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Hot Rod Forum : Hotrodders Bulletin Board > Tech Help> Hotrodding Basics> whats better 400 crank or a 383 crank in a 350 sbc when trying to build a 383?
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Topic Review (Newest First)
12-28-2012 08:27 AM
Mr. P-Body Gosh... (red-faced...)

Thanks. I DO try!

Jim
12-28-2012 06:02 AM
Bonneville462
Quote:
Originally Posted by Mr. P-Body View Post
Our experience with aftermarket CAST cranks has not been good. Most are flimsy and hard to "balance", Scat being amongst the MOST difficult. We tried them in the low-level circle track engines and they simply don't "hold up". The factory crank (400) is a good nodular iron unit. If you find one with no cracks, it's the 'better" casting to use.

If you plan any significant power, the aftermarket forgings are much better. We "like" the Eagle products, but others like Scat. These are both strictly "import". The American-made cranks are very good, and quite a bit more money.

NOTE: Many of the imported cranks are made of CAST steel. Even though they use the word "steel" in the description, these are NOT what we refer to as "steel" cranks. "Steel" implies a forging, NOT a casting.

FWIW

Jim
Jim knows his stuff....
12-27-2012 10:08 PM
cobalt327
Quote:
Originally Posted by BuzzLOL View Post
.. Hot Rod mag. said they changed the stock head gasket on their 2002 HT383 to a steel shim and bumped the compression ratio from 9.1 up to 9.6... they didn't mention the thickness of either gasket... they did say 18cc piston dish, .026" in the hole... I also read somewhere's there was a change made to the pistons (& head gaskets?) later... their engine was rated 325HP and 415 lb.-ft., but they got a baseline dyno of 338HP 444 lb.-ft., almost identical to the current ratings of 340HP 435 or 440 lb.-ft. (don't know why torque different at different sources)
There were two design pistons. The second design (and what I think is currently used) is +0.005" bigger.

I don't know if the reason for the bigger diameter piston was because of piston to wall fit or to bring the actual displacement up to 383 from 382.

If the piston is 0.026" down the hole and the CR is 9.1:1, the head gasket would need to be about 0.041". But that puts the quench at 0.067".

Using the same numbers, a 0.015" shim head gasket puts the CR at about 9.6:1- just what HR said they got.

This leads me to believe GM built the HT383 w/a very poor quench distance.
12-27-2012 09:52 PM
BuzzLOL .. Hot Rod mag. said they changed the stock head gasket on their 2002 HT383 to a steel shim and bumped the compression ratio from 9.1 up to 9.6... they didn't mention the thickness of either gasket... they did say 18cc piston dish, .026" in the hole... I also read somewhere's there was a change made to the pistons (& head gaskets?) later... their engine was rated 325HP and 415 lb.-ft., but they got a baseline dyno of 338HP 444 lb.-ft., almost identical to the current ratings of 340HP 435 or 440 lb.-ft. (don't know why torque different at different sources)
12-27-2012 08:32 PM
cobalt327
Thinking outloud

What I'm seeing is the 383HT's pistons give a 9.1:1 CR w/a 0.028" thickness HG and the Vortec chamber size. So that takes the block being built w/a zero deck out of the equation, I think.

Chevy shoots for a 9.0" stack of parts for SBC engines. Using a 9.025" block deck height, 3.8" stroke, and 5.7" rods, the piston CH would need to be 1.4” for the stack to equal 9”: 1.9 + 5.7 + 1.4 = 9.0

It appears the HG is 0.028" thick, and we know the CR is advertised as 9.1:1.

A 0.028" gasket using the numbers above will give a quench distance of 0.053". To bring the quench down to 0.040" would require a piston CH of about 1.413" (or a HG thickness of 0.015").

Approximate dish volume:
Using the quench distance of 0.053", a piston dish size of 22cc would be about right for a 9.1:1 CR.
Quench of 0.040" = 24cc dish.

383HT specs
12-27-2012 05:49 PM
BuzzLOL .. One place I looked by part number showed the HT383 #12499103 pistons to be for a Gen. 1 350" engine... Jegs shows them as 4-eyebrow flat tops in their picture... guess we won't know CH or deck clearance until someone measures one... maybe the reason the HT383 performs so well with what looks like a tiny cam is because it has -0- deck...
12-27-2012 01:24 AM
cobalt327 The GMPP 383 SBC crate engines have the 3.8" stroke crank and use 5.7" rods. What I can't find is the compression height (CH) of the pistons they use. Looked for both the first and second design part numbers, and found nothing on the CH.

One one forum a guy estimates the CH of his GMPP 383 to be 1.375", but that puts the piston too far down the hole (about 0.050" w/o a head gasket installed). I just can't see Chevy doing this- even though the pistons are crappy round dish, but w/2 instead of 4 valve reliefs. But that's small consolation for the round dish/narrow quench band design.

If the 3.75" stroke/5.7" rod piston CH of 1.425" is used, that puts the piston deck height at zero on an undecked 9.025" block. I really don't see Chevy doing that, either.

So does anyone know what the CH is for the GMPP 383?
12-26-2012 11:33 PM
BuzzLOL .. Chevy makes two 383" crate engines... the 325HP one will propel typical mid-size drag car into high 12's or low 13's... 425HP gives high 11's to low 12's...
12-26-2012 08:23 AM
Mr. P-Body Our experience with aftermarket CAST cranks has not been good. Most are flimsy and hard to "balance", Scat being amongst the MOST difficult. We tried them in the low-level circle track engines and they simply don't "hold up". The factory crank (400) is a good nodular iron unit. If you find one with no cracks, it's the 'better" casting to use.

If you plan any significant power, the aftermarket forgings are much better. We "like" the Eagle products, but others like Scat. These are both strictly "import". The American-made cranks are very good, and quite a bit more money.

NOTE: Many of the imported cranks are made of CAST steel. Even though they use the word "steel" in the description, these are NOT what we refer to as "steel" cranks. "Steel" implies a forging, NOT a casting.

FWIW

Jim
12-26-2012 04:06 AM
BuzzLOL .. Actually Chevy does make and sell a 383" engine with a 383 crankshaft... but it uses a different set of dimensions to get to 383" than what the aftermarket usually uses. Chevy's slightly longer stroke 383 crank gets 383" without having to overbore a 350 block...
02-07-2011 12:32 PM
cobalt327 If you are building a SBC 383, use a dedicated 383 crank w/the mains already sized for the SBC 350 main journals.
02-07-2011 12:32 PM
327NUT Just to clarify if this is what you're thinking (??) Chevy never made a 383 crankshaft. You get a "383" by boring a 350 sbc .030, having a 400 sbc crankshaft machined to fit the 350 block.

By the time you find a GOOD 400 crankshaft and have it machined you could buy a Scat 9000...ready to go.....with money left over. This of course depends on what power level you're aiming for and I hope you're not one of the dreamers that come on here wanting a 2000 hp engine.
02-07-2011 12:01 PM
onlychevy
400 crank or 383

so what you saying is that there is no difference other then haveing to machine the 400? but what would you recommend?
02-07-2011 11:46 AM
68NovaSS I don't understand your question fully. If you use a true 400 crank, you'll have to turn the mains down. If you're talking about a SCAT 383 crank, it's just that, they're own version of the 400 with smaller mains, same with any after market 383 crank. Keep in mind you'll need different pistons for the new stroke as well, the old 350 pin location no longer works.
02-07-2011 11:37 AM
onlychevy
whats better 400 crank or a 383 crank in a 350 sbc when trying to build a 383?

i am building a 383 with a 350 block but i want to know what crank is better a 400 scat or a 383 crank??? please help

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