turbotuner20v

I'm looking into building my LS3 and it seems like the block and the crank are good up to ~1,000whp (well over my goal). How many people are just replacing the rods and pistons and keeping the stock crank?

I'm not too familiar with the details of the engine building process, but I'd assume you can pull the stock rods/pistons and weigh each one, then mate the new rods/pistons and add/remove material so that they match the stock weights and drop them in. Does anything change w/ the rod bearings? Or how do you make sure you have the right size for the new rods that still fits the stock crank without machining? Or does the crank need to be removed for machining to fit the new rods/bearings?

The motor only has 10,500 mile, so I would assume there isn't much (if any) work that needs to be done to it if the pistons are stock sized.

old motorhead

Everything needs to be pulled out. Crank inspected. New and reused parts of the rotating assembly need to be balanced as a whole. Good time to add studs on the top and bottom end also. Especially if you're shooting for those type numbers. Pick a shop that knows their way around a boosted motor. You'll be much happier (and confident) with the end result

turbotuner20v

Yea, I just read some articles on balancing and it looks like while you can match rods and pistons to get similar weights, you need to either remove material or drill the crank and add mallory slugs to balance out the rotating assembly.

Is this part of the reason you see most people go with aftermarket cranks instead of rebalanced stock ones? The money/time involved in rebalacing the stock one is about the same as going with an aftermarket crank that requires less work? Then you get the option of pretty easily adding displacement (which I assume produces pretty decent power gains?)

I see a lot of 408's, 416's, 418's, but not many 376's.

NICK YOSKIN

[QUOTE=old motorhead;1579798588]Everything needs to be pulled out. Crank inspected. New and reused parts of the rotating assembly need to be balanced as a whole. Good time to add studs on the top and bottom end also. Especially if you're shooting for those type numbers. Pick a shop that knows their way around a boosted motor. You'll be much happier (and confident) with the end result[/QUOTE
Agreed! ERL is some of best in the biz!

5 Liter Eater

I looked into this. I thought I could break the glaze on the cylinders, put new 4.000" (stock LS2 bore) pistons and rings in along with new rods and call it good. Frist off the new rotating assembly needs to be balanced, second everyone said it was risky that bores were perfectly round and to go .005" over. Which opens up piston choices but there's more machine work. Everyone recommended checking and or align honing the mains even though the engine has been running fine for ~60k miles, many of them boosted. Main cap studs, head studs, new tri-metal bearings, head gaskets, it adds up QUICK! Having said all that, if I hadn't gotten a deal on my 408 rotating assembly and LQ block I think I would have chanced putting in 4.000" slugs in a newly crosshatched bore, forged rods and had the crank balanced to them, LS9 head gaskets, cheap-o head studs, called it good and rolled the dice. But I am a cheap *****.

turbotuner20v

Yea, I guess I just see all these LS3 motors in the 750whp range that live for quite a while, and when they do fail it seems related to the stock pistons cracking. Lots of the recommended work seems like overkill for an additional ~75whp.

filmjay

I'd say its less the pistons cracking, and more that the rings weren't gapped correctly. A boosted LS motor likes an additional .010'ish of ring gap for the added heat so the rings don't butt or overlap...which kills ring lands, and therefore, pistons.

I thought of doing the same thing as you, using the forged GMPP LSX pistons/rods with the stock crank. By the time all the necessary clean up machine work and balancing is done, it just a bit of a waste to not stroke it, or at least over bore a smidge.

All in all, it just cost-prohibitive is the reason you don't see many guys running forged stock cubes. You're buying a new rotating assembly, so you gotta have it balanced. You gotta clean up the block, probably including a cylinder hone, so the stock 4.065 bore, is now closer to 4.070 (4.067-8'ish), so those stock size pistons aren't going to fit quite right anymore...cylinders are getting honed, might as well take them to 4.070...youre buying new rods, might as well get the longer ones for the same price....see where the train is going here?

turbotuner20v

I hadn't really looked into how the extra displacement is achieved, but if it's from longer rods, does that impact how 'forgiving' the motor is? Does putting together a higher displacement motor reduce PTV clearance and increase the risk of damage when missing a shift or running up against the rev limiter?

And from a performance standpoint, does it limit the size of cam that can be used?

Is there an ideal size LS3 motor for supercharged applications?

old motorhead

The rod length doesn't change......but, oh wait....it usually does. Has nothing to do with displacement, but most any time the rods are upgraded, the rod length is increased from 6.098 (stock length) to 6.125. The only LS engine that uses a different lenth con rod is the 4.8LS truck engine. The con rod, in only that motor, is lengthened to be able to use 5.3L pistons (3.62 stroke) with a shorter stroke crank. All production LS motors except the 4.8L (shorter) and the LS7 (longer) use the same 3.62" stroke. If you figure out why aftermarket rods are usually .027" longer, let me know. Never understood that. Kept me awake many a night.....

The piston, when it's all said and done, is going to end up in the same place at the top of the compression stroke. No matter the stroke. Piston pin placement is changed to accomodate the stoke/rod length combination. Stoke does not change PTV clearance. The angle of the con rod in relation to piston movement does increase with a longer stroke. I prefer to keep that angle less with a shorter stroke. But I'm not as much of a power junkie as some on this site

If you're looking for big hp/tq, more cubes is usually better. If not, less cubes is easier on the short block. Kind of a balancing act. The beauty of f/i is that you don't need big cubes to make big power. You can build a motor that can make double stock type hp and still be well mannered and long lived.

turbotuner20v

I think I'd want to stick w/ the 376cid vs. the 416cid then. How much power would you really gain w/ an additional 40cid?

So if you go to a 4.0 stroke crank using the same 6.125 rod length, I assume the piston size gets smaller in order to keep the PTV clearance? Does this impact the reliability of the piston if it's smaller?

5 Liter Eater

The wrist pin location moves up with a stroker crank. This can lead to more piston skirt wear but not excessively so. It's not really worth worrying about.

As mentioned the stock rod is ~6.1. For some reason all aftermarket rods are 6.125" and all aftermarket pistons are modeled off that as far as wristpin height.

You'd have to balance the crank whether you stay stock or stroker so it's an additional $800 - $2000 for a stroker crank to add the extra cubes. Figure adding ~10% volume should add ~10% more power. Doesn't sound like much but a 376" making 700 RWHP would make ~770 with the additional displacement from the 416. This is a rough rough SWAG but IMO it works. So it's up to you whether thats a good HP/$ ratio.

turbotuner20v

Interesting, that's good to know about the general displacement/power increase.

5 Liter Eater

Thats just my SWAG. No actual data behind it.

Mr.Big

The main reason an aftermarket crank is advised in an S/C setup... is the Crank snout... the aftermarket forged canks have a much stronger snout... thus the reason I run a stock stroke after market crank in my setup. I stayed small cubes (370c.i.) because my goal was a boost only 1000+rwhp and a fully street friendly driver. I wanted a setup I could drive in 110*+ heat here in Texas in 5 o'clock traffic and not have issues, so that's exactly what we built. On a higer HP S/C/ build an aftermarket forged crank is highly recommended if TT a stocker would be fine at the 1000rwhp level imho...

turbotuner20v

Is it common for the extra belt tension on the crank pulley to pull the stock crank enough that it can bend or flex?

I'm probably looking at running a flip drive setup with a 4.0" pulley, so hopefully I don't need crazy tension to avoid slip.

old motorhead

You probably need to decide where you want to be power wise, where you want to be engine size wise, and then you'll know about what type load will be on the crank snout.

5 Liter Eater

It's not "typical", no. But it can happen. If I were staying stock cubes I would definitely use the stock crank. And if I were going with an aftermarket crank I would definitely stroke it.

turbotuner20v

I'd like to make 800whp on a D1SC w/ a 6-rib direct drive w/ a 376 motor.

winters97gt

Won't be an issue. I wouldn't worry about it and leave the stock crank at that power.