Originally Posted by Hubba_Bubba
Hey Guys! I am brand spanking new to the forum here and hoping to gain a plethora of knowledge. I am looking to build my first SBC and rearin' to make a nasty 383, hopping for the 450-500hp range with a budget of $3000-4000. What better motor to build right! I have been reading as much as possible, but still have a few questions I cant seem to get answered. I am looking at purchasing a rotating assembly, or possibly piecing it all together. I know SCAT is not the highest quality on the market, but I have no intentions with a blower or any giggle gas and figure it will hold up if everything is properly pieced together. My question is how much does crank and connecting rod weight affect a motors performance? For instance, I am looking at a Scat Series 9000 Cast Pro Comp Stroker Lightweight Crank that weights 42 lbs compared to the stock crank at 50ish lbs. Yes their website says 51lbs, but that is the shipping weight. My buddy bought one of these and weighed it on a scale without the packaging. Now for the connecting rods. Should I run H-Beam or I-Beam? Also curious as to what grams i should stay with. What would the difference between a 500gram connecting rod and a 700gram connecting rod? I realize heavy things rob horsepower, but lightweight stuff also goes SNAP. Extremely wordy, but in short how much does the weight of the crank and connecting rods affect HP and "REV", for lack of experience, of a motor.
We've used SCAT for years without problems, they have a broad range of quality parts that range in price from moderate to high depending upon the expected usage.
While the rods are the highest loaded part in the engine and most likely to fail, if you're going into the build with a nodular crank (what the advertising guys like to call cast steel) so there isn't any reason to get crazy on the rod. Yes to a point a heavier rod is stronger, but it also because of its weight generates higher loads so the crank needs to be stronger as well. There are also light weight rods out there for sportsman and claimer engines that cut back on the weight to reduce cost, but one has to remember that race engines see a lot of inspection that a street driver does not, so they have an advantage of heading off problems before they make a disaster or at least that's the plan. A heavy rod also causes balance problems in that if the rod and piston combination exceeds the weight of the counterweights then high density very heavy metal called Mallory metal has to be added into holes drilled into the counterweights. This is mighty expensive! So you want to stay at or a little under the factory component weight if possible. A forged crankshaft has more weight in the counterweights simply because forgings are more dense than castings so this gives some fudge factor on heavier rods and pistons. But for a cast crank, you want to stay close to the OEM part weights or you'll incur the cost of installing Mallory metal and after a little of that you'll realize for that cost you could have started with a forged crank.
The SBC has rod failure problems at elevated RPMs, the cheap solution is better rod bolts but the problem's root casue is bending of the rod cap which is resisted by the bolts. When the forces combine of tension to keep the rod cap on (it's first job) are added to resisting the deforamtion of the interface surface that is also bending the rod bolts (not desireable), then they fail. So while stronger rod bolts resist the bending load and delay failure the real cause is the bending from the cap deforming. So a better cap design, a better interface with the shank, and a stronger material for the rod are appropiate solutions. The factory rod is forged from 1030/1040 mild steel, or of couse the new ones are powered metal. The aftermarket cheepies start at 4130 or 5140 alloy steel which quickly gives way to a plethora of 4340 rods that run from reasonably to extremely expensive. SCAT makes a couple very nice rods for low end cost engines out of 4340 that feature ARP cap screws instead of a bolt and nut along with a doweled connection between the cap and shank and have an enlarged interface surface to react cap bending loads over more area which serves to a considerable extent to protect the bolt from these bending moments. These same similar rods are also available from Eagle and other brands, I'm not here to sell rods from anyone, only to point out the enginerring features you should be looking for. I'll also say that when you buy parts you must measure every dimension, that includes dissassembling oil pumps to review what's inside. You could think of this as engine assemblers just bolt parts together then wonder why they get failures, engine builders leave nothing to chance, they inspect and correct everything. The rods I'm refering to are these:
<<< Scat Crankshafts
>>> This is an excelent street /strip /claimer rod. It will usually provide clearance for stroker small blocks, if you need to dress it for cam clearance, it will at least not be on a bolt head.
<<< Scat Crankshafts
>>> This is very good strip/competition rod that won't bust the budget. It also provides the clearances for stroker SBC's. This rod configurationally is very similar to a CompStar and an Oliver piece that are made from super E4340 milled all over and cost 4 times as much, these are true NASCAR Cup level rods and cost like it
You will note the features I said to look for are present on both the linked rods of excellent ARP bolts, an enlarged interface for more support area, high alloy steel that is less likely to deform under loads, and in both of these dowels to locate the cap and help carry bending loads at the interface keeping them from the bolts; plus they are shot peened, this increases fatigue resistance by 50% or better compared to the same rod without shot peening. For rods; if it doesn't say "shot peened", just stay away from the part.
I beams or H beams either are suitable. The H beam is more tolerant of off center loads that result from wrist pin bending, and/or the piston deforming as a result of super high blower pressures, or the effects of high volume fuels like nitrous, methanol, or nitromethane and/or heavy detonation for long time periods. This is stuff a street/strip engine isn't likely to ever encounter, so an I beam is plenty adaquate and then some for street strip or claimer competition engine. You may see rods similar to these I referenced for a bit less money made from 4130 or 5140 forgings, these are also pretty darn good for a street/strip/claimer engine that runs under 6200 RPM and develops about 400-450 hp. The 41 and 51 series steels are a little given to work hardening so you really want these guys only in the shot peened condition and if you do any machine work on them that area needs to be re-shot peened. They hold up well after this treatment, otherwise treat them like a factory mild steel rod, OK but don't get crazy on 'em with lots of revs and power.