383/388 Chevy stroker

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All small block Chevy blocks are the same measurement from the centerline of the main bearing journals to the deck where the heads bolt on, ~9.025". When we mix and match components, we have to fit them into this "Block Deck Height" dimension of ~9.025". We would use half the stroke dimension, because only half of it swings toward the block decks. So, with a stock 350 Chevy, with a stroke of 3.480", we would use the radius of the stroke (3.480" times .5 = 1.74") to start putting the stack of parts together that will fit into the block. Next, we would use a standard 5.703" rod length, then a piston with a 1.560" piston compression height (measured from the centerline of the wrist pin to the top of the piston crown). Now, if we add up all the dimensions of our "stack" components, we find that we have room in the block for them, plus a little extra which will be between the crown of the piston and the block deck. 1.740" + 5.703" + 1.560" = 9.003". So, if there is 9.025" available and the stack is only 9.003", that leaves 0.022" space above the piston crown with the piston at top dead center. This space is called the piston deck height (not to be confused with block deck height).
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Now, we decide to stroke the motor, so we install a crankshaft that has a stroke of 3.750". This can be either a 400 Chevy crankshaft with the main journals turned down to a smaller diameter so that the crankshaft will fit into the main bearings of a 350 block or it can be a specialty aftermarket crankshaft that has been manufactured with the 350 main journal size and will drop right into the bearings with no machine work at all. Now, we have a choice to make. The radius of this crank is 1.875" (3.750" times .5 = 1.875"). Common sense will tell you that if you retain the 5.703" rod length and the 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 + 5.703" rod length + 1.560" piston compression height = 9.138"). This sticks the piston out past the block deck (remember, our block deck height is 9.025", so you cannot bolt in a stack of parts that measures 9.138") This prevents the head from being bolted on, so we must make other provisions.
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==Overview==
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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 350/355 SBC.  
  
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Most commonly, we would use a piston with a shorter compression height, since using a shorter rod is not the desired way to make the change. Most of the time, with a shorter rod, the skirt of the piston will strike the counterweights of the crankshaft with the piston at bottom dead center, so we want to use the longest rod we can within reason. We can continue to use the 5.703" rod and use a different piston, 1.425" compression height. Now, our stack looks like this......1.875" + 5.703" + 1.425" = 9.003" with a 0.022" piston deck height. OK, that works.  
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The following article will help the new engine builder to better understand the details involved in building this powerplant.
  
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Another way to do it is with a longer rod yet, a 6.000" rod length and a shorter yet piston, one with a 1.125" compression height. So, 1.875" + 6.000" + 1.125" = 9.000". The piston deck height becomes 0.025" with this stack of parts. I personally do not like the 6.000" rod in the 383 build because it puts the wrist pin up into the oil ring groove in the piston. Then you have to use a filler ring to make a platform for the oil ring rail to ride on and it just adds more parts to go wrong in the perplexity of an engine build. Many fellows have done the 6.000" rod stroker motors and have run them without a hitch. I just personally do not like them. For me, a 5.703" rod and a 1.425" piston works just fine. I will agree however, that there is more clearance between the skirts of the pistons and the counterweights of the crankshaft with the 6.000" rod. If you are building an internally-balanced motor, you need all the counterweight you can get, plus maybe some Mallory Metal, so a 6.000" rod begins to make sense. The least expensive way would be to build an externally-balanced motor using a 400 damper/balancer and a 400 flexplate/flywheel.  
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==SBC dimensions==
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===Block deck height===
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All small block Chevy blocks are manufactured with the same measurement from the centerline of the main bearing bores to the deck where the cylinder heads bolt on. This measures 9.025" nominal. A nominal dimension is the targeted dimension, the actual dimension may vary a small amount from this, say +/- 0.005". This 9.025" dimension is called the '''''block deck height''''' (or just "deck height").  
  
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One of the most commonly used combinations used for a 383/388 stroker is the Scat cast steel crankshaft and Scat I-beam forged Pro Stock rods. The counterweights will clear the pistons using a 5.703" rod and the rods will clear the cam, so no grinding on the rods for cam clearance will be necessary. You may need to do a little grinding on the inside of the block at the pan rail to clear the big end of the rod, but it will be minimal. 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.  
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==Parts stack height==
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When the various parts that make up the reciprocating assembly are selected, these parts have to fit into the SBC deck height dimension of ~9.025". When calculating the stack of parts that make up the reciprocating assembly, use the radius of the stroke, which is the same thing as 1/2 of the stroke, because only 1/2 of the crankshaft stroke swings ''above'' the crankshaft centerline. So for a stock 350 Chevy having a stroke of 3.48", use 1/2 of the stroke (3.480" times .5 = 1.74") to start putting the stack of parts together that will fit into the block.  
  
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It is commonplace for us hot rodders to cut the block decks to zero piston deck height and use a head gasket that compresses to around 0.040". This allows a squish of 0.040". Squish is the high speed jetting of fuel air mixture from the dead zone opposite the combustion chamber. When the piston approaches top dead center, the clearance between the crown of the piston and the underside of the cylinder head diminishes to just short of a collision. This squeezes or "squishes" the mixture that is there, across the cylinder toward the spark plug. This high-speed jetting of the mixture not only eliminates any dead spots in the chamber, but also creates turbulence to accomplish complete mixing of the fuel/air mixture so that there are no lean or rich areas in the chamber. Having the tight squish (0.035" to 0.045") will allow you to build a high performance street/strip motor that will run on pump gas without detonation.
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==Piston compression height and piston deck height==
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Next, if using a standard 5.7" rod length, use a piston with a 1.560" piston compression height (measured from the centerline of the wrist pin to the top of the piston crown). Now, if we add up all the dimensions of our "stack" components, we find that we have room in the block for them, plus a little extra which will be between the crown of the piston and the block deck. 1.740" + 5.703" + 1.560" = 9.00". So, if there is 9.025" available and the stack is only 9.00", that leaves 0.025" space above the piston crown with the piston at top dead center (TDC). This space is called the '''''piston deck height''''' (not to be confused with ''block'' deck height).
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==Stroke/rod/piston combos==
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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. 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 5.7" rod length and the 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, our 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.
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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 and the side loading of the piston to the cylinder wall, using a shorter rod can cause the skirt of the piston to strike the counterweights of the crankshaft with the piston 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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Another way to do this 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; there's more room for the crankshaft counterweights. Some builders prefer not to use a 6" rod, their thinking being the pin intruded 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, and the added complexity 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. The least expensive way to build a 383 stroker is to use an externally-balanced 400 harmonic balancer and a 400 flexplate/flywheel.
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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. Most builders agree the 5.565" rod/3.75" stroke ratio is borderline unacceptable at 1.48:1, compared to a stock SBC 350 ratio of 1.64:1, and 1.6:1 for the 6" rod 383stroker. For comparisons sake, the SBC 302 rod/stroke ratio is 1.9:1.   
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==Aftermarket parts combos==
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A popular parts combo for a 383/388 stroker is the Scat cast steel crankshaft and Scat I-beam forged Pro Stock rods. The Scat counterweights will clear the pistons using a 5.7" rod and the rods will clear the cam, so no grinding on the rods for cam clearance will be necessary. You may need to do a little grinding on the inside of the block at the pan rail to clear the big end of the rod, but it will be minimal. 
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==Zero decking the block==
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It is common engine building practice to cut the block decks to zero piston deck height and to use a head gasket that compresses to around 0.040". This allows a quench or squish, or "squench" of 0.040". Squish is the high speed jetting of fuel air mixture from the dead zone opposite the combustion chamber. When the piston approaches top dead center, the clearance between the crown of the piston and the underside of the cylinder head diminishes to just short of a collision. This squeezes or "squishes" the mixture that is there, across the cylinder toward the spark plug. This high-speed jetting of the mixture not only eliminates any dead spots in the chamber, but also creates turbulence to achieve a more homogeneous mixing of the fuel/air mixture so that there are no lean or rich areas in the chamber. When using steel rods on a street/strip performance engine, having a tight squish of 0.035" minimum to 0.045"-0.050" will allow a high performance street/strip motor that will run on pump gas without detonation, providing that all the other important areas are also covered- like the static and dynamic compression ratios.
  
 
[[Category:Engine]]
 
[[Category:Engine]]

Revision as of 07:24, 11 April 2012

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