1990 350 - Internally or externally balanced? Thanks guys. I just wanted to know which Streetdampr to buy!

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1990 IROC 350
Mods: Too busy trying to make it run right to mod it.
Airfoil, Dynomax cat-back, Accel coil, 180 t-stat, Bald Eagle tires,
Hypertech fan switch, Accel 23# injectors, Holley AFPR, ported plenum,
Ruger P95DC, hot wife, new oil filter, thick rubber floormats, no cats.
18.0 @ 85MPH since I'm one big-a$$ MF
"It's better to have and not need than to need and not have."

 

internally.

the only externally balanced smallblock is the 400 or any variant using a 400cid crankshaft (ie. 383)

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Webmaster: www.IROC-Z.org

 

thanks colin I have wondering this too. you don't know how many times I've asked this question and gotten conflicting answers.

 

Definitely a big thanks. Two engine guys out at work said all SBC 350's internal and 400's external. One guy said '86 and up 350 external, but I thought the reason he was thinking this is the one-piece rear main and the weight added to the flexplate. Again, thanks.

 

Jethro,

I can't see any reason why they would change the engine. The only major differences between a 1990 350 and a 1970 350 are the roller cam and lifters and the one piece rear seal..

i believe they still use the same rods, crank, and pistons, and even most of the same head parts, same externals even..

GM had the opinion that if it wasn't broke, don't fix it, and it makes it alot easier to find parts

the only reason the 400 was externally balanced was that the crank was too big to balance it correctly internally. That's the same reason the 396 and 427 are internal and the 454 is external

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Webmaster: www.IROC-Z.org

 

This is not entirelly correct! Although the 1990 350 (all blocks, even LT1/4s, after '86) is not balanced externally like conventional 400 engines, it is not internally balanced (like 2-piece rear main seal engines). But, to confuse you guys, it is not externally balanced like a 400 either. Do a search on this forum, we have discussed this before. It is not internal nor extrenal, but rather "late model externally balanced". This is because the mounting flange for the flywheel/flexplate is much smaller than the older design and does not provide enough material for counter balancing. So the flywheel/flexplate and (IIRC) the balancer assist in adding the needed counter weight.

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George P. Lara
1984 Z28
2001 SS #0391
SCCA, SCFB, SC3GFB

[This message has been edited by MRZ28HO (edited July 17, 2001).]

 

Exactly. GM balances its engines as an assembly now. The crank, balancer, and flywheel are balanced as an assembly. The pistons and rods are manufactured with certain tolerances to keep them as close to balanced as possible. This method resembles what people call 'balancing and blueprinting'. When an engine is balanced after internal parts have been changed (ie pistons, rods) the engine builder, if he is not worthless, will balance the rotating assembly as a unit with the balancer and flywheel bolted up and weights to represent the pistons and rods on each journal.

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1987 GTA L98 MD8 GH3
355 '97 Vortec 4-bolt block, TRW(suck) forged pistons 10:1 CR, TFS 23*, LT4 Hot Cam, Ported plenum and intake, SLP Siam Runners, Edelbrock TES, gutted Cat, Flowmaster, K+N cone, AFPR, TB bypass, AirFoil, !AIR, !A/C, !EVAP, ARAP, 2300 stall, Hotchkis STB.

 

man I knew this would happen...

 

The one guy is correct. The other two are partially correct, since the 400 is counterbalanced on both the harmonic damper and flexplate, while the 350 has a neutrally balanced damper regardless of the flexplate weight or lack thereof.

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82 Berlinetta, orig V-6 car, now w/86 LG4/TH700R4. 2.93 limited slip. Cat-back from '91 GTA, ZZ3 intake, Accel HEI SuperCoil. AMSOIL syn lubes bumper-to-bumper. Daily driver, work-in-progress (LG4 w/'87 LB9 block, ZZ3 cam, ported World 305 heads, Hooker headers & y-pipe, 3" Catco cat & 3" cat-back).
57 Bel Air, my 1st car. '66 396, 9.7 CR forged TRWs, Weiand Action+, Edelbrock 1901 Q-Jet, GK 270 cam, Magnum rockers, Jacobs Omnipack, 1-3/4" Hedders & 3" Warlocks, TH400 w/TCI Sat Night Special conv & Trans-Scat shift kit, MegaShifter, 3.08 8.2" 10-bolt w/Powertrax, AMSOIL syn lubes bumper-to-bumper. Idles smooth @ 600 RPM in D. Best 15.02/95.06 @ 5800' Bandimere (corrected 13.93/102.4 @ sea level).

 

I have often wondered about this topic and once thought I had a grasp of the sitiuation at hand but now I am confused.

 

I guess I should have said they're both half right. One-piece rear main seal engines have a weight on the flexplate that makes them "half-externally balanced". 383's & 400's are externally balanced on both ends.

If all you want to know is which damper to buy, a 350 needs an internally balanced piece.

[This message has been edited by five7kid (edited July 17, 2001).]

 

So, if you build an engine, with all aftermarket parts like Eagle rods, Speed Pro pistons, a lunati crank or whatever, you have to have it balanced at a machine shop?

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1987 Chevy Camaro IROC-Z
L98 TPI 350 (5.7L)
TH 700R-4 Transmission
Borg Warner 7.75" 9 Bolt Rear End with 2.77:1 Gears.

Current Mods: LT4 HOT Cam, Comp Cams 1.52:1 Roller Rocker Arms, Edelbrock TES 1 5/8" Headers, Catco 3" High-Flow Catalytic Converter, Hooker 3" Aerochamber Cat-Back System, Transgo Shift Kit, Performance Resource Chip, Accel Ignition, K&N Filters, JET TPI Air Foil, All Free Mods, Falken ZIEX Z-Rated Tires.

Best ET: 14.32 @ 97.7mph
(corrected for elevation)

 

You should always balance a new combo. They need the pistons, rings, pins, pin retainers (If not press fit), rods w/bolts, rod bearings (although they already know the weight), damper and flywheel/flexplate (for a more accurate job, and needed for a 400,383,454) and 2grams for oil weight.

I just got my reciprocating assm. today and I was surprised how much material they had to remove to bring it into balance. Looks like 4 - 3/4" holes going as much as 1-1/4" deep to just under 1/4" with a bunch of other smaller holes drilled here and there.

It couldn't of even have been close right out of the box.

 

Thanks guys. It's good to know all this since the one guy was saying I need an external balance rather than a neutral balance torsional damper. Glad I can now buy the cheaper damper! I understand where all the confusion comes from now.

 

The reason why people get so confused about this is the difference between a mass produced engine (like one from the factory) and a pieced together one that you buy a bunch of parts for and throw together. Since the stock engines are all built in the same plant with the exact same pieces for the same application, they can use tolerances in the manufacturing process of the internal componants to keep an engine relatively balanced.

When building your own engine with aftermarket parts, you MUST balance the entire rotating assembly or will most assuredly shorten the life of the engine and give away horsepower. The loss of power will be because of the increase in internal friction and vibrations from the mismatched componants making the engine work harder just to spin it's own rotating assembly.

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1987 GTA L98 MD8 GH3
355 '97 Vortec 4-bolt block, TRW(suck) forged pistons 10:1 CR, TFS 23*, LT4 Hot Cam, Ported plenum and intake, SLP Siam Runners, Edelbrock TES, gutted Cat, Flowmaster, K+N cone, AFPR, TB bypass, AirFoil, !AIR, !A/C, !EVAP, ARAP, 2300 stall, Hotchkis STB.

 

BTW,

What is the P95DC? I have seen plenty of regular P95s but never a DC, whatever that is.

Beretta 92FS for me