1996 - L31 Vortec Rebuild - Flat tops or Dish stockers? - Hot Rod Forum : Hotrodders Bulletin Board
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Old 02-21-2016, 12:16 PM
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1996 - L31 Vortec Rebuild - Flat tops or Dish stockers?

Hi guys, we pulled out the engine on the 1996 Yukon.
L31 Vortec SBC, We were doing the top end only but we decided
to go with a full rebuild. We already have some parts for it.

1 - 0912 AFR Heads
2 - LT4 Hotcam
3 - 1.6 Crane Cams Gold Race roller rockers
4 - 3600 Yank Torque Converter.

We already have 1 5/8 headers and other goodies
as the Taylor intake spacer. We want to make the truck
fun to drive, not looking to make it a race truck but closer
to the feel of it.

We are going .30 over, so it will be a 355 but here's the doubt.
Which kind of pistons to run? We are going with Hypereutectic
i rebuild my 87 Vette L98 eight years ago with great success using
Mahle Flat tops and runs really great so i want to repeat the formula.
But i'd rather ask than make a mistake, we still have the black box
and no 411pcm so we might purchase a custom tuned black box for now
and later maybe a fast system so we won't have to worry about tuning
issues.

Will .030 over Mahle Flat top pistons work on my build?
or should i go with the stock dished pistons that they sell specifically
for 1996-2002 Vortec engines?

Thanks in advance.

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Old 02-21-2016, 01:30 PM
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The LT4 hot cam will like the added compression of the flat tops.With factory dish pistons, your lower rpm power can be a little a little soft with that cam.It works best with at least 10:1, or,10.5:1.
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Old 02-21-2016, 01:39 PM
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Thanks Joker, there is a nice set for around $ 150.00
i think i will pick those Mahle Hypers.
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Old 02-21-2016, 02:06 PM
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Make certain they have a 1.56" pin hgt.Some aftermarket pistons for rebuilds will have a shorter pin hgt causing the piston to sit lower in the bore.This will lower your compression & cause problems with detonation. It will be impossible to get the quench to proper range of .035 to .045 without having the block cut.
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Old 02-21-2016, 02:17 PM
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Quote:
Originally Posted by hcompton View Post
Im a big fan of the flat top pistons in the sbc they work great for many applications. If you plan or running 92 octane gas it should be fine.

Are you running the lt4 hot cam that has .525 lift or the low lift version. I like the larger one and its often the only one most people see. I think the smaller one is basically for class racing and to fit low lift heads. Its under 500 lift so its not a good choice for larger heads.

The heads certainly look like a good match for that cam with there big cfm right at .500 and .525.

This cam will need a tuned ecu or different setup.

You could use this system with a nice big carb style manifold and make the most of the setup. Most of the gm manifolds are tightly configured for flow in the range the engines can they build use. These can work with the small victor or other high flow intake.

FiTech Fuel Injection

This engine sounds like a solid build and should be well matched. I think the flat tops will make it produce the best power and the most low end torque and snap out of the motor.

Hope this helps.
There is no smaller LT4 Hot cam.The .525"lift is with 1.6 rockers, which is what comes with the Hot cam kit.It was designed as a replacement cam for showroom stock racing for LT1/LT4 engines which came with 1.6's.Using 1.5's will give you approx .495.

Last edited by jokerZ71; 02-21-2016 at 02:31 PM.
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Old 02-21-2016, 02:23 PM
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Quote:
Originally Posted by jokerZ71 View Post
Make certain they have a 1.56" pin hgt.Some aftermarket pistons for rebuilds will have a shorter pin hgt causing the piston to sit lower in the bore.This will lower your compression & cause problems with detonation. It will be impossible to get the quench to proper range of .035 to .045 without having the block cut.

Is the 1.56" pin height something i see on specs?
Thanks!


Quote:
Originally Posted by hcompton View Post
Im a big fan of the flat top pistons in the sbc they work great for many applications. If you plan or running 92 octane gas it should be fine.

This cam will need a tuned ecu or different setup.

You could use this system with a nice big carb style manifold and make the most of the setup. Most of the gm manifolds are tightly configured for flow in the range the engines can they build use. These can work with the small victor or other high flow intake.

FiTech Fuel Injection

This engine sounds like a solid build and should be well matched. I think the flat tops will make it produce the best power and the most low end torque and snap out of the motor.

Hope this helps.
Yes we will need a tune.
Right now we will slap the terrible stock intake with the spider
(which we already upgraded and added a manifold spacer as well ) it might run good for now but i know we are leaving lots of potential power on the table, so thanksfor the link, we are thinking about those selftuning setups for the future if there is enough proof that we will pick some considerable power
to make the investment. Hotcam with those AFR Vortec heads and 1.6 will make it nice im sure, i have a similar build on my Vette using AFR195 heads with the Lingenfelter 219 cam all together packed into a 3200 Vigilante and i really love it.
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Old 02-21-2016, 02:37 PM
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The pin hgt should be listed in the specs.It may be listed as "compression hgt".This is the measurement from piston crown to the center of the wristpin.It determines the depth the piston will sit in the bore.Some replacement pistons will sit farther down by as much as .020".They are often referred to as "rebuilders pistons".
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Old 02-21-2016, 02:45 PM
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With aluminum heads, it's not good to use thinner style shim gaskets, so, you may need to have the block cut in order to obtain a good quench distance & still be able use a good gasket suitable for aluminum heads.IMO, I wouldn't go any thinner than a .028" gasket.
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Old 02-21-2016, 04:34 PM
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Thanks for the input, this is what i used with my L98 with great results
and i got this one for the L31 as well

Compressed Thickness (in): 0.026 in.
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Plus i have the Sealed Power set
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Old 02-21-2016, 04:45 PM
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For that heavy vehicle, you may want a piston with a D-dish cc size halfway between full dish and flat tops to prevent overheating/pinging... also, be aware the Hot Cam doesn't start pulling hard till 2,000 RPMs or so...
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Old 02-21-2016, 04:47 PM
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Yes, we got it covered with the 3600 Yank

Why is that Piston Choice Buzz?
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Old 02-21-2016, 05:28 PM
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A D cup piston has the benefits of a flat top, but, adds some volume to help lower compression. You'll be fine with the flat tops & your head/cam choice.D cups won't be needed.That cam can be really soft down low with less than 10:1.
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Old 02-22-2016, 06:47 AM
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Thanks hcompton, yes i saw the forged option as well
but ...if we are going that route we would use forged rods
and crank but we won't be using any forced induction on it.
Im leaned towards hypereutectics since they work real good
on my L98 it's just that i want to follow my proven formula
but i want to make sure if those pistons will work on the L31
setup we are doing now.
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Old 02-22-2016, 07:28 AM
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Speed Pro #H631CP is also a good piston for your build if the Mahle's your looking at don't come thru.Just stay away from Speed Pro H345's.The H631 is about the same price as the Mahles at $160 per set.

Last edited by jokerZ71; 02-22-2016 at 07:52 AM.
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Old 02-22-2016, 07:34 AM
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Thanks Joker, im asking the seller about compression height today.
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