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Thread: 4.x Stroker Long or Short rod?

  1. #1

    Default 4.x Stroker Long or Short rod?

    The way most folks build there strokers I've noticed is they use the 4.2 crank/rods which produces a 1.5:1 rod ratio which is to say not very good. Another thing I've noticed is the quench height this combo makes is far beyond the optimal .060"...some as high as .080"+.

    Would it be worthwhile to dish out the $ for the custom pistons to yield a zero deck clearance and be able to use the big 6.125" arm?
    1990 242 AW4 242
    1995 D30/8.25 4.10
    1997 Shortblock
    1999 Hogged out Cylinder head


  2. #2
    We Race, so you Win Evildriver-3's Avatar
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    Depends what you are looking for when you start playin with different length rods as they wil change the RR, shorter is better low end with a good head that can support it
    Less than 8 is like playing with half a deck


    www.LiMotorsports.org The voice of the racer

    71 Dart, 73 RR, 78 Mag GT, 85 Dodge W-350, 91 Dodge 250 c, 91 Daytona, 95 Jeep GC, 99 Jeep GC, 00 Dakota, 05 Jeep GC, 08 Dodge 2500hd

  3. #3
    Stroker Fever Dino Savva's Avatar
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    The rod length:stroke ratio is 1.51:1 with the 4.2 rods and 1.57:1 with the 4.0 rods so the difference is too small to be significant.
    As for the quench height, I think the importance of that is being overplayed. The 4.0 head and pistons already have a dual quench design so even with a quench height of 0.080" or slightly higher, the job still gets done.
    Strokers are mostly low revving torque monsters, not high revving race engines, therefore it's not worth the expense of going for longer rods with custom pistons when the difference they'll make to performance and octane tolerance is minimal.

    1992 XJ 4.6L "Poor Man's" Stroker
    202rwhp @ 4700rpm (248bhp)
    258rwtq @ 3400rpm (311lbft)
    1/4 mile: 14.63 @ 94.4, 3450lb curb
    AX15, NP231, D30/D35
    Jeep Performance, Jeep Tech, Junker to Stroker

  4. #4

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    Thanks for the input. I want what everybody else wants...gobs of power that will go the distance. The stock 4.0 has a RR of 1.79:1 which is excellent in terms of durability regarding the rotating assembly. Since I love to explore the entire range of the rpm band I don't think a stroker would be a good choice for me.:bewhew: Maybe a powertrain from ZJ is in order.
    1990 242 AW4 242
    1995 D30/8.25 4.10
    1997 Shortblock
    1999 Hogged out Cylinder head


  5. #5
    We Race, so you Win Evildriver-3's Avatar
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    Quote Originally Posted by Dino Savva View Post
    The rod length:stroke ratio is 1.51:1 with the 4.2 rods and 1.57:1 with the 4.0 rods so the difference is too small to be significant.

    Strokers are mostly low revving torque monsters, not high revving race engines, therefore it's not worth the expense of going for longer rods with custom pistons when the difference they'll make to performance and octane tolerance is minimal.


    Your saying it's not worth the expense to use a long rod, but you're leaving out why, race engines don't necessarily use long rods, unless you are looking for power at a high rpm and to stay there for prolonged periods of time.
    When you're increasing the RR you are making the motor want to breath better faster...
    The bigger the RR the slower you move air since you have now slowed down piston speed, piston speed is better higher in the rpm range, making it's peaks higher.
    So now you come up with a head that improves velocity at low end to overcome the motors inability to do it...
    This was the reason for the magnum head and sb revison to make both motors use 1 head that helped work out good on both motors considering the big difference in RR between 60 and 18...the result was a poor head for anything beyond that

    A Shorter RR makes the motor have the ability to move more air at lower rpm packing the cyl better, increasing low and mid range hp and tq.

    The problem is if the heads and intake can't make the use of it....it doesn't give as good a benefit and becomes a bad all around motor...

    Depending on RR of a motor and where it is being built for operation is where the head work and heads of use is very important along with the intake, and exhaust..
    Less than 8 is like playing with half a deck


    www.LiMotorsports.org The voice of the racer

    71 Dart, 73 RR, 78 Mag GT, 85 Dodge W-350, 91 Dodge 250 c, 91 Daytona, 95 Jeep GC, 99 Jeep GC, 00 Dakota, 05 Jeep GC, 08 Dodge 2500hd

  6. #6
    Bad Ass XJ
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    You cant go wrong no matter which way you go. I have the 4.2L rods and off the shelf 4.0L pistons and with a good cam and valvetrain my motor has absolutly no issues from idle all the way to the 5250 rev limiter. Sometimes I smack it with out thinking cause the motor does rev up pretty quick with lots of power to smoke the 35s

    AARON
    91 Comanche Truggy...Built for fun on 35s

  7. #7
    Stroker Fever Dino Savva's Avatar
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    Quote Originally Posted by Evildriver-3 View Post
    Your saying it's not worth the expense to use a long rod, but you're leaving out why
    You need custom forged pistons if you use 4.0 rods in a stroker. These cost $600+ per set and forged pistons tend to rattle in the bore when the engine's cold. They have a higher rate of thermal expansion than cast or hypereutectic pistons and require a greater piston to bore clearance. The result is cold start piston slap and a diesel-like ticking sound at idle until the engine warms up sufficiently to reduce the piston to bore clearance.
    The 4.0/stroker engine's also rev limited to 5250rpm so it's pointless trying to build the engine for higher rpm power when what most people really need is a buttload of low/medium rpm torque.

    1992 XJ 4.6L "Poor Man's" Stroker
    202rwhp @ 4700rpm (248bhp)
    258rwtq @ 3400rpm (311lbft)
    1/4 mile: 14.63 @ 94.4, 3450lb curb
    AX15, NP231, D30/D35
    Jeep Performance, Jeep Tech, Junker to Stroker

  8. #8
    We Race, so you Win Evildriver-3's Avatar
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    That depends on what piston manu you use, and your machinist, not any of my motors have piston slap. I think the point was missed of what i was saying though i never said to use a long rod.
    Less than 8 is like playing with half a deck


    www.LiMotorsports.org The voice of the racer

    71 Dart, 73 RR, 78 Mag GT, 85 Dodge W-350, 91 Dodge 250 c, 91 Daytona, 95 Jeep GC, 99 Jeep GC, 00 Dakota, 05 Jeep GC, 08 Dodge 2500hd

  9. #9

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    Well just playing with some #'s and it seems that a 'safe' redline rpm for the 4.2 crank/rods combo seems to be about 4500 rpm's producing an average piston speed of 2983 ft. per minute. The stock 4.0 at 5250 rpm's makes an average of 2914 ft. per minute. I've always been taught that going over 3k ft. per min. and the engine will have reliability issues, ones that I'm not trying to find out about.
    1990 242 AW4 242
    1995 D30/8.25 4.10
    1997 Shortblock
    1999 Hogged out Cylinder head


  10. #10
    Stroker Fever Dino Savva's Avatar
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    I've had no problem taking my stroker to the 5200rpm rev limiter in short bursts, but I rarely need to go above 4500rpm 'cause I've got so much torque to play with lower down.

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