383/388 Chevy stroker

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by: 1Gary, Cobalt327, Jon, Techinspector1
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Contents

[edit] 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 production 305 and 350 SBC.

The following article will help the new engine builder to better understand the details involved in building this powerplant.

[edit] SBC dimensions

[edit] Block deck height

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").

[edit] Parts stack height

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.

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.

[edit] 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.

The bottom line to all this is it's best to maintain an adequate quench figure of 0.040". There's nothing to be gained by going tighter, and a 0.040" quench distance will avoid unseen problems for the most part. If the quench is less than 0.040", be sure to double check clearances to be sure there is no contact between the piston and head- for obvious reasons.

[edit] 350 SBC piston compression height and piston deck height

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).

[edit] Stroke/rod/piston combos

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.

Piston compression height comparison between 6" (left) and 5.7" rods using a 3.75" stroke

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.

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.

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.

[edit] 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.

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

[edit] 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.

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.

ARP 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.

For clearance, I-beam rods are much easier to use than H-beam rods.

[edit] Crankshaft and rod to block

Bottom of block clearanced for rod nut

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.

We have found the thickness of the wire of a paper clip for clearances of the rods on the pan rail is just about right.

[edit] A word on rod/stroke ratios

Most SBC engine builders agree the 5.565" rod/3.75" stroke ratio is borderline unacceptable at 1.48:1. For comparisons sake, a stock SBC 350 has a ratio of 1.64:1, and the 6" rod 383 stroker is at 1.6:1. The SBC 302 rod/stroke ratio is 1.9:1. It is generally thought that a ratio of 1.5:1 to 1.55:1 is about as low of a rod/stroke ratio that should be used in a performance build that will redline at 5500 rpm or more.

BTW, this has little to 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 more than a horsepower or two possibly gained from a longer rod.

[edit] Aftermarket parts combos

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.

[edit] Quench

Squishband.jpg

One way to arrive at a ~0.040" quench distance is 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") measurement 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 to run on pump gas without detonation, providing that all the other important areas are also covered, like the static and dynamic compression ratios.

For a more detailed discussion, see Quench.

[edit] GMPP HT383 crate engine

This engine was introduced in response to the wildly popular SBC 383 stroker engine made popular by backyard mechanics, machine shops, and later in the aftermarket.

It uses a 4-bolt block and Vortec iron heads with 1.94" x 1.5" valves and an aluminum dual plenum intake.

It produces 340 HP @ 4500 rpm and It produces 435 ft/lb torque at 4000 rpm.

[edit] Differences

While the HT383 is not that different than many SBC engines, the differences are important ones:

[edit] Bore and stroke

  • Bore is 4.00" instead of the usual 4.030" or 4.060"
  • Stroke is 3.80" instead of the usual 3.75".

[edit] Rods

  • Rods are powdered metal with a pressed pin, but are clearanced for a stroker application. The retaining hardware consists of a threaded tapered stud in the rod (giving added clearance between the rods and the cam, etc.). This is different than the usual bolt and nut used on most other engines. Said to be good to 550 HP.

[edit] Crankshaft

  • Crankshaft is a quality, forged 4340 steel unit made for the 1-piece rear main seal configuration. It is externally balanced. Instead of the usual 3.75" stroke, it is a 3.8" stroke, so the displacement will still be 383 ci, but with a 4" bore block. Chevy saw no reason to bore out a new block just so a 3.75" crank could be used.
    • This crank in a 0.030" over block gives a displacement of 388ci.

[edit] Pistons

The HT 383 pistons are a letdown. They're the same old tired, cheap-to-make 4 valve relief, pressed pin, round dish pistons GM has been foisting off on the public since years ago when the drop in CR was mandated. These pistons have no redeeming qualities, unless you consider the aluminum alloy being hypereutectic a plus.

What is known about these pistons:

  • They're hypereutectic aluminum alloy.
  • They give a 9.1:1 CR with a 0.028" thickness head gasket and the Vortec chamber size of 64cc.
    • Chevy shoots for a 9.0" stack of parts for SBC engines. Using a 9.025" block deck height, 3.8" stroke, and 5.7" rods, the piston CH would need to be 1.4” for the stack to equal 9” (1.9 + 5.7 + 1.4 = 9.0). The actual compression height of the HT383 piston is presently unknown.
    • Using the quench distance of 0.053", a piston dish size of 21cc would be about right for a 9.1:1 CR. A 9.1:1 compression ratio using a quench of 0.040" will require a 24cc dish.

[edit] Head gasket

  • It appears the head gasket is composite and 0.028" thick. The CR is advertised as 9.1:1.
    • A 0.028" gasket using the numbers above will give a quench distance of 0.053". To bring the quench down to 0.040" would require a piston CH of about 1.413" (or a head gasket thickness of 0.015").

[edit] HT383 technical specifications

Part Number: 12499101
Engine type: Chevy small-block V8
Displacement (ci): 383
Bore x stroke: 4.00" x 3.80"
Block (P/N 88962516): Cast iron with 4-bolt main caps
Crankshaft (P/N 12489436): 4340 forged steel, externally balanced
Connecting rods (P/N 12497624): Heavy-duty powdered metal steel, screwed-in studs
Pistons (P/N 12499103): Hypereutectic aluminum, round dish w/4 valve reliefs
Camshaft type (P/N 14097395): Hydraulic roller
Valve lift: 0.431" intake/0.451" exhaust
Camshaft duration (@.050 in): 196º intake/206º exhaust
Cylinder heads (P/N 12558060): Vortec iron; 64cc chambers
Valve size: 1.94" intake/1.50" exhaust
Compression ratio: 9.1:1
Rocker Arms (P/N 10089648): Stamped steel
Rocker arm ratio: 1.5:1
Water pump (P/N 88894341): Cast iron
Recommended fuel: 87 octane
Ignition timing: 32º BTDC total @ 4000 rpm w/o vacuum advance
Maximum rpm: 5000

[edit] Resources



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