383/388 Chevy stroker
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==Overview== | ==Overview== | ||
| − | A 383 is a 350 block bored +0.030" with a 3.750" crank. A 388 is a 350 block bored +0.060" with a 3.750" crank. The 383/388 SBC stroker has become one of the most popular engines in the history of the small block Chevy. The only engine that surpasses it in volume is the 350 | + | A 383 is a 350 block bored +0.030" with a 3.750" crank. A 388 is a 350 block bored +0.060" with a 3.750" crank. The 383/388 SBC stroker has become one of the most popular engines in the history of the small block Chevy. The only engine that surpasses it in volume is the production 305 and 350 SBC. |
The following article will help the new engine builder to better understand the details involved in building this powerplant. | The following article will help the new engine builder to better understand the details involved in building this powerplant. | ||
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In a running engine, the oil clearance will create a slightly longer stack- a 0.003" rod bearing oil clearance will add something slightly less than 0.003". In this article, oil clearance will NOT be added into the stack height. If desired the oil clearance may be added; easiest way to do this would be to either add the oil clearance to the rod length, or simpler yet, just add the oil clearance after the stack height is calculated. The added height from the oil clearance would only be an issue if the engine is being built with a marginal amount of quench (<0.035" for steel rods); if built with the "ideal" 0.040" quench, the oil clearance can be basically ignored. | In a running engine, the oil clearance will create a slightly longer stack- a 0.003" rod bearing oil clearance will add something slightly less than 0.003". In this article, oil clearance will NOT be added into the stack height. If desired the oil clearance may be added; easiest way to do this would be to either add the oil clearance to the rod length, or simpler yet, just add the oil clearance after the stack height is calculated. The added height from the oil clearance would only be an issue if the engine is being built with a marginal amount of quench (<0.035" for steel rods); if built with the "ideal" 0.040" quench, the oil clearance can be basically ignored. | ||
| + | ===Piston rock=== | ||
Another consideration is piston "rock". At TDC as the piston transitions from upward to downward movement, the piston will tip on its wrist pin. This causes one edge of the piston to be a small amount higher than the other edge. The exact amount will vary with how much piston to wall clearance there is; more clearance means more piston rock. Forged pistons generally have a looser piston to wall clearance than cast pistons, but newer design forged pistons have tighter clearances than was used in days gone by. This is another thing that's basically accounted for if a 0.040" quench distance is maintained. Only if less than 0.035" would this possible be an issue. | Another consideration is piston "rock". At TDC as the piston transitions from upward to downward movement, the piston will tip on its wrist pin. This causes one edge of the piston to be a small amount higher than the other edge. The exact amount will vary with how much piston to wall clearance there is; more clearance means more piston rock. Forged pistons generally have a looser piston to wall clearance than cast pistons, but newer design forged pistons have tighter clearances than was used in days gone by. This is another thing that's basically accounted for if a 0.040" quench distance is maintained. Only if less than 0.035" would this possible be an issue. | ||
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==Stroke/rod/piston combos== | ==Stroke/rod/piston combos== | ||
| − | To stroke the SBC 350 to 383 cid requires a crankshaft having a stroke of 3.750". This can be a production SBC 400 Chevy crankshaft with the main journals turned down from 2.65" to 2.45" to allow the 3.750" stroke crankshaft to fit into the main bearing saddles of a 350 block. Or it can be a specialty aftermarket crankshaft that has been manufactured with the 350 main journal size that will fit into the 350 block with no machine work at all | + | To stroke the SBC 350 to 383 cid requires a 0.030" overbore plus a crankshaft having a stroke of 3.750". This can be a production SBC 400 Chevy crankshaft with the main journals turned down from 2.65" to 2.45" to allow the 3.750" stroke crankshaft to fit into the main bearing saddles of a 350 block. Or it can be a specialty aftermarket crankshaft that has been manufactured with the 350 main journal size that will fit into the 350 block with no machine work at all. |
| − | + | Now, we have a choice to make. The radius of the 3.750" stroke crank is 1.875" (3.750" times .5 = 1.875"). Common sense will tell you that if you retain the 350's 5.7" rod length and 1.560" piston compression height, that the additional length of the radius of the crank will push the piston out of the top of the bore at top dead center (1.875" stroke radius + 5.7" rod length + 1.560" piston compression height = 9.135"). This puts the piston crown 0.110" ''above'' the block deck (remember, an undecked block deck height is ~ 9.025"). This much piston protruding from the bore would prevent the head from being bolted on, so something has to be changed to shorten the stack height so the parts will fit into the block. | |
| − | + | Most often this problem is solved by using a piston having a shorter compression height to compensate for the stroke increase. Using a shorter rod is not the desired way to make the change. Besides the unwanted change in the rod/stroke ratio causing undue side loading of the piston to the cylinder wall, using a shorter rod can also cause the skirt of the piston to strike the counterweights of the crankshaft at bottom dead center. So instead of a shorter rod like the 5.565" factory SBC 400 rod, it is preferred to use at least the 5.7" rod combined with a piston made for this application having a compression height of 1.425". Now, the stack looks like this: 1.875" + 5.7" + 1.425" = 9.00" with a 0.025" piston deck height. This combination has been shown to work very well. | |
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| + | [[File:5.7 vs 6 in. CH.jpg|thumb|470px|right|Piston compression height comparison between 6" (left) and 5.7" rods using a 3.75" stroke]] | ||
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| + | Another option is to use a longer 6.00" rod combined with a piston having a 1.125" compression height. So, 1.875" + 6.000" + 1.125" = 9.000". The piston deck height is still 0.025" with this combination of parts. Using a 6" rod also has advantages if internally balancing is going to be done because there's more room for the crankshaft counterweights. | ||
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| + | Some builders prefer not to use a 6" rod, the thinking being because the wrist pin intrudes into the oil ring groove- meaning pinned rings or support rails must be used to allow the oil control ring package to do its job- isn't worth the possible advantages from a higher rod/stroke ratio and/or lighter pistons, etc. However, Ross offers a 383 stroker piston with a 1.120" compression height that doesn't require rail supports. | ||
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Yet another option, although one of the least popular, is to use the stock SBC 400 rod with a 350 SBC piston. The stack of parts looks like: 1.875" + 5.565" + 1.56 = 9.00". The main complaint about using this combo is the rod length. | Yet another option, although one of the least popular, is to use the stock SBC 400 rod with a 350 SBC piston. The stack of parts looks like: 1.875" + 5.565" + 1.56 = 9.00". The main complaint about using this combo is the rod length. | ||
==Clearances== | ==Clearances== | ||
| + | There should be a minimum of 0.050" clearance between the parts of the reciprocating assembly and anything else if it's a steel rod engine. You can go a little tighter than this between the crankshaft counterweights and the block. | ||
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| + | Generally a SBC 383 stroker will have the chance of hitting metal to metal in a three places: | ||
| + | * Camshaft to connecting rod | ||
| + | * Crankshaft to block | ||
| + | * Pan rail | ||
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| + | ===Camshaft to connecting rod==== | ||
| + | This is the most likely place to have interference. The rods that are most likely to be too close to, or to contact the cam are cylinder numbers 1, 2, 5, and 6. Using a reduced base circle cam will not necessarily help- the clearances have to be physically checked for all the rods. | ||
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| + | It is better to use an aftermarket 'stroker' rod from the get-go instead of having to grind on another type of rod/rod nut to gain enough clearance. Using a capscrew rod will sometimes be enough, but they still must be checked. A long, thick tie wrap can be used as a sort of feeler gauge. If it gets caught while checking, no damage. | ||
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| + | ARP [http://www.summitracing.com/parts/arp-134-6027 part number 134-6027] is for a set of rod bolts with additional clearance at their heads to help cam to rod clearance problems. They are for use with a factory 5.7" rod or the equivalent. | ||
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| + | ===Crankshaft to block=== | ||
| + | The places for clearance problems between the crank and block is at the bottoms of the cylinders and the pan rails. If any metal has to be removed, remove the least amount needed, or else the water jacket could be hit. | ||
| − | + | Same thing here as was said above, regarding stroker rods. They will have a much better chance of having enough clearance without any modifications to them '''or''' the block. The place where the rod hits is the nut (if a nut and bolt rod big end) or the capscrew. Grinding on either can be an iffy proposition, so only remove what's absolutely necessary and not a bit more. If using the stock SBC 400 rod (5.565"), it will not need any clearancing. But 383 stroker cranks that use the 400 rod are not that readily available and the rod length-to-stroke ratio is not favorable. BTW, this has nothing to do with making or not making power. The rod ratio concern in '''this''' case is a matter of frictional losses and side loading caused by a too-short rod. | |
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==A word on rod/stroke ratios== | ==A word on rod/stroke ratios== | ||
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