This iteration of “Ask The Experts” revolves around rotating assemblies, as you, the readers, submitted a wave of questions about all things crankshaft, connecting rod, and even piston-related for Scat Crankshafts’ Tom Lieb. Lieb reached back into his deep well of expertise in this field and answered the best 10 of those questions.
One thing Lieb was sure to mention in our conversations was that some of these questions ask for an opinion, and as he says, sometimes there’s more than one way to skin a cat. He’s been doing this for a long, long time, so we tend to value his opinion, as do you guys, as evidenced by the response we got.
Remember, for those of you whose questions we didn’t get to, Scat offers a host of tech resources, which can be found here, or you can call them at (310) 370-5501 or email the tech center at [email protected] Now, without further ado, we bring you the Q&A with Tom Lieb.
1) What are the unique requirements of the crank and rods when turbocharging a 1,000-horsepower street engine turning no more than 6,500 rpm. I am building a twin-turbo ’70 Boss 351C, and debating how much stroke to use, 3.750, 3.850, or 4.000. I still want decent gas mileage on long highway runs but need the torque to pull the twisty mountain roads. How much strength do you need in this application; cast, forged, or billet crank? Also, some say don’t use H-Beam rods, only use I-beam rods; which rods should be used?
Tom Lieb: David, for your application, a forged crank is a must, but a billet is “overkill.” There are various I-beam rod designs on the market, some of which can be marginal for strength. You can’t go wrong with an H-beam rod. Turbocharging is a substitute for cubic inches and for less oxygen available in mountain driving. The challenge is to get a piston, combustion chamber size and stroke combo to obtain a low-enough compression ratio to support your street application.
2) My question relates to the stroke length of modified LS engines. It seems that the most popular crankshafts listed as available (for stroker engines) are typically the 4.000-inch units. This seems true of both rotating assemblies and individual crankshafts. Yet I read most recently that the optimum crankshaft stroke that offers both performance, but also long-term reliability would not exceed 3.900-inches, due to the fact that the piston would not extend out of the bore. So, my question is, if this is the case then why don’t I see any offerings from any of the various manufacturers for crankshafts and rotating assemblies in the aforementioned 3.900-inch stroke?
Tom Lieb: Mike, stroke is half up and half down. Differences in 3.900 vs. 4.000 inches is 0.050 inch out of the top and 0.050 out of the bottom (more than 3.900-inch stroke.) Piston manufacturers and crank guys work together to furnish a combo of parts. Piston skirts that extend out the bottom of the cylinder–as in an LS–are designed differently. The piston manufacturers that know how to do this with crank guys like SCAT have the 4.000 stroke worked out to be very reliable. So why wouldn’t you run the 4.00-inch stroke?
3) I see so many stroker engines and kits for big-block amounts of displacement from small-block engines. I am concerned about the rod length to stroke ratios, piston acceleration and thrust angles. What do you consider good and what do you consider safe ratios and how much does the thrust angle and piston acceleration affect reliability?
– William Sargent
Tom Lieb: William, what you are seeing is a surface geometry problem. You have a triangle. One leg of triangle is half-stroke, and the hypotenuse of the triangle is rod length. The third angle of the triangle is crank rotation. Angle of load on the cylinder wall is what you are looking for, and piston position is the third leg of triangle you are looking for. Do the math on all the different combos that produce power and you will see ratios are interesting, but cannot be used as “gospel.”
4) I installed a Scat 383 internally balanced forged stroker kit in a first-gen SBC 350 four-bolt main block a while back. Sometime later, I was told by a keyboard cowboy on one of the car forums that the Scat crank required the use of a specific harmonic balancer which I was not using. My engine builder never said anything and I’m curious if there is in fact any problem running a Fluidamper harmonic balancer with a Scat 383 forged crank. If so, what is the preferred balancer?
– Steve Hayes
Tom Lieb: Steve, SCAT cranks do not require a specific harmonic balancer. Like all cranks they require a balancer that is properly designed for a performance engine. 90-percent of crank breakage in the front half of the engine is due to a balancer that is too large in diameter, too heavy, has moving parts, and cannot be balanced with the crankshaft. Moving parts include both fluid and mechanical pieces. These dampers cannot react fast enough to rapid acceleration and deceleration. Therefore, they create harmonics that will eventually fatigue a crank and break it. These dampers were designed for industrial applications that run constant speed in narrow RPM ranges. So, that being said, the smallest diameter, lightest weight, and an elastomer design is best.
5) Which is more effective or provides the biggest power gains? Knife-edging the crankshaft or a good oil scraper in the pan?
– Don C.
Tom Lieb: Don, this is not about power gain, it is about power loss. You must run a vacuum in the crankcase and you must have sufficient space to transfer the air mass between cylinder banks as the engine rotates. Looking at the nose of the crank, oil comes off of the crank at 6 o’clock to 9 o’clock as it rotates. Proper oil pan design (with a scraper, and windage tray, etc.) and knife edged crank both help this process.
6) I have a stroker kit with a 3.750-inch stroke and 6.000-inch 4340 H-beam rods. I recently changed rod bolts to 7/16-inch L19 bolts from the 8670 steel bolts that came in the kit. Do I need to rebalance the rotating assembly?
– Magne Aasheim
Tom Lieb: Magne, the bolts weigh the same. The issue is the torque of the rod bolt. In the case of the SCAT rods, we recommend torque to tighten bolts, not stretch them. To control roundness of the rod during manufacturing and engine builds this is a must. L19 bolts can be torqued to a higher number then 8740. If you do this you run the risk of the rod going out of round. You must resize the rod if you are going to change the torque to the higher torque spec. You could have bearing failure if you don’t resize the rod’s big end.
7) Can you tell me the RPM limit on the Scat 9000 windage-cut crankshaft for a Chevy 4.030-inch bore x 3.750-inch stoke with 6.000-inch rods? Which of those specs is the major determining factor in how high I can spin the engine? Thank you!
– Willie J. Price
Tom Lieb: Willie, a 9000-series crank is a casting and has one-third less strength and stiffness than a forging. Type, size and weight of damper and flywheel, clutch, torque converter, etc. all play into how high you can spin the engine. Typically, we’d say 5,500 to 6,500 rpm is the limit based on the specifics of the engine build.
8) With Ford putting out the 5.2-liter using a flat-plane crank, it brought back some questions that I’ve had for quite some time. I’ve been unsuccessful in finding technical data concerning the flat-plane V8 engine and I’m hoping you can shed some light and/or point me to any articles that would help me. My focus is with the small-block Chevrolet. Are there any inherent limitations for cubic-inches, stroke length, or connecting rod lengh? How will the torque curve be affected compared to a similar sized bore and stroke with a cross-plane engine? What are the pros and cons of converting to a flat-plane crank?
– Bill Murphy
Tom Lieb: Bill, if you check the firing order on one bank of a 90-degree crank you will see that it is 90 degrees to 180 degrees to 270 degrees, and 180 degrees back to start over. This makes the myth of equal-length headers useless. 180 degree cranks have a firing order per bank 180 degree – 180 degree – 180 degree – 180 degree. Equal-length headers now allow for tuning of intake and exhaust to be a reality. These engines have a higher and flatter torque curve. SCAT has done a lot of these cranks over the years. Longer strokes pose a balance issue with the flat-plane cranks, so most engines are less than 350 cubic inches.
9) What is the correct way to grind and polish a crankshaft that has been repaired, and what can be the consequences of an improperly ground or simply non-polished crankshaft? How critical is the quality of the finish on the bearing journals?
– Chiel van de Sande
Tom Lieb: Chiel, regrinding a crank is about three factors: size, proper radii, and surface finish. The polish is done in the opposite direction of the grind in order to remove any micro points left from grinding. You want to shoot for a micro finish of Ra 5 to Ra 8. If the surface finish and proper radii aren’t maintained, bearing failure or even crank breakage can result.
10) This past week I have been considering a new build for my ’67 El Camino. I wanted to go no bigger than 598 cubic inches. I am trying to achieve a motor that revs freely, and it is my opinion that a rod angle that is not severely pressing into the side of the bore has less friction, causes less piston rock and – in a small-block application at least – allows the piston to linger at TDC a fraction longer, allowing a better cylinder fill. I’m hoping you can help me find a combination that will allow my big-block to rev freely without a severe rod angle and make decent power.
– Steve Solo
Tom Lieb: Steve, refer to my answer to William Sargent as well. The piston is at TDC twice. Once on compression/firing and on overlap. I consider the overlap dwell most detrimental if the piston dwells at the top while the exhaust valve is open, allowing incoming mixture to flow out the exhaust port and not into the cylinder. The “power loss” is basically because the piston is in the way. Using the formula for rod length vs. stroke vs. rod angle for piston position during rotation, you can match the cam profile and cylinder head flow to the piston position for max power. I’m not sure why you are stuck on a maximum of 598 cubic inches. You need to do the math around your possible choices of crank stroke, rod length, cam profiles and cylinder head flow, and determine what the combinations like.