Streetiron85

I've been doing some searches and here I am posting again cause I couldn't find what I need.
Okaaay... So I'm trying to figure out which pistons to go with for this 383 truck motor project, and of course, first I'll need to know what CR I'm aiming for. Maybe someone can help out.
Basically, I'd like a dished piston that will give me the proper CR with a "vortec style" head. To be used in a work truck, with a TPI induction system.
I say "vortec style" head because preferably I would like to find a head that works like the vortec does, but has the regular SBC bolt pattern, that way I can keep my intake base.
I looked at the SDPC site to see what they have to offer and all I found out was that I can't trust their numbers at all, their ad says a flattop piston 383 with 64cc heads yields a 9.1:1 CR.
My block will be zero decked and I'm thinking that with vortec heads it's possible to go a bit higher on the CR than with older style iron heads.
The pistons I had in mind are the KB 18cc dished, and according to the CR calc, that will yield a 9.6:1 CR with a 64cc head and a .040 thick gasket.

http://www.kb-silvolite.com/temp/calc.php?action=comp
Use this link to check my math.

So how does that combo sound? Is the CR too high? Anyone tried it?
383 TPI truck motor
BTW I'll be using a hyd roller cam between 210* and 220* @ .050 (int)

Thanks in advance

RB83L69

"Vortec" = GM's trademark that means "we put the intake ports in a different place". Specifically, they raised them, so that the incoming air has a straighter shot into the cyl. Therefore, it's impossible to have a "Vortec" (or "Vortec style") head that a regular intake will fit, because the intake ports won't line up. By the definition of the word.

Those CR figures are way incredibly extremely far off. My calculations show a zero-decked 383 with 6cc valve relief flat-tops, a .039" gasket and 64cc heads to have a CR of right at 11:1. They show the same motor with 18cc dish pistons to be about 9.7:1. Those are about the same figures I'm accustomed to see elswhere.

Something is fishy here.

Streetiron85

What I meant by "vortec style", is the heart shaped chamber in an iron head that has performance similar to the vortec.
I swear there's an ad on the SDPC site for a 383 short block that claims 9.1:1 CR with 64cc heads and flattops. It just makes me not trust thier data.

Streetiron85

Is there such thing as an iron head that has std intake bolt pattern that performs like the vortec?

RB83L69

That part of the Vortec head design is only a very small part of the reason that they are what they are. The important part is the intake runner change. IMHO there's relatively little to be gained in the chamber shape.

But, Edelbrock heads I know have a similar chamber design, and maybe others do too. But they're aluminum.

9.1:1 is just wrong for that engine combo. Just as a simple smell test, a 350 with 64cc heads and the same specs is about 10.25:1, and a 383 is going to be higher than that, so there's no way it can be 9.1:1. Something's rotten somewhere.

Streetiron85

Yes, That's why I'm looking at the 18cc dished pistons.
So with a good set of Iron heads, like maybe some pro toplines is 9.6:1 CR too high for a TPI truck motor?

RB83L69

IMHO that's about the highest you'd want to run. Should be OK.

AJ_92RS

If this is the engine you're talking about http://www.sdpc2000.com/cart.asp?act...id=120&pid=465

then where do you see anything about flat-top pistons???

I don't see it anywhere.

Streetiron85

That isn't the one, it was a 383 shortblock.
I was just searching that site trying to find out what I could, and I couldn't find much data that I considered reliable, regarding piston selection. So I thought I'd post here.

http://www.sdpc2000.com/cart.asp?act...=514&pid=81632

AJ_92RS

OK. Thanks.

That is the same shortblock. But that helps someone who's more interested in what slugs came in it.

BTW, don't try to put that up against a typical 383. It isn't.

A "shade tree" 383 is a 350 block bored .030" over, then a 3.75" crank dropped in.

That engine has a standard 4" bore 350 block, but a NEW 3.80" stroke crank.

Who knows what length connecting rods they used, or what the compression height on the pistons is, or even how thick the gasket was/is/should be.

Ignore that engine when looking for ways to build your own 383. It will confuse you, obviously

EDIT: something else I just noticed..... the pic shows flat top, and the written description says flat top, but if you look at the brief description on the upper right, it says Hypereutectic dished.

Streetiron85

I noticed that myself... then at the bottom it says flat top... It just makes it more confusing. But that's sort of beside the point anyhow.
OK, so let's say I get some vortec heads, and matching intake base for my 383 TPI truck motor. Seems like a good choice, they perform well and they are, after all, truck heads. Do you think the 18cc dished piston be too high CR for that combo? (9.6:1)

AJ_92RS

It "should" be OK

One thing about the Vortec heads is they have excellent swirl, take advantage of the quench area, and they obviously flow well with smaller ports. Those all help prevent detonation.

Plus they don't like a lot of total timing. 32* usually does it. That allows you to get max efficiency out of them with less chance of detonation.

The problem is they usually check out at 62cc in the combustion chambers. I figured your compression at 9.623:1 using 62 cc combustion chambers with everything else you described. I also used 4.166" as the gasket diameter. That may be a little large, depending on the brand you use.

The disadvantage with using the common 18 cc pistons is the loss of the quench area. The latest craze is the 'd' shaped dished pistons which put the dish right under the head chamber. This allows the quench area to still be used. A lot of those pistons are noted to specifically work well with Vortec heads, or similarly shaped chambers.

Be cautious if you're buying used Vortec heads. I did and they have to be milled .007" to get them flat. I'm going to lose a little size in the already small chambers.

What cam are you going to use? Or, more specifically, what are ALL the specs on it?

Streetiron85

Thanks,
The KB 18cc Dshaped cup pistons are the ones I was looking at.
And the cam I'm thinking about is a hyd roller of between 210*- 220* @ .050 (int) possibly one of the Comp EFI grinds.
I called Comp Cams last wk and I was told the XR264HR or the XR269HR would be good. Both of those are 112* lsa, the 264 is 212/218*@.050, the 269 is 218/224*@.050.
At the time I talked to Comp tech tho, I was unsure of the CR, I was thinking back then that I might use 76cc heads. I probably ought to lean toward the bigger cam since the CR is borderline, yes?
Plus I guess the TPI will be helpful in making it more compression tolerant with good mixture dist.

Any idea what the actual CR is on the factory 350 vortecs truck motors ???