BURL

Considering doing a forged rotating assembly when I install a blower. I'm going to be looking at 10 - 15 PSI with meth. I haven't dealt with the newer forged pistons that say they have reduced expansion. Weisco, Mahle and others say their higher silicon pistons can run tighter clearances than older generation forged pistons.

I'm gonna call them but want to here what you guys are seeing. What clearances are you running. Are you having any cold start up piston slap? Are you taking into consideration increase heat due to blower?

Any info would be appreciated. Thanks

StatmanRN

Id definitly ask the particular piston manufacture for a specific recommendation.
I do know that the older design TRW forgings could be run much tighter than custom forgings for that reason.

inspector12

Still running about .003-.005" clearance Piston to cylinder wall clearance, depending on the HP level. I usually go by what the manufacturer recommends.

BLOWNBLUEZ06@RKT Performance

my .02 on pistons and clearances FWIW:

I'm not aware that Wiseco or Diamond has any more silicon in their 2618 forged pistons than before. I typically hear them calling for clearances around .0045". Of course they have you measure in a specific place in reference to the bottom ring. Seems like it's usually .5" below but I always ask the manufacturer to be sure. I've seen engine builders want to open up the piston to wall clearances to compensate for increased power, but it's something that I don't fully agree with. I might open it up more by .0005-.001" but my philosophy is that the piston taper should be increased in the design to compensate for increased power because increasing the piston to bore clearance increases the load on the skirts significantly due to the piston geometry. The geometry gets worse as stroke is increased.

BURL

I should have specified that I will be using 4340 pistons this will be a street/strip build. Both Wiesco and Mahle specifically reference the higher silicon for decreased expansion in their literature. The tendency to increase clearance is one of the reasons I posted this question. I know builders like to open up the clearance if for no other reason than unknowing drivers taking a new engine and going wide open from the git go. I am will to do a break in period to have tighter piston clearances. This is a street/strip build ad I don't like cold slap.

BLOWNBLUEZ06@RKT Performance

I'm assuming you mean 4032. I've never experienced any piston slap issues with the 2618 forged motors. I've always preferred 2618 since it's a less brittle material. I'm not sure if you've chosen or how well you know your builder, but my experience has been if you want your clearance set a certain way, you had better put it in writing in advance and come back and check it or have them set the block up to your specs and assemble it yourself. It's often the only way you can get what what you want.

BURL

Yes,4032. My bad again - must be the cold beverages. Anyway, I've read that 4032 is mch higher in silicon than 2618 and therefore has much less expaniso and is better for moderate powered street/strip engines. The Mahle paper at http://www.enginelabs.com/engine-tec...e-motorsports/
detals the deference. I've read this at Weisco and other sites also. Seems to say for all out race use 2618. For engines with street duty in mind, fit them closer and use 4032. Your thoughts?

MVP'S ZO6

OP,

Bret is pretty spot on here. It's nice to see people thinking about stuff like this. What I'm going to share with you is available online and are guidelines.

modern pistons are manufactured with the clearance built into them. i.e. Wiseco has a piston that calls for a 4.000" bore and .0035" piston to wall clearance; it's basically saying that the pistons actual diameter when measuring them will be in the neighborhood of 3.9965" at the point in which they say measure the piston.

First consideration you should take is the piston material and it's expansion rate and usage.

4032 will generally run a little tighter. (High silicon content)

As a general guide line; for I'd do this. (This should cover just about any bore size in the LS world.

4032 material (in most of these street application will be listed below. For the mild boost stuff a good starting point is with the looser end of the numbers below)

3.600 - 4.000" .0025" - .0030"
4.00 - 4.200" .0030" - .0035"
Additional clearance based on experience and application may be needed. As a guide line you may end up with .001 - .003 more clearance than the aforementioned guideline.

2618 material (low silicon content)

3.600 - 4.000" .0035" - .0040"
4.000" - 4.200" .0040 - .0045"

Additional clearance based on experience and application may be needed. As a guide line you may end up with .002 - .004 more clearance than the aforementioned guideline.
These manufacturers / engineers can't possibly know what's best for every single combo there for they give guidelines. This is where most enthusiast simply trust their engine builders experience and or education. However I think it's great when someone who don't normally build engines (an enthusiast) wants to understand this stuff and asks questions.
All these recommendations are considered without any coating.

Always use manufacturers recommended measuring points that are included with pistons when purchased from manufacturers. Typical wiseco shelf piston is measured 1.300" from the bottom of the oil ring land.

BURL

Excellent info from you and BlownBlue. My experience is with Gen 1 small blocks so I'm trying to get up to speed on the LS stuff. I planned to do this last year and health issues got in the way.

Anyway I've seen a lot of posts from you and BlownBllue and people like Bill Curlee and some others. Guys like you all can be be backbone of a forum where people are trying to learn from others knowledge ad experience. However in the past when I have asked technical questions I sometimes just get "crickets". And on other occasions I have gotten a couple of "Internet Engineers" that just stamp their feet and throw out online Engineering School formulas if you don't immediately agree with what they are saying. I've taken about a 3 month break because of one member who has done this a couple of times on my threads. I guess I'm saying I think more people would ask the technical stuff if people like you guys would frequently respond in your level headed and logical manner. But I guess it gets old addressing the same questions frequently.

Actually, I wish we has a Technical Building/Assembly section just for questions like materials, clearances, assembly methods, engine theory, head suggestions/flow numbers preferred parts/brands and such so those of us who enjoy the greasy hands can can communicate easily with the technical guys like you. I also have a feeling that the sponsors would really get involved at a more technical level. MVP I got a feeling that you and BlownBlue would be heard if you suggested the addition of such a section. Your thoughts? And many thanks.

Rkreigh

I'd love to see some manufacturer and vendor participation helping folks get great advice like these guys provide

I'm shopping for pistons right now and wiseco seems to be popular but I'm wondering about things like what stroke is best for my application

3.6 stock ls6 (stock crank)
lunati 4.0 forged (budget crank)

I'm looking for about 900 crank hp out of the combination and I'll be running e85 for the first time

I have lunati rods already

this is on a twin turbo ttix application running a haltech s2000 hopefully soon!

MVP'S ZO6

While it's true that outside the aftermarket blocks (which includes the Lsx$ the only factory block that has a longer cylinder is the LS7. Consequently the LS7 is the only engine that comes factory with a 4" stroke crankshaft.

Some argue that putting a 4" stroke in a block with shorter sleeves will trash the skirts or it will cause the piston ring to unload (not seal) because of how far it comes out of the bottom of the cylinder. Well my argument to the first part is piston skirt design and good machining practices all but eliminate beat up skirts and the idea that the ring unloading don't jive with me either. Even if it did, it would unload at BDC long after the ignited air and fuel mixture is still effective on forcing the piston down.

In retrospect; from the moment the compressed air and fuel ignite the pressure is chasing the piston down the bore after about an inch of travel well before it reaches BDC so the ring can unload at that point it won't really matter.


Look at it this way, if we had two engines one with a 3.905" bore and 3,622" stroke (347 CID) and the same exact engine with the only change being a 4.000" stroke (383) CID if the ring on the 4" stroke engine was unloading anywhere it mattered; you would see a massive decrease in power.

You ever hear about anyone building an engine with a 4" stroke saying hey man I'm down a ton of HP? Usually not. :-)

I do believe there is such a thing as too much stroke in a factory length sleeve / cylinder ( I'll call it a dirty engine)

Now on that note, can you make 1,000 wheel with a boosted 347 inch engine with readily available parts? Yep. Can you do the same with a 383" engine with readily available parts. Yep. All things the same it will be easier with a larger engine.






Hope that helps or gets the gears turning.

BURL

Glad you posted this question. You have received a great answer from MVP. He actually answered (or at least gave his opinion) on a question that been running around in my head. I,ll bet there are other opinions out there on the 383 stroker and some of them might come from vendors maybe even Weisco or Lunati who you want to do business with. My thought that has been beating around in my head is:

" I'm probably going to be doing a forged rotating assemble on my LS1 so if the cost is essentially the same why not just go the 383 stroker route? But wait a minute! Some builders say the LS1 sleeves (cylinder liners) are too short and the piston will be pulled too far out of the liner on the down stroke. If I'm gonna stroke maybe I need a later block with longer sleeves. Wonder if that's so. Maybe I should just keep the stock stroke. And how about cylinder side loading with the increased stroke with the thin sleeves? If the was a Gen I I'd stroke heck out of it for the same money cause the Gen I 383 really works but what about the LS engines? Different animal. Well h*ll, don't know what to do now"

MVP's post to you has given me some info to help me decide my question. And I'll bet it stirs up a little technical debate. I'm glad you posted and I glad he posted. Same thing that goes on all the time on the Forum but I just wish we could get more technically savvy people like MVP chiming in and get more vendor technical involvement by creating a section for the technical theory on clearances, tolerances materials, block/bore/stroke combination, etc. We could all learn a lot by having this info in one area. Kinda like the build and tech articles I and countless others poured over in Hot Rod magazine. I knew the old 1 8 4 3 6 5 7 2 Gen I firing order when I was in grade school from reading tech articles. For years we picked up all the good info on the "good ole Gen I small block. Problem is the Gen 3 & 4 stuff doesn't have years and years of Hot Rod mag articles to read and build out personal inventory of technical knowledge. I would like to see us start out own Gen 3 & 4 "knowledge base" here on the forum in a technical area.

Thanks again for your and MVP's post. Just my 2 cents worth....