3.9 cranks
04-17-2019, 09:35 PM
#1
3.9 cranks I cam across 3.9 cranks awhile ago but dont see a whole lot of them, not like the 4.00. Not as many people using them?
Seems like a good idea to me If you want to have a stroker crank and boost keeps the piston in the bore more. Or are boosted people staying with the stock stroke for that reason?
Seems like a good idea to me If you want to have a stroker crank and boost keeps the piston in the bore more. Or are boosted people staying with the stock stroke for that reason?
04-18-2019, 12:12 AM
#3
Theoretically it keeps .050 in the bottom of the sleeve. But actually the skirt is designed for the stroke and counterweight clearance so youre not keeping any part of the piston from being out of the bore honestly.
Plus, chicks dig the longer stroke i hear.
Plus, chicks dig the longer stroke i hear.
04-18-2019, 12:52 PM
#6
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04-18-2019, 04:37 PM
#9
3.9 cranks are now being made by K1 to help combat the issues a 4” crank can sometimes cause on a stock block with stock sleeves.
Going with a 3.9” vs a 4” crank on a ls1 or ls2 you only lose 10 cubic inches 373 vs 383 or 392 vs 402.
I’d say it’s worth looking into especially if you want to spray a lot or boost.
Going with a 3.9” vs a 4” crank on a ls1 or ls2 you only lose 10 cubic inches 373 vs 383 or 392 vs 402.
I’d say it’s worth looking into especially if you want to spray a lot or boost.
04-18-2019, 07:31 PM
#10
I'm not sure what issues you would see honestly.
Ive assembled more than i can count of a 383 and 402/408 on iron and alum and have never had issues.
Most of the issues I have seen were from poor machining or improper assembly/break in or bad ring gaps.
But also you dont have to jump to 2618 for everything. With the extra ptb clearance you can expect blowby and pcv issues.
Ive assembled more than i can count of a 383 and 402/408 on iron and alum and have never had issues.
Most of the issues I have seen were from poor machining or improper assembly/break in or bad ring gaps.
But also you dont have to jump to 2618 for everything. With the extra ptb clearance you can expect blowby and pcv issues.
04-18-2019, 08:37 PM
#11
Cool. It was mostly a curiosity question. On paper the 3.9 sounds like a great idea but like you said you only really gain 0.050 more piston in the bore and then clearance it not much of a gain.
I dont build performance engines daily so I dont see some of the smaller details.
I did find that with the 3.9 you have enough material on the piston the wrist pin doesn't go into the oil ring groove. But if done right I doubt it's a problem wither way.
I dont build performance engines daily so I dont see some of the smaller details.
I did find that with the 3.9 you have enough material on the piston the wrist pin doesn't go into the oil ring groove. But if done right I doubt it's a problem wither way.
04-19-2019, 07:01 PM
#12
You dont gain .050 more in the bore.
Theoretically it would just be the skirt anyway but the piston skirts are either cast, forged, or machined differently for the extra clearance.
You only gain theoretical peace of mind honestly. Nothing actual.
Theoretically it would just be the skirt anyway but the piston skirts are either cast, forged, or machined differently for the extra clearance.
You only gain theoretical peace of mind honestly. Nothing actual.
04-19-2019, 07:34 PM
#14
3.9 would be a good stroker crank for a boosted deal in a stock block.
K1 and Molnar both stock 3.9s and Compstar used to.
JE has shelf pistons for the 3.9" cranks too.
And Wiseco, CP, and Diamond can make em with a couple of week lead time.
CP may even have some stocking pistons in their catalog.
K1 and Molnar both stock 3.9s and Compstar used to.
JE has shelf pistons for the 3.9" cranks too.
And Wiseco, CP, and Diamond can make em with a couple of week lead time.
CP may even have some stocking pistons in their catalog.
04-19-2019, 07:47 PM
#15
As much power as 402 and 408s make on boost im not sure id spend so much on a 3.9 crank and trying to get pistons.
I mean im all for being unique, hell im building a 299ci 4.8 and a 333ci ls2 etc but those both use stock cranks and budget parts.
The theory that most use for the 3.9 argument is also thinking that every measurement would stay the same. They definitely dont. Of course you may lose a small amount of ring land real estate, but piston design is way different now vs 2010 even. Ring packs are 1.2/1.2/3.0 or 2.5 or 2.0 leaving much more real estate for area between the lands or adding thickness to the top of the piston by moving the 1st ring land down. I know CP and i think diamond do this now and maybe wiseco also. Mahle uses a 1mm 1mm 2mm ring pack so their total area is much smaller leaving much more room for strength between the lands.
Everyone used to build them all the same and use the same 1.5 1.5 3.0 Now the pistons have finally caught up with the rings so to speak and we see much more power now than before.
Using a .927 pin helps also of course. But youd be looking at a 1.165 vs 1.115 which seems like alot but that all depends on ring placement, design, thickness, and the oil rails may likely both have Supports for them.
I mean im all for being unique, hell im building a 299ci 4.8 and a 333ci ls2 etc but those both use stock cranks and budget parts.
The theory that most use for the 3.9 argument is also thinking that every measurement would stay the same. They definitely dont. Of course you may lose a small amount of ring land real estate, but piston design is way different now vs 2010 even. Ring packs are 1.2/1.2/3.0 or 2.5 or 2.0 leaving much more real estate for area between the lands or adding thickness to the top of the piston by moving the 1st ring land down. I know CP and i think diamond do this now and maybe wiseco also. Mahle uses a 1mm 1mm 2mm ring pack so their total area is much smaller leaving much more room for strength between the lands.
Everyone used to build them all the same and use the same 1.5 1.5 3.0 Now the pistons have finally caught up with the rings so to speak and we see much more power now than before.
Using a .927 pin helps also of course. But youd be looking at a 1.165 vs 1.115 which seems like alot but that all depends on ring placement, design, thickness, and the oil rails may likely both have Supports for them.
04-19-2019, 08:11 PM
#16
Question for ws6store, one of the things I was concerned with a 4” crank in a stock ls1 or 2 block is burning oil, which has been a problem in the past. Are you saying now this should not be an issue or less of an issue because of better piston design?
04-19-2019, 08:31 PM
#17
The only time it has ever really been an issue is with either improperly gapped rings, poor machine work or no machine work sometimes.
If you use a 4032 piston you'll have much less issue there vs 2618 and thats pretty much a fact due to the ptb clearances.
If you use a 4032 piston you'll have much less issue there vs 2618 and thats pretty much a fact due to the ptb clearances.