rustykey

iTrader: (2)

For all you guys that really know engines. I am planning on a 398LT1 build and all I lack is crank and rods. I bought most of my stuff from a forum member and would like to know if I changed from a 3.875 crank and 5.85 rod to a 3.75 crank and a 6" rod what would change besides cubic inches? I was going to run a .039 gasket and have a compression ratio of around 10.8.1 with AFR227 heads 60cc, -14cc nitrous pistons. Let me know any thoughts or suggestions. If this is too vague let me know what other info you might need and I will gather it up.

bufmatmuslepants

iTrader: (3)

You want more compression than 10.8, shoot for closer to 12:1. You can get that from a 0.026 inch head gasket.

duh

iTrader: (16)

Depending on your deck height you might have some issues since your piston top will be .0125" higher. If you're already a 0 deck you're now .0125" out of the top. That could be catastrophic. And are you implying a dish or dome? But by the compression I'd say it's a dish. Better off selling the pistons and getting a rotating assembly that is all matched. 3.75 stroke, 6" rid and a set of pistons that will put the crown where you want it and bump the compression up to around 12:1 if you're still using pump fuel.

97 6speed z

Duh is on the right track here, but .......... he is a little "fuzzy" on his engine math. If you kept the exact same piston compression height, and just changed the crank stroke and rod length as you indicated above your change would be given by:

1) 3.750"/2 + 6.000" = 7.8750"

2) 3.875"/2 + 5.850" = 7.7875"

for a difference of 7.8750" - 7.7875" = .0875" (NOT .0125". as Duh implied)

As Duh said though, if your block was ZERO decked for the 3.875 stroker crank and 5.850 rods ......... the 3.750 stoker crank and 6.000 rod switch would now put you eighty seven and a half thousandths, (i.e. .0875") "out of the hole" ......... without any change in piston compression height!!!

rustykey

iTrader: (2)

Yes the pistons are dished. I have been told they will work fine with the 3.875 crank and 5.85 rod combo with the .039 gasket. The member I purchased everything from was putting down very good numbers with his build (using 3.875 crank and 5.85 rod combo with the .039 gasket)but he changed over to LSX. I was just wanting to tweak it a bit and pull a little more natural HP out of it. Yes it will be pump gas tune. I have tried to get ahold of HKE here in Houston for the shortblock build but havent gotten a response. I know they are very busy these days. So now I am trying to gain knowledge on my end so I can be more eduacated when it finally comes to build time. Without selling my -14cc dish pistons what changes can I safely make? I dont have to keep them but would be alot easier to keep them. The block is already zero decked with splayed billet caps and all the goodies that go along with it.

97 6speed z

Maybe I'm not fully understanding what you are asking here, but let us try this another way. If your block was ZERO decked (i.e. at 9.000") with the 3.875" crank and 5.850" rod "set up", then ............. the pistons you currently own ......... have a compression height of:

9.000" - 7.7875" = 1.2125"

If you were to try and use these same pistons with a 3.750" stroker crank and 6.000" rods ....... they would "stick out":

7.8750" + 1.2125" = 9.0875" - 9.000" = .0875" inches above the block deck surface!!!!

Let's make this real simple ............ you CANNOT use these pistons with the 3.750" crank and 6.000" rod set up you asked about!!!!!!

If you want to build a higher compression 3.750" crank with 6.000" rod motor (i.e. a higher compression 383 (with a .030 over bore)) ........ since you would have to order NEW pistons anyway ........ order FLAT TOP (not dished) pistons for that engine "set up".

rustykey

iTrader: (2)

Ok, that makes sense. I tend to ramble a bit. I will stick with my original intent and build with the 3.875 crank and 5.85 rods.....I like more cubic inches anyway. If I wanted to build compression a bit, could I do with a different gasket as stated earlier in the first response to my thread? It makes sense that I could but I know it comes down to close tollerances and clearance issues.

97 6speed z

Aaaaaaah the questions get easier to answer now!

NO, you cannot build more compression by using a thiner head gasket for this particular engine build!

Since your block is already ZERO decked that .039" compressed height head gasket will give you ........... .039" piston to cylinder head clearence, and you DO NOT want much less than this! (Yes, fellow engine "guru's" I know .035"/.036" would be "ideal", but let's not argue about .004"/.003", here, okay).

The only way you are going to "bump up" that static compression ratio "a bit" is to have those 227 AFR 60 cc combustion chamber heads milled ........... "a bit".

rustykey

iTrader: (2)

okay bud, thanks for the info. What is the ideal compression for spraying a 200 to 300 shot?

Disregaurd, I will take this over to the Nitrous Section.

duh

iTrader: (16)

I want to apologize for my "fuzzy" math. I was actually in the middle of doing something and responding from my phone, so I was a bit rushed. Either way the answer is no, you cannot do what you were thinking. But since you have to buy rods and a crank anyway, why not sell those pistons, and purchase something that will do everything you want it to do...Bump compression and make it fit whatever CI displacement you want? I personally would think that would be a better way to build than to do everything around a set of low compression pistons that you already have. Just thinking out loud I suppose

97 6speed z

Duh, your math may have been a little "fuzzy" but your logic certainly isn't!!!

Seems like the OP is really "jumping through some hoops" here ......... just to keep a set of used -14cc dished pistons in his new N/A (or nitrous?) engine build.

Rustykey, it sounds like you've gotten a very nice 4-bolt splayed main, zero decked, torque plate bored and honed, LT1 block as part of your deal. Why not take Duh's advice and fill it with a freshly balanced completely new rotating assembly .......... that does everything you want???

rustykey

iTrader: (2)

I will most likely end up doing that. Been reading alot the last couple days and dont want to skimp. I would love to go boosted and lower the compression a bit but not sure its in my price range. Whats the ideal compression for a boosted engine? The lower the better right? You are correct, I have a very nice start at a good build here, gotta get my ducks in a row and do it right the first time.

97 6speed z

Okay, first you asked about increasing compression ratio for a naturally aspirated (or nitrous?) engine build, and now about lowering compression for a "boosted" build? I know you are still in the "planning stages" of what engine you really want to build here, but, as you have already figured out for yourself ....... you cannot build an "ideal" engine for BOTH boost AND spray.

Now back to your most recent question, I'm pretty sure the overwhelming majority of LT1 engine "guru's" on here would tell you that an "ideal" static compression ratio for a "boosted" application would tend to be in the 7.0:1 to 8.0:1 range. Remember "boost" can come from a supercharger OR tubrocharger, and it CAN make a difference how you produce that "boost" !

rustykey

iTrader: (2)

My original intent was nitrous because I know I can afford a good solid build for it. I have always wanted boost but it stretches the pocket book. I know its one or the other, was just curious as to whats ideal compression for boost. Thanks for being patient and helping me out. I have 7k left in my budget and need my rotating assembly, rearend and shortblock built. I really dont think I can do a forced induction build on my budget but can build a hella nasty NA/Nitrous build.

duh

iTrader: (16)

If you've only got 7k to play with, keep it na. Sell those pistons and you can finish up your shortblock