Chuck84TA

Engine performance goals in terms of priority-

1) driveability. I want a smooth, strong engine build.
2) Horsepower. Looking at 280-320 range. As much low-end torque as decent driveability will allow.

I plan to have Hooker 2055's, CCC qjet, and the PROM optimized for my final configuration.

Trying to learn more about options with respect to piston choice. I understand that the stock L98 (came out of a 1989 IROC) uses dished like pistons. If my stock pistons are checked out ok, should I keep them or go flat top?

Chuck

sofakingdom

iTrader: (1)

What heads?

Chuck84TA

Stock heads. Iron, not sure if they are 083 or 624?. Not at home now so i can't verify the exact specs now.

Chuck84TA

Thinking of this -


Cam- slightly more aggressive, LT4 or something ....still learning about options...

Heads - keep stock, have them cleaned and ported OR go with 113 heads.

sofakingdom

iTrader: (1)

Big difference there...

083 = excellent head, one of the best pre-Vortec iron stockers, came on L98 and certain of the non-"Goodwrench" GM crate motors; 64 cc chambers

624 = probably the lowest of the low, deepest of the deeeeeeeep smogger turd heads, poor flow, highly prone to cracking even before any valve jobs or other metal removal has occurred; 76 - 80 cc chambers

If I were building such a thing, I'd build a flat-top short block, 083 heads, pistons that DON'T have the extra .020" of "rebuilder" clearance (i.e. MEASURED compression height 1.56", NOT "catalog" compression height "designed in USA to fit 350"). IN NO EVENT would I EVER use those heads, or 882 or 993 or any other 76cc smoggers, with dome pistons just to fake the CR up.

If all I had was 624s, I'd settle on something else before choosing pistons. I might look real hard at Vortecs in that case.

Either way, definitely need to know what heads you're going to end up with, before accidentally making a mistake.

Chuck84TA

Thanks Sofa.

NagleMac

iTrader: (1)

Yup, piston choice will depend on the heads. I think you should be able to meet your goal with the LT4 cam, clean up the heads a little (depending on which ones you have). Are you also changing your timing chain, rockers, lifters and pushrods?

Chuck84TA

All if that. Complete rebuild. Figure to get it all done at once, the right way

Chuck84TA

I'll try to keep the parts that are still good. But new chain, water pump, oil pump ..etc

Johnny Blaze

iTrader: (15)

Using speed pro flattops on my l98 build. 083 heads, measured out to 64cc, Flattop with 4 valve relief,.020" in the hole, .015 gasket, came out to 10.3:1.

My new build is using probes, a bit lighter than speed pro.

InfernalVortex

iTrader: (4)

Be careful with the speed pros, many of them are 1.540 compression height. The ones with coated skirts are allegedly 1.560 compression height. I know I have the H345NCP and they're 1.560, yet the h345np (no skirt coating) are 1.540. I dont really know that I believe that, especially after what sofa said about measuring. Wish I could go back and build this engine again with all I know these days, I would have checked a whole lot more things a lot more closely.

skinny z

10.3:1 and iron heads? What cam?

Johnny Blaze

iTrader: (15)

^08-501-8

Johnny Blaze

iTrader: (15)

I was shooting for 9.5:1 and figured the speedpros would get me there, but according to the calculators it ended up much higher.
even with the pistons ending up .020" in the hole.

The listed ratios seem about useless.

Here is a thread disscussing my ratio and pump gas.

https://www.thirdgen.org/forums/tech...-pump-gas.html

Johnny Blaze

iTrader: (15)

My last build with the same pistons in my 69 c10 gave me 9.2:1 with the 76cc head and I ran on 87 all the time. With a small cam.

Johnny Blaze

iTrader: (15)

My DCR figures out to about 8.3:1.

sofakingdom

iTrader: (1)

To give you some idea of the variability that can be caused by deck clearance, here's what the CR works out to, for a .030" over 350 w 64cc heads and a .039" head gasket using flat-tops w 6cc valve reliefs.

The stock spec is .025"; most blocks have more than that, and it varies from side to side, and front to rear, on any given block; plus, crank journals aren't all perfect, so there's some variation also in both the stroke and the clearance. Rods vary in length, almost always to the short side of the "spec". Individual blocks sometimes vary by nearly .015" from cyl to cyl, virtually ALWAYS in the direction of GREATER THAN .025".

Reconditioning stock rods shortens them further. Any "crate" "rebuilt" engine may have one or more rods that have been reworked,, adding to the potential for variation.

The thinner the head gasket, the more critical the deck clearance becomes, because it is a greater proportion of the total "minimum" volume.

Deck clearance / static CR:
.000" 10.3
.025" 9.7
.030" 9.6
.040" 9.4
.050" 9.2
.060" 9.0

Virtually ALL cast pistons add the .020" of "rebuilder" extra clearance; the reason for this is, if a rebuilder is slapping motors together with minimal "checking" of anything, as is most often the case, (faster and cheeeeper to just assemble stuff with the least amount of measuring possible) then if the block has required heavy decking for example to repair a scour from a blown head gasket, having the pistons hit the heads would be fatal. But a loss of compression, not so much of a warranty problem. Therefore those guys prefer a piston that eliminates risk of catastrophic failure, at the expense of giving up a bit of compression, which doesn't affect the "rebuilder" in the slightest. As said, virtually ALL cast pistons are made this way; MOST hypereutectics; and MOST forged pistons except for racing designs, including MOST TRW/SpeedPro part #s. (since those are essentially "rebuilder" parts for truck engines)

NEVER EVER EVER "assume" anything about parts you are buying, and NEVER "assume" that just because the catalog "says" 1.56" CH or whatever, that that's what the pistons actually are. Often that just means, that's the "design height".

Easy enough to measure a piston and find out: the pin in a SBC is .927" dia; half of that is .4635"; a 1.56" CH piston will measure 1.56" - .4635" from crown to pin bore, or 1.0965". A "rebuilder" piston will measure closer to 1.0765".

Similarly, stock rods are 5.700" nominal. Big end bore is 2.225", pin bore is .927". Overall length you can measure with calipers is therefore 5.700" - 1.1125" - .4635", or 4.124" for a nominal-length 350 rod.

ALWAYS MEASURE EVERYTHING!!! NEVER ASSUME ANYTHING!!!! Attention to detail is what separates one motor built with a romantic-sounding parts list that turns out a disappointment, from the motor in the other lane built with the exact same parts list but that kicks the other one's butt.

Index-ground cranks (not ground "on the wear"), correct length rods, zero-decking the block, boring the block so all cylinders are perpendicular to the crank instead of "on the wear", torque plate boring & honing to guarantee that the cyls are round when heads are bolted on, and balancing, make a MASSIVE difference in how a motor runs. They are for example the difference between putting the "RPM kit" onto a "rebuilder" short block and getting the usual 280 HP or so, vs what Edelbrock did to get that "420 HP" "rating" they claim, which is a totally maxed-out DETAIL ORIENTED short block build. They don't "lie", mind you; they just kind of gloss over the fact that you don't open a box, pour whatever comes out all over your random short block, and magically, "420 HP" just appears out of nowhere.

1gary

Sofa.Very nice.Well said.

1gary

Good squish and SCR reduction with maximum flame travel .Kind of the best of two worlds.

skinny z

Have you ever done a compression test?

Johnny Blaze

iTrader: (15)

As in cranking pressure? Not on this engine.

skinny z

Yes. Cranking pressure.
What I'm finding are plenty of offerings for SCR/DCR based on the printed data but no one has provided compression test data to support the calculations.
I started (another) thread trying to collect some info so it can be posted as a guide.

https://www.thirdgen.org/forums/tech...sion-test.html

Without the compression test, all the info doesn't amount to much.

skinny z

Precisely what I found.
Stock crank, cut 10/10 (not indexed), resized stock rods, piston to deck height varies from .026" to .036" and so does the compression ratio and subsequent compression pressures.

skinny z

Why? What's the point of zero decking? Decking the block so it's uniform makes perfect sense but to go for a zero deck doesn't.
The block is limited in future rebuilds isn't it?
With a zero deck, the head gasket determines the piston to head clearence and forces the use of a gasket in the .035"-.040" range. The thicker gasket would increase the crevice volume and the spaces for trapped end gases wouldn't it? This is as compared to a .010" deck and a .026" gasket. Yes that's splitting hairs I suppose but I've never understood nor have I had anyone offer a reasonable explanation as to why a zero deck is considered the target spec.

sofakingdom

iTrader: (1)

Not really...

In a perfect world, the block should be cut to exactly 9.000" from crank CL to deck; perfectly square in all directions. That is, both decks parallel to the crank CL, perpendicular to the CL of the cylinders, perpendicular to each other.

(3.480" ÷ 2) + 5.700" + 1.560" = 9.000" (stock 350)

Or,

(3.750" ÷ 2) + 5.565" + 1.560" = 9.000" (stock 400)

Or,

(3.750" ÷ 2) + 5.700" + 1.425" = 9.000" (typical 5.7" 383)

Or,

(4.000" ÷2) + 6.000" + 1.000" = 9.000" (typical 434)

And so on. Then, it doesn't matter what you put in there; the block is simply .... CORRECT.

In the future, whenever it's rebuilt, as long as the crank is the CORRECT stroke, the rods the CORRECT length, and the pistons the CORRECT height, then the block will also be CORRECT.

I have NEVER, not in all my years of building motors, taken apart A SINGLE ONE that had been built with a shim gasket, that it sealed. NOT ONE. EVERY ONE I have ever seen has evidence of leakage around bores. Therefore I will not build a motor with those. I'll stick with composition.

As far as "crevices" or whatever, that's a function of the "fire ring" of the gasket, and of course the gasket's bore dia. As long as the CORRECT gasket is used (for example don't use a 1014 on a 4.03" bore) there aren't enough of that sort of thing to matter.

redneckjoe

Virtually ALL cast pistons add the .020" of "rebuilder" extra clearance; the reason for this is, if a rebuilder is slapping motors together with minimal "checking" of anything, as is most often the case, (faster and cheeeeper to just assemble stuff with the least amount of measuring possible) then if the block has required heavy decking for example to repair a scour from a blown head gasket, having the pistons hit the heads would be fatal. But a loss of compression, not so much of a warranty problem. Therefore those guys prefer a piston that eliminates risk of catastrophic failure, at the expense of giving up a bit of compression, which doesn't affect the "rebuilder" in the slightest. As said, virtually ALL cast pistons are made this way; MOST hypereutectics; and MOST forged pistons except for racing designs, including MOST TRW/SpeedPro part #s. (since those are essentially "rebuilder" parts for truck engines)

NEVER EVER EVER "assume" anything about parts you are buying, and NEVER "assume" that just because the catalog "says" 1.56" CH or whatever, that that's what the pistons actually are. Often that just means, that's the "design height".

Easy enough to measure a piston and find out: the pin in a SBC is .927" dia; half of that is .4635"; a 1.56" CH piston will measure 1.56" - .4635" from crown to pin bore, or 1.0965". A "rebuilder" piston will measure closer to 1.1165".







isnt this backwards? or maybe i'm reading the math wrong? wouldnt that make the "rebuilder" 0.020 more CH?

skinny z

That's what I'm referring to.
What of subsequent rebuilds and it's found that a few thou are needed to "clean up" the deck due to a gasket failure and other abuse?
Then again, I suppose if it was zero decked and 60 000 miles later it was found that a little material needed to removed to square things up, then the piston could be out of the bore a little and a thicker gasket used to compensate. Or a different piston spec I suppose. I just prefer the "what if" factor built into my personal projects.

sofakingdom

iTrader: (1)

No...

Maybe...

No...

CH is the distance from the wrist pin center, to the crown surface of the piston. The greater it is, the taller the piston, and therefore the less the deck clearance. So for example, the majority of "rebuilder" pistons for 350, have a CH of 1.54"; .020" less than the stock 1.56", and their top therefore sits .020" lower in the block, if all else is equal.

{edit} Sorry Joe, you're right, I musta got a brain cramp and added instead of subtracting, right there. Should be 1.0765". Sorry!!

skinny z

Another vote for composition over shim.

sofakingdom

iTrader: (1)

That's not unreasonable; given that a stock block core is all but free when considered as part of a max effort build, most people who are "going for it" would consider the block sacrificial. If you don't, ... then maybe leave .010" extra or something, and use the .028" Victorcore gasket or the like. But NOT, leave .020", and use a .016" shim gasket; at least, not if you want it to stay sealed indefinitely.

skinny z

That's something I've learned with reluctance. It's a shame to toss away what seems to be a useful core but in reality it's not worth much when put into the context of serious engine project.

redneckjoe

Easy enough to measure a piston and find out: the pin in a SBC is .927" dia; half of that is .4635"; a 1.56" CH piston will measure 1.56" - .4635" from crown to pin bore, or 1.0965". A "rebuilder" piston will measure closer to 1.1165".



1.1165 + 0.4635 = 1.58

isnt that an increase of 0.020? i'm trying to learn. it seems that example would put the piston up higher? maybe i'm not getting it?

sofakingdom

iTrader: (1)

You're right Joe, I had hosed it, and actually already fixed it.

redneckjoe

i actually learn a LOT from you and appreciate your time. i was thinking the rebuilder piston measurement would come out to 1.0765? kinda figured it was an honest mistake.

1gary

With the stacked parts method 0 deck rarely ends up exactly at 0.Often it is down just abit.

redneckjoe

all great information. i have some rebuilt rods i was considering using on a future build, and it never really crossed my mind to actually measure them. duhho!!!

Chuck84TA

Sofa,

They are 083's