Thread: virgin 454 build guestion

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Originally Posted by bear9820

sounds good, I'm not looking for a serious lope but just enough to turn heads with out having to figure out the brake issue you described. I'm sure there will probably be some there but as I would have no idea where to start with that issue I'll leave it alone until i have a bit more knowledge on just what I'm doing. As far as separation all I have found through top-end kits was edelbrock at I believe 112 or 114. but if there would be no noticeable sound difference then I assume I would have to go to a tighter separation until I get where I want to be with out braking issues. Now, as far as timing, I understand advancing and retarding the timing is this the same as "putting it more at the crank" or am I not even on the same page? sorry for the probably redundant questions but I have paid too much over the years having motors built and put in and no time working on them myself.

Yeah, the more cam you use, the more initial ignition timing at the crank the motor wants. If you increase the timing at the crank, you need to decrease the centrifugal timing in the distributor to keep the total initial plus centrifugal about the same.

I'm gonna talk about small block Chevies here because I am more familiar with them. Some of the other fellows on this board are more familiar with big blocks and hopefully they will chime in on this thread to customize it for your needs. Irregardless, every motor out there is just a big old air pump and they all respond pretty much the same way to modifications by us Troglodytes.

If, for instance, you are using L31 Chevy heads or other current technology cylinder heads that have a very efficient chamber, the motor will probably run best with about 34 degrees total initial plus centrifugal ignition timing under power. The stock distributor arrangement probably provides around 24 degrees centrifugal advance, so if you use more advance at the crank without changing your centrifugal curve in the distributor, you will likely have too much total initial plus centrifugal timing in the motor. The older design, less efficient heads with less efficient chambers may want a little more total ignition advance, up around 36 or maybe a little more. I know some fellows have told me they are running 40 with the older heads.

OEM timing curves call for the total initial and centrifugal to be all in by somewhere around 3600 rpm's or maybe a little higher. This will work swell for haulin' Grandma to Bingo, but is less than ideal for our modified ground pounder. Most fellows will agree that total initial and centrifugal should be in the mix by 2600 to 3000 rpm's.

Here's a general recommendation for initial timing at the crank for different camshafts and carbs.
Less than 220 @ 0.050" camshaft- 10-12 degrees ignition advance at the crank with enough centrifugal to make 34 at 2800. (assuming efficient chambers).
Less than 240 @ 0.050" camshaft- 14-16 degrees ignition advance at the crank with enough centrifugal to make 34 at 2800. (assuming efficient chambers).
Less than 240 @ 0.050" camshaft with big carburetor- 16-20 degrees ignition advance at the crank with enough centrifugal to make 34 at 2800 (assuming efficient chambers).
Less than 260 degrees @ 0.050" camshaft- 20-24 degrees ignition advance at the crank with enough centrifugal to make 34 at 2800 (assuming efficient chambers).
More than 260 degrees @ 0.050" camshaft- 24 plus degrees ignition advance at the crank. At this point, you may consider locking out the centrifugal and running initial advance at the crank only (34 at the crank, zero centrifugal, assuming efficient chambers).

These suggestions are only to get you in the ballpark. Each and every motor is unique and different unto itself, so tinker with carburetor and ignition timing adjustments until you get your particular combination dialed in. There are numerous kits available from Summit and others that will allow you to tailor the centrifugal ignition advance in your distributor.

Many of you fellows and gals are using the popular Edelbrock Performer carburetor, so I'll include the Owner's Manual here for you to dial in the carb....REMEMBER, these carbs do not like more than 5 psi fuel pressure at the carb inlet. More pressure than that will over-power the needle and seat and blow raw fuel into the intake manifold, providing you with a tuning nightmare that you may never find unless you know about the limit on fuel pressure. If you don't know what the pressure is on your combo, then rig up a mechanical fuel pressure gauge and mount it temporarily on the cowl of the vehicle so you can monitor it through the windshield while you drive. That's the only way you will know. Use duct tape or tie wraps to secure the 0-15 gauge to the cowl. It's only temporary.
http://www.edelbrock.com/automotive_...ers_manual.pdf

Somewhere along the way as you are increasing the initial ignition timing, you will encounter resistance against the starter. You can fix this by installing a momentary push button on the steering wheel or on the dash where you can get to it easily. The button should be installed so as to interrupt the power supply to the coil. Push the button, crank the motor up to speed with the starter key, then release the button. The motor will fire normally without the drag on the starter that would result from trying to turn the motor against so much initial ignition timing at the crank.

As far as the power brake issue, most guys will not talk about it. I think it must be human nature not to admit that you screwed up something. Power brakes will need a minimum of 18 inches of mercury (in hg) to operate correctly and efficiently. Suppliers and others will tell you that they will operate fine at 14 inches or 12 inches. BULL HOCKEY. They were designed by the engineers at the factory to work with stock cams that produce 20-22 inches of mercury. Hopefully you can see past the crap and realize that as you pull back on the suck, you pull back on the efficiency of the power brake booster. It's just that frickin' simple. No amount of BULL HOCKEY will make it any different.

If you find yourself with enough cam to negate the correct operation of power brakes, there is only one real fix. Add an electrically-operated vacuum pump to the system. Those Fosdick vacuum cans will hold enough vacuum for one good stop, then they are done until you run the motor to fill the can again (or un-fill the can, depending on how you look at it).
http://www.summitracing.com/search/S...p&autoview=SKU

You may encounter some pinging (detonation) with the increased ignition timing. Do not let this continue for even a micro-second. This is the sound of the motor coming apart from the inside out. Detonation can crack piston rings, break pistons in half, bend connecting rods and even break crankshafts. This problem can usually be side-stepped by building the motor with a very tight squish (0.035" to 0.045") and using the correct grade of fuel. Of course, you must match the camshaft timing to the static compression ratio of the motor in the first place or you are fighting a losing battle.

Once you get the initial and centrifugal dialed in, it is time to work on the vacuum advance. OEM systems will usually add about 10-12 degrees of vacuum advance to the system as the manifold vacuum comes up due to cruising or other light loads on the motor. There are 2 things to consider here. One is the total AMOUNT of advance applied to the system. The other is the RATE at which the advance is applied to the system. You can, of course, keep the OEM system and hope for the best, or you can purchase a Crane adjustable cannister and stop limiter plate and dial the vacuum advance in to where the motor wants it for best operation.
Crane number 99601-1 will modify a Delco point type distributor. Crane number 99600-1 will modify an HEI distributor. The limiter plate for each of these is Crane number 99619-1.
http://www.summitracing.com/parts/CRN-99601-1/
http://www.summitracing.com/parts/CRN-99600-1/
http://www.summitracing.com/parts/CRN-99619-1/

One other point to make. Most fellows will run the vacuum cannister off manifold vacuum. Some fellows will run it off ported vacuum. You will have to try both positions to see which works best for your application. Ported vacuum was originally introduced for the early emissions systems cars and can be found above the throttle blades on the carb. Manifold vacuum is taken from below the throttle blades or directly from a nipple screwed into an intake manifold runner.