I am purchasing a 400 SB chevy short block. It is a fully rebuilt motor. However, it has stock rods, crank, and pistons. I am looking at about 450 hp with a potential 100-150 shot of nitrous. From what I have been able to determine, I should be ok with the bottom end with the exception of the cast pistons, If I put forged pistons in, how would I swap pistons? Do I need the assembly to be balanced or can I just balance all the pistons to the same weight or...? And how do I adjust the pistons weight to balance them. Lastly who makes good forged pistons for the buck? The Speed pro power forged seem to be the best I can find. Will the rods and crank be able to handle the nitrous? Other wise I will have to skip it, but it is nice to have the motor able to handle it in the future from when I / if I decide to put a small shot in.

 

no idea how you figured out the stock bottom end is going to be ok with a power adder but if i were you i'd take another look. you can balance the pistons by making the heavier 7 match the lightest, but i'd have the rotating assembly balanced by a shop.

 

The motor will handle a 150hp shot with no problem. You worry way too much.
Swapping in forged pistons will require rebalancing the crank.

Just run it as is and concentrate on a proper N2O tune. Plenty of fuel pump delivery capacity and knock about 4-6 deg of timing out of it while spraying 'er.
Use the highest octane fuel available. 92+ race gas if you can get it.

what is the compression ratio and cam specs?

 

Ok heres my setup

Geting the 400 small block

Will be a stock rebuild bottom end.
Not sure what that is for CR but I was told will be around 9.9 - 10.2:1 with 64cc heads.

I am looking very heavily into the vortec heads from SDPC and port the exhaust side with a quick gasket match on intake.

I think my cam might be a touch small for my goals so I am leaning towards 1.6 rockers
The cam specs are
CraneCams
Lift:
Intake @Cam 315 @Valve 473 (.504 with 1.6)
Exhaust @ Cam 324 @Valve 486 (.518 with 1.6)

Cam Timing: TAPPET @.004
Lift: Opens Closes ADV Duration
Intake 30.0 BTDC 64.0 ABDC 274.0 °
Exhaust 73.0 BBDC 27.0 ATDC 280.0 °

Cam Timing: TAPPET @.050
Lift: Opens Closes Max Lift Duration
Intake 7.0 BTDC 37.0 ABDC 105 224.0 °
Exhaust 50.0 BBDC 0.0 TDC 115 230.0 °

THis is the Z cam so it lifts the valve a little quicker.


93 octane is readliy availble

What would be reccomended for fuel pump and ignition control? A Full msd setup is a little to much more my budget but if I have to I guess I must.

If I have the option to have the forged pistons installed shoud I? They are the stock cast pistons. This company Supplies like Tires plus and other garages with short blocks, only corprate places. So it could be difficult to do BUT I may be able to convince them.

Would some octane boost be a good Idea too?

I dont want a piston in my oil pan is my main worry, the next is staying off the throttle until new tranny, rear, and subframes get installed snap goes the axel. This is why the nitrous isnt till later =D

 

Measure and calculate the cr yourself. If you want a bullet proof motor, why did you buy one with a "stock" bottom end?

A motor with cast pistons will easily handle 150hp. If your tune up is all wrong you'll break the motor reguardless of what piston is in it.

Don;t expect the engine builder to do this upgrade for free. Forged pistons cost a lot more and require the motor be balanced. Forget about little cans of octane booster. If you want to *significanly* raise the octane of pump gas, mix it with 110-114 octane unleaded racing fuel.

Why not build the motor to perform without using Nitrous.
1.6 rockers will not make the cam "bigger" near enough. Get the right cam for the job.
A fair size solid lifter (eg comp cams 12-678-4) (could be roller) cam, better flowing heads (vortecs at a minimum) and exhaust combined with a racing torque converter and more rear gear will get you there without the worry.

 

usually stock type rebuilds fail under nitrous boost because the piston ring gaps are way to tight. The extra heat of combustion under nitrous causes the top ring gaps to close up lifting the top ring land off the piston. This has nothing to do with the strength of the piston. Nitrous motors need larger ring end gaps.

 

I havent purchased it yet. I will be soon. As for the ring gaps, that could be easily solved by putting in new rings is assume. So in otherwords F the nitrous. I Have already purchased the cam. I suppose It could go on ebay or I could call jegs and see if they will do and exchange.... I wasnt expecting it for free. I was going to supply the forged pistons for him.