lift in degrees before TDC and the intake opening point at degrees after TDC. This number came out to an astonishing 83 degrees. While this number is difficult to relate to by itself, let's compare it with the same Comp lobe family but with a shorter XE276HR cam. The overlap figure for that cam is 59 degrees or (drum roll, please) 24 degrees less. For every added degree of overlap, the idle quality suffers. So my guess is that the Holley is a little too lean to compensate for that much overlap, and that's why it appears you need so much timing. About the only thing you can do to help this situation is to degree the cam to ensure that the intake centerline is actually at 106 degrees after TDC. Assuming the cam was not retarded (which would lower idle manifold vacuum even more), you might consider advancing the cam by 2 to 4 degrees. This will improve idle vacuum slightly, requiring a little less initial timing.

You will also have to make the idle air/fuel ratio much richer with the transmission in Neutral or Park because once the engine drops into gear, it will create more load, and idle vacuum will drop even further. This means the air/fuel ratio has to already be rich in Neutral or Park because the fixed idle-feed restrictor that determines the mixture will not be able to compensate for the reduced signal from the manifold when the transmission is put in gear. This is part of the problem with a carburetor attempting to accommodate a long-duration camshaft with lots of overlap using an automatic trans that changes the idle load. You will probably have to drill holes in the primary throttle blades to allow more air into the manifold. This will allow you to close the throttle blades back down to their intended location so that the idle transfer slot will work the way it was intended. The new Holley HP carbs incorporate an idle bypass feature, located underneath the carb stud, that allows the tuner to leave the throttle blades properly located relative to the idle transfer slot, and adjust the idle speed by varying the air bypassed through that separate circuit. It's slick and it works very well, which means you don't have to drill holes in the throttle butterflies anymore.

More Info

Holley Performance Products
Bowling Green, KY
270/781-9741
www.holley.com


CC Quickies

We spotted this SOHC 5.4L Ford engine in the junkyard. The bore size and spacing is the same as a 4.6, but the 5.4 has a taller deck height. You can tell by looking at the distance between the top of the front-facing freeze plug on the driver side of the engine.


  4.6L 5.4L
Bore 3.552 3.552
Stroke 3.543 4.165
Deck Height 8.937 10.079
Bore Spacing 3.937 3.937

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