Nick Loesch; Hastings, MN: How do you find intake closing at the intake valve? I have a 383ci small block Chevy with Pro Topline Vortec heads and a Comp Extreme Energy 274H cam with 0.490-inch lift and 274 degrees of intake duration and 286 degrees on the exhaust. The intake valve opens at 29 degrees before top dead center (BTDC) and closes at 64 degrees after bottom dead center (ABDC). I would like to check intake closing at the valve with a 1.5:1 rocker arm ratio. I cannot find a formula for checking this. It is the most important valve event in the engine’s cycle.
Jeff Smith: This is a great question, Nick. I like the idea of checking the event at the valve rather than just using the cam numbers. First, let’s show you a little trick in computing duration that can come in handy, and then we’ll address your question. If you take the opening and closing points of the intake valve as you’ve listed them: 29 degrees BTDC and 64 degrees ABDC—simply add these two numbers together along with 180 degrees (29 + 180 + 64 = 273 degrees of intake duration) to determine duration. The intake valve opens before the piston reaches TDC and then closes after BDC, which is why you add the 180 degrees. The same technique can be used on the exhaust side. These are the numbers as expressed on the cam card for advertised duration. Comp uses 0.006 inch of tappet lift for hydraulic cams (0.020 inch for solids) as the checking point. Another little trick is to note the lobe-separation angle (110 degrees) and then look at the cam card for the intake centerline number. In your case, the intake centerline is 106 degrees, which indicates where the cam should be installed. That means Comp ground the cam with a built-in 4 degrees of advance since a “straight up” cam would have the same figure (110 degrees) for both lobe-separation angle and intake centerline.
Now let’s look at your question. Rocker ratio doesn’t really affect true opening and closing points on a theoretical basis, but if we’re using a checking point of 0.006 inch at the cam, we have to know what the actual rocker arm ratio is at that point to determine the lift at the valve. We can’t assume the rocker ratio is actually 1.5:1 because rocker ratios are not constant through the entire lift curve. This will become clear in a moment. Let’s first look at how a rocker arm creates ratio. The rocker is actually a very simple lever that, in this case, uses an arm that is 1.5 times longer from the centerline of the rocker stud to the centerline of the roller tip compared with the distance from the centerline of the pushrod to the center of the rocker stud. Looking at rocker arm operation from the side, it’s easy to see how the rocker tip scribes an arc as it travels through the valve lift curve. As valve lift is created, the rocker arm arc produces lateral movement across the valve tip. Assuming proper pushrod length, the rocker tip begins at the inboard side of the valve tip, then moves across the valve’s vertical centerline to a point outboard of center at max lift and then back again as valve lift nears its closing point. This lateral movement is important because the travel across the face of the valve tip changes the effective rocker ratio at the valve. Many rocker arms are set up to create the rated rocker ratio at mid-lift. This means that with your 0.490-inch-lift-rated camshaft (0.3267-inch lobe lift) that at the halfway point of valve lift you should see roughly 0.245-inch lift if the rocker ratio is truly 1.5:1 at that point. You could calculate the rocker ratio at half lift by limiting valve lift (with a stop placed over the valve stem) to 0.245 inch. To make things easier, you could use a checking spring and then place a dial indicator in line with the pushrod on that side of the rocker arm. This cam lift number could then be used to compute the rocker ratio. For example, let’s say we measured 0.163 inch on the pushrod side at 0.245 valve lift. Dividing 0.245 by 0.163 equals a ratio of 1.5:1. I doubt your rockers will come out that even.
Another variable in this equation is the height of the pushrod cup in the rocker arm. As mentioned earlier, the rocker ratio is determined by the relative distances between the centerlines of the pushrod to the stud and to the rocker tip. But the height of the pushrod cup in the rocker also affects the geometry. Crane makes a Nitro Carb stamped rocker arm with a repositioned pushrod cup that actually increases the rocker ratio at low lifts. When I was the editor at Chevy High Performance magazine, we tested a set of these 1.6:1, long-slot Nitro Carb rockers. We found that at 0.100 inch of lobe lift the rocker ratio was actually 1.62:1. By 0.300 inch of lobe lift, the ratio had climbed to 1.66:1! That means that at the same lobe lift on the cam, the valve jumps off the seat much more quickly with more lift at the same crank position. Or stated another way, at 0.300-inch lobe lift, the valve lift was 0.018 inch more than the “rated” 1.6: ratio. This also will affect valve position relative to intake closing within the valve opening and closing cycle since this type of rocker will be moving the valve much more quickly toward closing at whatever checking point you choose. Despite all this, the actual valve closing point will not be different regardless of the rocker ratio. What will be affected is the valve lift relative to the piston position with the different rocker ratios.
This was a very long explanation as to why there is no real mathematical way to determine actual valve closing point from your cam numbers because the advertised duration assumes a cam lift of 0.006 inch, which at a 1.5:1 rocker ratio means a valve lift of 0.009 inch. You could start there and see where the valve is in relation to the cam card numbers. But a true intake closing number will be the point where the valve actually connects to the seat. You’ll want your actual valvesprings in place for checking this because there is bound to be a small amount of deflection involved with all this lever action. If you really want to get crazy, you might want to check intake closing on all eight intake valves. I wouldn’t be surprised if the numbers vary 2 or 3 degrees between all eight cylinders—maybe more—I’ve never checked it. If the closing point scatter between the cylinders is more than 2 or 3 degrees, it could be variable rocker ratios are the culprit–assuming the cam is accurate. Who knows, you might find some power hidden in all this research. If nothing else, your high school geometry teacher should be pleased that you’ve finally found a practical application for all that material he tried to teach you.
Comp Cams; Memphis, TN; 800/999-0853; CompCams.com
Crane Cams; Daytona Beach, FL; 866/388-5120; CraneCams.com
The 3⁄8-inch pipe plug can be accessed once the flexplate or flywheel is removed and then
Pontiac Oil Pressure Problems
Brian Matte; Milwaukee, WI: I have a street/strip ’69 GTO that has low oil pressure when the engine is up to normal operating temperature (180-185 degrees) when idling. The cold oil pressure is 30-35 psi, then down to 8-9 psi at 180 degrees. I have tried three different oil pressure gauges, all reading the same. The engine is a ’69 Pontiac block that is stroked with an Eagle crank, steel H-beam rods, and forged JE dished pistons (9.5:1) to 468 ci with Federal-Mogul race bearings. The oil pump is a Melling M54DS with a welded-on Melling pickup set up to 3⁄8 inch clearance from the bottom of the stock Pontiac oil pan. The engine has maybe 200 miles on the new rebuild. I changed the conventional oil that I’m using from 10W-30 to 10W-40 to now straight 40 with Prolong in fear I’m going to ruin the crank again. Changing the oil has only raised the cold pressure a few pounds to what it is now. I’ve also tried shimming the oil pump spring 0.050 inch with no luck. I have a solid roller cam with restrictors in the lifters, aluminum Kauffman D port heads, a Victor intake, and a Holley 850 carb. The engine does not leak oil, and the main bearing clearances are 0.0024 to 0.0026 inch, while the rods are all 0.0019 to 0.0020 inch. The rod side clearances are between 0.024 and 0.026 inch. I only use 93-octane pump gas. The car is 3,500 pounds but runs hard, clocking an 11.51 at 115 mph on the motor in the quarter-mile. I also sprayed it for fun with a 100hp shot, and I was rewarded with a 10.84 at 125.
Jeff Smith: We talked to our Pontiac engine builder buddy Ken Crocie at H.O. Enterprises, and he clued us in on what Crocie says is a common Pontiac problem. There is an internal oil passage plug located at the rear of the engine that can be accessed (the good news) without having to disassemble the engine. But (here’s the not so bad news) you will either have to pull the engine out of the car or at least remove the trans. Yanking the trans might be easier, but that will depend on how easy it is to access the plug. Once you have the engine out, unbolt the flexplate/flywheel and remove the pressed-in 15⁄16-inch plug located on the passenger side of the camshaft. This will allow access to the 3⁄8-inch pipe thread gallery plug that is missing, but you will also have to remove the distributor to see the plug, or in your case the hole where the plug should go. This plug is usually removed by machine shops to ensure the block is adequately cleaned before machine work starts. It’s a common oversight to leave this plug out, which creates a massive internal oil leak. That you have some pressure once the engine warms up indicates that the oil pump has enough capacity to create a small amount of pressure despite the large internal oil leak. That means that once the leak is cured, the engine will make a lot more pressure.
This photo shows the relationship of the distributor gear to the plug.
The repair is simple. All you have to do is install the pipe plug, press a new 15⁄16-inch plug in the block, and you’re ready to go. We’d suggest leaving the distributor out and pressure-lubing the engine with a 1⁄2-inch drill motor to double-check oil pressure before you button everything back up. You should be able to generate 50 to 70 psi with the drill motor with cold oil, which will indicate that you have solved the problem. If the engine is out of the car, you might consider removing that 0.050-inch shim in the oil pump, as it will not be necessary now. We’d also suggest returning to the 10W-30 oil to reduce oil pressure. Even large, 3-inch main journals used on Pontiac and Oldsmobile engines do not require oil pressure in excess of 60 to 65 psi. Any pressure higher than that is merely greater load on the oil pump and camshaft distributor gears, which will be manifested in greater wear. If the oil pressure is higher than 70 psi with the oil at an operating temperature of around 210 to 220 degrees F, consider changing to thinner-viscosity 5W-20 oil. It will lower oil pressure with no sacrifice in lubrication protection.
Another related point concerns wide rod side clearances that are attributed to loss of oil pressure and excessive windage. The problem with this assumption is that the numbers don’t add up. Let’s assume a rod bearing clearance of 0.0025 inch on a rod journal of 2.5 inches in diameter. With a 0.025-inch rod side clearance (which is often considered excessive), if we do an area calculation of the circumference of the rod bearing clearance times the bearing clearance times Pi and compare that with the area calculation of the same circumference times the side clearance (we’ll spare you the boring math), the rod side clearance area ends up 10 times more than the rod bearing clearance (this is calculating one side of the rod with all the clearance on that side). So the rod bearing clearance determines the amount of oil flow (since it is the restriction) which means it should be obvious that the rod side clearance does not contribute to any additional oil flow from the bearing. You can use this little tech tidbit the next time some engine builder “expert” espouses on the evils of excessive rod side clearance.
H.O. Enterprises; Rancho Cucamonga, CA; 909/980-1451
The Cleveland Stall
Michael Roach; Fairfield, CT: First, Car Craft rocks. Here’s my problem. I bought a ’71 Mustang Mach I with a 351 Cleveland, a 650 Holley carburetor, and an automatic trans. The car’s in decent shape but it stalls out, especially when slowing down at a light, backing up, or anytime the engine is below 1,500 rpm. The guy who solid it to me said the cam is not stock. I’ve changed the plugs and wires, but so far no change. I’m not a mechanic and I’m a little short on cash, so if you can offer some advice I’d be eternally grateful.
Jeff Smith: Driveability problems are some of the most frustrating to diagnose, but thankfully we can offer a whole slew of suggestions to ensure decent throttle response. First we’re going to assume the engine is in good shape mechanically and that there’s probably only a few minor tweaks to perform. Most driveability issues are related to vacuum leaks, or more accurately, unmetered air entering the intake manifold. We’ll start with some simple diagnostic checks first to eliminate mechanical problems with the engine and then home in on the repairs.
First, let’s make sure the engine is in good shape. Borrow a compression tester and have a friend help you by warming the engine first, then yank all the spark plugs and disable the ignition by removing the positive lead to the coil so the spark plug wires won’t arc to ground. Using a compression tester, measure the compression in each cylinder and record your numbers. Make sure the throttle is open and crank the engine for each cylinder a minimum of three pumps against the gauge. Now compare the numbers from each cylinder; they should be within 10 to 15 percent. Assuming this is true, we now know the engine is in pretty good shape. Next, replace the spark plugs, enable the ignition, and fire the engine to check initial ignition timing. Be sure to remove and plug the vacuum-advance line from the carb to the distributor. At idle, you should have around 6 to 8 degrees of initial timing. If the number is less than 6 degrees, loosen the distributor hold-down bolt and twist the distributor to advance the timing. You might try 10 to 12 degrees initial timing. Now that we have the initial timing set, reconnect the vacuum advance line to the distributor.
Before we start this next test, borrow a vacuum gauge from a friend. Start the engine and read the vacuum in inches. Be sure to connect the gauge to manifold vacuum. The gauge should read between 12 and 18 inches of vacuum, but it can be more or less depending upon the camshaft. More than likely, the gauge needle will not be steady, but instead fluctuate between high and low readings. The more the needle moves, or hunts, the worse the idle quality. Now let’s try adjusting the idle mixture screws. On a 650-cfm Holley, these will generally be found in the primary metering block. With the engine shut off, first turn each idle mixture screw in until it lightly seats and count the turns to that point. Each idle mixture screw should be turned out roughly 1 1⁄2 turns. If not, you should set both idle mixture screws at that adjustment and see if the vacuum reading improves. If it does, turn both idle mixture screws in roughly 1⁄8 turn and reference the idle vacuum. If it improves, continue adjusting the idle mixture screws in this direction with 1⁄8 turns until the idle vacuum stops improving. If the vacuum drops on your first move inward (lean), then turn both screws outward 1⁄8 turn and run through a similar tuning operation.
If the idle does not stabilize or improve or it seems that the engine is not responding consistently to these adjustments, then the engine probably is suffering from a vacuum leak somewhere in the induction path. Start with a can of WD-40 or carb cleaner equipped with one of the red spray nozzles attached. With the engine running, squirt around the base of the intake manifold and watch for an instant increase in idle speed. If you discover an area around the intake manifold base, first try tightening the intake manifold bolts. If that doesn’t help, then it appears there is an intake manifold gasket vacuum leak, which is fairly common. This will require draining the coolant from the block, removing all the intake bolts, and prying off the intake. Luckily with a Ford, you don’t have to remove the distributor.
When installing a new intake gasket, be sure to use a dedicated gasket cement like Gasgacinch or that nasty dark brown gasket glue you can find in a bottle at the auto parts store. Don’t use RTV silicone around the intake ports because gasoline over time will dissolve the RTV and cause another leak. Make sure both the gasket surface on the heads and the intake are as clean as you can make them and carefully mate the manifold to the head and start all the bolts before beginning the torquing procedure. That should solve your driveability problems. If you continue to have problems, consider having the carburetor rebuilt, as it could be suffering from partially blocked idle passages or other common fuel delivery maladies.
The New Math
Bruce Baxter; Missoula, MT: A fellow is trying to sell me a Gen I small-block Chevy that he says is 427 ci. Is that possible? Maybe our definitions of stroke are different. When I measure my 383, I get 3.75 inches stroke from top of piston at TDC to top of piston at BDC. His same measurements are 3.52 inches, so I told him he has a 357. He claims I should measure from the top of the piston at TDC to the wristpin at BDC. His math looks like this: a ‘69 350 block, 0.040-inch over, decked 0.100 inch, offset-ground GM crank with a 4.00-inch stroke, 6.00-inch V6 rods, flat-top pistons, and shaved double-hump heads. He says it is a 3.52-inch piston stroke, plus 0.100 decking, plus 0.250-inch to tall piston wristpin makes 3.87-inch stroke. How far can we safely deck a Gen I SBC block? Who is correct here?
Jeff Smith: I’ve heard of creative math before, but this is particularly innovative—flat out wrong, but innovative. The simple truth is that displacement is determined by measuring the bore of the cylinder along with the stroke, which is easily determined just as you said by measuring how far the piston travels from bottom dead center (BDC) to top dead center (TDC). All that other stuff about compression height has no bearing on displacement. We’ll even give you a simple formula for determining displacement in cubic inches: bore x bore x stroke x 0.7854 x number of cylinders. One way to remember that oddball 0.7854 number is that these four numerals are located in the upper lefthand quadrant of most numerical keypads. Start with the number 7 and work clockwise. Using the measurements of your buddy’s engine we get: 4.04 x 4.04 x 3.52 x 0.7854 x 8 = 360.9 ci or a 361ci small-block. It’s as simple as that. Everything else is just horse hockey.
It appears that he may have confused the calculation necessary to determine proper piston compression height when mixing and matching different crankshaft stroke and connecting rod lengths, since the definition of compression height is the distance from the wristpin centerline to the flat portion of the piston. This distance is used to determine on paper whether a given connecting rod length, stroke, and piston compression height will work in your engine. As an example, the engine in question has a 6-inch connecting rod length and a stroke of 3.52 inches. The formula for engine deck height is: rod length + piston compression height + 1⁄2 stroke = combined height. This can then be compared with the block’s actual deck height to make sure the piston does not protrude from the top of the deck (unless that’s what you desire) or that it falls way short of the actual block’s deck height. Let’s say the engine is assembled, and we don’t want to remove the piston to measure its compression height. The first thing to do is measure the deck height of the piston in the cylinder. Let’s say we measure this engine at a 0.015-inch height below the block deck surface, if we assume our block is at the standard small-block Chevy deck height of 9.025 inches. To get an idea of the compression height, subtract the rod length, half the stroke, and the amount the piston is short of the deck from the normal deck height: 9.025 – (6.0 + 1.76 + 0.015) = 1.25 inches. The one thing we have to assume here is the block deck height. It may not actually be 9.025 inches. The only way to know for sure is to completely disassemble the engine and then measure from the crankshaft centerline to the block deck. Usually too, you quickly learn that the block is not consistent and not square to the crank centerline, but we’ll save that discussion for a later date.
On a production small-block Chevy, I would hesitate to deck a block more than 0.020 to 0.025 inch. Typically, a production small-block Chevy will measure the piston-to-deck clearance at around 0.020 to 0.025 inch in the hole. So it’s quite common for machine shops to machine the block to an even 9.00 inches of deck height. You can take a block farther than that, but it begins to drastically reduce the block deck thickness, which will reduce its ability to seal combustion pressure with the head gasket. You can use the above math to determine if the deck has been machined, but I would be highly suspect of an engine that has been decked 0.100 inch if that is indeed an accurate number. A stock small-block deck thickness is only around 0.375 at best, so removing almost 1⁄8 inch would be excessive. That would remove almost a quarter of the deck thickness. This is clearly an issue because Dart builds all its small-block iron castings with 0.500-inch deck thicknesses, as deck strength is a critical component of sealing the head and block from cylinder pressure. If the block has been decked 0.100 inch, that’s a great reason to avoid this engine.
Dart Machinery; Troy, MI; 248/ 362-1188; DartHeads.com
A Supercharger and Overdrive
Tim Graves; via Bucks County, PA: I’m looking for a supercharger and a TH350 with Overdrive for my daily-driven Cutlass. So far I have a rebuilt ’75 Olds 455. With 0.030-over pistons, a ported Port-O-Sonic manifold, ported Edelbrock heads, a 0.544-inch lift cam, a 750-cfm street D carb, and a serpentine accessory drive kit. The exhaust consists of Hooker Super Comp headers and a 3-inch exhaust through Hedman cross-pipes that leads to a pair of 3-inch, 50-series Flowmasters. I run a 2,800- to 3,200-rpm stall converter with the TH350 leading to a 12-bolt 3.73:1 Chevy posi rear. The biggest, widest BFGoodrich drag radials I can fit are approximately 28 inches tall. The front suspension has QA1 coilovers along with tubular upper and lower front control arms and Edelbrock rear shocks.
I cannot find any superchargers for an Olds 455. I’m also looking for an overdrive for my TH350 because I max out the rpm before I get to the end of Atco’s quarter-mile dragstrip. Best run a few years ago was 13.42 spinning the tires off the line, with no Second gear, and still had to let off the throttle before crossing the lights at the end of the track. I’ve been debating changing the 3:73:1 to a 3:42:1 or just getting a new trans with overdrive. I cannot fit any tire bigger than what is in there now. One last thing, I also have hand controls, because I’m a paraplegic. Where can I find more power for my Cutlass? I don’t want nitrous or turbos.
Jeff Smith: While Oldsmobile engines are well known for their torque, I’ve always thought a centrifugal supercharger package for an Olds would make major power (if you could hook it to the ground) and you wouldn’t have to spin the engine really hard to get there. Plus, the centrifugal supercharger would be hidden underneath a stock or near-stock hood. Alas, no one as yet builds the mount assembly for a Procharger F2 or Vortech centrifugal, so I went looking at more conventional 6-71- or 8-71-style Roots superchargers and a couple of companies that offer impressive aluminum lungs for a 455 Olds.
The first company I tried is perhaps the best known—Blower Drive Service (BDS) in Whittier, California. BDS offers a manifold, a drive kit, and an 8-71 supercharger in pieces that quickly added up to more than $5,200, and that’s before we talk about a pair of carburetors, linkage, air cleaners, scoops, and ignition. So we’re talking some serious coin here. I also found a company called Superchargers USA located in La Habra, California, that sells a complete Olds 455 kit for a 6-71 supercharger with an 8mm drivebelt kit for around $4,400, but again, you’re going to need the aforementioned accessories. You might consider a main web support girdle if you lean on this thing, and I wouldn’t recommend any kind of power-adder unless the engine has forged pistons. If you have hypereutectic or cast pistons, you’re just asking
for trouble. Blowers make heat, heat makes pressure, but heat also makes the engine more detonation prone, and detonation will turn cast pistons into piles of scrap in milliseconds. Ask me how I know.
Also, I’m surprised that a TH350 trans has survived behind your 455 for any length of time. Generally the standard trans is the TH400. You might consider stepping up to the larger TH400 trans for durability. Of course, if you’re going to buy a transmission, you might consider a 4L80E. It is a late-model electronic version of the TH400 with the same First-Second-Third gear ratios as the TH400 (2.48:1, 1.48:1, 1.00:1) but with an added overdrive of 0.75:1. The only problem with these transmissions is they are much heavier than typical TH400s, but worse, a new one will run more than $2,000 with a converter and you still have to purchase a separate controller that generally costs anywhere from $500 to almost $1,000. These controllers allow you to easily manipulate up- and down-shift points, shift firmness, line pressure, and several other parameters, but at the aforementioned cost. Most of these units are controlled via laptop computer software, which, while not difficult to learn, may be more technically challenging for some. You will also need a throttle position sensor to make most of these systems function.
Another option that might be best is the Gear Vendors overdrive unit. Gear Vendors has been making a bolt-on extension for many automatic and manual transmissions that incorporates a very rugged overdrive planetary system that will produce a 22 percent (0.78:1) overdrive in high gear. This virtually duplicates the 4L80E overdrive but does not require a black box/computer to operate. The biggest hassle is fitting the 12 1⁄4-inch-long and 7-inch-tall overdrive unit underneath the floor pan of your ’75 Olds. The good news is that the later-model cars added more elbow room for the TH400 compared with the ’64 through ’67 A-body cars, so the overdrive should fit without too much difficulty. Even if you left the 3.73:1 rear gears in the car, the overdrive would produce an equivalent final drive ratio of 2.90:1. Frankly, most mild Olds and Pontiac engines don’t respond well to deep gears because these engines like a taller gear such as the 3.42 or even a 3.31:1 gearset if you could find one. Combining the 3.42:1 ratio with overdrive becomes the equivalent of a 2.66:1 rear gear, which might be close to ideal. With a 26-inch-tall rear tire, cruising rpm in overdrive would be a little more than 2,400 rpm. That’s nice because you could just roll in on the throttle at that speed and that big Olds would pull forever. Consider changing the rear gear regardless of whether you go with an overdrive. Those big Olds motors use torque to accelerate the car, and a deeper gear adds leverage that isn’t necessary. Small-block Chevys need gear to get them moving; big-block Olds motors don’t need the extra leverage.
Gear Vendors; El Cajon, CA; 800/999-9555; GearVendors.com
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