Cat Timing; How and Why (by PM request)
This is the third one of these asking about timing on a Cat, so here be my first thread ;-) Starting with an A model, the early PC’s had a 3 degree slip weight advance, then went with a 5* on late PC’s and DI’s. It is a simple mechanism in which there are weights held inward by two coil springs. The weights themselves are held by two pivoting flat shafts that the weight surrounds, thereby from our point of view, the carrier drive is attached to the weights, not the body of the carrier. As RPM rises, the weights force outward against the springs, but at an angle (5* reversed) so the more the weights come out, the more the advance. They start to move around 1300rpm, and stop about 2000rpm, all according to the springs themselves, and how old they are.
To time an A to static (0rpm/ engine off), there are two ways. The first being the way all Cats “Field Time”. Roll engine to where you can see the timing pin hole in the pump cam just start to come around. All 32, 33, 34, and 3500 series engines are the same in this way. When you see the pin hole come around, drop in the correct timing pin in and slowly role the engine til the pin drops. Now loosen the bolts on the advance. Unless it’s a 3200 series, it is not locked (press fit) and will spin free. Now role the engine backwards three or four inches of damper travel. Remove 1/4” pipe plug from the flywheel housing and insert your 3/8” bolt in the hole. Have someone push in on the bolt while you bar the engine forward til the bolt drops in the flywheel hole (only goes in about 3/8” so when it locks the motor, you’re there). Torq the advance bolts, REMOVE BOTH PINS, and put the covers back on. This will get you within 1.5degrees static timing. All A DI’s are 24* engines, as are the first B’s. The other common service manual way to do an A is a dial indicator on #1 cylinder (thru the nozzle adapter). TDC it with pin in pump, lock it down, then role it back and recheck it til you’re right on. You can run up to 29* degrees dynamic (engine running at 1000rpm), but not with a hot turbo on an A. This includes the T1810 or a Holset from a Cummins (HT3B or 4LHR). It will rattle a bit if you do.
Now we’ll cover the hydraulic advance in general. Caterpillars’ 11 to 14 degree advance (depending on arrangement number [model number]) is a simple gadget. There are just a few necessary components to this apparatus. The body, the carrier, and the spool/ weight assembly. The inside of the main carrier is oring sealed to hold oil pressure. The gear on the inner carrier is on a helix, just as the front of the cam in the pump it drives. The degree of pitch on the carrier is the degree in which the advance itself can advance the timing. Then we get to the weight carrier. It is a flyweight controlled (by RPM) assy. that is hooked to a spool valve (a machined rod that slides in and out of the main carrier spool body) that will load the inside of the sealed portion of the timing advance with oil to either push it in or force it out, depending on RPM by flyweight position. The machining on the spool is basically an end block, and a front block. In the carrier cap is a huge hollow screw that the weight spring fits in (the start screw, as in how much pressure is on the weights), and a baby stop screw in the middle (same as a rack screw/ same tool to turn it). The stop screw limits the full travel of the spool valve by hitting the end of it. For reference purposes, we will use 1600rpm for this explanation. 1600rpm, give or take by model, is the mid point of the advance. At 1600rpm, the weights are half way out (open). This puts the spool valve in the center position, thereby filling the advance half way with oil pressure, pushing the carrier outward on a helix half way, that, by the earlier reference of 1600r’s, adds 1.2 degree’s per 100 rpm past the point of the start screw (spring pressure on weights). In the case of an early B, that = 29.5* (not including .2* scroll port effect. If you must know, this is the natural advance of any scroll injection pump that occurs when the rack turns the barrel to move the scroll to desired fuel, the scroll itself takes the fuel in further down [hence more fuel] and further down means on a basic 45 degree scroll you get .2 more degrees advance per 100rpm unintended by design. Most Cat guys don’t even understand that stuff. That part is hard, but not necessary to know for this these basic analogies). Easy enough without that part, yes?
Early B’s are different than later B’s and C’s. We will cover 7FB’s first, then 4MG and up to the latest as they will all be the same (basically). Early B’s are water jacket after-cooled 24* motors with the same 14.5:1 pistons as the A DI’s. The timing advance is hydraulic advance, spring return. This means that whenever the rpm’s go down, the oil is forced out of the carrier by spring pressure, not spool valve location. These can be timed by pin method and is usually within 1.5degrees this way. It’s a 24* motor, so leave it that way. No need to go higher unless you are building a true race truck (nope, not in this thread ;-). When the air to air 425hp came out in Oct ’86, they used all the same parts but dropped the pin location in the cam to 21*. To check the weight carrier is these, the entire advance must be removed and disassembled from the rear to get to the weight carrier itself. Check the weight tips, replace the races always, check the C clip for tightness (use 093101 clip on all advances) and sometimes replace the orings. They are quadraseal orings, so they last a very long time. One of the main focuses on all hydraulic advances is that the first ones pressed together, then the later and ReMan’s screwed together with no fault. If you can turn the inside while holding the outside, the press fit slipped, and needs updated with the ReMan or simply weld it together (also for another thread if ness.). If they slip inside themselves, the weights are not turning the same RPM as the cam, so timing is incorrect.
Now the later ones. These are hydraulic advance/ hydraulic return. This means the front end can be accessed by removing the front cover of the advance and you don’t have to automatically reset timing. You did not move the carrier. The bad part is when you do remove the advance, you must stick a screwdriver in one of the windows of the weight carrier and push (collapse) the carrier back to zero and hold it when you torq the advance bolts. It has no big spring to return it to zero. The build of these is basically the same. If you’ve seen one, they all make since. The spool is a bit different, it has better races, yada yada, and the C’s are even better than that, also with the same exact design, just a beefier weight carrier. These all fix and time the same as the aforementioned, you just have to hold the carrier collapsed when torqing.
Now to the guts ya’ll inquired about in the earlier post. The C’s are slow timed to meet emissions (we won’t go there now but it worked for stockers, and sucked on turned up motors, even a little). The 8.75 and 11 degree engines were the worst, but basically every Cat made, if static timing is less than 16*, needs bumped to the 16* spec. That is, 17.5 to 18* dynamic (on the meter), with the start screw at 1240 to 1260, and remove the stop screw. This will give it a much more efficient burn, in where the piston is at less compression when the fuel is injected, it still ignites but continues to burn as it compresses more towards TDC, and has a slower more consistent drive pressure on the piston instead of one big boom to push the piston down. The more pressure over time you get, the lower but longer the cylinder pressure (i.e. heat pressure) well past TDC, actually burning more fuel longer, but not the acids and particulates. This takes more load away from critical components adding life to the beast, but also lets the engine rely more on boost from air than “blast cylinder pressure” from a super-heated explosion. Ambient is always cooler than ignition heat pressure, hence the advent of air to air or aftercoolers.
To answer the last part, yes, the best method to check and/ or time a Cat or any scroll pump on any engine is with a meter. It is exact. It does not lie. It lets you check all variables and see for yourself exactly what is going on in that oil soaked mess in there.
I hope this answered most of the questions surrounding timing, which applies to most everything. Kinda feels like I wrote a book (God help us :-)…T
Last edited by nevrenufhp; 03-13-2009 at 03:01 AM.
Reason: Returning original post