I'm not talking about rebuilding a motor with stock pistons. I'm talking about taking a known good running motor and just boosting it without rebuilding it. No need to take apart a good running motor to gap rings for 600 whp. My current motor is at the 600 whp range with nothing but a cam change, headers, and a procharger at 12 psi with a completely stock as it came from GM bottom end.

I'm with you on putting good pistons and rods in if you are going to rebuild it anyway like the OP is doing. For the other guy with a good running motor I'd go ahead and put the procharger on it without any worries. Stock cranks have been way beyond 600 whp with no issues. I will never tell someone not to use the best parts they can afford even if it's overkill for their goals, but lets not mislead someone into thinking a forged crank is necessary for 600 whp either. If it gives you piece of mind then sure go ahead and put a forged crank in it, but can a thin sleeved ls1 block actually handle more boosted power than a stock crank anyway? Boost doesn't really care about the extra 40 cubic inches either. If you are producing 1500 cfm with a procharger they say that will support 1000 horse power. It doesn't matter if it's a 347 or a 383 that 1500 cfm will support the same 1000 horse power. The larger cubic inches will move that dyno curve to the left in other words making more power at a lower rpm. The issue then is at the 600 whp and above level is that extra low end from the longer stroke even needed or useable? Also at what power level does the longer stroke due to the effect on piston design actually become weaker than the pistons in a 347? There is a reason some high power boosted guys prefer 3.9" or shorter strokes over the 4"+ strokes.

 

 

 

Agreed. Doing a SBE LS with a Procharger, good heads, reasonable cam at ~600 whp much easier now and probably last way longer too.

The Procharger and a SBE LS or Forged 347 sound like choices that meet the OP's goals. Definitely a lot of good choices and paths forward.

I will confess l like that instant response and torque on the bottom end at low rpm the stroker gives in a daily driver. Most of the Procharger sets ups I've ridden in are pulling vacuum on the boost gauge at cruising rpm on the street.

Well said!!! +++.

​​​​​Agreed 100% based on experience with two 4 inch crank strokers.

Math is scary to some folks.

 

 

I've had 4" stroker engines everyone knows that can be done, but rather than actually address my comments you'd rather just insult me that I don't know math? The issue is you are trying to sell parts he doesn't need. For 600 rwhp he doesn't need a forged crank what so ever. My comment was in relation to the fact that at the power level in which a forged crank becomes necessary a 3.9" stroke motor allows a stronger piston. At 1000+ horse where a forged crank is actually needed boost cares less about 0.1" of stroke and more about stronger piston ringlands.

Go ahead and insult me I don't know math blah blah blah, but I have nothing to sell. I don't want people wasting money on parts they don't need while I'm over here making 600 rwhp on a stock motor. I've had cars with a lot more power than my current one as well and various combos NA, nitrous, turbo v6, and now procharged ls. I get the value of low end torque in a street car, but at a certain level it become unusable anyway as you just blow the tires off on the street. The proper gear and/or stall for the combo goes along way also, but I don't know math so you all enjoy selling this guy stuff he doesn't need to reach his 600 rwhp goal.

 

 

No offense intended. As we stated, the factory crank with forged rod and piston are desirable at the O.P.'s power level. You'll notice that Dart shortblocks come in 387 Cid to suit people with the need for the powerband and 4" stroke 427 Cid. as well. Cubic inches for blower cars do the same that they do for N.A. cars, namely allow the rpm to be turned down to keep the valvetrain in control. The car doesn't need to be geared as hard or require as much of a stall converter. You could accomplish this with a smaller pulley (and less belt wrap), but then you're just adding heat and running it closer to the limits of the fuel. For Turbo people, going to a much larger Turbine housing and A/R can be used and still spools more quickly. This reduces reversion and makes more power there as well.

Here's the math many people don't know. We've attached a (very) rough sketch of a typical 1.110 Compression Height Wiseco piston with a 1.2 1.2 3mm ring pack in a factory iron block. These have the shortest cylinder lengths of all the blocks at 5.430 effectively and aluminum are typically 5.550. The ring lands are maximized at nearly .300 top and .165 second. The third land could be pulled back to .070 without issue and add more meat to the top or second land. A .928 pin bore is within .009 of the top of the oil ring groove and the rest of the oil ring groove hangs below. From there we have piston skirt that remains riding in the bore at BDC. Skirt taper varies not only in the amount, but more importantly where it's introduced. A piston needs to have at least .080" of piston skirt at full diameter up above the liner at bdc with taper being introduced above there. This avoids the sharp intersection that would occur, act as a razor blade, and destroy the skirt in short order.

Wiseco's typically have a break point .650 below the oil ring groove or .525 depending on the model. The .525 is always found on the 1.050 C.H. pistons used with 4.125 strokes and serves well with 4.000 in. as well. From there, it adds a slight amount more stability too, but most of that comes from the amount of taper above the break point.

Herein lies the rub. Many piston companies didn't have a high enough break point with 4" strokes first became popular. This is what wiped out peoples engines 15 years ago. Wiseco and Diamond were the first to start publishing the actual reason for this skirt profile to be correct. Other companies were behind (and may still be). A lot of piston companies used the same break point for non-strokers as their stroker pistons at about .900 in. below the oil ring groove. You can see that didn't fare so well with either a 4.000 in. (or 3.900 in. for that matter) as it was definitely introduced far below the bottom of the cylinder liner at BDC.

This break point theory applies to all stroker engines. Ford, Chevy, Chrysler. etc. It's pretty simple for a good piston designer to look at the ring stack and the intake pocket etc. and determine the maximum stroke an engine builder should pursue. As a side note, we've done 2.200 rod journal/.990 pin 1.5/.043/3mm assemblies for the 2k hp plus guys, but the same theory applies and they're often tall deck, raised and larger cam journal anyway. The industry sells a ton of 4" stroker rotating assemblies a year and the cars aren't mosquito foggers or breaking ring lands.

 

 

That probably should have been our first reply, but it was late. Thanks for the opportunity to explain the rationale more fully and thank you for your Patronage. Let us know when you're building your next bullet.

 

 

It can and has been done and it's a balancing act as the top of the piston is undergoing a lot more thermal expansion (from combustion) than the bottom (mostly subject to oil temps). Basically the piston needs to be straight up and down when it's up to temp.

You don't want to make a big change all at once and seize a piston due to lack of taper. This takes careful reading of used piston skirts to evolve the design over time. You would think it was all done through FEA, but with the differences in blocks and sleeve materials and thickness...good old fashioned skirt profile reading is still the primary method of getting it right. The better companies have in-house dynos to get it right the first time, but others work through evolution. This works too, but at a slower pace. Often based what customers send back from the field as they develop engines.

The skirt profile (cam and taper) can be affected by things like water jacket temperature differences across all 8 cylinders. Because of this, pistons are made to be happy in the cylinder that has the least heat transfer. #7 in a LS often to blame for ring butting because the top ring cant transfer heat out quickly enough, but we've seen evidence of 6 getting hot as well.

The very top of a piston's top land may be .054-.060 smaller than the bore in many cases. This can be more of less depending on the app. and bore size. The arc to the bottom of the break point up isn't constant. This is because the top, second, and oil rings are all transferring heat into the water jackets with the top ring doing the most work. You'll usually see round lands that are tapered from the bottom of the third to the top of the top. Sometimes you'll see stepped lands, but his takes a little more development.

On high end pistons used in Pro Stock, Nascar etc., oval ring lands and even other shapes can be found. This is based on structure (mass) through the pistons differing (like the pin towers at the sides for instance). It requires a lot of testing over many many successive builds to get this level of resolution.

Some engine builders will have a go/no-go for cold piston rock before they start seeing signs of oil consumption. This is something easily modeled and there are guidelines that create a stable piston whether it's a tall standard stroke piston or shorter stroker piston. A company's stocking piston will have this all worked out (usually).

Thanks and let us know if you have any more questions.